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LSU Historical Dissertations and Theses
Graduate School
1992
Genetics of Stalk and Seed Quality Traits in Maize
(Zea Mays L.) With Lfy Gene.
Sridhar Dronavalli
Louisiana State University and Agricultural & Mechanical College
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Dronavalli, Sridhar, "Genetics of Stalk and Seed Quality Traits in Maize (Zea Mays L.) With Lfy Gene." (1992). LSU Historical
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G en etics o f sta lk an d seed q u a lity tr a its in m a ize ( Zea mays L.)
w ith L fy gen e
Dronavalli, Sridhar, Ph.D .
The Louisiana State University and Agricultural and Mechanical Col., 1992
UMI
300 N. Zeeb Rd.
Ann Arbor, MI 48106
GENETICS OF STALK AND SEED QUALITY
TRAITS IN
MAIZE (ZEA MAYS L.) WITH L F Y GENE
A Dissertation
Submitted to the Graduate Faculty of the
Louisiana State University and
Agricultural and Mechanical College
in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy
in
The Department of Agronomy
by
Sridhar Dronavalli
B.Sc. (Agri.), University of Rajasthan, 1983
M.Sc. (Agri.), Tamil Nadu Agricultural University, 1985
May 1992
ACKNOW LEDGEM ENTS
I express my sincere gratitude to my major professor, Dr. Manjit Kang for his
valuable advice and encouragement during the course of this study. I thank the members
o f my advisory committee, Dr. Stephen Harrison, Dr. Bobby Harville, Dr. Freddie
Martin, and Dr. Arnold Saxton for their suggestions in preparation of this dissertation.
I am indebted to Dr. Manjit Kang and the Department o f Agronomy for
providing me financial support. I acknowledge the valuable assistance rendered by my
friends Dr. Danny Gorman, Mr. W arner Hall, and Mr. Deng Dexiang in collecting data
from field experiments.
I owe a great deal to my wife Sarada Vani, my son Sunny, and my parents for
their patience, constant support, inspiration, and encouragement.
TABLE OF CONTENTS
ACKNOWLEDGEMENTS................................................................................
Page
ii
LIST OF TABLES...............................................................................................
v
ABSTRACT..........................................................................................................
viii
INTRODUCTION................................................................................................
1
LITERATURE REVIEW...................................................................................
4
Stalk Quality............................................................................................
4
Stalk Rots................................................................................................
4
Stalk Borers...............................................................................................
5
Stalk Strength..........................................................................................
6
Parenchyma Cell Death...........................................................................
8
Genetics of Pith Cell Death.....................................................................
11
Leaf Midrib Cell Death vs.
Stalk Pith Cell Death...............................................................................
12
pH vs. Pith Cell Death...........................................................................
13
Late Season Plant Health........................................................................
13
Fertility Imbalance...................................................................................
15
Plant Populations....................................................................................
15
Seed Quality...............................................................................................
15
References...............................................................................................
19
iii
CHAPTER 1. GENERAL AND SPECIFIC COMBINING ABILITY
FOR STALK INTERNODAL PARENCHYMA CELL
DEATH IN CORN WITH Lfy GENE..................................................
25
Introduction................................................................................................
26
Materials and Methods..............................................................................
31
Results and Discussion..............................................................................
33
References..................................................................................................
47
CHAPTER 2. DIALLEL ANALYSIS OF PARENCHYMA CELL
DEATH IN LEAF MID-RIBS OF CORN
POSSESSING TH E Lfy GENE..............................................................
51
Introduction................................................................................................
52
Materials and Methods..............................................................................
54
Results and Discussion..............................................................................
56
References...........................................
67
CHAPTER 3. GENERAL AND SPECIFIC COMBINING ABILITY FOR
SEED QUALITY TRAITS IN M AIZE.................................................
69
Introduction.................................................................................................
70
Materials and Methods..............................................................................
72
Results and Discussion..............................................................................
75
References....................................................................................................
82
SUMMARY AND CONCLUSIONS....................................................................
84
VITA.........................................................................................................................
86
iv
LIST OF TABLES
Page
CHAPTER 1.
Table 1.1.
Table 1.2.
Table 1.3.
Table 1.4.
Table 1.5.
Table 1.6.
Table 1.7.
Table 1.8.
Estimates of R2, C.V., and mean statistics for parenchyma
cell death (PCD) in each of four stalk intemodes from a
diallel cross among seven Lfy synthetics grown in three
environments during1988-90.....................................................
34
Estimates of R2 and C.V. statistics for parenchyma
cell death (PCD) in each of four stalk intemodes from a
diallel cross among seven Lfy synthetics grown at Ben Hur
Farm in year 1988......................................................................
35
Estimates of R2 and C.V. statistics for parenchyma
cell death (PCD) in each of four stalk intemodes from a
diallel cross among seven Lfy synthetics grown at Perkins
Road Farm in year 1989............................................................
36
Estimates of R2 and C.V. statistics for parenchyma
cell death (PCD) in each of four stalk intemodes from a
diallel cross among seven Lfy synthetics grown at Ben Hur
Farm in year 1990......................................................................
37
Analysis of variance for parenchyma cell death ratings for
third stalk intemode above brace roots from a diallel cross
among seven Lfy synthetics rated in three environments
(1988-90)....................................................................................
38
Mean parenchyma cell death ratings for third stalk intemode
above brace roots from a diallel cross among seven Lfy
synthetics grown in threeenvironments during1988-90....
40
Analysis of variance for parenchyma cell death ratings for
third stalk intemode above brace roots from a diallel cross
among seven Lfy synthetics rated in two environments
(1989-90)....................................................................................
42
Effect of Lfy gene on parenchyma cell death in each of four
stalk intemodes from a diallel cross among seven Lfy
synthetics grownin two environments (1989-90)....................
43
v
Table 1.9.
Mean parenchyma cell death ratings o f F, crosses (above
diagonal), general combining ability (GCA) effects (on
diagonal), specific combining ability (SCA) effects (below
diagonal), and line means o f F, crosses (F, from a diallel
cross among seven Lfy synthetics grown in three environments
(1988-1990).................................................................................
45
Estimates of R2, C .V ., mean, and correlation coefficients
(r) for parenchyma cell death in leaf midribs (LMCD) in eadh
o f five leaves, beginning with the sixth leaf from base of
a plant, from a diallel cross among seven leafy (Lfy)
synthetics grown in three environments during 1989 and
1990..............................................................................................
57
Estimates of R2 and C.V. statistics for parenchyma cell death
in leaf midribs (LMCD) in each o f five leaves, beginning with
the sixth leaf from base of a plant, from a diallel cross among
seven leafy (Lfy) synthetics grown at two environments
in year 1989................................................................................
58
Estimates of R2 and C.V. statistics for parenchyma cell death
in leaf midribs (LMCD) in each o f five leaves, beginning with
the sixth leaf from base of a plant, from a diallel cross among
seven leafy (Lfy) synthetics grown at Ben Hur Farm
in year 1990.................................................................................
59
Analysis of variance for ratings (mean of five leaves) for
parenchyma cell death in leaf mid-rib, beginning with the
sixth leaf from the base of a plant, from a diallel cross
among seven leafy (Lfy) synthetics evaluated in three
environments during 1989 and 1990........................................
60
Overall effect o f the leafy (Lfy) gene on mean parenchyma
cell death in leaf mid-rib in each of five leaves, beginning
with the sixth leaf from the base of a plant, from a diallel
cross among seven leafy (Lfy) synthetics grown in three
environments (1989-90)..............................................................
62
CHAPTER 2.
Table 2.1.
Table 2.2.
Table 2.3.
Table 2.4.
Table 2.5.
Table 2.6.
Mean rating for parenchyma cell death in leaf mid-rib of
Fj crosses (above diagonal), general combining ability
(GCA) effects (on diagonal), specific combining ability
(SCA) effects (below diagonal), and line means o f F,
vi
crosses (Fj) from a diallel cross among seven leafy {Lfy)
synthetics grown in three environments during 1989 and
1990.............................................................................................
65
Marginal means of Fi crosses for seven seed quality traits
in Lfy maize...............................................................................
76
Table 3.2.
Mean squares for seven seed quality traits of Lfy maize....
77
Table 3.3
General (g;) and specific combining ability (S^)
effects for seven seed quality traits in Lfy maize...................
80
CHAPTER 3.
Table 3.1.
vii
ABSTRACT
Parenchyma cell death (PCD) in stalk intemodes of maize (Zea mays L.) is
related to stalk rot. Spread of stalk rotting fungi is restricted by living parenchyma
cells. A previously reported correlation coefficient of 0.70 between leaf midrib cell
death (LMCD) and PCD indicated that selection for LMCD might result in low PCD,
thus improving stalk quality. Inheritance o f PCD and LMCD in maize possessing the
leafy (Lfy) gene needs to be investigated. Experiments were conducted in three
environments to estimate general (GCA) and specific combining ability (SCA) effects
for PCD and LMCD via a diallel cross among seven Lfy synthetics and to determine
the effect of Lfy gene on PCD and LMCD. Eight weeks after planting, five consecutive
leaves per plant, beginning with the sixth leaf from the base o f a plant, were visually
rated for LMCD. Twenty one days after mid silk, the first four stalk intemodes above
brace roots of 10 leafy and 10 non-leafy plants in each o f the 21 F, crosses were split
longitudinally and visually rated for PCD. The PCD of third stalk intemode and LMCD
of third leaf were reliable in differentiating genotypes. SCA was more important than
GCA for PCD, whereas GCA was more important than SCA for LMCD. A632 syn,
B73 syn, M ol7 syn, and Wf9 syn showed negative GCA effects for PCD, whereas
A632 syn, B73 syn, and Hy syn showed negative GCA effects for LMCD. The Lfy
plants had significantly lower PCD and LMCD than the non -Lfy plants, suggesting that
extra leaves in the Lfy plants probably produced supplemental photosynthate and thus
had favorable effect on PCD and LMCD.
The current knowledge o f inheritance o f maize seed quality traits, though
important to breeders, is limited. Seed from the 21 F t crosses were evaluated for seven
seed quality traits. SCA was more important than the GCA for most seed quality traits.
Significant additive genetic correlations (r > 0.84) among seed weight, dry matter,
and vigor index suggested that selection for heavier seed may result in improved
seedling vigor. Results revealed superiority o f Hy syn and Hy syn x Mo 17 syn cross
for seed quality.
INTRODUCTION
Stalk lodging in maize (Zea mays L.), a reflection of poor stalk quality, is
affected by several stalk rot-causing organisms. Diplodia maydis is one of the major
causal organisms of stalk rot. Stalk rot and associated stalk lodging result in loss of
millions of bushels of maize. Estimated annual yield losses due to stalk lodging vary
from 5 to 25 percent (Zuber and Kang, 1978).
Considerable progress has been made in stalk quality improvement as evident
from the fact that hybrids developed in the 1940s and 1950s were nearly three times
more prone to stalk lodging than the hybrids developed in the 1970s and 1980s (Zuber,
1982). However, stalk lodging continues to be a major production problem, especially
with an increase in use of fertilizers and high plant populations for higher productivity.
Therefore, stalk quality improvement has been and probably will remain an important
and challenging issue for maize breeders.
The stalk of maize contains parenchymatous tissue, commonly called pith. The
vascular strands of xylem and phloem elements surrounded by sclerenchyma sheaths are
located in the pith and offer strength to the stalk. Further, the pith of lower intemodes
is usually filled with fluids, thus providing maximum strength at the point of greatest
strain. Stalk rots cause lodging chiefly by destruction of pith, allowing stalk failure by
collapse of unsupported peripheral rind tissue (Duncan, 1975). Host plant resistance is
the most effective means o f reducing losses due to stalk rots (Hooker, 1957).
Stalk rot incited by D. maydis was found to be related to death of parenchyma
cells in the stalk, and stalk rot resistance was related to the presence of living cells in
1
that tissue (Pappelis, 1957). A highly significant positive correlation between
parenchyma cell death (PCD) in stalk intemodes and stalk rot ratings was reported by
Pappelis and Williams (1966). Selection of maize genotypes with low PCD and their
use in a breeding program may result in hybrids resistant to stalk rot. However,
inheritance of PCD has not been thoroughly investigated.
The methods currently used for evaluating stalk quality are cumbersome and
destructive in that stalks are split longitudinally or stalk sections are removed for
evaluation. Maize breeders are interested to know whether stalk quality can be
improved by using an easily measurable trait as a non-destmctive method. A significant
correlation coefficient (r= 0 .7 0 ) between leaf midrib cell death (LMCD) and PCD in
stalk intemodes was reported (Kang, 1971; Colbert, 1971). Pappelis and Myers (1970)
indicated that LMCD could easily be measured and there was a considerable variation
among inbreds for this trait. The use of LMCD could facilitate selection for PCD in
stalk intemodes and stalk rot. Knowledge of the relative magnitudes o f the additive,
dominance, and epistatic components of genetic variance for LMCD would help maize
breeders in choosing an appropriate selection strategy.
Seed quality in maize hybrids is an important consideration for the seed industry
as well as farmers. Both genetic and physiological factors determine seed quality
(Pollock and Roos, 1972). The genetic component includes differences between two or
more genetic lines; whereas, the physiological component includes differences between
seed lots within one genetic line (Krishnasamy and Seshu, 1989). There has been much
work on the physiological aspects of seeds, but relatively little work has been done on
3
genetics of various traits influencing seed quality. Breeders need information on
genetics of seed quality traits to improve seed quality.
Breeders are continually interested in obtaining genetic information on new
germplasm that may have resistance to stalk rots, and exhibit good stalk and seed
quality, high yield and/or other desirable attributes. Information on genetic effects
conditioning PCD in stalk intemodes, LMCD, and seed quality traits o f genetic stocks
containing the leafy (Lfy) gene is lacking. The Lfy gene, dominant to the non-leafy (lfy)
allele, was discovered by Robert C. Muirhead, Hughes Hybrids Inc., in 1971 (Shaver,
1983). The Lfy gene materials are characterized by extra leaves above the ear, low ear
placement, highly lignified stalk and leaf parts, and high yield potential (Shaver, 1983).
In yield trials, hybrids developed from Lfy inbreds recorded relatively higher yields and
low stalk lodging than those developed from the non-leafy (lfy lfy) inbreds (Shaver,
1983). In a field study, aflatoxin accumulation on maize kernels was less in the Lfy
maize than the corresponding maize without the Lfy gene (Kang and Lillehoj, 1987).
Extra leaves in Lfy materials might produce supplemental photosynthate and thus have
a favorable effect on PCD in stalk intemodes, LMCD, and seed quality. Therefore, the
objectives o f the following studies were: 1) to estimate general combining ability (GCA)
and specific combining ability (SCA) for PCD in stalk intemodes, LMCD, and seed
quality traits, 2) to determine the effect of Lfy gene on PCD in stalk intemodes and
LMCD via a diallel mating design among seven maize synthetics containing the Lfy
gene, 3) to identify the most reliable stalk intemode and leaf for possible selection of
PCD in stalk intemodes and LMCD, respectively.
LITERATURE REVIEW
Stalk Quality
Stalk quality in maize (Zea mays L.) is an important consideration for breeders.
Estimated annual yield losses due to stalk lodging vary from 5 to 25 percent (Zuber and
Kang, 1978). In a survey o f public and private maize breeders, a majority o f the
respondents indicated that stalk strength was one of the most important selection traits
in an inbred line development program (Bauman, 1981). It has been a continuing
challenge for maize breeders to develop hybrids with improved stalk quality, especially
with an increase in use of fertilizers and high plant populations for higher productivity.
Stalk lodging in maize, a reflection o f poor stalk quality, is affected by
interaction of factors such as stalk rots, stalk borers, inherent differences among
genotypes (for stalk strength, parenchyma cell death, and late-season plant health),
fertility imbalance, and plant populations. The major factors that are responsible for
stalk lodging are stalk rot resistance, stalk strength, and parenchyma cell death in stalk
intemodes. A review of the interacting factors that affect stalk quality with an emphasis
on research relevant to parenchyma (pith) cell death in stalk intemodes o f maize, an
important stalk quality, is presented.
Stalk Rots
Stalk lodging in maize is enhanced by several stalk rot inciting organisms.
Diplodia maydis (Berk) Sacc. is one o f the major causal organisms o f stalk rot. Others
4
5
include Gibberella zeae (Schw) Petch, Fusarium moniliforme Sheld., Colletotrichum
graminicola (Ces.) G.W. Wils, and Macrophomina phaseoli (Maubi) Ashby. Stalk rot
causing fungi weaken stalk by restricting the flow of moisture and nutrients to the plant
parts. Weakening o f the stalk leads to lodging. The development of stalk rots is favored
by dry weather early in the growing season followed by extended periods of rainfall
shortly after silking. Disease prevalence and spread of stalk rot are influenced by insect
damage, plant maturity, soluble solid content, stalk pith condition, unbalanced fertility,
plant density, and genotype. Host plant resistance is the most effective means of
reducing losses due to stalk rots (Hooker, 1957). Nature of resistance and spread of
Gibberella zeae are the same as those of Diplodia maydis (Pappelis, 1965). Resistance
to both the fungi appears to be quantitatively inherited. Both additive and non additive
gene effects have been reported and at least nine genes for resistance to D. maydis were
reported by El-Rouby and Russell, 1966.
Stalk Borers
Severity of stalk rot is higher when maize plants are infested heavily with
European com borer (Ostrinia nubilalis Hubner (ECB)). ECB larvae bore into stalks,
tunnel extensively through stalk pith, and cause stalk lodging. Stalk rot damage was
lower when maize plants were kept free from ECB with an insecticide. Conversely,
stalk rot damage was higher when ECB egg masses/plant increased (Jarvis et al., 1984).
Resistance to ECB was conditioned by relatively few partially dominant genes (Guthrie
and Dicke, 1972). Resistance to D. maydis stalk rot was conditioned by several genes.
6
S, recurrent selection, a strategy suited to increase the frequency o f alleles with additive
genetic effects was found to be effective in improving resistance to ECB and Diplodia
stalk rot (Nyhus et al., 1988).
Stalk Strength
The maize stalk consists o f rind and pith. The relative contributions of rind and
pith to total stalk strength are approximately 60 and 40% respectively (Zuber and Kang,
1978). The rind of maize stalk consists of a layer of sclerified cells (hypodermis) just
below the stalk epidermis followed by thick-walled parenchyma cells that are present
in between vascular bundles with sclerified bundle sheaths. Moving from the edge to
the center o f the stalk, the transition from rind to pith is characterized by a reduction
in thick-walled parenchyma cells and in the size o f bundle sheaths. There are
anatomical differences between strong and weak stalks. The hypodermal cells of strong
stalks have relatively thicker walls than the hypodermal cells o f weak stalks. In
addition, rind-parenchyma cells are relatively thicker in strong stalks than in weak
stalks. Vascular bundle sheath cells extend farther into stalk (pith) in strong stalks than
in the weak stalks (Berzonsky et al., 1986). These anatomical differences partially
explain differences in stalk strength in strong and weak stalks.
Techniques that can effectively differentiate genotypes for stalk strength are
needed. Various methods of evaluating stalk strength have been developed (Cloninger
et al., 1970). Crushing strength of a stalk section was shown to be negatively
correlated with stalk lodging (Zuber and Grogan, 1961). Recurrent selection for stalk
7
crushing strength was effective in improving stalk quality (Zuber, 1973). Crushing
strength appears to reside principally in the rind tissue as evident from a study
involving twelve morphological and anatomical traits in two synthetics of com that had
undergone selection for low and high crushing strength (Chang et al., 1976). Two or
three cycles of recurrent selection for rind strength should be sufficient to improve stalk
quality of most maize populations to a desirable level for subsequent inbred
development (Martin and Russell, 1984). Other methods of evaluating stalk strength
include weight of 2-inch stalk section, rind
thickness, and rind puncture (Zuber,
1973.). Rind puncture is a non-destructive method to evaluate stalk strength. It
measures the force required to penetrate a needle of a rind penetrometer through the
rind. There is a good agreement between rind puncture and crushing strength (Zuber
and Kang, 1978). Significant correlations were reported for rind puncture vs. stalk
strength ( r = 0.86), rind puncture vs. stalk rot rating (r = -0.93), and rind puncture
vs. stalk lodging (r = -0.86) (Martin and Russell, 1984). The rind puncture method
should be useful to breeders because selections can be made before flowering. This
would allow the selected plants to be intermated in the same generation, thus
completing a cycle of selection in only one generation.
Effects of selection for stalk-crushing strength on agronomic traits have been
studied (Zuber, 1973; Colbert et al., 1984; Martin and Russell, 1984; and Berzonsky
and Hawk, 1986). Results on effect of selection for stalk strength on yield are not
conclusive. Large deleterious effects on grain yield as the result of selection for high
stalk-crushing strength, were not detected through four cycles of recurrent selection
8
(Zuber, 1973; Colbert et al., 1984). Kernel yield increased with selection for high
stalk-crushing strength through eight cycles of recurrent selection (Berzonsky and
Hawk, 1986; Moentono et al., 1984). On the other hand, three cycles of Sj recurrent
selection for stalk strength resulted in populations with significantly reduced yields.
However, most of yield reduction occurred in the second and third cycles o f selection,
whereas most of the progress for stalk quality was made during the first two cycles of
selection (Martin and Russell, 1984). Selection for stalk strength also resulted in taller
and later flowering plants (Zuber, 1973; Colbert et al., 1984; and Martin and Russell,
1984).
Parenchyma Cell Death
Stalk rot incited by D. maydis was found to be related to parenchyma cell death
in the stalk, and stalk rot resistance was related to the presence of living cells in that
tissue (Pappelis, 1957). Pith cell death in stalk intemodes has been implicated as an
important stalk quality trait (BeMiller and Pappelis, 1965; Pappelis and Williams,
1966), Pappelis et al., 1971; Kang et al., 1974; Kang et al., 1986; and Colbert et al.,
1987).
The relationship o f pith condition to the spread of stalk-rotting fungi has been
studied by several researchers (Craig and Hooker, 1961; Pappelis and Smith, 1963;
Pappelis and Williams, 1966; Wysong and Hooker, 1966; Gates, 1970; Pappelis, 1970;
Pappelis et al, 1971; Kang et al., 1974). A highly significant positive correlation
between pith cell death and stalk rot ratings was reported by Pappelis and Williams
9
(1966). A decrease in reducing sugars in pith of stalk with an increase in pith cell
senescence was observed (Craig and Hooker, 1971). A reduction in soluble stalk solids
occurred as pith senesced, and tissues low in soluble solids were more susceptible to
Diplodia stalk rot (Wysong and Hooker, 1966). Stalk rot susceptible tissue was
characterized by low water content per unit volume and density (fresh weight/cc. of
tissue) (Pappelis and Smith, 1963). Conversely, resistant tissue was characterized by
a high moisture content and selection of genotypes with high water content for stalk rot
resistance has been suggested. Resistance to spread of Diplodia maydis in stalk
intemodes is conditioned by a fungistatic compound, DIMBOA (2,4 dihydroxy-7methoxy-1,4 benzoxazine-3-1), apparently active in living host cells that are resistant
to fungi (BeMiller and Pappelis, 1965). Pith condition ratings were higher in plants
treated with nitrogen or phosphorous, and addition of potassium or limestone decreased
pith condition ratings (Pappelis and Boone, 1966).
Most important natural points of entry of stalk rot fungi in the stalk, in
decreasing order of importance, were at the nodes through the sheath junction, at root
junction with the stalk through insect wounds, or directly through stalk buds (Koehler,
1960). Pith condition ratings and stalk rot ratings of the first intemode were never
greater than those of the fourth intemode (Pappelis and Smith, 1963).
High yielding varieties were more stalk rot susceptible than low yielding
varieties (Koehler, 1960). It was hypothesized that plants with a greater number of dead
cells in the lower stalk, prior to ear development may translocate more photosynthate
to the ear than do plants in which all of the stalk cells are alive. It appears that both
10
high yield and stalk rot susceptibility may be associated with early death of cells in the
lower stalk intemodes (Pappelis, 1965). This hypothesis was proved to be valid by
demonstrating that cell death in stalk-intemodes and nodes was delayed by removal of
ear or sink (Pappelis and Katsanos, 1969). Inoculation of cob and shank with Diplodia
maydis about 10 days after silking delayed cell death in stalk intemodes (Kang et al.,
1974). Later, "Photosynthetic stress-translocation balance concept of stalk rot" was
proposed (Dodd, 1977 and 1980), which appears to be based on the above hypothesis.
According to the photosynthetic stress-translocation balance concept of stalk rot, limited
photosynthate that is available under stress conditions leads to competition between
grain filling ear (sink size) and stalk. The cells in the stalk intemode senesce with
reduction in photosynthate (carbohydrates), lose resistance to invading microorganisms,
and allow the spread of stalk rotting fungi.
Selection for lodging resistance, has been effective in reducing percentage
lodged plants (Thompson, 1972 and 1982). However, selection for stalk lodging
resistance is affected by a substantial genotype X environment interaction.
Stalk
strength, stalk-rot resistance, and parenchyma cell death are highly correlated with
lodging resistance and are less affected by environmental fluctuations (Zuber and
Loesch, 1966). In addition, stalk strength (measured by stalk-crushing strength and rind
penetrometer), stalk-rot resistance (after inoculation with stalk rotting fungi), and
parenchyma cell death in stalk intemodes are highly heritable and are controlled by
additive type of gene action (Kappelman and Thompson, 1966 and Colbert et al.,
1987). Heritability estimates for percent stalk lodging, stalk crushing strength, and rind
11
puncture resistance were estimated to be 0.36, 0.78, and 0.83, respectively (Albrecht
and Dudley, 1987). Stalk quality traits that contribute to stalk strength, viz., crushing
strength, weight o f a stalk section, and rind thickness are conditioned by polygenes
(Kang, 1979). However, inheritance of parenchyma cell death in stalk intemodes has
not been thoroughly investigated.
Genetics of Pith Cell Death
In a study o f nine F, crosses involving synthetic parents, cell death ratings of
seven crosses showed dominance toward the parent with lower cell death rating (Miller,
1971). Inheritance o f stalk cell death appears to be complex. Susceptibility to stalk cell
death is conditioned by a small number of major genes and modified by several minor
genes (Pappelis et al., 1971). A study involving reciprocal chromosomal interchanges
indicated that gene loci conditioning pith cell death in stalk intemodes o f maize were
many in number and located on several different chromosomes (Kang, 1991
unpublished data).
In a diallel study (Colbert et al., 1987) involving six inbred parents tested at two
locations in one year, broad sense heritability for stalk pith cell death was estimated to
be 86%. General combining ability effects were found to be more important than
specific combining ability effects. Pith cell death and spread of stalk rotting fungi are
closely related. A cyclic selection for low pith cell death would result in stalk-rot
resistance (Colbert et al., 1987). Identification of desired lines for low pith cell death
prior to flowering, so that a cycle of selection can be completed in the same generation,
12
is feasible. The rating system used in pith cell death evaluation was shown to be precise
(Pappelis and Williams, 1966; Pappelis and Katsanos, 1969).
Leaf Midrib Cell Death vs. Stalk Pith Cell Death
Identifying simple, easily measurable traits that are correlated to pith cell death,
would be of great use to commercial breeders. Leaf midribs might serve as selection
guides in differentiating between strong and weak stalk inbred lines of maize (Hunter
and Dalbey, 1937). The differences between strong and weak stalk inbred lines reflect
in the anatomy of ear-leaf midribs. Vascular bundles o f the ear-leaf midrib of strong
stalk inbred are associated with more dark staining, thick walled cells than weak stalk
inbred line midrib vascular bundles (Berzonsky et al., 1986). Significant correlation
coefficients between cell death in leaf midribs and cell death in stalk intemodes were
reported, indicating that white leaf midribs (dead cells) could predict stalk rot
susceptible plants and possibly eliminate the need for inoculation of stalks to determine
resistance of an inbred line or hybrid (Kang , 1971; Colbert, 1971). Genetic variability
for leaf midrib cell death existed among maize inbred lines. Leaf midrib cell death is
highly heritable (Kang et al., 1988). Genotypes with low ratings can be identified prior
to flowering and desired crosses can be made in the same generation (Kang et al.,
1988). Therefore, literature supports the real possibility of using leaf midrib cell death
ratings to select against stalk cell death or stalk rot.
13
pH vs. Pith Cell Death
pH of stalk, leaf, and husk components increases exponentially with a decrease
in moisture content. Senesced com stalks commonly had a pH of 8 or higher. Increase
in pH was commonly observed during oxidative, saprophytic decomposition of
biological organs or organisms (Muldoon et al., 1984). pH of maize hybrids shows
similar pattern of changes, although significant differences were present among hybrids
for absolute values of pH. A significant positive correlation (r= 0 .6 7 , P = 0.01) was
observed between pH of lower stalk intemodes and visual pith condition ratings (Leask
and Daynard, 1976). The phenomenon o f pH increase could serve as a useful indicator
o f genetic differences in stalk quality. Since all stalks decompose with time, lodging
resistance could be interpreted as a delay in decomposition for a maximum period of
time following grain maturity. On the contrary, stalk pH, in a study involving 14
hybrids tested in four environments, correlated negatively with stalk lodging recorded
on the same 14 hybrids, grown earlier, in four years of field testing (Muldoon et al.,
1984).
Late Season Plant Health
Late-season plant health or "stay green" (absence of premature cell death) is a
desirable trait in relation to yield and stalk quality. Resistance to root and stalk lodging,
favorable stay green scores, and high tolerance to second-generation com borer were
highly correlated with grain yields. Stay green character is associated with later
maturity or increased plant height. Taller and later materials are generally higher
14
yielding and more tolerant to stalk rots than earlier and shorter materials (DeLeon and
Pandey, 1989). Amount of total non-structural carbohydrates in stalk tissue at
physiological maturity reflects late-season plant health. However, there was no study
conducted to determine the critical level of total non-structural carbohydrate required
for the maintenance of late-season plant health that is associated with resistance to
premature senescence (stay green) and/or stalk rot. Premature stalk senescence and rot
were common in varieties susceptible to lodging. Lodging was negatively correlated
with grain yield and with percentages of total non-structural carbohydrate, protein and
potassium in the stalks (Esechie, 1985). Lodging in maize was associated with
parenchyma disintegration in stalk intemodes with exhaustive translocation of
carbohydrates from the lower stalk and roots to the developing ear in potassium
deficient maize. Identification of maize hybrids that have capability to produce more
photosynthate will result in high grain yield and good stalk quality.
Breeders are in search of genetic material that is characterized by late-season
plant health and capability to produce extra photosynthate. Robert C. Muirhead, Hughes
Hybrids, Inc., discovered in a breeding nursery a single mutant plant that had extra
leaves above the ear. The extra leaves above the ear were found to be conditioned by
a single dominant gene, which was later designated as Lfy gene (Shaver, 1983). Lfy
material is also characterized by low ear placement and highly lignified stalk and leaf
parts. In yield trails, hybrids developed from Lfy inbreds recorded relatively higher
yields and low stalk lodging than the normal high yielding hybrid checks (Shaver,
1983).
15
Fertility Imbalance
Use of balanced fertilizers is important for standability of maize. Excess use
of nitrogenous fertilizers leads to excessive vegetative growth and results in stalk
lodging. Increased pith cell death in stalk intemodes of maize was observed in plots
treated with high nitrogen rates. On the contrary, a decrease in pith cell death with an
increase in potassium level was reported (Pappelis and Boone, 1966). Organic fields
(fields that are not supplemented with synthetic fertilizers) are characterized by low
nitrogen content (Kang, 1986). In a comparative study of pith cell death in stalk
intemodes of com grown on organic and conventional fields, pith cell death, stalk rot,
and lodging were found to be lower in the organic fields than the conventional fields
(Kang, 1986).
Plant Populations
High plant density is known to increase stalk lodging in maize. However, newer
hybrids have been bred for improved standability at increased plant densities (Duvick,
1984). Selection for stalk crushing strength reduced the adverse effects of high plant
densities on stalk lodging and yield (Moentono et al., 1984).
Seed Quality
Seed quality in maize hybrids is an important consideration for the seed industry
as well as farmers. Quality of seed can be improved genetically as well as by favorable
agronomic and developmental factors (Gupta and Basak, 1983). Both genetic and
16
physiological factors determine seed quality (Pollock and Roos, 1972). The genetic
component includes differences between two or more genetic lines; whereas, the
physiological component includes differences between seed lots within one genetic line
(Krishnasamy and Seshu, 1989). There have been many studies on seed physiology, but
relatively little work has been done about the genetics of various traits influencing seed
quality. Information is needed on genetics of seed quality traits. Knowledge of the
relative magnitudes of the additive, dominance, epistatic and maternal components of
genetic variance and of magnitudes of heritabilities and genetic correlations of maize
seed quality traits would be useful to maize breeders in choosing an appropriate
breeding strategy.
Germinability and vigor are two important facets of seed quality. Germination
capacity estimated out of standard germination test is undoubtedly an important
indicator of seed quality. To base an estimate of the quality of seed lot merely on its
germination percentage is a short sighted shortcut which may not be meaningful since
the weak, deteriorated seeds are capable of producing a normal seedling in the standard
germination test in the absence of microorganisms, herbicides, fungicides and
unfavorable conditions of germination that a seed encounters in the field (Desai and
Agrawal, 1979). More meaningful comparative forecasts of plant performance could
be made if they were based on more sensitive and more representative measurable
characteristics of seeds or germinating seedlings (Heydecker, 1972).
Seed vigor is a physiological property determined by genotype modified by the
environment (Perry, 1972). "Seed vigor is the sum total of those properties of the seed
17
which determine the potential level of activity and performance of the seed or seed lot
during germination and field emergence". Vigor cannot be quantified because it is a
concept (Perry, 1978) and only specified components or manifestations can be
expressed numerically (Heydecker, 1972). Various components of vigor may respond
differently to the environmental influence, so vigor cannot be estimated with only one
vigor test (Delouche, 1975). Hence, to estimate the seed vigor various tests such as root
(Grabe, 1965) and shoot length of seedlings (Egli and Tekrony, 1973), dry matter
weight (Krishnasamy, 1977), vigor index (Abdul-Baki and Anderson, 1973), rate of
germination (Agrawal and Kaur, 1977), electrical conductivity (Grabe, 1967), and cold
test germination (Isley, 1950) can be employed.
Inbred lines o f maize differ in seed quality traits such as germination, vigor and
longevity and these differences are heritable (Lindstrom, 1942; Haber, 1950; Neal and
Davis, 1956; Rowe and Gorwood, 1978). Limited genetic studies conducted on maize
seed quality indicated inheritance o f seed quality traits to be quantitative in nature.
Inheritance of five seed quality traits (germination, electrical conductivity, accelerated
aging, complex stressing vigor, and cold test germination) was due to additive gene
effects with relatively high broad sense heritability estimates (Odiemah, 1989).
Recurrent selection for these traits in a breeding program, based on additive genetic
effects, should be effective. Seedling vigor (as measured by field emergence percentage
and rate of emergence) and grain yield in maize were quantitatively inherited with
preponderance of additive gene effects over non-additive effects
at two levels of
inbreeding (S2 and S5). Since similar genetic factors control the inheritance of seedling
18
vigor and grain yield at both generations o f inbreeding, selection practiced for seedling
vigor at early stages o f growth would influence grain yield at later stage (Azala and
Fakorede, 1988).
Ability of the seeds to germinate in cold and wet soils is an important aspect
o f seed quality. Cold test is one o f the oldest methods of stressing seeds and is most
often employed for evaluations o f seed vigor in com . Cold test can predict emergence
o f inbred lines in cold and wet environments (Martin et al., 1988). Studies have shown
the existence of genetic variability in maize for germination at low temperatures (Eagles
and Hardacre, 1979).
Little or no literature is available in maize on inheritance of other seed quality
traits such as root and shoot length of seedlings, dry matter, and vigor index.
19
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Paper presented at workshop on Seed Testing organized by Ministry o f Agric.
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crop plants. Spec. Pub. 7. Crop science Society of America, Madison, WI.
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germination and emergence at 10 ”C. Euphytica 28:287-295.
Egli, D .B ., and D.M . Tekrony 1973. Laboratory measurements of soybean seed quality
for predicting field emergence. Am. Soc. Agron. Abstr. p. 52.
Elrouby, M .M ., and W .A.Russell. 1966. Locating genes determining resistance to
Diplodia maydis in maize by using chromosomal translocations. Can. J. Genet.
Cytol. 8:233-240.
21
Esechie, H.A. 1985. Relationship of stalk morphology and chemical composition to
lodging resistance in maize (Zea mays L.) in a rain forest zone. J. Agric. Sci.
Camb. 104:429-433.
Gates, L.F. 1970. Relationships between pith cell death conditions as assessed by
tetrazolium chloride and incidence of Gibberella stalk rot in com. Can. J. Plant
Sci. 50:674-684.
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Seedsman short course, pp. 79-85. Mississippi State University, Miss.
Gupta, D ., and S.L. Basak. 1983. Genetics of germination and seedling growth of flax
(Linum usitatissimwn). Seed Sci. Technol. ll(2):251-256.
Guthrie, W .D ., and F.F.Dicke. 1972. Resistance of inbred lines of dent com to leaf
feeding by lst-brood European com borers. Iowa State J. Sci. 46:339-357.
Haber, E.S. 1950. Longevity of the seed of sweet com inbreds and hybrids. Amer.
Soc. Hort. Sci. Proc. 55:410-412.
Heydecker, W. 1972. Vigor. In E.H. Roberts (ed.) Viability of seeds. Syracuse
University Press, New York. pp. 209-252.
Hooker, A.L. 1957. Factors affecting the spread of Diplodia zeae in ioculated com
stalks. Phytopathology 25:1113-1114.
Hunter, J.W ., and N.E. Dalbey. 1937. A histological study of stalk breaking in maize.
Am. J. Bot. 24:492-494.
Isley, D. 1950. The cold test for com. Proc. Int. Seed Test. Assoc. 16:299-311.
Jarvis, J.L ., R.L. Clark, W .D. Guthrie, E.C. Berry, and W.A. Russell. 1984. The
relationship between second-generation com borers and stalk rot fungi in maize
hybrids. Maydica 29:247-263.
Kang, M.S. 1971. Inheritance of cell death patterns in the leaf mid-rib of Zea mays L.
Southern Illinois University, Edwardsville. 34 pp.
Kang, M.S. 1986. A comparative study of pith cell death in stalk intemodes of com
grown on organic and conventional fields. Amer. J. Alter. Agric. 1:165-168.
22
Kang. M .S., and E.B. Lillehoj. 1987. Aflatoxin resistance in com . Report of Projects
for 1987, Department o f Agronomy, Louisiana Agricultural Experiment Station,
Baton Rouge, Louisiana, p. 48.
Kang, M .S ., O. Myers, Jr., and A.J. Pappelis. 1988. Heritability o f cell death in leaf
midribs of maize (Zea mays L .). Cereal Res. Commun. 16:25-29.
Kang, M .S., A.J. Pappelis, P. Mumford, J.A . Murphy, and J.N . BeMiller. 1974.
Effect o f cob and shank inoculations (Diplodia maydis) on cell death in stalk
intemodes o f com . Plant Dis. Rep. 58:1113-1117.
Kang, M .S., A.J. Pappelis, and M.S. Zuber. 1986. Effect o f stalk inoculation (Diplodia
maydis) on parenchyma cell death in cob and shank intemodes of maize. Cereal
Res. Commun. 14:267-272
Kang, M .S ., M .S. Zuber, O. Myers, and T.R. Colbert. 1979. Genetic studies on three
stalk traits in maize using using chromosomal interchanges. Can. J. Genet.
C ytol., 21:139-144.
Kappelman, A .J., Jr., andD . L. Thompson. 1966. Inheritance o f resistance to Diplodia
stalk rot in com. Crop Sci. 6:288-290.
Krishnasamy, V. 1977. Studies on the standardisation of seed production techniques in
CSH 5 hybrid sorghum. M .Sc.(A g.) Thesis, Tamil Nadu Agrl. University,
Coimbatire, India.
Krishnasamy, V ., and D .V . Seshu. 1989. Seed germination rate and associated
characters in rice. Crop Sci. 29:904-908.
Koehler, B. 1960. Com stalk rots in Illinois. Illinois Agr. Exp. Sta. Bull. 658. 90 pp.
Leask, W .C ., and Daynard, T.B. 1976. Changes in pH o f com stover following grain
maturity. Crop Sci. 16:505-507.
Lindstrom, E.W . 1942. Inheritance o f seed longevity in maize inbreds and hybrids.
Genetics 27:154.
Martin, B .A ., O.S. Smith, and M .O ’Neil. 1988. Relationships between laboratory
germination tests and field emergence o f maize inbreds. Crop Sci. 28:801-805.
Martin, M .J., and W .A.Russell. 1984. Response o f a maize synthetic to recurrent
selection for stalk quality. Crop Sci. 24:331-337.
Miller, T.L. 1971. Correlations of pith cell death in two synthetic populations o f maize
23
with various stalk quality characteristics. M.S. Thesis. Southern Illinois
University, Carbondale. 41 pp.
Moentono, M .D., L.L.Darrah, M.S. Zuber, and G.F. Krause. 1984. Effects of
selection for stalk strength on responses to plant density and level of nitrogen
application in maize. Maydica 29:431-452.
Muldoon, J.F ., W.C.Leask, T.B. Daynard, and M.S. Zuber. 1984. Potential use of
stalk pH and stalk percent dry matter as estimators of lodging susceptibility in
com. Can. J. Plant Sci. 64:559-564.
Neal, N.P. and J.R. Davis. 1956. Seed viability of com inbred lines as influenced by
age and condition of storage. Agron. J. 48:383-384.
Nyhus, K.A., W.A.Russell, and W.D.Guthrie. 1988. Response of two maize synthetics
to recurrent selection for resistance to first-generation European com borer
(Lepidoptera: Pyralidae) and Diplodia stalk rot. J. Econ. Entomol. 81:17921798.
Odiemah, M. 1989. Quantitative inheritance of seed quality characteristics in maize
(Zea mays L.). Cereal Res. Commun. 17:245-251.
Pappelis, A.J. 1957. Nature of resistance to Diplodia stalk rot of com. Ph. D. thesis,
Iowa State University, Ames, IA ., 204 pp.
Pappelis, A.J. 1965. Relationship of seasonal changes in pith condition ratings and
density to Gibberella stalk rot of com. Phytopathology 55:623-626.
Pappelis, A.J. 1970. Effect of root and leaf injury on cell death and stalk rot
susceptibility in com. Phytopathology 60:355-357.
Pappelis, A.J., J.N. BeMiller, W.E. Schmid, O. Myers, and J.A. Murphy. 1971. Stalk
rot of com. Proc. Annu. Com Sorghum Res. Conf. 28: 110-122.
Pappelis, A.J., and L. V. Boone. 1966. Effect of planting date on stalk rot susceptibility
and cell death in com. Phytopathology 56: 829-831.
Pappelis, A.J., and R.A. Katsanos. 1969. Ear removal and cell death rate in stalk
tissue. Phytopathology 59:129-131.
Pappelis, A.J., and O. Myers, Jr. 1970. Parenchyma cell death patterns in the stalk,
shank, cob, and leaf midrib of maize. Agron. Abstr. 27.
24
Pappelis, A .J., and F.G . Smith. 1963. Relationship of water content and living cells
to spread of Diplodia zeae in com stalks. Phytopathology 53:1100-1105.
Pappelis, A .J ., and J.R. Williams. 1966. Patterns of cell death in the elongating com
stalks. Trans. 111. State Acad. Sci. 59:195-198.
Perry, D. A. 1972. Seed vigor and field establishment. Hort. Abstr. 42:334-342.
Perry, D.A. 1978. Report of the vigor committee. 1974-77. Seed Sci. Technol. 6: 159182.
Pollock, B.M ., and E.E. Roos. 1972. Seed and seedling vigor. /n T .T . Kozlowski (ed.)
Seed biology. Volume 1, pp. 313-387. Academic Press, New York.
Rowe, D .E. and D.L. Gorwood. 1978. Effects of four maize endosperm mutants on
kernel vigor. Crop Sci. 18:709-712.
Shaver, D. 1983. Genetics and breeding of maize with extra leaves above the ear. Proc.
Annu. Com sorghum Res. Conf. 38:161-180.
Thompson, D .L. 1972. Recurrent selection for lodging susceptibility and resistance in
com. Crop Sci. 12:631-634.
Thompson, D .L. 1982. Grain yield of two synthetics o f com after seven cycles of
selection for lodging resistance. Crop Sci. 22:1207-1210.
Wysong, D .S ., and A.L. Hooker. 1966. Relation of soluble solids content and pith
condition to Diplodia stalk rot in com hybrids. Phytopathology 56:26-35.
Zuber, M.S. 1973. Evaluation o f progress in selection for stalk quality. Proc. Annu.
Com Sorghum Res. Conf. 28:110-122.
Zuber, M.S. 1982. Challenges for maize breeders-today’s challenges for increased
maize production tomorrow. Proc. Annu. Com Sorghum Res. Conf. 37:88-102.
Zuber, M .S., and C.O. Grogan. 1961. A new technique for measuring stalk strength
in com . Crop Sci. 1:378-380.
Zuber, M .S., and M .S. Kang. 1978. Com lodging slowed by sturdier stalks. Crop
Soils, 30(5) N: 13-15.
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in com {Zea mays L.). Agron. J. 58:173-175.
Chapter 1
General and Specific Combining Ability for Stalk Intemodal
Parenchyma Cell Death in Corn with LJy Gene
25
26
Introduction
Stalk quality improvement in com (Zea mays L.) has been an important and
challenging objective for breeders. Considerable progress has been made in stalk quality
improvement as evident from the fact that hybrids developed in the 1940s and 1950s
were nearly three times more prone to stalk lodging than the hybrids developed in the
1970s and 1980s (Zuber, 1982). However, stalk lodging continues to be a major
production problem, especially with an increase use of fertilizers and high plant
populations. Estimated annual yield losses due to stalk lodging vary from 5 to 25
percent (Zuber and Kang, 1978).
Stalk lodging in maize, a reflection of poor stalk quality, is affected by several
interacting factors. The major factors that are responsible for stalk lodging are a lack
of heritable resistance to stalk rot, poor stalk strength, and increased parenchyma (pith)
cell death (PCD) in stalk intemodes. Stalk rotting fungi such as Diplodia maydis (Berk)
Sacc. and Gibberella zeae (Schw) Petch, weaken stalk by restricting the flow of
moisture and food to plant parts. Host plant resistance is the most effective means of
reducing losses due to stalk rots (Hooker, 1957).
Various methods have been developed to identify and select maize genotypes
with superior stalk quality (Cloninger et al., 1970). Crushing strength of a 5.1-cm stalk
section was shown to be highly correlated with stalk lodging (Zuber and Grogan, 1961).
Recurrent selection for stalk crushing strength was effective in improving stalk quality
(Zuber, 1973; Colbert et al., 1984). Crushing strength appears to reside principally in
27
the rind tissue as evident from a study o f 12 morphological and anatomical traits in two
com synthetics that had undergone selection for low and high crushing strength (Chang
et al., 1976). Two or three cycles o f recurrent selection for rind strength should be
sufficient to improve stalk quality o f most com populations to a desirable level for
subsequent inbred development (Martin and Russell, 1984). Other methods of
evaluating stalk strength include weight of a 5.1-cm (2-inch) stalk section and its rind
thickness. Inheritance and procedures for improvement of three important stalk quality
traits that contribute to total stalk strength, viz., crushing strength, weight o f stalk
section, and rind thickness have been reported by several researchers (Loesch et al.,
1963; Singh et al., 1969; Loesch, 1972; Zuber and Kang, 1978). These three stalk
quality traits are conditioned by polygenes (Kang et al., 1979). In addition to these
three important stalk quality traits, PCD in stalk intemodes has been implicated as an
important stalk quality trait (BeMiller and Pappelis, 1965; Pappelis and Williams, 1966,
Pappelis et al., 1971; Kang et al., 1974; Kang et al., 1986; and Colbert et al., 1987).
Stalk rot incited by D. maydis was found to be related to death of parenchyma
cells in the stalk, and stalk rot resistance was related to the presence o f living cells in
that tissue (Pappelis, 1957). The relationship of pith condition to the spread of stalkrotting fungi has been studied
by several researchers (Craig and Hooker, 1961;
Pappelis and Smith, 1963; Pappelis and Williams, 1966; Wysong and Hooker, 1966;
Gates, 1970; Pappelis, 1970; Pappelis et al., 1971; Kang et al., 1974). A highly
significant positive correlation between PCD and stalk rot ratings was reported by
Pappelis and Williams (1966). With an increase in PCD, a decrease in reducing sugars
28
in parenchymatous tissue of stalk was observed (Craig and Hooker, 1971). A reduction
in soluble solids in stalks occurred as PCD increased, and tissues low in soluble solids
were more susceptible to Diplodia stalk rot (Wysong and Hooker, 1966). Resistance to
spread of D. maydis in stalk intemodes was found to be conditioned by a fungistatic
compound, DIMBOA (2,4 dihydroxy-7methoxy-l,4 benzoxazin-3-one), apparently
active in living host cells that are resistant to fungi (BeMiller and Pappelis, 1965).
High yielding cultivars are reportedly more stalk rot susceptible than low
yielding cultivars (Koehler, 1960). It was hypothesized that plants with a higher number
of dead cells in the lower stalk, prior to ear development, may translocate more
photosynthate to the ear than do plants with a lower number of dead cells in stalk
intemodes. Both high yield and stalk rot susceptibility may be associated with early
death of cells in the lower stalk intemodes (Pappelis, 1965). This hypothesis was
proved to be valid by Pappelis and Katsanos (1969), demonstrating that cell death in
stalk intemodes and nodes was delayed with removal of ear or sink. Inoculation of cob
and shank with D. maydis about 10 days after silking delayed cell death in stalk
intemodes (Kang et al., 1974).
The relative contributions of rind and pith to total stalk strength are
approximately 60 and 40 %, respectively (Zuber and Kang, 1978). Zuber (1980)
indicated that pith component plays an important role in stalk quality and emphasized
its importance in developing strains with stalk rot resistance. The PCD was found to
be negatively correlated with crushing strength and rind thickness of 5-cm stalk sections
from the second stalk intemode (Miller and Myers, 1974). However, inheritance of
29
PCD has not been thoroughly investigated. Inheritance of PCD appears to be complex.
In a study of nine F, crosses involving synthetic parents, cell death ratings of seven
crosses showed dominance toward the parent with lower cell death rating (Miller,
1971). Susceptibility to stalk PCD was reported to be conditioned by a small number
o f major genes and modified by several minor genes (Pappelis et al., 1971). A study
involving reciprocal chromosomal interchanges indicated that gene loci conditioning
PCD in stalk intemodes of com were many in number and located on several different
chromosomes (Kang, 1991 unpublished data). In a diallel study involving six inbred
parents tested at two locations in one year, broad sense heritability for stalk PCD was
estimated to be 86 % (Colbert et al., 1987).
Late-season plant health or stay green (absence of premature cell death) is a
desirable trait in relation to yield and stalk quality (DeLeon and Pandey, 1987).
Breeders continually search for genetic materials that are characterized by late-season
plant health and capability to produce extra photosynthate. Robert C. Muirhead, Hughes
Hybrids, Inc., discovered in a breeding nursery a single mutant plant that had extra
leaves above the ear (Shaver, 1983). The extra leaves above the ear were found to be
conditioned by a single dominant gene that was later designated as leafy {Lfy) gene
(Shaver, 1983). Lfy material is also characterized by low ear placement, highly lignified
stalk and leaf parts, and late-season plant health. Extra leaves in Lfy materials might
produce supplemental photosynthate and thus have a favorable effect on PCD in stalk
intemodes. The objectives of this study were: 1) to estimate GCA and SCA effects for
PCD, 2) to determine the effect of Lfy gene on PCD in stalk intemodes via a diallel
30
mating design among seven maize synthetics containing the Lfy gene, and 3) to
determine the most reliable stalk intemode for studying PCD.
31
Materials and Methods
Seven com synthetics (A619 syn, A632 syn, B73 syn, Hy syn, M ol7 syn, Wf9
syn, and 914 syn) containing the Lfy gene were crossed in all possible combinations in
1987 in the com breeding nursery at Baton Rouge, LA. The seven synthetics were
developed by Comnuts, Inc., Oakland, California and provided as subscription
material, under licensing, to M. S. Kang. Seed of the resulting 21 F, crosses
(reciprocals were not included because of insufficient seed for some crosses) were
planted on 13 April 1988 at Ben Hur Plant Research Farm (BHF), Baton Rouge; on 4
April 1989 at Perkins Road Research Farm (PRF), Baton Rouge; and on 26 March
1990 at BHF, Baton Rouge. A randomized complete block design with four replications
and one-row plots was used in 1988, and with three replications and two-row plots in
1989 and 1990. The soil type at BHF was a Commerce silt loam (Aerie Fluvaquent,
fine-silty, mixed, nonacid, and thermic), whereas the soil type at PRF was an Olivier
silt loam (Aquic Fragiudalf, fine-silty, mixed, nonacid, and thermic). The F t crosses
were grown, using standard cultural practices, in 6.1 m length plots with a 30 cm
spacing between plants and a 102-cm spacing between rows.
Mid-silking dates for all plots were recorded. On the basis of number of leaves
above the ear, 10 leafy and 10 non-leafy plants in the two-row plots of each Ft cross
segregating for Lfy and lfy phenotypes were identified and tagged at mid-silk date in
1989 and 1990. In 1988, plants in each plot were not classified as leafy and non-leafy.
All the plants in a plot were rated for PCD. In 1989 and 1990, 21 days after mid-silk,
32
the 10 leafy and 10 non-leafy plants in each plot were split longitudinally using a knife.
First four stalk intemodes, above the brace roots, of each marked plant in a plot were
visually rated for PCD, using the following rating scale developed by Pappelis (1957):
0.0, no white tissue (dead cells) in the pith; 0.1, less than 1 % white; 0.5, 2-12% white;
1.0, 13-25% white; 2.0, 26-50% white; 3.0, 51-75% white; 4.0, 76-100% white; 5.0
same as 4.0 with white tissue in the node linking adjacent intemodes; and 6.0 same as
5.0 but plant dead. Data for PCD in each of the four stalk intemodes and mean PCD
(mean rating of four stalk intemodes) in each plot in each environment were recorded.
Data from the three environments were organized into two data sets; the first
set included 1988, 1989, and 1990 data, whereas plants in a plot were not separated
into leafy and non-leafy classes; the second set included only 1989 and 1990 data,
wherein plants in a plot had been classified as leafy and non-leafy. Both data sets were
analyzed according to Griffing’s Method 4, Model 1 (Griffing, 1956), with genotypes
treated as fixed effects. Hallauer and Miranda (1988) suggested that, in Model 1,
estimation o f components of variance and their subsequent use in calculation of
heritability was not appropriate, whereas estimation of GCA and SCA effects was
appropriate and valid. We restricted our analysis to estimation of GCA and SCA effects
and to determination of the relative importance of GCA and SCA for PCD by
computing
{2a1glQ.dlg + o2s)} ratio suggested by Baker (1978) using mean square
components (fixed model) associated with variance of GCA (o*g) and SCA (c^s).
33
Results and Discussion
Estimates of R2 (the percent of corrected total sum of squares accounted for by
the sum of squares regression in an analysis of variance for a variable), C.V.
(coefficient of variation), and mean statistics for PCD in each of the four stalk
intemodes are presented in Table 1.1. The R2 values for PCD were highest for the third
intemode. The C.V. values progressively decreased from 51.6% for the first intemode
to 4.4% for the fourth intemode. Mean values for PCD were lowest for the first
intemode and highest for the fourth intemode. This is in agreement with Pappelis and
Smith (1963), who reported that PCD and stalk rot ratings of the first intemode were
never greater than those of the fourth intemode. Correlation coefficients (P=0.001)
between PCD of first through fourth stalk intemode with mean PCD of all four stalk
intemodes ranged from r= 0 .6 7 to r= 0.97. On the basis of a relatively high R2 (68.6%)
and a low C.V. (9.5%), we concluded that the third intemode PCD was more reliable
in differentiating genotypes than PCD of the other three stalk intemodes. A relatively
high R2 and a low C.V. value for the third intemode PCD in years 1988 (Table 1.2),
1989 (Table 1.3), and 1990 (Table 1.4) substantiated the above conclusion. A practical
implication of this is that breeders could save time and resources by reliably evaluating
only one intemode.
A combined analysis of variance for the third stalk intemode PCD using the first
data set that included 1988, 1989, and 1990 data, wherein plants in a plot were not
separated into leafy and non-leafy classes, is presented in Table 1.5. Mean squares for
34
Table 1.1. Estimates of R2+, C.V.*, and mean statistics for parenchyma cell death
(PCD) in each of four stalk intemodes from a diallel cross among seven Lfy
synthetics grown in three environments during 1988-90.
Intemode
above brace roots
R2
------------
C.V.
Mean
------------
rating
r*
First
58.52
51.62
1.01
0.91***
Second
60.73
23.14
2.20
0.97***
Third
68.59
9.45
3.35
0.91***
Fourth
63.78
4.44
3.83
0.67***
Mean PCD
65.38
12.63
2.60
1.00***
*** Significant at the 0.001 probability level.
+ R2 = Coefficient of determination.
* C.V. = Coefficient of variation.
* r = Correlation coefficients between PCD of first through fourth stalk
with mean PCD of all four stalk intemodes.
intemode
35
Table 1.2. Estimates o f R2t and C.V.* statistics for parenchyma cell death (PCD) in
each o f four stalk intemodes from a diallel cross among seven Lfy synthetics
grown at Ben H ur Farm in year 1988.
Intemode
above brace roots
R2
C.V.
-------- %--------
First
48.42
46.54
Second
57.69
18.04
Third
60.97
8.97
Fourth
57.80
2.67
Mean
60.31
9.85
+ R2 = Coefficient o f determination.
* C.V . = Coefficient o f variation.
36
Table 1.3. Estimates of R2+ and C.V.* statistics for parenchyma cell death (PCD) in
each of four stalk intemodes from a diallel cross among seven Lfy synthetics
grown at Perkins Road Farm in year 1989.
Intemode
above brace roots
R2
C.V.
------- %---------
First
57.22
48.59
Second
63.76
23.64
Third
65.95
10.38
Fourth
45.34
5.98
Mean
65.33
14.02
+ R2 = Coefficient of determination.
* C.V. = Coefficient of variation.
37
Table 1.4. Estimates of R2t and C.V.* statistics for parenchyma cell death (PCD) in
each of four stalk intemodes from a diallel cross among seven Lfy synthetics
grown at Ben H ur Farm in year 1990.
R:2
Intemode
above brace roots
C.V.
------- %---------
First
44.92
80.43
Second
51.81
37.61
Third
63.71
16.39
Fourth
55.09
10.21
Mean
56.33
21.17
t R2 = Coefficient of determination.
* C.V. = Coefficient of variation.
38
Table 1.5. Analysis of variance for parenchyma cell death ratings for third stalk
intemode above brace roots from a diallel cross among seven Lfy synthetics
rated in three environments (1988-90).
df
Source
Mean squares
Environments (E)
2
2.034**
Replications / E
7
0.351“
Crosses (C)
20
0.677**
GCA
6
1.150“
SCA
14
0.475“
40
0.243“
E x GCA
12
0.330“
E x SCA
28
0.205“
139
0.100
Ex C
Pooled error
{2o2g/(2o2g + o 2s)}
** Significant at 1 % level of probability.
0.32
39
environments were significant. The relatively higher amount of variation among
environments was probably due to differential rainfall in 1988, 1989, and 1990 (143,
178, and 147 cm, respectively). Mean PCD at PRF in 1989 (3.5) was higher than PCD
from the other environments (Table 1.6). High amount of soil moisture might have been
responsible for increased PCD in stalk intemodes. Melis and Rijkenberg (1988)
concluded that high soil moisture during plant development, prior to tasseling, increased
stalk rot.
Mean squares for PCD due to crosses were significant. Corrected sum of
squares for crosses was partitioned into GCA and SCA. Both the GCA and SCA mean
squares were significant. Baker (1978) suggested that use of relative sizes of mean
squares to assess the relative importance o f general and specific combining ability may
be misleading, and recommended the use of { 2 ^ / ( 2 ^ + o 2s)} ratio that can be applied
to fixed models as well as random models. The closer this ratio is to unity, the greater
the predictability of progeny performance based on GCA alone (Griffing, 1956; Baker,
1978). Recently, this ratio was used to estimate the relative importance of GCA in
explaining progeny performance in a diallel analysis (fixed model) o f relative growth
rates (Nevado and Cross, 1990) and aflatoxin production on com grain (Gorman and
Kang, 1991). A ratio of 0 .3 2 suggested that SCA effects were more important than
GCA effects in the seven Lfy synthetics studied and indicated dominance and epistatic
effects for PCD to be preponderant. The significance of a low ratio is that genetic gain
from recurrent selection for PCD would be relatively small and breeders should strive
to identify specific crosses for low PCD. However, a genetic study by Colbert et al.
40
Table 1.6. Mean parenchyma cell death ratings1 for third stalk intemode above brace
roots from a diallel cross among seven Lfy synthetics grown in three
environments during 1988-90.
Environment
Year
Parenchyma
cell death
Ben Hur Farm
1988
3.397 a
Perkins Road Farm
1989
3.498 a
Ben Hur Farm
1990
3.149 b
LSD (P=0.05)
0.107
t Mean parenchyma cell death ratings followed by the same letter are not significantly
different according to LSD (P=0.05).
(1987), which included a set o f seven non-Lfy parents (inbreds), revealed that GCA
effects were more important than SCA effects. The contradicting results between this
study and that o f Colbert et al. (1987) may be explained by the differences in
experimental material and environments used. The significant environment x cross
interaction for PCD suggested that PCD was highly influenced by environmental
factors, and it emphasized the need for rating genotypes in more than one environment.
Further partitioning o f environment x cross interaction indicated that this interaction
was composed o f significant environment x GCA and environment x SCA interactions.
Analysis of variance for the third stalk intemode PCD using the second data
set (1989 and 1990), wherein plants in a plot had been classified as leafy and nonleafy, revealed that difference between the leafy vs. non-leafy plant type was significant
(Table 1.7). The significant environment x plant type interaction (Table 1.7) suggested
that the effect o f Lfy gene on PCD in stalk intemodes was modified by environmental
factors. PCD in each of the four stalk intemodes and mean of all four intemodes of Lfy
plants were significantly lower than those o f non -Lfy plants in BHF-90 environment
(Table 1.8), substantiating that extra leaves in Lfy plants would supplement
photosynthate production and have a favorable effect on PCD in stalk intemodes.
Shaver (1983) reported that, in yield trials, hybrids developed from Lfy inbreds had
relatively higher yields and lower stalk lodging than the normal high yielding checks.
These findings are in agreement with Dodd’s theory that the "photosynthetic stresstranslocation balance" plays an important role in development of stalk rot. According
to photosynthetic stress-translocation balance concept o f stalk rot (Dodd, 1977 and
42
Table 1.7. Analysis o f variance for parenchyma cell death ratings for third stalk
intem ode above brace roots from a diallel cross among seven Lfy synthetics
rated in two environments (1989-90).
Source
df
Means squares
Environments (E)
1
6.780’*
Replications/E
4
1.195”
Plant Type Lfy+ vs. non-L^y* (PT)
1
7.770”
20
0.928”
GCA
6
1.043”
SCA
14
0 .8 7 9 "
E x PT
1
2 .1 4 3 "
Ex C
20
0.469”
E x GCA
6
0 .9 8 6 "
E x SCA
14
0.247
20
0.526”
PT x GCA
6
1.018”
PT x SCA
14
0.316
20
0.130
E x PT x GCA
6
0.287
E x PT x SCA
14
0.063
161
0.201
F t Crosses (C)
PT x C
ExPTxC
Pooled error
(2<r2g/(2a2g-l-a2s)} ratio
0.18
** Significant at 1 % level o f probability.
+ Lfy type plants are characterized by more leaves above the ear.
* Non -Lfy {lfy lfy) type plants have normal number o f leaves above the ear. 1980),
43
Tabic 1.8. Effect of Lfy gene on parenchyma cell death in each of four stalk intemodes from a diallcl cross among seven
Lfy synthetics grown in two environments (1989-90).
Intcmode number
One
Lea fines s
P89*
Two
B90*
P89
Three
B90
P89
B90
Four
Mean PCDf
P89
B90
P89
B90
Cell death ratings
Non-Lfy'
1.52 a*
1.07 a
2.55 a
2.34 a
3.58 a
3.43 a
3.90 a
3.85 a
2.89 a
2.67 a
Lfy«
1.29 a
0.64 b
2.36 a
1.74 b
3.41 b
2.89 b
3.82 a
3.57 b
2.72 b
2.21 b
LSD(P=0.05)
0.24
0.24
0.20
0.27
0.13
0.18
0.08
0.13
0.14
0.18
t Mean PCD: Mean parenchyma cell death in all four stalk intemodes.
* Means (over replications and crosses) in each column followed by the same letter arc not significantly different according to
LSD (P=0.05).
• P89 = Perkins Road Research Farm-1989.
I B90 = Ben Hur Plant Research Farm-1990.
# Non-£# (lfy lfy) plants have normal number of leaves above the ear.
II Lfy type plants are characterized by more number of leaves above the car.
44
and 1980), limited photosynthate that is available under stress conditions leads to
competition between grain filling ear (sink size) and stalk.
Plant type x crosses interaction was significant (Table 1.7). Further partitioning
o f plant type x crosses interaction indicated that this interaction was mainly due to plant
type x GCA interaction, and GCA was differentially modified by the Lfy and non -Lfy
characteristics o f a plant.
The GCA effects (Table 1.9) for A632 syn, B73 syn, M ol7 syn, and Wf9 syn
were negative, suggesting that use of these synthetics might be effective in reducing
PCD and possibly stalk rot and associated stalk lodging. A619 syn, Hy syn, and 914
syn showed positive GCA effects and use o f these synthetics might cause an increase
in PCD. The line means for A619 syn and 914 syn were significantly higher than those
o f the other synthetics. Among all the Fj crosses, A632 syn x M ol7 syn had the lowest
PCD and A619 syn x A632 syn had the highest PCD.
Eleven out o f 21 crosses showed negative SCA effects. The cross A619 syn x
Hy syn showed the highest negative SCA effects (Table 1.9), followed by A632 syn x
M ol7 syn, Wf9 syn x 914 syn, A619 syn x 914 syn, B73 syn x M o l7 syn, M o l7 syn
x Wf9 syn, B73 syn x 914 syn, Hy syn x Wf9 syn, A632 syn x B73 syn, A632 syn x
914 syn, and A619 syn x A632 syn. Use of crosses with positive SCA effects, for
instance, M ol7 syn x 914 syn, A619 syn x M017 syn, A632 syn x W f9 syn, and B73
syn x Hy syn, etc. should be avoided.
In conclusion, PCD was under genetic control and was conditioned by
quantitative trait loci. Use o f the genetic material possessing the Lfy gene in breeding
Table 1.9. Mean parenchyma cell death ratings'! of Ft crosses (above diagonal), general combining ability (GCA) effects (on diagonal), specific combining ability (SCA) effects
(below diagonal), and line means of Ft crosses (F,) from a diallel cross among seven Lfy synthetics grown in three environments (1988-1990).
Synthetic
A619
A632
B73
Hy
Mol7
Wf9
914
F,
A619
0.199
3.720
3.693
3.366
3.151
3.392
3.604
3.487
A632
-0.012
-0366
3.298
3.569
2.613
3.250
3.703
3.358
B73
0.413
-0.116
-0.185
3.268
3.214
3.397
3.703
3.428
Hy
-0.611
0.187
0.575
0.116
3.159
3.040
3.208
3.268
Mol7
0.720
-0.604
-0.495
0.110
-0.217
3.279
3.327
3.123
Wf9
0.039
0.593
0.016
-0.353
-0.462
-0.076
3.439
3.280
914
-0.549
-0.047
-0.392
0.091
0.732
-0.555
0.530
3.497
L.S.D. (P=0.05) values for testing differences among Ft crosses and line means (F() are 0.282 and 0.126, respectively.
Standard error for: g; = 0.134, ft - gj = 0.205,
— 0.265, s^ -
= 0.411, and
- Sy = 0.356.
r Mean parenchyma cell death ratings (averaged over replications and environments) are based on third stalk intemode above brace roots.
46
programs would be expected to reduce PCD. We propose that identification and use of
genetic materials that have capability to produce more photosynthate or have increased
photosynthetic rate per unit leaf area, and late season plant health may result in a
desirable combination o f high grain yield and good stalk quality in the leafy com.
47
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48
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49
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50
Zuber, M .S., and M .S. Kang. 1978. Com lodging slowed by sturdier stalks. Crop
Soils 30(5) N: 13-15.
C hapter 2
Diallel Analysis of Parenchyma Cell Death in Leaf
M id-ribs of C orn Possessing the Lfy Gene
51
52
Introduction
Stalk quality improvement in com (Zee mays L.) continues to be an important
consideration for breeders as estimated yield losses due to stalk rot and associated
lodging vary from 5 to 25 % (Zuber and Kang, 1978). The methods currently used for
evaluating stalk quality are destructive in that stalks are split longitudinally or stalk
sections are removed for evaluation. Com breeders are interested to know whether stalk
quality can be effectively improved by using a non-destructive method. Leaf mid-ribs
might serve as selection guides in differentiating strong- and weak-stalk inbred lines
(Hunter and Dalbey, 1937). Differences between strong- and weak-stalk inbred lines
may be reflected in the anatomy of ear-leaf mid-ribs. Vascular bundles of the ear-leaf
mid-ribs o f strong-stalk inbreds are associated with more dark- staining, thick-walled
cells than are those of weak-stalk inbreds (Berzonsky et al,, 1986). A significant
correlation coefficient (r=0.70) between leaf midrib cell death (LMCD) and stalk
intemodal cell death (SICD) in com has been reported (Kang, 1971; Colbert, 1971).
Pappelis and Myers (1970) indicated that LMCD could be easily measured, and there
was considerable variation among inbreds for this trait. The use of LMCD could
facilitate selection for SICD and stalk rot. However, inheritance o f LMCD has not been
thoroughly investigated. A genetic study, conducted in a single environment, by Kang
et al. (1988) with 15 inbreds, 14 F ,’s, and 14 F2’s revealed that LMCD rating was
highly heritable in some populations but not in others.
53
Breeders are continually interested in obtaining genetic information on new
germplasm that may have resistance to stalk rots, and exhibit good stalk quality, high
yield and/or other desirable attributes. Information on genetic effects conditioning
LMCD of genetic stocks containing the leafy (Lfy) gene is lacking. The Lfy gene,
dominant to the non-leafy (lfy) allele, was discovered by Robert C. Muirhead, Hughes
Hybrids Inc., in 1971 (Shaver, 1983). The Lfy gene materials are characterized by extra
leaves above the ear, low ear placement, highly lignified stalk and leaf parts, and high
yield potential (Shaver, 1983). In yield trials, hybrids developed from Lfy inbreds
recorded relatively higher yields and low stalk lodging than those developed from
normal (lfy lfy) inbreds (Shaver, 1983). Extra leaves in the Lfy material may have a
favorable effect on LMCD by production of more photosynthate. Therefore, Lfy genecontaining materials were deemed desirable for studying LMCD from the genetic
standpoint. The objectives of this study were: 1) to estimate GCA and SCA effects for
LMCD, 2) to determine the effect of Lfy gene on LMCD via a diallel mating among
seven com synthetics containing the Lfy gene, and 3) to identify the most reliable leaf
for possible selection for SICD.
54
Materials and Methods
Seven maize synthetics (A619 syn, A632 syn, B73 syn, Hy syn, M ol7 syn, Wf9
syn, and 914 syn) containing the Lfy gene were crossed in all possible combinations in
1987 in the com breeding nursery at Baton Rouge, LA. The seven Lfy synthetics were
developed by Comnuts, Inc., Oakland, California and provided as subscription
material, under licensing, to M. S. Kang. Seed of the resulting 21 F, crosses
(reciprocals were not included because of insufficient seed for some crosses) were
planted in three environments: Ben Hur Plant Research Farm (BHF) on 13 April 1989
and 26 March 1990 and Perkins Road Research Farm (PRF) on 4 April 1989. A
randomized complete block design was used with three replications and two-row plots
in each environment. The soil type at BHF was a Commerce silt loam (Aerie
Fluvaquent, fine-silty, mixed, nonacid and thermic), whereas the soil type at PRF was
an Olivier silt loam (Aquic Fragiudalf, fine-silty, mixed, nonacid, thermic). The F!
crosses were grown in 6.1-m length plots with a 30-cm spacing between plants and a
102-cm spacing between rows using standard cultural practices.
Eight weeks after planting, i.e., approximately one week prior to mid silking,
five leafy plants and five non-leafy plants were identified on the basis o f the number
o f leaves above the ear from the two-row plots of each F, cross segregating for Lfy and
lfy phenotypes. In each category, five consecutive leaves per plant, beginning with the
sixth leaf from the base of a plant, were visually rated for LMCD, using the following
rating scale reported by Kang et al. (1988): 0.0, no white tissue composed o f dead
cells; 0.1, less than 1% white; 0.5, 2 to 12%; 1.0, 13 to 25% white; 2.0, 26 to 50%
white; 3.0, 51 to 75% white; and 4.0, 76 to 100% white. Data for LMCD for
individual leaves and mean LMCD (mean rating of five leaves) from each plot in each
environment were analyzed according to Griffing’s Method 4, Model 1 (Griffing,
1956), with genotypes treated as fixed effects. Hallauer and Miranda (1988) suggested
that in Model 1, estimation of components of variance and their subsequent use in
calculation o f heritability was not appropriate, but estimation o f GCA and SCA effects
was appropriate and valid. We restricted our analysis to estimation o f GCA and SCA
effects and to determination of the relative importance o f GCA and SCA for LMCD by
computing the {2dlglQ.6lg + 6 1&)} ratio suggested by Baker (1978) using mean square
components (fixed model) associated with variance of GCA (c^g) and SCA (t^s).
56
Results and Discussion
Estimates of coefficient of determination (R2) (corrected sum of squares
accounted for by the sum of squares regression in an analysis of variance for a
variable), coefficient of variation (C. V.) (a relative measure of variation for a variable),
and mean statistics for LMCD for each of five consecutive leaves in a plant, beginning
with the sixth leaf from the base of a plant, are presented in Table 2.1. R2 value was
highest for the third leaf. C.V. values progressively decreased from 27.6% to 13.2%
as the leaf position varied from first to fifth. Mean LMCD value for the first leaf was
lowest and that for the fifth leaf highest. Correlation coefficients (P =0.001) between
LMCD of first through fifth leaf with mean LMCD of all five leaves (Table 2.1) were
high (r^ 0 .9 2 ). The correlation coefficient involving the third leaf (r=0.98) was the
highest among all leaves. On the basis of a relatively high R2 (87.4%) and low C.V.
(17.3%), LMCD for the third leaf was more reliable in differentiating genotypes than
LMCD of the other four leaves. A relatively high R2 and a low C.V. value for LMCD
o f the third leaf in years 1989 (Table 2.2) and 1990 (Table 2.3) substantiated the above
conclusion. A practical implication of these results is that LMCD for the third leaf
would eliminate the need to rate the other four leaves, thus saving researchers’ time.
A combined analysis of variance for LMCD based on the mean of all leaves is
presented in Table 2.4. Mean squares for environments were significant. The relatively
high amount of variation among environments may be due to differences in soil type.
The mean LMCD ratings at BHF in 1989 and 1990 were higher than the ratings at PRF
57
Table 2.1. Estimates of R2t, C.V.*, mean, and correlation coefficients (r) for
parenchyma cell death in leaf midribs (LMCD) in each of five leaves, beginning
with the sixth leaf from base of a plant, from a diallel cross among seven leafy
(Lfy) synthetics grown in three environments during 1989 and 1990.
Leaf position
R2
C.V.
Mean
r®
---------------- %. ------- ------------
Rating
First leaf
86.0
27.6
1.49
0.94*”
Second leaf
86.7
23.4
1.83
0.97*”
Third leaf
87.4
17.3
2.31
0.98*"
Fourth leaf
84.5
15.0
2.74
0.96*”
Fifth leaf
79.9
13.2
3.09
0.92”*
Mean LMCD
87.8
15.7
2.29
1.00*”
*** Significant at the 0.001 probability level.
f R2 = Coefficient of determination.
* C.V. = Coefficient o f variation.
* r = Correlation coefficients between LMCD of first through fifth leaf with
LMCD of all five leaves.
mean
58
Table 2.2. Estimates of R2+ and C.V.* statistics for parenchyma cell death in leaf
midribs (LMCD) in each of five leaves, beginning with the sixth leaf from base
of a plant, from a diallel cross among seven leafy (Lfy) synthetics grown at two
environments in year 1989.
Leaf position
R2
C.V.
------------------------- %..
First leaf
79.6
35.8
Second leaf
79.2
29.2
Third leaf
77.9
21.9
Fourth leaf
75.5
18.0
Fifth leaf
70.6
16.0
Mean LMCD
79.4
19.4
+ R2 = Coefficient of determination.
* C.V. = Coefficient of variation.
59
Table 2.3. Estimates of R2+ and C.V.* statistics for parenchyma cell death in leaf
midribs (LMCD) in each of five leaves, beginning with the sixth leaf from base
of a plant, from a diallel cross among seven leafy (Lfy) synthetics grown at Ben
Hur Farm in year 1990.
Leaf position
R2
C.V.
---% ---------------
First leaf
66.7
19.9
Second leaf
66.1
17.4
Third leaf
69.7
12.0
Fourth leaf
60.9
10.8
Fifth leaf
50.6
8.9
Mean LMCD
68.4
11.3
+ R2 = Coefficient of determination.
* C.V. = Coefficient of variation.
60
Table 2.4. Analysis of variance for ratings (mean of five leaves) for parenchyma cell
death in leaf mid-rib, beginning with the sixth leaf from the base of a plant,
from a diallel cross among seven leafy {Lfy) synthetics evaluated in three
environments during 1989 and 1990.
Source
df
Mean squares
Environments (E)
2
77.569”
Replications/E
6
0.659”
Leaf Type (LT)
1
5.356”
F] Crosses (C)
20
1.166”
GCA
6
2.833”
SCA
14
0.452"
E x LT
2
2.382”
Ex C
40
0.291"
E x GCA
12
0.622”
E x SCA
28
0.149
20
0.363”
LT x GCA
6
0.698”
LT x SCA
14
0.220
40
0.101
LT x E x GCA
12
0.166
LT x E x SCA
28
0.073
234
0.130
LT x C
LT x E x C
Pooled error
2<r2g/(2a2g+cr2s)} ratio
** Significant at 1 % level of probability.
0.59
61
in 1989 (Table 2.5). The soil at PRF was well drained, whereas the soil at BHF was
poorly drained. High amount o f soil moisture at BHF, due to poor water drainage,
might have increased LMCD. Melis and Rijkenberg (1988) concluded that high soil
moisture during plant development, prior to flowering, increased stalk rot in com.
Variation due to leaf type was significant (Table 2.4). The significant
environment x leaf type interaction (Table 2.4) suggested that the effect of Lfy gene on
LMCD was modified by environmental factors. LMCD o f individual leaves and mean
o f all five leaves was significantly lower for the Lfy than the non -Lfy plants in PRF-89
and BHF-90 environments (Table 2.5), which substantiated that extra leaves of Lfy
plants probably produced supplemental photosynthate and had a favorable effect on
LMCD in leaves. Shaver (1983) reported that Lfy plants had highly lignified stalk and
leaf parts and excellent stalk quality. These findings are in agreement with Dodd’s
theory that the "photosynthetic stress-translocation balance" plays an important role in
development o f stalk rot. According to photosynthetic stress-translocation balance
concept of stalk rot (Dodd, 1977 and 1980), limited photosynthate that is available
under stress conditions leads to competition between grain filling ear (sink size) and
stalk. A relatively higher LMCD rating for each o f the five leaves of rvon-Lfy plants
might have been due to competition for limited photosynthate between grain filling ear
and leaves (source) under stress conditions.
Mean squares for both the GCA and SCA were significant (Table 2.4). Baker
(1978) suggested that the use of relative sizes o f mean squares to assess the relative
importance of general and specific combining ability may be misleading, and recommended
Table 2.5. Overall effect of the leafy (Lfy) gene on mean parenchyma cell death in leaf mid-rib in each of five leaves, beginning with the sixth leaf from the base of a plant,
from a diallel cross among seven leafy (Lfy) synthetics grown in three environments (1989-90).
Leaf
Leaf tvpc
One
P89*
B89*
Two
B901
P89
B89
Three
B90
P89
B89
Five
Four
B90
P89
B89
B90
P89
B89
Mean LMCD1
B90
P89
B89
B90
Cell death rating
Non-Lfy 0.62 a# 1.53 a 2.58 a
0.99 a 1.81 a 3.04 a
1.62 a 2.28 a 3.47 a
2.17 a 2.69 a 3.74 a
2.70 a 3.08 a 3.89 a
1.62 a 2.28 a 3.34 a
0.77 b 1.93 a 2.42 b
1.25 b 2.33 a 2.91b
1.79 b 2.76 a 3.29 b
2.26 b 3.06 a 3.55 b
1.32 b 2.35 a 2.84 b
0.10
0.13
0.15
0.16
0.10
Lfy
0.53 b
1.67 a 2.02 b
LSD
0.06
0,19
0.16
0.17
0.17
0.16 0.13
1 Mean LMCD: Mean cell death in leaf mid-ribs of all five leaves.
* P89 = Perkins Road Farm-1989.
1 B89 = Ben Hur Farm-1989.
1 B90 = Ben Hur Farm-1990.
1 Means followed by the same letter are not significantly different according to LSD (P=0.05).
0.15
0.13
0.15
0.11
0.15
0.12
the use of the {2o2g/(2cr2g + ^ s)} ratio that can be applied to fixed models as well as
random models. The closer this ratio is to unity, the greater the predictability of
progeny performance based on GCA alone (Griffing, 1956; Baker, 1978). Recently, this
ratio was used to estimate the relative importance of GCA and SCA in explaining
progeny performance in a diallel analysis (fixed model) of relative growth rate in com
(Nevado and Cross, 1990) and aflatoxin production on com grain (Gorman and Kang,
1991). A ratio of 0.59 suggested that GCA effects were more important than the SCA
effects for LMCD, and that additive effects were preponderant over non-additive
effects. The relative importance of GCA and SCA can also be assessed by computing
the amount of corrected sum of squares among the F, crosses accounted for by the
GCA and SCA. The GCA accounted for 73 % of the corrected sum of squares among
F| crosses, and the other 27% were attributable to SCA. Both the approaches of
assessing the relative importance of GCA and SCA lead to a similar conclusion that
GCA effects were more important than SCA effects in the seven LJy synthetics studied.
The significance of additive effects being more important than non-additive effects for
LMCD is that a recurrent selection program for low LMCD might be effective in
reducing LMCD by concentrating favorable genes. Since LMCD was rated one week
prior to flowering, genotypes with low ratings can be identified and desired crosses can
be made in the same generation, which would allow completion of a cycle of selection
in one generation. Selection for low LMCD could result in genotypes with low
intemodal parenchyma cell death and possibly low stalk rot and associated lodging.
64
The significant environment x crosses interaction for LMCD suggested that the
trait was highly influenced by environmental factors, and it emphasized the need for
evaluating genotypes in more than one environment. Further partitioning of environment
x crosses indicated that this interaction was mainly due to environment x GCA
interaction. GCA was also differently modified by the Lfy and the non-Lfy
characteristics, which implied that progress from selection for LMCD would be
expected to be different in Lfy and non -Lfy populations derived from the seven Lfy
synthetics studied.
Correlation coefficient (P =0.001) between LMCD o f third leaf and third stalk
intemodal pith cell death (PCD) was significant and positive (r= 0 .3 3 ). The GCA
effects (Table 2.6) for A632 syn, B73 syn, and Hy syn were negative suggesting that
use o f these synthetics might be effective in reducing LMCD and possibly stalk rot and
associated stalk lodging. Crosses involving Lfy genotypes, especially A632 syn and B73
syn, would be expected to have low PCD and LMCD. It would be desirable to avoid
the use o f A619 syn, M ol7 syn, Wf9, and 914 syn in a breeding program as they
showed positive GCA effects. The GCA effects of the seven synthetic parents were in
the same rank order as their line means, except Wf9 syn. Among all crosses, the Fj
cross W f9 syn x Hy syn has the lowest LMCD and Wf9 syn x M ol7 syn had the
highest LMCD.
Ten out of 21 crosses showed negative SCA effects. The cross B73 syn x 914
syn showed the highest negative SCA effects (Table 2.6) followed by A619 syn x A632
syn, A619 syn x Wf9 syn, Hy syn x M ol7 syn, Hy syn x Wf9 syn, M o l7 syn x 914
Tabic 2.6. Mean rating* for parenchyma cell death in leaf mid-rib of F ( crosses (above diagonal), general combining ability (GCA) effects (on diagonal), specific combining
ability (SCA) effects (below diagonal), and line means of F, crosses (F,) from a diallel cross among seven leafy (LJy) synthetics grown in three environments during
1989 and 1990.
Synthetic
A619
A632
B73
Hy
Mo 17
W©
914
F,
A619
0.021
2.17
2.50
2.02
2.23
2.10
2.55
2.26
A632
-0.604
-0.226
2.08
1.99
2.21
2.19
2.29
2.15
B73
0.353
-0.175
-0.207
1.97
2.41
2.20
2.42
2.18
Hy
0.466
-0.128
0.245
-0.833
2.24
1.87
2.24
2.05
Mol7
-0.047
0.118
0.048
-0.370
0.311
3.08
2.69
2.47
W©
-0.557
0.251
0.232
-0.294
0.462
0.552
2.76
2.37
914
0.389
0.539
-0.704
0.082
-0.212
-0.047
0.382
2.49
L.S.D. (P=0.05) values for testing differences among F, crosses and line means are 0.24 and 0.11, respectively.
Standard error for g; = 0.149, gj - gj = 0.228, s, = 0.293, s, - sa = 0.456 and s- -
= 0.394.
* Leaf mid-rib cell death ratings are based on mean of five leaves beginning with the sixth leaf from the base of a plant.
66
syn, A632 syn x B73 syn, A632 syn x Hy syn, A619 syn x M ol7 syn, and Wf9 syn
x 914 syn. Use o f crosses with positive SCA effects, for instance, A632 syn x 914 syn,
A619 syn x Hy syn, M ol7 syn x W f9 syn, and A619 syn x 914 syn etc. should be
avoided.
In conclusion, LMCD was under genetic control and was conditioned by
quantitative trait loci. Use o f the genetic material possessing LJy gene in breeding
programs would be desirable to reduce LMCD. O f course, this conclusion is applicable
only to the LJy synthetics used in this study, as they were regarded as fixed effects.
Additive genetic effects were more important than non-additive genetic effects for
LMCD. A recurrent selection practiced in a population derived especially from three
LJy synthetics, viz., A632 syn, B73 syn, and Hy syn, might be effective in reducing
LMCD. Evaluation of genotypes for LMCD via a visual rating of the third leaf
provides a non-destructive, rapid method for indirect selection o f genotypes with low
SICD and might result in reduced parenchyma cell death in stalk intemodes, thus
possibly lowering stalk rot and associated stalk lodging.
67
References
Baker, R.J. 1978., Issues in diallel analysis. Crop Sci. 18:533-536.
Berzonsky, W .A, J.A. Hawk, and T.D.Pizzolato. 1986. Anatomical characteristics of
three inbred lines and two maize synthetics recurrently selected for high and low
stalk crushing strength. Crop Sci. 26:482-488.
Colbert, T.R. 1971. Inheritance of cell death patterns in stalk, shank, and cob in maize
(Zea mays L.). M.S. Thesis, Southern Illinois University, Carbondale. 39 pp.
Dodd, J.L. 1977. A photosynthetic stress-translocation balance concept of stalk rot.
Proc. Annu. Com Sorghum Res. Conf. 32:122-130.
Dodd, J. L. 1980. Grain sink size and predisposition of Zea mays to stalk rot.
Phytopathology 70:534-535.
Gorman, D .P., and M.S. Kang. 1991. Preharvest aflatoxin contamination in maize:
Resistance and genetics. Plant Breeding 107:1-10.
Griffing, B. 1956. Concept of general and specific combining ability in relation to
diallel crossing systems. Aust. J. Biol. Sci. 9:463-493.
Hallauer, A.R., and J.B. Miranda. 1988. Quantitative genetics in maize breeding. 2nd
edition, Iowa State University Press, Ames, Iowa.
Hunter, J.W ., and N.E. Dalbey. 1937. A histological study of stalk breaking in maize.
Am. J. Bot. 24:492-494.
Kang, M.S. 1971. Inheritance of cell death patterns in the leaf mid-rib of Zea mays L.
Southern Illinois University, Edwardsville. 34 pp.
Kang, M .S., O. Myers, Jr., and A.J. Pappelis. 1988. Heritability o f cell death in leaf
midribs of maize (Zea mays L.). Cereal Res. Commun. 16:25-29.
Melis, M ., and F.H .J. Rijkenberg. 1988. Moisture stress in the screening of maize
cultivars for stalk rot resistance and yield. Plant Dis. 72:1061-1064.
Nevado, M .E ., and H.Z. Cross. 1990. Diallel analysis of relative growth rates in maize
synthetics. Crop Sci. 30:549-552.
68
Pappelis, A .J., and O. Myers, Jr. 1970. Parenchyma cell death patterns in the stalk,
shank, cob, and leaf midrib o f maize. Amer. Soc. Agron. Abstracts, p. 27.
Shaver, D. 1983. Genetics and breeding o f maize with extra leaves above the ear. Proc.
Annu. Com sorghum Res. Conf. 38:161-180.
Zuber, M .S ., and M .S. Kang. 1978. Com lodging slowed by sturdier stalks. Crop Soils
30(5) N: 13-15.
Chapter 3
General and Specific Combining Ability for
Seed Quality Traits in Maize
69
70
Introduction
Seed quality in maize (Zea mavs L.) hybrids is an important consideration for
the seed industry as well as farmers. Seed quality is determined by both genetic and
physiological factors (Pollock and Roos, 1972). Inbred lines of maize differ in seed
quality traits such as germination, vigor, and longevity (Lindstrom, 1942; Haber, 1950;
Neal and Davis, 1956). Breeders need information on inheritance of seed quality traits
to improve seed quality. Limited genetic studies conducted on maize seed quality
indicated inheritance of seed quality traits to be quantitative in nature. Azala and
Fakorede (1988) reported that seedling vigor in maize, as measured by field emergence
percentage and rate of emergence, was inherited quantitatively with additive and
nonadditive gene effects being important. Odiemah (1989) studied inheritance o f four
seed quality traits (germination, electrical conductivity, complex stressing vigor, and
cold test germination) and reported that a selection procedure based on additive genetic
effects should be effective in improving these characters. Despite these studies, little
or no information is available on inheritance of seed weight, root and shoot length o f
seedlings, dry matter, and vigor index. Genetic knowledge of maize seed quality traits
would be useful to maize breeders in choosing an appropriate breeding strategy for
improving seed quality.
Breeders are continually interested in obtaining genetic information on new
germplasm that may have a potential for high yield, improved seed quality, and/or other
desirable attributes. Information on genetic effects conditioning seed germination and
71
vigor of genetic stocks containing the leafy (Lfy) gene is lacking. The Lfv gene,
dominant to the non-leafy (lfv') allele, was discovered by Robert C. Muirhead, Hughes
Hybrids Inc., in 1971. The Lfv gene materials are characterized by extra leaves above
the ear, low ear placement, highly lignified stalks and leaf parts, earliness, and high
yield potential (Shaver, 1983).
According to Dodd’s (1977) photosynthetic stress
translocation balance concept of maize stalk rot, a direct competition between grain sink
and stalk for carbohydrate, which was limited by stress conditions, leads to poor grain
and stalk quality. Extra leaves in Lfv material may have a favorable effect on seed
quality by production o f more photosynthate. In a field study, aflatoxin accumulation
on maize kernels was less in the Lfy maize than the corresponding maize without the
Lfv gene (Kang and Lillehoj, 1987). A literature search revealed no studies on
inheritance o f seed quality traits in the Lfy maize.
The objective of this study was to estimate general (GCA) and specific
combining ability (SCA) effects for 100-seed weight, percent germination, percent cold
test germination, root length, shoot length, dry matter, and vigor index through a diallel
mating design among seven maize synthetics containing the Lfy gene.
72
Materials and Methods
Seven maize synthetics (A619 syn, A632 syn, B73 syn, Hy syn, M ol7 syn, Wf9
syn, and 914 syn) containing the Lfy gene were crossed in all possible combinations in
1987 in the maize breeding nursery at Baton Rouge, LA. The seven synthetics were
developed by Comnuts, Inc., Oakland, California and provided as subscription
material, under licensing, to M. S. Kang. The resulting 21 F, crosses were hand
harvested, dried for a week at 43° C in a forced hot air dryer, and machine shelled.
Seed was stored in a freezer at -18° C. Reciprocal crosses were not included in the
study because of insufficient seed. Seeds from the 21 F, crosses were evaluated in a
seed testing laboratory for the following seed quality traits:
a)
One hundred seed weight: Six replications of 100 seeds each from the 21 F t
crosses were weighed and the mean weight was expressed in grams (g).
b)
Percent germination: Eight replications of 50 seeds each were germinated in
rolled paper towels at 25° C and > 9 5 % relative humidity in a germinator.
After seven days, the seedlings were evaluated and normal seedlings were
counted and expressed as percent germination (International Seed Testing
Assoc., 1976).
c)
Root and shoot length and seedling dry matter: Seven days after planting, 10
seedlings from paper towel medium were carefully removed at random from
each of four replications, and root and shoot length were measured. The mean
values were calculated and expressed in cm. The seedlings were dried in a hot
73
air oven at 85° C for 24 h, cooled in a desiccator for 45 minutes, and weighed
to determine seedling dry matter (mg).
d)
Vigor index: vigor index was calculated using the following formula:
Vigor Index = Percent germination x mean dry matter o f seedling (mg).
e)
Percent cold test germination: Jiffy peat line mix (1.13 kg) was placed in each
18.75 cm x 26.25 cm x 10 cm plastic pots and spread evenly in the bottom.
One hundred seeds were placed in each pot and pressed down to keep the seeds
in place. W ater was added in a calibrated amount to moisten the medium to
70% saturation. Two replications of 100 seeds were planted for each cross.
The pots were covered with lids and placed, for seven days, in an insulated
chamber cooled to 10° C. The pots were removed from the chamber and placed
on growing racks at room temperature with continuous light for four days and
the number o f normal seedlings was recorded (Association o f Official Seed
Analysts, 1983).
Data were analyzed according to Griffing’s Method 4, Model 1 (Griffing, 1956),
with genotypes treated as fixed effects. Hallauer and Miranda (1986) suggested that in
Model 1, estimation o f components o f variance and their subsequent use in calculation
of heritability was not appropriate but estimation o f GCA and SCA effects was
appropriate and valid. We restricted our analysis to estimation o f GCA and SCA effects
and relative importance of GCA and SCA for each trait by computing a
(2cr2g/(202g+ a 2,)} ratio as suggested by Baker (1978) using mean square components
74
associated with variance of GCA (o^) and SCA (o2,). Genetic correlations of GCA
effects among the observed seed quality traits were calculated.
75
Results and Discussion
Significant differences were detected among marginal means o f F, crosses for
seed weight, percent germination, percent cold test germination, root length, shoot
length, dry matter, and vigor index (Table 3.1). Marginal means o f B73 syn and A632
syn for root and shoot length were higher than those of the other synthetics. The Hy
syn had highest mean values for all seed quality traits except shoot length. Seedling
vigor, expressed as vigor index, was superior for Hy syn, Mo 17 syn, and Wf9 syn
because o f their greater seed weight. The 914 syn had relatively lower seed weight and
recorded relatively lower vigor index. Superiority of the synthetics with heavy seed,
for seedling vigor, may be due to their "initial food-capital" (Heydecker, 1972). A
practical implication of this finding is that selection for genotypes possessing heavier
seed may result in improved seedling vigor in the Lfy material used in this study.
Analysis of variance (Table 3.2) indicated that GCA and SCA mean squares
were significant for all traits. Baker (1978) suggested that use o f relative sizes o f mean
squares to assess the relative importance o f general and specific combining ability may
be misleading, and recommended the use o f {2o2g/(2o2g+ o 2J} ratio that can be applied
to fixed models as well as random models. The closer this ratio is to unity, the greater
the predictability o f progeny performance based on GCA alone (Griffing, 1956;
Baker, 1978). Recently, this ratio was used to estimate the relative importance o f GCA
in explaining progeny performance in a diallel analysis (fixed model) o f relative growth
rates (Nevado and Cross, 1990) and aflatoxin production on grain (Gorman and Kang,
76
Table 3.1. Marginal means of F, crosses for seven seed quality traits in Lfy maize.
100 Seed
weight
Germination
Cold test
germination
Root
length
Shoot
length
Dry
matter
<g)
(%)
<%)
(cm)
(cm)
(mg)
A619 syn
24.67
88.08
86.50
16.55
16.63
143.04
12599
A632 syn
23.26
92.90
90.00
17.93
17.98
133.10
12365
B73 syn
24.23
93.37
91.50
19.67
17.43
133.00
12418
Hy syn
26.23
97.17
94.00
18.35
14.82
162.58
15798
Mo 17 syn
25.40
87.47
90.50
17.29
15.36
152.37
13328
WF9 syn
27.46
86.00
80.00
16.22
16.72
153.00
13158
914 syn
23.02
90.25
90.66
17.78
16.87
129.17
11658
Mean
24.32
91.14
89.70
17.78
16.75
139.86
13046
0.95
4.90
7.75
1.57
1.51
17.64
675
Synthetic
LSD (0.05)
Vigor
index
Table 3.2. Mean squares for seven seed quality traits o f Lfy maize.
Source of
variation
df
100 Seed
weight
df
Germination
df
%
g
Cold test
germination
df
*
Root
length
Shoot
length
Dry
matter
cm
cm
mg
Genotypes
20
9.51**
20
180.92**
20
82.33**
20
11.77**
8.84**
GCAt
6
10.22**
6
227.59**
6
55.11**
6
17.75**
SCA*
14
9.20**
14
160.92**
14
94.00**
14
147
33.73
0.20
21
12.75
0.08
63
Error
CV(% )
105
0.14
0.14
1.5
6.3
3.8
Vigor
index
1517.15*
16607199**
18.01**
1480.46**
16632821**
9.21**
4.91**
1532.87**
16596219**
1.32
1.85
0.50
142.92
0.2i
6.5
8.2
8.6
0.14
*, ** Significant at the 5 % and 1% level o f probability, respectively. |GCA = general combining ability; {SCA = specific combining ability.
1574578
0.14
9.9
1991) in maize synthetics. The relatively low ratios (Table 3.2) suggested that SCA
effects were more important than GCA effects for all seed quality traits studied except
shoot length (for shoot length, both GCA and SCA effects were equally important),
indicating preponderance of dominance and epistatic effects for these seed quality traits.
The significance of these results is that genetic gain from selection for a majority of
seed quality traits would be relatively small. Breeders should strive for identification
of specific crosses with desired seed quality. Odiemah (1989) reported that inheritance
of germination and cold test germination was conditioned mainly by additive genetic
effects. However, our results from the Lfy material suggested an important role of
nonadditive effects for germination and cold test germination. The differences relative
to inheritance in the two studies might be attributable to the differences in the
experimental materials used.
Correlations among GCA effects for different traits are additive genetic
correlations (Griffing, 1956) and are due to pleiotropic effects of genes and not due to
linkage. When an additive genetic correlation between two traits is high and significant,
it suggests that improvement in one trait would result in improvement o f the other trait
(correlated response). Genetic correlations among GCA effects for seed weight vs. dry
matter (rA = 0.89), and seed weight vs. vigor index (rA = 0.84), and dry matter vs.
vigor index (rA = 0.90) were high and significant (P = 0.01). These correlations
substantiated our previously stated conclusion that selection for genotypes possessing
heavier seed weight may result in improved seedling vigor. The only other significant
correlation was between percent germination and cold test germination (rA = 0.74, P
79
= 0.05). The implication here is that due to pleiotropic effect, selection for genotypes
possessing high percent germination may improve cold test germination.
According to Sprague and Tatum (1941), GCA is the average performance of
a line in hybrid combinations, whereas, SCA is deviation in performance of a hybrid
combination from that predicted on the basis o f the GCA o f parents involved in the
hybrid. Both GCA and SCA effects are important indicators of the potential value of
parents in hybrid combinations. All the GCA effects for seed quality traits are presented
(Table 3.3), however, only the significant SCA effects are presented.
Different
synthetics showed positive or negative GCA effects for different seed and seedling
traits. Use of parents with positive GCA effects for seed quality traits would improve
seed quality in resulting populations. Parents B73 syn, Hy syn, M ol7 syn, and Wf9 syn
had significant, positive GCA effects for seed weight.
Hy syn and Wf9 syn had
significant, positive GCA effects for vigor index. These synthetics also had relatively
higher seed weight (Table 3.1). Hy syn showed significant, positive GCA effects for
germination, whereas B73 syn had significant, positive GCA effects for cold test
germination. Overall, Hy syn consistently showed positive GCA effects for all traits
except shoot length. A632 syn and B73 syn showed positive GCA effects for shoot
length. The Hy syn x 914 syn cross consistently had positive SCA effects for all seed
quality traits. The Hy syn x M ol7 syn cross exhibited significant positive SCA effects
for seed weight, germination, cold test germination, dry matter, and vigor index.
Therefore, use of Hy syn in hybrid combinations would aid in improving seed
germination and vigor.
80
Table 3.3. General (g j t and specific combining ability (Sy) effects for seven seed quality traits in Lfv maize.
Effect
100 Seed
weight
Germination
Cold test
germination
Root
length
Shoot
length
Dry
matter
(cm)
(n>8)
Vigor
index
(8)
(»)
(* )
(cm)
0.1 0
0.33
-0.10
-1.65+*
0.59
-8.06
-779.00
62
-0.98*+
-0.84
-2.70+
-0.34
1.48*+
-12.73**
-1383.45+*
Ej
0.25**
0.64
3.08**
1.25++
0.74+
0.92
414.21
84
0.18**
4.17++
1.83
1.42++
-1.30++
6.84+
1284.20+*
Sj
0.44**
-5.48+*
-3.91
-0.41
-1.69**
13.25*+
So
0.77**
0.75
-0.65
-0.44
0.38
8.96*+
724.10++
St
-0.57+*
0.41
2.46
0.17
-0.19
-9.16**
-750.35*+
s„
Sa
0.54+*
-6.70*+
-2.73
1.49+
-0.73
20.78*+
1948.89++
1.27+*
4.93**
4.87*
-2.17**
0.88
18.95++
-2263.36++
-16.62
-719.70
1910.47+*
Si
-
490.27
7.09**
12.86*
-1.78*+
1.60
1.06+*
2.13
1.60
1.99+*
-1.14*
12.92+
S„
-1.47
4.12
-2.92
0.87
2 .5 1+*
-33.30++
-2794.37*+
S«
-1.10**
1.15
7.82**
1.53**
1.33**
-16.99**
-1654.21**
S 43
2.38**
7.08++
6.33*
0.22
-1.49
37.05++
4499.64+*
4.23
12.61**
1.63+
1.19
11.33
1535.57*
S 23
-0.14
s*
S 47
0.37*
*t ** Significant at the 5% and 1% probability level, respectively,
t Standard error for:
Si
0.15
2.38
1.42
0.48
0.57
4.96
16.75
S.J
0.29
4.70
2.81
0.95
1.12
9.78
33.02
& - Ej
0.23
3.64
2.17
0.73
0.87
7.56
25.58
0.46
7.29
4.35
1.47
1.74
15.15
51.61
0.39
6.31
3.77
1.27
2.43
42.89
141.12
S<j ■
S|j - su
81
In conclusion, seed quality traits were under genetic control and were
conditioned by quantitative trait loci. Since SCA effects were important for most of the
seed quality traits studied, genetic gain, via a cyclic selection program, for seed quality
would be small. We propose that use of genotypes or parents possessing high seed
weight in a breeding program would result in improved seed germination and vigor. Hy
syn would be a useful source of favorable alleles for improving seed quality traits.
Further research is needed to establish the effect of the Lfy gene on seed quality traits
by including normal maize along with their isogenic Lfy counterparts for comparative
purposes.
82
References
Association o f Official Seed Analysts. 1983. Cold test. Seed Vigor Testing Handbook,
pp. 56-62.
Azala, S.O ., and M .A.B. Fakorede. 1988. Inheritance o f seedling-vigor and its
association with mature plant traits in a maize population at two levels of
inbreeding. Maydica 33:121-129.
Baker, R.J. 1978. Issues in diallel analysis. Crop Sci. 18: 533-536.
Dodd, J.L . 1977. A photosynthetic stress-translocation balance concept o f com stalk
rot. Proc. Annu. Com Sorghum Res. Conf. 32:122-130.
Gorman, D .P . and M .S. Kang. 1991. Preharvest aflatoxin contamination in maize:
Resistance and genetics. Plant Breeding 107:1-10.
Griffing, B. 1956. Concept o f general and specific combining ability in relation to
diallel crossing systems. Aust. J. Biol. Sci. 9:463-493.
Haber, E.S. 1950. Longevity o f the seed o f sweet com inbreds and hybrids. Amer.
Soc. Hort. Sci. Proc. 55:410-412.
Hallauer, A .R ., and J.B. Miranda. 1988. Quantitative genetics in maize breeding.2nd
edition, Iowa State University Press, Ames, Iowa.
Heydecker, W. 1972. Vigor. In: E.M . Roberts (ed.) Viability o f seeds. Syracuse
University Press, New York. pp. 209-252.
International Seed Testing Association. 1976. International rules for seed testing. Seed
Sci. Technol. 4:3-49.
Kang, M .S ., and E.B. Lillehoj. 1987. Aflatoxin resistance in com . Report o f Projects
for 1987, Department of Agronomy, Louisiana Agricultural Experiment Station,
Baton Rouge, Louisiana, p. 48.
Lindstrom, E.W . 1942. Inheritance o f seed longevity in maize inbreds and hybrids.
Genetics, 27:154.
Neal, N .P ., and J.R. Davis. 1956. Seed viability o f com inbred lines as influenced by
age and condition of storage. Agron. J.48:383-384.
Nevado, M .E ., and H .Z . Cross. 1990. Diallel analysis of relative growth rates in maize
synthetics. Crop Sci. 30:549-552.
83
Odiemah, M. 1989. Quantitative inheritance of seed quality characteristics in maize
(Zea mays L.). Cereal Res.Commun. 17:245-251.
Pollock, B.M., and E.E. Roos. 1972. Seed and seedling vigor. In: T.T. Kozlowski
(ed.) Seed biology, volume 1, pp. 313-387. Academic Press, New York.
Shaver, D. 1983. Genetics and breeding of maize with extra leaves above the ear. Proc.
Annu. Corn Sorghum Res. Conf. 38:161-180.
Sprague, G .F., and L.A. Tatum. 1941. General vs. specific combining ability in single
crosses of corn. J. for Amer. Soc. Agron. 34:923-932.
SUMMARY AND CONCLUSIONS
Field studies were conducted in three environments to elucidate genetics of
parenchyma cell death (PCD) in stalk intemodes and leaf midrib cell death (LMCD) in
maize possessing the leafy {LJy) gene via a diallel cross among seven LJy synthetics, to
determine the effect of Lfy gene on PCD and LMCD, and to identify the most reliable
stalk intemode and leaf for possible selection of PCD in stalk intemodes and LMCD,
respectively.
Specific combining ability (SCA) effects were more important than general
combining ability effects (GCA) for PCD, whereas GCA effects were more important
than SCA effects for LMCD. A recurrent selection program for low LMCD might be
effective in reducing LMCD by concentrating favorable genes. LJy plants had
significantly lower PCD and LMCD than the non-LJy plants, suggesting that extra
leaves in the Lfy plants probably produced supplemental photosynthate and thus had
a favorable effect on PCD and LMCD. A632 syn, B73 syn, M ol7 syn, and Wf9 syn
showed negative GCA effects for PCD, whereas A632 syn, B73 syn, and Hy syn
showed negative effects for LMCD. Crosses involving LJy genotypes, especially A632
syn and B73 syn, would be expected to have low PCD and LMCD. Both the PCD and
LMCD were highly influenced by environmental factors. Because of a significant
genotype x environment interaction, selection of genotypes should be based on ratings
from more than one environment. The PCD of third stalk intemode and LMCD of third
leaf were reliable in differentiating genotypes. Selection of genotypes for good stalk
quality and resistance to stalk rot based on either third stalk intemode PCD or third leaf
84
85
LMCD would be reliable. Since, LMCD can be visually rated with ease by examining
the leaf mid-rib without destroying any plant part, unlike stalk intemodal cell death
where stalks need to be split longitudinally, it could be used as a practical method to
screen genotypes for resistance to stalk rot and lodging.
In a laboratory experiment, seed from the 21 F, crosses (diallel) were evaluated
for seed weight (g), percent germination, root length (cm), shoot length (cm), dry
matter (mg), and vigor index. SCA was more important than GCA for all seed quality
traits except for shoot length, indicating dominance and epistatic effects to be
preponderant for these traits. The significance of these results is that genetic gain from
selection for a majority of seed quality traits would be relatively small. Since additive
genetic correlations (r > 0.84) for dry matter and vigor index with seed weight were
significant, it is suggested that selection of genotypes or parents possessing relatively
higher seed weight would result in improved seed germination and vigor.
VITA
Sridhar Dronavalli was bom in Vijayawada, Andhra Pradesh, India on
December 7, 1960. H e graduated from high school in 1976. The author completed
intermediate from Siddhartha College, Vijayawada in 1978. He studied one year of
Biology at Loyola college, Vijayawada. In August, 1979, the author joined D .A .V .
College, Ajmer (affiliated with the University o f Rajasthan, Jaipur), India for a
Bachelors degree in Agriculture and graduated in August, 1983. Later, he entered
Tamil Nadu Agricultural University, Coimbatore, India for a Master o f Science degree
in Agriculture with major in Seed Technology and graduated in December, 1985. In
April, 1986, the author accepted a position as a Research Officer (Seed Quality Control
Officer) with Pioneer Seed Company, Hyderabad (subsidiary o f Pioneer Hi-Bred
International, U.S.A) and worked until August, 1987. He married Sarada Vani in
August, 1986. In August, 1987, the author joined a Ph.D . program in Agronomy (Plant
Breeding) at Mississippi State University, Starkville. In August, 1988, he transferred
to Department o f Agronomy, Lousiana State University to continue his Ph.D . He
accepted a graduate research assistantship in January, 1989. The author is now a
candidate for the Ph.D. degree in Agronomy with Plant Breeding as major and Genetics
as minor.
86
DOCTORAL EXAMINATION AND DISSERTATION REPORT
Candidate: Sridhar Dronavalli
Major Field:
Agronomy
Title of Dissertation:
Genetics of Stalk and Seed Quality Traits in Maize
(Zea mays L.) with Lfy Gene
Approved:
Major Professor and Chaxrman
Dean of the Graduate School
EXAMINING COMMITTEE:
lV\
Date of Examination:
November 18, 1991