Drought Effects on Adventitious Root Development in Blue Grama Seedlings
D.D BRISKE
AND A.M. WILSON
Abstract
Crowns of blue grama (Bouteloua gracilis) seedlings of three
ages were exposed to drought treatments for 2 days, in constant
humidity environments, and were then planted in moist soil for a
lo-day growth test at 25°C. Percentage survival of crowns
decreased with a decrease in water potential during the temporary
drought treatment and with a decrease in crown age at time of
treatment. The percentage survival rates of 21-, 28-, and 35-day-old
crowns treated at -180 bars were 5, 54, and 83, respectively.
Crowns exposed to the 2-day drought treatment subsequently
produced shorter adventitious roots than untreated crowns. Thus,
a drought-induced
inhibition of adventitious root growth may
reduce the probability of successful seedling establishment.
Establishment
of blue grama (Bouteloua
lings begins with germination
and extension
gracih) seedof the seminal
primary root, which originates from the lower portion of the
embryo
(Gould
1968). After approximately
2 weeks of
favorable
environmental
conditions
following emergence,
seedlings develop the capacity for adventitious
root development from crowns of the primary shoot and tillers located
at or very near the soil surface (Hyder et al. 197 1; van der
Sluijs and Hyder 1974). If seedlings do not initiate and
develop adventitious
roots within 6 to 10 weeks after emergence, they often die. Seedling death at this time does not
result from an inherent limit to longevity of the seminal
primary root; rather, seedlings expand leaf area to a maximum that can be supported by water from the seminal root
and then succumb to an increase in transpiration
stress
(Wilson et al. 1976). Adventitious
roots are effective in
seedling establishment
because they are longer-lived
and
have a greater capacity for water uptake than the seminal
primary root. Thus, successful adventitious
root establishment provides seedlings with an effective means of drought
avoidance.
In the more arid portions
of the Central Plains, blue
grama
plantings
often fail because
the environmental
requirements
for development
of adventitious
roots are not
met. These requirements
include a moist soil profile at the
time of planting, temperatures
between 15 and 30°C, and
two periods in which the soil surface remains continuously
period for emergence and one for
moist for 2 to 4 days-one
development
of adventitious
roots (Briske and Wilson 1977,
1978; Fults 1944; Olmsted 1941, 1942; Riegel 1941; Wilson
and Briske 1979).
The authors are, respectively, former graduate research assistant, Department of
Range Science, Colorado State University, and plant physiologist, U.S. Department
of Agriculture, Science and Education Administration,
Agricultural Research, Crops
Research Laboratory,
Colorado State University, Fort Collins, Colorado 80523.
Briske currently is assistant professor, Department of Range Science, Texas A&M
University, College Station, Texas 77843.
The study involved cooperative investigations of the Agricultural Research, Science
and Education Administration,
U.S. Department of Agriculture, and the Colorado
State University Experiment Station, Fort Collins 80523 (Scientific Series Paper NO.
2413). The authors thank Michael A. Orrino, former biological aid, for photographs
of seedlings and assistance in plant measurements. We thank John A. Dickerson, of
the Soil Conservation
Service, for providing the blue grama seed used in the study.
Manuscript received February 23, 1979.
JOURNAL
OF RANGE MANAGEMENT
33(5), September
1980
In addition
to the direct effects of environment,
morphological stage and physiological
condition
may influence the
capacity
of seedlings
to initiate
and extend
adventitious
roots. On the Central Plains, seedlings surviving on only the
seminal
root are often exposed
to periods of drought,
which
may adversely
affect
their physiological
condition.
The
objective
of this research
was to determine
the capacity
of
seedlings
to prod&e
adventitious
roots as affected
by age
and exposure
to temporary
drought.
This capacity may
determine
whether or not seedlings will become successfully
established
once
the
rather
restrictive
environmental
requirements
for adventitious
root initiation
and elongation
are met.
Methods
Blue grama accession PM-K-1483 is a synthetic blend of seed
from 12 accessions originating in southern Kansas and Texas. Seed
for this study was harvested in 1974 at the Plant Materials Center,
Manhattan,
Kansas.
Seeds were planted at a depth of 2 cm in plastic pots (15 cm
diameter by 15 cm deep) filled with autoclaved sandy loam soil.
Planting of seeds was scheduled so that seedling crowns of three
age classes (2 1, 28, and 35 days) would be ready for temporary
drought treatment on the same date. During initial seedling growth
in the greenhouse, the pots were subirrigated to promote growth of
the seminal primary root and to maintain a 1.5-cm layer of dry
surface soil that would prevent the initiation of adventitious roots.
After emergence, seedlings were thinned to 12 per pot.
At 2 1,28, and 35 days after planting, the seminal root system and
subcoleoptile
internode were removed just below the coleoptilar
node. All leaf tissue was removed 1 cm above the coleoptilar node
to simulate a drought-induced
die-back of all exserted leaf tissue
and to facilitate placement of crowns within the constant humidity
tray. These procedures were accomplished
to accurately control
crown drought stress. Uniform stress generally is not achieved
through drying soil in the entire rooting zone of the seminal root.
Therefore, only the seedling crown (lowest l-cm section of stem
base) was retained to evaluate drought tolerance.
Seedling crowns were exposed to temporary drought for 2 days
in constant humidity trays (Wilson 1971). Water potentials of -30,
-60, -90, - 120, - 150, and - 180 bars were maintained in the trays
with various concentrations
of NaCl solution (Lang 1967; Robinson and Stokes 1949). Seedling crowns lost water vapor to the
constant humidity air but did not touch the NaCl solutions. The
capillary flow of NaCl solution above and below the constant
humidity tray was sufficient to compensate for evaporation from
the moist paper.
After 2 days of exposure to drought, seedling crowns were
planted at a depth of about 0.7 cm in plastic pots (15 cm diameter
by 15 cm deep) filled with autoclaved sandy loam soil. Thus, the
upper portion of the crowns extended about 0.3 cm above the soil
surface. The soil was covered with a 0.5-cm layer of fine gravel to
reduce evaporation
from the soil surface. The soil was irrigated
daily by surface application to maintain favorable moisture conditions (about -0.3 bars) for the growth of leaves and adventitious
323
roots from treated crowns. After the planting of crowns, pots were
placed in a growth chamber at a constant temperature
of 25OC,
daylength of 15 hours, relative humidity of SO%, and photosynthetic photon flux density of 480 microeinsteins
mZ2 set-‘. The
experiment
also included a control consisting of 21-, 28, and
35-day-old
crowns that had not been exposed to temporary
drought (untreated).
The seminal root system and leaves were
removed just as in the treated crowns, but they were then immediately planted on the same date as the treated crowns.
After a IO-day growth performance
test, all pots were removed
from the growth chamber, soil was washed from the roots with a
fine spray of water, and seedlings were placed in a 10% solution of
acetic acid and stored at 5°C until measurements
of growth could
be made. The number of shoots (primary shoot and all tillers) that
had developed before the drought treatment, the number of shoots
that produced regrowth during the lo-day test, and the number of
adventitious roots were counted. The lengths of all leaf blades and
adventitious
roots were also measured. New shoots, leaf blades,
and adventitious
roots were counted and measured if they had
reached lengths of 0.5, 0.5, and 0.1 cm, or more respectively.
Percentage survival of treated and untreated crowns was based on
the number of crowns in which one or more shoots exhibited leaf
growth of at least 0.5 cm during the lo-day growth test under
favorable temperature
and moisture conditions.
The number of
surviving 21-day-old crowns in the -180 bar treatment was inadequate for an evaluation
of drought effects on selected growth
measurements.
Consequently,
only crown survival was compared
for all three age classes at the -180 bar drought treatment.
A randomized
complete block experimental
design with six
replications
was used. The six replications
were six sequential
repetitions of the experiment. Each value (within a replication) for
seedling survival was based on a sample of 15 treated or untreated
crowns planted in an individual pot. Each value for the other
growth criteria was based on the average of all surviving seedlings
from one pot. Analysis of variance, linear regression, and polynomial regression were used to evaluate relationships among age
classes, drought treatments,
and growth responses. All observations were included in statistical analyses, but only means were
reported.
Additional seedlings (21,28, and 35 days of age) were grown for
measurement
of xylem water potential with a pressure chamber
(Waring and Cleary 1967). The main shoot and large tillers were
used for these measurements.
The objective was to determine
seedling water potential
before harvest compared
with crown
water potential during the 2-day drought treatment.
Extra seedling crowns (35 days old) were exposed to water
potentials of -30, -60, -90, -120, -180 bars in constant humidity
trays to determine if they had reached equilibrium after 2 days of
treatment. The water potential of these crowns was measured with
a thermocouple
psychrometer.
Because standard procedures are
not useful for measuring water potentials below about -60 bars, a
modified procedure was used (Campbell and Wilson 1972). The
thermocouple junction was cooled for 10 minutes over a sample of
NaCl solution (-10 bars) to allow water condensation
on the
junction. The sample of seedling crowns was then quickly positioned below the junction, and the thermocouple
output (microvolts) was recorded. A similar procedure was used with standard
NaCl solutions for developing a calibration curve. Measurements
of crown water potentials were made for three of the replications.
growing was kept at about -0.3 bars. Therefore, differences
in seedling water potential were not due to differences in soil
moisture conditions,
but were probably due to greater leaf
area and transpirational
water loss in older seedlings than in
younger seedlings. Wilson et al. (1976) observed that an
increase in leaf area at a time when there was little or no
increase in capacity for water uptake created a condition of
seedling drought, even though the seminal root was growing
in moist soil. Apparently,
water uptake was restricted by a
threadlike nature of the subcoleoptile
internode connecting
the seminal primary root and the shoot.
Thermocouple
psychrometer
measurements
indicated
that seedling crowns had reached equilibrium
with constant
humidity environments
during the 2-day treatment (Table
1). Because water potentials
represent average values for
individual crowns, the possibility of minor water potential
gradients within the crown has not been eliminated. Shoot
apices and adventitious
root primordia of large 35-day-old
crowns might have been protected from dehydration
more
than the apices and primordia of comparatively
small 21day-old crowns. However, the close agreement
between
solution
osmotic
potential
and average
crown water
potential
suggests that differences among age classes in
crown injury and survival resulted mainly from differences
in drought tolerance rather than drought avoidance (Levitt
1964).
Table 1. Osmotic potentials of controlling solutions (bars) and water
potentials for 35-day-old crowns for each of the six drought treatments
after a 2-day equilibration period in constant humidity trays.
Solution
osmotic
potentials
Crown
-30
-60
-90
-120
-150
-180
l&indicates
standard
water potentials
-3Of2.1’
-62f 1.5
-87f7.9
-116f7.5
-152f7.5
-1801t9.2
deviation
of the sample.
20
35
DAYS --oDAYS --A-
-
Results
Seedling and Crown Water Potential
Before harvest
and treatment
of crowns, 21-, 2%, and
35-day-old blue grama seedlings exhibited average xylem
water potentials of -9.5, -14.5, and -17.0 bars, respectively.
Thus, crown water potentials
in the least severe drought
treatment were about 13 to 20 bars lower than xylem water
potentials of seedlings before harvest.
Water potential of soil in which the seminal root was
324
-?*o
-150
I
-120
I
DROUGHT
-901
-601
TREATMENT
-301
0I
(bars)
Fig. 1. Effects of age and temporary drought treatments on thepercentage
of blue grama crowns that produced regrowth during a IO-day growth
test underfavorable soil moisture conditions. Correlation coefficients (r)
SEE =
labeled with two asterisks p*) are significant at the 0.01 level.
standard error of estimate.
JOURNAL
OF RANGE MANAGEMENT
33(5), September 1980
Table 2. Number of shoots per crown of 35-, 2%, and 21-day-old blue grama seedlings, before temporary drought treatments; and number of shoots
crown producing regrowth during a lo-day growth test following temporary drought treatments.
Growth and criteria
crown age class
Temporary
and
drought
treatments
(bars)
-180
-150
-120
-90
-60
-30
0
Mean
Number of shoots/crown
before treatment
35 days
28 days
21 days
Treatment
Mean
6.3’
3.9
2.6
4.3 a
6.1
4.5
3.1
4.6 a
6.3
4.2
2.7
4.4 a
6.6
3.9
2.8
4.4 a
6.9
4.5
2.8
4.7 a
6.4
4.2
2.8
4.5 a
6.6
4.4
2.7
4.6 a
6.5 c
4.2 b
2.8 a
Number of shoots/crown
producing
regrowth
35 days
28 days
21 days
Treatment
Mean
3.5
2.1
1.0
2.2 a
4.0
2.6
1.6
2.7 c
4.2
2.5
1.2
2.6 b
4.8
2.8
1.6
3.1 cde
5.0
3.1
1.7
3.3 de
4.8
3.0
2.0
3.3 se
5.5
3.3
2.0
3.6 e
4.5 c
2.8 b
1.6 a
IValues represent an average of six replications.
level.
Effects of Temporary
Means in columns or rows, within each growth criterion,
Drought on Survival
Percentage
survival of crowns exhibited
a significant
(P<O.Ol) age times drought
interaction,
indicating
that
crowns of younger
seedlings
were more susceptible
to
drought injury than crowns of older seedlings (Fig. 1). Percentage survival rates of treated (-180 bars) and untreated
crowns were 5 and 86 for 2lday-old
crowns, 54 and 95 for
28-day-old
crowns, and 83 and 99 for 35day-old
crowns,
respectively.
Drought
tolerance
of 28day-old
crowns
approached that of 35day-old
crowns but had not yet developed sufficiently for a high percentage survival in the severe
drought treatments.
Twenty-one-day-old
crowns had developed neither the capacity for regrowth (control treatment)
nor the drought tolerance exhibited by the 28- and 35-dayold crowns.
Seedlings randomly
assigned to various treatments
did
not differ in number of shoots per crown before harvest and
treatment (Table 2). The number of shoots that survived the
drought treatments
and produced regrowth during the loday test decreased with a decrease in water potential during
temporary
drought treatments.
50
Leaf Development
and Growth
Leaf growth resulted mainly from the expansion of immature leaf blades that had been clipped at the time crowns
were harvested;
it resulted, to a lesser degree, from the
initiation
of new tillers from axillary buds. Twenty-six percent of the crowns produced one new tiller, and only rarely
did crowns produce two new tillers. Data indicated that
both leaf intercalary meristems and axillary buds possessed
a high degree of drought tolerance.
Total leaf length produced
during
the IO-day test
decreased with a decrease in water potential
during the
2-day drought treatment (Fig. 2). Total leaf length produced
bY 21-, 28-, and 35-day-old-crowns
was reduced 56,35, and
30%, respectively,
as a result of treatment
at -150 bars.
Number of surviving shoots per crowns was positively associated with total leaf length produced per crown (r = 0.96).
s I
Adventitious
Root Development
and Growth
Adventitious
roots were not initiated in crowns of any age
class until several centimeters
of leaf had developed. Thus,
leaf tissue was the first regrowth observed. These results
agree with those of Hyder et al. (1976), who found that the
JOURNAL
OF RANGE MANAGEMENT
33(5), September
1980
labeled with the same letter are not significantly
different at the 0.0
I
capacity for root growth in mature blue grama crowns was
more sensitive to drying than the capacity for shoot growth.
Only 3% of the crowns produced leaf growth but no root
growth in this study.
The number of roots per crown produced during the
IO-day test decreased with a decrease in age of crowns at
time of treatment (Fig. 3). The large number of roots produced by older crowns was mainly a result of the large
number of surviving shoots from which adventitious
roots
could develop.
Number of roots per crown also tended to decrease with a
decrease in water potential during the temporary drought
treatments.
In relation to the control treatment, the number
of roots in 21-, 28-, and 35-day-old crowns was reduced 24,
8, and S%, respectively, as a result of the - 150 bar treatment.
Only in the 21day-old
crowns was the reduction in root
numbers significant (KO.05).
Total length of adventitious
roots per crown decreased
with a decrease in age of crowns at the time of drought
treatment (Fig. 4 and 5). Greater total length of roots in the
older crowns can be attributed both to a greater number of
Drought on Growth
Effects of Temporary
per
r
O-180
I
-150
1
I
1
35, WIYS --o--
-30
-120
-90
-60
DROUGHT TREATMENT (bars)
0
Fig. 2. Effects of age and temporary drought treatments on total leafblade
length produced by the surviving blue grama crowns during a IO-day
growth test under favorable soil moisture conditions. Correlation coefficients (r) labeled with two asterisks p*) are significant at the 0.01
level. SEE = error of estimate.
325
IO -
5
p
6-
E
k
, = 0.16
4’31
-
r= 0.40*
2-
-7eo
SEE = *_ 1.0 ro;t
.__~____-__-
:
z
3
________-~
~
A
-________-&.------‘1
26
g
21 days
Untreated
21 days
-180 bars
0
SEE’
o
? 0.5
rOOt
0
AGE CLASS
‘21
DAYS +
20 DAYS -m35 DAYS --A-
0
-150
0
-120
DROUGHT
-90
-60
TREATMENT
-30
-
2, DAYS 28 DAYS -m35 DAYS --A--
-120
-90
-60
DROUGHTTREATMENT
\
(bars)
shoots from which adventitious
roots could be produced
and to a greater length per root.
Total length of adventitious
roots per crown also
decreased with a decrease in water potential during the
temporary
drought treatments
(Fig. 4). Total root length
produced from 21., 2X-, and 35.day-old crowns was reduced
52, 34, and 27%, respectively,
by the -150 bar treatment.
Number of live shoots per crown was positively associated
with total length of roots per CTOW~(I = 0.94).
The length of the longest root per crown decreased wifh a
decrease in age of crowns at the time of treatment (Fig 6).
The maximum
rate of root elongation
(averaged over the
entire IO-day growth test) was about 1.0 cm per day and
occurred in 35-day-old, untreated crowns. This is a low rate
_qKl
I
-I 50
\
0
-30
(bars1
0
28 days
-180 bars
‘\
28 days
,ptreoted
of elongation,
compared with an elongation
rate of 2.3 cm
per day found in a previous experiment (Briske and Wilson
1977). The low rate of root elongation was probably a result
of the long period (2 to 6 days) required by seedling crowns
to produce photosynthetic
tissue. Crowns required several
centimeters of leaf length before the initiation and development of adventitious
roots.
The length of the longest root per crown decreased with a
decrease in water potential during the temporary drought
treatment (Fig. 6). A significant (KO.01) age timesdrought
interaction
indicated that drought inhibited root elongation
in younger crowns more than in older crowns. The length of
the longest root produced
by 21-, 2%, and 35day-old
crowns was reduced 44,25, and 21% respectively, as a result
Ih
1:j , , ijiii~, ,
-180
-I 50
-30
-60
-90
-120
DROUGHT TREATMENT (bars)
0
Fig. 6. Effects of age and temporary
drought treatments on the longest
adventitious
root produced by blue grama crowns during a IO-day
growth test under favorable soil moisture conditions. Correlation coefficients (r) labeled with two asterisks (**) are significant at the 0.01
level. SEE = standard error of estimate.
of the -150 bar drought treatment.
The relatively small
percentage decrease iniength
of roots produced by %-dayold crowns is similar to the results of Hyder et al. (1976),
who found that root lengths of mature blue grama crowns
were not affected by drying treatments.
The crowns of
mature blue grama plants-and of older seedlings apparently
are more drought tolerant than crowns of younger seedlings.
Discussion
Drought may affect the capacity for root development
from blue grama seedling crowns in two ways: (1) by reduction of seedling leaf area, and (2) by direct injury of root
primordia and other crown tissues. This study simulated the
conditions in which drought causes die-back of exserted leaf
tissue and, in addition, causes desiccation
of crowns.
The combined
clipping and mild drought
treatments
probably
injured seedlings more than a similar degree of
drought (without clipping) in the field. For example, one
would expect that only portions of leaves would die back at
a water potential
of -30 to -60 bars (Wilson and Sarles
1978). The retention of leaves by seedlings at such water
potentials would probably result in a greater development of
roots than in the present experiment,
in which all exserted
leaf tissue was removed. However, under severe drought
conditions in the field, all exserted portions of leaves would
die back; the effects of drought in that respect, would be
similar to the effects observed in this study:
In relation to soil moisture requirements
for development
of adventitious
roots, the test conditions following temporary drought were less severe than conditions generally found
in the field. Favorable
moisture conditions
during the loday growth test were adequate for development
of leaves
and roots from many of the crowns, in spite of drought
injury. On the Central Plains, periods of 2 or more days with
a continuously
moist soil surface are rare (Briske and Wilson 1977), and seedlings often fail to develop adventitious
JOURNAL
OF RANGE
MANAGEMENT
33(5), September
1980
roots because of rapid drying of the soil surface. Even under
conditions in which seedling-drought
is not severe, the initial
rate of root extension is sometimes just sufficient to keep the
root tip ahead of the drying soil front (Wilson and Briske
1979). Thus, a drought-induced
delay of root initiation and
an inhibition
of root growth may reduce the probability
of
successful seedling establishment.
Injury and death of seedlings might be reduced by conserving soil moisture and by
planting when the soil profile is moist and the probability of
precipitation
and favorable temperatures
is high.
On the basis of leaf and root growth following temporary
drought, the results indicate that drought tolerance of blue
grama seedlings increases with increasing age. However,
both the mechanism by which drought tolerance increases
and the stage of seedling development
at which drought
furtherinvestigation.
tolerance reaches a maximum remain unknown and warrant
Literature
Cited
Briske, D.D., and A.M. Wilson. 1977. Temperature effects on adventitious
root development
in blue grama seedlings. J. Range Manage. 30. 276280.
Briske, D.D., and A.M. Wilson. 1978. Moisture and temperature
requirements for adventitious
root development
in blue grama seedlings. J.
Range Manage. 31: 174-178.
Campbell, G.S., and A.M. Wilson. 1972. Water potential measurements of
soil samples, p. 142-149. In: R.W. Brown and B.P. Van Haveren (ed.)
Psychrometryin
Water Relations Research. Utah Agr. Exp. Sta., Logan.
Fults, J.L. 1944. Some factors affecting the establishment of perennial grass
for erosion control in eastern Colorado.
J. Amer. Sot. Agron. 36:
615-625.
Gould, F.W. 1968. Grass Systematics. McGraw-Hill
Book Co., New York.
382 p.
Hyder, D.N., A.C. Everson, and R.E. Bement. 1971. Seedling morphology
and seeding failures with blue grama. J. Range Manage. 24: 287-292.
Hyder, D.N., W.R. Houston, and J.B. Burwell. 1976. Drought resistance of
blue grama as affected by atrazine and N fertilizer. J. Range Mnage. 29:
214-216.
Lang, A.R.G. 1967. Osmotic coefficients and water potentials of sodium
chloride solutions from 0 to 4O’C. Aust. J. Chem. 20: 2017-2023.
Levitt, J. 1964. Drought p. 57-66. In: M. Stelley and H. Hamilton (ed.).
Forage plant physiology
and soil-range
relationships.
Amer. Sot.
Agron., Madison, Wisconsin.
Olmsted, C.E. 1941. Growth and development
in range grasses. I. Early
development of Bouteloua curtipendula in relation to water supply. Bot.
Gaz. 102: 499-519.
Olmsted, C.E. 1942. Growth and development
in range grasses. II. Early
development
of Bouteloua curtipendula as affected by drought periods.
Bot. Gaz. 103: 531-542.
Riegel, A. 1941. Life history and habits of blue grama. Trans. Kans. Acad
of Sci. 48: 119-127.
Robinson, R.A., and R.H. Stokes. 1949. Tables of osmotic and activity
coefficients of electrolytes in aqueous solution at 25OC. Trans. Faraday
Sot. 45: 612-624.
van der Sluijs, D.H., and D.N. Hyder. 1974. Growth and longevity of blue
grama seedlings restricted to seminal roots. J. Range Manage. 27: 117119.
Waring, R.H., and B.D. Cleary. 1967. Plant water stress: evaluation by
pressure bomb. Science 155: 1248-I 254.
Wilson, A.M. 1971. Amylase synthesis and stability in crested wheatgrass
seeds at low water potentials. Plant Physiol. 48: 541-546.
Wilson, A.M., D.N. Hyder, and D.D. Briske. 1976. Drought resistance
characteristics
of blue grama seedlings. Agron. J. 68: 479-484.
Wilson, A.M., and J.A. Sarles. 1978. Quantification
of growth drought
tolerance and avoidance of blue grama seedlings. Agron. J. 70: 231-237.
Wilson, A.M., and D.D. Briske. 1979. Seminal and adventitious
root
growth of blue grama seedlings on the Central Plains. J. Range Manage.
32: 209-213.
327