Agric. Rev., 26 (2) : 114 - 123, 2005
CHEMICAL HYBRIDIZING AGENTS (CHA) - A TOOL FOR
HYBRID SEED PRODUCTION - A REVIEW
Yogendra Sharma and S.N. Sharma l
National Bureau of Plant Genetic Resources.
Regional Station, Phagli, ShimJa - 171 004, India
ABSTRACT
The failure of a plant to produce functional gametes is known as sterility. Sterility induced by
application of certain chemicals like, auxins and antiauxins (NM, TlBA, 2, 4-0, MH, etc.), halogenated
aliphatic acid (FW-450, daIapon, etc.), gibbereUic acid (GAJ, etheophon, OPX-3718, arsenicals
(MSMA, OM, ZMA, etc.), RH-531, RH-532 etc. These chemicals are called gametocides since
they lead to pollen abortion and there by cause male sterility, some times it results in female sterility
also. So the term chemical hybridizing agents (CHA) are used since after the entire primary objective
is to produce a hybrid. A number of CHAs have been reported to cause male sterility for production
of hybrids. Therefore, no need to developed maintainer or restorer lines and save time, labor and
money. In the present review, different aspects of CHA viZ., characteristic, mode of action, stage of
treatment, application doses, etc. for different CHAs and also for different crop wise.
The failure or inability of a plant to
produce functional gametes or viable pollen
under a given set of environmental conditions
is known as sterility. The term sterility covers
all cases of infertility or barrenness that result
from irregularities in the sexual reproductive
system. This is different from incompatibility,
for the male or female gametes are abnormal
or non-functional. Male sterility to be a
condition resulting frorn defects leading to the
non-forrpation of pollen or to the lack of
functional power in it when formed. Thus in a
male sterile lines either pollen is not formed
or if formed it is non-viable. Male sterility can
be classified as follows based on the factors
which cause them.
MALE STERIUlY
Genetic male sterility
1. Phenotypic - Sporogenous
- Structural
- Functional
2. Genotypic
- Genic
- Cytoplasmic
Cytoplasmic Genic
Chemical induction of sterility in plants
has been of interest since 1950 when the
potential for selective male sterility was first
demonstrated. It was recogn!zed that while
there may be disadvantages with chemicals
there could also be advantages, especially in
terms of time required to discover economically
viable hybrids. The chemical method for
I
Induced male sterility
1. Chemical
- Male Gametocides
2. Physiological 3. Ecological
Temperature
Photoperiod
Enzyme balance
Other biochemical's
Climatic
inducing sterility can obviate the often lengthy
time period required to obtaip male-sterile and
restorer lines, which usually must precede
evaluation of hybrid performance.
Consequently, chemical became of interest
both for use as breeding tools as a means to
produce hybrids on a commercial scale.
Various terms have been used since
Present address: Agriculture Research Station, Rajasthan Agricultural University, Durgapura, Jaipur-302 01B,lndia.
Vol. 26, No.2, 2005
115
1950 to describe chemicals that induce male
These criteria are essential to
sterility in plants. The most commonly used maximize the efficiency of hybrid seed
term is gametocide (or) selective gametocide. production, a requirement for commercial
This terminology was introduced by Eaton hybrid seed production. However, all the
(1957) who demonstrated the potential of criteria are not required to the same degree in
producing Gossypium h,irsutum hybrids order to produce hybrid seed in smaller
through the use of sodium-a, ~-dichloro- quantities, only limited quantities of hybrid seed
isobutyrate (FW-450). Over years many are required for breeding purposes. Thus, even
investigator have used such terms as male though a CHA may have some adverse affect
sterilant, selective male sterilant, pollen on female fertility, it may still be possible to
suppressant, pollenicide and androcide. The produce seed in quantities sufficient for
companies involved in producing these breeding programs. Thus, a CHA may not·
chemicals desist from using such terms because have potential for large-scale commercial seed
the male sterility and site of action is not known production may have potential for use in
until well after the chemical begins to be breeding programme.
investigated. Further, it results in female sterility Characters of an Ideal CHA are:
also. 'So, the term Chemical Hybridizing Agents
• Induction of male but not female sterility.
(CHAs) is used after the entire primary
• Complete inhibition of pollen
objective is to produce a hybrid.
development.
CHA Technology
• Independent from environmental
The purpose of a CHA is to facilitate conditions.
the production of hybrid seed. The technology
• Independent from genotypic differences.
for making hybrids with a CHA is essentially
• Wide flexibility of rate and time of
identical to cytoplasmic male-sterile (CMS) application.
• Absence of phyto-toxicity or other adverse
method. The only difference is that functional
male sterility in the female parent is obtained effects.
with a chemical rather than by genetic
• Non mutagenic
manipulations. Both the CMS and CHA
• Environmeptal safety
methods are dependent upon the following:
• Minimum side effects, and
• A source of viable pollen from a male
• Cost effectiveness.
parent that can outcross with male sterile, Mode of Action:
female parent.
The major disturbances that result
• A female planting configuration that will ultimately in non-functioning of male gametes
maximize out crossing.
include the following:
• An agency such as wind or insects to move
1. Disruption of meiosis lending of arrest of
pollen from the male to the female parent.
anther development and degeneration of
• Female and male parents that are in pollen mother cells (PMC) or curly microspores.
synchrony i.e. pollen from the male parent. 2. Disruption of exine formation resulting
must sheds than male-sterile female parent is in thin walled, irregular and non-viable
receptive.
microspores.
3. Decrease of starch deposition and
• An abundance of pollen from the male
parent.
appearance of abnormal vacuoles in the
• Good flower-opening characteristics in the microspores making them non-viable.
female parent.
4. Persistence and abnormal growth of
116
AGRICULTURAL REVIEWS
tapetal layers.
5. Indehiscence or delayed dehiscence of
normally developed anthers with viable pollen.
6. Non-germination of pollen on stigma or
cessation of elongation of pollen tube resulting
in non-fertilization.
Tapetal cells surrounding the developing pollen
cells play an essential role in the development
of pollen grains by serving as nutrition channel.
Any disturbance to tapetal layer, therefore,
leads to male sterility. Even extreme protrusion
of tapetal cells crushing microspores has also
been reported to cause sterility (Frankel and
Stage of Treatment
Galun,
1977).
The nature of damage listed above depends
largely on the stage of treatments:
At the end of interphase, microspore
1. Treatment at panicle initiation stage (pre- nuclell~ .divides unequally forming vegetative
meioticl affects.normal development ofamners and generative cells. This division being a very
and PMCs.
unusual type, it becomes vulnerable to CHA's
2. Treatment at bolting stage (meiotic phase) which are specific to post-meiotic phase. The
and flowering stage (post-meiotic) disturbs generative cell divides once more in the pollen
normal development and functioning the grain forming two sperm cells. Normally, the
microspores.
generative cell is near the intine wall, and if
any
chemical renders it impossible either to
Any disturbance to plasmodesmata,
be
at
that place, or to divide, the pollen
which connects sporogenous and tapetal cells,
becomes sterile (Colhoun and Steer, 1982).
may lead to non-development of PMC's. Once
Anther dehiscing is a pre-request for
PMC's are formed, tapetum plays a significant
the
release
of pollen. This is mainly due to
role in causing sterility. Over growth of tapetum
pressure
exerted
by microspores which in turn
in terms of either formation of double layer or
is
due
to
microspore
size. Indirectly, in other
clone action or tapetal cells prior to callus
words,
normal
development
and functioning
formation of PMC's leads to crushing of
of
tapetum
is
important
for
anthesis. If the
PMC's. A blockage in synthesis of calluses may
microspores are of small size due to poor
also render the PMC's sterile. All the foregoing
nutrient supply from tapetum, their pollen
is-a result of pre-meiotic disturbance.
tubes may not be long enough to fertilize the
Once the developmental process egg, resulting in sterility against.
reaches meiotic phase, treatment effect is
Frankel et al. (1969) reported (in
reflected in the form of varied cytological
Petunia) that proper timing of the activity of a
anomalies such as chromosome breakage,
particular enzyme, callase, during
irregular pairing, orientation and disfunction
microsporogenesis may be critical for the
of chromosomes, occurrence of laggards and
normal development of pollen. Early or delayed
uneven distribution of chromosomes, all
activity of callase in different plasma type leads
affecting normal development and viability of
to male sterility. Later on, Izhar and Frankel
pollen.
.(1971) further indicated that pH within the
Post-meiotic disturbance starting from anther locule regulates the callase activity.
tetrad stage extend till dehiscence of anther Differences in the free amino acid balance in
and release of pollen. Hindrance to microspore fertile and sterile anthers which are reflected
separation during tetrad stage causing sterility at early meiotic stage are also responsible for
is due to lack of calluse formation. Chemicals male sterility. Thus, alternative cytoplasmic and
specifically disrupting the callose synthesis chromosomal heredity elements exist for male
induce sterility through this phenomenon. sterility (Sharma, 1994).
Vol. 26, No.2, 2005
The gametocides RH-531 and RH532 were developed primarily for use in
cereals. The mode of action of RH-531 in
wheat was studied by Rajendra and Bates
(1981) and they reported that its action was
ascribed to its ability to make the tapetum
persistent which results in carbohydrate
starvation and subsequent microspores
degeneration. No meiotic abnormalities were
detected in the treated plants. Libale (1974)
obtained similar results in corn by using another
gametocide (DEX-3773), a symmetrical
triazinc which prevented anthers from
dehiscing. Mc Rae (1985) have reported that
in plant treated with 100 ppm and above of
RH before the emergence of the flag leaf,
abnormalities such as varying number of
univalents, cells with unbalanced chromosome
number, stickiness at metaphase laggards and
sticky bridges at first anaphase were observed
during meiosis in microsporocyte. Extreme
stickiness both at metaphase and anaphase was
observe~ at the second division also.
Based on their studies Wang and Lund
(1975) postulated that sterility was due to
microspore degeneration or of carbohydrate
starvation in treated barley plants that
prevented the tapetum from dehiscing and thus
caused microspore degeneration. Rajendra and
Bates (1981) postulated three possible modes
of gametocidal action.
(1) They could be involved in carbohydrate
metabolism, which some how starves the
tapetum and makes it persistent. But the pollen
and meiotic study suggest that microspore
degeneration is more likely due to meiotic
abnormalities.
(2) The pollen abnormalities could have
formed by direct action of the gametocide on
the genetic process of meiosis. The
gametocides could act directly on the gene (or)
genes for pole determinants and (thus disrupt
the meiotic spindle formation, which in turn
would result in aberrant cytokinesis). Any
117
disruption of pole and pore determinants would
lead to unequal distribution of chromatin,
resulting in polymorphic pollen and multiporate
(or) sporate pollen. The gemetocide also
disturbs the gene expression for exine
differentiation also.
(3) The third potential mode of action could
be directly on DNA synthesis prior to entry of
cells into meiosis. The gametocides may act
on some of the chromosomes before entry of
cells into prophase-I which could delay the
condensation of the chromosomes which in
turn delays bivalent formation. The final result
of this abnormality would be expressed as
partial
cleavages,
"Aporateness"
"Multiporateness" and other pollen
abnormalities.
HYBRIDIZING CHEMICALS
(A) An~i-auxins and Auxins
1. Maleic Hydrazide (MH): Auxins
and antiauxins were intensively studied as plant
growth regulators in the 1950s and maleic
hydrazide which is classified as an antiauxin
which initiated interest in the chemical control
of sterility. In spite of being the most studied
chemical for over 30 years, it is not adequately
. male/female selective.. Warren and Dimmock
(1985) established that MH did not prevent
pollen production in the field.
2. Other Auxins: Varying degrees of
success of male sterilants for various auxins
and antiauxins such as TIBA, 2, 4-dichloro
phenoxy acetic acid (2,4-D) and Naphthalene
acetic acid (NAA) has been reported. However,
there is no indication that any of these has
been effective either as a breeding tool (or) for
commercial production of hybrid seed (Tue and
Bang, 1998).
(B) Halogenated Aliphatic Acid
1. FW-450 (Mendck): FW-450 is
chemically sodium, il, a-dichloro isobutyrate
and its gametocidal effect was discovered by
Eaton (1957) in cotton. It was extensively
evaluated on many plant species and was found
118
AGRICULTURAL REVIEWS
t6 be often too phytotoxic and was not
adequately male/female selective. However,
this is still being used in many crops and more
often in cotton and also in pearl millet, barley
and triticale.
2. Other Halogenated Aliphatic
Acids: Seventeen halogenated aliphatic acids,
including FW-450 and dalapon were
investigated in Capsicum annum, and FW450,
dalapon and sodium- achloropropionate were
most effective (Hirose and Fujime 1973), but
for FW450, there is no evidence for the usage
of other chemicals of this category.
(C) Gibberellins
Interest is,Gibberellin~ (GA) as male
sterilants developed as a consequence of
studies in maize by Nelson and Rossman
(1958) who found potassium gibberellate
induced 100% male sterility without damage
to female fertility. Later Hansen et at (1976)
reported complete sterility in maize when
applied at pre meiosis growth stage. But, if
the treatment growth stage was too early,
adequate male sterility could not be obtained,
if too late, weakening of stems resulted. Later
similar gametocidal effect was noticed in
sunflower by many workers and these
established the potential of GA as a breeding
tool.
Ethephon cannot sterilities all tillers
in populations, in which tillers are at variable
development stages at treatment time. This
\
deficiency can be alleviated to some degree by
using high seed rates which tend to improve
tillers stage uniformity. Another weakness of
ethenphon is that it inhibits calm elongation,
so spikes may not fully emerge from the flag
leaf sheath. This problem can be minimized
by combining ethephon with GA (Dotlacil and
Apltauerova, 1980). In spite of all these
limitations, it is being used to pre use hybrid
seeds in many crops.
(E) DPX-3778
Chemically this is 13-P-chloro-phenyl6-methoxy-S-triazine-2, 4-OH-3H dionetri
ethanol amine]. Its gemetocide action was
reported in maize by Long et ai. 1973 and
was produced by DU-Pont. But because of its
weaknesses such as high application rate
requirement and variable sterility in commercial
planting its production was discontinued. It has
been studied extensively in China under the
name KM5-1 and Gametocide No.-2.
(F) Arsenicals.
Various arsenicals such as
monosodium methane arsonate (MSMA).
Dimethyl arsenic acid (OM) and Zinc methyl
arsonate (73010, Gemetocide No.1) have
been studied for their gemetocidal action in
People's Republic of China out of these zinc
(D) Ethephon [2-Chloroethyl Phosphonic
methyl arsonate was successful in rice (Jiang
Acid]
Xi Communism Labor University 1977 , South
This chemical is also called as Ethrel
China Agricultural College 1978), but it
(or) Camposan. Its gametocidal potential has
required 2-3 applications.
been extensively studied in many crops such
as wheat, barley, oats, rice, triticale, frosomillet, (G) Patented Compounds
Many
chemical
compounds
flax, brinjal, maize, rye, lettuce and sugar beet.
extensively used as gametocides have been·
Wheat has been the primary crop for
patented by multinational agiants such as
ethephon investigations. It has been found to
Rohm and Hass, and Shell chemical company.
induce adequate male sterility in both winter
The important chemicals patented are
and spring wheat when applied at the
considered under this section.
appropriate pre meiosis growth stage.
1) RH-531: Chemicalo/ this is Sodium
Adequate female fertility could also maintain.
119
Vol. 26, No.2, 2005
1- P-chloro phenyl-1, 2-dihydro-4, 6-dimethyl2 oxonicotinate. RH-531 induced male sterility
was first discovered by Yih et al. (1971) in
barley. Additional studies established that RH531 was not adequately male/female selective
in barley (or) in wheat. Another disadvantage
to RH-531 is its growth inhibitory character.
In cereals it inhibits stem elongation and spike
length. This affects the opening of the male
sterile florets.
2) RH-532: RH-532 is another Rohm
and Hass chemical and it differs slightly form
RH-531. This chemical initially was not
distributed to public (or) private investigators
to any extent better than RH-531. It could
induce 100% male sterility in a broad spectrum
of wheat genotype at application ratio of 1-3
kglha. However, like RH-531, it too was
inhibitory and was not adequately male/female
selective for large scale use. Nevertheless,
many wheat hybrids in the range of 85-100%
purity were obtained in USA and France. Thus,
RH-531 was a significant improvement from
Ethephon and DPX-3778.
3) RH-2956 and RH-4667: These
chemicals were developed by Rohm and Haas
between 1976 and 1981. Their important
feature is that they are significantly less
inhibitory to growth than RH-532, indicating
that inhibition is not obligatorily associated with
male sterility. Spike length inhibition is minimal
and male sterile florets have good flowering
opening characteristics with these two
. compounds. Further they are better than RH532, with regard to female fertility. The major
disadvantage is the relatively high application
rate (about 10 kglha).
4) Hybrid (RH-0007): This isa Rohm
and Haas chemical with d common name.
Fenridaxon-potassium and a trade mark
HYBRED. This induces 95-100% male sterility
in a very broad spectrum of hexaploid winter
and spring wheat. Genotypes responded to
dosages as low as 0.5 to 2.0 kglha. This is
. very slightly inhibitory, and florets on sterile
plants have good opening and closing
characteristics. Though fenridaxon-potassium
appears to be close to ideal gametocide,
improper treatment timing can have
unfavorable impact. Large number of hybrids
has been produced at the experimental level
in sufficient quantities for extensive yield trials.
The seeds produced using this agent is being
marketed in the US by Rohm and Haas seeds
inc. as HYBREX Hybrid seed wheat.
In the shell chemical company,
research was started in early 1990s to develop
gametocides of atleast 3 chemicals classes. (1)
Derivatives of the acid from which RH-5148
was derived (2) 3-azabicycle (3.1.0)-hexane2-Carboxylic acids and (3) azaetidine
derivatives eg-2carboxy-3methyl azetidine (Day
et al. 1983 and Devlin, 1981).
CROP WISE STUDY OF VARIOUS
GAMETOCIDE TREATMENTS
1. Rice: Huang Qun-Ce and Wang
Li-Zhu (1990) used different concentrations
(30, 40, 50 and 60 ppm) of Zinc methyl
arsenate (CH:AS03 Xn HP) to induce male
sterility in an incompletely male sterile
population of CIS 28-15 TGMS line. They
reported that complete pollen sterility could
be obtained in all the concentration levels by
spraying 5 days before heading which is at the
pollen exine stage. Successful commercial
cultivation of hybrid rice in China has created
enough interest in research and heterosis
breeding throughout the World including India.
But existing Indian environmental conditions
and instability of CHS line under Indian
conditions made it difficult to exploit
hybridization programme. Under such
conditions, gametoCides can be effective.
Sathyanarayana et al. (1995) used
three male gametocides Ethrel, sodium methyl
arsonate (SMA) and Natrium arsonate to
induce complete male sterility in three partially
sterile CMS, line viz., V20 AIR 54753A and
\
120
AGRICULTURAL REVIEWS
IR58053A along with a normal fertile variety
BPT 1235. They reported that among the
treatments SMA at 500 ppm in the genotype
IR 54754A was found to be the best treatment
as in this 100% pollen sterility could be
obtained with least phytoxicity. Higher
concentrations of chemicals resulted in more
plant damage by reduction in plant height and
panicle length.
Gangarao et aI. (1996) studied the
effect of a four chemicals on rice and reported
that, Ethrel was most eff(lctive in inducing
pollen sterility (94-95%) at 10,000 ppm.
Natrium arsonate was found to cause 49.9%
sterility at 600 ppm. The other chemicals
tested, TIBA and streptomycin were less
effective according to them, further
investigations in this direction by greatly helpful
to standardize the chemical hybridization
technology to supplement the three line
breeding system.
(MS) and 80% more seed set (55) could be got
by spraying of 7000 ppm for EK or 6000 ppm
for ES, during the pollen mother cell stage.
They also observed that the both CHAs have
a side effect on the plant heig!)t as well as on
the length of last internode, i.e. shorting of
them at after spraying.
Mahajan and his associates (1998)
evaluated different formulations for inducing
MS in wheat (WH 542) out of 41 formulations
studies. Seven amino acids analogues, viz., CH
97101, CH 9702, CH 9708, CH 9714, Ch
9731, CH 9732 and CH 9737 have exhibited
male sterility.
Female sterility was lowest with CH
9732 (10.7%) and this also caused auto
induction of floret opening, that promoted out
crossing with no apparent deformity on plants.
Effect of pollen source on weed setting
indicates that a proper selection of a male
parent can result in a better seed set. It has
2. Wheat: Miller and Lucken (1977) been confirmed that CH 9732 is an ideal CHA
evaluated several chemicals for their candidate.
gemetocidal action on 3 spring wheat cultivars.
3. Maize: Foliage of corn was sprayed
The chemicals were applied in two growth with gibberellic acid (GA ) in an attempt to
stages, first when the plants were beginning regulate sex expression of 3corn tassels. Tassels
to elongate or joint and the second treatment of inbred lines A 619 and Gaspe flint became
just before the appearance of flag leaf legule. pistillate male sterile (or) remained male fertile
RH-532 was the most effective of the four depending on time of application of GA .
3
gametocides, reducing fertility to 0-10% level Complete male sterility without pistil
for all the cultivars.
development was observed when applications
Rajendra and Bates (1981) reported
that the gametocides RH-531 and RH-532 are
highly potent and appearently act directly on
the genetic material of wheat to bring about
various gross pollen abnormalities and thus
sterility. According to them, not only was the
tapetum shown to be persistant but a total
disruption of meiosis and pollen exine different
action was observed.
were made 1 to 3 days before the onset of •
meiosis. Ear sex expression was not modified
by any of the treatments. (Hansen et aJ. 1976).
4. Pearl Millet: Sharma (1979)
reported that 0.5% of the Gametocide FW450 was effective in inducing complete male
sterility when applied on plants with their
panicles wrapped in the flag leaf on the main
tiller. It caused indehiscence of another. But
Zhao and his associate (1993) study pollen fertility was normal in panicles
on EK and ES and observed that both were developing later in tillers. Similar works were
ideal CHAs on wheat. 95-100% male sterile reported in Barley (Kumar et aJ. 1976), haploid
Vol. 26, No.2, 2005
Triticale (Sapra et.. al., 1973) etc.
5. Rapeseed: According to Guan ,et
al. (1981) a spray of 0.03% male gametocide
No-I (Zinc methyl arsenate) at floral initiation
stage caused 60-90% completely sterile plants,
partially sterile plants, plants with abortive
flower, buds and dead plants. They reported
that adhesion of PMCs and shriveling of micro
spores were responsible for failure of pollen
development. Amino acid composition of
anthers from completely sterile plants was
similar to that of plants with inherited male
sterility with only Alanine, Aspreatic acid and
Aspargin present in small amounts. The major'
disadvantage was that the emasculation rate
was not stable.
6. Cotton: In cotton the gametocide
often used in hybridizati<:m programme is FW450(Mendok)LadymanetaJ.(1990)evaiuated
the male sterilant potential of L-O-Methyl
threonnine (OMn by comparing its activity
with TD-1123 (3-4 di chloro-5-isothiazole
carboxylic acid) and SD-227559 (1,2-methyl
-4-difluoromethoxy-phenyi-1,4-dihydr0-4-oxo6-methyl-pyridazin-3yl) carboxylic acid. Hybrid
seeds were produced by using a gladded male
parent and the hybrid seeds were distinguished
from self by the gladded trait, it \:Vas found that
TD 1123 and OMT exhibited selective
gametocidal activity at dose rates 0.9 and 0.45
kgiha, but were phytotoxic at higher doses
such as 1.8 kgiha. According to them OMT
warrants further testing for its commercial
application.
Advantages of Gametocides
• The long and cumbersome process of
developing male-sterile lines and their
maintenance through B-lines can be over
come.
• The danger of narrow genetic base for
cytoplasmic male-sterility being experienced in
3 line hybrid breeding programme would cases
to be a problem.
• Any two parent which are heterotic in a
121
cross can be directly used for hybrid seed
production.
• Unstable but otherwise best combining
CMS lines can be made use of by ensuring
complete sterility through the use of
gametocides.
• Gametocides are very useful in cases
where induction of EGMS is incomplete due
to deviations in temperature and/or'
photoperiod as the sensitive phase of panicle
development for EGMS and gametocides.
• If synchronization of flowering or'
consecutive rainy days proves a limitation to
the use of gametocides, hybrids seeds may not
be produced. However, there will not be heavy
yield losses as the yield of female parent would
compensate it to a greater extent.
• It can be an effective substitute for hand
emasculation thereby reducing very
.significantly the time normally required. for
emasculation in greenhouse or field breeding
programs.
Disadvantages
• The gametocide spray should betaken up
at the most responsive stage during plant
growth for maximum sterility to be caused.
• Only certain specific developmental stages
are sensitive.
• The need for repeated application increase
the cost of hybrid seed.
• In cereals since there is difference in the
development of spikelets within and between
panicles, single treatment of gametocide even
at the most responsive stage would not ensure
sterility in flowers. In some crops the flowering
in tillers does not coincide with that of the main
stem flowering and such cases warrants
repeated spraying.
• High doses of these chemicals have been
found to be phytotoxic.
• In many cases it has been'found to cause
some amount of female sterile.
• The responses to chep'litals by the plants
have been found to be genotype dependent.
122
AGRICULTURAL REVIEWS
• This also is dependent on environment ~.g.
if heavy rainfall occurs immediately after the
spray, the gametocidal action would be
nullified.
• Carry over effect of some chemicals has
also been reported e.g. In triticale treated with
Ethrel, F1 seed germination reduced from 93
% in control to 73 % when the concentration
was 500 ppm. Higher concentration of 1000,
2000 and 4000 ppm further reduced seed
germination to 57,42 and 30% respectively
(Sapra et. al., 1971).
'. In cases where it causes incomplete
sterility, pollen shedders may result.
• Cost involved is more especially when
repeated spraying is done.
• In plants having c1eistogamous flower,
where selfing is the rule, gametocide usage in
limited only for academic purpose.
• These chemical are teratogenic and must
be handled with utmost care. Rajendra and
Bates (1981) based on his observation on the
effect of RH-531 and RH-532 on human
peripheral blood lymphocytes suggested that
these chemicals disturb human chromosomes
also.
Future Perspective
• Considering the relative advantageous and
disadvantageous effects of high concentrations
of gametocide on the one hand and differential
phase of development of spikelets within and
between panicles on the other, there is great
scope for gametocides possessing stable and
systemic action. The systemic nature would
facilitate slow and continuous release of the
gametocide at low concentration. As believed
by some workers, the granular from of Ethrel
might prove effective. Research efforts directed
towards the discovery and extensive use of such
chemical as well as detailed studies on the stage
specificity, environmental and genotype
influence on the efficiency of the gametocide
would enhance the prospects of hybrid seed
programme in self pollinated crops like wheat
and Rice.
• Many plants produce exudates which have
suppressing action on other plants. Such
chemicals can be tested for gametocide
potential thereby producing cheap alternative
for the growth regulation and also the risk of
their toxic effects on human beings can be
negated e.g. plants of Brassica sp. Produce
brasino steroids which have been identified as
having growth regulator potential. Similar other
substances can be studied for gametocidal
action.
• The concept of using chemicals to restore
fertility in genetic male sterile has been around
for about as long as there has been interested
in chemical hybridizating agents. The
advantages orchemical restoration of genetic
male sterility are at least three fold. Firstly the
chemical need not be 100% effective since its
use is only to increase supplies of female parent
seed. But if it is more effective more efficiently
seeds could be multiplied. Secondly, all seeds
produced is exactly what is wanted. It is a fail
safe method. Thirdly since the chemical dose
not have to be used in the hybrid seed
production, a lower volume is required
compared with the amount of gametocides
required. This in turn has favourable impact
on crop and environmental residues. Once an
adequate male sterile female parent seed
supply is available, hybrid seed production
using the current production techniques can
be followed.
• In crops that produce tillers, breeding work
should be carried out to produce lines in which
tillers do not deviate from main stem for days
to flowering characters.
CONCLUSION
Allusions have already been made to
the fact that Gametocide can be valuable
breeding tools. Perhaps their greatest
advantage in this context is the relative ease
with which kilogram quantities of hybrids seed
can be produced with chemical hybridizing
Vol. 26, No.2, 2005
agents techniques in contrast to the gram
quantities normally obtained by conventional
hand crossing producers. From the afore-stated
literature it is vivid that, in spite of its innate
advantages, research on development of
gametocides is very limited and also there is a
virtual vacuum for gametocidal research during
the past decade. However, in recent years,
many foreign firms have tested and developed
123
proprietary sterility inducing hybridizing agents
and have either not made them available (or)
made them available on a very restricted basis
to public and seed company breeders. Hence,
there is a paucity of published information on
the most recent chemicals. Research works
needs to be intensified in Research stations to
develop efficient gametocides and their
potential needs to be exploited.
REFERENCES
Colhoum, c.w. an~Steer, MW. (1982). Rev. Cyto/. Bio/. Veg., 5: 282-301.
Day, PR. et al. (1983). Crop Imp., 10: 217-225.
Devlin, R.N. (1981). Can. J. PI. Sci.,61: 465-470.
Dotlacil, A. and Apltauerova, C.E. (1980). Field Crop Res., 13: 68-75.
Eaton (1957). Science, 29: 1174-1175.
Frankel, R. eta/. (1969). Biochem. Genet., 3: 451-455.
Frankel, R. and Galum, E. (1977). Pollination Mechanisms, Reproduction and Plant Breeding. Springer-Verlog.
New York. pp: 66-69.
Gangarao, N.V.P.R. et a/. (1996). Seed Tech. News., 26(8): 1-2.
Guan, C.Y. etal. (1981). Hereditas, Chine. 3(5): 15-17.
Hansen, D.J. eta/. (1976). Crop Sci., 16: 371-374.
Hirase, BV and Fujime, M.J. (1973). Can. J. PI. Sci., 53: 460-464.
Huang, Q un-ee and Wang Li-Zhu (1990). lRRI News/., 15: 5, 6-7.
Izhar, S. and Frankel, R.(1971). Theoret. Appl. Genet., 41: 104-108.
Kumar, J. et a/. (1976). Crop Imp., 3: 106-114.
Ladyman et a/. (1990). Field Crop Res., 23: 177-196.
Libala, G.H. (1974). Can. J. Genet. Cytol., 16: 273-278.
Mahajan, V. et a/. (1998). lCAR News/., 4(4): 1-2.
Mc Rae, D.H. (1985). Plant Breeding Rev., 3: 169-191.
Miller, J.F. and Lucken, K.A. (1977). Euphytica, 28: 103-112.
Nelson and Rosman (1958). Science, 30: 42-68.
Rajendra, B.R. and Bates, L.S. (1981). J. Heredity, 72: 52-56.
Sapra, et a/. (1971). Crop Sci., 11: 108-116.
Sapra, et al. (1973). Crop Sci., 13: 273-280.
Sathyanarayana, KVV. et a/. (1995). Seed Tech. News/., 26(3): 7-8.
Sharma, J.R. (1994). Principles and Practices of P1a'nt Breeding. Tata McGraw-Hili Pub. Co. Lt., New Delhi.
Sharma, YP. (1979). Indian J. Agril. Sci., 49(4): 294-295.
Tue, Z.P. and Bang, SK (1998). In: Hybrid Cultivar Development. (Banga, 5.5. and Banga, S.K. Eds.). Narosa
Publ. House, New Delhi. pp: 160-180.
Wang, G.H. and Lund, H.Z. (1975). Crop Sci., 15: 321-325.
Warren and Dimmock (1985). Rev. Cytol. BioI. Veg., 8: 725-733.
Yih et a/. (1971). Sci. Sin.. 14: 227·236.
Zhao, F. et a/. (1993). Wheat Information Service. 77: 29-32.