Novel Approaches and Techniques of Cost-saving Potential in Crop Improvement
March 03, 2010; Accepted: May 08, 2010;
Published: July 10, 2010
Crop improvement is cost-intensive. It demands in ample measure inputs like money and manpower, land and labour and most important of all, time. Though apparently a non-monetary input, time contributes to cost in diverse ways. Breeding improved crop varieties through conventional approaches and techniques is slow. New approaches and novel techniques need be adopted to counteract constraints of cost and time (particularly, in resource-poor situations), which will also help to enhance efficiency in crop improvement. Rapid progress in cereal improvement in recent times through the ideotype approach and the use of in vitro culture of plant tissue for accelerating crop improvement in general illustrates this point. Even simple or small innovations can appreciably contribute to saving time and cost in breeding crops, particularly those of long duration. A few potentially useful approaches and techniques are briefly discussed below, basing on our work and experience in different crops. Hopefully these will contribute a cost-effectiveness in crop improvement.
THE USE OF MINIATURE PLANTS
Roots can be induced from isolated leaves of many plants using auxins. When
planted in pots these rooted leaves can be maintained for a long time as miniature
plants, which show certain aspects of growth and development like secondary
growth, lamina expansion (as in Basella), bud formation (as in Vitis),
tuberization (as in Sweet potato), nodulation (as in legumes) etc. (Samantarai
and Sinha, 1955). These phenomena differ depending on species. Intra-species
variation has also been observed.
As suggested long back by Sinha (1964), the miniature
plants can be put to multiple uses in potato breeding. A simple lab technique
was developed for screening potato varieties for nematode resistance using rooted
leaflets. Thus a lot of labour, time and field space could be saved (Nirula
and Pushkarnath, 1966). This simple technique is also useful in breeding
for disease resistance (Poehlman and Borthakur, 1969).
Through the use of miniature plants, it should be possible to undertake laboratory
evaluation of germplasm for a number of characters (e.g., resistance to different
pathogens and pests and physiological characters like abiotic stress tolerance,
photosynthetic efficiency, mineral stress response etc.) along with simultaneous
field evaluation for agronomic characters of breeding interest.
The miniature plant approach can be profitably used for other tuber crops, legumes (for evaluation of germplasm for nodulation and nitrogen-fixation capacity) and tree species. There is one limitation however, since all plants are not amenable to induction of rooting by auxins. To this recalcitrant group belong most monocots and a few dicots.
THE USE OF SHORT-TERM LEAF CULTURE AND DARK RESPONSE
The limitation due to lack of rooting response of most monocot leaves to auxins
can be at least partially circumvented through the use of short-term culture.
Infact, excised cereal leaves can be easily cultured for a very short period
of at least 72 h using simple culture media (contrasted with complex media for
the in vitro tissue culture). In this context, another well-known phenomenon
thought to be useful is dark-induced bleaching or etiolation. Light regulates
the expression of a number of chloroplast genes and also certain nuclear genes
associated with chloroplast development and function. Hence, absence of light
affects chloroplast development and causes etiolation. Grover
et al. (1986) examined changes in various components of photosynthetic
activity during the dark-induced senescence of detached wheat leaves. Saulescu
and Kronstad (1998) found large and visible genotypic differences in dark-induced
senescence in wheat by exposing seedlings of wheat cultivars at the 2-leaf stage
to continuous darkness. They also observed that chlorophyll content decreased
linearly with dark treatment duration and the regression slopes varied significantly
among varieties. Saulescu et al. (1998) studied
genotypic differences in dark-induced senescence among fourteen hybrids of maize
and reported that dark-induced senescence symptoms were correlated with field
stay green scores (the percentage of green plants at physiological maturity)
of maize hybrids.
The dark response approach can also be adopted for GP (germplasm) evaluation
for breeding cereals, using intact seedlings. Intra-species variation in etiolating,
whether in vitro or in vivo is an unexplored area. Saulescu
et al. (2001) conducted a study to investigate the feasibility of
a seedling test for estimating genotypic differences in senescence rate of wheat.
They studied the response of several wheat genotypes to dark exposure of seedlings,
by measuring the rate of chlorophyll loss in the first leaf. They found that
the rate of chlorophyll loss varied significantly among genotypes, the highest
rate being more than double of the lowest. Linear regression accounted for most
of the chlorophyll content variation during dark treatment in all cultivars.
They concluded that a seedling test for dark-induced senescence is a potential
tool in breeding wheat for optimum senescence pattern. For physiological characterization
of stay green mutants in durum wheat, Spano et al.
(2003) excised the fifth youngest fully expanded leaves after booting and
incubated the excised leaves in dark at 20°C for 6 days. They excised the
leaves of both parental and mutant plants. Leaves were sampled every 3 days
for chlorophyll content. They found that the chlorophyll content of mutant plants
were more than the parental plants (cv. Trinakaria), indicating that senescence
of detached youngest leaves in darkness was faster in parental line than in
the mutants. The chlorophyll contents, measured by using SPAD meter, of mature
flag leaves of control plants grown in the green house also decreased much earlier
in parental line than in mutants. The yield data indicated that mutant lines
had higher seed weights and grain yield per plant than the parental line. They
concluded that dark response might indirectly help in selection of high yielding
genotypes. Saulescu and Mustatea (2002) reported that
a good correlation existed between the rate of dark-induced chlorophyll loss
in wheat seedlings and the rate of age-induced senescence of flag leaves in
the field in absence of disease. Slow senescence seems to be a characteristic
of wheat cultivars adapted to more favourable environments, whereas fast senescence
can be found in cultivars that are more adapted to stress environments. The
seedling test for dark-induced senescence could facilitate selection for optimum
senescence pattern suitable for target environment.
||Genotypic differences in dark-induced senescence in rice (Oryza
The relevance of dark induced bleaching in breeding has been indicated in our
laboratory in rice and ragi (Fig. 1). Pattanaik
(1994) examined dark-response of nineteen short duration ragi varieties
and its bearing on adaptation. To study dark-response, fully expanded first
leaves were excised from 7-day old seedlings and incubated in dark for 48 h.
Senesced leaves were scored as: green (score 0), partially green or yellow (score
1) and fully yellow (score 2). Dark response of a variety was evaluated by an
etiolation index, derived by dividing the total score by twice the number of
leaves. On the basis of etiolation index (EI) value, the nineteen varieties
were classified into 3 dark-response classes - low, moderate and high. MLT yield
data were used to evaluate the adaptation pattern of varieties. It was observed
that varieties falling in moderate dark-response class showed most desirable
pattern of adaptation. He concluded that genotypic differences for dark-induced
senescence could be used to identify high-yielding short duration ragi varieties
with desirable type of adaptation.
Mohapatra (1997) studied dark-response of 14 medium and
9 late duration ragi varieties in both in vivo and in vitro conditions.
In in-vitro condition, fully expanded first leaves were excised from
7 days old seedlings and exposed to dark for a period of 48 h. In in vivo
condition, 7 day old seedlings were incubated in dark for 48 h and senescence
of fully opened first leaves were recorded. Dark-response of varieties were
determined by calculating etiolation index in both in vivo and in
vitro conditions. On the basis of in vivo EI and in vitro
EI values, the varieties were classified as Dark-S (Sensitive), Dark-MR (moderate
response) and Dark-R (resistant). In the medium duration group, the Dark-MR
and Dark-R classes contained all top yielders and most above-average yielders
(yield data from All India Coordinated Trial was used for field evaluation).
In the late duration group, Dark-S class contained more above-average yielders
and top yielders than Dark-R or Dark-MR class. He suggested that dark response
(Dark-induced senescence) could be used to identify high- yielding varieties.
Das et al. (2008a) studied dark response of thirty
six rice genotypes and found that high sensitivity to dark treatment in terms
of senescence (SI) index may indicate better adaptation to rich environments
and low sensitivity to dark treatment may indicate better adaptation to poorer
PREFERENTIAL INDUCTION OF MICRO-MUTATIONS (PIMM)
Gregory (1956) focused attention on the utility of
induced polygenic variation or micro-mutations involving quantitative characters
for improvement of yield. The conventional method adopted for this purpose comprises
elimination of macro-mutatants and selection of only normal looking plants and
evaluation of lines derived from them for yield and desirable agronomic traits.
Micro-mutation breeding thus entails large investment of many inputs and carries
a huge load of garbage to be shed gradually through field testing over generations.
Hence arises the need of preferential induction of mciro-mutations, i.e., without
In an attempt to economize and accelerate breeding of late duration winter
rices, Maleic Hydrazide (MH) was found useful as a mutagen through PIMM, which
helps to eliminate wastage in mutation breeding for yield. This novel technique
was developed by Mahapatra (1967) and used for rapid improvement
of a number of popular rice varieties. In each case, an appreciable number of
higher yielding micro-mutants (HYM) could be produced in a much shorter period
and with much less cost than otherwise possible by conventional method.
Most gratifying results were obtained with the popular winter rice, T90. Among
several HYMs, three out yielded the parent by more than 20% and one gave 29%
higher yield. This suggested the potential of PIMM, which may not be realized
in all cases, since the response to MH varies with the genotype (due to genetic
control of mutability). The degree of response to the mutagen MH can be ascertained
through preliminary seedling growth response. In fact, in one study T90 was
most sensitive (Das and Sinha, 1965). Das
and Sinha (1965) treated seeds of nineteen varieties of rice with 25 and
100 ppm aqueous solution of MH for 24 h and also seeds were soaked in water
to serve as control. Growth response of seedlings was studied at regular intervals
for five days. They found that in most varieties growth of shoots and roots
at any interval was retarded due to treatment with MH and the maximum was recorded
at 100 ppm after 5 days. Variation was found in varietal response to MH. In
case of variety T 90, root and shoot growth with 100 ppm MH after 5 days was
40 and 70%, respectively, of the control. The variety T 1242 appeared to be
quite tolerant to MH, in which 5 days root growth with 25 and 100 ppm MH were
108 and 96%, respectively, of the control and the corresponding shoot growth
were 109 and 125%. The differential response of varieties pointed to a genotypic
control of sensitivity to MH. They also found that MH-resistant varieties were
high yielder than sensitive varieties.
Dash (1988) studied the response of wheat varieties to
MH (250, 500 and 1000 ppm) in the laboratory. Observations were recorded on
shoot length and length of the longest root. Using numerical taxonomic method,
the varieties were classified as moderate MH-response and extreme- response
groups. To compare the results of laboratory study, a field trial was conducted.
It was observed that moderate-response types were moderately stress- tolerant
(heat/drought) in the field and extreme-response types were extreme in stress
response (being more tolerant or more sensitive). Das et
al. (2008b) tested MH sensitivity of 36 rice and 30 ragi genotypes in
terms of growth inhibition index to predict yield and adaptability at early
seedling stage. Laboratory result was compared with multilocation trial result
and it was suggested that low sensitivity to MH could be used as an indicator
of high yield potential.
PIMM can also be applied to high yielding varieties of rice and other self-pollinated
cereals like wheat as suggested by Sinha and Mohapatra (1994).
Its potential use for other crops is yet to be tested. Preferential attack of
MH on heterochromatin makes it possible for a cytological prediction of the
quantum of improvement through PIMM. The tomato genome with high heterochromatin
content might be most responsive to PIMM. The importance of cytology in this
context is emphasized.
NOVEL USE OF CHEMICALS
The antibiotic streptomycin: It is known since long that streptomycin
(SM) induces bleaching/albinism in barley (Von Euler, 1947).
It brings about ultra structural changes at the cellular level (Behn
and Arnold, 1974) and causes induction of cytoplasmic organellar mutation
including male-sterility (Burton and Hanna, 1982). It
also affects gene expression by virtue of its effect on the translation process
during protein synthesis. The diverse effects of this chemical has been thoroughly
reviewed by Kirk and Tilney Bassett (1978).
SM can induce mutations in plants. Maize plants treated with 0.1 to 0.2% SM
for 48 h produced multi-tillering mutant lines in maize (Choudhary
and Minocha, 1989). SM treatment in sorghum produced mutants with improved
capacity for phosphorous utilization (Spivakov and Klimashevski,
1989). Plants exposed to SM showed reduction in growth (Kirk
and Juniper, 1955; Gerhard, 1967; Mukherjee
and Bag, 1975; Nageswar Rao et al., 1980)
and chlorophyll synthesis (Khudairi, 1961; Babayan
et al., 1975; Mancinelli et al., 1975:
Pretova, 1980; Zubko and Dey, 1998).
SM is mostly used in somatic hybridization as a chloroplast marker for selection
of fusion products (Kinoshita and Reiko, 2001).
However, its possible use in crop improvement was realized much later when
evidence for genetic variation in SM-induced bleaching was observed in maize
in our laboratory (Sinha and Satpathi, 1977). Inbred
lines of maize showed variation in SM-response, which could be roughly measured,
using a Bleaching Index (BI) developed by us. Thus genotypes of maize and other
crops could be characterized for SM-sensitivity.
Of what use however is such characterization? The finding in maize gained further
significance, when rice varieties were characterized for bleaching response
(Sinha and Satpathi, 1977). It was of great interest
to note that semi-dwarf High Yielding Varieties (HYV) were highly SM-sensitive
and tall low yielding varieties (LYV) were resistant. This important finding
was confirmed, using a large number of varieties in repeated experiments (Satpathi,
1991). It was inferred that screening germplasm of rice for SM-response
may have some practical value.
There are two categories of characters for which germplasm characterization
and evaluation is rather deficient: (1) physiological characters and (2) cytoplasm
characters/organelle behaviour. A simple and rapid lab technique like bleaching
response to SM would hence be of some value. Satpathi (1991)
made a comparison of bleaching response of several crops (major cereals, millets,
pulses and oilseeds), using a few varieties for each crop. Certain findings
of these studies are worth mentioning (1) C4 species are generally
more SM-sensitive than C3 species (2) There are however, exceptions,
particularly in C3 crops, which is of some significance (for example.
SM-sensitive rice varieties may have high yield potential), (3) Drought tolerant
species and varieties are highly SM-resistant Horsegram which is highly drought
tolerant is almost SM-resistant.
The earlier finding as to HYV-LYV difference in bleaching response to SM (Sinha
and Satpathi, 1977) formed the basis of a laboratory approach to an indirect
(laboratory) evaluation of adaptation, in subsequent studies on different crops
e.g., rice, wheat, ragi and green gram. Rath (1977) examined
the possible relationship between SM-induced albinism in seedlings and high
yield in wheat varieties and found that high yielding varieties of wheat were
sensitive to SM.
Sinha and Swain (1978) used SM-induced bleaching as
a criterion of selection in seedling stage of EMS-derived populations of ragi
(Fig. 2). They found that the SM-sensitive lines were earlier
in maturity, shorter in height and higher yielding than SM-resistant lines.
Das and Sinha (1986) analysed SM-sensitivity of 22 mutant
lines of a finger millet strain (T25-1) at an early seedling stage.
To compare the results of laboratory study, a field trial was conducted. Correlation
analysis between SM-response and plant characters indicated that SM-sensitivity
in terms of bleaching index was positively correlated with maturity duration
and plant height. They suggested that selection for SM-sensitivity in seedling
stage might lead to selection of lines with shorter height, earlier maturity
and higher number of tillers per plant. Selection for SM-resistance may lead
to selection of late maturing lines with taller height and fewer tillers.
SM-response could be used as a criterion in preliminary laboratory evaluation
of broad adaptation pattern of new high-yielding rice varieties and for germplasm
screening for drought tolerance (Das and Sinha, 1992).
The high-yielding varieties of wheat having low sensitivity to SM in terms of
seedling growth were found to be of general adaptation (Das
and Sinha, 1995). Sinha et al. (1996) while
working in ragi suggested that it was possible to identify ragi varieties having
general adaptability by selecting the low SM-response class. In selecting greengram
genotypes for bleaching sensitivity and resistance to growth inhibition, the
selected group is likely to contain high frequency of superior genotypes with
desirable pattern of adaptation (Singh and Nanda, 1997).
Das (2001) found that the high yielding varieties of
rice having low sensitivity to SM in terms of bleaching response were drought
tolerant. Das (2008) evaluated SM sensitivity of 36 rice
and 30 ragi genotypes in terms of bleaching index (Fig. 2,
|| Genotypic differences in SM-induced bleaching effect of ragi
|| Genotypic differences in SM-induced bleaching effect of rice
To compare the results of laboratory study, multilocationa trials were conducted.
And it was suggested that high sensitivity to SM could be used as an indicator
of high yield potential.
Use of gibberellic acid (growth regulators): The use of plant growth
regulators as an aid to screen crop germplasm has been reported by many authors.
Sip et al. (1986) used Gibberellic Acid (GA)
to identify desirable genetic dwarfs in wheat. Obrein and
Pugsley (1981); Boyd and Wade (1989) suggested the
use of GA to screen wheat germplasm for yield potential. The relationship between
GA response and rust resistance in wheat reported by Syme
and Thompson (1986) would suggest the possible utility for screening for
GA response in indirect evaluation of certain types of disease resistance. As
an aid to rapid evaluation for dwarfism, seedling tests involving GA have been
developed in rice (Matsuanger et al., 1981).
Use of digitonin: Singh et al. (2008)
studied the digitonin sensitivity of high yielding mungbean varieties. Sensitivity
to this chemical was measured by using a suitable index based on visual rating
of abnormal hypocotyls growth. They found that high sensitivity to digitonin
could be used as an indicator of high yielding ability and adaptation to favourable
Of the different chemicals suggested here for chemo-response studies one or more could be used. Being well tested in different crops, SM is a chemical of choice, which may be supplemented with one or more of the remaining for better productivity. These studies showed that the simple lab technique (seedling response to chemicals) helps in preliminary selection of (1) varieties with high yield potential and (2) stable - yielding varieties with a greater probability. No other techniques is available today to be used as a simple, low-cost alternative to the cost-intensive field evaluation through multilocation/ environment testing.
NOVEL USES OF NUMERICAL TAXONOMY (NT)
Orthodox taxonomy or classification as is usually practiced basing on a few
qualitative characters is of little application in crop improvement except for
correct identification of plants. Numerical Taxonomy (Taxometry), which adopts
a mathematical approach to classification, can make use of any number of quantitative
characters besides qualitative characters. Its applicability in crop improvement,
which deals with both kinds of characters, is evident. One use that has been
suggested (Sneath and Sokal, 1973) and often adopted is
selection of parents for hybridization in breeding for higher yield.
We have conceived and shown much wider application of NT in problems of crop
improvement e.g., evaluation of adaptation (Das and Sinha,
1992, 1995), mutation breeding (classification of micro-mutants and mutagens;
Singh and Sinha, 2010; Muduli and
Misra, 2008), classification of environments (trial sites/environment in
multilocation testing, crop zonation etc., Karamura, 1998),
maintenance of genetic purity, cytogenetic studies related to breeding (Badr
and Elkington, 1978; Szlachciak and Boron, 2003;
Jackson and Crovello, 1971), evaluation of inbreds for
heterosis breeding (Qi et al., 2010; Koutsos
et al., 2000; Fan et al., 2009) use
in resistance breeding (Kim et al., 2004; Karuri
et al., 2009), variety identification (Portis
et al., 2004; Saric-Kundalic et al., 2009;
Shuaib et al., 2007) and other possible uses
(in germplasm collection and utilization, growth pattern analysis to detect
sub-types within plant types etc).
NT may be especially useful in management of massive data as in the case of
All India Coordinated Trial (AICT) for different crops. We strongly feel more
information, relevant to breeding could be extracted from these data sets than
is done at present. NT can thus help in effective utilization of data, assiduously
gathered with heavy investment. The significance of the AICT data stems from
one important aspect of NT, which is the principle that the greater the
information content in the taxa of a classification and the more characters
on which it is based, the better the classification will be (Sneath
and Sokal, 1973). The term better means greater productivity,
which should be a goal in plant breeding research. The goal, however, is difficult
to achieve, in any case, AICTs do provide a large number of characters to work
towards a predictive (more useful) classification of the taxa (varieties/locations
or sites of testing etc.).
The role of molecular markers is a novel and recent approach in crop improvement.
Among the various markers, RAPD (Random amplified polymorphic DNA) is methodologically
simple and less expensive. Therefore, possible use of RAPD markers was tried
in our lab for preliminary evaluation of yield and adaptability. Das
et al. (2008c) studied the relationship of RAPD banding pattern with
yield potential and adaptability of rice genotypes. The experimental results
revealed that absence of polymorphic band ~400 bp in OPD-08 gave indication
about high yield potential; presence of polymorphic band ~750 bp in OPD-05,
~400 bp in OPD-08 and ~2000 bp in OPD-18 gave indication about genotypic adaptation
to poorer environments and absence of these markers gave indication about genotypic
adaptation to rich environments. The experimental investigation of Das
et al. (2009) revealed that the presence of OPA4~1800 bp and absence
of OPN15~900 bp bands in finger millet may give some indication about specific
adaptability of genotypes to poor environments. Absence of OPA4~1800 bp and
presence of OPN15~900 bp in finger millet would indicate specific genotypic
adaptation to rich environments. Finally they concluded that these studies may
help to predict the genotypic adaptation of different genotypes before going
for multilocation trial.
Epilogue: It is not possible to present all potentially useful approaches/techniques, except for a few important observations. We conclude with our conviction that integration of the novel and the conventional approaches/techniques can contribute to cost-effectiveness in crop improvement. This economic consideration should also be the concern of plant breeders and germplasm workers.
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