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Developing salt-tolerant Mustard Hybrid: Challenges and Opportunities

Jogendra Singh and Vijayata Singh

Central Soil Salinity Research Institute, Karnal (Haryana) -132001, India

Soil salinity, one of the major abiotic stresses reducing agricultural productivity,

affects large terrestrial areas of the world; the need to produce salt-tolerant crops is evident.

In India, nearly 6.73 million hectare area is affected with salinity and sodicity stresses

covering various states of the country. Further, the arid and semiarid areas in different states

are associated with saline underground water, which have to be used for irrigation purpose.

India ranks second in the world with regard to production ofBrassicas and supplies nearly 7

percent of the worlds edible oil. However, Indian mustard production still remains

insufficient to meet even the daily requirement of its people let alone fulfilling prospects of

fruitful export. This low economic yield can be attributed to the crops susceptibility to anumber of abiotic and biotic stresses, among which of alarming concern, is the salt stress.

High salinity adversely affects germination, growth, physiology and productivity by

causing ionic and osmotic stresses as well as oxidative damage. Moreover, salt stress has also

been found responsible for an increased respiration rate, ion toxicity, changes in C and N

metabolism, mineral distribution, membrane instability and permeability, decreased

biosynthesis of chlorophyll and inefficiency of photosynthesis all of which ultimately leading

to lowered economic productivity. Salt stress effectively decreases the availability of water in

the soil to plants, and hence there is a substantial overlap between plant responses to drought

and to salinity. Generally, varieties developed to be more tolerant to drought and that usewater more efficiently, will also be more resilient to salt stress. However, in addition to

affecting the water balance of the plant, salt poses another problem to plants: excess

accumulation of salt ions in cells is toxic, and potentially fatal. Salt ions impair enzyme

function, inhibit protein synthesis, affect the structure and permeability of cell membranes,

inhibit photosynthesis, and lead to the production of toxic reactive oxygen species.

Two main approaches are being used to improve salt tolerance: (i) the exploitation of

natural genetic variations, either through direct selection in stressful environments or through

mapping quantitative trait loci and subsequent marker-assisted selection; and (ii) the

generation of transgenic plants to introduce novel genes or to alter expression levels of theexisting genes to affect the degree of salt stress tolerance. The achievements made are not

enough. Low yield levels, instability in productivity and production, low oil recovery,

pesticide residues, likely introduction of new pests and diseases alongwith the seed imported

under the new seed policy, international competition, etc., are the challenges ahead to be

looked into and more emphasis should be given to develop hybrid which is the goal for

breaking yield plateau of mustard.

Hybrid mustard breeding along with salt tolerance should provide farmers with an

opportunity to improve productivity, particularly in potential high-yield areas and where

conventional breeding has apparently reached a yield plateau (Dhillon et al. 1996). Reports

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on availability of heterosis in rapeseed-mustard in early forties (Sun 1943), raised interest

amongst plant breeders to develop the means for harnessing hybrid vigour.

Here, we discuss the challenges and opportunities provided by recently developed

functional tools for the development of salt-tolerant Mustard Hybrid.

Outline of the problems that hindered the success and need to rectify

Limited choice of parental lines

In CMS system, the problems with female parents are by their maintenance behaviour

and combining ability and in case of pollen parent by restoration ability; these limit the

choice of parents. Joint study of the restore lines showed that only 15-24% was effective

restorer and bulk of materials is partial restorer. The choice of parental lines can be greatly

broaden by using back cross method, pre-breeding or Chemical Hybridizing Agents (CHAs).

Diversification of cytoplasmic male sterility systems has been accomplished through somatic

hybridization (Kirti et al. 1995, Armugam et al. 1996, Gaikwad et al. 1996).Low magnitude of heterosis:

As compare to cross pollination crops, the magnitude of heterosis in self pollinated

crops is much lower. To make hybrid technology commercially viable, it is better to

channelize more diverse germplasm i.e. development of interspecific using in vitro

techniques and wide compatibility genes. Development of synthetic lines which gave >50%

heterosis in mustard may be used in hybrid development.

Need to develop heterotic pool

The development and improvement of maintainer and restorer lines by variousbreeding efforts or by growing F1s and identify superior cross for specific traits and harvested

in bulk, this yields a diverse heterotic pool.

Recycling maintainer and restorer lines

When these lines are become susceptible to any calamities, the back cross and

pedigree method should be required for recycling. This is a long tern project so used other

methods like Genetic Male sterility System and Chemical Hybridizing Agents (CHAs).

Opportunities available for Mustard hybrid development:

Following methods are available for hybrid mustard development:

Cytoplasmic male sterility system

Introgression of cytoplasmic male sterility developed in radish (Raphanus sativus L.)

by Ogura (1968) to Brassica juncea (Banga and Labana 1984) raised hopes to develop

hybrids in rapeseed-mustard. Systematic and coordinated efforts for developing hybrids in

rapeseed-mustard started in 1989 under the ICAR sponsored project on promotion of research

and development efforts on hybrids in crops with 2 CMS systems in B. juncea, i.e. ogu and

tour, and one in B. napus, i.e. polima. Major emphasis was given to the simultaneous

development of CMS-fertility restorer systems. The approach led to the development of 7

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new CMS systems, viz siifolia, oxyrrhina, muralis, catholica, nigra, moricandia and

trachystoma till 1995.

The CMS systems in brassica had several limitations. Moreover, full fertility

restoration could not be identified for any of the systems. Due to the failure of conventional

breeding methods in identifying the restorer gene (s) in B. juizcea, it was felt to find outrestorer gene (s) in the same species from which cytoplasm had been introgressed. Concerted

efforts at the National Research Centre for Plant Biotechnology, New Delhi, resulted in the

development of fertility restorer formori and trachy CMS systems. Mori CMS and fertility

restorer were found to be associated with severe chlorosis and retarded growth. Chlorosis

associated with mori CMS systems was rectified and has been diversified into the improved

background of identified heterotic combination.

In B. napus, CMS development programme has been in progress with 3 CMS

systems, viz polinza, tournejbrrii and lyratus. The three-line technology has already been

perfected with release of first commercial hybrid 'PGSH 51' based on tour- CMS system bythe PAU, Ludhlana. Anorher hybrid developed by Advanta Limited. Canada, 'Hyoia PAC

401 ' has also been released in India. During 1999-2004, the hybrid development programme

concentrated on evaluation, refinement and diversification of mori CMS-fertility restorer

system in B. juncea and identification or introgression of marker gene as well as

diversification of GMS system inB.rapa var. yellow sarson.

Fig. The three-line technology for commercial hybrid mustard development

CHAs (Chemical hybridizing agents)

Since late 1970s, efforts have been made to use chemical hybridization agents or male

gametocides for selective abolition of male sex. These chemicals selectively sterilized the

stamens without much affecting the normal function of female system of plant when applied

rfrfS

rfrfF

Male sterile

A-line (Female)

Maintainer

B-line (Male fertile)

rfrfS

RfRf F /

S

Restorer

R-line (Male Parent)

RfrfS

F1 Hybrid (fertile)

x

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at specific stage of growth and development and thus can be used to produce female parents

for Mustard hybrid production. Very few researchers have evaluated the use of male

gametocides in brassica (Chopra et al. 1960,Banga and Labana 1983,Banga et al. 1986). It

was not found promising. Thus the male gametocides do not seem to have immediate

practical utility. The major problems associated with these are:(i) Reduced female fertility: need extensive testing before utilization.(ii) Doses and durability: required extensive testing before utilization.(iii) Development stage specificity: CHAs are generally applied at PMC formation or

pre-booting stage thats why they require critical observation.

(iv) Chronic spray required: to signify the results of CHAs, spray should be repeated.(v) Phytotoxicity: caused epinasty, hybrid necrosis and lipid deterioration.Although, CHAs have some advantages like -

There is no need of maintenance of male sterile and restorer parent. Sufficient large amount of seed can be produced. Save the time needed for transferring male sterility to agronomically useful variety

through back crossing.

Conclusion and future prospects

The use of salt tolerant hybrids brassica is a strategy to lift the yield ceiling of brassica

to help the Indias meet the future projected demand, which will increase due to increasing

populations and rising incomes. Though, we have very much succeeded in developing and

releasing the first commercial hybrid cultivar in B. Juncea (NRC HB 506), for normal land

which has very large acreage and could make much of dent on oilseeds production in the

country, is not yet in sight. So, the work on this area of research has to be accelerated. More

basic work is needed on the male sterility, fertility restoration, heterosis, combining ability

and hybrid seed production techniques.

Hybrids of mustard with tolerance to high salt along with consumers acceptance and

good oil quality are required for obtaining optimum yield and expansion of cultivated area

under such stress situation. The adoption of Salt tolerant Indian mustard hybrids will

ultimately depend on (1) the magnitude of the yield advantage obtained, (2) the most/benefit

ratio of using hybrid versus pure line seed, and (3) the efficiency of seed production,

certification and distribution agencies available in the country. Progress during the next

decade will determine how much hybrids in this major oilseed crop can help to increase India

oilseed production.

References

Arumugam, N., Mukhopadhyay, A., Gupta, V., Pental D. and Pradhan, A.K. 1996. Synthesis

of hexaploid (AABBCC) somatic hybrid :. a bridging material for transfer of tour

cytoplasmic male sterility to different Brassica species. Theoretical and Applied

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Banga, S.S. and Labana, K.S. 1983. Production of F1 hybrids using ethrel-induced male

sterility in Indian mustard.Journal of Agricultural Science Cambridge. 101: 453-455.

Banga, S.S. and Labana, K.S. 1984. Heterosis in Indian mustard. Zeitscriftfiir

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Banga, S.S., Labana, K.S., Banga, S.K. and Singh, B. 1986. Experimental evaluation of male

gametocides in Indian mustard. SABRAO Journal. 18: 31-35.

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Gaikwad, K., Ambika, B., Vageeshbabu, H.S., Kirti, P.B., Prakash, S. and Chopra, V.L.1996. Somatic hybridization in Brassicaceeae : cytoplasmic genome organization in the

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