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| Drought stress in vegetable crops | | Division of Vegetable Science and Floriculture, SKUAST-Jammu | |
Ravneet Kour, R.K. Samnotra and Sandeep Chopra
Drought is actually
a meteorological
event which implies the absence of rainfall for a period of time, long enough to cause moisture-depletion in soil and water deficit with a decrease of water potential in plant tissues. But from agricultural point of view, its working definition would be the inadequacy of water availability, including precipitation and soil-moisture storage capacity, in quantity and distribution during the life cycle of a crop plant, which restricts the expression of full genetic potential of the plant (Sinha 1986) . It acts as a serious limiting factor in agricultural production by preventing a crop from reaching the genetically determined theoretical maximum yield. The effect of drought on crop production and overall economy is well known. Most of the crops are sensitive to water deficits, particularly during flowering to seed development stage.
In agriculture, drought resistance refers to the ability of a crop plant to produce its economic product with minimum loss in a water-deficit environment relative to the water-constraint-free management. The ability of a plant to produce its economic product with minimum loss under water deficit environment in relation to the water-constraint-free management is referred as drought tolerance (Mitra, 2001). In other words, drought can be described as a climatic hazard which implies the absence or very low level of rainfall for a period of time, long enough to cause moisture depletion in soil with a decline of water potential in plant tissues. Drought is often accompanied by relatively high temperatures, which promote evapotranspiration and affects photosynthetic kinetics, thus intensifying the effects of drought and further reducing crop yields (Mir et al., 2012). Drought stress is the major abiotic stress for many Indian states viz. Rajasthan, parts of Gujarat, Haryana and Andhra Pradesh (Mitra, 2001). About two thirds of the geographic area of India receives low rainfall (less than 1000 mm), which is also characterized by uneven and erratic distributions. Out of net sown area of 140 million hectares about 68 % is reported to be vulnerable to drought conditions and about 50 % of such vulnerable area is classified as 'severe', where frequency of drought is almost regular (http://www.dsc.nrsc.gov.in/). Being succulent in nature, most of the vegetable crops are sensitive to drought stress, particularly during flowering to seed development stage. Moreover, the legume vegetables, for instance cowpea, vegetable pea, Indian beans etc., grown in arid and semi-arid regions are generally affected by drought at the reproductive stage. Drought stress modifies photosynthetic rate, relative water content, leaf water potential, and stomatal conductance. Ultimately, it destabilizes the membrane structure and permeability, protein structure and function, leading to cell death (Bhardwaj and Yadav, 2012).
Several physiological and biochemical processes essential for plant growth and development are significantly affected by drought stress, and plant develops various defense mechanisms against moisture stress at the molecular, cellular and whole plant levels. An understanding of genetic basis of drought tolerance in vegetables is a pre-requisite for plant breeders to evolve superior genotype by adopting conventional breeding methodology. In view of the fact that there is no single mechanism by which stress can be alleviated.
MECHANISM OF DROUGHT ; In genetic sense, the mechanisms of drought resistance can be grouped into three categories, viz. drought escape, drought avoidance and drought tolerance. However,crop plants use more than one mechanism at a time to resist drought.
DROUGHT ESCAPE; Drought escape is defined as the ability of a plant to complete its life cycle before serious soil and plant water deficits develop. This mechanism involves rapid phenological development (early flowering and early maturity), developmental plasticity (variation in duration of growth period depending on the extent of water-deficit) and remobilization of preanthesis assimilates to grain.
DROUGHT AVOIDANCE; Drought avoidance is the ability of plants to maintain relatively high tissue water potential despite a shortage of soil-moisture, whereas drought tolerance is the ability to withstand water-deficit with low tissue water potential. Mechanisms for improving water uptake, storing in plant cell and reducing water loss confer drought avoidance. The responses of plants to tissue water-deficit determine their level of drought tolerance. Drought avoidance is performed by maintenance of turgor through increased rooting depth, efficient root system and increased hydraulic conductance and by reduction of water loss through reduced epidermal (stomatal and lenticular) conductance, reduced absorption of radiation by leaf rolling or folding and reduced evaporation surface (leaf area). Plants under drought condition survive by doing a balancing act between maintenance of turgor and reduction of water loss.
DROUGHT TOLERANCE ; The mechanisms of drought tolerance are maintenance of turgor through osmotic adjustment (a process which induces solute accumulation in cell), increase in elasticity in cell and decrease in cell size and desiccation tolerance by protoplasmic resistance.
GENETICS
Drought resistance is a complex trait, expression of which depends on action and interaction of different morphological (earliness, reduced leaf area, leaf rolling, wax content, efficient rooting system, awn, stability in yield and reduced tillering), physiological (reduced transpiration, high water-use efficiency, stomatal closure and osmotic adjustment) and biochemical (accumulation of proline, polyamine, trehalose, etc., increased nitrate reductase activity and increased storage of carbohydrate) characters. Very little is known about the genetic mechanisms that condition these characters.The identification of genes responsible for morphological and physiological traits and their location on chromosome have not been possible, but their inheritance pattern and nature of gene action have been reported.
ADAPTATION RESPONSES TO WATER DEFICIT; They can be divided into following heads;
1) Physiological adaptations to water deficit
2) Biochemical responses
3) Responses at molecular level
PHYSIOLOGICAL ADAPTATIONS TO WATER DEFICIT
The most striking adaptations are accumulation of water to delay or escape water deficit. Other diminish their metabolic activities which are resumed once the water potential increases. Some are able to maintain their biological functions at low water potential though with little development. Other strategies include minimizing water loss by ABA mediated stomatal closure. Stomata are highly specialized cells invovled in gas exchange. .Similar strategies are rolling of leaves,floral abscission and alteration in cuticular permeability.
BIOCHEMICAL RESPONSES; The common adaptation is osmotic adjustment which is result of newly synthesized metabolites.These are hydrophilic molecules having solvation surface that capture water molecules to be later available during water limitation.Osmolytes,besides turgidity maintenance, have additional functional to reduce oxidative stress by quelling ROS species. The various metabolites are as follows;
RESPONSES AT MOLECULAR LEVEL: When drought stress is perceived by the plant, changes in the expression level have been monitored ,ranging from genes whose products are involved in early response such as signal transduction, transcription and translational factors; to late response genes such as water transport, osmotic balance, oxidative stress and damage repair. Adaptive responses are observed as a consequence of such changes
BREEDING METHODS FOR DROUGHT TOLERANCE
CONVENTIONAL METHODS;
1)Introduction
2)Selection
3)Hybridisation
NON-CONVENTIONAL METHODS;
1)Genetic engineering, MAS, QTL's, Molecular cytogenetics
METHODOLOGY OF BREEDING;
The yield potential of an already resistant material may be a more promising approach, provided there is genetic variation within such a material .
Pedigree and bulk method could be used for self-pollinated crops .
Recurrent selection for cross-pollinated crops.
For transferring few traits relating to drought resistance to a high-yielding genotype, then back cross is the appropriate methodology. On the other hand, biparental mating (half sib and full sib) maintains the broad genetic base as well as provides the scope to evolve the desired genotype of drought resistance.
FUTURE STRATEGIES; The future research programmes for drought resistance should consider the following strategies;
Exploration of the plant genetic resources and their transfer of desired traits through conventional plant breeding or biotechonological methods.
Breeding programme for pyramiding a number of relevant traits in a crop
Many different genes responsible for biosynthesis of different solutes and osmolytes conferring drought resistance should be considered for transfer in a crop plant at a time.
Attention should be concentrated on better understanding of genetic basis of drought resistance
A comparative assessment of various polypeptides produced in response to drought, between sensitive and tolerant genotypes may be used in identification of protein marker, which could help in producing transgenic drought resistant plants.
A multidisciplinary approach involving genetics, biochemistry, biotechnology, physiology, plant breeding and crop science will be appropriate to evolve superior drought resistant genotypes.
CONSTRAINTS; The lack of success probably results from a combination of following factors:
Lack of efforts through multidisciplinary approach
Lack of repeatable and precise screening techniques.
Knowledge is incomplete about reliable attributes as indices of drought resistance and selection criteria
Several adaptations have a negative effect on crop productivity. These traits are not useful in breeding drought resistance.
Drought reduces nutrient uptake and is associated with temperature stress and at higher elevation with cold. This association makes the breeding programme more complicated. Since dry matter production is strongly associated with total transpiration, any reduction in transpiration results in reduced crop growth rate.
Limitation in application of genetic engineering in this aspect is lack of information on availability of the most appropriate gene. |
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