RESISTANCE BREEDING TO DROUGHT

ABIOTIC STRESS AND BREEDING FOR DROUGHT RESISTANCE

Introduction:

                 Stress is an altered physiological condition caused by factors that tend to disrupt the equilibrium. Strain is any physical and chemical change produced by a stress (Gaspar et al., 2002). The term stress is used with various meanings, the physiological definition and appropriate term as responses in different situations. The flexibility of normal metabolism allows the response initiation to the environmental changes, which fluctuate regularly and are predictable over daily and seasonal cycles. Thus every deviation of a factor from its optimum does not necessarily result in stress. Stress being a constraint or highly unpredictable fluctuations imposed on regular metabolic patterns cause injury, disease or aberrant physiology. Plants are frequently exposed to many stresses such as drought, low temperature, salt, flooding, heat, oxidative stress and heavy metal toxicity, while growing in nature.

                                      Drought is a meteorological term and is commonly defined as a period without significant rainfall. Generally drought stress occurs when the available water in the soil is reduced and atmospheric conditions cause continuous loss of water by transpiration or evaporation. Drought stress tolerance is seen in almost all plants but its extent varies from species to species and even within species. Water deficit and salt stresses are global issues to ensure survival of agricultural crops and sustainable food production (Jaleel et al., 2007b-e; Nakayama et al., 2007). Conventional plant breeding attempts have changed over to use physiological selection criteria since they are time consuming and rely on present genetic variability (Zhu, 2002). Tolerance to abiotic stresses is very complex, due to the intricate of interactions between stress factors and various molecular, biochemical and physiological phenomena affecting plant growth and development (Razmjoo et al., 2008). High yield potential under drought stress is the target of crop breeding. In many cases, high yield potential can contribute to yield in moderate stress environment. Drought stress is considered to be a moderate loss of water, which leads to stomata closure and limitation of gas exchange. Desiccation is much more extensive loss of water, which can potentially lead to gross disruption of metabolism and cell structure and eventually to the cessation of enzyme catalyzed reactions (Smirnoff, 1993; Jaleel et al., 2007d).

                                      Drought stress is characterized by reduction of water content, diminished leaf water potential and turgor loss, closure of stomata and decrease in cell enlargement and growth. Severe water stress may result in the arrest of photosynthesis, disturbance of metabolism and finally the death of plant (Jaleel et al., 2008). Water stress inhibits cell enlargement more than cell division. It reduces plant growth by affecting various physiological and biochemical processes, such as photosynthesis, respiration, translocation, ion uptake, carbohydrates, nutrient metabolism and growth promoters (Jaleel et al., 2008). In plants, better understanding of the morpho-anatomical and physiological basis of changes in water stress resistance could be used to select or create new varieties of crops to obtain a better productivity under water stress conditions (Nam et al., 2001; Martinez et al., 2007).

              

         The reactions of plants to water stress differ significantly at various organizational levels depending upon intensity and duration of stress as well as plant species and its stage of growth. Understanding plant responses to drought is of great importance and also a fundamental part for making the crops stress tolerant.

Breeding for drought Resistance:

                Drought seems to be rather difficult to define more difficult to quantify. For example , the common criteria used in the various definitions are precipitation, air temperature, relative humidity, evaporation from free water surface, transpirations from plant , wind, air flow , soil moisture and plants conditions. A working definition of drought may be “the inadequacy of water availability, including precipitation and soil moisture storage capacity, in the quantity and distribution during the life cycle of a crop to resist expression of its full genetic yield potential”(Sinha, 1986). Therefore, under conditions of drought , water stress develops in the plants as the demand exceeds supply of water; this may occur due to atmospheric or soil  conditions, and is reflected in a gradient of water potentials developed between the soil/soil-root interference and leaf , the transpiring organ. Thus moisture stress may be defined as the inability of plant to meet the evapotranspirational demand.

                      Moisture stress is likely to develop to a different rate in different plant organs along this gradient (Blum, 1988).Phenotype is the result of genotype and environmental interaction. Therefore, assessment of desired genotypes is highly dependent on proper environmental conditions. Abiotic stresses (particularly drought, high temperature, salinity and others) generally reduce crop productivity. These stresses are location-specific, exhibiting variation in frequency, intensity and duration. Stresses can occur at any stage of plant growth and development, thus illustrating the dynamic nature of crop plants and their productivity.

                                          Drought is the primary abiotic stress causing not only differences between the mean yield and potential yield but also causing variation from year to year, resulting in yield instability. Although selection for genotypes with increased productivity in drought-prone environments has been an important aspect of many plant breeding programs, the biological basis for drought tolerance is still poorly understood. Also, drought stress is highly heterogeneous in time, space, degree of stress, growth stage and time of stress exposure and is unpredictable. Due to their secondary mode of life, plants resort to many adaptive strategies in response to different abiotic stresses such as high salt, dehydration, cold and heat, which ultimately affect the plant growth and productivity (Gill et al., 2003).

                                   Against these stresses, plants adapt themselves by different mechanisms including change in morphological and developmental pattern as well as physiological and biochemical responses (Bohnert et al., 1995). Drought tolerance comprises drought escape (the ability of a plant to escape periods of drought, especially during the most sensitive periods of its development), drought avoidance (the ability of a plant to withstand a dry period by maintaining a favorable internal water balance under drought) and drought tolerance mechanisms (the ability of a plant to recover from a dry period by producing new leaves from buds that were able to survive the dry spell (Blum, 1988).

Effect of drought on plant growth and development:

                 Water stress has marked effect on cellular processes, plant growth, development and economic yield. Water stress is usually measured as leaf –water potential since leaves are directly involved with the production of assimilates for growth and yield. As water potential declines, pressure or torgor potential also declines; the decline in the turgor potential is much more when there is no osmoregulation in response to water stress. Osmoregulation or osmotic adjustment refers to the active accumulation of solutes in the cell during the period in which water stress develops. The solutes accumulated by different plants are considerably different: they ranges from photosynthetic products like sugar,fructans etc through important . At the cellular level, it affects the flowering structures processes:

1.      Structures and membranes and organelles.

2.      Hydration and structures of macro molecules like proteins and nucleic acids.

3.      Pressure differential across the membranes-cell wall complex, which in turns affect cell expansion.

Stomatal and non-stomatal limitation on photosynthesis of droughted plants:

                     The rate of CO2 assimilation in the leaves is depressed at moderate leaf water deficits or even before leaf water status is changed in response to a drop in air humidity or in soil water potential. The relative part of stomatal limitation of photosynthesis depends on the severity of water deficit. Under mild stress it is a primary event, which is then followed by adequate changes of photosynthetic reactions (Cornic and Briantais, 1991). Stomatal control of water loss has been identified as an early event in plant response to WD under field conditions leading to limitation of carbon uptake by the leaves (Chaves, 1991; Cornic and Massacci, 1996). Stomata close in response either to a decline in leaf turgor and/or water potential, or to a low-humidity atmosphere (Maroco et al., 1997). As a rule, stomatal responses are more closely linked to soil moisture content than to leaf water status. This suggests that stomata are responding to chemical signals (e.g. ABA) produced by dehydrating roots.

                      A clear time dependency in stomatal responsiveness to air humidity and water status was also found, suggesting that some of diurnal changes in stomatal function may result from metabolic processes with a circadian rhythm (Chaves et al., 2002). Changes in cell carbon metabolism are also likely to occur early in the dehydration process as shown by Lawlor (2002). The drought-tolerant species control stomatal function to allow some carbon fixation at stress, thus improving water use efficiency, or open stomata rapidly when water deficit is relieved. Although stomatal closure generally occurs when plants are exposed to drought, in some cases (severe stress) photosynthesis may be more controlled by the chloroplast’s capacity to fix CO2 than by the increased diffusive resistance. Non-stomatal  responses of carbon fixation such as PS2 energy conversion and the dark reaction of Rubisco carbon fixation are resistant to WD (Chaves, 1991). In addition, stomatal closure occurs before inhibition of photosynthesis and restricts CO2 availability at the assimilation site in chloroplasts.

                                          When WS is imposed slowly as is generally the case under field conditions a reduction in the biochemical capacity for carbon (C) assimilation and utilization may occur along with restriction in gaseous diffusion. For example, in grapevines grown in the field, CO2 assimilation was limited to a great extent due to stomatal closure as summer drought progress, but there was also proportional reduction in the activity of various enzymes of the reductive Calvin cycle.The tight correlation between mesophyll photosynthesis and stomatal aperture may reflect a down-regulation of photosynthetic apparatus by low C availability. According to Ort et al. (1994) the response of photo- Plant responses to drought and stress tolerance190 synthesis to internal cell CO2 (Ci) indicates that the biochemical demand for CO2 was downregulated in response to declining CO2 availability.

Drought stress and PS2 activity:

                 Photosynthetic carbon reduction and carbon oxidation cycles are the main electron sink for PS2 activity during mild drought. It was shown that PS2 functioning and its regulation were not quantitatively changed during desiccation. The CO2 molar fraction in the chloroplasts declines as stomata close in drying leaves. As a consequence, in C3 plants RuBP oxygenation increases and becomes the main sink for photosynthetic electrons. Depending on the prevailing  photon flux density (PFD), the O2 through photorespiratory activity can entirely replace CO2 as an electron acceptor or not. Havaux (1992) has investigated the impact of various environmental stresses (drought, heat, strong light) applied separately or in combination on the PS2 activity.The existence of a marked antagonism between physicochemical stresses (e.g. between water deficit and HT) was established, with a water deficit enhancing the resistance of PS2 to constraints as heat, strong light .Similar results were obtained on bean plants (Yordanov et al., 1999). The data show that quantum yield of PS2, as related to Calvin cycle metabolism, is reduced only under drastic water deficit.

                      Long-term drought reduction in water content led to considerable depletion of pea PS2 core. The remaining PS2 complex appeared to be functional and reorganized with a unit size (LHCP/PS2 core) twofold greater than that of well irrigated plants, and enhanced degradation of CP43 and Dl proteins (Girardi et al., 1996). In addition,PS2 complexes are able to change their location and structure as in PS2-β centers and state-transitions. The decline in PS2 efficiency is regulatory, serving a photoprotective role. Increased levels of energy dissipation which decrease ΨPS2 may help to protect PS2 from over-excitation and photo damage studied the metabolic consumption of photosynthetic electrons and dissipation of excess light energy in tomato plants under WS. They established that O2 evolution, O2 uptake, net CO2 uptake and CO2 evolution declined. It was concluded that PS2, the Calvin cycle and mitochondrial respiration are down regulated under WS. The same authors calculated the percentages of photosynthetic electrons dissipated by CO2 assimilation, photorespiration and the Mehler reaction in control leaves more than 50% of the electrons were consumed in CO2 assimilation, 23% in photorespiration and 13% in Mehler reaction. Under severe stress the % of electrons dissipated by CO2 assimilation and the Mehler reaction declined while the % of electrons used in photorespiration doubled. The consumption of electrons in photorespiration may reduce the likelihood of damage during WS.                                Noctor et al. (2002) provided quantitative estimation of the relative contributions of the chloroplast electron transport chain and the glycolate oxidase load placed on the photosynthetic leaf cell.

                Assuming a 10% allocation of photosynthetic electron flow to the Mehler reaction, photo respiratory H2O2 production would account for about 70% of total H2O2 formed. When chloroplastic CO2 concentration rates are decreased photorespiration becomes even more predominant in H2O2 generation. At the increased flux through photorespiration observed at lower ambient CO2 the Mehler reaction would have to account more than 35% of the total photosynthetic electron flow in order to match the rate of peroxisomal H2O2 production. According to the authors, the interac-tion between oxidants, antioxidants and redox changes in draughted plants can modify gene expression and photorespiratory H2O2 can play role in signaling and acclimation.

Rubisco, specific proteins and drought stress:

                        The mechanism by which Rubisco may be down regulated in the light due to tight binding inhibitors could be pivotal for tolerance and recovery from stress and may be central to integrating the midday depression of photosynthesis.Additionally, enhanced rates of oxygenase activity and photorespiration maintain the ET rate in response to drought and are quantitatively much more important than the Mehler reaction  found a close relationship between Rubisco content and maximal O2 evolution rate measured at high photosynthetic photon flux density (PPFD) during leaf dehydration. It was established that below –2.0 MPa inhibition of photosynthesis in two maize cvs is in part attributed to stomatal conductance but mostly to the decreased activities of carbonic anhydrase, phosphoenol pyruvate carboxylase and Rubisco (Prakash and Rao, 1996). As mentioned above, the primary site of limitation of maximal O2 evolution rate, measured at high PPFD, seemed related to significantly reduced RuBP content, not to the amount of Chl or Rubisco. But as mentioned above, Rubisco is not a prime target of water deficit and is not limiting net CO2 assimilation of leaves submitted to desiccation. Decreased supply of CO2 to Rubisco under both mild and severe water deficit is primarily responsible for the decrease in CO2 fixation (Lal et al., 1996).

                    Specific proteins display particular structural features such as the highly conserved domain predicted to be involved in hydrophobic interaction leading to macromolecular stabilization. The majority of new proteins belong to dehydrin-like proteins,which are abundantly induced during embryo maturation of many higher plants as well as in water stressed seedlings. Dehydrins are synthesized by the cell in response to any environmental influence that has a dehydration component,such as drought, salinity or extracellular freezing. Dehydrins may stabilize macromolecules through detergent and chaperone like properties and may act synergistically with compatible solutes. The steady state levels of major PS2 proteins, including the Dl and D2 proteins in the PS2 reaction center, declined with increasing water deficit possibly as a result of increased degradation. The effects of WD on PS2 protein metabolism, especially on the reaction center proteins may account for the damage to PS2 photochemistry.

Drought stress and lipids:

                          Along with proteins, lipids are the most abundant component of membranes and they play a role in the resistance of plant cells to environmental stresses (Kuiper, 1980; Suss and Yordanov, 1986). Strong water deficit leads to a disturbance of the association between membrane lipids and proteins as well as to a decrease in the enzyme activity and transport capacity of the bilayer  established that for Arabidopsis, polyunsaturated trienoic fatty acids may be an important determinant of responses of photosynthesis and stomatal conductance to environmental stresses such as vapour pressure deficit. When Vigna unguiculata plants were submitted to drought the enzymatic degradation of galacto- and phospholipids increased. The stimulation of lipolytic activities was greater in the drought-sensitive than in drought-tolerant cvs.  Drought stress provoked considerable changes in lipid metabolism in rape (Brasica napris) plants (Benhassaine-Kesri, 2002). The decline in leaf polar lipid was mainly due to a decrease in MGDG content. Determination of molecular species in phosphatidylcholine (PC) and MGDG indicated that the procaryotic molecular species of MGDG (C18/C16) decreased after DS while eukaryotic molecular species (C18/C18) remain stable.

                            It was suggested that the prokaryotic pathway leading to MGDG synthesis was strongly affected by DS while the eukaryotic pathway was not. Strong WD results in a profound overall drop in MGDG, the major leaf glycolipid. In drought sensitive seedlings of Lotus corniculatus the ratio of MGDG/DGDG declined 3-fold, while the relative part of MGDG was 12-fold lower. The lipid composition of desiccated Ramonda leaves is profoundly modified: the ratio of phospholipids (PLs) to galactolipids (GLs) increased and the relative proportion of MGDG to DGDG drastically decreased. An increase in the PLs relative to GLs in leaves indicate a preferential degradation of the chloroplast membranes.

Oxidative stress and antioxidant defense systems:

                              It was established a link between tolerance to oxidative stress induced by WD and rise in antioxidant concentration in photosynthetic plants (Winston, 1990; Prince and Hendry, 1991). This shows that plants are well endowed with antioxidant molecules and scavenging systems (Larson, 1988). Enzymatic free radical processing systems include SOD, catalysing the dismutation of superoxide (O2–) into H2O2 and O2 and those involved in the detoxification of H2O2 – catalase, peroxidase, glutathione reductase (GR-ase). In optimal conditions leaves are rich in antioxidant enzymes and metabolites and can cope with activated O2, thus minimizing oxidative damage. Antioxidant metabolites as ascorbate and glutathione are present in chloropiasts in very high concentrations (Iturbe-Ormaetxe et al., 1998) and apart from their obvious role as enzyme substrates, they can react chemically with almost all forms of activated O2 (Halliwell and Gutteringe, 1989).

                          The hydrophilic antioxidants ascorbate and glutathione are effective chemical scavengers of oxygen radicals. Enzymatic detoxification systems either quench toxic compounds or regenerate antioxidants with the help of reducing power provided by. Foyer et al. (1997) showed that overexpression of GRase in chloropiasts doubled the concentrations of ascorbate and glutathione (GSH) in leaves and conferred increased resistance to oxidative stress. According to their results drought caused a decrease in the content of reduced glutathione and an increase in that of vitamin E. Carotenoids and vitamin E are the main lipid soluble antioxidants of plant cells. In stressed leaves vitamin E increased significantly.

Types of drought environment:

             The breeding methodology as well as the resistance mechanism that should be developed will depend, to a large extent, on the type of drought environment to which the crop will be subjected to. In general following three types of environment s can be associated with drought: a) stored moisture environment, b) variable moisture environment and c) optimal moisture environment. It may, however, be pointed out that numerous combinations of these environment occur in reality.

a)      Stored moisture environment: In this type of environment, the crop completes it life cycle on the moisture stored in the soil during a prior wet or rainy season.  As a result the level of moisture stress will depend upon the amount of moisture stored in the soil, the duration of the crop and the rate of evapotranspiration. In such environments crops become subjected to moisture stress during their terminal phase of their growth and development. The likelihood of success of breeding for drought resistance is rather high, and a spectrum of traits can be exploited for this purpose.

b)      Variable moisture environment: This type of drought environment is characterized by alternate drying and wet periods of varying lengths. Plant grow in such environments can be able to take the advantages of the periodic rainfall and also to survive, with minimum detrimental effects, the periods of the water stress. The periodic and variable nature of water stress is likely to reduce the chance of breeding programmed for drought resistance.

c)      Optimal moisture environment: The crop grown with adequate moisture during of its life cycle; drought occurs occasionally at highly unpredictable stages of growth and development. The period of drought may be limited to a part of one day when evapotranspiration greatly exceeds the root uptake. But ordinarily it is associated with a period less than normal precipitation. The effect of drought in such environment are likely to be rather serve in view of the inadequate time available for the plants to become adjusted to water stress. Breeding for drought resistance for such environment would be extremely difficult.

Drought Resistance:  Drought resistance may be defined as the mechanism causing minimum loss of yield in drought environment relative to the maximum yield in a constraints free environment for the crop. However it does not exist as a unique heritable plants attribute. The various mechanism by which a crop can minimize the loss in the yield during to drought are grouped into the following three categories 1)Drought escape 2) dehydration avoidance 3) dehydration tolerance; these are briefly disused below.

1.      Drought escape:

Drought escape describes the situation where a drought susceptible variety performs well in a drought environment simply by avoiding the period of drought. Early maturity is an important attribute of drought escape, and is suitable for the environments subjected to late –season drought stress. Early varieties generally have lower leaf area index, lower evapo-transpirational and lower yield potential. Therefore, this attributes is not suitable for variable moisture and optimal moisture drought environments. Seasonal length for maize under rainfed conditions is often defined as that time when precipitation is equal to or exceeds 50% of potential evapotranspiration, as determined by radiation, wind, and temperature. A major goal of breeding is to develop cultivars that can escape drought by being sufficiently early in maturity as to complete their life cycle within a given season length. In the lowland tropics, the lower limit of average seasonal rainfall for successful maize cultivation (> 1 t/ha) is around 400-500 mm; in mid altitude areas the minimum is about 350-450 mm; in the highlands it is around 300-400 mm.

                                                                                                                                                                        

                              Because WUE is lower in the warmer lowlands, maize requires more rain fall than in the highlands. Selection for earliness matches the phenology of the crop to the pattern of water availability. Since the time from sowing to flowering or physiological maturity is a highly heritable trait, selection for earliness can easily be accomplished. However, earliness carries a yield “penalty” when rainfall is higher than average. Under those circumstances, the yield of an early maturing cultivar is limited by the amount of radiation the cultivar can capture—normally less than that for a later maturing cultivar.

2. Dehydration Avoidance:

               Dehydration avoidance is the ability of a plant to retain a relatively high level of hydration under conditions of soil or atmospheric water stress. The results of various   physiological, biochemicals and metabolic process of plants those are involved in the growth and yield not being internally exposed to stress, and there, by they are protected from water stress (Blum, 1988). The common measures of dehydration aviodence are tissue water status as expressed by water turgor potential under condition of water stress. This can be achieved by reducing the transpiration rate or increased by water uptake. Wild species are readily classifiable as “water savers” and water spender but crops plants ordinarily exhibits a combination of both features, probably as a result of selection by man. 

·         Reduced Transpiration: Water saving mechanism is common in xerophytes, which have evolved for survival under extreme water stress conditions; ordinarily, they show poor biomass production. Water saving species reduces transpiration mostly by closure of their stomata in response to water deficit well before wilting. Stomata are responsible for the bulk of transpiration, and also for gas exchange in respiration and photosynthesis. Therefore, stomatal closure is likely responsible for the interference with photosynthesis, and drought resistance mechanism based on the stomatal sensitivity and reduced transpiration are generally opposed to maintainance of a higher yield potential.in water stress plant, stomata may open early in the morning hours and close during the day time as solar radiation increases.

·         Osmotic adjustment: It is also important mechanism responsible for the drought avoidance. Osmoregulation is positively associated with higher yield under water stress conditions, as it allows growth and result in delayed leaf death by maintaining torgour pressure and possibly, some other unknown mechanism, but this mechanism of dehydration avoidance may reduce photosynthesis upon recovery and could lower potential yield if it is associated with smaller cell size. The role of different mechanism may change with the stage of plant development. for example in sorghum, stomatal sensitivity to water stress seems to be the main mechanism during vegetative phase, while after flowering osmoregulation and torgour maintenance were important.

·          Abscisic Acid (ABA): ABA is known as stress hormone as its concentration increases in response to stress, including water stress. Water deficit is sensed by roots. This begins to synthesis ABA within one hour of the onset of water stress. ABA is transported via xylem from roots to leaves within minutes to hours, its half life in leaf being 30 minutes. Xylem ABA concentration decrease sharply and stomata open in less one day after watering of the stressed plants. ABA plays a major role in water stress avoidance by effecting stomata closure reduction in leaf expansion and promotion of root growth. As a result mutants partly deficient in ABA biosynthesis are more stressed at the cellular level then are normal plants, when both are subjected to the same level of water stress. In some crops, ABA accumulation was positively associated with yield under stress, while in several other the association was not clear.

·         Cuticular wax: Transpiration also occurs through cuticle; the amount of transpiration depends mainly on the wax deposited within and over the cuticle. The genotypic potential for wax deposition is best in evaluated in plants subjected to water stress. But the effect of Cuticular wax on transpiration is small and, for a given plant, increase wax in load beyond a given threshold would not reduce transpiration.  The shape and angle of wax deposition may affect leaf reflectance within the spectrum range of 400 to 700nm, which in turn may affect net radiation and leaf temperature. For example increase in glaucousness in wheat and sorghum reduced net radiance and leaf temperature, which improves their yield under water stress.

·         Increased water uptake:  water uptake depends mainly on the characteristics of root system, which may be described and measured in various ways, e.g., root- length density, root axial resistance , root radial resistance etc. some broad generalization about root system and its possible role in water stress resistance are as follows; When soil moisture is unlimited in deeper soil horizon, a deep root system is a distinct and    effective component of drought resistance. Root distribution pattern is affected by water status of soil. In a situation of transient soil drying and wetting, a dense root system and or a low root resistance is important in the maintenance of higher leaf water potential.

3. Drought tolerance:

Dehydration tolerance describes the ability of plants to continue metabolizing at low leaf water potential and to maintain growth despite dehydration of the tissue or to recover after release from stress conditions. According to Hsiao, (1973) and Boyer, (1976), translocation is one of the more dehydration-tolerant processes in plants. It would proceed at levels of water deficit sufficient to inhibit photosynthesis. Ample information has been accumulated in the cereals to show that grain growth is partially supported by translocated plant reserves stored mainly in the stem during the pre-anthesis growth stages. When water stress occurs and the current photosynthetic source is inhibited, the role of stem reserves as a source for grain filling increases, both in relative and absolute terms. Stem reserves may therefore be considered as a powerful resource for grain filling in stress affected plants during the grain filling stage.

Genetics of drought tolerance:

                            Existence of different variation for drought resistance has been demonstrated in many crops. Drought resistance was estimated as yield stability (e.g. in wheat, rice, maize, barley, sorghum), lead water potential (sorghum, wheat, rice, soya bean, cotton), leaf rolling (rice), root growth (sorghum, rice, oat, wheat, maize,) root xylem diameter (wheat), osmotic adjustment (wheat, sorghum), stomatal conductance, ABA accumulation, canopy temperature, seedling establishment and growth, seeding recovery after stress, growth under stress , resistance to flower shedding and sustained pod formation under stress and prolie accumulation (barley and Brassica sp.).

                                  The genetic control of these traits ranges from polygenic to oligogenic. Generally, leaf character like waxy bloom, glossy trait, and glaucousness, glabrous leaves are oligogenic control. Some other traits like ABA accumulation in wheat, constitutive proline accumulation in barley mutant and resistance to flower abscission and ability to support pod formation in rajma seem to be determined by oliogenes.

Sources of drought resistance: Drought resistance may be available in cultivated varieties, landraces, related wild species, or may be introduced by genetic engineering in different plants called transgenic.

           Selection criteria: A good selection criteria should have following attributes; it should be easy to estimate\score, it should be have high heritability, a large genetic variability should exists for the trait, it should exhibit a significant association with yield under stress. A major factor that prevented progress for improving yield in water limited environment is the lack of knowledge of the critical traits that should be selected for achieving the goal. The various selection criteria used in breeding for drought resistance crops are written as follows:

1.      Dehydration avoidance:

Leaf rolling: leaf rolling is visually scored from 0-5 in rice either in the morning or at mid day. It is extensively used as selection criteria at IRRI, Philippines. It is likely that leaf rolling will predict leaf water potential in species of low osmotic adjustment in rice.

A combination of leaf rolling and leaf firing is being extensively used in maize and sorghum. Leaf firing is the drying of leaves due to insufficient transpiration cooling. In maize leaf senescence under stress was negatively correlated with yield.

Canopy temperature is readily measured with infrared thermometer. In maize it was negatively correlated with yield under water stress and in rice it was negatively correlated with spikelet fertility. A sufficient level of water stress is a major pre-requisite for applying the method to selection work. The crop must fairly dense and free of skips measurements should made immediately after noon, and windy conditions should be avoided. Measurement should be repeated 2-3 times a week as stress progress.

Leaf attributes like dense pubescence, glaucousness etc. are scored visually. Epicuticular wax load can also measure relatively rapidly. But these traits are unreliable indicators of drought resistance; it is desirable that be incorporated into an integrated selection index with greater weight given to the visual symptoms of wilting/leaf rolling/canopy temperature.

Leaf water retention may be useful in some materials, and its usefulness as the sole criterion of selection is opened to question.

Root characteristic are very difficult to evaluate in masses. In any cases, root attributes should be reflected in canopy response to stress, which are far easier to evaluate.

2.      Dehydration tolerance:

Seedling growth under PEG stress is a useful selection criterion. In alfalfa, a successful selection program for drought tolerance had delayed wilting in a PEG-containing solution as one of the components. Depending on the species, the extent of genetic variation and PEG concentration, a visual ranking of the response to PEG may be sufficient for selection.

Growth under stress in the field may be used as a selection index provided the measure is sufficiently simple and rapid.

Plant phenology may be used as a an index of stress tolerance as drought stress delays or accelerates flowering depending on the growth stage at which stress occurs and on stress intensity, delayed heading under stress has been used a selection index in rice. The time interval between pollen shedding and silking in maize under drought stress is shorter in selections with higher yield under stress, and can be used as selection index.

Grain filling by translocated stem reserve is an important attribute of resistance of cereals to drought after anthesis. In wheat, it may be used as follows: plants are sprayed to complete wetting by a solution of magnesium/sodium chlorate at 14 days after anthesis. The entire plant surface is bleached without killing the plant. Grain filling is now proceeding only due to reserve materials. The 1000-kernals weight of the treated plots is compared with that of the corresponding untreated plots. A smaller difference between the treated and untreated plots indicates a greater reserve translocation. This approach is used experimentally in Israel and Australia.

 Difficulties in breeding for drought resistance:

1.      The moisture regime or drought environment prevailing in the region for which the variety is to be developed must be clearly defined. This is essential since it would often determine the combination of drought resistance traits that should be incorporated in the new variety.

2.      Selection for drought resistance has to be performed under moisture stress. Creation of controlled moisture stress in the field/greenhouse is usually difficult, and greenhouse results have to e confirmed in the field.

3.      Drought resistance in plants usually is the consequence of a combination of characters. As a consequence, no single character can be used for selection index may developed.

4.      Measurement of many drought resistance traits is difficult and problematic.

5.      Many drought resistance traits reduce yield; e.g., earliness, stomatal sensitivity, etc. incorporation of such traits may reduce yields of the varieties. In any case, it would at least require additional breeding effort to enhance the yield of the varieties in such cases where the drought resistance trait is not positively associated with yield.

6.      Selection for yield for yield has to be performed under optimal moisture, while that for drought resistance must be done under moisture stress. This makes it necessary to develop a suitable and elaborate breeding scheme to develop a drought resistant variety with high potential.

7.      The use of wild relatives as sources of drought resistance is problematic and their value for improving the drought resistance of crops is questionable.

8.      Several transgenes holds promise for the development of drought resistant varieties. The results from such work are quite encouraging but so far a commercial example is not available.

Conclusion:

                         Drought seems to be rather difficult to define and more difficult to quantify. For example, various definition used the term i.e., precipitation, air temperature, relative humidity, winds, air flow, soil moisture condition, transpiration, soil moisture storage capacity and plant condition. A working definition may be drought is inadequacy of supply of water to the plant to meet their physiological activity for the growth and development. Increasing the drought area is increasing day by day due to many cause e.g. climate change, global warming. Against these stresses, plants adapt themselves by different mechanisms including change in morphological and developmental pattern as well as physiological and biochemical responses. Drought tolerance comprises drought escape i.e., the ability of a plant to escape periods of drought, especially during the most sensitive periods of its development, drought avoidance i.e., the ability of a plant to withstand a dry period by maintaining a favorable internal water balance under drought, and drought tolerance mechanisms the ability of a plant to recover from a dry period by producing new leaves from buds that were able to survive the dry spell.

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