DROUGHT

2. PLANT PARAMETERS RELATED TO SALINITY AND DROUGHT STRESSByFARGHAMA KHALILReg. No: 09-US-AGR-16PLANT BREEDING & GENETICS

3. DROUGHT • “The inadequate water availability during the life cycle of a crop that restrict the expression of its full genetic yield potential.” • Occurs when demand exceeds supply of water or due to atmospheric or soil conditions, • It is the gradient of water potentials between soil/soil-root interface and leaf.

4. EFFECTS OF DROUGHT • Affect cellular processes, plant growth, development and yield. • Initially, photosynthesis continues but leaf expansion ceases which inhibits plant growth, increases root/shoot ratio & prevents increase in leaf area. • If water stress develops in meristem, differentiation of organs contributing to yield will affect directly. • If water stress continues to increase, stomata close fully, growth ceases, live leaves roll up, gas exchange drops to zero, tissue water continues to decrease slowly, and plant enters prelethal, non-reproductive stage of survival.

5. DROUGHT RESISTANCE “Mechanisms causing minimum yield loss in a drought environment relative to maximum yield in optimum environment for the crop”. A crop can minimize yield loss due to drought by following mechanisms: • Drought Escape. • Dehydration Avoidance. • Dehydration Tolerance.

6. DROUGHT ESCAPE • “Describes situation where a susceptible variety performs well in a drought environment simply by avoiding period of drought”. • Early maturity is an important attribute of drought escape, and is suitable for environments subjected to late-season drought stress. • Early varieties have low leaf area index and lower yield potential.

7. Early flowering (left) and late flowering (right) sorghum cultivars under late-season drought stress. The late cultivar will not flower at all due to stress.

8. DEHYDRATION AVOIDANCE • “Ability of a plant to retain a relatively higherlevel of hydration under soil or atmospheric water stress.” • This protects various physiological, biochemical and metabolic processes of plants from water stress. • Common measure is tissues water status as expressed by water or turgor potential under water stress. • Other factors responsible for it are; Osmotic adjustment: An important mechanism of dehydration avoidance. Osmoregulation is +vely associated with yield under stress, allows growth and results in delayed leaf death by maintaining turgor pressure and other mechanisms.

9. Reduced transpiration: Water saving mechanism. Species reduce transpiration by closure of their stomata in response to water stress before wilting. • Conc. of Abscisic acid (ABA): ABA known as ‘stress hormone’ its conc. increase under stresses, plays an imp role in water stress avoidance by effecting stomata closure, reduction in leaf expansion and root growth promotion. • Cuticular wax: Transpiration also occurs through cuticle; the amount of transpiration depends on the wax deposited within and over the cuticle. Has small effect on transpiration control.

10. Leaf characteristics: Leaf pubescence generally reduces net radiation resulting in lower leaf temperature. This trait shows +ve association with yield under stress. Net radiation can also be reduced by altering the leaf angle from ‘horizontal’, which receives maximum radiation. • Increased water uptake: Water uptake depends on characteristics of root system e.g., root length density, dense and deep root system etc. • Deep root system: desirable when there is unlimited soil moisture at deeper soil layers. • Larger root-length density: no additional moisture reserves at deeper soil layers.

11. DEHYDRATION TOLERANCE “Dehydration tolerance of a genotype means that significantly lower level of changes induced in it than those in another genotype when both are subjected to same level of stress.” Various measurements of dehydration tolerance are; • Maintenance of membrane integrity determined by solutes leakage ( e.g. amino acids, hormones etc) from cells. • Plant growth • Seedling growth and survival after stress • Seed germination under water stress • Presence of large amount of awns ( a drought adaptive attribute). • Proline (a cell compatible solute) accumulation, its conc. Increases under stresses which helps in osmotic adjustment.

12. SOURCES OF DROUGHT RESISTANCE: • Cultivated varieties • Land races • Wild relatives • Transgenes BREEDING SCHEME: Step 1: • Multilocation evaluation under stress to identify stable and drought resistant lines. • Crosses made between drought resistant lines and agronomically superior cultivars to combine high yield potential with drought resistance.

13. F1, F2 andF3 grown under non-stress conditions. In F3 individual plant progenies are evaluated for yield and selection is done. • In F4 selected progenies are evaluated under stress as well as non-stress conditions. Stress resistant progenies are identified and grown in F5. • Multilocation tests of selected lines and release for commercial cultivation. Step 2: • Selected F5 lines are crossed to identify sources for desired drought resistant traits.

14. SELECTION CRITERIA Should have following attributes: • Easy to estimate. • Have high heritability. • Large genetic variability for trait. • Should exhibit significant association with drought resistance. • Should show a +ve association with yield under stress. Various criteria used in breeding for drought resistance in different crops are;

15. Dehydration Avoidance: • Leaf rolling: used extensively as selection criteria at IRRI in rice. It predict leaf-water potential in species of low osmotic adjustment. • Canopy temperature: measured with infrared thermometer, is the most relevant screen for drought resistance. Lower canopy temp. were generally found to be correlated with higher yield under stress. • Leaf attributes: like dense pubescence, epicuticular wax load etc can be scored easily. • Leaf water retention: may be useful in some materials. It should be used as a component of an integrated selection index. A higher water retention is the +ve response. • Root characteristics: their penetration etc

16. Sorghum leaf epicuticular wax by the scanning electron microscope; left normal (Bm genotype); right low wax (bm genotype). Wheat seedlings grown in vermiculite and severely desiccated after which they were irrigated. The seedling on the right received 0.1 µmol of ABA in the irrigation water before the onset of stress. Control seedlings are on the left.

18. Dehydration Tolerance: • Seedling growth under PEG stress: most imp. for measuring plant growth rate under a given root medium moisture stress. Depending on spp. and PEG conc. response to PEG may be sufficient for selection. • Plant phenology: used as index of stress tolerance as drought stress delays or accelerates flowering depending on growth stage at which stress occurs and on stress intensity. • Grain filling by stem reserve utilization: When demand by the grains is not fully supplied then plant reserves provide the balance. Carbohydrates stored in diff. plant parts especially in stem are then translocated to grain for grain filling. • Cellular membrane stability under stress: has been shown to be +vely correlated with yield under stress. Can be assessed by measuring the leakage of cellular electrolytes.

19. Grain of two wheat cultivars subjected to sever drought stress during grain filling (right). Top: cultivar with superior capacity for stem reserve utilization; bottom: normal cultivar. Note the shriveled grain under stress in the latter.

20. SALINITY • “Excessive accumulation of soluble salts in the root zone that leads to detrimental effects on plant growth and development.” • Salt affected soils are mainly of 2 types: Saline soils: contain soluble salts mainly chlorides and sulfates of Na, Ca, Mg, and K. Their EC is always more than 4. Alkaline soils: contain >15% exchangeable Na, common salt is sodium carbonate. • Soil salinity has a number of causes which may be natural, be due to clearing of vegetation (‘dryland salinity’), or due to irrigation.

21. EFFECTS OF SALINITY • Symptoms include slow and spotty seed germination, sudden wilting, stunted growth, marginal burn on leaves, leaf yellowing, leaf fall, restricted root development, and sudden or gradual death of plants. Plants growing in saline conditions face 3 types of stress: • Water stress generated by osmoticum i.e. salts in solution. • Mineral toxicity stress caused by salt. • Disturbances in mineral nutrition of plant.

23. MANAGEMENT PRACTICES • Soil Reclamation: replacing Na ions in soil with Ca by applying gypsum (calcium sulfate) to the soil followed by water ponding. • Scraping and removal of surface soil • Mulching with crop residue such as straw etc • Deep Tillage • Incorporation of Organic matter • Breeding for increased salt tolerance in crops

24. SALINITY RESISTANCE Resistance to Salinity-induced water stress: • Osmoregulation is common response, helps in turgor maintenance, leaf desiccation and other consequences of turgor loss. • Osmotic adjustment is through proline accumulation in halophytes • In glycophytes it is through sugars, proline and ions (Na, K, Cl) accumulation • Organic solutes involved in osmoregulation also protect cellular membranes from damage due to stress.

25. Resistance to salinity-induced ion toxicity: • Ion toxicity avoidance involves mechanism which maintains a low nontoxic level of salts in the cytoplasm. • Achieved by 2 ways: (1) Ion exclusion (2) Salt tolerance by cellular compartmentation/ salt excretion • Ion exclusion: “when some spp./ genotypes take up smaller quantities of injurious ions i.e. Na and Cl so that conc. of these ions in their tissues is much lower than those of other spp./ genotypes.” • Occurs in rice, halophytes, soybean, tomato • Salt exclusion at root is an efficient mechanism of avoiding ion toxicity. Roots size is +vely associated with salt load on roots due to ion flow.

26. Salt tolerance: • “Differential effect on various life processes of same tissue conc. of salt in diff. genotypes of a spp.” • Genotypes differ in tolerance to same amount of salt. • Halophytes have the ability to accumulate Na+ to very high conc. in vacuoles and Glycophytes only to some extent. • Response to salinity may change with plant age and crop. • Salt stress increases as plant continue to grow under saline conditions

28. SELECTION CRITERIA • Cell survival: Better index of salt resistance especially at range of salinity levels. • Seed germination: Desirable selection criteria in spp. where germination is more salt sensitive than later stages of plant growth. • Leaf death: Can be estimated by total dead leaf area or by no. of dead leaves.

29. Leaf necrosis: Caused by accumulation of Na, K and Cl ions, used as selection criteria based on ion exclusion. • Root growth: Expresses resistance of plants to mineral toxicity. • Osmoregulation: may be measured as proline or carbohydrate accumulation and is determined as turgor maintenance under stress. • Yield: also an imp selection criteria.

30. SOURCES OF SALINITY RESISTANCE: • Cultivated varieties • Germplasm collections • Wild relatives • Transgenes • Molecular markers BREEDING APPROACHES: • Selection • Hybridization • Interspecific hybridization • Genetic engineering

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