Transcript PLANT PARAMETERS RELATED TO SALINITY AND DROUGHT

PLANT PARAMETERS RELATED TO SALINITY AND
DROUGHT STRESS
By
FARGHAMA KHALIL
Reg. No: 09-US-AGR-16
PLANT BREEDING &
GENETICS
• “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.
• 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.
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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, nonreproductive stage of survival.
“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.
• “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.
Early flowering (left) and late flowering (right) sorghum cultivars under lateseason drought stress. The late cultivar will not flower at all due to stress.
• “Ability of a plant to retain a relatively higher level 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.
• 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.
• 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.
“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.
SOURCES OF DROUGHT RESISTANCE:
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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.
• F1, F2 and F3 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 nonstress 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.
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;
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
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.
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.
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.
• “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.
• 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.
• 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
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.
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
1. 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.
2. 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
• 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.
• 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.
SOURCES OF SALINITY RESISTANCE:
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Cultivated varieties
Germplasm collections
Wild relatives
Transgenes
Molecular markers
BREEDING APPROACHES:
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Selection
Hybridization
Interspecific hybridization
Genetic engineering
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