Drought Tolerance in Wheat

8/3/2019 Drought Tolerance in Wheat

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DROUGHT

Deficiency of a precipitation over an extended period of time usully a season or more

result in water shortage for some activity, groups, or environmental sector is drought.

Types of drought

There are three types of drought

I. Metrological drought

It is defined on the bases of degree of dryness and duration of dry spell. Various

meteorological characteristics are linked to agricultural impact, focusing on

precipitation shortage, soil water deficit, reduced groundwater and /or reservoir levels .

II. Agricultural drought

Agriculture is usually the first economic sector to be effected by drought. Agricultural

drought is typically seen after meteorological drought but before a hydrological drought

i.e. when rainfall decreases it effects agriculture, then water levels of river lakes and

reservoirs decreases

III. Hydrological drought

Effect of periods of precipitation shortfalls on the surface or subsurface water supply.

Although climate is the primary contributor to hydrological drought, others factors are

changes in land use, Land degradation, construction of dams. All effects thehydrological characteristics of basins .The frequency and severity of hydrological

drought is often defined on the watershed basin scale.

Effects of drought

Effects of drought are different in irrigated and in rainfed agriculture. In region which

rely on irrigation, the impacts of short lived agricultural droughts are lower than in

regions where crops are not irrigated. Irrigated agriculture relies on stocks of water so if

it does not rain , these crops still gets the water they need (until the reservoirs get run

dry). However in non irrigated agriculture crops depends directly on the rain as their

water source. If it does not rain, the plants don’t get the water they need to survive.

Mega environment

Wheat is grown on about 110 million hectares in more than 70 developing countries so

breeders must understand how factors such as temperature , rainfall, diseases and pests

vary. They need to know which characteristic is essential in wheat varieties intended

1



for specific parts of the world. CIMMYT ‘wheat breeders began to codify their vision

of developing world`s wheat growing areas into standard set of “mega environments”.

The mega environments were defined by crop production factors i.e temperature,

rainfall, sunlight, latitude, elevation, soil characteristics and diseases, consumer

preferences i.e. the colour of grain and how it would be used and wheat growth habit

i.e. wheat has three distinct growth habits

1. Spring habit wheat

Spring habit wheat have a continuous growth cycle with no inactive period. In areas

where winters are severe such as northern Kazakhstan or Canada, wheat is planted in

the spring after there is no risk of frost. In areas with very mild winters, such as India or

Austrailia, spring wheat is sown in autumn and grows through the winter

2. Winter habit wheat

Winter habit wheat evolved to withstand low winter temperatures, such as those that

prevailin North Korea or Northwestern Europe. To flower they require exposure to cold

during their early growth. Winter wheat are sown in autumn and start to grow before

winter sets in, when they become inactive

3. Facultative habit wheat

Facultatie habits wheat tolerate cold more then spring wheats and less than winter

wheats,but they donot require extended exposure to cold temperatures to reproduce.

These wheat are found in transition zones between true spring and winter wheat regions

Description of global wheat mega environment

CIMMYT has categorized wheat growing world into twelve mega environments out of

which three are irrigated and nine are rainfed Researchers identified six mega

environments for spring wheats ahd three each for facultative and winter wheat.

Climatic criteria are based on conditions during coolest, warmest or wettest consecutive

three months of the year and annual means or total

Mega enviroment Description Representative sites

Spring wheat:

ME1:Favorable,irrigated

well irrigated, low rainfall

Well irrigated low rainfall

regions. Conditions during

the cropping season range

from temperate to late heat

stress, specially with late

sowing. Predominently

Gangetic Valley, India;

Indus valley,Pakistan;

Nile valley, Egypt;

Yaqui Valley,Mexico

2



winter’ sown tropical to

sub tropical. Rarely spring

sown cool temperate

regions

White grained type

predominate

ME 2 High rainfall Regions where crops

experience no or minor

moisture deficits

ME 2A :Highland ,

summer rain

Highland regions of the

tropics and subtropics

where crops are grown on

summer rainfall

Red grain type except

white for Ethopia

Kulumsa,Ethopia

Toluca,Mexico

ME 2B:Lowland, winter

rain

Highland regions of

subtropical and warm

temperate regions where

crops are grown on winter

rainfall.

Red grain type

Izmir, Turkey;

Pergamino, Argentina

ME 3:High rainfall ,acid

soil

Similar to ME2 but for

regions with acid soils

Red grain is generally

preferred except in

Himalayas

Passo Fundo, Brazil;

Mpika,Zambia

ME 4:Low rainfall Three types of moisture

deficits,based on

developmental stage when

moisture deficits occur, are

recognized as sub-

enviroments

ME4A:Winter rain or

Meditteranean type

Regions with a

Meditteranean climate

Aleppo, Syria;

3



with post flowering

moisture deficits and heat

stress typical.Late season

frosts

White grain is mostly preferred

Settat,Morocco

ME 4B: Winter drought or

Southern cone-type rainfall

Associated with pre-

flowering moisture

deficits.

Red grain is mostly

preffered to reduce

sprouting

Marcoz Juarez, Argentina

ME 4C: Stored moisture Sown after monsoon rains,resulting in continuous,

Indian Subcontinent-type

drought. Only white grain

is accepted

Dharwae,India

ME 5: Warm

ME 5A: Warm, humid Warm, humid lowland

tropical to subtropical

regions

Joydebpur,Bangadesh;

Encarnaci & oacute;n

Paraguay

ME 5B:Warm, dry Warm,semiarid to arid

tropical to subtropical

regions

Kano, Nigeria;

Wad Medani, Sudan

ME 6:High latitude Cool temperate regions of

North America,Europe,

and Asia where wheat is

spring sown as winters are

too severe survival of even

winter wheat

ME 6A: High rainfall Humid regions of Western

and Central Euorope and

of eastern Asia with winter

conditions too severe for

Harbin, Heilongjiang,China

4



winter wheat

ME 6B:Semi arid Dry regions of Central and

eastern Asia and the

northern plains of Canada

and USA with winter conditions too severe for

winter wheat

Astana, Kazakhstan

Facultative wheat

ME7:Favorable,moderate

cold, irrigated

Ahenxhou,Henan,China

ME 8: High rainfall Temuco,Chile;

Corvallis, Oregon, USAME 9:Semi arid, moderate

cold, irrigate

Diyarbakir,Turkey;

Vernon,Texas,USA

Winter wheat

ME 10:Favourable ,cold,

irrigated

Biejing ,China.

ME 11: High rainfall,cold Cambridge,

UK,Krasnodar,Russia

ME 12: semi arid, low

rainfall, cold

Ft.Collins,Colorado;

Manhattan,Kansas,USA

DROUGHT

5



Deficiency of precipitation over an extended period of time usually a season or more

result in water shortage for some activity, group, or environmental sector is drought.

Types of drought


I. Meteorological drought

It is defined on the basis of degree of dryness and duration of dry spell. Various


precipitation shortage, soil water deficit, reduced groundwater and /or reservoir levels.

II. Agricultural drought


drought is typically seen after meteorological drought but before a hydrological droughti.e. when rainfall decreases it effects agriculture, then water levels of river lakes and


III. Hydrological drought

Reduction in number of rain spells and duration of precipitation results in reduction of

surface or subsurface water supply. Although climate is the primary contributor to

hydrological drought, others factors are changes in land use, Land degradation,

construction of dams. All effects the hydrological characteristics of basins. The

frequency and severity of hydrological drought is often defined on the watershed basinscale.

Effects of drought

Effects of drought are different in irrigated and in rainfed agriculture. In regions, which



it does not rain, these crops still gets the water they need (until the reservoirs get run



Photosynthesis, together with cell growth, is among the primary processes to be

affected by drought (Chaves, 1991) or by salinity (Munns et al., 2006).

The effects can be direct, as the decreased CO2 availability caused by diffusion

limitations through the stomata and the mesophyll (Flexas et al., 2004, 2007) or the

6

http://aob.oxfordjournals.org/content/103/4/551.full#ref-7










alterations of photosynthetic metabolism (Lawlor and Cornic, 2002) or they can arise as

secondary effects, namely oxidative stress. The latter are mostly present under multiple

stress conditions (Chaves and Oliveira, 2004) and can seriously affect leaf

photosynthetic machinery (Ort, 2001).

Photosynthetic response to drought and salinity stress is highly complex. It involves the

interplay of limitations taking place at different sites of the cell/leaf and at different

time scales in relation to plant development.

The intensity, duration and rate of progression of the stress will influence plant

responses to water scarcity and salinity, because these factors will dictate whether

mitigation processes associated with acclimation will occur or not.

Acclimation responses under drought, which indirectly affect photosynthesis, include

those related to growth inhibition or leaf shedding that, by restricting water expenditure

by source tissues, will help to maintain plant water status and therefore plant carbon

assimilation.

Osmotic compounds that build up in response to a slowly imposed dehydration also

have a function in sustaining tissue metabolic activity.

These responses will eventually lead to restoration of cellular homeostasis,detoxification and therefore survival under stress.

Drought indices

1. Percent of normal

Percentage of normal precipitation is one of the simplest measurements in rainfall for a

location.

7









The percent of normal is a simple calculation well suited to the needs of TV

weather casters and general audiences. It is quite effective for comparing a single

region or season.

Easily misunderstood, as normal is a mathematical construct that does not necessarily

correspond with what we expect the weather to be.

2. Standardization Precipitation Index

a) Overview

The SPI is an index based on the probability of precipitation for any time scale

b) Who uses it?

Many drought planners appreciate the SPI‘s versatility

c) Pros

The SPI can be computed for different time scales, can provide early warning

Strategies to mitigate drought stress

Effect of drought can be mitigate by

1) Supplemental Irrigation

The canal systems, tube wells and other water resources are included in supplementalirrigation

2) Managing the dryland crop environment

o Improved soil and water conservation practices and the associated tillage

systems

o Optimization of the fit between crop growth cycle and the available moisture

o Weed control

o Soil fertility management

o Optimized plant population density and spatial arrangement of plants with

respect to the expected soil moisture regime

o Control of soil biotic stress factors that reduce root development

8



o Avoidence of mono cropping and the diversification of farming

o The increase of precipitation by cloud seeding ,as an on going experiment

3) Drought resistance

Drought resistance may be defined as the mechanism causing minimum loss of yield in

a drought environment relative to the maximum yield in a constraint free, i.e optimal

environment for the crop. However it doesnot exist as a unique heritable plant attribute.

The various mechanisms by which a crop can minimize the loss in yield due to drought

are grouped into following three categories

1. Drought escape

It describes the situation where an otherwise 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 environments subjected to late

season drought stress.Early varieties generally have lower leaf area index,lower total

evapotranspiration and lower yield potential

2. Dehydration avoidance

It is the ability of a plant to retain a relatively higher level of hydration under conditions

of soil or atmospheric water stress. This result in various physiological, biochemical

and metabolic processes of plants that are involved in growth and yield not being

internally exposed to stress and thereby , they are protected from water stress. The

common measure of dehydration avoidance is the tissue water status as expressed bywater or turgor potential under conditions of water stress. This can be achieved either

by reducing transpiration (water savers) or increasing water uptake (water spenders)

a. Reduced transpiration

Water saving species reduce transpiration mostly by closure of their stomata in

response to water deficit well before wilting( stomatal sensitinity to water

stress ).Stomata are responsible for the bulk of transpiration , and also for gas exchange

in respiration and photosynthesis. Therefor stomatal closure is likely to interfere with

photosynthesis, and drought resistance mechanisms based on stomatal sensitivity andreduced transpiration are generally opposed to to the maintainance of a higher yield

potential. In water stressed plants stomata may remain remain open in the early

morning hours, and close as solar radiation increases. An extreme example is provided

by the crassulacean acid metabolism (CAM) in some succulents, where stomata close

during day and open during night, where CO2 is fixed by them.

9



I. Osmotic Adjustment

It is an important mechanism of dehydration avoidance Osmoregulation is positively

associated with yield under stress conditions ,as it allows growth and result in delayedleaf death by maintaining turgor pressure and possibly, some other unknown

mechanisms. But this mechanisms of dehydration avoidance may reduce

photosynthesis upon recovery and could lower potential yields if it is associated with

smaller cell size . Osmotic adjustment positively affects growth and yield under stress.

II. Abscisic Acid (ABA)

ABA is known as “stress hormone” as its concentration increases in response to

stresses, including water stress. Water deficit is sensed by roots, which begin to

synthesize ABA within 1 hour of the onset of water stress. ABA concentration

decreases sharply and stomata open in less than 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.

III. 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 evaluated in plants subjected to water stress. But the effect of

cuticular wax on transpiration is small and, for given plant, increase in wax load

beyond a given threshold would not reduce transpiration.

IV. Leaf Characteristics

Leaf Pubescence generally increases leaf reflectance and reduces net radiation resulting

in lower leaf temperature under high irradiance. This trait shows positive association

with yield under stress in some cases but not in others.Net radiations can also br

reduced by altering the leaf angle from horizontal which receives the maximum

radiation. Thus in cereals ‘erect leaf’ lines perform better than ‘lax leaf’linesunder

moisture stress. Leaf rolling is the most familiar response to water stress ,especially in

cereals. Leaf rolling is induced by loss of turgor; therefore, it is delayed by osmotic

adjustment. Hence delayed leaf rolling is an indication of tugor maintainance as a

component of dehydration avoidance

b. Increased water uptake

10



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, Some broad generalization about root system and its possible role

in water stress resistance are as follows

o When soil moisture is unlimited at deeper soil horizons, a deep root system is adistinct and effective component of drought resistance.

o But when there are no additional moisture reserves at deeper soil layers, large

root system density and small root (hydraulic)resistance would contribute to the

maintenance of higher water potential

o Root distribution pattern is affected by water status of soil .In situations 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

o In stored moisture environments, it is important to minimize evapo-transpiration

during the early season. This can be achieved either by reducing leaf-area index

or by the development of a greater hydraulic resistance in the root. A selection

for smaller root xylem vessel diameter resulted in increased root axial hydraulic

resistance.

3. Dehydration tolerance

When cell loose turgor and dehydrate there is

(1) Reduced chemical activity of water

(2) Increased concentration of solutes and macromolecules

(3) Removal of water of hydration from macromolecules

(4) Alteration in cellular membranes

Dehydration tolerance of a genotype means that a significantly lower level of changes

are induced in it than those in another genotype when both of them are subjected to the

same level of dehydration .The various mearsuements of dehydration tolerance are

a) Maintainence of membrane integrity

11



It is usually determined by the leakage of solutes including various electrolytes

,aminoacids, sacchrides,organic acids, hormones e.t.c from the cell. Cell membrane

stability, in water shortage condition due to rigidity of cell wall , cell membrane remain

stable upto certain level of water shortage condition It should be noted that all test plants

must be subjected to the same stress historyand its association with drought resistance at

plant levels remains to be shown

b) Stem reserves

Stem reserves are powerful source of grain filling in stress affected plants during grain

filling sage The capacity of stem reseve translocation appears to be related to drought

toleranceand could be signaled by the ABA accumulated in response to water

stress.If,however photosynthesis continues during stress , the translocation of stem

reserve is reduced.

a. Proline accumulation

Proline, a cell compatible solute ,is accumulated to very high levels during stress; its

concentration declines rapidly upon rehydration .Proline also accumulates in response

to other stresses like salinity and exreme temperature and contributes plants tolerance to

them. The proline accumulation appears to be involved in tolerance to water and other

stresses.Accumulation of very lare amount of proline contributes to osmotic

adjustment; it may also serve as cytoplasmuc osmotic balance for potassium

accumulation as the main osmoticum in vacule.

It serves as a protectant of various enzymes and biological membranes subjected to

dessication and heat stress. However it is extremely important that the proline contents

of different genotypes/ lines are measured when their tissue water potentials are

practically the same. This is essential to avoid wrong conclusions as proline

accumulates exponentially with the reduction in tissue water potential.

Stress adaptation

o The signal transduction

o Osmotic adjustment(OA)

o Cellular membranes

o Antioxidation

o Stress protein or Chaperons

o ABA(abscisic acid ) accumulation and its consequences

12



Breeding for Drought Resistance

It is important that resistance be incorporated into materials with high genetic potential

for yield

a. Selection for drought

A good selection criteria should have the following attributes.

o It should be easy to score

o It should have high heritability

o A large genetic variability should exist for trait

o It should exhibit significant association with drought resistance

o It should show positive association with yield under stress

Direct method

It is the method in which those genotypes are selected which perform better under drought conditions To see cultivar differences in yield under drought, an experiment is

there i.e Drought Susceptanility index DSI in which the mean yield of all cultivars

under drought, corresponding to irrigation cut-offs and at high temperatures are taken to

check the yield potential. If DSI is greater than 1 then the plant is susceptible or viceverca.

Indirect method

In this approach , the material is not deliberately developed for stress resistance after it

has been developed. During selection phase of breeding there is no conscious effort toselect for stress resistance,testing is only during the evaluation phase of the breeding

programme .Evaluation of materials under stress often helps in the identification of a

resistant line .In this method the traits are incorporated which increase the yield under drought.

b. Breeding methods

It is important that materials that drought resistance be incorporated into the materials

with high genetic potential for yield.Thus selection for stress response need to be

integrated with selection and testing for potential yield under non stress conditions

.Various approaches for breeding for drought resistance are

• Hybridization followed by selection

13



In first generation F1 s are produced by crossing parents that are widely adapted

and high yielding with lines possessing proven tolerance to specific drought

environments.Use of winter wheats and synthetic germplasm is emphasized .F1s

is space planted under irrigated condition

In next generation,F2 is space planted under irrigated and optimum conditions,and inoculated with a wide spectrum of rust virulence; only resistant plants are

selected.F3 and F4 are handled according to modified pedigree bulk scheme

In F3 generations ,individual plants progenies are grown at commercial rates

under rainfed conditions or very low water availability. Selection is based on

progeny lines rather than individual plants

The operations of F3 generations are repeated

In F5 generations the progenies selected in F4 are evaluated under opimum

conditions. Best spikes for best progenies are selected and bulked for each

progeny

In F7 generations , selected progenies are evaluated simultaneously under both

moisture stress and optimum environments. Lines showing outstanding

performance under both environments are selected

In F8 generation, operations of F7 generation are repeated

In F9 generations, multilocational trials are initiated for verification of input

responsiveness and input efficiency, and for evaluation required for release as avariety

• Marker assisted selection

Drought tolerance stress is a quantitatively inherited trait, controlled by several

genetic loci. QTL analyses in segregating populations facilitate the dissection of the

genetic basis of drought tolerance. Successful marker identification efforts would

facilitate integration of marker assisted selection procedures in the breeding

programmes, enabling the pyramiding of favourable alleles and target loci

14



DROUGHT

Deficiency of precipitation over an extended period of time usually a season or more

result in water shortage for some activity, groups, or environmental sector is drought.

Types of drought


IV. Meteorological drought

It is defined on the basis of degree of dryness and duration of dry spell. Various


precipitation shortage, soil water deficit, reduced groundwater and /or reservoir levels.

V. Agricultural drought


drought is typically seen after meteorological drought but before a hydrological drought

i.e. when rainfall decreases it effects agriculture, then water levels of river lakes and


VI. Hydrological drought

Reduction in number of rain spells and duration of precipitation results in reduction of

surface or subsurface water supply. Although climate is the primary contributor to

hydrological drought, others factors are changes in land use, Land degradation,construction of dams. All effects the hydrological characteristics of basins. The

frequency and severity of hydrological drought is often defined on the watershed basin

scale.

Effects of drought

Effects of drought are different in irrigated and in rainfed agriculture. In regions, which



it does not rain, these crops still gets the water they need (until the reservoirs get run



Photosynthesis, together with cell growth, is among the primary processes to be

affected by drought.

15



The effects can be direct, as the decreased C O2 availability caused by diffusion

limitations through the stomata and the mesophyll or the alterations of photosynthetic

metabolism or they can arise as secondary effects, namely oxidative stress. The latter

are mostly present under multiple stress conditions and can seriously affect leaf

photosynthetic machinery.

Photosynthetic response to drought and salinity stress is highly complex. It involves the

interplay of limitations taking place at different sites of the cell/leaf and at different

time scales in relation to plant development.

The intensity, duration and rate of progression of the stress will influence plant

responses to water scarcity, because these factors will dictate whether mitigation

processes associated with acclimation will occur or not.

Acclimation responses under drought, which indirectly affect photosynthesis, include

those related to growth inhibition or leaf shedding that, by restricting water expenditure

by source tissues, will help to maintain plant water status and therefore plant carbon

assimilation.

Osmotic compounds that build up in response to a slowly imposed dehydration also

have a function in sustaining tissue metabolic activity.

These responses will eventually lead to restoration of cellular homeostasis,

detoxification and therefore survival under stress.

Drought indices

Percent of normal

Percentage of normal precipitation is one of the simplest measurements in rainfall for a

location.

The percent of normal is a simple calculation well suited to the needs of TV

weather casters and general audiences. It is quite effective for comparing a single

region or season.

Easily misunderstood, as normal is a mathematical construct that does not necessarily

correspond with what we expect the weather to be.

16



Standardization Precipitation Index (SPI)

The SPI is an index based on the probability of precipitation for any time scale

Many drought planners appreciate the SPI‘s versatility

The SPI can be computed for different time scales, can provide early warning

Strategies to mitigate drought stress

Effect of drought can be mitigate by

1) Supplemental Irrigation

The canal systems, tube wells and other water resources are included in supplementalirrigation

2) Managing the dryland crop environment

o Improved soil and water conservation practices and the associated tillage

systems

o Optimization of the fit between crop growth cycle and the available moisture

o Weed control

o Soil fertility management

o Optimized plant population density and spatial arrangement of plants with

respect to the expected soil moisture regime

o Control of soil biotic stress factors that reduce root development

o Avoidence of mono cropping and the diversification of farming

o The increase of precipitation by cloud seeding ,as an on going experiment

3) Drought resistance

Drought resistance may be defined as the mechanism causing minimum loss of yield in

a drought environment relative to the maximum yield in a constraint free, i.e optimal

environment for the crop. However it doesnot exist as a unique heritable plant attribute.

The various mechanisms by which a crop can minimize the loss in yield due to drought

are grouped into following three categories17



1. Drought escape

It describes the situation where an otherwise 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 environments subjected to late

season drought stress. Early varieties generally have lower leaf area index, lower total

evapo-transpiration and lower yield potential

2. Dehydration avoidance

It is the ability of a plant to retain a relatively higher level of hydration under conditions

of soil or atmospheric water stress. This result in various physiological, biochemical

and metabolic processes of plants that are involved in growth and yield not being

internally exposed to stress and thereby , they are protected from water stress. The

common measure of dehydration avoidance is the tissue water status as expressed by

water or turgor potential under conditions of water stress. This can be achieved either by reducing transpiration (water savers) or increasing water uptake (water spenders)

a. Reduced transpiration

Water saving species reduce transpiration mostly by closure of their stomata in

response to water deficit well before wilting (stomatal sensitivity to water stress).

Stomata are responsible for the bulk of transpiration, and also for gas exchange in

respiration and photosynthesis. Therefore, stomatal closure is likely to interfere with

photosynthesis, and drought resistance mechanisms based on stomatal sensitivity and

reduced transpiration are generally opposed to the maintenance of a higher yield potential. In water stressed plants stomata may remain open in the early morning hours,

and close as solar radiation increases. An extreme example is provided by the

crassulacean acid metabolism (CAM) in some succulents, where stomata close during

day and open during night, where CO2 is fixed by them.

a. Osmotic Adjustment

It is an important mechanism of dehydration avoidance. Osmoregulation is positively

associated with yield under stress conditions, as it allows growth and result in delayed

leaf death by maintaining turgor pressure and possibly, some other unknown

mechanisms. But this mechanisms of dehydration avoidance may reduce

photosynthesis upon recovery and could lower potential yields if it is associated with

smaller cell size. Osmotic adjustment positively affects growth and yield under stress.

18



b. Abscisic Acid (ABA)

ABA is known as “stress hormone” as its concentration increases in response to

stresses, including water stress. Water deficit is sensed by roots, which begin to

synthesize ABA within 1 hour of the onset of water stress. ABA concentration

decreases sharply and stomata open in less than 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.

c. 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 evaluated in plants subjected to water stress. But the effect of

cuticular wax on transpiration is small and, for given plant, increase in wax load

beyond a given threshold would not reduce transpiration.

d. Leaf Characteristics

Leaf Pubescence generally increases leaf reflectance and reduces net radiation resulting

in lower leaf temperature under high irradiance. This trait shows positive association

with yield under stress in some cases but not in others.Net radiations can also be

reduced by altering the leaf angle from horizontal which receives the maximum

radiation. Thus in cereals ‘erect leaf’ lines perform better than ‘lax leaf’ lines under

moisture stress. Leaf rolling is the most familiar response to water stress ,especially in

cereals. Leaf rolling is induced by loss of turgor; therefore, it is delayed by osmoticadjustment. Hence delayed leaf rolling is an indication of tugor maintenance as a

component of dehydration avoidance

b. 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, Some broad generalization about root system and its possible role

in water stress resistance are as follows

o When soil moisture is unlimited at deeper soil horizons, a deep root system is a

distinct and effective component of drought resistance.

19



o But when there are no additional moisture reserves at deeper soil layers, large

root system density and small root (hydraulic) resistance would contribute to the

maintenance of higher water potential.

o Root distribution pattern is affected by water status of soil. In situations of

transient soil drying and wetting, a dense root system and/or a low rootresistance is important in the maintenance of higher leaf water potential

o In stored moisture environments, it is important to minimize evapo-transpiration

during the early season. This can be achieved either by reducing leaf-area index

or by the development of a greater hydraulic resistance in the root. A selection

for smaller root xylem vessel diameter resulted in increased root axial hydraulic

resistance.

3. Dehydration tolerance

When cell loose turgor and dehydrate there is

1. Reduced chemical activity of water

2. Increased concentration of solutes and macromolecules

3. Removal of water from macromolecules

4. Alteration in cellular membranes

Dehydration tolerance of a genotype means that a significantly lower level of changes

are induced in it than those in another genotype when both of them are subjected to thesame level of dehydration. The various measurements of dehydration tolerance are

a) Maintainence of membrane integrity

It is usually determined by the leakage of solutes including various electrolytes, amino

acids, sacchrides, organic acids, hormones etc from the cell. Cell membrane stability, in

water shortage condition due to rigidity of cell wall , cell membrane remain stable upto

certain level of water shortage condition It should be noted that all test plants must be

subjected to the same stress levels.

b. Stem reserves

Stem reserves are powerful source of photosynthates in stress affected plants during

grain filling stage. The capacity of stem reserve translocation appears to be related to

drought tolerance and could be signaled by the ABA accumulated in response to water

20



stress. If, however photosynthesis continues during stress, the translocation of stem

reserve is reduced.

c. Proline accumulation

Proline, a cell compatible solute, is accumulated to very high levels during stress; its

concentration declines rapidly upon rehydration. Proline also accumulates in response

to other stresses like salinity and extreme temperature and contributes plants tolerance

to them. The proline accumulation appears to be involved in tolerance to water and

other stresses. Accumulation of very large quantity of proline contributes to osmotic

adjustment; it may also serve as cytoplasmic osmotic balance for potassium

accumulation as the main osmoticum in vacuole.

It serves as a protectant of various enzymes and biological membranes subjected to

desiccation and heat stress. However it is extremely important that the proline contents

of different genotypes/ lines are measured when their tissue water potentials are

practically the same. This is essential to avoid wrong conclusions as proline

accumulates exponentially with the reduction in tissue water potential.

Stress adaptation

o The signal transduction

o Osmotic adjustment(OA)

o Cellular membranes

o Antioxidation

o Stress protein or Chaperons

o ABA(abscisic acid ) accumulation and its consequences

Breeding for Drought Resistance

It is important that resistance be incorporated into materials with high genetic potential

for yield

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Direct method

It is the method in which those genotypes are selected which perform better under drought conditions To see cultivar differences in yield under drought, an experiment is

there i.e. Drought Susceptibility index DSI in which the mean yield of all cultivars

under drought, corresponding to irrigation cut-offs and at high temperatures are taken tocheck the yield potential. If DSI is greater than 1 then the plant is susceptible or viceversa.

Indirect method

In this approach, the material is not deliberately developed for stress resistance. During

selection phase of breeding there is no conscious effort to select for stress resistance,testing is varied out only during the evaluation phase of the breeding programme.

Evaluation of materials under stress often helps in the identification of a resistant line.

In this method the traits are incorporated which increase the yield under drought.

Selection Criteria for drought tolerance traits

A good selection criteria should have the following attributes.

o It should be easy to score

o It should have high heritability

o A large genetic variability should exist for the trait

o It should exhibit significant association with drought resistance

o It should show positive association with yield under stress

Breeding methods

It is important that materials that drought resistance be incorporated into the materials

with high genetic potential for yield. Thus selection for stress response need to be

integrated with selection and testing for potential yield under non stress conditions.

Various approaches for breeding for drought resistance are practiced.

• Hybridization followed by selection

In first generation F1 s are produced by crossing parents that are widely adapted

and high yielding with lines possessing proven tolerance to specific drought

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environments. Use of winter wheats and synthetic germplasm is emphasized. F1

is space planted under irrigated condition

In next generation, F2 is space planted under irrigated and optimum conditions,

and inoculated with a wide spectrum of rust virulence; only resistant plants are

selected.

In F3 generations, individual plants progenies are grown at commercial rates

under rainfed conditions or very low water availability. Selection is based on

progeny lines rather than individual plants.

The operations of F3 generations are repeated in F4.

In F5 generations the progenies selected in F4 are evaluated under opimum

conditions. Best spikes for best progenies are selected and bulked for each

progeny

In F7 generation, selected progenies are evaluated simultaneously under both

moisture stress and optimum environments. Lines showing outstanding

performance under both environments are selected

In F8 generation, operations of F7 generation are repeated

In F9 generations, multilocational trials are initiated for verification of input

responsiveness and input efficiency, and for evaluation required for release as a

variety

• Marker assisted selection

Drought tolerance stress is a quantitatively inherited trait, controlled by several

genetic loci. QTL analyses in segregating populations facilitate the dissection of the

genetic basis of drought tolerance. Successful marker identification efforts would

facilitate integration of marker assisted selection procedures in the breeding

programmes, enabling the pyramiding of favourable alleles and target loci.

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Documents

Drought Tolerance in Wheat