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 d rought. 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 me teor ol ogi cal char acte ri st ic s are li nke d to agri cult ur al impact , focus ing 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 la nd us e, Land deg radati on, cons tr ucti on of dams . Al l ef fect s the hydro logic al charac teris tics of basin s .The frequ ency and severi ty of hydro logica l drought is often defined on the water shed basin scale. Effects of drought Effects of drought are di fferent in irrigat ed and in rainfed agriculture. In r egion 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 ifit 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 theirwater 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
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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
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
There are three types of drought
I. Meteorological drought
It is defined on the basis 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 droughti.e. when rainfall decreases it effects agriculture, then water levels of river lakes and
reservoirs decreases
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
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.
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
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.
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
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
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
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
There are three types of drought
IV. Meteorological drought
It is defined on the basis 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.
V. 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
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
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.
Photosynthesis, together with cell growth, is among the primary processes to be
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.
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.
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
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