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The
Mitigation of Drought Stress |
By
Dr. Abraham Blum |
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NOTE: Numbers in parenthesis throughout text are
ID number for searching that reference
in the Reference Database on this site. Sometimes it is also a
live link to the report.
MITIGATION OF DROUGHT
STRESS BY CROP MANAGEMENT
Irrigation, where economically available, is the major means
for combating drought conditions. It is a prime approach to the intensification
of agriculture and the generation of stable income. The development of
irrigation depends on various environmental, economical and social factors on
both the macro and micro scales.
There are hazards in irrigation if practiced
indiscriminately, such as soil erosion, soil salination,
soil leaching and soil disease infection. Irrigation
as such is not an important topic in this site. Links to irrigation sites
throughout this section should provide additional information.
General Crop Irrigation Guidelines
The
key to planning irrigation system and scheduling is knowledge of the crop, the
soil properties and the potential evapotranspiration
General Irrigation Links
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(PET) of the specific crop
at the site. This information can also be used to estimate dryland crop water use and deficit at any given time during
the crop cycle, which is actually an index of crop drought stress.
The Penman-Monteith potential evapotranspiration equation is recommended by the FAO as
the standard method for estimating reference and crop evapotranspiration.
The new method has been proved to have a global validity as a standardized
reference for grass evapotranspiration and it has
been recognized by both the International Commission for Irrigation and
Drainage and by the World Meteorological Organization.
The
(FAO) Penman-Monteith method was developed by
defining the reference crop as a hypothetical crop with an assumed height of 0.12 m having a surface resistance of 70 s m-1 and an albedo of 0.23, closely resembling the evaporation of an
extensive surface of green grass of uniform height, actively growing and
adequately watered.
Further
educational information and guidelines on the applications of the Penman-Monteith method and the general approaches to prediction of
crop water requirements and the supply of requirement is provided by:
The FAO Methodology for Crop Water Requirements.
FAO CROPWAT, downloadable software to carry out standard
calculations for design and management of irrigation.
Irrigation to Control Drought in Various Crops
Supplemental irrigation is the more common irrigation
practice for crops not designated a priori for fully irrigated
conditions. It is also recently being labeled as "deficit irrigation"
since it does not supply the full seasonal crop water requirement.
Supplementary irrigation is a practice dictated by constraints, which can be
derived from the limited availability of water, irrigation equipment, the cost
of water, or other economical and technical constraints. With supplemental
irrigation the amount of water applied to the crop in irrigation is well below
the full requirement of the crop. The water-use efficiency of supplemental irrigation
is generally high if applied logically. It can be applied to save the crop in
case of un-expected drought or as a planned practice to supplement the expected
total seasonal rainfall. The practice may vary extensively with crop and
region. In many environments, and especially the Mediterranean region, if only
a single supplementary irrigation is given it is usually more effective if
applied pre-planting. As such the crop enters the season with a stored supply,
which can insure growth despite unexpected transient rainfall fluctuations.
There are numerous computer models based on simulations of
crop growth combined with field water balance computations (see above), which
allow the grower to input his environmental data set in order to develop
recommendations for irrigation at different scenarios.
Managing the Dryland Crop
Environment
Modern dryland farming is a system
of low inputs combined with soil and water conservation practices and risk
reducing strategies. The system can be sustainable if practiced properly. Water shortage is the main limiting factor,
but successful dryland systems also maintain
reasonable practices to eliminate other limiting factors (poor nutrient status,
weeds, biotic stresses, etc'), which can reduce the efficiency by which the
crop uses the limited moisture. However, as water shortage a priori dictates a
limit on yield, all other inputs must be carefully adjusted downwards to fit
the expected low economic return. Except for subsistence agriculture, economical
dryland farming depends on a large area per farmer.
The most advanced systems have been developed in the
The lesson learned from the American and Australian
experience is that the development of a sustainable dryland
farming system involves the following principles, not necessarily in their
order of importance:
1. Improved
soil and water conservation practices and the associated tillage systems.
2. Optimization
of the fit between crop growth cycle and the available moisture.
3. Weed
control
4. Soil
fertility management.
5. Optimized
plant population density and spatial arrangement of plants with respect to the
expected soil moisture regime.
6. Control
of soil biotic stress factors that reduce root development.
7. Improved
forage/livestock/grains integration and rotation.
8. Avoidance
of mono cropping and the diversification of farming.
9. The
increase of precipitation by cloud seeding, as an ongoing experiment.
Some of the above principles of the dryland
farming system constitute general agronomic knowledge, which can be explored in
our Web Resources page as well as in
standard agronomy textbooks and other publications (e.g. Drought Management of Farmland - by Joan Sydney Whitmore,
2000, 360 pp., Springer, SBN 0792359984). Here only several topics will be
touched upon.
The fallow system is designed to conserve
soil moisture from one season to another or from one year to the other,
depending on climate and crop. Increasing storage of soil moisture by the
fallow system with or without conservation tillage is standard agricultural
practice in dryland farming. The benefit of fallow
and conservation tillage in terms of increasing available soil moisture to the
crop depends on soil water-holding capacity, climate, topography and management
practices. Fallow efficiency, in terms of percent increase in soil moisture
availability to the crop measured at planting date normally ranges from about
5% to 30%. While these amounts are not impressive they can make a difference
between crop failure and success. The fallow carries additional benefits such
as improved soil nutrients availability and the eradication of certain
soil-born pests, such as nematodes.
Conservation tillage involves the
principle of minimized tillage operations to conserve soil structure and to
maintain ground cover by mulch, such as stubble. These practices reduce water
runoff and increase soil infiltration. Conservation tillage has become the
cornerstone of dryland systems in certain regions of
the
In certain soils deep tillage was found
very useful to improve soil moisture storage, especially when hard soils or
hardpans are a problem. This is an expensive operation that cannot be deployed
regularly in dryland farming.
For further online information on dryland farming and its research visit the following sites
and the links therein:
§
Soil and
Water conservation in Semi Arid Areas – (FAO Manual, 1987)
§ USDA-ARS Bushland Texas Experiment Station – dedicated to
conservation research
§ Soil, water and reclamation publications --From
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These
techniques constitute a field surface tillage manipulation to minimize runoff
away from the field.
Furrow
dikes
are furrows, which are divided into short basins by small dikes (see right side
photograph). This is achieved by special
equipment. The system is very amenable to row crops
such as cotton, corn and sorghum and it can be integrated with or without
furrow irrigation. It is generally considered effective for increasing rainfall
capture and raising dryland yield where annual
rainfall ranges between 500 and 800 mm.
Soil
pitting (left side photograph) involves the formation of small depressions at
close proximity to reduce runoff from rainstorms. The crop is planted over this
modified surface. Experiments performed with wheat in nine farmer demonstration
plots in Southern Israel during 1988 showed that pitting increased yield by an
average of 7.5% at a mean yield of 3.25 ton/ha. Unlike furrow dikes these
system is not limited to row crops.
This is
a broad term to describe various methods to collect runoff from large
contributing areas and concentrate it for use in smaller crop area. This is an
ancient practice already adopted by Nabatian desert settlements in the
For more detailed
information see ‘Water Harvesting’,
and Runoff Farming.
Diversification of farming is an ancient but an effective approach to
reduce the risk associated with farming in unpredictable environments. Reduced
diversification to the extent of mono-cropping is possible only if a high level
of control is possible over the crop environmental conditions. Such control
method (irrigation, chemical pest control, etc’) are among the main reasons for
the more recent environmental quality problems found to be associated with
mono-cropping.
Diversification
of cropping to reduce risk is especially important under dryland
conditions. It is achieved on several levels, as described by Pandey et al (#4194) for the case of traditional rain-fed
rice in Eastern India.
1. Spatial
diversification of fields. The farmer’s land is divided into
several fields or plots which may differ in their topography, soil and
hydraulic properties. Some fields may be prone to flooding while others do not
hold water. Certain fields may be on a warmer slope while others on a cooler one.
The different field conditions allow to achieve a better fit between the crop
and the environment and to reduce the general probability of stress affecting
the farmer.
2. Crop
diversification is an important feature of traditional farming. It takes
an advantage of the generally low correlation between crops in performance when
grown in a single stress environment. Crops differ in their response to a given
environment and this difference is used to reduce the risk associated with
growing one crop. “Mixed cropping” or “intercropping” is an example of a
traditional and a successful approach to crop diversification on a single
parcel of land, where two or more crops are grown together in various possible
configurations. If for some reason only one crop is grown, a certain (though
lower) level of risk reduction can be achieved by varietal
diversification. Planting of several crop varieties offer a better
probability for reducing loss due to environmental stress, as compared with
growing one variety only. For environmental stress conditions varietal diversification is based mainly on differential phenology, primarily flowering date. A typical example is a
transient frost or heat wave that is likely to occur around flowering time of
the specific crop. Damage reduction can be achieved when the crop is sown to
several varieties of different flowering dates.
3. Temporal
diversification may achieve the same result as varietal
diversification, when phenology is concerned. The
purpose of setting a distinct planting date is to optimize crop development
with respect to seasonal climate, mainly rainfall in rain-fed agriculture.
Ideally the crop is planted at the beginning of the rainy season, rainfall
peaks when crop evapotranspiration peaks and it
terminates just before harvest time. When such conditions are reasonable
predictable, planting date can be set to optimize production. Where the timing
of rainfall is very unpredictable, adopting more than one planting date for the
given crop can reduce the risk involved with untimely rainfall and a given
planting date.
Cloud seeding is a form of weather modification attempt.
The process of cloud seeding involves deposition of cloud condensation nuclei
(CCN) into a specific region of the cloud. Seeding may be achieved from above
or through the clouds by aircraft, and from below where CCN are carried into
the cloud by updrafts. With either method, the CCN must reach the super cooled
cloud region, where water molecules remain unfrozen at temperatures below 0C.
Experiments in cloud seeding have been performed for the last 50
years. The results and benefits of this practice are still under debate.
Information is available in the report on 'Weather
Modification by Cloud Seeding-A Status Report
1989-1997 by William R. Cotton,
MITIGATION OF DROUGHT
STRESS BY CROP PLANT RESISTANCE
The
nature of Drought Resistance
Drought
Resistance and Crop Yield
Crop plant breeding for drought
resistance has long been part of the breeding process in most crops that have
been or are being grown under dryland conditions.
During the period of the pre-scientific agriculture the genetic improvement of
plant adaptation to dry conditions was simply attained by repeatedly selecting
plants that appeared to do well when drought stress occurred. As a result of
many generations of selection by generations of farmers we now encounter such
materials, which are defined as “landraces” of the crop. Such landraces were
shown to posses distinct drought resistance traits. Later, as scientific
agriculture developed and following the emergence of Mendelian
genetics, elaborate biometrical and statistical methods for quantitative
genetics analysis were developed to enable selection for yield stability. An
important factor of yield stability is coping with drought and other abiotic
plant stresses. Subsequently, yield-based selection programs were augmented by
observing plants under carefully managed stress environments, followed by the
development of physiological selection criteria for stress resistance. More
recently, molecular methods, such as marker-assisted selection are being
adopted to facilitate more efficient selection for distinct components of
stress resistance. Finally, biotechnology is experimenting with genetic
transformation, which has not yet been applied as a solution to breeding for
drought resistance. Some applications may be forthcoming in the private seed
business sector.
Looking
at crop drought resistance from a botanical perspective it must be realized at
the onset that there is a vast difference between drought resistance in natural
vegetation and in crop plants. Natural vegetation has evolved to conserve the
species. Henceforth, plant survival and the capacity to produce at least one
seed per life cycle despite stress is the most powerful component of natural
selection. On the other hand, drought resistance in modern agriculture requires
sustaining economically viable plant production despite stress. Crop survival
is of a lesser consequence to economical farming. On the other hand, plant
survival can be a critical factor in subsistence agriculture, where the ability
of a crop to survive drought and produce some yield at all may translate into a
difference between famine and livelihood. Breeding for drought resistance is
therefore very tightly linked to the target environment of the crop, not only
with respect to its physical and chemical features but also its social grounds.
The recent developments in GIS technology are extremely important as a tool for
defining a target environment for the breeding program. GIS has been discussed
under impact of stress above.
John B. Passioura’s
(#6030) general description of yield and water-use is widely
accepted by agronomists. It is based on the initial model of C.T. de Wit and in
its simple form it can be expressed as:
Yield = T x WUE x HI,
Where, T= total seasonal crop
transpiration, WUE= crop water-use efficiency and HI= crop harvest index (the
ratio of economic yield to total aboveground biomass). T is a component of ET
(crop evapotranspiration), where E denotes direct
evaporation from the soil.
It can be immediately seen that yield is
proportional to crop water-use. The older concept of formulating drought
resistance by designing a plant that is “water-saving” will not work for most
crop environments. Moderated water-use by design can be important in systems
where the crop grows mostly on pre-seasonal stored soil moisture. Here, fitting
crop-water use to the limited available seasonal water supply is crucial.
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Generally, crops must use water under stress
in order to sustain yield. Drought resistance is a finite trait. Life cannot be
sustained without water. Drought resistance in modern farming is based on the
plant ability to obtain water or to use water efficiently, when water is
limited by drought.
A most preoccupying issue for the dryland crop breeder, and not only the beginner, is the
formulation of a plant ideotype for the target
environment addressed by his program. The ideotype is
a detailed morphological, physiological, developmental and conceptual
description of the desirable plant type. The construction of an ideotype can be based on accumulated knowledge,
consultation, biased opinion and even intuition. Lately, serious attempts are
made at adopting crop growth simulation models to assist in developing a
breeding ideotype. The formulation of an appropriate
and relevant ideotype for a drought target
environment is probably the most crucial component of planning a breeding
effort for improved drought resistance. It is hoped that this contribution will
help to understand what is involved in formulating a drought resistant ideotype and hopefully assist in applying such knowledge to
practice.
In order to further understand what
constitutes drought resistance in terms of crop production, one should closely
examine how different cultivars may respond to changing moisture availability (Fig.7).
Wheat cultivar C
is different from A and B in that it has a lower yield potential (yield at high
moisture conditions) but as moisture becoming deficient C turns out to be
superior to A and B. In terms of yield, C may be defined as drought resistant
while cultivars A and B are of high yield potential but are relatively drought
susceptible. The “crossover” where the advantage of C over A and B under stress
begins to be expressed is at about 300 mm or at a yield level of about 300 g m-2.
Hence, drought resistance of C is expressed only when stress is sever (<300
mm). Still, it is extremely important to realize that the high yielding
cultivars A and B are superior to the drought resistant C when drought stress
is moderate (e.g. at 400 to 500 mm). A high yield potential therefore ascribes
an advantage under moderate stress conditions. On the other hand Fig.7 seems to
indicate that drought resistant wheat cultivars have lower yield potential,
which in a sense is a penalty for drought resistance. This point will be
discussed later.
Constitutive plant traits affecting
drought response
As discussed above
under the “Impact of Stress”, moisture stress signals certain stress responsive
genes, which are responsible for a chain of events, expressed at various levels
of plant organization. It has been assumed almost axiomatically that stress
responsive genes are involved in adaptation; henceforth that they are ‘stress
adaptive’. It was later realized that not every stress responsive gene is
necessarily adaptive in terms of drought resistance or survival.
Irrespective of
the role and function of stress adaptive genes in plant drought resistance, it
should be recognized that not only stress adaptive genes condition plant
performance under drought stress. Genes that are expressed irrespective of the
environment also condition plant function and performance under stress. These
genes are expressed constitutively. For example, genes conditioning early
flowering are basically expressed in any environment. Their expression is not
conditioned by stress. Early flowering may have a critical role in plant
response to and performance under stress (see below). Constitutive plant traits
can be modified by stress, and the rate of modification can be considered to
involve stress responsive elements. Using the above example, stress may cause
one early variety to flower a little later than normal while another similarly
early variety will not be affected by stress and will maintain its normal
flowering time. The delay in flowering may be generally considered as
un-desirable and an expression of drought susceptibility.
Plant Phenology
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Short growth
duration (generally defined by early flowering) constitutes an important
attribute of ‘drought escape’, especially for conditions of a
late-season drought stress (Fig.8). On the other hand, longer growth duration
is often associated with high yield potential. Consequently, using drought
escape as a solution may involve a cost in terms of reduced yield potential.
This is serious, especially when the moisture environment is unpredictable and
may vary to a large extent between years. The more predictable the environment
is, the easier it becomes to optimise phenology. The unpredictability of the
environment may reach a state where short growth duration is a drawback,
especially in indeterminate plants that offer a potential for regrowth and
productivity upon recovery. A longer growth duration in both determinate and
indeterminate plants would improve the probability for regrowth upon recovery
simply because, on the same calendar day, late-maturing genotypes are younger
than early ones and younger plants recover better. The final decision on the
optimum growth duration has, of course, to consider additional factors, such as
late-season disease and insect pressure or periods of frost.
Early maturity
lead to reduced, total seasonal evapotranspiration
simply because of the shorter time in the field. However, as growth duration is
genetically linked with leaf number, early genotypes have a small transpiring
leaf-area index. Thus, early genotypes show reduced evapotranspiration
during most growth stages, up to the point where a full ground cover is
achieved. At most growth stages, root-length density and total root length per
plant is generally greater in a late than in an early cultivar. This should be
reflected in an advantage for the late genotype under conditions where
extensive rooting is required.
A phenological trait specific to maize is
the timing of anthesis with respect to silking, defined as anthesis-to-silking
interval (ASI). Evidently a short interval is desirable whereas a large
interval results in poor pollination. The maize program at CIMMYT dedicated
many years of work to research the trait and explore its significance in
tropical maize breeding for stress environments (#3347). Maize germplasm can vary for ASI irrespective of the effect of
stress; a short ASI is a universally important trait for maize production.
However, stress, and especially drought during the reproductive stage may
extend ASI and thus reduce yield. Maize genotypes may vary in ASI under drought
stress from few days up to around 60 days. The effect on yield of change in ASI
between null and 10 days is exponential. Selection for short ASI under drought
stress proved to be the most effective approach to improve drought resistance
of tropical maize. QTLs (quantitative trait loci)
controlling ASI were identified
and marker-assisted selection is possible.
Different crop plants may advance (e.g.
wheat) or delay (e.g. rice) their flowering when stress occurs before flowering.
The rate of delay is a function of plant water deficit and probably also ABA
signalling. The rate of change in flowering time under stress can be taken as
an index of genotypic rate of stress in the field.
Plant development and size
Plant size as expressed mainly in terms
of single plant leaf area or leaf area index (LAI) has a major control over
water-use, as explain under Impact of Stress.
Small plants and reduced leaf area are generally conducive to low productivity
while they limit water use. Botanists have long recognized small plants bearing
small leaves as typical ecotypes of xeric environments. While such plants
withstand drought very well their growth rate and biomass are relatively low.
In the domain of plant breeding,
cultivars developed for dryland conditions by
selecting mainly for yield under such conditions often result in plants of
moderate size and productivity. For example this can be seen in dryland temperate cereals, which tend to have moderate
tillering. On the other hand researchers in the CSIRO Australia have concluded (#3903)
that early plant (and seedling) vigour are important traits for dry conditions.
The reason is in the rapid ground cover achieved and the subsequent decrease in
water loss by direct soil evaporation at this stage. However, other benefits
for seedling vigour were also noted, such as the nitrogen status of the plant
(#7059).
As for other constitutive traits,
irrespective of the genetic control of plant size, drought stress signals a
reduction in leaf area to effectively reduce water use. Drought stress reduces
the transpiring leaf area by leaf expansion inhibition, leaf wilting (such as
leaf rolling in the cereals) and leaf death and abscission. While leaf
desiccation is caused by dehydration, leaf growth inhibition and abscission can
result at least partly from stress induced ABA accumulation.
The Root
The most important control of plant water
status is with the root, whereas roots are the main engine for meeting transpirational demand. Two major dimensions describe the
root: root depth and root-length density (Fig.9). The more practically
important dimension for most breeding scenarios is root depth, which facilitate
deep soil moisture extraction where such moisture is available. It is a primary
component of drought resistance. The development of lateral roots at very
shallow soil depth may have a role in capturing small amount of intermittent
rainfall.
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Root depth in the cereals is generally associated
with a small number of main thick axes. Such fibrous root system is often seen
in upland rice, which has a deeper root system, in contrast to lowland rice
with the shallower roots. The control of root growth is not only in the root.
In the cereals, tillering is associated with production of new crown roots from
each developing tiller. Such profuse rooting is at the expense of the growth of
existing roots, deeper into soil. Hence, limited tillering in cereals and
grasses has been repeatedly observed to be associated with relatively deeper
root extension.
In certain soils a hardpan can limit deep
root growth and the capacity for hardpan penetration by roots becomes a
critical factor in drought resistance. The factors, which may support axial
root force and hardpan penetration, are not known and most research in this
area has been performed with seedlings only. In mature plants the penetration
of hardpan by roots seems to be better in plants that constitutively develop
fibrous and thick roots.
Many drought environments present a
situation where rainfall is low and soil depth that contains moisture is
permanently shallow. For example, in many of the drier Mediterranean
wheat-growing regions the wetted soil depth of around 60-80 cm is shallower
than the maximum root depth of wheat (>100 cm). Under such conditions a deep
root is not an issue. Other plant factors may then become far more important in
the control and use of the limited soil moisture, such as shoot developmental
characteristics (e.g. leaf area development or growth duration), osmotic
adjustment, leaf surface properties, etc.'
Another scenario of seasonal soil
moisture status is where the crop is grown on stored soil moisture and there is
little effective rainfall during the growing season. Under such conditions the
main consideration is to manage seasonal soil moisture use such that sufficient
moisture will remain for carrying the crop to maturity. It is to be expected
that with the available moisture the crop might grow luxuriously leading to a
large leaf area and an even greater water requirement towards the latter part
of the season. Hence, short growth duration, small leaf area and perhaps a
higher root hydraulic resistance can achieve the control of seasonal water use.
The last option has been researched at the CSIRO Australia and an increase
wheat root hydraulic resistance was effectively attained by selection for
smaller xylem element diameter. It is not known whether this approach has found
its way into actual application in wheat breeding
Whatever may be the constitutive form and function of roots, the environment can modify the root in a pronounced way. Offcourse, soil conditions in terms of topsoil moisture and deep soil hardness alter root growth and depth. Drought stress generall
