Principles of Plant Genetics and Breeding

388 CHAPTER 21
duration of drought is variable, sometimes lasting for
a short time and without severe adverse physiological
impact, sometimes lasting throughout an entire growing
season or even years, resulting in complete devastation
of crops. The efforts of breeders are directed at
short-duration drought that often is experienced when
crop production is rainfed. Under rainfed conditions,
rainfall is often erratic in frequency, quantity, and distribution.
To avoid disruption in growth and development
processes, and consequently in crop performance
(yield), plants need to maintain a certain level of physiological
activity during the adverse period.
The effect of drought varies among species and also
depends on the stage of plant growth and development
at which the moisture stress occurs. Drought at flowering
may cause significant flower drop and low fruit set.
Similarly, when drought occurs at fruiting, there will
be fruit drop and/or partially filled or shriveled fruits.
In the end, both quality and product yield will be
decreased.
Overview of drought stress concepts
Scientists have developed crop simulation models that a
researcher may use to estimate and quantify the impact
of specific drought stress conditions on crop productivity.
Models are available for cereals (e.g., maize, wheat,
rice), grain legumes (e.g., soybean, dry bean, peanut),
root crops (e.g., cassava, potato), and other crops
(e.g., tomato, sugarcane).
As the demand for water exceeds supply, a plant
develops what is called plant water deficit. The supply
of water is determined by the amount of water held
in the soil to the depth of the crop root system. The
demand for water is determined by plant transpiration
rate or crop evapotranspiration (both plant transpiration
and soil evaporation). The rate of transpiration is
influenced by solar radiation, ambient air temperature,
relative humidity, and wind. These factors control transpiration
at the single leaf level. The most important
factor that controls transpiration at the whole-plant or
crop level is total leaf area.
It is difficult to sense and estimate plant water stress
at whole-crop levels because of the need to integrate
an estimate based on the whole canopy. Various plantbased
methods are used to obtain direct measurements
of water status, stress status, and other physiological
consequences of water deficit. These include leaf water
potential, relative water content, and osmotic potential.
Soil moisture is measured in a variety of ways. The soil
moisture content (volume) is measured by gravimetric
methods (soil is weighed before and after drying to
determine water content). The soil water status is measured
in terms of potential and tension. The amount
of water a given crop can extract from the soil at a
given water potential and depth is called the extractable
soil moisture. Most crops usually extract between
50% and 80% of the extractable soil moisture before
crop transpiration is reduced and symptoms of water
deficit occur.
The relative importance of the major mediators of
cellular response to water deficit are not exactly known.
Plant hormones (e.g., abscisic acid) are believed to be
a key factor in water stress response. Numerous water
stress responsive genes have been identified. At the
whole-plant or crop level, water deficit effects manifest
in various ways – phenology, phasic development,
growth, carbon accumulation, assimilate partitioning,
and reproductive processes. These manifestations are
primarily responsible for the variations in crop yield
attributable to drought stress. Water deficit causes
reduced cell expansion, reduced plant water use, and
reduced plant productivity. Reduced cell expansion
also adversely impacts meristematic development of
yield components (e.g., inflorescence or tiller initials in
cereals), leading to potentially small reproductive
organs and hence reduced yield. Damage to the meristem
is usually irreversible and cannot be corrected by
irrigation. Water deficit also causes advanced or delayed
flowering, depending on the species. This alteration in
phasic development is known to be critical to maize
yield under stress. Water deficit can cause reproductive
failure whereby the pollen may desiccate, creating an
effect similar to male sterility, and leading to reduced
grain set. The duration of grain filling is reduced under
stress. The root : shoot dry weight ratio increases as
plants slip into water stress.
Managing drought stress
Irrigation is the primary means of addressing drought in
crop production, provided this approach is economical.
Crop production may be designed for irrigated or dryland
(rainfed) environments. In irrigated production,
the common practice is to implement a supplemental
irrigation regime, whereby irrigation is applied when
rainfall is inadequate. Various soil and water conservation
practices may be adopted to conserve soil moisture
from one season to another. Practices such as fallow
and the use of ground cover are recommended practices
for soil and water conservation.