Agriculture Reference
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of reduced water use. Genotypic variation in WUE was driven mainly by
variations in water use rather than by variations in plant production or as-
similation per unit of water use. If low water use is the breeder's target, it
is highly probable that selection for the same can be achieved by directly
selecting for these plant traits, without measuring WUE (Blum, 2005). The
enhancement of biomass production under drought stress can be achieved
primarily by maximizing soil water capture while diverting the largest part
of the available soil moisture towards stomatal transpiration. This is de-
fined as effective use of water, and it is the major engine for agronomic or
genetic enhancement of crop production under a limited water condition.
High WUE is a critical characteristic of drought-tolerant species, and is a
water-saving strategy of plants in arid regions. However, there are many
relative physiological traits affecting leaf WUE expressing wide variations
in leaf WUE under normal and water stress conditions. Intrinsic water use
efficiency (IWUE) estimated as a ratio of photosynthesis/transpiration has
been recognized as a measure of carbon gain per unit of water loss and
found to be inversely proportional to the ratio of intercellular and ambi-
ent CO 2 concentrations (C i /Ca) (Martin et al., 1992). Large variability in
WUE has been reported among several species as well as cultivars within
a species including cowpea (Condon et al., 2002). Because higher rates of
leaf photosynthesis are often associated with faster crop growth rates, a
combination of higher photosynthesis and improved WUE mayplay a vital
role for yield enhancement of crops under drought stress conditions (Parry
et al., 2005).
9.3.5 EVAPOTRANSPIRATION (ET)
The onset of stress may initially cause a loss of cell turgor, which in turn
reduces gaseous exchange, and leaf elongation sinceboth are turgor-de-
pendent processes. ET is known to positively correlate with yield of the
crops, since it is a direct measure of crop water loss. Water stress causes a
decrease in transpiration, an increase in foliage temperature and closure of
stomata. Canopy temperature is dependent on climatic factors and internal
plant water status. There seems to be a positive link between yield and
transpiration rate. Important increases in crop yield might be possible if
irrigation water is applied at the most appropriate time to prevent exces-
sive and nutrient leaching. In order to improve irrigation efficiency, it is
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