GROWTH AND PRODUCTIVITY OF CROPS IN RELATION TO WATER STATUS

Effects of water stress on crop productivity and yield are discussed in terms of the photosynthetic acquisition of carbon and the partition of the carbon gained to the harvested organ. Leaf growth is extremely sensitive to water stress. When crop canopy is incomplete and intercepts only a part of the incident radiation, even mild water stress slows leaf growth and the development of canopy, leading to lessened radiation absorption and photosynthesis per unit land area. This effect compounds with time and results in a larger reduction in biomass accumulation for a small reduction in relative growth rate. On the other hand, if canopy is already complete, mild water stress should have little or no effect on biomass production rate. With more severe water stress, stomatal opening and photosynthesis per unit leaf area are reduced, with the consequent reduction in biomass accumulation. Intercellular CO₂ often remains constant in spite of the partial closure of stomata, but the adaptive advantage of this behavior is not clear. Moderate to severe water stress usually accelerates the senescence of leaves, especially when the crop is approaching maturity. Biomass accumulation slows earlier because of the reduced leaf area duration. The number of fruits or grain is often reduced by water stress. Although often attributed to the inhibition of pollination by water stress, the more likely causes are the reduced number of fruiting sites associated with the smaller size of the plant and the abortion of the younger fruits because of lessened assimilate supply when water stress occurs at the time of fruit or grain filling. If water stress is very severe at the time of anthesis, however, pollination can be prevented and the number of reproductive sinks for assimilates greatly reduced. Harvest index and yield are reduced more than the reduction in total biomass. In contrast, mild to moderate water stress before and during anthesis can actually enhance the partition of assimilate to the reproductive bodies and promote the development of embryos formed from the earlier flowers in some species. The result is the earlier maturation of at least a portion of the fruits.