Crop yield response to water - Cra

estricted transpiration rate. Contrary to earlier opinions, maize under favourable conditionsresponds positively to increases in atmospheric CO 2 , as shown by increases in leaf area (Hsiaoand Jackson, 1999) and biomass at least up to 520 ppm CO 2 . Hence, AquaCrop adjustmentsnormalize water productivity (WP*) according to atmospheric CO 2 concentration, year‐by‐year.For example, maize WP* was adjusted from 32.4 g/m 2 in 1990 to 33.7 g/m 2 in 2000.Responses to StressesWater stress develops when rain and irrigation are absent and the water stored in the rootzone is depleted to the point where plant processes are affected. In AquaCrop, the thresholdlevel that triggers stress responses are set for different key processes. As for virtually all crops,leaf expansive growth of maize is the most sensitive of all the stress responses (Bradford andHsiao, 1982; Hsiao and Xu, 2000) and its stress response threshold in AquaCrop is set not farbelow soil field capacity. Very mild water stress, lasting for many days can lead to a muchsmaller canopy cover during the vegetative stage. If stress is sufficiently more severe, stomatalconductance are also reduced, and at a similar stress level senescence of older leaves beginsto accelerate. Crop transpiration and photosynthesis would be reduced both as the result ofless green canopy cover (because of reduced growth or more senescence) and lower stomatalconductance. This of course leads directly to reduced rate of biomass production, and hencereduced grain yield. An added negative effect is that the acceleration of canopy senescencewould reduce the canopy duration and shorten the grain-filling period. There would not besufficient time for the harvest index to build up and reach its normal maximum. The end resultis that the percentage reduction of grain yield would be even more than the percentagereduction of biomass.As a result of the monoecious nature of maize, fairly severe to severe water stress can cause apeculiar problem of reproduction. In addition to expansive growth of leaves, expansive growthof stems as well as silk and tassel are also inhibited by the stress. The slower silk growth orelongation leads to delayed emergence of silk from the husk. Tassel emergence is also delayedby water stress, but the delay is less than that for silk (T.C. Hsiao, personal observation). Thisdifference in delay can cause pollination failure as, by the time the silk emerges, there may notbe sufficient pollen left to fully pollinate the crop. On the other hand, the failure to pollinate thelate silks on an ear and the very late ears of a plant population must be substantial to negativelyimpact yield because in dense plantings the number of grains (kernels) a plant is able to matureis only 65 to 75 percent of its number of silks, and the late emerging silks do not form maturekernels even when pollens are ample (Duncan, 1975; T.C. Hsiao, unpublished). Also, the verylate ears are formed by the smallest plants in the population and their contribution to yield isminimal even when pollinated. The time interval between tassel emergence and silk emergenceappears to vary with different genetic lines (Bolaños and Edmeades, 1996), but is minimal forlines well-adapted to the local environment. As the severity of water stress increases, however,this interval is lengthened more and more and grain yield can be drastically reduced as a resultof pollination failure (Bolaños and Edmeades, 1996). As mentioned, modern maize is highlydeterminant with a narrow time interval for pollination. This means there is no opportunity tomake up for reduced pollination with later flowers when rain or irrigation comes.Overall, and relative to other crops, maize is considered to be sensitive to water stress. Maizedoes not osmotically adjust as well as cotton, sorghum or wheat to low water status. In118crop yield response to water