Saini and Westgate (2000) point out that early reproduction is highly phasic, with each phase showing susceptibility to water deficits. Meiosis, anthesis, pollen fertility, pollination, female fertility, and early zygote development are susceptible, and their failure diminishes the number of kernels that the plant produces. Later in development, water deficits tend to reduce kernel size rather than number, and size seems to be determined in large part by the available photosynthetic reserves that can be moved to the grain. For example, McPherson and Boyer (1977) worked with maize subjected to water deficits beginning 2 weeks after pollination and extending until grain maturity. This long-term deficit occurred during the linear phase of dry mass accumulation by the kernel. The kernel size was reduced with little effect on kernel numbers. Similar results were obtained in a field study by Jurgens et al. (1978).
The water deficits studied by McPherson and Boyer (1977) caused low enough water potentials (w) to inhibit photosynthesis substantially, and scarcely any dry matter accumulated as a result. Despite the lack of dry matter accumulation in the parent, the kernels continued to fill for some time. The dry matter for the kernels came from the parental reserves. In recent work in rice, the importance of reserves was also demonstrated (Yang et al., 2001a, b) when low w imposed during the linear phase of grain filling caused little loss in yield. Large amounts of carbohydrate were moved from the stems to the grain that made up for the lack of current photosynthesis. As a result, there is often a relationship between the dry matter in the grain at the end of the season and that in the parent (McPherson and Boyer, 1977; Yang et al., 2001a, b). Sinclair (1990) collected data from many maize plots subjected to water deficits and also found a correlation between kernel yield and shoot dry matter accumulation during a water deficit.
For low w around the time of pollination, however, the yield/dry matter relationship fails because kernel numbers diminish. Westgate and Boyer (1985) found a result similar to that of McPherson and Boyer (1977) and Sinclair (1990), but also tested earlier phases around the time of pollination. The early phases were unable to access plant reserves sufficiently to maintain reproductive development when current photosynthesis was inhibited by limited water. And unlike the situation during grain filling, a larger pool of reserves in the plant as a whole did not help to maintain ovary growth (Schussler and Westgate, 1994). Kernel number decreased irreversibly, i.e. abortion occurred. These results indicate that the response to a water deficit early in development (i.e. during pollination) differs from the response later in development (i.e. during grain filling). Large agricultural losses can occur during both phases, but the irreversibility of the early events is especially damaging.