Grain yields with limited water.

Plant reproduction is sensitive to water deficits, especially during the early phases when development may cease irreversibly even though the parent remains alive. Grain numbers decrease because of several developmental changes, especially ovary abortion in maize (Zea mays L.) or pollen sterility in small grains. In maize, the water deficits inhibit photosynthesis, and the decrease in photosynthate flux to the developing organs appears to trigger abortion. Abscisic acid also increases in the parent and may play a role, perhaps by inhibiting photosynthesis through stomatal closure. Recent work indicates that invertase activity is inhibited and starch is diminished in the ovaries or affected pollen. Also, sucrose fed to the stems rescues many of the ovaries otherwise destined to abort. The feeding restores some of the ovary starch and invertase activity. These studies implicate invertase as a limiting enzyme step for grain yields during a water deficit, and transcript profiling with microarrays has identified genes that are up- or down-regulated during water deficit-induced abortion in maize. However, profiling studies to date have not reported changes in invertase or starch synthesizing genes in water-deficient ovaries, perhaps because there were too few sampling times. The ovary rescue with sucrose feeding indicates either that the changes identified in the profiling are of no consequence for inhibiting ovary development or that gene expression reverts to control levels when the sugar stream recovers. Careful documentation of tissue- and developmentally specific gene expression are needed to resolve these issues and link metabolic changes to the decreased sugar flux affecting the reproductive organs.