Although it is well established that maize is vulnerable to drought during flowering and early kernel development, the underlying biology of the response and the genes involved are only weakly understood.  Drought during flowering and early post-pollination decreases kernel set and yield, and a better understanding may help us identify potential targets for future crop improvement.  Studies have indicated that at least part of the response is due to limited photosynthate supply.  In addition, lowered water potential and elevated abscisic acid (ABA) are thought to play roles in the tissue growth and metabolic responses. Profiling of gene transcripts with DNA microarray and related methods has been used to obtain a global view of the regulated gene expression and the physiological basis of the effects, as well as identify specific genes that might be candidates for manipulation in a genetic improvement program. These studies have indicated that distinctly different gene expression occurs in the pedicel versus endosperm of effected kernels.  Several studies indicate that in the highly vascularized pedicels, certain invertase genes are downregulated, while starch is depleted, thus identifying potential candidate genes with involvement in responses related to the decreased photosynthate supply and flux in the region.  Another approach is to identify QTLs related to carbohydrate and ABA levels in the kernel tissues, and to associate these with yield or yield components in stress trials.  These studies have generally indicated that genotypes with elevated ABA have lower yield performance, perhaps due to growth inhibitory functions of the hormone, but also due to its indirect association with the extent of root development. The mechanisms by which carbohydrate levels or flux regulate gene expression remain elusive, perhaps reflecting the complexity of intracellular compartmentation and tissue-specific responses.