Drought stress is most damaging to grain yield when it occurs during a 2-week period either side of silk emergence. Stress at this period results in barrenness and reduction in kernels per ear. Reduced kernel set is associated with an increased anthesis-silking interval (ASI), lower ear growth rates, and an increase in kernel abortion, especially near the ear tip. Fifty years of breeding at Pioneer have resulted in significant gains in yield and yield stability, ASI, and staygreen. Preliminary results under moderate drought stress indicate a change in ASI of -14 oCd (vs. °V9 oCd under no stress) per decade of hybrid selection, no change in staygreen when stress was terminal, and a significant increase in leaf rolling. Evaluation of elite pre-commercial Pioneer hybrids under a range of water availability has displayed significant genetic variation for drought tolerance.
To better understand the biochemical pathways and genes that limit yield potential under transient water deficits, we have conducted over 25 mRNA expression-profiling experiments on maize tissues using either high-density oligonucleotide or cDNA microarray platforms. We have focused our efforts on expression patterns in developing ears and kernels exposed to shade, drought, and density stresses. By imposing water deficits around anthesis and analyzing leaf, whole ear, pedicel and seed tissues during the lag phase of kernel development, we have discovered that overall differential gene expression („d 2-fold change) in seeds is typically 2 to 3 times less than in other plant parts under drought. It appears that maternal tissues (leaf, pedicel, and ear) show greater changes in gene expression under drought stress relative to the mostly zygotic tissues of the developing seed. In other experiments we compared gene expression patterns in female reproductive tissues of hybrids that were drought stressed either via a rainout shelter in the field or in buckets in the growth chamber. The exposure of tissues to a more chronic vs. a sudden stress resulted in a reduction in the number of genes showing differential expression, a finding with important implications for interpreting results from the literature and in the future design of experiments. For example, studies using potted plants have shown that sucrose metabolism (e.g., cell wall invertase), starch synthesis (e.g., ADP-glucose pyrophosphorylase) and hormone biology play critical roles in regulating kernel set and abortion. Detailed analyses of these pathways and others are underway using field-grown plants.