Maize Response to Drought At Flowering.

Water deficit during the flowering and post-pollination phases severely affects kernel set and yield.  In maize, yield in drought environments has been improved by selecting for genotypes with better ability to sustain floral growth and development during stress.  The available evidence suggests that sensitivity to drought at this stage is regulated by response to kernel carbohydrate supply, and its interaction with abscisic acid (ABA).  However, the basis of genotypic differences in this response is not known.  To gain improved understanding of these differences, we have examined the levels of carbohydrate and ABA, and gene expression, in maize hybrids with contrasting tolerance to drought at flowering.  From a diverse set of 400 tropical maize hybrids that were previously tested in multi-location field trials under well-irrigated control and water-stress at flowering, a set of 3 drought-susceptible and 3 tolerant hybrids were selected that yield similarly in control conditions but are susceptible or tolerant of stress at flowering, as indicated by anthesis silking interval (ASI) and yield.  In greenhouse trials, we subjected potted plants to water stress for 5 days during silking; pollination was prevented by bagging ears.  Ear tips were analyzed for metabolite levels and gene expression by oligo-nucleotide microarray slides (Agilent).  Stress decreased grain dry weight to a much greater extent in susceptible than tolerant ears.  Stress increased ABA levels similarly in all lines, and ABA-glucose ester, phaseic acid and glucose were not differentially affected, but sucrose levels were decreased more in susceptible than tolerant lines.  The expression profiles in eartips showed that the susceptible lines had relatively higher transcript levels for stress genes (dehydrins, ethylene metabolism, and stress transcription factors) whereas tolerant lines had higher levels of growth-associated genes.  These studies support the hypothesis that better yield performance is related to maintenance of growth-related processes in the face of stress  rather than ability to have a high expression of stress-stabilizing gene-products.