DROUGHT RESISTANCE OF Sorghum bicolor. 5. GENOTYPIC DIFFERENCES IN THE CONCENTRATIONS OF FREE AND CONJUGATED ABSCISIC, PHASEIC AND INDOLE-3-ACETIC ACIDS IN LEAVES OF FIELD-GROWN DROUGHT-STRESSED PLANTS.

Concentrations of free and conjugated abscisic acid (AbA), phaseic acid (PA) and indole-3-acetic acid (IAA) were measured in leaves of sorghum (Sorghum bicolor (L.) Moench) genotypes grown in the field. Hormone levels were compared and related to grain yield stability under drought, expressed as the percentage reduction in grain yield (percent RGY) of drought-stressed compared to irrigated plants. Although hormone concentrations were similar in irrigated plants, there was considerable genotypic variation in drought-stressed plants. In a four genotype comparison during the panicle initiation stage, mean leaf AbA concentrations in drought-stressed plants were positively related to percent RGY. Furthermore, the slopes of regression lines of AbA on leaf water potential in stressed genotypes were also positively related to percent RGY. In contrast, PA and total AbA metabolite concentrations were negatively related to percent RGY, implying a higher efficiency of conversion of AbA to its metabolites in drought r...