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.

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Abstract
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 resistant than in drought-susceptible genotypes. There was genotypic variation in free and conjugated IAA concentration in leaves of stressed plants, but these concentrations were not directly related to percent RGY. Nevertheless, high levels of free and conjugated IAA were found at some periods in leaves of drought-susceptible genotypes. The positive relationship between free AbA concentration and percent RGY was confirmed in a nine genotype comparison. Mean leaf AbA concentrations during flowering and early grain filling in drought-stressed plants were found to be a significantly correlated (r = 0.86**) with percent RGY. It is concluded that it is possible to evaluate genotype drought resistance to a given stress treatment in sorghum by examination of AbA, PA and IAA concentations in leaves. The potential of the method as a tool for plant breeders is discussed.Key words: Sorghum bicolor, drought stress, abscisic acid, phaseic acid, indole-3-acetic acid, yield