A study was carried out using three grain sorghum genotypes which differ in their response to drought, in order to determine the capacity of the leaves to accumulate proline. The genotypes tested were UAT 124 and UAT 152 ( drought resistant), and UAT 30 (drought sensitive). Total leaf water potential and its components and leaf proline content were determined at the end of drought treatment, applied during either of three different phenological stages. Results showed that drought stress increased leaf proline content, but the increase depended on the phenological stage during which the stress was applied and also on the genotype. The differences between genotypes were significant only when drought was applied during the flag leaf stage. The drought resistant genotype UAT 152 showed the largest increase in leaf proline content when water stress was applied during the flag leaf stage and during panicle initiation. Our data support the idea that higher proline accumulation in grain sorghum exposed to water stress acts as a physiological mechanism that confers drought resistance due to its positive effect on leaf turgor maintenance, explained by a reduction in osmotic potential and its contribution to osmotic adjustment.

Key words: drought, osmotic adjustment, proline, Sorghum bicolor (L. Moench), water stress