REGULATION OF SPIKELET DEVELOPMENT IN RICE BY HORMONES

Four indica rice varieties belonging to two maturity groups were cultivated in medium irrigated land during the wet season of 1989. In all varieties, the primary branches of the panicle developed basipetally. Poor development of spikelets on the basal branches resulted in a high percentage (50-60%) of degeneration and sterility and, consequently, grain yield was poor. Application of gibberellic acid (GA3) and kinetin (6-furfuryl amino purine) improved development and grain yield on all branches. The treatments were more effective on the proximal branches and reduced the existing gradient in growth, development and yield between the distal and proximal branches. In contrast to this, 3-indole acetic acid (IAA) treatment increased spikelet growth and development in the distal branches, but suppressed them in the proximal branches thus increasing the heterogeneity in primary branch growth and development. The soluble carbohydrate and amino acid content per unit dry weight of the primary branches increased at the time of anthesis following the application of growth regulators. The assimilate level declined rapidly at the post-anthesis stage. Associated with the depressed growth and development in the lower branches of the panicle by IAA treatment, was a significantly higher level of assimilates in these branches in the post-anthesis period. The physiological mechanism of spikelet growth regulation is discussed.