HETEROGENEITY OF PRIMARY BRANCH DEVELOPMENT AND SPIKELET SURVIVAL IN RICE PANICLE IN RELATION TO ASSIMILATES OF PRIMARY BRANCHES

Two high yielding semi-dwarf rice varieties namely IR-36 and Jaya, and one traditional tall indica rice variety, Akul, were cultivated in the dry and wet seasons, respectively, in 1987 under irrigated field conditions. A few days after visible panicle initiation, the panicle on the main shoot was dissected out to measure development, growth, soluble carbohydrates, amino acid and phosphate contents of individual primary branches. Primary branch development was quantified by noting the average time to anthesis of spikelets on the branch. The pattern of development was hierarchical with each successive branch dominating the one immediately below it and hence, the measure of development increased in an acropetal fashion from the bottom to the top in all varieties. However, in comparison to the developmentally advanced distal branches, the branches at the proximal region had the potential to generate larger numbers of spikelets, although actual potential to produce grains in them was quite poor. Almost 2 weeks prior to mean anthesis, the gradient in assimilate concentration of the primary branches was found to increase acropetally from bottom to top and correlated negatively with the developmental score (high score indicates less development and vice versa). During the following week, the situation reversed and a higher concentration of assimilates was found in the lower branches than in the upper. Subsequent observations taken before mean anthesis showed assimilate contents to be correlated positively with the developmental score. The role of assimilates in spikelet development on the primary branches has been discussed and it is concluded that there is no clear causative relationship between assimilate concentration and spikelet development. In the post anthesis period, little difference in assimilate concentration of the branches was observed. The presence of a higher concentration of assimilates did not help spikelet survival on the proximal branches any more than that on the distal.