Starch synthesizing enzymes and sink strength of grains of contrasting rice cultivars

ADPglucose pyrophosphorylase (AGP) and sucrose synthase (SS) are important enzymes in starch biosynthesis pathway of rice endosperm. In this work we test the hypothesis that difference in individual grain weight among rice cultivars or within a cultivar is directly related to the variation in activities of these enzymes and hormonal manipulation of enzyme activity can enhance grain yield. One potential mechanism for yield increases is that reduction of ethylene concentration at anthesis may improve assimilates partitioning to grains and increases its weight. In the experiment, three indica rice cultivars differing in grain size and weight were grown in the field conditions during the dry season of 2007. Dry matter growth, rate of division of endosperm cells, starch and sugar concentrations as well as AGP and SS activities of endosperm and ethylene evolution of spatially separated developing spikelets of panicle of the three cultivars were measured during the early part of grain filling period. Growth and cell division rates as well as activities of enzymes were higher in a big sized seed compared to a small sized seed in sequel to the difference in either cultivars or position on the panicle axis. Growth and enzyme activities correlated negatively with ethylene concentration. It is concluded that seed weight of inferior spikelets can be improved in rice panicle by increasing activities of starch synthesizing enzymes through manipulation of ethylene production, while cultivar difference in seed weight remains beyond the purview of such manipulations.