Author : Phyo La Pyae Won

Summary : An analysis on historical data and four field experiments at the Zeigler Experiment Station of the International Rice Research Institute (IRRI) were conducted from January 2016 to April 2017 to identify the key yield-limiting factors of short duration rice varieties in the tropics. The study put emphasis on the sink-source relationship and availability of non-structural carbohydrate (NSC) during the reproductive and grain filling stages. The yield level of current short- duration genotypes is low compared with medium-duration genotypes in the current IRRI's irrigated breeding program. Yield superiority in elite short-duration genotypes to PSBRc10 was attributable to biomass accumulation during the grain filling stage (?W) rather than to apparent translocation of assimilates at the flowering to grain (Tr). Low percentage of filled grains due to higher spikelet sterility was observed in short-duration genotypes, particularly in the wet seasons. When PSBRc10, a popular short-duration rice variety, and SL8H, a hybrid rice variety, were compared, the grain filling in PSBRc10 was poorer than SL8H. This could be attributed to the difference in source availability whereas the sink size (spikelets m-2) of PSBRc10 and SL8H were the same but sink capacity (spikelets m-2 x grain weight) of PSBRc10 was smaller than that of SL8H. PSBRc10 had lower content of stem NSC at flowering and lower Tr and ?W. The results suggest source limitation is likely a possible reason for poor grain filling of PSBRc10. A sufficient NSC accumulation around heading and photoassimilate supply to the developing spikelets as well as a large sink size will be required for further yield improvement in short-duration varieties in the tropics.