Physiological characterization of high yielding genotypes of rice, Oryza sativa L. in the tropical irrigated system

Breeding programs for new plant type and hybrid are currently the leading strategies for high yielding of rice. This study examined the physiological factors affecting yield potential and grain filling of irrigated lowland rice with the specific aims to 1) compare yield potential among different genotypic groups in dry and wet season rice crop, 2) determine grain filling patterns and related processes and 3) assess the response of rice plants to low radiation. Hybrid rice varieties produced 11% higher grain yield than inbred in dry season due to greater sink size and higher harvest index. New plant type, however, did not out-yield inbred lines due to small particle size. The higher grain yield recorded in dry season than in wet season is attributed to higher radiation during ripening as well as greater biomass accumulation after flowering and harvest index. Genotypic variation in yield stability across dry and wet season was well correlated with variation in grain filling period, not grain filling rate. Daily grain development rates from 10 to 20 days after flowering and from 30 to 35 days after flowering independently contributed to grain yield in dry season and from 20 to 25 days after flowering in wet season. It was observed that low radiation during reproductive and ripening stages significantly reduced grain yield. Rice plants responded to low radiation with increases in height, chlorophyll accumulation, leaf expansion, biomass partitioning to leaves and inhibition in tillering. When exposed to sun, shade-grown plants maintained their photosynthetic potentials although showing increased sensitivity to high light stress. In conclusion, grain filling was a critical process to yield performance of rice and biomass accumulation during ripening stage was a major contributing factor to grain filling. Slow degradation of chlorophyll a content during ripening, low leaf sugar and high grain starch content, as well as longer grain filling period all contributed to better grain filling and higher grain yield of hybrid rice in dry season.