High temperate effects on crop and grain growth of four rice cultivars

High temperature (chambers) reduced days to flowering and days to harvest more in early sowing (January) than late sowing (March). Later flowering with late planting increased tiller production, leaf area index, dry matter, and crop growth rate, and reduced panicle weight. In the phytotron, temperatures above 33/25 deg C reduced panicle and grain weight, increased the shoot dry matter, and shoot:root ratio. The fraction of unfilled spikelets was directly related to temperature hence, high temperature reduced grain yield. N22 was more tolerant to high temperatures than the other genotypes. At 41/33 deg C the grain filling period was 7 days for temperature sensitive genotypes and 14 days for N22. Starch content in the grains and apparent contribution of N22 were 40% and 25% greater, respectively, at 41/33 deg C than the other genotypes. Apparent translocation of N22 was, however, lower than the other genotypes at 41/33 deg C indicating more assimilate translocation to the grains than to straw as compared to the other genotypes. Nitrogen and chlorophyll contents of all genotypes increased with increasing temperature above 33/25 deg C. Net CO2 assimilation showed a similar direct relationship with temperature. Net assimilation per unit leaf area was strongly associated with N assimilation in the leaves. Net CO2 assimilation of N22 was lower than that of the other genotypes. Respiration of N22 was inversely proportional with temperature and was less than that of the other genotypes. Net leaf CO2 assimilation increased with increase of light levels, whereas the leaf intercellular CO2 decreased and leaf conductance at 29/21 deg C and 33/25 deg C increased. However, above 33/25 deg C, the response of the leaf conductance to light became less. Light saturation for net leaf CO2 assimilation was 580 MJm raised to the negative 2 power for all genotypes. The net leaf CO2 assimilation at 41/33 deg C was twice that of the control (29/21 deg C) for all genotypes