Photosynthetic characteristics in rice plants grown under the conditions of low temperature and irradiance.

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In the northern area of Japan, cool weather damage in summer is always caused not only by low temperature but also by a shortage of sunshine. Under such weather conditions, photosynthesis is : strongly suppressed. However, little is known about how growth under such conditions affects photosynthesis at the level of a single leaf and the growth at the level of the whole plant.The photosynthetic characteristics and plant growth were examined in rice plants grown at a day/night temperature of 20/18ﾟC and an irradiance of 350 mutmol quanta m-2 s-1 as the low-temperature/irradiance treatment. The photosynthetic rate measured at 20ﾟC was a little higher in rice grown under low-temperature/irradiance than in rice grown under 25/20ﾟC and 1000 mumol m^<-2> s^<-1>, whereas there was no difference in the photosynthetic rate measured at 25ﾟC between rice grown in these two ways. This difference was suggested to be caused by a slightly enhanced RuBP regeneration capacity in the low-temperature/irradiance-grown plants. The RGR (relative growth rate) were much smaller in the low-temperature/irradiance grown plants, and it was caused by a decreased NAR (netassimilation rate). The LAB (leaf area ratio) was slightly greater. We, thus, concluded that although rice plants are partially able to acclimate to the conditions of low temperature and low irradiance, the suppressed photosynthesis under such conditions directly leads to a decreased biomass production.