Growth and carbon utilization in rice plants under conditions of physiologically low temperature and irradiance

The effects of low temperature and irradiance on growth, N allocation, and the diurnal turnover of carbo-hydrates in rice plants ( Oryza sativa L.) were examined. The plants were grown hydroponically using two growth treatments, i.e. a day/night temperature of 20/18°C under an irradiance of 350 µmol quanta m−2 s−1 and a day/night temperature of 25/20°C under an irradiance of 1000 µmol quanta m−2 s−1. The biomass production decreased remark-ably under conditions of low temperature and low irradiance. This decreased biomass was caused by an extreme decrease in the net assimilation rate. Several adaptive phenomena were observed, similar to those observed in response to low light environments such as the enlargement of leaf area relative to plant mass and preferential N-investment in the leaf blades. In addition, although the diurnal turnover of carbohydrates in the shoots per unit plant mass during the night were greater in the low-temperature/irradiance-grown plants than in the control plants, the amount of daily assimilated photosynthate which is respired by the plant was almost the same during the night. These results indicate that the efficiency with which photosynthates had been used for growth during the night was also enhanced under such conditions. However, they were insufficient to compensate for the decrease in photosynthesis caused by the low temperature and irradiance. The results indicated that a decrease in photosynthesis due to the com-bination of low temperature and low irradiance directly suppresses the growth of the whole plant.