THE INTERACTION OF HIGH-TEMPERATURE AND ELEVATED CO2 ON PHOTOSYNTHETIC ACCLIMATION OF SINGLE LEAVES OF RICE IN-SITU

Rice (Oryza sativa L. cv. IR72) was grown at three different CO2 conce ntrations (ambient, ambient + 200 mu mol mol(-1), ambient + 300 mu mol mol(-1)) at two different growth temperatures (ambient, ambient + 4 d egrees C) from sowing to maturity to determine longterm photosynthetic acclimation to elevated CO2 with and without increasing temperature. Single leaves of rice showed a cooperative enhancement of photosynthet ic rate with elevated CO2 and temperature during tillering, relative t o the elevated CO2 condition alone. However, after flowering, the degr ee of photosynthetic stimulation by elevated CO2 was reduced for the a mbient + 4 degrees C treatment, This increasing insensitivity to CO2 a ppeared to be accompanied by a reduction in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity and/or concentration as evide nced by the reduction in the assimilation (A) to internal CO2, C-1) re sponse curve. The reproductive response (e.g. percent filled grains, p anicle weight) was reduced at the higher growth temperature and presum ably reflects a greater increase in floral sterility. Results indicate that while CO2 and temperature could act synergistically at the bioch emical level, the direct effect of temperature on floral development w ith a subsequent reduction in carbon utilization may change sink stren gth so as to limit photosynthetic stimulation by elevated CO2, concent ration.