EVIDENCE FOR THE OCCURRENCE OF FEEDBACK INHIBITION OF PHOTOSYNTHESIS IN RICE

The response of photosynthesis in the flag leaf of rice (Oryza sativa) to elevated CO2 or reduced O-2 was investigated relative to other env ironmental factors using steady-state gas exchange techniques. We foun d under moderate conditions of temperature and photosynthetic flux den sity (PFD) (26 degrees C and 700 mu mol quanta m(-2)s(-1), similar to growth conditions) photosynthesis in the flag leaf of rice during head ing and grain filling saturated at near ambient levels of CO2, with a concomitant loss of O-2 sensitivity, when a high stomatal conductance was maintained by high humidity (low vapor pressure deficit). Under 18 degrees C there was near complete loss of O-2 sensitivity of photosyn thesis at normal ambient levels of CO2, This is in contrast to the lar ge enhancement of photosynthesis by supra-atmospheric levels of CO2 an d sub-atmospheric levels of O-2 by suppression of photorespiration whe n there is no limitation on utilizing the initial product of CO2 assim ilation (triose-beta) as predicted from Ribulose-1,5-bisphosphate carb oxylase/oxygenase (Rubisco) kinetic properties. Thus, loss of sensitiv ity to CO2 and O-2 has been previously explained as a limitation on ut ilization of triose-beta to synthesize carbohydrates. Under high PFD a t 25 degrees C, the rate of photosynthesis in rice declined over a per iod of hours around midday, while the intercellular levels of CO2 rema ined constant suggesting a limitation on utilization of photosynthate, Short-term fluctuations in climatic factors including temperature, li ght and humidity could result in a feedback limitation on photosynthes is in rice which may be exacerbated by rising CO2.