Evidence for the Occurrence of Feedback Inhibition of Photosynthesis in Rice

Abstract

The response of photosynthesis in the flag leaf of rice (Oryza sativa) to elevated CO2 or reduced O2 was investigated relative to other environmental factors using steady-state gas exchange techniques. We found under moderate conditions of temperature and photosynthetic flux density (PFD) (26°C and 700μmol quanta m−2s−1 similar to growth conditions) photosynthesis in the flag leaf of rice during heading and grain filling saturated at near ambient levels of CO2, with a concomitant loss of O2 sensitivity, when a high stomatal conductance was maintained by high humidity (low vapor pressure deficit). Under 18°C there was near complete loss of O2 sensitivity of photosynthesis at normal ambient levels of CO2. This is in contrast to the large enhancement of photosynthesis by supra-atmospheric levels of CO2 and sub-atmospheric levels of O2 by suppression of photorespiration when there is no limitation on utilizing the initial product of CO2 assimilation (triose-P) as predicted from Ribulose-l,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetic properties. Thus, loss of sensitivity to CO2 and O2 has been previously explained as a limitation on utilization of triose-P to synthesize carbohydrates. Under high PFD at 25°C, the rate of photosynthesis in rice declined over a period of hours around midday, while the intercellular levels of CO2 remained constant suggesting a limitation on utilization of photosynthate. Short-term fluctuations in climatic factors including temperature, light and humidity could result in a feedback limitation on photosynthesis in rice which may be exacerbated by rising CO2.