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.
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