Photosynthesis and ribulose-1,5-bisphosphate carboxylase/oxygenase in rice leaves from emergence through senescence. Quantitative analysis by carboxylation/oxygenation and regeneration of ribulose 1,5-bisphosphate

Changes in gas-exchange rates during the life span of the leaves of rice (Oryza sativa L.) were analyzed quantitatively by measuring changes in the carboxylation/oxygenation and regeneration of ribulose 1,5-bisphosphate (RuBP) at photon fluence rates of 2000 (saturating) and 500 (subsaturating) μmol quanta·m^-2·s^-1 under ambient air conditions. The RuBP levels were always higher than the active-site concentrations of RuBP carboxylase (EC 4.1.1.39), irrespective of the irradiance supplied. Analysis of the CO₂-assimilation rate as a function of intercellular CO₂ concentration indicated that RuBP regeneration does not limit CO₂ assimilation. The estimated RuBP-carboxylase/oxygenase activity in vivo was linearly correlated to the rate of CO₂ assimilation at each level of irradiance. This enzyme activity was just enough to account for the rate of CO₂ assimilation at the saturating irradiance and was 35% more than the rate of CO₂ assimilation at the subsaturating irradiance. Analysis of the assimilation rate at subsaturating irradiance as a function of intercellular CO₂ concentration indicated that a limitation caused by enzyme activation comes into play. The results indicate that the rate of CO₂ assimilation in rice leaves under ambient air conditions is limited during their entire life span by the RuBP-carboxylation/oxygenation capacity.