EFFECTS OF O2 AND CO2 ON NONSTEADY-STATE PHOTOSYNTHESIS - FURTHER EVIDENCE FOR RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE LIMITATION

The effects of CO2 and O2 on nonsteady-state photosynthesis following an increase in photosynthetic photon flux density (PPFD) were examined in Spinacia oleracea to investigate the hypotheses that (a) a slow ex ponential phase (the ribulose-1,5-bisphosphate carboxylase/oxygenase [ Rubisco] phase) of nonsteady-state photosynthesis is primarily limited by Rubisco activity and (b) Rubisco activation involves two sequentia l, light-dependent processes as described in a previous study (I.E. Wo odrow, K.A. Mott [1992] Plant Physiol 99: 298-303). Photosynthesis was found to be sensitive to O2 during the Rubisco phase in the approach of photosynthesis to steady state. Analyses of this sensitivity to O2 showed that the control coefficient for Rubisco was approximately equa l to 1 during this phase, suggesting that Rubisco was the primary limi tation to photosynthesis. O2 had almost no effect on the kinetics (des cribed using a relaxation time, tau) of the Rubisco phase for leaves s tarting in darkness or for leaves starting in low PPFD, but tau was su bstantially higher in the former case. CO2 was found to affect both th e rate of photosynthesis and the magnitude of tau for the Rubisco phas e. The tau value for the Rubisco phase was found to be negatively corr elated with intercellular CO2 concentration (c(i)), and leaves startin g in darkness had higher values of tau at any c(i) than leaves startin g in low PPFD. The effects of CO2 and O2 on the Rubisco phase are cons istent with the existence of two sequential, light-dependent processes in the activation of Rubisco if neither process is sensitive to O2 an d only the second process is sensitive to CO2. The implications of the data for the mechanism of Rubisco activation and for the effects of s tomatal conductance on nonsteady-state photosynthesis are discussed.