RUBPCO KINETICS AND THE MECHANISM OF CO2 ENTRY IN C3 PLANTS

The CO2 partial pressure in the chloroplasts of intact photosynthetic C3 leaves is thought to be less than the intercellular CO2 partial pressure. The intercellular CO2 partial pressure can be calculated from CO2 and H2O gas exchange measurements, whereas the CO2 partial pressure in the chloroplasts is unknown. The conductance of CO2 from the intercellular space to the chloroplast stroma and the CO2 partial pressure in the chloroplast stroma can be calculated if the properties of photosynthetic gas exchange are compared with the kinetics of the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBPCO). A discrepancy between gas exchange and RuBPCO kinetics can be attributed to a deviation of CO2 partial pressure in the chloroplast stroma from that calculated in the intercellular space. This paper is concerned with the following: estimation of the kinetic constants of RuBPCO and their comparison with the CO2 compensation concentration; their comparison with differential uptake of 14CO2 and 12CO2; and their comparison with O2 dependence of net CO2 uptake of photosynthetic leaves. Discrepancy between RuBPCO kinetics and gas exchange was found at a temperature of 12.5-degrees-C, a photosynthetic photon flux density (PPFD) of 550-mu-mol quanta m-2 s-1, and an ambient CO2 partial pressure of 40 Pa. Consistency between RuBPCO kinetics and gas exchange was found if CO2 partial pressure was decreased, temperature increased and PPFD decreased. The results suggest that a discrepancy between RuBPCO kinetics and gas exchange is due to a diffusion resistance for CO2 across the chloroplast envelope which decreases with increasing temperature. At low CO2 partial pressure, the diffusion resistance appears to be counterbalanced by active CO2 (or HCO3-) transport with high affinity and low maximum velocity. At low PPFD, CO2 partial pressure in the chloroplast stroma appears to be in equilibrium with that in the intercellular space due to low CO2 flux.