RuBPCO kinetics and the mechanism of CO₂ entry in C₃ plants

Abstract

Abstract. The CO 2 partial pressure in the chloroplasts of intact photosynthetic C 3 leaves is thought to be less than the intercellular CO 2 partial pressure. The intercellular CO 2 partial pressure can be calculated from CO 2 and H 2 O gas exchange measurements, whereas the CO 2 partial pressure in the chloroplasts is unknown. The conductance of CO 2 from the intercellular space to the chloroplast stroma and the CO 2 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 CO 2 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 CO 2 compensation concentration; their comparison with differential uptake of 14 CO 2 and 12 CO 2 ; and their comparison with O 2 dependence of net CO 2 uptake of photosynthetic leaves. Discrepancy between RuBPCO kinetics and gas exchange was found at a temperature of 12.5 °C, a photosynthetic photon flux density (PPFD) of 550 μmol quanta m −2 s −1 , and an ambient CO 2 partial pressure of 40 Pa. Consistency between RuBPCO kinetics and gas exchange was found if CO 2 partial pressure was decreased, temperature incresed and PPFD decreased. The results suggest that a discrepancy between RuBPCO kinetics and gas exchange is due to a diffusion resistance for CO 2 across the chloroplast envelope which decreases with increasing temperature. At low CO 2 partial pressure, the diffusion resistance appears to be counterbalanced by active CO 2 (or HCO 3 ) transport with high affinity and low maximum velocity. At low PPFD, CO 2 partial pressure in the chloroplast stroma appears to be in equilibrium with that in the intercellular space due to low CO 2 flux.