Compartmental modelling of photorespiration and carbon metabolism of water stressed leaves

Abstract

Abstract Carbon fluxes in photosynthesis and photorespiration of water stressed leaves have been analysed in a steady state model based on the ribulose diphosphate carboxylase (RuDP carboxylase) and RuDP oxygenase enzyme activities and the CO2 and O2 concentrations in the leaf. Agreement between predicted and observed photorespiration (Lawlor & Fock, 1975) and C flux in the glycollate pathway is good over much of the range of water stress, but not at severe stress. An alternative source of respiratory CO2 is suggested to explain the discrepancy. The model suggests that resistance to CO2 fixation is mainly in the carboxylation reactions, not in CO2 transport. Using the steady state model, the kinetics of 14C incorporation into photosynthetic and photorespiratory intermediates are simulated. The predicted rate of 14C incorporation is faster than observed and delay terms in the model are used to simulate the slow rates of mixing and metabolic reactions. Inactive pools of glycine and serine are suggested to explain the observed specific activities of glycine and serine. Three models of carbon flux between the glycollate pathway, the photosynthetic carbon reduction cycle and sucrose synthesis are considered. The most satisfactory simulation is for glycollate pathway carbon feeding into the PCR cycle pool of 3-phosphoglyceric acid which provides the carbon for sucrose synthesis. Simulation of the specific activity of CO2 released in photorespiration suggests that a source of unlabelled carbon may contribute to photorespiration.