Abstract

Rate-limiting processes for C₄ photosynthesis were examined in Sorghum bicolor, an NADP-ME type species, and Amaranthus cruentus, an NAD-ME type C₄ species, by studying the kinetics of transient changes in photosynthetic rates following rapid changes in CO₂ or temperature. Primary responses (faster than 15 s) to increasing CO₂ or temperature are considered direct effects on the turnover rate of the C₄ cycle, whereas medium transient changes (2–3 min) are considered due to build-up of C₄ cycle intermediates, and the slowest transient changes (20–30 min) are thought to be related to end product synthesis. Reciprocal plot of carboxylation rates versus cell wall (dissolved) CO₂ concentration (C_w) gives an apparent K_m (CO₂) of 8 µM and a V_m of 200 µmol m^-2 s^-1 for PEP carboxylase, which is about 4 times higher than the maximum rate of photosynthesis. Under strictly limiting CO₂, the rate of PEP carboxylation in C₄ photosynthesis is independent of temperature (20–35°C), suggesting a physical rather than a biochemical limitation. It is suggested that the rates of C₃ and C₄ cycles are coordinated through the pool sizes of the C₄ cycle, which are in equilibrium with the pool of 3-phosphoglyceric acid. At low CO₂, the C₄ pools decrease and are slowly regenerated at elevated CO₂, restricting the CO₂ response of C₄ photosynthesis.