CO2 and Temperature-dependent Induction in C4 Photosynthesis: an Approach to the Hierarchy of Rate-limiting Processes

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 (Cw) gives an apparent Km (CO2) of 8 µM and a Vm 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.