CO2 and temperature-dependent induction in C-4 photosynthesis: an approach to the hierarchy of rate-limiting processes

Rate-limiting processes for C-4 photosynthesis were examined in Sorghum bicolor, an NADP-ME type species, and Amaranthus cruentus, an NAD-ME type C-4 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 C-4 cycle, whereas medium transient changes (2-3 min) are considered due to build-up of C-4 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 mu M and a V-m of 200 mu mol 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 C-4 photosynthesis is independent of temperature (20-35 degrees C), suggesting a physical rather than a biochemical limitation. It is suggested that the rates of C-3 and C-4 cycles are coordinated through the pool sizes of the C-4 cycle, which are in equilibrium with the pool of 3-phosphoglyceric acid. At low CO2, the C-4 pools decrease and are slowly regenerated at elevated CO2, restricting the CO2 response of C-4 photosynthesis.