Post-illumination CO2 Exchange and Light-induced CO2 Bursts during C4 Photosynthesis

Detailed kinetics of the post-illumination CO2 exchange, and darklight transients following post-illumination exchange, were measured in leaves of Sorghum bicolor , a NADP-malic enzyme (NADP-ME), and Amaranthus cruentus , a NAD-malic enzyme (NAD-ME) type C4 plant using a gas system that has a full-response time of 3.5 s. The amount of CO2 fixed in the dark (assimilatory charge, AC ) was up to 200 µmol m-2 for A. cruentus and 350-450 µmol m-2 for S. bicolor . AC was at its maximum value at CO2 concentrations close to the inflection of the CO2 response curve, and decreased when photosynthesis was limited by low light intensity. The kinetics of post-illumination CO2 fixation indicate that the rate of carboxylation in the C4 cycle is limited by the supply of phospho enol pyruvate. In A. cruentus , under saturating CO2 the post-illumination CO2 uptake was replaced by a burst (68 µmol m-2). In S. bicolor , the dark-light induction commenced with a rapid CO2 burst (less than 5 s) of 46 µmol m-2, followed by a gulp. The observed CO2 transients show imbalances in the C4 and C3 cycles. In S. bicolor the lack of a post-illumination burst, and the presence of the light- induced CO2 burst is taken as evidence for strict coupling of malate decarboxylation to PGA reduction in NADP-ME species; the opposite response in A. cruentus indicates the lack of strict coupling between the C4 and C3 cycle in NAD-ME species.