POST-ILLUMINATION CO2 EXCHANGE AND LIGHT-INDUCED CO2 BURSTS DURING C-4 PHOTOSYNTHESIS

Detailed kinetics of the post-illumination CO2 exchange, and dark-ligh t transients following post-illumination exchange, were Measured in le aves of Sorghum bicolor, a NADP-malic enzyme (NADP-ME), and Amaranthus cruentus, a NAD-malic enzyme (NAD-ME) type C-4 plant using a gas syst em 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 mu mol m(-2) for A. c ruentus and 350-450 mu mol m(-2) for S. bicolor. AC was at its maximum value at CO2 concentrations close to the inflection of the CO2 respon se curve, and decreased when photosynthesis was limited by low light i ntensity. The kinetics of post-illumination CO2 fixation indicate that the rate of carboxylation in the C-4 cycle is limited by the supply o f phosphoenolpyruvate. In A. cruentus, under saturating CO2 the post-i llumination CO2 uptake was replaced by a burst (68 mu mol m(-2)). In S . bicolor, the dark-light induction commenced with a rapid CO2 burst ( less than 5 s) of 415 mu mol m(-2), followed by a gulp. The observed C O2 transients show imbalances in the C-4 and C-3 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 d ecarboxylation to PGA reduction in NADP-ME species; the opposite respo nse in A. cruentus indicates the lack of strict coupling between the C -4 and C-3 cycle in NAD-ME species.