Abstract

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 phosphoenolpyruvate. 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.