Heat stress and photosynthesis: Contributions of the photosynthetic electron transport chain to the decline in carbon dioxide assimilation and effects on Rubisco kinetics

Photosynthesis is inhibited by high temperatures that plants are likely to experience under natural conditions. Several observations have indicated a role for the thylakoid membranes during moderate heat stress. Membrane conductance, measured by the electrochromic shift at 518 nm, demonstrated that the thylakoids have higher conductance at higher temperatures. However, whole leaf ATP levels remained constant. This indicated that the increase in cyclic electron transport was able to maintain an adequate energy gradient. However, the increase in cyclic electron transport led to an oxidation of the stroma as measured by NADP-dependent malate dehydrogenase (NADP-MDH) activation. The decline in NADP-MDH activation was highly correlated with the decline in photosynthesis and Rubisco activity. The decline in Rubisco activity during prolonged heat stress is generally associated with an increase in RuBP levels. However, rapid heating caused an initial decline in RuBP which was subsequently followed by a decline in Rubisco carbamylation. The ratio of RuBP to Rubisco sites declined from a saturating concentration to a sub-saturating concentration, providing a possible mechanism for the decarbamylation of Rubisco. Measurements of triose-phosphate levels and NADP-MDH activity further indicated that the regeneration of RuBP by the Calvin cycle was limited by the availability of NADPH.; Rubisco activity is strongly correlated with the decline in photosynthesis during moderate heat stress. This led to the suggestion that activase is the limitation during moderate heat stress. Rubisco activase activates Rubisco by removing inhibitors from Rubisco's catalytic site. Measurements of Rubisco fallover demonstrated that heat stress relieved the inhibition of Rubisco by bound inhibitors. Further, the rate of activation of Rubisco complexed with XuBP was greater at higher temperatures. This led to the conclusion that Rubisco's catalytic site becomes looser at higher temperatures, which allows bound inhibitors to dissociate more readily. It was concluded that the effects of heat stress on photosynthesis were primarily on electron transport and effects on Rubisco were secondary.