Rubisco activase constrains the photosynthetic potential of leaves at high temperature and CO2

Net photosynthesis (Pn) is inhibited by moderate heat stress. To elucidate the mechanism of inhibition, we examined the effects of temperature on gas exchange and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activation in cotton and tobacco leaves and compared the responses to those of the isolated enzymes. Depending on the CO2 concentration, Pn decreased when temperatures exceeded 35-40 degreesC. This response was inconsistent with the response predicted from the properties of fully activated Rubisco, Rubisco deactivated in leaves when temperature was increased and also in response to high CO2 or low O-2. The decrease in Rubisco activation occurred when leaf temperatures exceeded 35 degreesC, whereas the activities of isolated activase and Rubisco were highest at 42 degreesC and >50 degreesC, respectively. In the absence of activase, isolated Rubisco deactivated under catalytic conditions and the rate of deactivation increased with temperature but not with CO2, The ability of activase to maintain or promote Rubisco activation in vitro also decreased with temperature but was not affected by CO2, Increasing the activase/Rubisco ratio reduced Rubisco deactivation at higher temperatures. The results indicate that, as temperature increases, the rate of Rubisco deactivation exceeds the capacity of activase to promote activation. The decrease in Rubisco activation that occurred in leaves at high CO2 was not caused by a faster rate of deactivation, but by reduced activase activity possibly in response to unfavorable ATP/ADP ratios. When adjustments were made for changes in activation state, the kinetic properties of Rubisco predicted the response of Pn at high temperature and CO2.