Rubisco activase constrains the photosynthetic potential of leaves at hightemperature 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) acti vation 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, Rubi sco deactivated in leaves when temperature was increased and also in respon se 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, respect ively. In the absence of activase, isolated Rubisco deactivated under catal ytic conditions and the rate of deactivation increased with temperature but not with CO2, The ability of activase to maintain or promote Rubisco activ ation in vitro also decreased with temperature but was not affected by CO2, Increasing the activase/Rubisco ratio reduced Rubisco deactivation at high er temperatures. The results indicate that, as temperature increases, the r ate of Rubisco deactivation exceeds the capacity of activase to promote act ivation. The decrease in Rubisco activation that occurred in leaves at high CO2 was not caused by a faster rate of deactivation, but by reduced activa se activity possibly in response to unfavorable ATP/ADP ratios. When adjust ments were made for changes in activation state, the kinetic properties of Rubisco predicted the response of Pn at high temperature and CO2.