Effect of heat stress on the inhibition and recovery of the ribulose-1,5-bisphosphate carboxylase/oxygenase activation state

Experiments were conducted to determine the relative contributions of ribul ose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) activatio n state vis-g-vis Rubisco activase and metabolite levels to the inhibition of cotton (Gossypium hirsutum L.) photosynthesis by heat stress. Exposure o f leaf tissue in the light to temperatures of 40 or 45 degreesC decreased t he activation state of Rubisco to levels that were 65 or 10%, respectively, of the 28 degreesC control. Ribulose-1,5-bisphosphate (RuBP) levels increa sed in heat-stressed leaves, whereas the 3-phosphoglyceric acid pool was de pleted. Heat stress did not affect Rubisco per se, as full activity could b e restored by incubation with CO2 and Mg2+. Inhibition and recovery of Rubi sco activation state and carbon dioxide exchange rate (CER) were closely re lated under moderate heat stress (up to 42.5 degreesC). Moderate heat stres s had negligible effect on Fv/Fm, the maximal quantum yield of photosystem II. In contrast, severe heat stress (45 degreesC) caused significant and ir reversible damage to Rubisco activation, CER, and Fv/Fm. The rate of Rubisc o activation after alleviating moderate beat stress was comparable to that of controls, indicating rapid reversibility of the process. However, modera te heat stress decreased both the rate and final extent of CER activation d uring dark-to-light transition. Treatment of cotton leaves with methyl viol ogen or an oxygen-enriched atmosphere reduced the effect of heat stress on Rubisco inactivation. Both treatments also reduced tissue RuBP levels, indi cating that the amount of RuBP present during heat stress may influence the degree of Rubisco inactivation. Under both photorespiratory and non-photor espiratory conditions, the inhibition of the CER during heat stress could b e completely reversed by increasing the internal partial pressure of CO2 (C i). However, the inhibition of the CER by nigericin, a K+ ionophore, was no t reversible when the Ci was increased at ambient or high temperature. Our results indicate that inhibition of photosynthesis by moderate heat stress is not caused by inhibition of the capacity for RuBP regeneration. We concl ude that heat stress inhibits Rubisco activation via a rapid and direct eff ect on Rubisco activase, possibly by perturbing Rubisco activase subunit in teractions with each other or with Rubisco.