RECOVERY OF NET CO2 ASSIMILATION AFTER HEAT-STRESS IS CORRELATED WITHRECOVERY OF OXYGEN-EVOLVING-COMPLEX PROTEINS IN ZEA-MAYS L

Photosystem 2 (PS2) in general, and the oxygen-evolving complex (OEC) in particular, is one of the most thermolabile components of photosynt hesis, We examined the effects of heat stress on net photosynthetic ra te (P-N) and content of several stromal and thylakoid-membrane protein s (including OEC proteins) in maize (Zea mays L.) in order to determin e if decreases in P-N during, and especially after, heat stress were c orrelated with decreases in the content of OEC proteins, The P-N decre ased with heat stress in maize, and post-heat stress recovery of P-N r equired 4 d following the second of two heat-shocks. The decrease in P -N was not the result of stomatal closure. Cellular levels of the 33, 23, and 16 kDa OEC proteins decreased with heat stress, and the decrea ses were greatest and most closely correlated with decreases in P-N fo r OEC16. Following the second heat stress, full recovery of OEC levels (especially OEC 16 and 33) coincided with full recovery of P-N, more so than with other photosynthetic proteins examined. For example, decr eases in levels of the 32-kDa Q(B)-binding protein of the PS2 reaction center (D1), ribulose-1,5-bisphosphate carboxylase/oxygenase large su bunit, and phosphoenolpyruvate carboxylase were generally smaller than for the OEC proteins and full recovery of these proteins occurred at least 2 d prior to full recovery of photosynthesis, These results are consistent with previous fluorescence and in vitro studies by others i n suggesting that heat-related effects on PS2 and the OEC are an impor tant limitation to P-N during heat stress. Additionally, these results suggest that heat-related decreases in the content of OEC proteins ma y limit post-heat stress recovery of carbon fixation.