Reversal of heat-induced alterations in photochemical activities in wheat primary leaves.

Chloroplasts isolated from elevated temperature treated 8-day-old continuous-white-light-grown wheat primary leaves lost the ability to photo-oxidize water. Also, the ability of ascorbate to donate electrons to photosystem II declined. However, a significant increase in reduced dichlorophenolindophenol-supported photosystem-I-mediated methylviologen photo-reduction activity was observed. The plants stressed at 45°C and 47°C were subsequently grown at 25°C and the partial photochemical activities were measured in chloroplasts isolated from the plants at 24-h intervals. The post stress alterations observed are (1) a significant restoration of water oxidation capacity in 45°C- and partial restoration in 47°C-treated leaves. Ascorbate-supported photochemical activities recovered more or less in similar fashion; (2) reversal of enhanced photosystem I activity in both 45°C- and 47°C-treated leaves. These results suggest that the restoration in water oxidation capacity is possible in 45°C-treated leaves and is limited by the severity of heat stress in 47°C-treated leaves. Restoration of water oxidation capacity vis-à-vis to the reversal of heat-enhanced photosystem I activity also indicates the existence of possible endogenous control for repair of alterations during the post stress.