Protection of Photosystem II by Light in Heat-Stressed Pea Leaves

M odulated 685-nm chlorophyll fluorescence was measured under steady-state conditions in pea leaves adapted to m oderate actinic light. W hen the m easurem ents were perform ed after a short exposure to heat (42 °C) in darkness, the maximal (Fm) and steady-state (Fs) fluorescence levels were dram atically and irreversibly quenched whereas the basic fluorescence F0 rem ained unchanged. C oncom itantly , photosynthetic 0 2 evolution was irreversibly inhibited. Analysis o f the fluorescence data suggested tha t the heat treatm ent affected prim arily the PS II reaction center, with the main effect being presum ably an increased therm al dissipation o f the excita­ tion energy transferred to the reaction centers. In contrast, when heat stress was im posed in the presence o f light, the loss o f variable fluorescence (Fm—F0) was much less m arked and was fully reversible. In addition , no inhibition o f in vivo 0 : evolution was observed when the samples heated in the light were recooled at 25 °C. The results indicate tha t light acted as an efficient p ro tector o f PS II against heat injury.