Antagonistic Effects of Red and Far‐red Lights on the Stability of Photosystem Ii in Pea Leaves Exposed to Heat

The presence of light during exposure of intact pea leaves to high temperature (40°C) protects Photosystem II (PSII) against inactivation, as indicated by the preservation of the maximal variable 685 nm chlorophyll fluorescence and the photosynthetic oxygen evolution. This photoprotection was observed (i) to be saturated at low fluence rates (ca 10 W m‐2) and (ii) to be strongly dependent on the spectral characteristics of the light. It was specifically induced by red light (630–670 nm) whereas other wavelengths were much less protective. A strong antagonism between red and far‐red lights was also observed, with PSII stabilization by red light being partially cancelled by additional far‐red light.