Influence of Light on the Heat Sensitivity of the Photosynthetic Apparatus in Isolated Spinach Chloroplasts

The most heat-sensitive functions of chloroplasts in Spinacia oleracea L. including the stromal carboxylation reaction, the light-induced electrical field gradient across the thylakoid membrane, as well as the overall photosynthetic CO(2) fixation were less affected by heat if chloroplasts were heated in the light: 50% inactivation occurred around 35 degrees C in the dark and around 40 degrees C in the light. Relative low light intensities were sufficient to obtain optimal protection against heat. In contrast, the light-induced DeltapH across the thylakoid membrane, the photophosphorylation, and the photochemical activity of photosystem II which were less sensitive to heat in the dark (50% inactivation above 40 degrees C) were not protected by light. Photosystem II even was destabilized somewhat by light.The effect of light on the heat sensitivity of the water-splitting reaction was dependent on the pH in the medium. Protection by light only occurred at alkaline pH, in which case heat sensitivity was high (50% inactivation at 33 degrees C in the dark and at 38 degrees C in the light). Protection was prevented by uncouplers. At pH 6.8 when the heat sensitivity was low in any case (50% inactivation at 41 degrees C in the dark), light had no further protecting effect.Protection by light has been discussed in terms of light-induced transport of protons from the stroma to the thylakoid space and related ion fluxes.