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

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Abstract
The most heat-sensitive functions of chloroplasts in S. oleracea L. including the stromal carboxylation reaction, the light-induced electrical field gradient across the thylakoid membrane, as well as the overall photosynthetic CO2 fixation were less affected by heat if chloroplasts were heated in the light; 50% inactivation occurred around 35.degree. C in the dark and around 40.degree. C in the light. Relative low light intensities were sufficient to obtain optimal protection against heat. The light-induced .DELTA.pH across the thylakoid membrane, the photophosphorylation and the photochemical activity of photosystem [PS] II which were less sensitive to heat in the dark (50% inactivation above 40.degree. C) were not protected by light. PS II 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.degree. C in the dark and at 38.degree. 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.degree. C in the dark), light had no further protecting effect. Protection by light was discussed in terms of light-induced transport of protons from the stroma to the thylakoid space and related ion fluxes.