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The responses of photosynthesis to high light and low temperature were studied in vines cultivated in the greenhouse in low light. Exposure to high light (1000//mol m~^ s"') or low temperature (5 °C) alone had no measurable effect on the photosynthetic processes, hut the combination of high light and low temperature caused rapid loss of photosynthetic capacity and a decrease in the efficiency of photosynthetic energy conversion. After a 15h exposure to 5°C at high light, the FJFm ratio had decreased hy 80% and the apparent quantum yield hy 75%. Nevertheless, when the leaves were returned to low light at 22 °C, these parameters recovered rapidly. The foliar pools of ascorhate and glutathione decreased in the first hours of photoinhihitory treatment while the zeaxanthin content increased from negligible levels to about 50% of the total foliar xanthophyll pool. There was a clear correlation between the zeaxanthin content of the leaves and their FJF^ ratio during both photoinhibition and recovery. However, there was also a good correlation between the decrease in the F^Fr atio and the measured decrease in the total foliar levels of the antioxidants ascorbate and glutathione. The amount of D, protein diminished over the same period as the zeaxanthin levels were increasing. This approach, involving simultaneous measurements of several parameters considered to influence photosystemy II activity, clearly demonstrates that measured decreases in FJF^ may not simply be related to zeaxanthin levels or to amounts of D, protein alone but result from multifactoral influences. 1358 assimilation measured in air; ^^pp, apparent quantum yield of CO2 assimilation rate.

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