Effects of photoinhibitory treatment on CO2 assimilation, the quantum yield of CO2 assimilation, D1 protein, ascorbate, glutathione and xanthophyll contents and the electron transport rate in vine leaves

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 /umol m^?2 s^?1) or low temperature (5 °C) alone had no measurable effect on the photosynthetic processes, but 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 15 h exposure to 5°C at high light, the F_v/sb/F_mratio had decreased by 80% and the apparent quantum yield by 75%. Nevertheless, when the leaves were returned to low light at 22°C, these parameters recovered rapidly. The foliar pools of ascorbate and glutathione decreased in the first hours of photoinhibitory 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 F_v/F_m ratio during both photoinhibition and recovery. However, there was also a good correlation between the decrease in theF_v F_m ratio 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 F_v/F_m may not simply be related to zeaxanthin levels or to amounts of D₁ protein alone but result from multifactoral influences.