Effects of high temperature coupled with high light on the balance between photooxidation and photoprotection in the sun-exposed peel of apple

The sun-exposed peel of 'Gala' apple with or without sunburn was compared in terms of photooxidation and photoprotection, and a controlled experiment was conducted to probe the initial responses of PSII to high light and high temperature. The content of carotenoids, lutein and xanthophylls on a chlorophyll basis was higher in the sunburned peel although they were lower expressed on a peel area basis. Significant loss of beta-carotene and neoxanthin was observed relative to chlorophylls in the sunburned peel. O-2 evolution rates and the activity of key enzymes in the Calvin cycle were lower in the sunburned peel, but the activity of these enzymes decreased to a lesser extent than the O-2 evolution rates. The activity of antioxidant enzymes in the ascorbate-glutathione cycle and the level of total ascorbate, total glutathione, and reduced glutathione were higher in the sunburned peel. However, the sunburned peel had higher H2O2 and malondialdehyde contents. Fruit peels treated with high temperature (45 degrees C) alone showed a clear K step in their chlorophyll fluorescence transients whereas high temperature coupled with high light (1,600 mu mol m(-2) s(-1)) led to the disappearance of the K step and a further decrease in F-V/F-M (similar to what was observed in the sunburned peel). We conclude that high temperature coupled with high light damages the PSII complexes at both the donor and acceptor sides. Although both the xanthophyll cycle and the antioxidant system are up-regulated in response to the photooxidative stress, this up-regulation does not provide enough protection against the photooxidation.