The sun-exposed peel of apple fruit has higher xanthophyll cycle-dependent thermal dissipation and antioxidants of the ascorbate–glutathione pathway than the shaded peel

Abstract

The objective of this study was to determine how xanthophyll cycle-dependent thermal dissipation and the antioxidant system in the peel of apple fruit respond to the natural light exposure within the tree canopy. Fruit from exterior and interior canopies of both mature ‘Gala’ and ‘Smoothee’ apple trees were sampled at noon and/or predawn to measure chlorophyll fluorescence, xanthophyll cycle pool size and composition, and enzymatic and non-enzymatic antioxidants. Compared with the shaded side, the sun-exposed peel of the fruit had more excess absorbed photon flux density (PFD) as a result of a lower photosystem II operating efficiency and a higher incident PFD at noon. The efficiency of excitation transfer was lower in the sun-exposed peel than in the shaded peel, indicative of higher thermal dissipation. The sun-exposed peel had a larger xanthophyll cycle pool size and a higher conversion state. It also had higher activities of ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase, and a larger size and a higher reduction state of the ascorbate pool and the glutathione pool. However, catalase activity was lower in the sun-exposed peel than in the shaded peel. Superoxide dismutase did not show significant trend with regard to fruit peel type or position in the canopy. We conclude that both the xanthophyll cycle and the ascorbate–glutathione pathway in the apple fruit peel are acclimated to the prevailing light exposure within the tree canopy to meet the respective needs for dissipating excess absorbed PFD and detoxifying reactive oxygen species.