The effect of high UV-B dosage on apple fruit photosystems at different fruit maturity stages

- High dosage UV-B stress does not cause photodamage in sun-exposed apple fruit peel.
- The UV-B stress did cause photodamage in shaded apple peel.
- Apple peel photosystems were most responsive to UV-B at the juvenile stage.
- The PS II light use efficiency and oxygen evolution decrease with fruit maturity.

Apple fruits (Malus domestica Borkh) of the cultivars Granny Smith, Fuji, Cripp's Pink, Braeburn, Golden Delicious and Topred were harvested at three stages during fruit growth. Previously sun-exposed peels of the apple fruits were exposed to a high ultraviolet radiation-B (UV-B) dosage for 150 min at each stage. In a second experiment mature 'Granny Smith', 'Fuji' and 'Cripps' Pink' fruits previously sun or shade exposed were also exposed to the UV-B stress. The effect of UV-B stress on fruits photosystem components was assessed by measuring the change in maximum light use efficiency and light reflection of fruit peels. UV-B induced pigment changes were analysed for 'Braeburn', 'Fuji' and 'Cripps' Pink'. The UV-B stress did not cause photoinhibition to any of the cultivars during fruit growth. However, UV-B stress did cause photoinhibition to previously shaded mature 'Granny Smith' and 'Fuji' fruits. Previously shaded 'Cripps' Pink' fruits were conversely as insensitive to UV-B stress as the previously sun exposed fruits. 'Braeburn' showed no major pigment response to UV-B stress throughout the season. However, in 'Fuji' and 'Cripps' Pink' fruits, total phenolic content increased at mid-season and maturity, while decreasing at the juvenile stage. All cultivars appear to have a stronger light reflection response to UV-B stress at the juvenile stage than later in the season. Photosystem II (PS II) units (as indicated by the Fm values) and the oxygen evolving complex activity (as indicated by the Fv values) in all the cultivars decreased with fruit maturity. Shaded 'Cripps' Pink' fruits seemed to use the xanthophyll cycle as a photoprotective mechanism after UV-B stress. Photosynthetic systems in sun-exposed, therefore acclimatised, apple fruit peel are possibly not sensitive to UV-B stress in isolation. The fruits are probably well screened against UV light. Conversely, shaded peel may be less adapted and therefore more sensitive to high UV-B exposure. The light reflection response to UV-B stress at the juvenile stage could be due to the reduced phenolic content after stress and the presence of more PS II units at this stage compared to the mature stage.