Photosynthetic Efficiency of Apples under Protected Shade Nets
Sunburn in apples is caused by excessive solar radiation and high temperatures. Shade netting is used to control sun damage in apple (Malus domestica Borkh.) orchards. The physiological aspects of the effectiveness of this technique have not yet been fully understood. The objective was to study the variation in chlorophyll fluorescence in apples exposed to environmental conditions of radiation and temperature under netting, to better understand the physiological aspects in the effectiveness of this technique to control sun damage. From 50 and up to 150 d after flowering (DAF), maximum photochemical quantum efficiency of photosystem II (Fv/Fm), incidence of photosynthetically active radiation (PAR, μmol m-2 s-1 ), and fruit surface temperature (FST, °C) were evaluated in netted and non-netted ‘Fuji’ and ‘Gala’ apple orchards with 20% shade netting, as well as at harvest in fruit picked from the tree. Netting reduced sunburn by 9% for ‘Gala’ and 5% for ‘Fuji’. For ‘Gala’, netting reduced FST from 2 to 5 °C and 2 to 3 °C for ‘Fuji’ compared to the control (P < 0.05). The PAR decreased by 27% for ‘Gala’ and 22% for ‘Fuji’ compared to the control (P < 0.05). The mean value of Fv/Fm was 12% in ‘Gala’ and 3% in ‘Fuji’ lower than under netting. This difference in Fv/Fm was maintained in fruit picked from the tree and was 13% in ‘Gala’ and 8% in ‘Fuji’. The effectiveness of nets as a measure to control sun damage in apples is related to the ability of this technique to reduce the photo-inhibition process on the fruit skin.