Sunburn control by nets differentially affects the antioxidant properties of fruit peel in ‘Gala’ and ‘Fuji’ apples

Netting has been widely used in recent years to protect apple fruit from solar injury. There is little information on the biochemical aspects underlying the effectiveness of netting against sunburn in apples. The objective of this study was to quantify the effects of netting on polyphenols, anthocyanins, and antioxidant activity in apples with different sunburn degrees in terms of fruit peel color variation. Orchards of ‘Gala Brookfield’ and ‘Fuji Raku Raku’ apples were covered in red and pearl nets (providing 20% shading), respectively. Uncovered trees served as controls for the experiments. Spectral light transmission was measured under the nets from 350 to 1700 nm. Photosynthetically active radiation (PAR), ultraviolet radiation (UV), and fruit surface temperature (FST) were evaluated from 20 to 175 day after full blooming. Red netting increased the amount of red (600–700 nm) and blue (400–500 nm) wavelengths of the visible spectrum. Both red and pearl nets reduced infrared (900–1700 nm) transmission by 8%, which resulted in a decrease around 5 °C in FST. Similarly, PAR radiation was reduced by 25 and 21% on average, while the reduction in UV radiation reached 29 and 25% with red and pearl netting, respectively. In moderate (browning; SB-3) and severe (browning with necrosis; SB-4) damage categories, sunburn decreased 15% in ‘Gala Brookfield’ under red netting, and only 4% in ‘Fuji Raku Raku’ under pearl netting. In ‘Gala Brookfield’, red netting significantly increased anthocyanin content and ferric ion-reducing antioxidant power (FRAP) antioxidant capacity for the SB4 sunburn category. In contrast, pearl netting significantly reduced the amount of polyphenols, anthocyanins, and the FRAP antioxidant capacity in SB-4 sunburn group of ‘Fuji Raku Raku’ apples. Collectively, our results suggest that sun protection provided by netting result in changes in the composition and antioxidant capacity of fruit peels, whose response varies depending on the cultivar and type of net used.