Early detection of sun injury on apples (Malus domestica Borkh.) through the use of crop water stress index and chlorophyll fluorescence

•Sunburn on apples can be predicted using the CWSI index and chlorophyll fluorescence•Both indexes detect early stress-related physiological events in sunburn development•The CWSI predicted severe sunburn as early as 42 days before symptoms appeared•The CWSI increased as sunburn severity increased, with a threshold value of 0.5•Fv/Fm detected sun damage before symptoms appeared at 100 DAFB, threshold value of 0.6

Sun-injury (=sunburn) on apples is an important physiological disorder affecting all cultivars grown in semi-arid climates. Both, high light and elevated temperatures during the fruit-growing season are responsible for the symptoms. Therefore, a crop water stress index (CWSI), calculated from infrared images, and chlorophyll fluorescence were assessed as early predictors, when no symptoms of sun-damage were yet visible. Fifty fruit of each cv. (Royal Gala, Granny Smith, Fuji) were tagged at 50 days after full bloom (DAFB) in different orchards. Infrared images were taken weekly and used for CWSI calculation. A similar number of fruit (Royal Gala, Fuji) were tagged after 80 DAFB for chlorophyll fluorescence measurements carried out every 2 weeks.The proposed CWSI was able to predict the occurrence of severe sunburn symptoms in Granny Smith apples as early as 86 DAFB, 42 days before symptoms were visible. In Fuji, this occurred as early as 122 DAFB, 25 days prior visible symptoms. The threshold value of CWSI required for sunburn development on the fruit was 0.5. The maximum quantum efficiency of photosystem II (Fv/Fm) was also able to detect early sun damage, at 100 DAFB, before symptoms appearance, with a threshold value of 0.6. The results would imply that early events in photooxidative stress leading to sun injury have a profound effect on tissue physiology and biochemistry, which can be assessed using chlorophyll fluorescence and a thermography-based index, such as the CWSI. Both, the CWSI and Fv/Fm ratio could be used as non-destructive, fast and reliably field tools to monitor sun-injury when no symptoms have yet developed.

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