Sunburn can be caused either by heat stress or by UV-B radiation. Under natural conditions fruit sunburn is usually caused by the combination of both heat stress and UV-B radiation. To evaluate the use of reflectance spectroscopy in early detection of fruit sunburn, 5-year-old `Fuji' apple bearing trees growing in pot-in-pot system at the Lewis-Brown Horticulture Farm of Oregon State Univ. were used in the experiment. Fruit sunburn was monitored either under natural conditions or treated with UV-B, heat or both under controlled conditions after detached from the tree. Under natural conditions, the sun-exposed side of Fuji fruit has much higher anthocyanins than the shaded side. The increase in anthocyanins at the sun-exposed side is to protect the fruit from sunburn. When the temperature higher than 40 °C the sunburn symptom initiated first by change the color of the sun-exposed side from red to tan, then to yellow patches, and finally turn to brown and dark brown patches. The peel pigments analysis results indicated that anthocyanins decreased earlier than chlorophyll in the symptom development. The results of detached fruit exposure to 30,000μW/cm2 UV-B lights or to 40 °C at control conditions for 2, 4, 6, 8, 10, and 12 hours indicated that high intensity UV-B lights or high temperature alone can cause sunburn respectively. The UV-B and temperature combination treatment enhanced the sunburn processes. The apples with sunburn caused under natural conditions or under UV-B and temperature controlled conditions were scanned by using reflectance spectroscopy of FOSS NIR system at different stage of the sunburn. The results indicted that Fuji apple sunburn can be efficiently detected at the early stage of sunburn in both natural and controlled conditions.
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