Orchard Management in Sustainable Fruit Production

Sunburn of apple fruit is a surface injury caused by solar radiation that in the initial phase results in a light corky layer, golden or bronze discolouration, and other injuries to the epidermal tissue on the surface exposed to the sun. According to the definition of Retig and Kedar (1967), sunburn is a physiological injury that significantly affects fruit quality. Meteorological elements, the variety and the physiological condition of the plant may all play a role in sunburn. The damage occurs mainly in the surface and sub-surface layers. American apple growers divide the phenomenon into three types: "sunburn", "sunscald" and "delayed sunscald". Previously, Walker (1952, 1957) had used "sunscald" to describe all injuries caused by solar radiation. On the basis of cause, Schrader et al. (2001) proposed two main types of sunburn injuries. The first is "sunburn necrosis", necrosis of the epidermal and sub-epidermal cells caused by heat. This phenomenon causes spotting on the sunlit side of the fruit. The second type is "sunburn browning", which causes yellowish or brownish patches on the sunlit side of the fruit. Schrader et al. (2001) also determined the physiological reasons for the two phenomena. Sunburn necrosis happens when the fruit surface reaches 52±1oC, which damages the permeability of cell membranes. Sunburn browning happens when the fruit surface reaches 46o to 49o C, but sunlight also plays a decisive role in its formation. In sunburn browning, the membranes of the surface cells are injured to a lesser degree (Schrader et al., 2001; 2003).