Photo-oxidative sunscald of apple: Effects of temperature and light on fruit peel photoinhibition, bleaching and short-term tolerance acquisition

Sunscald of apple (Malus×domestica Borkh.) fruit is a major cause for economical losses. It is widely accepted that exposure to high temperature and light increases the incidence and severity of this fruit injury, but the separate contribution of each factor and the mechanisms involved in the injury development in apples are not well understood. The present study aimed at the quantification of both temperature and light required for photo-oxidative sunscald development and for tolerance acquisition, under controlled conditions. In mid-summer, immature green 'Smoothie' apples from the inner (shaded) part of the canopy were picked biweekly and placed in the dark for 48h at 25°C for decondidting. Fruit were then exposed to a variety of temeprature and light regimes. Sunscald injury developed only when peel temperature exceeded 40°C and was light-dependent. Maximum tolerance to sunscald was acquired in fruit pre-exposed to 38°C for 24h. This temporary tolerance gradually diminished with storage time at 25°C. Development of brown stains within the bleached peel areas increased with temperature. F_v/F_m was negatively correlated with PFD and the duration of exposure to temperatures above 40°C. Threshold values of F_v/F_m required for bleaching and browning of the peel were 0.11 and 0.072, respectively. A 'photo-destructive quantum' (PDQ) was characterized as the minimum absorbed energy required for peel bleaching. The calculated PDQ diminished with the rise in temperature. Pre-treatment with 5 and 10mgL^-1 methyl viologen increased fruit susceptibility to sunscald compared to controls, whereas 1mgL^-1 increased fruit tolerance to the combined injurious effect of light and heat. We conclude that sunscald is an expression of photo-oxidative stress in the fruit peel which is facilitated by peel temperatures above 40°C. Short-term field tolerance is acquired by insolation at sub-injurious temperatures, but this tolerance is rather weak and cannot accommodate the harsh atmospheric conditions which prevail in the Israeli summer.