The Mediterranean-type climate apple production areas of the Western Cape of South Africa are subject to high irradiation and high summer temperatures. These environmental conditions are conducive to the development of sunburn (Schrader et al., 2003; Van den Ende, 1999), resulting in the downgrading of fruit and lower income for producers.
Almost all cultivars are susceptible to sunburn, although some are seemingly more sensitive than others. Of the cultivars grown in South Africa, Granny Smith seems to be particularly sensitive to sunburn, Golden Delicious, Braeburn, Fuji, and Delicious are less sensitive while Cripps' Pink is least sensitive (Racsko and Schrader, 2012 and references therein). Schrader et al. (2001) demonstrated that threshold temperature for sunburn development is cultivar dependent and ranged from 47 °C to 51 °C. ‘Delicious’ and ‘Jonagold’ had the highest and lowest thresholds, respectively, while ‘Gala’, ‘Fuji’ ‘Braeburn’, and ‘Golden Delicious’ were intermediate. This differential susceptibility could be a function of the physiochemical properties of fruit peel (Wünsche et al., 2004), but may also relate to indirect factors that may modulate the occurrence of sunburn. This could include factors such as when during the season the fruit ripens, the tree architecture and bearing habit, and the accumulation of anthocyanin in the peel toward harvest that may mask the visible symptoms of sunburn (Evans, 2004; Racsko and Schrader, 2012; Schrader et al., 2001). Masking of sunburn by anthocyanin in blushed and red cultivars may result in the misconception of cultivar susceptibility to sunburn.
As anthocyanins mostly accumulate in the epidermal and hypodermal tissue of apple peel (Gross, 1987), red color development may conceal the two superficial forms of sunburn viz sunburn browning and photo-oxidative bleaching. These two types of sunburn entail the loss of chlorophyll and accumulation of phenolic compounds and carotenoids (in the case of browning) in the hypodermis (Felicetti and Schrader, 2009; Wünsche et al., 2001). Sunburn necrosis, however, is not hidden by red color development, as it results in the death of epidermal and sub-epidermal tissue, causing the formation of sunken, brown to black necrotic spots on the affected fruit area (Schrader et al., 2003). The ability of apple fruit to produce anthocyanin during ripening differs among cultivars (Curry, 1997; Saure, 1990). The logical inference is that the more intense the pigmentation, the less severe the sunburn browning and bleaching on an affected fruit is likely to appear, irrespective of the underlying damage to the peel.
Although red color development may mask underlying sunburn symptoms, some sources argue that the accumulation of additional pigment should increase sensitivity to sunburn. Woolf and Ferguson (2000) reasoned that dark pigmented fruit would attain higher temperatures than green pigmented fruit under the same conditions. Anthocyanin accumulation makes the fruit darker, reducing the reflective capacity (albedo) (Evans 2004; Schroeder, 1965; Smart and Sinclair, 1976). The resultant increased radiant heating due to low albedo, increases FST (Smart and Sinclair, 1976). Considering the involvement of high temperature in the development of sunburn (Schrader et al., 2008), increased FSTs should increase the risk of sunburn. In agreement, Hetherington (1997) reported increased sunburn in purple compared with green mango cultivars. However, contrary to the above, Li and Cheng (2009) found that anthocyanins in the shaded peel of red ‘Anjou’ pears made it more tolerant to photothermal stress (high light and high temperature).
By acting as a superficial light screen attenuating irradiation impinging on chloroplasts, anthocyanins have been shown to reduce short-term photoinhibition (Gould et al., 1995; Hada et al., 2003). Rabinowitch et al. (1983) argued that green pepper and cucumber cultivars are more predisposed to sunburn development compared with red cultivars, because their fruit peel contains more chlorophyll, which plays a role in the photo-oxidative processes of sunburn.
To fully understand the differential sunburn susceptibility between different apple cultivars, a comparative progressive assessment of sunburn development between these cultivars is necessary. The objective of this study was to investigate the effect of red color development on sunburn incidence and severity in red, blushed, and green apple cultivars. We hypothesized that accumulation of anthocyanin toward harvest in blushed and red cultivars would mask sunburn browning resulting in a lower perceived incidence and severity of sunburn at harvest compared with nonblushed cultivars and a resultant underestimation of the sunburn sensitivity of these cultivars. Anthocyanin accumulation should, however, not affect the incidence of sunburn necrosis.
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