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Red color development toward harvest may conceal superficial blemishes such as sunburn browning in apple peel. Masking of sunburn may result in the underestimation of sunburn incidence in full red and blushed cultivars and may result in inaccurate assessments of sunburn susceptibility of various cultivars. However, anthocyanin accumulation may potentially also increase sunburn by decreasing the albedo and thereby increasing the fruit surface temperature (FST). Conversely, it has been proposed that anthocyanins may protect peel against photothermal stress. We assessed the effect of anthocyanin accumulation on the visible sunburn incidence in two trials. In the first trial, hue angle, blush coverage, sunburn incidence, and sunburn severity were assessed on fully exposed apples of the green cultivars, Granny Smith and Golden Delicious, as well as the blushed cultivars, Royal Gala, Fuji, Braeburn, and Cripps’ Pink, and the full red cultivar Topred, a month before harvest and again at harvest. Increases in sunburn toward harvest were greater in green than in red and blushed cultivars. Accumulation of anthocyanins seemed to decrease the conspicuousness of sunburn browning, which is a superficial form of sunburn while sunburn necrosis, which manifests as sunken black spots, was not masked by anthocyanins. In the second trial, ‘Granny Smith’, ‘Fuji’, and ‘Cripps’ Pink’ apples with moderate sunburn browning, sunburn necrosis, or no sunburn were tagged 1 month before the expected harvest date. Sunburn browning was apparent and significantly increased in severity in ‘Granny Smith’ while the severity of visible sunburn browning symptoms decreased in ‘Fuji’. Sunburn severity increased at the same level in the control and sunburn browning ‘Cripps’ Pink’ fruit. Measurement of maximal photochemical efficiency of the originally undamaged apples and the sunburn browning treatment indicated comparable damage levels in the peel of the sunburn browning treatment in all three cultivars. No masking of sunburn necrosis occurred. Our data thus provide clear evidence that anthocyanin accumulation masks sunburn browning in blushed and red cultivars, but does not seem to increase the susceptibility or protect these cultivars from sunburn damage. The incidence of sunburn browning is likely to be underestimated in red and blushed cultivars, with compounding effects on comparative studies of sunburn susceptibility between cultivars and sunburn physiology. 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 (W€unsche et al., 2004), but may also relate to indirect factors thatmaymodulate 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; W€unsche 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 Received for publication 13 Feb. 2015. Accepted for publication 1 Apr. 2015. This article is based upon the work supported by the THRIP program of the National Department of Trade and Industry and the South African Apple and Pear Producers’ Association. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Research Foundation. This article forms part of a thesis submitted by Brian Makeredza in fulfilling an MSc. Agric degree requirement. To whom reprint requests should be addressed; e-mail wiehann@hortgro.co.za. 814 HORTSCIENCE VOL. 50(6) JUNE 2015 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. Materials and Methods Trial 1.This experiment was carried out at De Rust Estate (34 10# S; 19 4# E) in Elgin, South Africa during the 2009–10 season. Seven apple cultivars were used viz green Granny Smith and Golden Delicious, blushed Royal Gala, Fuji, Braeburn, and Cripps’ Pink, and the full red Topred. ‘Royal Gala’, ‘Granny Smith’, and ‘Fuji’ were all on M793 rootstock. The remaining cultivars were all on seedling rootstock. ‘Granny Smith’ and ‘Fuji’ were planted in 2000 at a tree spacing of 4 m · 1.5 m, while ‘Royal Gala’ was planted in 1987 at a tree spacing of 4 m · 2 m. ‘Topred’ and ‘Golden Delicious’ were planted in 1984, whereas ‘Cripps’ Pink’ and ‘Braeburn’ were planted in 1991 and all at a tree spacing of 4 m · 2 m. ‘Fuji’, ‘Granny Smith’, and ‘Cripps’ Pink’ had a northeast by southwest row orientation, while ‘Royal Gala’, ‘Braeburn’, ‘Golden Delicious’, and ‘Topred’ had an east to west orientation. All trees were trained to a central leader. For each cultivar, four 30-fruit plots were selected at random within an orchard on the northern or north-western side of the row. Fruit that were fully sun-exposed, but showed no prior sunburn were tagged and marked on 12 Jan. 2010 for repeated evaluation. Sunburn incidence and sunburn severity were evaluated a month before expected commercial harvest and at commercial harvest on the same fruit. Sunburn severity was assessed using a 0–5 scale developed by Schrader and McFerson (Schrader et al., 2003), where 0 represented no sunburn and five, sunburn necrosis. At harvest, sunburn severity was assessed using the same Schrader and McFerson scale and a ‘Granny Smith’ sunburn evaluation chart (Deciduous Fruit Board Set A33) for all cultivars. Assessments for a month before harvest were made on 12 Jan. (‘Royal Gala’), 3 Feb. (‘Topred’), 11 Feb. (‘Golden Delicious’), 25 Feb. (‘Fuji’ and ‘Braeburn’), 6 Mar. (‘Granny Smith’), and 19 Mar. (‘Cripps’ Pink’) 2010. Fruit were harvested on 11 Feb. (‘Royal Gala

Ripening Associated Red Color Development Masks Sunburn Browning in Apple Peel