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.
Contreras, C., Zoffoli, J.P., Alcalde, J.A. & Ayala, M. 2008 Evolution of sunburn in ‘Granny Smith’ apples during storage Cien. Inv. Agr. 35 113 122
Felicetti, D.A. & Schrader, L.E. 2009 Changes in pigment concentrations associated with sunburn browning of five apple cultivars. I. Chlorophylls and carotenoids Plant Sci. 176 78 83
Fouché, J.R., Roberts, S.C., Midgley, S.J.E. & Steyn, W.J. 2010 Peel color and blemishes in ‘Granny Smith’ apples in relation to canopy light environment HortScience 45 899 905
Gross, J. 1987 Pigments in fruit. Acad. Press, London
Hada, H., Hidema, J., Maekawa, M. & Kumagai, T. 2003 Higher amounts of anthocyanins and UV-absorbing compounds effectively lowered CPD photorepair in purple rice (Oryza sativa L.) Plant Cell Environ. 26 1691 1701
Knee, M. 1971 Anthocyanin, carotenoid and chlorophyll changes in the peel of Cox’s Orange Pippin apples during ripening on and off the tree J. Expt. Bot. 23 184 196
Li, P. & Cheng, L. 2009 The elevated anthocyanin level in the shaded peel of ‘Anjou’ pear enhances its tolerance to high temperature under high light Plant Sci. 177 418 426
Pietrini, F. & Massacci, A. 1998 Leaf anthocyanin content changes in Zea mays L. grown at low temperature: Significance for the relationship between quantum yield of PSII and the apparent quantum yield of CO2 assimilation Photosynth. Res. 58 213 219
Rabinowitch, H.D., Friedman, M. & Ben-David, B. 1983 Sunscald damage in attached and detached pepper and cucumber fruits at various stages of maturity Sci. Hort. 19 9 18
Racsko, J. & Schrader, L.E. 2012 Sunburn of apple fruit: Historical background, recent advances and future perspectives Crit. Rev. Plant Sci. 31 455 504
Ritchie, G.A. 2006 Chlorophyll fluorescence: What is it and what do the numbers mean? In: L.E. Riley, R.K. Dumroese, and T.D. Landis (Technical coordinators). 2006. National Proceedings: Forest and Conservation Nursery Associations–2005. Proc. RMRS-P-43. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 160 p
Schrader, L.E., Sun, J., Zhang, J., Felicetti, D. & Tian, J. 2008 Heat and light-induced apple skin disorders: Causes and prevention Acta Hort. 772 51 58
Schrader, L.E., Zhang, J. & Duplaga, W.K. 2001 Two types of sunburn in apple caused by high fruit surface (peel) temperature. Plant Health Prog. 1 Dec. 2010. <http://hort.tfrec.wsu.edu/les/Temperature.pdf>
Steyn, W.J., Holcroft, D.M., Wand, S.J.E. & Jacobs, G. 2004 Anthocyanin degradation in detached pome fruit with reference to preharvest red color loss and pigmentation patterns of blushed and fully red pears J. Amer. Soc. Hort. Sci. 129 13 19
Wünsche, J.N., McGhie, T., Bowen, J., Ferguson, I. & Woolf, A. 2004 Sunburn on apples-causes and control mechanisms Acta Hort. 636 631 636