Sunburn of apples is a physiological disorder caused by heat and/or light stress (Felicetti and Schrader, 2008a; Schrader et al., 2001, 2003a, 2004). Sunburn markedly increases cullage and results in large economic losses to growers (Schrader et al., 2003b). Sunburn of apples is a longstanding problem first described by Brooks and Fisher (1926). They inserted a thermometer into the cortex for temperature measurements and reported that exposure to the sun increased fruit surface temperatures (FST) of apples up to 14 °C on the sun-exposed side and often caused sunburn of those apples. More recent reports using thermocouples inserted into the fruit (Ferguson et al., 1998; Parchomchuk and Meheriuk, 1996) and attached to the fruit surface (Schrader et al., 2003b) indicated that FST could exceed air temperature by 15 °C and sometimes by 17 °C, respectively. Schrader et al. (2003b) reported that maximum FST was highly correlated with the maximum daily air temperature (r = 0.90**), the mean of maximum hourly air temperatures between 1100 and 1700 hr (r = 0.88**), and mean solar radiation between 1100 and 1700 hr (r = 0.65**). However, mean wind velocity and mean relative humidity between 1100 and 1700 hr were inversely correlated (r = −0.24** and −0.66**, respectively) with maximum FST.
Three distinct types of apple sunburn have been identified and characterized: photo-oxidative sunburn, sunburn necrosis, and sunburn browning. Photo-oxidative sunburn results from the sudden exposure of shaded peel to the visible range of solar radiation and can occur at FST below 31 °C. It is characterized by a photobleaching of the affected area with the possibility of brown, necrotic tissue appearing within the photobleached area (Felicetti and Schrader, 2008a). Schrader et al. (2001) described sunburn necrosis and sunburn browning, and later classified sunburn browning into four classes according to the extent of damage (Felicetti and Schrader, 2008b; Schrader et al., 2003a). The specific FST at which sunburn necrosis and browning occurred were determined experimentally on attached apples by Schrader et al. (2003b). Sunburn necrosis occurred when the FST reached 52 ± 1 °C for only 10 min, whereas sunburn browning varied with cultivars and occurred when FST of attached fully exposed apples was 46 to 49 °C for 1 h under environmental conditions prevailing in Wenatchee, WA. Induction of sunburn necrosis required only a high FST with no direct solar irradiance, whereas induction of sunburn browning required ultraviolet-B radiation and a high FST (Schrader et al., 2008). In Southern Hemisphere regions that have more ultraviolet-B radiation because of a thin ozone layer or the ozone hole, the threshold FST required for sunburn browning may be lower than the FST required under our conditions in the Pacific northwestern United States. This could explain the high incidence of sunburn browning observed in New Zealand where maximum air temperatures and FST are lower than those measured in Washington State.
Schrader et al. (2001) reported that the relative electrical conductivity (REC) for apples with sunburn necrosis was higher (P ≤ 0.05) than REC for fruit with no sunburn or with sunburn browning. Elevated REC in fruit with sunburn necrosis indicated that membrane integrity was destroyed, permitting electrolytes to leak freely. The REC for fruit with no sunburn and fruit with sunburn browning did not differ significantly, indicating that membrane integrity was unaffected by sunburn browning. These findings indicated that thermal death occurred with sunburn necrosis, but not with sunburn browning. Sunburn necrosis, but not sunburn browning, also caused serious changes in the cuticle and in epidermal and subepidermal tissues (Schrader et al., 2001).
Andrews and Johnson (1996) used electron microscopy to reveal that the wax cuticle on an apple's epidermis was dissipated by the intense sunlight associated with sunburn. They reported that cell walls thickened and lignified even before peel color changes were visible. However, they did not distinguish between sunburn browning and sunburn necrosis.
Apple trees are frequently exposed to air temperatures exceeding 35 °C in several apple-growing regions of the world. Under these conditions, FST of sun-exposed apples can reach 45 to 50 °C. Exposure of apples to these high FST required for sunburn browning raises questions about what effects these high temperatures have on internal fruit quality. Yet very little has been done to systematically investigate the effects of these high FST on internal fruit quality of apples.
Watkins et al. (1993) reported that sunburn may result in loss of fruit quality in ‘Braeburn’ apples, but provided no supporting data. Racskó et al. (2005a) reported that sunburn of apples increased flesh firmness, decreased water content, and increased soluble solids concentration. However, they did not indicate what type of sunburn they studied. They did indicate that plant cells suffering from sunburn died and that plant pathogens infected some fruit through the injured epidermal tissue, making the fruit unmarketable (Racskó et al., 2005b). Based on their description, it seems likely that Racskó et al. (2005a) studied sunburn necrosis or photo-oxidative sunburn, as necrosis (i.e., cell death) is a characteristic of photo-oxidative sunburn and sunburn necrosis, but not a characteristic of sunburn browning. We define sunburn browning as a peel disorder in which epidermal cells neither die nor incur damage that permit pathogens to infect the fruit. In contrast, sunburn necrosis results from thermal death, and can damage the apple epidermis so seriously that invasive pathogens can gain entry.
Although apples with slight to moderate sunburn browning are marketed, little is known about the impact sunburn browning has on apples as they move through storage, packing, and distribution. Hence, our objective was to investigate changes in internal fruit quality over time in regular atmosphere (RA) cold storage associated with increasing severity of sunburn browning of apples. We compared flesh firmness, soluble solids concentration (SSC), titratable acidity (TA), and the ratio of SSC to TA (SAR) in apples with different classes of sunburn damage that ranged from no sunburn (Sb-0) to severe sunburn browning (Sb-4).
Brooks, C. & Fisher, D.F. 1926 Some high-temperature effects in apples: Contrasts in the two sides of an apple J. Agr. Res. 32 1 16
Dussi, M.C. , Giardina, G. , Sosa, D. , Junyent, R.G. , Zecca, A. & Reeb, P. 2005 Shade nets effect on canopy light distribution and quality of fruit and spur leaf on apple cv. ‘Fuji’ Span. J. Agric. Res. 3 253 260
Elfving, D.C. 1990 Growth and productivity of ‘Empire’ apple trees following a single heading-back pruning treatment HortScience 25 908 910
Felicetti, D.A. & Schrader, L.E. 2008a Photooxidative sunburn of apples: Characterization of a third type of apple sunburn Int. J. Fruit Sci. 8 3/4 160 172
Felicetti, D.A. & Schrader, L.E. 2008b Changes in pigment concentrations associated with the degree of sunburn browning of ‘Fuji’ apple J. Amer. Soc. Hort. Sci. 133 27 34
Ferguson, I.B. , Snelgar, W. , Bowen, J.H. & Woolf, A.B. 1999 Preharvest field heat and postharvest fruit response Acta Hort. 485 149 154
Ferguson, I.B. , Snelgar, W. , Lay-Yee, M. , Watkins, C.B. & Bowen, J.H. 1998 Expression of heat shock protein genes in apple fruit in the field Aust. J. Plant Physiol. 25 155 163
Harker, F.R. , Marsh, K.B. , Young, H. , Murray, S.H. , Gunson, F.A. & Walker, S.B. 2002 Sensory interpretation of instrumental measurements 2: Sweet and acid taste of apple fruit Postharvest Biol. Technol. 24 241 250
Klein, J.D. , Dong, L. , Zhou, H.-W. & Lurie, S. 2001 Ripeness of shaded and sun-exposed apples (Malus domestica) Acta Hort. 553 95 98
Nilsson, T. & Gustavsson, K.E. 2007 Postharvest physiology of ‘Aroma’ apples in relation to position on the tree Postharvest Biol. Technol. 43 36 46
Parchomchuk, P. & Meheriuk, M. 1996 Orchard cooling with pulsed overtree irrigation to prevent solar injury and improve fruit quality of ‘Jonagold’ apples HortScience 31 802 804
Racskó, J. , Thurzó, S. , Szabó, Z. & Nyéki, J. 2005b Schadwirkung des Sonnenbrands auf das Gewebe des Apfels (Malus domestica Borkh.) (in German with English abstract) Gesunde Pflanzen 57 47 52
Racskó, J. , Szabó, Z. & Nyéki, J. 2005a Importance of the supraoptimal radiance supply and sunburn effects on apple fruit quality. Proc. 8th Hungarian Congr. Plant Physiol. and 6th Hungarian Conf. Photosyn Acta Biologica Szegediensis 49 111 114
Schrader, L.E. , Sun, J. , Felicetti, D. , Seo, J.-H. , Jedlow, L. & Zhang, J. 2003a Stress-induced disorders: Effects on apple fruit quality 21 Nov. 2008 <http://postharvest.tfrec.wsu.edu/PC2004E.pdf>.
Schrader, L.E. , Sun, J. , Zhang, J. & Seo, J.-H. 2004 Fruit skin disorders 21 Nov. 2008 <http://postharvest.tfrec.wsu.edu/PC2003A.pdf>.
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. 21 Nov. 2008 Plant Health Prog.
Triantaphylidés, C. , Krischke, M. , Hoeberichts, F.A. , Ksas, B. , Gresser, G. , Havaux, M. , Van Breusegem, F. & Mueller, M.J. 2008 Singlet oxygen is the major reactive oxygen species involved in photooxidative damage to plants Plant Physiol. 148 960 968
Tustin, D.S. , Hirst, P.M. & Warrington, I.J. 1988 Influence of orientation and position of fruiting laterals on canopy light penetration, yield, and fruit quality of ‘Granny Smith’ apple J. Amer. Soc. Hort. Sci. 113 693 699
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Tustin, D.S. Hirst, P.M. Warrington, I.J. 1988 Influence of orientation and position of fruiting laterals on canopy light penetration, yield, and fruit quality of ‘Granny Smith’ appleJ. Amer. Soc. Hort. Sci. 113 693 699 10.1660/0022-8443(2007)110[259:CIFQPO]2.0.CO;2
Warmund, M.R. , Starbuck, C. & Kadir, S. 2007 Changes in fruit quality parameters of ‘Jonathan Rasa’ and ‘Delicious Flanagan’ apples in response to elevated temperatures Trans. Kans. Acad. Sci. 110 259 267
Watkins, C. , Harker, R. , Brookfield, P. & Tustin, S. 1993 Maturity of Royal Gala, Braeburn and Fuji: The New Zealand experience Proc. Washington Tree Fruit Postharvest Conf. 9 16 19
Woolf, A.B. & Ferguson, I.B. 2000 Postharvest responses to high fruit temperatures in the field Postharvest Biol. Technol. 21 7 20
Wu, J. , Gao, H. , Zhao, L. , Liao, X. , Chen, F. , Wang, Z. & Hu, X. 2007 Chemical compositional characterization of some apple cultivars Food Chem. 103 88 93