HEAT AND LIGHT-INDUCED APPLE SKIN DISORDERS: CAUSES AND PREVENTION

In Washington State (USA) and many other major fruit-growing regions of the world, several fruit skin disorders are induced by heat and/or light stress in apples (Malus × domestica Borkh.). Three types of apple sunburn have been characterized by the authors. The first type (sunburn necrosis) is caused by heat alone [high fruit surface temperature (FST) of 52±1°C] and results in thermal death followed by necrosis. The second type (sunburn browning) is caused by both high FST and UV-B radiation. The third type (photooxidative sunburn) appears to be caused by light alone and occurs on green peel (non-acclimated to light) that is suddenly exposed to full sunlight so that photobleaching occurs first, followed by necrosis. This third type is thought to be photooxidative damage and can occur at much lower FST and without UV-B radiation. Another disorder that results from stress is ‘Fuji’ stain. This skin disorder appears only after a period of cold storage. This stain disorder appears primarily in sunburned fruit, and its incidence rises sharply as the severity of sunburn increases. This indicates that stress conditions that cause sunburn, also predispose certain fruit prior to harvest to develop this skin disorder during cold storage. We have evidence that UV-B radiation is involved. Another disorder that results from heat and light stress is lenticel marking. Its incidence also increases in fruit that have more severe sunburn. All of these disorders can be significantly decreased by use of RAYNOX, a sunburn protectant. Evaporative cooling or a combination of RAYNOX and evaporative cooling are also effective. Evaporative cooling reduces the FST, and RAYNOX decreases both FST and UV-B absorption by fruit. INTRODUCTION Several skin disorders appear in apples (Malus × domestica Borkh.) as a result of environmental stress. The emphasis in this paper will be on those disorders associated with damaging levels of solar radiation and heat (especially as they relate to fruit surface temperature). In addition to the skin disorder called sunburn, we also discuss other heat and/or light-induced disorders that appear on apples shortly after they are sunburned, later in maturity, or even in postharvest cold storage. The literature on sunburn is confusing as the terms sunburn and sunscald have been used interchangeably over the years to describe disorders of various fruits caused by solar radiation. According to the Compendium of Apple and Pear Diseases published by the American Phytopathological Society (Jones and Aldwinckle, 1990), the term sunburn refers to a disorder of the fruit that is caused by solar radiation, while the term sunscald refers to disorders of the bark caused by freezing. Accordingly, the authors of this paper use the term sunburn to describe the disorder of apple fruit that is caused by solar radiation. Sunburn has been reported in many fruits and vegetables, but much of the research has been done on detached fruits, and sunburn has been induced under artificial lighting in some studies. We found out that detached apples respond very differently to apples attached to trees. Detached apples sunburn more easily, and develop symptoms that differ from the most common sunburn symptoms described below. Thus, sunburn research reported herein was done using attached apples that became sunburned on the tree. Proc. XXVII IHC Enhancing Econ. & Environ. Sustain. of Fruit Prod. in a Global Econ. Ed.-in-Chief: J.W. Palmer Acta Hort. 772, ISHS 2008 52 Sunburn is usually the largest source of cullage in apples with losses averaging about 10% of the crop under the environmental conditions prevailing in the applegrowing regions of Washington State. Over the past decade, we have characterized three types of sunburn that occur in the field in attached apples (Fig. 1). We have reported the causes of each type elsewhere (Schrader et al., 2001, 2003a, 2004; Felicetti and Schrader, 2007) so we will review the causes of each type here only briefly. Sunburn necrosis occurs when the apple fruit surface temperature (FST) reaches 52±1oC for only 10 min (Schrader et al., 2001). At this FST, membranes lose their integrity so that electrolytes leak from the cells and thermal death occurs on the skin (Schrader et al., 2003a) a dark brown or black necrotic spot appears within 1 to 4 d, and necrotic cells often collapse. Sunburn necrosis can be naturally induced by high FSTs (i.e., ~52°C) attained in sunlight, but also can be induced experimentally at a FST of 52°C imposed in the dark (Schrader et al., 2001). Hence, sunlight is not directly involved in its induction. Sunburn browning is the most prevalent type of sunburn occurring on attached sun-exposed apples (i.e., fruit acclimated to high light) in the field. It results in a yellow, brown, or dark tan spot on the sun-exposed side of the apple. Sunburn browning is induced under high solar radiation when the apple FST reaches 46-49oC either naturally or after 45-60 min in a temperature-controlled chamber under full sunlight. When solar radiation is excluded, sunburn browning does not occur in similar attached apples heated to similar FST (Schrader et al., 2001). Ultraviolet-B (UV-B) radiation and a certain threshold or minimum FST are required to induce sunburn browning. This threshold or minimum FST varies with cultivar. ‘Cameo’ and ‘Honeycrisp’ have the lowest threshold temperature (46oC) and ‘Pink Lady’ has the highest threshold temperature (49oC) we have observed. The difference between maximum air temperature and maximum FST on the sunexposed side of apples is normally at least 11°C, and can be as much as 17°C. However, maximum FST cannot be predicted accurately with air temperature alone. Other meteorological factors such as solar radiation, relative humidity, and wind speed also affect the FST on a given day (Schrader et al., 2003b). We have found that the maximum FST is usually attained between 1300 and 1600 h (standard time), and sunburn browning normally occurs within this time interval. The third type (Photooxidative sunburn) occurs on apples that lack acclimation to high light and high temperatures (i.e., shaded apples). When these “non-acclimated” apples are suddenly exposed to full sunlight (e.g., after thinning, after summer pruning, or after shifting of a branch as fruit load increases), they are easily sunburned (Felicetti and Schrader, 2007). Initial damage is seen within 24h as bleaching or whitening of the sunexposed apple skin surface. With continued exposure to sunlight, the photobleached area turns brown (Fig. 1) and cells become necrotic. Electrolyte leakage from cuticular cells that are photobleached is comparable to that observed with sunburn necrosis. However, photooxidative sunburn can occur at much lower air and FST (i.e., can occur at FST below 31°C) than the other types of sunburn. Instead of requiring UV-B radiation, this type of sunburn appears to be dependent on visible radiation that produces reactive oxygen species that are highly destructive to lipids, proteins, and DNA. Apples protected from UV-B radiation with Mylar and apples protected from UV-A and UV-B with UVblocking plexiglass still developed photooxidative sunburn (Felicetti and Schrader, 2007). Detached apples often develop symptoms of photooxidative sunburn and appear similar to sunburn observed in “non-acclimated” apples. It is possible that the presence of little or no heat shock proteins (HSPs) in the non-acclimated apples makes them more vulnerable to photooxidative sunburn when suddenly exposed to sunlight. In apple peel and outer cortex cells, HSPs can be induced when the fruit are exposed to high temperatures. Ferguson et al. (1998) reported that mRNA’s for HSPs are induced when the flesh temperature is 34°C. Western immunoblot analyses showed that small HSPs (e.g., HSP 22) are low or undetectable in peel of ‘Fuji’, ‘Gala’, ‘Jonagold’, ‘Criterion’ and ‘Delicious’ growing shaded (shade apples) within the tree canopy, but are high in apples