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Sunburn, a major type of solar radiation injury, causes large economic losses of several fruits, vegetables, and ornamentals. Sunburn necrosis occurs in attached apple (Malus domestica Borkh.) when excess solar radiation is converted to heat energy and causes a high fruit surface temperature (FST). When the FST reaches ≈52 °C, thermal death of cells is induced and sunburn necrosis appears later. A second type of sunburn, sunburn browning, is caused by the combination of high FST (46 to 49 °C) and high solar radiation. Cell death is not induced, but several pigment changes occur and the apple peel typically turns yellow or brown. The objective of this study was to develop a unique technology for protection of apples from sunburn necrosis and sunburn browning. Natural waxes in the cuticle and pigments in the upper epidermis of apple, which reflect and absorb ultraviolet radiation, attenuate only part of the harmful ultraviolet radiation. I hypothesized that applying additional wax as a sprayable emulsion would augment natural waxes in the cuticle and that addition of reflective compounds to the wax emulsion would reduce the heat load on the fruit. Synthesis of a formulation of this composition would be unique for plants and would be akin to sunscreens used by humans that generally contain organic (chemical) compounds to absorb harmful ultraviolet radiation and inorganic (physical) compounds to scatter, block, and/or reflect solar radiation. Emulsified carnauba wax, a natural plant wax, decreased transmission of ultraviolet radiation; addition of emulsified, organically modified clay to the wax emulsion enhanced reflectivity. When combined into a sprayable formulation, the carnauba wax and organoclay emulsion was more effective in protecting apples from sunburn browning than from sunburn necrosis. Spraying the unique formulation on apple trees caused no phytotoxicity on either leaves or fruit and did not decrease leaf chlorophyll fluorescence or whole-tree gas exchange, indicating no effect on photosynthesis or transpiration of apple trees. The unique formulation is patented as a sunburn protectant for fruits and vegetables and is marketed in several countries as RAYNOX®.

Significant changes in pigments were identified, quantified, and correlated to the changes in color associated with sunburn browning (SB) of ‘Fuji’ apples (Malus domestica Borkh.). Apples were sorted into five classes: NB, no sunburn; SB-1 to SB-4, increasing severity of sunburn browning. A decline in chlorophylls a and b and reduced anthocyanin accumulation with increased sunburn severity were observed. A significant increase in total quercetin glycosides was seen with slight sunburn (i.e., SB-1) with most of the increase resulting from increased quercetin 3-galactoside and quercetin 3-glucoside + quercetin 3-rutinoside. Quercetin glycosides increased modestly from SB-1 to SB-4, but few differences were statistically significant. β-carotene increased in sunburned apples both years, but changes in SB-3 and SB-4 were inconsistent between the 2 years. The xanthophylls were significantly higher (P < 0.05) in SB-1 than in NB, but no difference was detected from SB-1 to SB-4. Lutein, a xanthophyll, showed no change as a result of sunburn in either year. Hue angle was highly correlated (P < 0.05) to the concentrations of total chlorophylls, idaein, and total quercetin glycosides. Despite minor differences between growing seasons, the overall trends of decreased chlorophyll and idaein, and increased quercetin glycosides and carotenoids persisted. Lower chlorophyll and anthocyanin concentrations observed in the sunburned apples allowed the yellows from the carotenoids and quercetin glycosides to be more prominent. Additionally, the increases in carotenoid and quercetin glycoside concentrations of sunburned apples made the change in color more striking.

Western immunoblot analyses showed that small heat shock proteins (smHSPs) are low or undetectable in the peel of `Fuji', `Jonagold', `Criterion', `Gala', and `Delicious' apples [(Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.] growing shaded within the tree canopy (shade apples), but are high in apples growing exposed to direct sunlight (sun apples). `Fuji', `Jonagold', and `Gala' sun apples sampled biweekly between 1 July and 21 Oct. 1997 were highest in content of smHSPs on 31 July, 13 Aug., and 10 Sept., corresponding to some of the warmest periods of the sampling period. The smHSPs started to disappear first in `Gala', the earliest maturing cultivar, and last in `Fuji', the latest maturing cultivar indicating that maturity might play a role in regulating smHSP accumulation. In sun apple fruit left on trees for 60 to 120 days beyond commercial maturity and exposed to field temperatures as low as -4 °C, a 71.7 ku (u = unified atomic mass unit) polypeptide was detected with a polyclonal antiwheat (Triticum aestivum L.) HSP70 in the peel and cortex of all five cultivars. While no smHSPs were detected in these apples, three smHSPs, as detected by antibodies against pea (Pisum sativum L.) cytosolic HSP18.1, could be induced in the same fruit 24 hours after heating to 45 °C for 4 hours. In `Fuji' shade apples heated at 40 °C, smHSP accumulation was detected after the second hour of a 4-hour heat treatment and continued to increase over the next 48 hours at 22 °C. Levels of HSP70 did not change in `Fuji' shade apples heated at 45 °C for 2, 4, or 6 hours, but smHSPs became detectable immediately after each of these heat treatments and further increased over the next 24 hours at 22 °C. Accumulation of smHSPs was maximal in the 4-hour heat treatment. After a 4-hour heat treatment at 45 °C, smHSPs increased during the next 48 hours at 22 °C and then declined by 72 hours. Using two-dimensional electrophoretic analysis, as many as 17 proteins ranging from 15 to 29 ku were found to accumulate in the peel 48 hours after a 4-hour heat treatment. Thus, apples can respond rapidly to high temperature stress, even at advanced stages of maturity, by synthesizing smHSPs, which likely play an important role in protecting cellular biochemical processes during these periods of stress.

Postharvest changes in flesh firmness, soluble solids concentration (SSC), and titratable acidity (TA) associated with increasing severity of sunburn browning of apples (Malus ×domestica Borkh.) were investigated over time in regular atmosphere cold storage. In 2005, ‘Gala’ apples were harvested at maturity and sorted into five classes ranging from no sunburn (Sb-0) to severe sunburn browning (Sb-4). Flesh firmness, SSC, and TA were measured on flesh tissue beneath the sunburned area of the apples at harvest and at monthly intervals for 6 months. In 2006, a similar protocol was used for ‘Gala’, ‘Golden Delicious’, ‘Jonagold’, ‘Granny Smith’, and ‘Fuji’ apples, except that these fruit quality traits were determined at harvest and after 3 and 6 months in regular atmosphere cold storage. Flesh firmness and SSC increased, but TA decreased both years in all cultivars as sunburn severity increased from Sb-1 to Sb-4. The patterns of higher firmness with more severe sunburn browning persisted during cold storage even though firmness in all classes of fruit decreased gradually with time in cold storage. The patterns of lower TA as severity of sunburn increased also persisted during cold storage, but TA declined more markedly in apples with more severe sunburn. The ratio of SSC to TA (SAR) increased as the severity of sunburn increased in all five cultivars. The SAR also increased dramatically from harvest to 3 and 6 months after harvest with ratios exceeding 200 in Sb-4 of some cultivars after 6 months storage. This occurred because TA declined very markedly as time in cold storage increased. Relative water concentration of tissue in sunburned areas of apples declined as the severity of sunburn increased, and helps to account for the higher SSC and firmness observed in sunburned apples.