Sunburn Browning Research Articles

Sunburn disorder in tropical and subtropical fruits. A review

The increase in solar radiation and temperature as a result of climate change and variability has increased sunburn damage to fruits, which highly affects quality and yield in tropical and subtropical fruit plants. Solar injuries increase because of conditions with low relative humidity, fruits left uncovered by foliage, and plantations at higher altitudes as a result of increased UV radiation. Three different types of sunburn on fruits are distinguished: photooxidative sunburn, sunburn browning, and necrosis on the epidermis. Fruits employ self-protection mechanisms against this stress through the production of enzymatic and non-enzymatic antioxidants. Fruit growers try to mitigate the impact of sunburning by planting species and varieties that are tolerant to this stress, pruning and training plants, leaving enough leaves above the fruit, efficient irrigation and intercropping for shading. More technical sunburn mitigation strategies include 1) improving the microclimate (shading nets, fruit bagging, evaporative cooling), 2) suppressors (kaolinite clay particle films, calcium carbonate) or 3) chemical protection (tocopherol, abscisic acid, ascorbic acid, anti-transpirants). This paper presents the state of research and its results for this abiotic stress in some tropical and subtropical fruit trees, such as avocado, banana, citrus, mango, pineapple and pitaya, along with observations for other fruit trees in tropical altitude zones. Continued research is recommended for this stress in different varieties with the use of environmentally friendly protective materials, along with studies on molecular mechanisms that direct the acclimatization of plants to a combination of these two types of stress, excessive radiation and temperature.

Characteristics of Sunburn Browning Fruit and Rootstock-Dependent Damage-Free Yield of Ambrosia™ Apple after Sustained Summer Heat Events.

The 2021 summer heat waves experienced in the Pacific Northwest led to considerable fruit damage in many apple production zones. Sunburn browning (SB) was a particularly evident symptom. To understand the mechanism underlying the damage and to facilitate the early assessment of compromised fruit quality, we conducted a study on external characteristics and internal quality attributes of SB ‘Ambrosia’ apple (Malus domestica var. Ambrosia) and evaluated the fruit loss on five rootstocks. The cell integrity of the epidermal and hypodermal layers of fruit skins in the SB patch was compromised. Specifically, the number of chloroplasts and anthocyanin decreased in damaged cells, while autofluorescent stress-related compounds accumulated in dead cells. Consequently, the affected sun-exposed skin demonstrated a significant increase in differential absorbance between 670 nm and 720 nm, measured using a handheld apple DA meter, highlighting the potential of using this method as a non-destructive early indicator for sunburn damage. Sunburn browning eventually led to lower fruit weight, an increase in average dry matter content, soluble solids content, acidity, deteriorated weight retention, quicker loss of firmness, and accelerated ethylene emission during ripening. Significant inconsistency was found between the sun-exposed and shaded sides in SB apples regarding dry matter content, firmness, and tissue water potential, which implied preharvest water deficit in damaged tissues and the risk of quicker decline of postharvest quality. Geneva 935 (G.935), a large-dwarfing rootstock with more vigor and higher water transport capacity, led to a lower ratio of heat-damaged fruits and a higher yield of disorder-free fruits, suggesting rootstock selection as a long-term horticultural measure to mitigate summer heat stress.

Sunburn in Grapes: A Review.

Sunburn is a physiological disorder that affects the visual and organoleptic properties of grapes. The appearance of brown and necrotic spots severely affects the commercial value of the fruit, and in extreme cases, significantly decreases yield. Depending on the severity of the damage and the driving factors, sunburn on grapes can be classified as sunburn browning (SB) or as sunburn necrosis (SN). Sunburn results from a combination of excessive photosynthetically active radiation (PAR) and UV radiation and temperature that can be exacerbated by other stress factors such as water deficit. Fruit respond to these by activating antioxidant defense mechanisms, de novo synthesis of optical screening compounds and heat-shock proteins as well as through morphological adaptation. This review summarizes the current knowledge on sunburn in grapes and compares it with relevant literature on other fruits. It also discusses the different factors affecting the appearance and degree of sunburn, as well as the biochemical response of grapes to this phenomenon and different potential mitigation strategies. This review proposes further directions for research into sunburn in grapes.

A robust impact assessment that informs actionable climate change adaptation: future sunburn browning risk in apple.

Climate change impact assessments are predominantly undertaken for the purpose of informing future adaptation decisions. Often, the complexity of the methodology hinders the actionable outcomes. The approach used here illustrates the importance of considering uncertainty in future climate projections, at the same time providing robust and simple to interpret information for decision-makers. By quantifying current and future exposure of Royal Gala apple to damaging temperature extremes across ten important pome fruit-growing locations in Australia, differences in impact to ripening fruit are highlighted, with, by the end of the twenty-first century, some locations maintaining no sunburn browning risk, while others potentially experiencing the risk for the majority of the January ripening period. Installation of over-tree netting can reduce the impact of sunburn browning. The benefits from employing this management option varied across the ten study locations. The two approaches explored to assist decision-makers assess this information (a) using sunburn browning risk analogues and (b) through identifying hypothetical sunburn browning risk thresholds, resulted in varying recommendations for introducing over-tree netting. These recommendations were location and future time period dependent with some sites showing no benefit for sunburn protection from nets even by the end of the twenty-first century and others already deriving benefits from employing this adaptation option. Potential best and worst cases of sunburn browning risk and its potential reduction through introduction of over-tree nets were explored. The range of results presented highlights the importance of addressing uncertainty in climate projections that result from different global climate models and possible future emission pathways.

Ripening Associated Red Color Development Masks Sunburn Browning in Apple Peel

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.

Sun damage risk of Royal Gala apple in fruit-growing districts in Australia

This study estimated minimum air temperatures for potential sun damage for sunburn browning (non-netted and netted) and sunburn necrosis (non-netted) for Royal Gala apple in Australia. The approach estimated when conditions may be conducive to the development of sun damage in some fruit. The approach provides a measure of potential damage. This allows for more flexible analyses of potential sun damage which current models are unable to produce due to data limitations. The air temperature thresholds determined were 34.1 and 38.7 °C, respectively, for browning and necrosis for non-netted fruit and 37.9 °C for browning under netting. These air temperature thresholds were applied across southern Australia from 1911–2013 demonstrating different risk profiles between sites, inter-annual variability and the benefit of installing netting via a reduction in potential damage days. The results can be further extended to estimate impacts from climate change and assess the benefit of installing netting to adapt to increasingly extreme hot weather.

Sunburn of Apple Fruit: Historical Background, Recent Advances and Future Perspectives

Sunburn is a physiological disorder of apples and other fruit species caused by excess solar radiation. Damage occurs in practically all growing regions of the world, causing severe crop loss every year. Direct factors required for induction of the three currently-known types of sunburn (i.e., sunburn necrosis, sunburn browning, and photooxidative sunburn) include excess radiant heating and/or exposure to excess sunlight. Several other factors (e.g., relative humidity, wind velocity, acclimation of fruit, and cultural management practices), which alone cannot induce sunburn damage, indirectly influence the induction of sunburn by interacting with the direct factors to influence the appearance and severity of the symptoms. Sunburn affects apple fruit at many levels; it causes structural and morphological changes, alters pigment composition, influences adaptive mechanisms, impairs photosynthesis, and consequently decreases fruit quality. Fruits employ multiple physiological and biochemical mechanisms as complex defense systems to minimize damage. Photoprotective pigments, antioxidant enzymes and metabolites, heat-shock proteins, and the xanthophyll cycle help mitigate damage, but are often inadequate under field conditions to fully protect from sunburn. Quality loss significantly affects postharvest behavior, marketing and consumer acceptance of fruit. Internal fruit quality (e.g., firmness, soluble solids concentration, and titratable acidity) is affected by sunburn, and changes in these traits continue during cold storage. Sunburn-related disorders (e.g., sunburn scald in ‘Granny Smith’ and ‘Fuji’ stain) can appear in cold storage. There are several methods with various modes of action (e.g., climate ameliorating techniques, and sunburn suppressants) available to growers to decrease sunburn under field conditions. At the end of this review, the potential impact of a changing climate on sunburn incidence is considered, as both UV-B radiation and temperature are projected to change. Finally, several topics that need further research are discussed.

Scientific Basis of a Unique Formulation for Reducing Sunburn of Fruits

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®.

Characterization of sunburn damage to apple fruits and leaves

The specific conditions of the formation of three different types of sunburn (sunburn browning, sunburn necrosis, and photooxidative sunburn) have been recently characterized on apple fruit. However, no information is still available on leaf damage. Therefore, the aims of this study were i) to extend the knowledge on fruit damage, ii) characterize leaf damage and iii) find relationship between fruit and leaf damage. The observations were made on 586 apple accessions in a gene bank orchard located in Hungary. The incidence of the three different types of fruit symptoms were recorded and based on the visual symptoms, two different types of leaf sunburn (sunburn yellowing and sunburn necrosis) were characterized. The most frequent type of fruit sunburn observed was sunburn browning. Photooxidative sunburn was found for less number of accessions, and only some accessions were affected by sunburn necrosis. Fruit were far more susceptible than leaves; (>60%) of the examined accessions were affected by fruit damage and (<3%) by leaf damage. Although a large number of accessions were affected, the percentage of fruit damaged within accessions was not that excessive; ~6% of the fruit assessed showed the symptoms of sunburn browning. Significantly fewer fruit were damaged by sunburn necrosis (~1%) or photooxidative sunburn (~1.4%) than sunburn browning. The percentage of leaves damaged within accessions were simlarly very low (~1%). Close relationship between fruit and leaf damage was found. Accesions with relatively heavily sunburned leaves usually had severe fruit damage as well. Leaves showing sunburn symptoms were usually closely located around those fruit which were sunburned severely. Leaf damage of sunburn was found on spur leaves in a great majority of the accessions damaged, shoot leaves did not seem to be susceptible to sunburn.

Sunburn Browning Decreases At-Harvest Internal Fruit Quality of Apples (Malus domestica Borkh.)

Changes in internal fruit quality associated with increasing severity of sunburn browning of apples were investigated at harvest in five cultivars. Apples were sorted into five classes ranging from no sunburn (Sb-0) to severe sunburn browning (Sb-4). Fruit quality of flesh tissue was compared beneath the sunburned area, the shaded side, and the shoulder side of each apple. All two-way interactive effects (i.e., sunburn classes vs. sides of fruit, sunburn classes vs. cultivars, and cultivars vs. sides of fruit) were significant (P ≤ 0.05) for all fruit quality traits. For the treatment combination of sunburn class and side of fruit, flesh firmness, and soluble solids concentration increased on the sun side, but titratable acidity averaged over cultivars for the sun side generally declined as sunburn severity increased from Sb-1 to Sb-4. Starch cleared more quickly on the shaded and shoulder sides as compared to the sun-exposed side of all fruit with sunburn browning. Starch also generally cleared more rapidly in apples with more severe sunburn browning, suggesting that sunburned apples were more mature than nonsunburned apples harvested on the same day. For the treatment combination of side of fruit and cultivar, the sun side was higher than shade or shoulder sides in firmness and soluble solids concentration for all cultivars when averaged over sunburn classes. Starch index was lowest for the sun side. For the treatment combination of sunburn class and cultivar, firmness, soluble solids concentration, and starch index, when averaged over sides of fruit, generally increased in all cultivars as severity of sunburn increased; titratable acidity decreased.

Sunburn incidence of apples is affected by rootstocks and fruit position within the canopy but not by fruit position on the cluster

Authors investigated sunburn incidence of apples on the combinations of three different growth inducing rootstocks (M.9,MM.106 and seedling) and five varieties (‘Smoothee’, ‘Golden Reinders’, ‘Granny Smith’, ‘Gloster’ and ‘Jonagold Jonica’). Symptoms were classified as sunburn browning, sunburn necrosis and photooxidative sunburn. The frequency of symptoms was recorded at various parts of the canopy (N, E,W, S, and lower canopy, upper canopy) and on the cluster (terminal, lateral). Cultivar susceptibility varied between 0.30 and 5.65% on M.9 rootstock, ‘Granny Smith’ seemed to be the most susceptible cultivar whereas relatively low percentage of damaged fruit was observed for ‘Gloster’. On MM.106 and seedling rootstocks, damage level was significantly lower than on M.9. Remarkable differences were not observed in the share of the three sunburn types between cultivars. The most common symptom observed was sunburn browning. Far less fruit was affected by sunburn necrosis and photooxidative sunburn. Photooxidative sunburn symptoms were not found on ‘Granny Smith’ and ‘Gloster’ fruits on MM.106 rootstock. Latter cultivar did not show sunburn necrosis symptoms either. With increasing growing vigor of the rootstocks the share of sunburn browning increased. Fruits with sunburn symptoms were found in a great majority on theW quadrant of the trees. This was true for all cultivars. Remarkable differences in the location within the canopy of affected fruits between the three types of sunburn were not observed. Specific distribution of sunburned fruit was observed along the vertical axis of the canopy, too. Most of the damaged fruit were found in the upper canopy. This is particularly true for trees on vigorous stocks such as MM.106 and seedling. On M.9 rootstock, depending on cultivars 5.9 to 38.9% of sunburned fruit was located in the lower canopy. Most common symptom in the lower canopy was the sunburn browning, however symptoms of sunburn necrosis were not found at lower canopy level. Low rate of photooxidative sunburn was observed such lower canopy conditions. Sunburn incidence was very similar on king or side fruit. Significant differences were not found in the share of each sunburn types between fruit positions on the cluster. This was not influenced by rootstocks either.

Postharvest Changes in Internal Fruit Quality in Apples with Sunburn Browning

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.

Photooxidative Sunburn of Apples: Characterization of a Third Type of Apple Sunburn

ABSTRACT Photooxidative sunburn, a third type that is distinctly different from sunburn necrosis and sunburn browning, is described for apple (Malus domestica Borkh.). Shaded (nonacclimated) apples suddenly exposed to sunlight became photobleached and eventually necrotic. Electron micrographs and electrolyte leakage studies indicated that cell death occurred with photobleaching. Photobleaching occurred at fruit surface temperatures (FST) below 31°C. Sunburn browning and sunburn necrosis require FST above 45°C. Ultraviolet-B radiation is involved in induction of sunburn browning, but blocking UV-A and UV-B radiation did not protect nonacclimated apples from photobleaching and necrosis. Infrared radiation (>700 nm) was ruled out as an induction factor. We suggest that visible radiation and the production of reactive oxygen species are the induction factors for photooxidative sunburn.

Changes in pigment concentrations associated with sunburn browning of five apple cultivars. I. Chlorophylls and carotenoids

Peel disks were taken from the sunburned area (SB-2), the area around the sunburned area (halo), the area around the halo (OH), and the center of the sun-exposed side of non-sunburned (NB) ‘Fuji’, ‘Gala’, ‘Delicious’, ‘Golden Delicious’, and ‘Granny Smith’ apples ( Malus domestica Borkh). Chlorophyll a and b concentrations in SB-2 peels of all five cultivars were generally significantly ( P ≤ 0.05) lower than those in the halo, OH and NB peels. Significantly higher concentrations of β-carotene, violaxanthin, and antheraxanthin were observed in ‘Fuji’ and ‘Delicious’ SB-2 peel as compared to OH and NB peels. Lower concentrations of β-carotene and L were observed in ‘Granny Smith’ SB-2 and halo peels than in OH and NB peels, with no differences observed in violaxanthin and antheraxanthin concentrations. The results indicate uniform changes in chlorophyll concentrations as a result of sunburn, while changes in carotenoid concentration are cultivar specific.

Changes in pigment concentrations associated with sunburn browning of five apple cultivars. II. Phenolics

Changes in pigment concentrations associated with sunburn were studied by taking peel disks from the sunburned area (SB-2), the area around the sunburned area (halo), the area around the halo (OH), and the center of the sun-exposed side of non-sunburned (NB) ‘Fuji’, ‘Gala’, ‘Delicious’, ‘Golden Delicious’, and ‘Granny Smith’ apples ( Malus domestica Borkh). Idaein concentrations in the red cultivars (i.e. ‘Fuji’, ‘Gala’, and ‘Delicious’) decreased from NB to OH to halo to SB-2. Total quercetin glycosides increased from OH to halo to SB-2 in all cultivars except ‘Delicious.’ Chlorogenic acid (CA) concentrations were typically highest in the SB-2 peel and lowest in NB, except in ‘Granny Smith’ where no CA was detected in any peel type.

Changes in Pigment Concentrations Associated with the Degree of Sunburn Browning of ‘Fuji’ Apple

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.

ENVIRONMENTAL STRESSES THAT CAUSE SUNBURN OF APPLE

ENVIRONMENTAL STRESSES THAT CAUSE SUNBURN OF APPLE

Two Types of Sunburn in Apple Caused by High Fruit Surface (Peel) Temperature

Sunburn of apple costs growers millions of dollars in economic losses annually. We have identified two types of sunburn in apple and their causes. The first (sunburn necrosis) is caused by thermal death of epidermal and subepidermal cells (peel), and causes a necrotic spot on the sun-exposed side of the fruit. Thermal death occurs at 126 ± 2°F (52 ± 1°C). Electrolyte leakage increases significantly with necrosis, indicating that membrane integrity is lost during thermal death. The second type (sunburn browning) is sublethal and results in a yellow, bronze, or brown spot on the sun-exposed side of the fruit. Sunburn browning occurs at a fruit surface temperature of 115 to 120°F (46 to 49°C) and has little effect on membrane integrity, but sunlight is required for sunburn browning.Accepted for publication 24 September 2001. Published 4 October 2001.