HEAT AND LIGHT STRESS/SUNBURN Schrader, L. [email protected]Professor Emeritus, Washington State University, Wenatchee, WA KEYWORDS Apple; skin disorders; sunburn necrosis; sunburn browning; photo-oxidative sunburn; fruit surface temperature (FST); Fuji stain; lenticel marking; sunburn scald in Granny Smith; water core in Honeycrisp ABSTRACT Several fruit skin disorders are induced by heat and/or light stress in apples (Malus x domestica Borkh.). Some disorders appear on fruit prior to harvest, whereas others do not appear until after harvest and cold storage. Fruit do not utilize much light energy, so excess light energy is converted to heat injury. On a hot day, the fruit surface temperature (FST) of the sun-exposed side of apple can be 20 to 30 °F above air temperature. Sunburn is usually the major cause of cullage. It appears before harvest and its incidence often provides an early signal that other disorders will appear later. Three types of apple sunburn have been characterized (Figure 1). One type (sunburn necrosis) is caused by heat alone. A high FST of ~126 °F results in thermal death followed by necrosis. A second type (sunburn browning) occurs with high FST (115 to 120 °F) and damaging UV-B radiation. A third type (photooxidative sunburn) appears to be caused by visible 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 results from photooxidative damage and can occur at much lower FST and without UV-B radiation. Another disorder that results from heat and light Necrosis Browning Type 3 Figure 1: Three types of fruit skin disorders induced by heat and/or light stress
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Professor Emeritus, Washington State University, Wenatchee, WA
KEYWORDS Apple; skin disorders; sunburn necrosis; sunburn browning; photo-oxidative sunburn; fruit surface temperature (FST); Fuji stain; lenticel marking; sunburn scald in Granny Smith; water core in Honeycrisp ABSTRACT Several fruit skin disorders are induced by heat and/or light stress in apples (Malus x domestica Borkh.). Some disorders appear on fruit prior to harvest, whereas others do not appear until after harvest and cold storage. Fruit do not utilize much light energy, so excess light energy is converted to heat injury. On a hot day, the fruit surface temperature (FST) of the sun-exposed side of apple can be 20 to 30 °F above air temperature. Sunburn is usually the major cause of cullage. It appears before harvest and its incidence often provides an early signal that other disorders will appear later. Three types of apple sunburn have been characterized (Figure 1). One type (sunburn
necrosis) is caused by heat alone. A high FST of ~126 °F results in thermal death followed by necrosis. A second type (sunburn browning) occurs with high FST (115 to 120 °F) and damaging UV-B radiation. A third type (photooxidative sunburn) appears to be caused by visible 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 results from photooxidative damage and can occur at much lower FST and without UV-B radiation. Another disorder that results from heat and light
Necrosis Browning Type 3
Figure 1: Three types of fruit skin disorders induced by heat and/or light stress
stress is lenticel marking. Its incidence increases in fruit that have more severe sunburn. Fuji stain is a skin disorder that 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. We have evidence that UV-B radiation is involved; incidence of stain is higher in orchards with excess Nitrogen. Another disorder that is enhanced by heat stress is bitter pit (especially in Jonagold). Although Calcium deficiency is reported to cause bitter pit, we observed increased bitter pit in Jonagold apples that were exposed to high temperatures and water stress as they neared maturity. Color of the peel did not develop normally, but had a blotchy appearance. High temperatures near maturity also increased water core in ‘Honeycrisp’. This physiological disorder is associated with internal moisture stress, and high temperatures cause premature conversion of starch to sugar and pronounced leakage from cells into intercellular spaces. Sunburn scald in ‘Granny Smith’ develops during cold storage on the sun-exposed side of fruit that showed sunburn browning earlier in the season. If sunburn browning is prevented, the skin disorders listed above seldom appear. This suggests that management practices are needed to reduce absorption of UV-B radiation and to keep FST below critical temperatures at which sunburn browning occurs. Such practices include overhead evaporative cooling; sunscreen such as RAYNOX; sun blockers such as Eclipse or Surround WP; and photo-selective netting.