Fruit water relations and osmoregulation on apples (Malus domestica Borkh.) with different sun exposures and sun-injury levels on the tree

• We investigated fruit water relations and osmotic adjustment in sun injured apples.
• Skin water and solutes potentials decreased with fruit development and sun damage.
• Sun exposure induced sorbitol and glucose accumulation in the skin and flesh.
• Tissue toughness increased with sun exposures although internal ethylene increased.
• Sun exposure activates acclimation mechanisms, perhaps, via sugars and ethylene.

Sun-injury (=sunscald, sunburn) in fruit crops is caused by photodynamic reactions on heated fruit sections exposed to direct sunlight. There is evidence supporting the oxidative nature of this disorder. Nevertheless, there are other physiological events, typically observed in water-stressed plants, such as water relations and osmoregulation that greatly influence external and internal quality of sun-injured fruit and have poorly addressed in the literature. Apple tissues (skin and flesh) with different levels of sun exposure and sun-injury (mild, moderate (Mod), severe (Sev)) were sampled at different growing stages during 2011 and 2012, in Royal Gala and Fuji. Water, solutes, and turgor potentials, relative water content (RWC), carbohydrates concentration (sucrose, sorbitol, glucose, fructose), internal ethylene concentration (IEC), and textural curves were determined. Skin water potential decreased with fruit development and it was significantly more negative in sun-injured tissue compared to unexposed ones. Solutes potential also decreased in skin and flesh with increasing sun damage, but the opposite was found in turgor potential. Fruit RWC at harvest decreased with increasing sunburn severity. Sun exposure induced sorbitol and glucose accumulation in fruit peel and flesh. Tissue with Mod and Sev sunscald showed higher IEC early in the season. Fruit shape and firmness were also altered by sun exposure. The results might indicate that sun-exposed tissue via more negative water potentials activates an abiotic stress-response cascade, perhaps mediated by sugars and ethylene, to cope with the environmental stress caused by high irradiance and heat, whose consequences are peculiar fruit quality traits.