Sun injury on apple fruit: Physiological, biochemical and molecular advances, and future challenges

Climate change negatively influences many human activities and one of the most affected is agriculture. In the apple industry, water availability, elevated temperatures and altered phenology will transform fruit production in traditional growing regions. Extended periods of intense solar radiation and high temperatures during the growing season cause problems in fruit quality increasing losses and reducing sustainability and profitability. Photooxidative and heat stress stimulate sunburn development on apple fruit in the field growing under increasingly stressful conditions. In particular, apples growing in semi-arid conditions are frequently exposed to high solar irradiance and elevated temperature during the growing season that promote the development of sun-related skin disorders. Furthermore, regions that have traditionally not faced sunburn pressure may begin to experience losses in this area. Apple cultivars differ in their susceptibility to sun damage, which is evidenced, in part, by the timing of symptom development and severity. Some studies attribute genotypic variation to physiological and morphological differences while others do to antioxidant-related metabolic differences between them. Here, we discuss the physiological and molecular progress and gaps in knowledge of sunburn damage and the development of sunburn resistance in apple fruit. This information will help develop stronger sunburn mitigation strategies and enhance breeding efforts to address challenges associated with sunburn in apple production.