Sun-related physiological disorders have a substantial annual economic impact on the worldwide apple (M. domestica) industry, particularly when grown in semiarid climates where losses can be from 10% to 50% of total production (Racsko and Schrader, 2012; Yuri et al., 2000b). Climatic conditions in these regions can elevate photooxidative and heat stress throughout the growing season affecting the tree and fruit physiology. Skin browning or “stain” can develop on ‘Fuji’ apples during cold storage (Mattheis, 1996; Schrader et al., 2008). “Stain” symptoms are typically browning or discoloration that begin to appear 1–2 months after harvest (Schrader et al., 2008) on the periphery of sunburned or sun-exposed areas of the peel (Felicetti and Schrader, 2010; Kupferman, 1994). Symptoms are very superficial and only occur in the first epidermal cell layer of the peel. To date, there are no known postharvest crop protectant or storage regimes that reduce this problem. “Stain” is not prevented by postharvest diphenylamine or AsA (0.2%) treatments at harvest (Kupferman, 1994).
As with sunscald of ‘Granny Smith’ apples (Contreras et al., 2008; Hernandez et al., 2014; Lurie et al., 1991), “stain” development is associated with sun exposure in the orchard as well as sunburn incidence and severity (Fan and Mattheis, 1998; Felicetti and Schrader, 2010; Schrader et al., 2003, 2008). Fuji apples are highly susceptible to “stain,” although it can occur on other cultivars including Royal Gala and Honeycrisp apples. Limiting sunlight exposure on ‘Fuji’ apples using bags prevents “stain” development (Fan and Mattheis, 1998). Raynox® is an orchard-applied carnauba-based coating that blocks ultraviolet-B and, consequently, reduces sunburn and “stain” incidence (Schrader et al., 2008).
Sun injury or sunburn of fleshy fruit is caused by absorption of excess solar energy by exposed tissue, leading to photoinhibition and oxidative stress (Ma and Cheng, 2003; 2004; Torres et al., 2006). Defense mechanisms activated in tissue exposed to direct sunlight or during early stages of sunburn include antioxidants (i.e., AsA, GSH) and enzymes functioning as antioxidants or regeneration of antioxidants such as, ascorbate peroxidase (APX; EC 18.104.22.168), dehydroascorbate reductase (DHAR; EC 22.214.171.124), monodehydroascorbate reductase (MDHAR; EC 126.96.36.199), glutathione reductase (GR; EC 188.8.131.52), catalase (CAT; EC 184.108.40.206), and superoxide dismutase (EC 220.127.116.11) (Chen et al., 2008; Ma and Cheng, 2003, 2004). However, activity of these recycling enzymes is not always higher in sun-damaged apple peel (Zhang et al., 2014).
Levels of certain pigments, including chlorophylls and carotenoids, decrease as sun-injury symptoms increase, whereas others, including xanthophylls, increase with light stress, possibly as a photoprotective mechanism (Chen et al., 2008; Ma and Cheng, 2003; Tartachnyk et al., 2012; Torres et al., 2006; Wünsche et al., 2001). Phenolic compounds rapidly accumulate on sun-exposed peel in response to direct sunlight (Felicetti and Schrader, 2008; Yuri et al., 2010). Phenolic compounds also appear to be directly involved in sunscald development in ‘Granny Smith’ apples during cold storage (Hernandez et al., 2014). The antioxidative capacity and regeneration of ascorbate by the AsA–GSH pathway ostensibly has an indirect role in sunscald development by improving defense against postharvest oxidative stresses (Hernandez et al., 2014). During this process, quercetin oxidation and accumulation of its brown products may be responsible for the symptoms (Jimenez and Garcia-Carmona, 1999). Flavonols, including quercetin can be substrates for polyphenol oxidase from a variety of plant species (Gasic et al., 2004; Jimenez and Garcia-Carmona, 1999) indicating that this defense-related process may, likewise, have a similar function in apple in this instance. We hypothesize that, while mechanisms resulting in sunscald may be similar, they can culminate in different symptoms on ‘Fuji’ peel. Accordingly, our objective was to evaluate antioxidant metabolism to compare “stain” and sunscald development.
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