Roles of phenolics, ethylene and fruit cuticle in scald development of apples (Malus domestica Borkh).

Fruit ethylene was manipulated by harvest maturity, 2-dichloroethylphosphonic acid (ethephon), aminoethoxyvinylglycine (AVG), and storage conditions. Advanced maturity or ethephon reduced scald on 'Delicious'. AVG-treated fruit scalded like controls in a commercial room (high ethylene), but developed no scald in a low-ethylene room. Free phenolics were present in wax scraped from apples, and in enzyme-isolated fruit cuticle. Free phenolics were 8 to 45 $\\rm\\mu g{\\cdot}g\\sp{-1}$ cuticle. Advanced maturity and ethephon increased, and AVG decreased, cuticular phenolics. They increased during early storage, then remained unchanged. Bound phenolics were 50 to 110 $\\rm\\mu g\\cdot g\\sp{-1}$ cuticle, and were unaffected by fruit maturity, AVG or ethephon, remaining constant during storage. In a linoleic acid system, antioxidant activity of standard flavonoids was higher than that of phenolic acid standards. Antioxidant activity of free cuticular phenolics was equivalent to that of flavonoid standards, and activity of bound phenolics approximated phenolic acid standards. Both were lower than diphenylamine and higher than $\\alpha$-tocopherol. In hexane, cuticular phenolics inhibited $\\alpha$-farnesene oxidation more than diphenylamine. Free cuticular phenolics correlated negatively with conjugated trienes (CT281) and scald. At harvest epidermal and hypodermal cells contained 50-fold higher phenolics than did cuticle, correlating positively with scald. Advanced maturity or ethephon reduced cellular phenolics and scald. AVG increased them, but fruit did not scald in a low-ethylene room. In a linoleic acid system, cuticular lipid-soluble antioxidant activity was 10-15% of total activity. Cuticular lipid-soluble antioxidants did not reduce $\\alpha$-farnesene oxidation in hexane. Bagging fruit mid-July until harvest reduced cuticle thickness and phenolic contents, but did not affect ethylene, $\\alpha$-farnesene or cellular phenolics. Bagging increased CT281 and scald. Waxing at harvest did not inhibit $\\alpha$-farnesene oxidation or reduce scald. Conclusions. Cuticlar free phenolics are relatively stable, protecting $\\alpha$-farnesene from oxidation and reducing scald susceptibility. Minimally, four factors are involved in scald development: cuticular $\\alpha$-farnesene, phenolics in vacuole, cuticular phenolics, and antioxidants in living cells. The former two are enhancing; the latter two, protective. With low ethylene, fruit synthesize little $\\alpha$-farnesene and develop no scald. Ethylene increases $\\alpha$-farnesene, stimulates cuticular phenolic accumulation and lipid-soluble antioxidant activity, and reduces free phenolics in vacuoles. The balance determines scalding.