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- Author or Editor: William J. Bramlage x
Preharvest temperature (hrs. below 10°C) is a predictor of scald development on North American apples after long-term storage. In Mass., these variables are highly negatively correlated for both `Delicious' and `Cortland' apples. However, this predictor was not generally applicable for scald development on `Delicious' or `Granny Smith' apples in New Zealand. There, the relationship between temperature and scald varied greatly among districts for both cultivars, with scald resistance often occurring with far fewer hrs. below 10° than in Mass. Yet, in two cases when the temperature: scald relationship for `Granny Smith' was similar to that for `Cortland' and `Delicious' in Mass., temperature patterns during fruit ripening were similar to that in Mass. In all other cases, more moderate temperature changes occurred. These results suggest that the true base temperature for developing resistance to scald may be between 10° and 15°C, rather than 10°.
`Cortland' and `Delicious' apples were stored at 0C for up to 25 weeks, and at 20C in either open boxes or non-sealed poly bags for up to 8 weeks. At 20C, lesions occurred mostly around lenticels, but with some calyx bronzing and scald-like symptoms on shaded areas. At 0C, typical scald symptoms occurred. At both temperatures, high concentrations of alpha-farnesene and conjugated trienes occurred in conjunction with symptom appearance, and both these concentrations and discolorations decreased with later harvest of fruit. Scald development appeared to be chilling-enhanced, but not chilling-dependent.
We expect that a plant of tropical or subtropical origin is chilling sensitive. A corollary might be that a plant not of tropical or subtropical origin is not chilling sensitive. Although there are exceptions to such a corollary, which I shall be describing, this evolutionary distinction is largely true because even those temperate crops that are chilling sensitive have developed survival mechanisms to circumvent this genetic liability.
Effects of fruit maturity, aminoethoxyvinylglycine (AVG) and 2-chloroethylphosphonic acid (ethephon) preharvest treatments, and storage conditions on cuticular phenolic concentration, α-farnesene oxidation, and scald susceptibility of `Delicious' apple were studied. Advanced maturity and ethephon reduced scald. AVG totally inhibited scald when the AVG-treated fruit were stored in low-ethylene room (<1 μL•L-1). In commercial room (ethylene >5 μL•L-1), however, AVG did not reduce scald. Advanced maturity and ethephon did not alter α-farnesene accumulation but significantly reduced conjugated triene (CT281) formation. AVG reduced α-farnesene and CT281 accumulation to very low levels in low-ethylene room but not in commercial room. Both advanced maturity and ethephon increased free phenolics in fruit cuticle, while AVG reduced them. Free cuticular phenolics increased during early storage in ethephon-treated and control fruit but not in AVG-treated fruit. Overall, free phenolics in fruit cuticle negatively correlated with formation of CT281 and scald susceptibility of apples. Neither fruit maturation nor AVG or ethephon treatment significantly affected lipid-soluble antioxidant concentration in fruit cuticle.
Developmental changes in total cuticle and cuticular constituents were studied with `Delicious' fruit. Total wax (0.31 mg/cm2) and total cutin (0.54 mg/cm2, including carbohydrate polymers) were low in young fruit. They increased during fruit growth and reached 1.41 and 2.47 mg/cm2 of fruit peel at harvest, respectively. During fruit ripening at 20 °C, total cutin did not change, but total wax increased rapidly and reached 2.15 mg•cm-2 at 6 weeks. The increase of cuticular wax paralleled the increase of internal ethylene in fruit. Wax was separated by column chromatograph into four portions, hydrocarbons and wax esters, free alcohols, free fatty acids, and diols. More than half of the diols was ursolic acid. During fruit development, more hydrocarbons and diols accumulated in cuticle than free fatty acids and alcohols. During fruit ripening, all of the four portions increased, coincident with the climacteric rise in ethylene, but the increase rates of free fatty acids and alcohols were higher than those of other portions. Aminoethoxyvinylglycine (AVG, 220 mg•L-1) preharvest treatment inhibited internal ethylene synthesis to below 0.5 μL•L-1 during 6 weeks at 20 °C, and also inhibited wax accumulation. Ethephon (200 mg/L) preharvest treatment increased ethylene production and accelerated wax accumulation. α-farnesene accumulation coincided with increased internal ethylene and paralleled free fatty acid and alcohol accumulation.
Three experiments were conducted using `Cortland' and `Delicious' apples (Malus domestica Borkh.). Fruit varying widely in superficial scald susceptibility, because of either endogenous or experimentally induced conditions, were stored for various intervals at 0C and evaluated for scald development at 20C. Samples were extracted periodically in hexane, and ultraviolet absorption spectra of the extracts were used to evaluate α-farnesene and conjugated triene (CT) relationships to scald development. CT concentrations were calculated using each of the three CT absorption maxima (258-290 nm, 269-290 nm, and 281-290 nm) and expressed accordingly as CT258, CT269, and CT281. The poor association of CT281 concentrations with scald development led us to propose that metabolic products of CT281 species are more likely to be associated with scald development than the species themselves.
When Cortland apples were stored at 0 and 20C, 0C reduced ethylene production and increased accumulations of ∞-farnesene and conjugated trienes (CTs) in fruit peel, but it resulted in a lower CT258: CT281 ratio than did 20C. At 20C no fruit developed scald, but at 0C, 84% of the fruit scalded. When fruit were stored at 0C but transferred to 20C for 5 days after 0 to 8 weeks at 0C, only transfer after 2 or 4 weeks reduced scald. During warming, ethylene production and accumulations of ∞-farnesene and CTs increased, but after 20 weeks of storage, when scald developed, warming at 2 and 4 weeks reduced CT281 and increased the CT258: CT281 ratio. Warming at other intervals had no effect.
Ethephon and diphenylamine (DPA) were used to examine the role of ethylene production in biochemical changes that precede development of superficial scald on `Cortland' apples (Malus domestica Borkh.) after cold storage. Treatments modified α-farnesene and conjugated triene (CT) accumulations in fruit peel, and their effects on CTs differed depending on whether CTs were measured at 258 nm (CT258) or 281 nm (CT281). Ethephon induced rapid and delayed effects on fruit, the former being stimulation of ethylene production and α-farnesene and CT accumulation in fruit peel, which could increase scald development, and the latter being a disproportionately higher accumulation of CT258 than of CT281 during prolonged cold storage, which was associated with reduced scald development. DPA treatment at harvest also produced rapid and delayed effects. It immediately reduced ethylene synthesis and α-farnesene and CT accumulation. In addition, during fruit storage at 0C, DPA reduced accumulation of CT281 more than that of CT258. The rapid and delayed effects of DPA should contribute to less scald development. These results showed that ethylene probably was involved in effects of ethephon and DPA on scald development and suggest that ethylene has a fundamental role in changes associated with superficial scald development.
Much correlative data support the hypothesis that superficial scald on apples results from oxidation of α farnesene to conjugated trienes (CT) in the coating of apples. However, these associations are poorly defined both chemically and physiologically. α Farnesene and CT are measured as OD 232 and OD 281-290, respectively, of a hexane extract of the fruit surface. During assays, we observed anomalies in absorbance characteristics of extracts from fruit with different scald potentials, particularly in the region of 258 nm. Results suggest that absorbance near 258 nm might represent a metabolite of CT, which may be further metabolized. It appeared that under different conditions, CT metabolism could be altered, resulting in changed ratios of OD 258/OD 281. Higher ratios correlated with lower scald development, regardless of CT concentration. Thus, CT metabolism, rather than its concentration, may determine if scald occurs.