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.
J. Song and R.M. Beaudry
Superficial scald is still one of the most important postharvest physiological disorders in apples. Commercial control of this disorder has been accomplished by selecting resistant cultivars, treating fruit with DPA and ethoxyquin, using oil-soaked fruit wraps and storing the fruit under low O2. However, the causal reason for scald development is still a mystery. Research has indicated that the scald-promoting factor or inducing compound may be formed or accumulated in apple cuticle then rediffused back into the hypodermis, thereby causing damage. Hydroperoxides, auto-oxidative product from α-farnesene, have been thought to be the toxic compounds, inducing scald; however, it is not explained how the hydroperoxides move from the cuticle to the hypodermis. The identification and dynamic changes of 6-methyl-5-heptene-2-one as a natural volatile in apple fruit during ripening were made, which accumulated in higher quantitaty in cuticular wax than in headspace. The close relationship between the chloroplast breakdown and amount of α-farnesene changes, the induction of scald-like symptom on the surface of apple fruit by 6-methyl-5-heptene-2-one and the sensitivity of fruit to this ketone damage were investigated. Our results suggest that the accumulation of 6-methyl-5-heptene-2-one in the cuticular wax of apple fruit might be the causal reason for scald development in apples.
Steven W. Pechous, Bruce D. Whitaker*, and Christopher B. Watkins
Fruit of different apple cultivars vary widely in susceptibility to superficial scald. The genetic and biochemical factors involved in this variation are unknown. Conjugated trienol (CTol) oxidation products of alpha-farnesene have been linked with scald induction, and a high rate of farnesene synthesis in peel tissue of scald-prone apples early in storage is often associated with development of the disorder. Pre-storage treatment of apple fruit with 1-methylcyclopropene (1-MCP) inhibits the early burst of farnesene production and prevents scald, suggesting that ethylene induces transcription of genes involved in farnesene synthesis. We recently cloned a gene from apple peel tissue, AFS1, which encodes alpha-farnesene synthase, the last enzyme in the farnesene biosynthetic pathway. In this study, expression of AFS1 was compared in scaldsusceptible Law Rome (LR) and scald-resistant Idared (IR) apples at harvest and over 20 weeks of storage at 0.5 C. AFS1 transcript levels were closely correlated with accumulation of farnesene and CTols. In fruit of both cultivars, a sharp increase in AFS1 mRNA during the first 4 to 8 weeks of storage preceded a proportional rise in farnesene and a subsequent increase in CTols. However, maximum levels of AFS1 transcript, farnesene, and CTols were, respectively, 2.5-, 4-, and 33-fold greater in LR than in IR apples. Treatment of fruit with 1-MCP at harvest suppressed the increases in AFS1 transcript and farnesene early in storage, but AFS1 expression and farnesene synthesis recovered in LR fruit after 20 weeks. Scald incidence in LR apples after 20 weeks at 0.5 °C plus 1 week at 20 °C averaged 86%, whereas IR fruit had no scald. 1-MCP treatment reduced scald incidence in LR to <1%.
N.A. Mir, M. Wendorf, and R.M. Beaudry
Chlorophyll fluorescence of three cultivars of apple, representing fruits that are considered highly susceptible (`Cortland'), moderate- to highly susceptible (`Red Delicious'), and resistant (`Empire') to superficial scald were studied in relation to scald development during storage. The preclimacteric harvested fruits from each variety were divided into two equal lots, lot one was treated with DPA (1000 ppm) and all the fruits (treated and untreated) were air-stored in separate bins at 0°C for 4 months. Chlorophyll fluorescence parameters, minimal fluorescence (Fo), maximal fluorescence (Fm) and the ratio of (Fm – Fo)/Fm, and various quenching components of variable fluorescence were measured at regular intervals during storage. The maximal level of fluorescence (Fm) at harvest varied between varieties; it was highest in `Empire', followed by `Red Delicious' and `Cortland', respectively. DPA dip treatment seemed to have no influence on chlorophyll fluorescence at harvest. Decline in Fm was found to be related to scald development during storage. The data on fluorescence quenching pattern and kinetics in relation to development of storage scald will be discussed. Changes in
H.P.V. Rupasinghe, G. Paliyath, and D.P. Murr
α-Farnesene is an acyclic sesquiterpene hydrocarbon that is a constituent of the surface wax of apples (Malus domestica Borkh.). Although, oxidation products of α-farnesene have been implicated in the development of the physiological disorder superficial scald in apple, the mechanism of α-farnesene biosynthesis has not been studied in detail. We are currently investigating α-farnesene biosynthesis in relation to superficial scald development in apples. Radiolabelled feeding experiments using isolated tissue segments indicated that α-farnesene is derived from trans,trans-farnesyl pyrophosphate (FPP), mainly in the skin rather than cortex. Among the other labeled products detected, farnesol level was over a hundred-fold higher compared to α-farnesene. However, [1-3H] trans,trans-Farnesol was not incorporated into α-farnesene. Feeding radiolabelled FPP to skin tissue segments of scald-developing and normal apples showed differential incorporation of radiolabel into various products. Though the incorporation into α-farnesene was nearly the same, there was higher levels of incorporation into farnesyl esters in normal apples. As well, the levels of radiolabelled in the farnesol fraction was three times higher in scald-developing regions. These results indicate that there are potential difference in the biosynthesis and metabolism of farnesyl components between scald-developing and normal apples. In studies using cell-free extracts, farnesol formation was observed from labeled FPP and was two-fold higher in crude membrane extract compared to crude cytosol. Our results indicate that α-farnesene formation in apple fruit tissue is through FPP and is possibly catalyzed by a single sesquiterpene synthase enzyme. Purification and characterization of this enzyme are in progress.
Morris Ingle, Mervyn C. D'Souza, and E.C. Townsend
Firmness, soluble solids concentration (SSC), starch index (SI), internal ethylene concentration (IE), and titratable acid concentration (TA) of `York Imperial' apple (Malus ×domestica Borkh.) fruit changed linearly with harvest date between 152 and 173 days after full bloom (DAFB). Firmness was positively correlated with TA, SSC was correlated with SI, and SI was negatively correlated with TA. After 150 days of refrigerated-air (RA) storage, there was no relationship between DAFB at harvest and firmness or superficial scald, but the malic acid concentration declined linearly and storage decay increased linearly with DAFB. Firmness had declined to a plateau and was not correlated with any variable at harvest. Malic acid concentration after CA storage was correlated with DAFB, firmness, SSC, and SI; scald was correlated with firmness and SI; and decay was correlated with DAFB, firmness, SSC, and SI. During 150 days of controlled-atmosphere (CA) storage (2.5% O2, 1.0% CO2), firmness and TA decreased as a linear function of DAFB. Percentage of fruit with scald and scald rating changed quadratically with DAFB, and decay increased linearly with DAFB. After 150 days of CA, firmness was correlated with DAFB, SI, and IE at harvest; TA was correlated with DAFB, firmness, SSC, TA, and SI; scald was correlated with firmness and SI; and decay was correlated with DAFB, SSC, and scald index at harvest. During 250 days of CA storage, firmness, TA, scald, and decay changed linearly with DAFB in only 1 or 2 years out of 3. Formulas were created to predict firmness after CA within 10 to 12 N (2.0–2.5 lb-f) and TA to within 25%.
Joshua D. Klein and Susan Lurie
`Anna' and `Granny Smith' apples (Malus domestics Borkh.) that were kept at 46C for 12 hours or at 42C for 24 hours before storage at 0C were firmer at the end of storage and had a higher soluble solids: acid ratio and a lower incidence of superficial scald than unheated fruit. These heat regimes produced results similar to those obtained by keeping fruit at 38C for 72 or 96 hours before storage. Prestorage regimes of 46C for 24 hours or 42C for 48 hours resulted in fruit damage after storage.
Joshua D. Klein and Susan Lurie
Apple (Malus domestica Borkh. `Grand Alexander') fruit were stored immediately at 0C after harvest or after being held at 38,42, or 46C for 72,24, or 12 h, respectively. Half of each fruit lot was dipped in 1.5 % CaCl before storage. Heating did not appreciably affect Ca uptake into epidermal or cortical tissue. Calcium and heat treatments acted synergistically in reducing the severity of superficial scald and in retaining fruit firmness after 5 months of storage, relative to nontreated or nonheated Ca-dipped fruit.
Xuetong Fan and James P. Mattheis
Enclosing `Fuji' apple (Malus ×domestica Borkh.) fruit in paper bags 2 months after full bloom delayed the increase in internal ethylene concentration at the onset of fruit ripening, and increased the respiration rate early in the bagging period. Bagging delayed and reduced red color development, especially on the blush side, but did not affect fruit resistance to gas diffusion. External surface color changed significantly within the first 4 days after bags were removed. Exclusion of UV-B from sunlight by Mylar film after paper bag removal impaired red color development. Bagging during fruit development increased superficial scald but eliminated stain during cold storage. Exposure to sunlight for 19 or 20 days before harvest reduced scald incidence in comparison with leaving bags on until harvest.
Cynthia L. Barden and W. J. Bramlage
Superficial scald development on apples is related to preharvest environmental conditions, perhaps through effects on endogenous antioxidant concentrations In 1989 we examined effects of maturity, light, and preharvest temperatures (< 10°C) on endogenous antioxidant levels in the fruit at harvest and on scald development after long-term storage in 0°C air. Cortland apple trees were sprayed with 500 ppm ethephon 1 month before normal harvest to create maturity differences. Fruit on other Cortland trees were bagged 1 month prior to harvest to reduce light interception. Samples also were harvested from other Cortland trees after exposures to different numbers of hours < 10°C, Hours < 10°C before harvest were negatively correlated to scald development. Ethephon treatment decreased scald incidence, and bagging increased it, The total lipid-soluble antioxidant activity increased with increasing hours < 10°C and with ethephon treatment, while bagging of fruit slightly decreased this antioxidant activity. To better understand the relationships between preharvest factors and antioxidant levels, individual antioxidants, including ascorbic acid, α tocopherol, anthocyanins and glutathione, are being analyzed.