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John R. Stommel and Bruce D. Whitaker

Eggplant (Solanum melongena L.) is ranked among the top ten vegetables in terms of oxygen radical absorbance capacity due to its fruit's phenolic constituents. Several potential health promoting effects have been ascribed to plant phenolic phytochemicals. We report here a first evaluation of phenolic acid constituents in eggplant fruit from accessions in the USDA eggplant core subset. The core subset includes 101 accessions of the cultivated eggplant, S. melongena, and 14 accessions representing four related eggplant species, S. aethiopicum L., S. anguivi Lam., S. incanum L., and S. macrocarpon L. Significant differences in phenolic acid content and composition were evident among the five eggplant species and among genotypes within species. Fourteen compounds separated by HPLC, that were present in many but not all accessions, were identified or tentatively identified as hydroxycinnamic acid (HCA) derivatives based on HPLC elution times, UV absorbance spectra, ES-—MS mass spectra, and in some cases proton NMR data. These phenolics were grouped into five classes: chlorogenic acid isomers, isochlorogenic acid isomers, hydroxycinnamic acid amide conjugates, unidentified caffeic acid conjugates, and acetylated chlorogenic acid isomers. Among S. melongena accessions, there was a nearly 20-fold range in total HCA content. Total HCA content in S. aethiopicum and S. macrocarpon was low relative to S. melongena. A S. anguivi accession had the highest HCA content among core subset accessions. Chlorogenic acid isomers ranged from 63.4% to 96% of total HCAs in most core accessions. Two atypical accessions, S. anguivi PI 319855 and S. incanum PI500922, exhibited strikingly different HCA conjugate profiles, which differed from those of all other core subset accessions by the presence of several unique phenolic compounds. Our findings on eggplant fruit phenolic content provide opportunities to improve eggplant fruit quality and nutritive value.

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Bruce D. Whitaker and Gene E. Lester

Increases in phospholipase D [PLD (EC 3.1.4.4)] and lipoxygenase [LOX (EC 1.13.11.12)] activities are thought to play a critical role in senescence of mesocarp tissues in netted and nonnetted muskmelon (Cucumis melo L.) fruits. We have cloned and characterized two full-length cDNAs, CmPLDα1 and CmLOX1, encoding PLDα and LOX proteins in honeydew melon (C. melo Inodorus Group cv. Honey Brew). Relative levels of expression of the corresponding genes were determined by semi-quantitative RT-PCR in developing and mature fruit mesocarp tissues [20-60 d after pollination (DAP)], as well as in roots, leaves, and stems from 4-week-old and flowers from 6- to 7-week-old plants. The coding regions of CmPLDα1 and CmLOX1 cDNAs are, respectively, 2427 and 2634 nucleotides long, encoding proteins 808 and 877 amino acids in length. CmPLDα1 is very similar to PLDα genes from castor bean (Ricinis communis L.), cowpea (Vigna unguiculata L.), strawberry (Fragaria ×ananassa Duch.) and tomato (Lycopersicon esculentum Mill.) (77% nucleotide identity), and is the first PLD gene cloned from a cucurbit species. CmLOX1 has 94% nucleotide identity to a cucumber (Cucumis sativus L.) LOX gene expressed in roots and 80% identity to cucumber cotyledon lipid body LOX. In general, transcript of CmPLDα1 was much more abundant than that of CmLOX1, but relative levels of transcript in the various organs and tissues were similar for the two genes. Expression was highest in roots, flowers, and fruit mesocarp tissues. CmPLDα1 expression in fruit was essentially constitutive throughout development, although maximum levels occurred at 50 and 55 DAP, respectively, in middle and hypodermal mesocarp. CmLOX1 expression was generally higher in middle than in hypodermal mesocarp with maximum transcript levels occurring at 55 and 50 DAP, respectively. Overall, the patterns of expression of CmPLDα1 and CmLOX1 are consistent with a model in which their encoded enzymes act in tandem to promote or accelerate senescence in fruit mesocarp tissues.

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Bruce D. Whitaker, Joshua D. Klein, and William S. Conway

Postharvest heat treatment of apples maintains fruit firmness and reduces decay during storage. Four days at 38C are beneficial, but 1 or 2 days are detrimental. The cellular basis of these effects may involve changes in cell wall and membrane lipid metabolism. Lipids from hypodermal tissue of `Golden Delicious' apples were analyzed after 0, 1, 2, or 4 days at 38C. Major lipids included phospholipids (PL), free sterols (FS), steryl glycosides (SG), and cerebrosides (CB). Galactolipids (GL) were minor components. PL content fell ?10% after 1 day at 38C, was unchanged after 2 days, and began to rise again after 4 days. PL class composition did not change with heating, but fatty-acid unsaturation declined throughout. FS and CB content and composition changed little, whereas SG content cropped by ≈20% over 4 days. GL fell ≈50% during 1 day at 38C, with no change at days 2 or 4. A burst of PL catabolism followed by recovery of synthesis may in part explain the different effects of 1-, 2-, or 4-day heat treatments. GL loss (in plastids) may be related to the effect of heat on fruit color (yellowing).

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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%.

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Joshua D. Klein, William S. Conway, Bruce D. Whitaker, and Carl E. Sams

`Golden Delicious' apples (Malus domestica Borkh.) were treated postharvest with heat (38C/4 d or 42C/24 h) or 2% CaCl2 (applied as a dip or pressure-infiltrated) or a combination thereof and then stored. Decay caused by Botrytis cinerea was virtually eliminated in fruit heated at 38C after inoculation prior to storage, regardless of Ca treatment. Apples inoculated upon removal from storage were almost completely protected from decay if they had been previously pressure-infiltrated with Ca, regardless of heat regime. Heating at 42C or Ca dips were only partially effective in preventing decay. Pressure infiltration of Ca (regardless of heat regime) or heating at 38C (regardless of Ca treatment) resulted in firmer fruit (68 N) than Ca dips or heating at 42C (56 N), which were firmer than nontreated fruit (52 N).

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Joshua D. Klein, William S. Conway, Bruce D. Whitaker, and Carl E. Sams

`Golden Delicious' apples (Malus domestica Borkh.) were treated after harvest with heat (air at 38 °C for 4 days or 42 °C for 1 day) or 2% CaCl2 (w/v; applied as a dip or pressure-infiltrated) or a combination of the two and stored at 0 °C for ≤6 months. Decay caused by Botrytis cinerea Pers.:Fr. after inoculation to a depth of 2 mm with a conidial suspension virtually was eliminated in stored fruit heated at 38 °C, regardless of Ca treatment. Apples punctured to a depth of 0.5 mm (but not 2 mm) and inoculated with B. cinerea on removal from storage were almost completely protected from poststorage decay if they had previously been pressure-infiltrated with 2% CaCl2, regardless of the heat regime. Heating fruit at 42 °C and dipping in 2% CaCl2 were only partially effective in preventing decay from either pre- or poststorage inoculations. Fruit firmness was not related to resistance to decay.

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Jinhe Bai, Xinhua Yin, Bruce D. Whitaker, Kristi Deschuytter, and Paul M. Chen

Superficial scald of ‘Anjou’ pears (Pyrus communis) usually develops after cold storage of ≥3 months. Ethoxyquin has been used to control scald commercially. However, only a small amount of fruit can be treated within 7 days after harvest as recommended, and sometimes ethoxyquin causes phytotoxicity. Application of 1-methylcyclopropene (1-MCP) showed excellent scald control potential, with rapid and mass treatment feasible. However, fruit may lose normal ripening ability at a dosage of 1-MCP as low as 30 nL·L−1, whereas a dosage of ≤20 nL·L−1 is not enough to control scald. In this investigation, ‘Anjou’ pears treated with 25 nL·L−1 1-MCP immediately after harvest were stored at −1 °C for up to 5 months. After 1, 7, 30, or 60 days of cold storage, part of the fruit were treated with 1000 μL·L−1 ethoxyquin and the remainder was left untreated as nonethoxyquin controls. The incidence of superficial scald, the concentrations of α-farnesene and its conjugated triene (CT) oxidation products, and the ripening ability of fruit were measured after 3-, 4-, and 5-month storages. All fruit ripened properly within 7 days of shelf life at 20 °C regardless of treatment. 1-MCP treatment at harvest or ethoxyquin alone applied within 7 days adequately controlled scald for only 3 months. By contrast, 1-MCP + ethoxyquin controlled scald for 5 months, regardless of when ethoxyquin was applied from 1 to 60 days after the start of cold storage. Thus, a combination of 25 nL·L−1 1-MCP, which is easily applied and does not influence ripening ability, and a delayed application (up to 60 days) of 1000 μL·L−1 ethoxyquin, which is a low dosage that does not cause phytotoxicity on fruit, controlled scald sufficiently. Scald is linked with accumulation of CT oxidation products of α-farnesene. 1-MCP and ethoxyquin inhibited accumulation of CT in fruit peel by different mechanisms. 1-MCP inhibited the production by reducing α-farnesene synthesis and the oxidation to CT, whereas ethoxyquin worked by inhibiting the latter.