| Abstract
| - Relationships among α-farnesene synthesis and oxidation, ethylene production and perception,antioxidative enzyme activities, and superficial scald development in fruit of three commercial applecultivars were investigated at the biochemical and gene transcriptional levels. Scald-susceptibleCortland and Law Rome and scald-resistant Idared apples were untreated or treated with the ethyleneaction inhibitor 1-methylcyclopropene (1-MCP) and stored for up to 25 weeks at 0.5 °C. Separateblushed (red) and unblushed (green) peel tissue samples were taken at harvest and after 2, 4, 6, 10,15, 20, and 25 weeks of storage. Large increases in peel tissue concentrations of α-farnesene andits conjugated trienol (CTol) oxidation products occurred in untreated Cortland and Law Rome andwere about 4−9-fold greater than those in Idared. In both Cortland and Law Rome, accumulation ofCTols in green peel was nearly twice that in red peel. 1-MCP treatment delayed and attenuatedα-farnesene and CTol accumulation in each cultivar. Activities of peroxidase (POX) and catalase(CAT) were lower in red peel than in green peel, with the exception of CAT in Law Rome, whereasno effects of 1-MCP on enzyme activities were detected except for Cortland. In control fruit, internalethylene concentrations (IECs) increased during the first 4−6 weeks to reach highest levels in Cortland,intermediate levels in Law Rome, and low levels in Idared. In 1-MCP-treated fruit, IECs increasedgradually to modest levels by 25 weeks in Cortland and Law Rome but were almost nil in Idared.Expression patterns of the α-farnesene synthase gene MdAFS1, the ethylene receptor gene MdERS1,and the ethylene biosynthetic genes MdACS1 and MdACO1 were generally in accord with the patternsof α-farnesene and ethylene production. In particular, MdAFS1 and MdACS1 showed similar patternsof expression in each cultivar. Among the controls, transcript levels increased more rapidly in Cortlandand Law Rome than in Idared during the first few weeks of storage. In 1-MCP-treated fruit, transcriptabundance in Cortland and Law Rome rose to untreated control levels after 10−15 weeks but remainedlow in Idared. Scald symptoms were restricted to unblushed skin, and the incidence in controls after25 weeks was nearly 100% in Cortland and Law Rome compared with 1% in Idared. 1-MCP treatmentreduced scald incidence to 14, 3, and 0% in Cortland, Law Rome, and Idared, respectively. Overall,the results support the proposed role of CTols in scald induction and indicate that α-farnesenesynthesis is tightly regulated by ethylene. However, gene transcription alone does not account forthe big differences in ethylene and α-farnesene production in Cortland, Law Rome, and Idared apples. Keywords: Apple; Malus domestica; ethylene; α-farnesene synthesis; conjugated trienes; geneexpression; superficial scald; physiological disorder
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