The low temperature normally applied to prevent fruit decay during the storage of apples, can also triggers the onset of a chilling injury disorder known as superficial scald. In this work, the etiology of this disorder and the mechanism of action of two preventing strategies, such as the application of 1-MCP (1-methylcyclopropene) and storage at low oxygen concentration in ‘Granny Smith’ and ‘Ladina’ apple cultivars were investigated. The metabolite assessment highlighted a reorganization of specific metabolites, in particular flavan-3-ols and unsaturated fatty acids, while the genome-wide transcriptomic analysis grouped the DEGs into four functional clusters. The KEGG pathway and GO enrichment analysis, together with the gene-metabolite interactome, showed that the treatment with 1-MCP prevented the development of superficial scald by actively promoting the production of unsaturated fatty acids, especially in ‘Granny Smith’. ‘Ladina’, more susceptible to superficial scald and less responsive to the preventing strategies, was instead characterized by a higher accumulation of very long chain fatty acids. Storage at low oxygen concentration stimulated a higher accumulation of ethanol and acetaldehyde together with the expression of genes involved in anaerobic respiration, such as malate, alcohol dehydrogenase and pyruvate decarboxylase in both cultivars. Low oxygen concentration, likewise 1-MCP, through a direct control on ethylene prevented the onset of superficial scald repressing the expression of PPO, a gene encoding for the polyphenol oxidase enzyme responsible of the oxidation of chlorogenic acid. Moreover, in ‘Granny Smith’ apple, the expression of three members of the VII subgroups of ERF genes, encoding for elements coordinating the acclimation process to hypoxia in plants was observed. The global RNA-Seq pattern also elucidated a specific transcriptomic signature between the two cultivars, disclosing the effect of the different genetic background in the control of this disorder.

After harvest, apples are stored at low temperature to slow down the ripening physiological processes. This strategy can, however, also promote the development of superficial scald, a chilling injury-related disorder showing brown-discoloured areas on the fruit skin, totally compromising its marketability. To examine thoroughly the underlying physiological mechanisms and genetic control of superficial scald, the “Scald-Cold” project, a three-year Interegional Project Network, was granted by the European Region Tyrol, South Tyrol and Trentino (EGTC). The project is centred on a comprehensive approach, integrating into a joint effort different scientific disciplines, ranging from genetics and transcriptomics to NIR spectroscopy and hyperspectral imaging, together with the employment of distinct postharvest storage technologies distinguished by a specific regime of low oxygen. The core of this project aims to disclose novel regulatory processes of this disorder, providing new series of tools important for both the scientific and technical communities interested in apple breeding and postharvest. The “Scald-Cold” project will identify new molecular markers suitable for the selection of new apple varieties genetically resistant to superficial scald, while developing tools for an early detection of this phenomenon.

No abstract