Postharvest metabolomics of soft scald and soggy breakdown of Malus domestica cv. 'Honeycrisp' fruit

Rachel Leisso

‘Honeycrisp' apple fruit marketability is negatively impacted by postharvest physiological disorders, including soft scald and soggy breakdown. These non-pathogenic disorders are thought to be provoked by environmental stresses, including postharvest chilling. Advanced harvest maturity also enhances disorder risk. The goal of this research was to understand metabolism associated with development of soft scald and soggy breakdown of ‘Honeycrisp in order to develop risk management tools for their control. Initially, field weather station data and standard quality/ maturity metrics were evaluated to determine their utility for disorder risk assessment. These metrics, including Brix, internal ethylene concentration, starch, firmness and titratable acidity, do not reliably predict fruit susceptibility to soft scald and soggy breakdown. Further tests characterized the metabolome associated with symptomatic fruit tissue, adjacent healthy tissue, and tissue from entirely healthy fruit. Metabolites associated with oxidative stress, membrane disruption, and fermentative metabolism were linked with soggy breakdown symptoms, providing a metabolic fingerprint for this disorder. A separate test sought to determine if internal CO2 injury is distinct from soggy breakdown visually as well as metabolically by evaluating the impact of elevated CO2 partial pressures on internal disorder development. The metabolic profile of high CO2-induced cortex browning was not different from cortex browning provoked by chilling. However, visual symptoms sometimes differed between ostensible high CO2 CA injury compared with soggy breakdown, and succinic acid was elevated in fruit stored in high CO2. Finally, to identify candidate metabolites/transcripts for possible risk management tools, metabolites and gene expression differing prior to soft scald development were determined using metabolomic and transcriptomic profiling. Several classes of metabolites and potentially associated transcripts were differently regulated according to soft scald susceptibility, including hydroxylated triterpenoids, sesquiterpenoids, phytosterols, and their conjugates. Overall, results affirm that pre- and postharvest environmental factors, CA and chemical treatments, and physiological maturity alter fruit metabolic activities related to soft scald/soggy breakdown and CA injury. Fruit at risk for these postharvest disorders may not be detectable with standard quality metrics, and results indicate that monitoring a set of metabolites and transcripts could lend accuracy to soft scald/soggy breakdown risk assessment and enable better storage management decisions.

Postharvest metabolomics of soft scald and soggy breakdown of Malus domestica cv. 'Honeycrisp' fruit

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