1-Methylcyclopropene differentially regulates metabolic responses in the stem-end and calyx-end flesh tissues of ‘Empire’ apple during long-term controlled atmosphere storage

Firm flesh browning, a postharvest disorder of ‘Empire’ apples, can develop during controlled atmosphere (CA) storage. The disorder is initiated in tissues at the stem-end tissue and progressively develops through the calyx-end region of fruit. The susceptibility of fruit to the disorder can be provoked by the application of 1-methylcyclopropene (1-MCP) prior to storage. To gain a better understanding of the metabolic changes associated with disorder development, we coupled metabolic profiling with measures of flesh tissue color in partial least squares-discriminant analysis (PLS-DA) to determine changes in metabolic profile occurring in stem and calyx tissues of untreated and 1-MCP treated fruit during CA storage. Flesh tissues were less light (L*) in the stem-end tissues than in the calyx-end tissues and to a greater extent in 1-MCP treated than untreated fruit. Reduction in L* in 1-MCP treated fruit corresponded to metabolic divergence between stem-end and calyx-end tissues. Furthermore, tissues of 1-MCP treated fruit had lower levels of volatile aromatic compounds (VACs), but less so in the stem-end tissues than in the calyx-end tissues. The levels of most amino acids and VACs were higher in the stem-end tissues of 1-MCP treated fruit than in untreated fruit. Of the VACs, methanol, acetaldehyde and ethanol concentrations were much greater in the stem-end tissues than in the calyx-end tissues of untreated fruit but reduced by 1-MCP treatment, irrespective of tissues. In summary, metabolic profiling indicates that 1-MCP treatment induces greater physiological changes in the stem-end tissues than in the calyx-end ones under hypoxic storage condition. While the underlying metabolic processes that result in firm flesh browning remain to be elucidated, the accumulation of small molecule compounds, such as amino acids, in the stem-end tissues suggests that 1-MCP affects ethylene related pathways in this region to a greater extent, and may be associated with higher incidence of flesh browning during long-term CA storage.