A metabolomics approach to elucidate apple fruit responses to static and dynamic controlled atmosphere storage

The response of apple fruit to storage conditions based on low oxygen protocols depends on their genetic background. In order to elucidate common and divergent processes characterizing the metabolic changes under hypoxia, fruit of two apple (Malus domestica) varieties ('Granny Smith', GS, and 'Red Delicious', RD) were stored under two different low oxygen protocols (Ultra Low Oxygen, ULO, at 0.9 kPa oxygen, and Dynamic Controlled Atmosphere based on chlorophyll fluorescence, DCA-CF, between 0.2 and 0.55 kPa oxygen) for up to 200 and 214 days of storage for GS and RD samples, respectively. Through an integrated metabolomics approach (1H NMR, GC-MS, HS-SPME-GC-MS analyses) a total of 130 metabolites (volatiles and non-volatiles) were identified. Most of them (117) were common to both cultivars; 95 were significantly different between both cultivars when comparing the whole set of data (ULO + DCA-CF), whereas 13 volatile compounds, identified via HS-SPME-GC-MS, were specific for either GS or RD. Multivariate analyses (PCA and PLS) of the whole dataset allowed to clearly discriminate between GS and RD samples. When storage condition was used as a categorical response variable, a lower percentage explained variance was obtained as this effect was overshadowed by the large effect of storage time. After 4 months of storage, RD underwent more pronounced metabolic compositional changes of the cortex, possibly associated with the evolution of ripening. Based on the accumulation pattern of pyruvate-derived metabolites (ethanol, acetaldehyde, lactate, alanine) it can be hypothesized that there are two main metabolic reconfiguration strategies in GS and RD to regenerate NAD+ and cope with energy crisis under hypoxia. GS showed more pronounced responses through changes in the nitrogen metabolism and limited induction of the ethanol fermentation while the latter was highly induced in RD under both ULO and DCA-CF. Marked differences were detected between the VOC profiles of the two cultivars regardless storage conditions. Ethyl esters and 2-methylbutyl derivatives appeared finely modulated by the oxygen level in GS and RD apples, respectively.