Fermentative metabolism of three apple cultivars during storage under low partial pressures of oxygen

The effects of low partial pressures of oxygen (pO2) on metabolism and fermentation-related gene expression of 'Empire', 'Jonagold' and, 'Fuji' apples have been investigated. Fruit were stored in 0.5, 1.0, and 1.5 kPa O2 (with 1 kPa CO2) at 0.5 °C for up to 6 months. During storage the lowest internal ethylene concentrations were observed in the 0.5 kPa O2 for ‘Empire’ and ‘Fuji’, but the pO2 did not affect those of ‘Jonagold’. pO2 did not affect flesh firmness of ‘Empire’ and ‘Fuji’ because little softening occurred in pO2, but for ‘Jonagold’ firmness was inversely related to pO2. In general, fermentation increased during storage in 0.5 kPa O2 but not in 1.5 kPa O2. However, 0.5 kPa O2 led to up-regulation of MdPDC2 and MdADH1 and down-regulation of MdADH3 in ‘Empire’ and ‘Fuji’, while expression levels of these genes at all pO2 in ‘Jonagold’ were similar. Metabolic responses to pO2 were affected by genetic background. We observed that regulation of MdADH1 is a key factor in control of ethanol accumulation of 'Empire' and 'Fuji' apples. On the other hand, MdADH1 in 'Jonagold' may be associated with stress tolerance to low O2 rather than with fermentation product biosynthesis.