Responses of ‘Red Delicious’ apples to exogenous ethylene applied after normoxic or hypoxic storage

The quality and ripening behavior of apples (Malus domestica Borkh.) after long-term storage are strongly influenced by the atmospheric conditions. One of the applied protocols is based on oxygen levels below 0.7 kPa pO2 (extremely low oxygen, ELO) that is effective in slowing down metabolic activity and altering ripening physiology. Since at the end of storage apples might be exposed to ethylene sources, this study aimed at evaluating how long-term ELO storage (0.3 kPa pO2 for 180 days) affects apple (‘Red Delicious’) peel metabolic responses to post-storage ethylene exposure (50 ppm for 6 h). After ethylene application, fruit stored under regular atmosphere showed, compared to ELO samples, more advanced ripening levels, evidenced by a lower firmness and high accumulation of volatile esters. In contrast, ELO samples showed higher flesh firmness values together with a pronounced and distinct alteration of polyphenol metabolism, coupled with a depletion in sugars and organic acids. Our findings suggest that the metabolic response of ‘Red Delicious’ apples to exogenous ethylene is strongly influenced by prior storage conditions and based on preliminary data on the expression of ethylene receptor genes, this may be potentially due to modifications of ethylene perception.