Phytosterol Conjugation in Cold-Stored Apple Fruit Is Linked to Oxidative Stress and Ripening

Low temperature stress is implicated in a wide-range of apple fruit postharvest necrotic disorders. Previously, untargeted metabolic profiling identified alterations in multiple metabolic processes that precede superficial scald symptom development. Metabolites with free sterol (FS) –like mass spectra were involved in this phenomenon. Subsequently, the identity of these compounds have been confirmed as glucose conjugates (SG), acylated glucose conjugates (ASG), and esters (SE) of campesterol and ß-sitosterol. Different levels of each of these metabolites are linked to storage longevity, ethylene insensitivity, and oxidative stress demonstrating divergent control of phytosterol conjugation associated with these processes. Reduced SE and elevated ASG levels over 6 months of cold storage in air are associated with storage stress and scald symptom development.