Distribution, developmental and stress responses of antioxidant metabolism in Malus

A comprehensive study was carried out to examine the interactions between the two major hydrophilic antioxidants L-ascorbate (vitamin C, L-AA), and glutathione (gamma-glutamyl cysteinylglycine, GSH), and other antioxidant pools in tissues of Malus, to identify factors affecting steady-state cellular concentrations. We show that in Malus, each tissue type has a characteristic and different L-AA/GSH ratio and that in fruit, exocarp (epidermal) tissue acclimated to high light has higher L-AA levels but lower GSH levels than shaded (green) areas. Maturing seeds were characterized by the highest concentrations of GSH and a highly oxidized L-AA pool. It is demonstrated that fruit seeds are capable of L-AA biosynthesis, but that this occurs exclusively by means of the Smirnoff-Wheeler pathway. By contrast, foliar tissue was also able to synthesize L-AA using uronic acid substrates. Unlike the fruit of some other plant species however, the remaining fruit tissues are incapable of de novo L-AA biosynthesis. The observed differences in the steady-state concentrations of L-AA and GSH and the capacity to withstand stress in fruit, were also independent of the rates of uptake of photosynthate or of L-AA, but were correlated with the protective effect provided by phenolic compounds in these tissues. During development and maturation, L-AA and GSH levels in apple fruit declined steadily while foliar levels remained essentially constant throughout. However there was no apparent relationship between the free sugar contents of the fruit and antioxidant concentrations.