Regulation of ethylene, ACC, MACC production, and ACC-oxidase activity at various stages of maturity of apple fruit and the effect of exogenous ethylene treatment

Summary The regulation of ethylene production of apple fruit ( Malus domestica Borkh. , cv. 'Golden Delicious') at different maturity stages was investigated by measuring the concentration of 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, its conjugation to N-malonyl-ACC (MACC), and its oxidation by ACC-oxidase to ethylene. The concentration of these metabolites was determined in the peel and pulp of the fruits. Both ACC and ACC-oxidase were rather low in the peel and in the pulp and increased only slightly before the onset of ripening, indicated by the start of autocatalytic ethylene production. In contrast, MACC levels were considerably higher in the peel and pulp tissue. With the start of autocatalytic ethylene production and the onset of the respiratory climacteric, a concomitant rapid increase in ACC, MACC and ACC-oxidase was detected. However, this increase very much depended an the maturity stage at which the fruits were harvested: those harvested very early did not attain similar high levels/activities compared to those harvested close or even after the beginning of the climacteric. Even a continuous treatment with saturating concentrations of exogenous ethylene did not overcome the inability of prematurely harvested fruit to produce "sufficient" ACC and ACC-oxidase activity and thus autocatalytic ethylene production of these fruit was very much delayed. There was considerable compartmentalization of ACC, MACC and ACC-oxidase between peel and pulp of the fruit. The ACC-oxidase activity was e.g. considerably higher in the pulp then in the peel, whereas ACC and particularly MACC were higher in the peel. The experiments revealed that the pulp rather than the peel played a dominant role in the regulation of autocatalytic ethylene production and subsequent ripening.