Metabolism of 1‐aminocyclopropane‐1‐carboxylic acid in ripening apple fruits

In preclimacteric apple fruits (Malus × domestica Borkh. cv. Golden Delicious) ethylene production is controlled by the rates of 1‐aminocyclopropane‐1‐carboxylic acid (ACC) synthesis, and by its metabolism to ethylene by the ethylene‐forming enzyme and to 1‐(malonylamino)cyclopropane‐1‐carboxylic acid (MACC) by malonyl CoA‐ACC transferase. The onset of the climacteric in ethylene production is associated with an increase in the activity of the ethylene‐forming enzyme in the pulp and with a rise in the activity of ACC synthase. Malonyl transferase activity is very high in the skin of immature fruit, decreases sharply before the onset of the climacteric, and remains nearly constant thereafter. More than 40% of the ACC synthesized in the skin and around 5% in the flesh, are diverted to MACC at early climacteric. At the climacteric peak there are substantial gradients in ethylene production between different portions of the tissue, the inner cortical tissues producing up to twice as much as the external tissues. This increased production is associated with, and apparently due to, increased content of ACC synthase. Less than 1% of the synthesized ACC is diverted to MACC in the flesh of climacteric apples. In contrast, the skin contains high activity of malonyl transferase, and correspondingly high levels [1000 nmol (g dry weight)−1] of MACC.