NADP+-Malic Enzyme and Organic Acid Levels in Developing Tomato Fruits

Abstract

Tomato (Lycopersicon esculentum Mill., cv. Ohio 7814) fruits were harvested weekly following flowering to relate changes in NADP+-malic enzyme (NADP-ME) activity and major organic acids (malate and citrate) to fruit development. Specific activity of NADP-ME and fresh weight concentrations of the acids reached maximal levels at the mature-green stage. During ripening, a decline in malate concentration was followed by decreases in NADP-ME activity and citrate concentration. Activity of NADP-ME and organic acid concentrations were highest in the locular gel, but activity also occurred in tomato leaves, stems, and roots. The data do not exclude a role for NADP-ME in the metabolism of organic acids during fruit ripening. However, it is also possible that the enzyme is involved in cytoplasmic pH regulation.