Fruit photosynthesis and pome fruit quality

Abstract

Fruit taste in terms of fruit quality is dominated by the sugar:acid ratio. Photosynthesis and carbon metabolism of fruit differ from that of leaves in that organic acids contribute to the carbon storage pool despite starch and sugar. Malic acid is the predominant organic acid in pome fruits, induces their acidic taste and is produced by a two-step enzymic reaction. It involves the synthesis of (1) the instable oxaloacetic acid (OAA) by phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) or phosphoenolpyruvate carboxykinase (PCk; EC 4.1.1.49), and (2) malic acid by malic dehydrogenase (MDH). These enzymes were purified from apple fruit cvs Golden DeUcious'and 'Cox's Orange Pippin' at Long Ashton, University of Bristol, England. This collaborative work has shown that the C02 concentrating mechanism by PEPC and the kinetic properties of apple fruit PEPC, both resulting in malic acid synthesis, suggests the definition of a new type of 'fruit photosynthesis' rather than its categorisation within an existing type.