Organic acid concentration is little controlled by phosphoenolpyruvate carboxylase activity in peach fruit

Abstract

Changes in phosphoenolpyruvate carboxylase (PEPC) activity were studied during the fruit development of two peach cultivars (Prunus persica (L.) Batsch) with normal (‘Fantasia’) and low (‘Jalousia’) organic acid concentration. PEPC activities were measured in fruit mesocarp at two stages of development, corresponding to malate accumulation and citrate accumulation phases in ‘Fantasia’, respectively. In vitro activity, measured under optimal conditions, was significantly higher in ‘Fantasia’ than in ‘Jalousia’ during the malate accumulation phase but lower during the citrate accumulation phase. In vivo activity was estimated using ¹⁴CO₂ labelling. The total incorporated radioactivity was higher in ‘Fantasia’ than in ‘Jalousia’ during the malate accumulation phase but lower during the citrate accumulation phase. During the malate accumulation phase, the partitioning of incorporated ¹⁴C into malate was significantly reduced in ‘Jalousia’ compared to ‘Fantasia’ after 20 min labelling. During the citrate accumulation phase, 14 C partitioning into malate was not significantly different between varieties, but partitioning into citrate was significantly reduced in ‘Jalousia’ compared to ‘Fantasia’. Therefore, PEPC activity does not seem to be the controlling step for the absence of organic acid accumulation in ‘Jalousia’ fruit. The other metabolic causes of the difference in organic acid accumulation are discussed; these may be connected with vacuolar storage.