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

Changes in phosphoenolpyruvate carboxylase (PEPC) activity were studied dur ing the fruit development of two peach cultivars (Prunus persica (L.) Batsc h) with normal ('Fantasia') and low ('Jalousia') organic acid concentration . PEPC activities were measured in fruit mesocarp at two stages of developm ent, corresponding to malate accumulation and citrate accumulation phases i n 'Fantasia', respectively. In vitro activity, measured under optimal condi tions, 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 (CO2)-C-14 labelling. The total incor porated radioactivity was higher in 'Fantasia' than in 'Jalousia' during th e malate accumulation phase but lower during the citrate accumulation phase . During the malate accumulation phase, the partitioning of incorporated C- 14 into malate was significantly reduced in 'Jalousia' compared to 'Fantasi a' after 20 min labelling. During the citrate accumulation phase, C-14 part itioning 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 controllin g step for the absence of organic acid accumulation in 'Jalousia' fruit. Th e other metabolic causes of the difference in organic acid accumulation are discussed; these may be connected with vacuolar storage.