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The identification of genes involved in variation of peach fruit quality would assist breeders in creating new cultivars with improved fruit quality. Major genes and quantitative trait loci (QTLs) for physical and chemical components of fruit quality have already been detected, based on the peach [ Prunus persica (L.) Batsch] cv. Ferjalou Jalousia((R)) (low-acid peach) x cv. Fantasia (normally-acid nectarine) F(2) intraspecific cross. Our aim was to associate these QTLs to structural genes using a candidate gene/QTL approach. Eighteen cDNAs encoding key proteins in soluble sugar and organic acid metabolic pathways as well as in cell expansion were isolated from peach fruit. A single-strand conformation polymorphism strategy based on specific cDNA-based primers was used to map the corresponding genes. Since no polymorphism could be detected in the Ferjalou Jalousia((R)) x Fantasia population, gene mapping was performed on the almond [ Prunus amygdalus ( P. dulcis)] cv. Texas x peach cv. Earlygold F(2) interspecific cross from which a saturated map was available. Twelve candidate genes were assigned to four linkage groups of the peach genome. In a second step, the previous QTL detection was enhanced by integrating anchor loci between the Ferjalou Jalousia((R)) x Fantasia and Texas x Earlygold maps and data from a third year of trait assessment on the Ferjalou Jalousia((R)) x Fantasia population. Comparative mapping allowed us to detect a candidate gene/QTL co-location. It involved a cDNA encoding a vacuolar H(+)-pyrophosphatase ( PRUpe;Vp2) that energises solute accumulation, and QTLs for sucrose and soluble solid content. This preliminary result may be the first step in the future development of marker-assisted selection for peach fruit sucrose and soluble solid content.

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As in many other fleshy fruits, the predominant organic acids in ripe peach (Prunus persica (L.) Batsch) fruit are malic and citric acids. The accumulation of these metabolites in fruit flesh is regulated during fruit development. Six peach fruit-related genes implicated in organic acid metabolism (mitochondrial citrate synthase; cytosolic NAD-dependent malate dehydrogenase, and cytosolic NADP-dependent isocitrate dehydrogenase) and storage (vacuolar proton translocating pumps: one vacuolar H+-ATPase, and two vacuolar H+-pyrophosphatases) were cloned. Five of these peach genes were homologous to genes isolated from fruit in other fleshy fruit species. Phylogenetic and expression analyses suggested the existence of a particular vacuolar pyrophosphatase highly expressed in fruit. The sixth gene was the first cytosolic NAD-dependent malate dehydrogenase gene isolated from fruit. Gene expression was studied during the fruit development of two peach cultivars, a normal-acid (Fantasia) and a low-acid (Jalousia) cultivar. The overall expression patterns of the organic acid-related genes appeared strikingly similar for the two cultivars. The genes involved in organic acid metabolism showed a stronger expression in ripening fruit than during the earlier phases of development, but their expression patterns were not necessarily correlated with the changes in organic acid contents. The tonoplast proton pumps showed a biphasic expression pattern more consistent with the patterns of organic acid accumulation, and the tonoplast pyrophosphatases were more highly expressed in the fruit of the low-acid cultivar during the second rapid growth phase of the fruit.

Changes in metabolites were studied during the fruit development of two greenhouse grown peach [Prunus persica (L.) Batsch] cultivars with low acidity (`Jalousia') or normal acidity (`Fantasia'). Both cultivars had the same sucrose concentration in fruit mesocarp at maturity. In the fruit juice, pH was higher and titratable acidity was lower for `Jalousia' than for `Fantasia' from 80 days after bloom to maturity. At four different times during fruit development, in vivo 13C NMR spectroscopy was used to measure the vacuolar pH of fruit mesocarp. At 55 days after bloom, the vacuolar pH of fruit mesocarp was not significantly different between `Jalousia' and `Fantasia', whereas the juice pH was different between cultivars. The three major organic acids in fruit mesocarp were malic, citric, and quinic acids for both cultivars. Citric acid concentrations were similar in both cultivars until ≈85 days after bloom and then became significantly higher in `Fantasia'. A significantly higher concentration in malic acid in `Fantasia' than in `Jalousia' was observed from the end of the first growth phase to maturity. At maturity, `Fantasia' fruit had two and five times more malic and citric acid, respectively, than `Jalousia' fruit. The differences observed between `Jalousia' and `Fantasia' fruit for malic and citric acid concentrations accounted for the difference in titratable acidity. The differences in acid concentration appeared during the plateau between the two rapid growth phases of the fruit, i.e., far before the onset of maturation. The three major amino acids were asparagine, glutamic acid, and proline for both cultivars. Their concentration followed similar patterns in acid and low-acid fruit.