Fructose Content and the Activity of Fructose-related Enzymes in the Fruit of Eating-quality Peach Cultivars and Native-type Peach Cultivars

Significant differences in fructose content were detected among different cultivars of the peach (Prunus persica (L.) Batsch). To investigate the factors that regulate fructose concentrations in peach fruit, we measured the activities of fructose-related enzymes in two eating-quality peaches, 'Akatsuki' and 'Kawanakajima hakuto', and two native-type peaches, 'Nagano yaseito Early' and 'Noto zairaito No. 2'. Fructose contents in the fruit of the eating-quality peach cultivars, measured from 3 months after flowering to the ripe stage, ranged from 7 to 12 mg · g-1 FW, whereas those of the native-type peach cultivars were less than 1 mg · g-1 FW. There were no clear differences between the two groups with respect to fructokinase, fructose-6-phosphatase, and phosphoglucose isomerase (PGI) enzyme activities although they were low in 'Noto zairaito No. 2'. In all cultivars, the activity of PGI was much higher than that of the other enzymes. This finding suggests that isomerization by PGI is not the rate-limiting step in the synthesis of fructose. The concentration of fructose-6-phosphate (Fru6P) in the fruit did not differ between the two groups. These results suggest that the differences in fructose content in different peach cultivars are related to different capacities for fructose production rather than to differences in the ability to convert fructose to Fru6P and then to glucose-6-phosphate. NAD-dependent sorbitol dehydrogenase (SDH) activity was found to be higher in the eating-quality peach cultivars than in the native-type peach cultivars during all fruit development stages tested in this study. It is therefore likely that SDH, which contributes to the production of fructose from sorbitol, is responsible for the regulation of fructose concentrations in peach fruit.