LEVELS AND ROLE OF SUCROSE SYNTHASE, SUCROSE-PHOSPHATE SYNTHASE, AND ACID INVERTASE IN SUCROSE ACCUMULATION IN FRUIT OF ASIAN PEAR

Soluble sugar content and activities of the sucrose-metabolizing enzymes sucrose synthase (SS) (EC 2.4.1.13), sucrose-phosphate synthase (SPS) (EC 2.4.1.14), and acid invertase (EC 2.4.1.26) were analyzed in the pericarp of fruit from pear cultivars that differed in their potential to accumulate sucrose to identify key enzymes involved in sucrose accumulation in Asian pears. The Japanese pear 'Chojuro' [Pyrus pyrifolia (Burm. f.) Nakai] was characterized as a high-sucrose-accumulating type based on the analysis of mature fruit, while the Chinese pear 'Yali' (P. bretschneideri Rehd.) was a low-sucrose-accumulating type throughout all developmental stages. The activity of SS and SPS in 'Chojuro' increased during maturation concomitant with sucrose accumulation, whereas the activity of these enzymes in 'Yali' did not increase during maturation. The activity of SS and SPS in the former were seven and four times, respectively, higher than those in the latter at the mature stage. Further, among 23 pear cultivars, SS activity was closely correlated with sucrose content, while SPS activity was weakly correlated. Soluble acid invertase activity in 'Chojuro' and 'Yali' decreased with fruit maturation, but the relationships between soluble invertase activity and sucrose content were not significant. The results indicate that SS and SPS are important determinants of sucrose accumulation in Asian pear fruit and that a decrease of soluble acid invertase activity is not absolutely required for sucrose accumulation.