Characterization of Soluble Sugar Content, Related Enzyme Activity and Gene Expression in the Fruits of ‘Minihyang’ Mandarin on Different Rootstocks

‘Minihyang’ mandarin bears fruits with small size and high sugar content. ‘Minihyang’ mandarin grafted on trifoliate orange (TO) tends to be vigorous and develops water sprout open. It is associated with insufficient floral differentiation and fruit set. Recently, the use of Flying Dragon (FD) as rootstock with a high dwarf effect has been proposed to improve this situation. Therefore, this study was conducted to evaluate the effect of two different rootstock genotypes on tree growth, fruit yield, and fruit quality at the physiological, biochemical, and molecular levels. As a result of the study, in FD, tree vigor was stably maintained, fruit size was large, and the sugar content was high compared to the TO. Fructose, glucose, and sucrose of fruit continued to increase from development to maturity. In particular, fructose and sucrose were significantly higher in the fruits of the FD than those in TO at 150 and 220 days after anthesis. The total sugar content was also significantly higher in the fruit of the FD. The activities of SPS and SS associated with sucrose synthesis tended to be increased during the fruit maturity season, but there was no significant difference between the two rootstocks. On the other hand, the activities of SS and AI breaking down sucrose were high in FD at 150 and 220 days after anthesis. These results suggest that the unloading of sucrose might be increased and affect the sugar content. However, the results of real-time PCR analysis of gene expression related to sucrose metabolism did not show an association with changes in enzyme activity affecting sugar content. Therefore, further detailed studies on the process after the regulation of gene expression are likely to be needed.