[Carbohydrate metabolism during fruit development of bayberry (Myrica rubra Sieb. et Zucc.)]

The dynamics of dry and fresh weight, the glucose, fructose, sucrose, titratable acid contents, and activities of sucrose-metabolizing and hexose-metabolizing enzymes were examined in developing fruits of bayberry (Myrica rubra Sieb. et Zucc. cvs. 'Wuzi' and 'Biqi'). The results showed the dry and fresh weight of bayberry fruit increased with fruit development and maturation (Fig. 1), with the highest increase rate of dry matters and water occurring during later stage of fruit development (about 10 d before maturation). The change in titratable acid followed a course of "low-high-low" in developing bayberry fruits (Fig. 3). The titratable acid content reached its peak at about 18 d before fruit maturation, and then decreased rapidly. The sugar compositions in fruits of bayberry cv. 'Wuzi' were different from those in fruits of bayberry cv. 'Biqi'. The main sugar accumulated in fruits of bayberry cv. 'Wuzi' was sucrose, accounting for 2/3 of total sugars but the sucrose content in fruits of bayberry cv. 'Biqi' was below 50% of total sugars. The fructose content in fruits of bayberry cv. 'Wuzi' was 4% higher, but that in fruits of bayberry cv. 'Biqi' was 12% lower than glucose content (Fig. 2). The activities of sucrose cleavage enzymes (invertase and cleavage activity of SS) in the fruit of bayberry cv. 'Biqi' increased with fruit development and maturation, but those activities in fruit bayberry cv. 'Wuzi' were almost stable during fruit development with lower levels of enzyme activities in fruit of cv. 'Wuzi' than in cv. 'Biqi' throughout fruit development (Fig. 4 and Fig. 5A). The SPS activity increased during fruit development (Fig. 6), however, the activity peak of synthetic activity of SS occurred at the middle stage of fruit development (Fig. 5B). The FRK activity in fruit of bayberry cv. 'Wuzi' was higher than that of HXK, but the reverse was in fruit of bayberry cv. 'Biqi' (Fig. 7). These results suggested that the 2-3 weeks before fruit maturation was a key phase for the bayberry development and the formation of fruit quality. There was a correlation between water transport and dry matter accumulation. The different sucrose constitutions between two varieties may be attributed to the differences in the activity levels of the sucrose cleavage enzymes while the difference in the ratio of glucose content to fructose content may be caused by the different activity levels of the hexose-metabolizing enzymes.