Sucrose accumulation and its related enzyme activities in the juice sacs of satsuma mandarin fruit from trees with different crop loads

Abstract

Sucrose accumulation and its related enzyme activities in the juice sacs were compared between the fruit from conventional crop load (CCL, leaf–fruit ratio is about 25) and low crop load (LCL, leaf–fruit ratio is more than 50) trees in satsuma mandarin ( Citrus unshiu Mark). Sucrose in the juice began to increase in September and the increase continued to harvest (late November) in fruit on trees with both crop loads, but the rate of increase was significantly higher in CCL fruit. Synthetic sucrose synthase (SS, EC 2.4.1.13) increased greatly as sucrose accumulated in the juice sacs, whereas its cleavage activity decreased. In spite of lower accumulation of sucrose in LCL fruit, synthetic SS activity was higher than in CCL fruit. Acid invertase (EC 3.2.1.26) activity, which decreased with fruit development, was significantly higher in LCL fruit than in CCL ones until late October. Thus, sucrose synthesized by SS may be broken through this higher activity of acid invertase in LCL fruit, resulting in repression of sucrose accumulation. When diurnal changes in SS activity in juice sacs were measured under orchard conditions in mid-November, sucrose increase was estimated to be 0.4% per fruit per day. This activity is enough to accumulate sucrose to harvest level (7%) within 20 days. In satsuma mandarin fruit, therefore, sucrose concentration in the juice may be regulated by both synthetic SS and acid invertase activities in the juice sacs, and crop load of the tree may greatly affect sucrose accumulation by controlling these enzyme activities.