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

Sucrose accumulation and its related enzyme activities in the juice sacs we re 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 bega n to increase in September and the increase continued to harvest (late Nove mber) 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 it s cleavage activity decreased. In spite of lower accumulation of sucrose in LCL fruit, synthetic SS activity was higher than in CCL fruit. Acid invert ase (EC 3.2.1.26) activity, which decreased with fruit developments was sig nificantly higher in LCL fruit than in CCL ones until late October. Thus, s ucrose synthesized by SS may be broken through this higher activity of acid invertase in LCL fruits resulting in repression of sucrose accumulation. W hen diurnal changes in SS activity in juice sacs were measured under orchar d 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 le vel (7%) within 20 days. In satsuma mandarin fruit, therefore, sucrose conc entration in the juice may be regulated by both synthetic SS and acid inver tase activities in the juice sacs, and crop load of the tree may greatly af fect sucrose accumulation by controlling these enzyme activities. (C) 2001 Elsevier Science B.V. All rights reserved.