Translocation and accumulation of fruit-fixed photosynthate in Satsuma mandarin

Abstract

Satsuma mandarin fruit (Citrus unshiu Mark.) photosynthesizes as comparable to leaf at about 100 days after full bloom (DAFB). In this study, translocation and accumulation of fruit-fixed photosynthate were investigated by using 14CO2. When fruit at 108 DAFB was exposed to 14CO2 for 48 h under 135 photosynthetic photon flux density (PPFD), 14C-sucrose, 14C-glucose and 14C-fructose were detected not only in flavedo but juice sac; more than 50 % of fruit assimilated 14C-sugars were present in juice sac. Thus, majority of rind-fixed photosynthate are infiltrated into juice sac and accumulated there within 48 h after assimilation. Although 14C-sucrose was predominant at flavedo where high SS (sucrose synthase) activity toward synthesis was present, the amount decreased gradually from the outside (flavedo) to the inside (juice sac) of fruit. In vascular bundle, strong SS toward cleavage and soluble acid invertase activities were involved, and 14C-fructose was predominant in juice sac. Accordingly, rind-fixed photosynthate is once converted to sucrose, the translocated sugar in Citrus, at flavedo by SS toward synthesis, and loaded on vascular bundle through symplastic and/or apoplastic movement in the albedo tissue. In the vascular bundle, sucrose may be degraded by SS toward cleavage and invertase, and resulting hexoses transported symplastically to the juice sac through juice stalk.