Intercellular transport of 14C-labelled photoassimilates, both in isolated upper shoots and in isolated internode-branchlet complexes of Chara corallina, was measured. The isolated upper shoots were composed of a primary apex, two mature internodes, and three branchlet whorls. A 10 min loading of the isolated upper shoot with H14CO3- resulted in a greater accumulation of 14C in the apical complex and branchlets than in the internodes, while a subsequent 50 min chase with unlabelled solution in the light resulted in a greater accumulation of 14C in internodes than in other parts of the shoot.In the isolated internode-branchlet complex, when the apex was not detached, the amount of 14C transported from branchlets to internodes was about fives times that transported from internodes to branchlets. Removal of the apex resulted in a decrease in transport from branchlets to internodes and an increase in transport in the opposite direction.In an attempt to explain the mechanism of the polar transport of photosynthetically fixed carbon between branchlets and internodes, photosynthetic activities of both types of cells were investigated. Detached branchlets have higher photosynthetic 14C-fixation activities than those of internodes. Chlorophyll contents, measured in terms of surface area, in internodes and branchlets were almost identical. The ribulose-l,5-bisphosphate carboxylase (RuBPCase) activity of branchlets was 1.6 times that of internodes, and the rate of ferricyanide-dependent evolution of oxygen in branchlets was 1.4 times that in internodes.
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