INTERCELLULAR TRANSPORT AND SUBCELLULAR-DISTRIBUTION OF PHOTOASSIMILATES IN CHARA-CORALLINA

The mechanism of polar transport of C-14-photoassimilates between Chara corallina branchlets and internodes was investigated using an intracellular perfusion procedure. When the apex remained, more C-14-photoassimilates were transported from branchlet to internode than in the opposite direction. When the apex was detached, this polarity disappeared. To study the polar transport mechanism, we examined the subcellular distribution of C-14-photoassimilates in the three main intracellular compartments, the cortical chloroplast layer, streaming sol endoplasm and vacuole. When the internode or branchlet of an internode-branchlet complex was exposed to C-14-bicarbonate for 10 min, 70 to 80% of the total fixed carbon was found in the sol endoplasm. In the subsequent 3 h chase, the C-14-photoassimilates in the sol endoplasm of the source cell decreased greatly to 20 to 40% of the total labelled substances. When the apex remained, the branchlet showed higher photosynthetic activity than the internode, which resulted in a C-14 concentration gradient of 13.5 mol m-3 in the sol endoplasm from branchlet to internode after photosynthesis for 10 min. In the 3 h chase period, this gradient decreased to about 1.0 mol m-3. In the other three cases, involving transport from branchlet to internode when the apex was detached, or transport from internode with or without apex to branchlet, the intercellular gradient of C-14 after 10 min photosynthesis was about 7.0 mol m-3 in the sol endoplasm. Thus, when the apex remains, the greater gradient of photoassimilates between the sol endoplasm of branchlet and internode seems to be one of the reasons for polar intercellular transport.