THE CELLULAR PATHWAY OF SUCROSE TRANSPORT IN DEVELOPING COTYLEDONS OFVICIA-FABA L AND PHASEOLUS-VULGARIS L - A PHYSIOLOGICAL ASSESSMENT

The cellular pathway of sugar uptake in developing cotyledons of Vicia faba L. and Phaseolus vulgaris is L. seed was evaluated using a physi ological approach. The cotyledon interface with the seed coat is chara cterised by a specialised dermal cell complex. In the case of Vicia fa ba cotyledons, the epidermal component of the dermal cell complex is c omposed of transfer cells. Sucrose is the major sugar presented to the outer surface of both cotyledons and it is taken up from the apoplasm unaltered. Estimated sucrose concentrations within the apparent free space of Vicia and Phaseolus cotyledons were 105 and 113 mM respective ly. Rates of in-vitro uptake of [C-14]sucrose by cotyledon segments or by whole cotyledons following physical removal or porter inactivation of the outer cells demonstrated that, for both Vicia and Phaseolus co tyledons, the dermal cell complexes are the most intense sites of sucr ose uptake. Accumulation of [C-14]sucrose in the storage parenchyma of whole cotyledons was directly affected by experimental manipulation o f uptake by the outer cell layers and plasmolytic disruption of the in terconnecting plasmodesmata. These findings indicated that sucrose acc umulated by the dermal cell complexes is transported symplasmically to the storage parenchyma. Overall, it is concluded that the dermal cell complexes of the developing legume embryo, irrespective of the presen ce or absence of wall ingrowths, are the major sites for the uptake of sucrose released from the maternal tissues to the seed apoplasm. Ther eafter, the accumulated sucrose is transported radially inward through the symplast to the storage parenchyma.