CELLULAR PATHWAY OF PHOTOSYNTHATE TRANSPORT IN COATS OF DEVELOPING SEED OF VICIA-FABA L AND PHASEOLUS-VULGARIS L .1. EXTENT OF TRANSPORT THROUGH THE COAT SYMPLAST

The extent of post-phloem solute transport through the coat symplasts of developing seeds of Vicia faba L. and Phaseolus vulgaris L. was eva luated. For Vicia seed coats, the membrane-impermeant fluorochrome, CF , moved radially from the chalazal vein to reach the chlorenchyma and thin-walled parenchyma transfer cell layers. Thereafter, the fluorochr ome moved laterally in these two cell layers around the entire circumf erence of the seed coat. Transfer of CF from the chatazal vein was inh ibited by plasmolysis of attached 'empty' seed coats. In contrast, the spread of phloem-imported CF was restricted to the ground parenchyma of Phaseolus seed coats. Fluorochrome loaded into the outermost ground parenchyma cell layer was rendered immobile following plasmolysis of excised seed-coat halves. Phloem-imported [C-14]sucrose and the slowly membrane permeable sugar, L-[C-14]glucose, were partitioned identical ly between the vascular and non-vascular regions of intact Vicia seed coats. For C-14-photosynthates, these partitioning patterns in attache d 'empty' Vicia seed coats were unaffected by PCMBS, but inhibited by plasmolysis. Tissue autoradiographs of intact Phaseolus seed coats dem onstrated that a pulse of C-14-photosynthate moved from the veins to t he ground tissues. In excised Vicia seed coats, preloaded with C-14-ph otosynthates, the cellular distribution of residual C-14-label was una ffected by PCMBS, In contrast, PCMBS caused the C-14-photosynthate lev els to be elevated in the veins and ground parenchyma relative to the branch parenchyma of Phaseolus seed coat halves. Based on the above fi ndings, it is concluded that the phloem of Vicia seed coats is interco nnected to two major symplastic domains; one comprises the chlorenchym a, the other the thin-walled parenchyma plus thin-walled parenchyma tr ansfer cells. For Phaseolus seed coats, the phloem forms a major sympl astic domain with the ground parenchyma.