PATHWAY OF PHOTOSYNTHATE TRANSFER IN THE DEVELOPING SEED OF VICIA-FABA L - A STRUCTURAL ASSESSMENT OF THE ROLE OF TRANSFER CELLS IN UNLOADING FROM THE SEED COAT

Photosynthate movement within the coat of the developing seed of Vicia faba occurs radially inward from the restricted vascular system and l aterally through the non-vascularized region of the seed coat prior to exchange to the seed apoplast. Thin-walled parenchyma/transfer cells line the entire inner surface of the seed coat and thus are located at the terminus of the photosynthate transfer pathway. The principal cel lular route of transfer within the seed coat and the role of the thin- walled parenchyma/transfer cells in membrane exchange to the seed apop last has been investigated. Sucrose fluxes, computed from estimates of the plasma membrane surface areas of the cell types of the pathway, t he plasmodesmatal cross-sectional areas interconnecting contiguous cel ls and the observed rate of sucrose delivery to the embryo indicate th at sieve element unloading and subsequent transfer to the thin-walled parenchyma/transfer cells is through the symplast. For the cells of th e ground tissue, plasmodesmatal density is consistently higher on thei r anticlinal walls. This observation supports the reported pattern of lateral transfer through these tissues in the non-vascular regions of the seed coat. Wall ingrowths are initiated sequentially in the thin-w alled parenchyma cells to maintain 1-3 rows of thin-walled parenchyma/ transfer cells. The development of these wall ingrowths results in a 5 8% increase in the plasma membrane surface area of these cells and pro vides them with the capacity to act as the principal cellular site for membrane exchange of sucrose to the seed apoplast. This cellular rout e of symplastic transfer from the sieve elements to the ground tissues where membrane exchange to the seed apoplast occurs is consistent wit h that reported for Phaseolus vulgaris.