Cellular Structures, Plasma Membrane Surface Areas and Plasmodesmatal Frequencies of Seed Coats of Phaseolus vulgaris L. In Relation to Photosynthate Transfer

Abstract

In the developing ovule of P. vulgaris, the rapid lateral dissemination of photosynthates throughout the seed coat occurs within a reticulate vasculature of uniform sized veins embedded in a parenchymatous layer. Radial movement of photosynthate from the sieve elements to the cotyledonary apoplast is through vascular, ground and branch parenchyma, epidermis and residual endosperm. For this cellular pathway, there appears to be no barrier within the apoplast to photosynthate transfer and symplastic continuity exists from the sieve elements to the branch parenchyma-epidermal boundary. However, based on estimates of possible sucrose fluxes through the apoplastic and symplastic routes, it seems likely that photosynthates move symplastically from the sieve elements, through the vascular parenchyma to either the adjoining ground parenchyma or inner band of branch parenchyma where membrane transfer to the apoplast occurs. The resemblance of the ultrastructure of the branch parenchyma to that of known secretory cells favours this tissue as the main site of photosynthate exchange to the apoplast. The significance of these findings is discussed in relation to photosynthate transfer within the developing ovule.