Cellular Structures, Plasma Membrane Surface Areas and Plasmodesmatal Frequencies of the Stem of Phaseolus vulgaris L. in Relation to Radial Photosynthate Transfer

Abstract

At an early stage of secondary development, the metaphloem sieve elements appeared to be the only functional axial transport conduit in fully elongated stems of P. vulgaris plants. There is no apparent barrier to the radial transfer of solutes in the stem apoplast. However, radial transfer through the stem symplast could be limited by discontinuities resulting from protoplast degeneration of the protophloem fibres and developing secondary xylem fibres. Estimates of possible sucrose fluxes through the apoplastic and symplastic routes indicated that radial photosynthate transfer from the sieve element-companion cell (se-cc) complexes of the stem metaphloem could follow either cellular route. In the case of apoplastic transfer, the plasma membrane surface area of the se-cc complexes is only sufficient to support some form of facilitated movement of sucrose. In contrast, the plasma membrane surface area of the phloem parenchyma is sufficient to permit passive diffusion of sucrose to the apoplast. Plasmodesmatal frequencies suggest that any symplastic transfer to the phloem parenchyma from the sieve elements would be via the companion cells.