The Cellular Pathway of Short-distance Transfer of Photosynthates and Potassium in the Elongating Stem ofPhaseolus vulgarisL. A Structural Assessment

The potential cellular pathway of radial transfer of photosynthate and potassium delivered in the phloem to the elongation zone (apical 0.5–2.5 cm) of internode 2 of Phaseolus vulgaris L. seedlings was elucidated. This was achieved using ultrastructural observations of the cell types that constitute the radial pathway and estimates of potential sucrose and potassium fluxes through the cross-sectional area of interconnecting plasmodesmata and across the plasma membrane surface areas of selected cell types. The investigation relied on predicting the relative roles of the mature and developing sieve elements as conduits for the axial delivery of solutes to the elongation zone. In turn, these predictions led to formulation of two transport models which were subsequently evaluated. It was found that unloading of sucrose and potassium from the protophloem sieve elements cannot be through the symplast due to the absence of plasmodesmata. On the other hand, mature metaphloem sieve element-companion cell complexes have the potential capacity to unload either through the stem symplast or apoplast. The potential symplastic route is proposed to be via the companion cells to the adjacent large phloem parenchyma cells. Continued radial transfer could occur either by exchange to the stem apoplast from the large phloem parenchyma cells or continue in the symplast to the ground tissues. It was further predicted that sucrose utilized for the development of the procambial/small phloem parenchyma cells could be delivered axially by them and not by the mature sieve elements.