THE CELLULAR PATHWAY OF SHORT-DISTANCE TRANSFER OF PHOTOSYNTHATES ANDPOTASSIUM IN THE ELONGATING STEM OF PHASEOLUS-VULGARIS L - 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 elucidat ed. This was achieved using ultrastructural observations of the cell t ypes that constitute the radial pathway and estimates of potential suc rose and potassium fluxes through the cross-sectional area of intercon necting plasmodesmata and across the plasma membrane surface areas of selected cell types. The investigation relied on predicting the relati ve roles of the mature and developing sieve elements as conduits for t he axial delivery of solutes to the elongation zone. In turn, these pr edictions led to formulation of two transport models which were subseq uently evaluated. It was found that unloading of sucrose and potassium from the protophloem sieve elements cannot be through the symplast du e to the absence of plasmodesmata. On the other hand, mature metaphloe m sieve element-companion cell complexes have the potential capacity t o unload either through the stem symplast or apoplast. The potential s ymplastic route is proposed to be via the companion cells to the adjac ent large phloem parenchyma cells. Continued radial transfer could occ ur either by exchange to the stem apoplast from the large phloem paren chyma cells or continue in the symplast to the ground tissues. It was further predicted that sucrose utilized for the development of the pro cambial/small phloem parenchyma cells could be delivered axially by th em and not by the mature sieve elements. (C) 1997 Annals of Botany Com pany