The Cellular Pathway of Radial Transfer of Photosynthates in Stems of <italic>Phaseolus vulgaris</italic> L.<subtitle>Effects of Cellular Plasmolysis and p-Chloromercuribenzene Sulphonic Acid</subtitle>

Abstract

Cellular plasmolysis with l M solutions of mannitol appeared to sever plasmodesmatal interconnections between all cells of the stems of Phaseolus vulgaris plants except the sieve element-companion cell (se—cc) complexes. Phloem loading and uptake of [14C]sucrose by the storage cells of the stems was unimpaired by cellular plasmolysis followed by rehydration of the stem tissues. Accumulation of phloem-transported 14C-photosynthates of the treated stems was inhibited in summer-grown plants and unaffected in winter-grown plants indicating that phloem unloading follows a symplastic and a free-space route respectively depending on growth season. At a concentration that did not interfere with cellular metabolism, p-chloromercuribenzene sulphonic acid (PCMBS) applied to the stems blocked [14C]sucrose loading into the phloem and storage cells of the stem, but had no effect on the pool size of free-space sugars. This latter response is consistent with a facilitated mechanism of sugar unloading to the stem free-space. Accumulation of phloem-transported 14C-photosynthates was stimulated by PCMBS and this effect was most pronounced in winter-grown plants. Cellular plasmolysis followed by rehydration abolished the PCMBS action on 14C-photosynthate accumulation. This effect is consistent with a PCMBS induction of phloem unloading through the stem symplast. It is proposed that phloem unloading in bean stems may follow either a free-space or symplastic route and that the latter route is entrained under sink-limited conditions.