THE CELLULAR PATHWAY OF SHORT-DISTANCE TRANSFER OF PHOTOSYNTHATES ANDPOTASSIUM IN THE ELONGATING STEM OF PHASEOLUS-VULGARIS L - A PHYSIOLOGICAL ASSESSMENT

Plasmolytic disruption of plasmodesmata interconnecting metaphloem sie ve element-companion cell complexes with small and large phloem parenc hyma cells in the elongating region of internode 2 of Phaseolus vulgar is L. seedlings did not affect accumulation of phloem-imported C-14-ph otosynthates and (86)rubidium. The membrane-impermeant dye, 5(6) carbo xyfluorescein, loaded into leaf phloem as the membrane-permeant diacet ate eater, was found not to move radially out of the importing sieve e lements in the internode elongation region. In contrast, the apoplasmi c tracer, Calcuofluor White, rapidly moved laterally throughout all ti ssues of the elongation zone. Hexoses, sucrose and potassium were iden tified as the main osmotica in internode apoplasmic sap. Label asymmet ry in [C-14]-(fructosyl)sucrose was retained on accumulation by excise d stem segments. Uptake of [C-14]sucrose and (86)rubidium by stem segm ents exhibited saturation kinetics. Sucrose uptake was inhibited by th e slowly penetrating sulphydryl reagent, para-chloromercuribenzenesulp honic acid. In vitro rates of sucrose uptake, at apoplasmic concentrat ions, corresponded to its predicted in vivo rate of delivery to the st em ground tissues from mature sieve elements when respiratory losses w ere assumed to be confined to the stem phloem. For potassium, the tota l delivery rate could be accounted for by its in vitro rate of uptake. Overall, it was concluded that radial transport, in the elongation zo ne of internode 2 of Phaseolus vulgaris L. seedlings, follows an apopl asmic route from mature sieve elements to stem ground tissues. (C) 199 8 Annals of Botany Company.