The cellular pathway of short-distance transfer of photosynthates and potassium in the elongating stem of Phaseolus vulgaris L. A physiological assessment

Abstract

Plasmolytic disruption of plasmodesmata interconnecting metaphloem sieve element-companion cell complexes with small and large pbloem parenchyma cells in the elongating region of internode 2 of Phaseolus vulgaris L. seedlings did not affect accumulation of phloem-imported 14C-photosynthates and 86rubidium. The membrane-impermeant dye, 5(6) carboxyfluorescein, loaded into leaf phloem as the membrane-permeant diacetate ester, was found not to move radially out of the importing sieve elements in the internode elongation region. In contrast, the apoplasmic tracer, Calcuofluor White, rapidly moved laterally throughout all tissues of the elongation zone. Hexoses, sucrose and potassium were identified as the main osmotica in internode apoplasmic sap. Label asymmetry in [14C](fructosyl)sucrose was retained on accumulation by excised stem segments. Uptake of [14C]sucrose and 86rubidium by stem segments exhibited saturation kinetics. Sucrose uptake was inhibited by the slowly penetrating sulphydryl reagent, para-chloromercuribenzenesulphonic acid. In vitro rates of sucrose uptake, at apoplasmic concentrations, corresponded to its predicted in vivo rate of delivery to the stem ground tissues from mature sieve elements when respiratory losses were assumed to be confined to the stem phloem. For potassium, the total delivery rate could be accounted for by its in vitro rate of uptake. Overall, it was concluded that radial transport, in the elongation zone of internode 2 of Phaseolus vulgaris L. seedlings, follows an apoplasmic route from mature sieve elements to stem ground tissues.