Localized inhibition of translocation of 14C-assimilates in the phloem by valinomycin and other metabolic inhibitors

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Abstract
Translocation of 14C-labelled assimilates down the petioles was studied in intact plants of Pelargonium zonale (L.) L'Hérit ex Ait. The central bundle of the petiole was dissected out and treated with solutions of various inhibitors. Whereas cytochalasin B had no effect on 14C-translocation, a distinct and localized inhibition was caused by CCCP (10-7 M), antimycin (5×10-5 M), atractylate (5×10-5 M), and valinomycin (10-5 M) without any significant change in the proportion of [14C]sucrose in the translocate. The inhibition of translocation is inferred both by accumulation of 14C distal to and a decrease in 14C concentration basal to the treated petiolar region. If valinomycin was fed into the transpiration stream by flapping the peripheral bundles of the petioles an increased labeling of sugar phosphates occurred in the 14C fed leaf. Plasmolysis tests indicated that whereas CCCP interfered with the semipermeability of phloem cell membranes, valinomycin had no such effect. The results with valinomycin suggest a compartmentation of potassium ions for the translocation process but are ambiguous as to whether or not a potassium pump is involved.