Translocation of C14 Metabolites in the Phloem of the Bean Plant

Abstract

An auto-radiographic study coupled with phloem fluorescence permitted a detailed study of metabolite movement from leaves of differing ages and positions. Lower leaves exported primarily to the root, upper leaves to the apex, and intermediate leaves in both directions. Flow was confined to phloem bundles except for radial loss to the xylem. The loss was least (4%) for metabolite from older leaves and most (49%) for metabolite from younger leaves. The metabolite from mature leaves moved downward one node where anastomosing of phloem bundles permitted a division into a downward and an upward moving component. The upward moving component was in bundles distinct from those carrying the downward moving component from the next higher leaf, i. e., bidirectional movement was in separate phloem bundles. Some metabolite from young leaves was transferred directly from the leaf traces at the node into cauline bundles wherein it ascended. A downward flow in the leaf traces developed as the leaf matured. Exportable metabolite from young leaves was rich in steroids; from older leaves it was principally sucrose. The export from older leaves resembled a mass flow while that from younger leaves, wherein a transfer from leaf traces to cauline bundles occurred, was not readily explained by this mechanism. The fluorescence method should prove valuable for studies of phloem anatomy.