Studies on Translocation of Metabolites in the Xylem of Grapevine Shoots

Abstract

Fluid extracted from grapevine shoots by suction contained up to o-2 per cent (w/v) of sugar. The total sugar concentration and the relative concentrations of sucrose, glucose, and fructose changed along the length of a shoot. Whereas sucrose is absent from or present only in traces in fluid bleeding from cut stumps, it was found to be a major constituent in fluid extracted from sections of wood by suction. 14C-glucose administered to the wood of an excised shoot moved up in the transpiration stream, moved laterally, into the bark, and was partly converted to sucrose and fructose. When 14C02 was supplied to a leaf of an intact vine, radioactive sugars were found in the fluid afterwards extracted from the wood by suction. When a ring of bark was removed from the stem at a point above the 14C02-treated leaf most of the 14C in the bark, wood, and extracted fluid below the ring was in the form of sugar but 14C in tissues above the ring was mainly in organic acids and amino acids. It is suggested that a barrier within the wood prevents diffusion of sugars into the transpiration stream, and that the fluid extracted from the wood by suction cannot be regarded as ascending sap since it contains considerable amounts of non-moving material.