Fine veins of dicotyledon leaves as sites for enrichment of solutes of the xylem sap

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Abstract
The apoplastic tracer sulphorhodamine G (SR) was used as an indicator of the flumes, the sites where water left the apoplast and entered the symplast, in a selection of dicotyledon leaves. At these flumes the dye is deposited as crystals after a pulse of dye is fed to the transpiration stream, followed or not by a water chase. In contrast to wheat, the dicotyledons showed SR cystals inside the tracheary elements of the finest leaf veins. At short pulse times the crystals were in the stems of the branch-trees of the fine veins, but after longer pulses, had moved to the vein termini. The dye solution was moving very slowly in the tracheary elements as it approached the ends of the branch-trees, since the axial flow there is nearly balanced by radial leakage. These results are interpreted as evidence that most of the transpiration water enters the symplast in the vein sheaths of the fine veins, and that these veins are places where many of the natural solutes of the xylem sap will be enriched to quite high concentrations.