5.2.7  Solute recycling: phloem export

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All solutes in the transpiration stream will be subject to the general rules worked out for dyes or K+: a tendency to become concentrated as water passes rapidly through the symplasm prior to transpirational loss, entry of some solutes into the symplasm from a slow-moving exchange bed of fine veins, or, for ions not accepted by any living cells, a slow diffusive spread in the cell wall apoplasm. Details for all solutes except K+ are still largely unknown.

Potassium is known to be re-exported via the phloem (Section 5.1). In leaves, the concentrated sap flowing through the narrow xylem vessels of fine veins (Figures 5.4 and 5.6) is separated from sieve tubes of the phloem by only two or three parenchyma cells (e.g. Figure 5.5). Exchange from xylem to phloem is probably made by this direct path. Solutes travel in the phloem back to stems, either distally to younger developing leaves and the stem apex or proximally towards roots. In dicotyledonous stems, solutes can re-enter the xylem by crossing the cambium and being used on the way as an osmotic engine to swell the young secondary vessel elements. These solutes re-enter the transpiration stream when vessel elements mature and die to form functional xylem vessels (Canny 1995).