Concentrations and Transport of Solutes in Xylem and Phloem along the Leaf Axis of NaCl-treatedHordeum vulgare

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Abstract
Hordeum vulgare cv. California Mariout was established in sand culture at two different NaCl concentrations (0.5 mol m−3 ‘control’ and 100 mol m−3) in the presence of 6.5 mol m−3 K +. Between 16 and 31 d after germination, before stem elongation started, xylem sap was collected by use of a pressure chamber. Collections were made at three different sites on leaves 1 and 3: at the base of the sheath, at the base of the blade, i.e. above the ligule, and at the tip of the blade. Phloem sap was collected from leaf 3 at similar sites through aphid stylets. The concentrations of K +, Na+, Mg2+ and Ca2+ were measured. Ion concentrations in xylem sap collected at the base of leaves 1 and 3 were identical, indicating there was no preferential delivery of specific ions to older leaves. All ion concentrations in the xylem decreased from the base of the leaf towards the tip; these gradients were remarkably steep for young leaves, indicating high rates of ion uptake from the xylem. The gradients decreased with leaf age, but did not disappear completely. In phloem sap, concentrations of K+ and total osmolality declined slightly from the tip to the base of leaves of both control and salt-treated plants. By contrast, Na+ concentrations in phloem sap collected from salt-treated plants decreased drastically from 21 mol m−3 at the tip to 7.5 mol m−3 at the base. Data of K/Na ratios in xylem and phloem sap were used to construct an empirical model of Na+ and K+ flows within xylem and phloem during the life cycle of a leaf, indicating recirculation of Na+ within the leaf.

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