We investigated the mechanism of Na+ reabsorption in exchange for K+ at the xylem/symplast boundary of soybean roots (Glycine maxvar Hodgson). The xylem vessels of excised roots were perfused with solutions of defined composition to discriminate between entry of ions into or reabsorption from the xylem vessels. In the presence of NaCI, the transport systems released K+ into the xylem sap and reabsorbed Na+. l h e Na+-K+ exchange was energized by proton-translocating AlPases, enhanced by external K+ concentration, and dependent on the anion permeability. Evidence was presented for the operation of H+/Na+ and H+/K+ antiporters at the xylem/symplast interface.The existence of electrogenic ion pumps working at the xylem/symplast interface, which provided the driving force for ion exchange, has been envisaged in pea hypocotyls (Okamoto et al., 1979) and Plantago roots (De Boer et al., 1983) and demonstrated in onion roots (Clarkson and Hanson, 1986) and Vigrza unguiculata hypocotyls Mizuno et al., 1985) using techniques of perfusion of xylem vessels. For example, in the halophyte Plantago mauitima, ion pumps mediate K t uptake from and Na+ release into the xylem (De Boer and Prins, 1985). These processes are a general property of halophyte roots, which maintain high Na+/K+ ratios in the shoot. On the contrary, in Fabaceae (particularly in saltsensitive species), it has been shown that any Na+ that has crossed the root to the xylem may be removed from the xylem stream in exchange for K+ (Lauchli, 1984), particularly in the proximal region of the root and the base of the stem.In a previous paper (Durand and Lacan, 1994) we showed the effectiveness of the root in excluding Na+ from the soybean (Glycine max L.) shoot. For a moderate range of sodium concentrations in the outer solution, Naf is removed from the xylem stream by surrounding tissues and subsequently excreted to the external medium. A low Na+ concentration in the transpiration stream is important because salt sensitivity in soybean may be mainly dependent on the control of the Na' concentration in the xylem entering the leaf. It was possible to discriminate with the technique of perfusion * Corresponding author; e-mail lacan9gbsaarpb-montp2.fr; fax 33-67-14-30-31. (Clarkson et al., 1984) among the absorption of Nat from the external medium, the reabsorption of Na+ from the transpiration stream, and the consequent release into the xylem. We have shown that even in the presence of mild salinity the access of Na+ to the xylem stream of the root is relatively unrestricted, and the role of reabsorption of Nat released into the xylem may be of importance (Lacan and Durand, 1995). Thus, the purpose of this work was to examine the reabsorption of Na+ from the xylem vessels and propose a model for the mechanisms of Na+-K+ exchange at the symplast/xylem interface.
MATERIALS AND METHODS
Material PreparationSoybean (Glycine max L. Merr. var Hodgson) seeds were surface sterilized for 20 min in 3% (w/w) Ca2+ hypochlorite and then germinated at 27°C on filter paper moistened...
