title: Radial transport of salt and water in roots of the common reed (Phragmites australis Trin. ex Steudel).
abstract: To understand the root function in salt tolerance, radial salt and water transport were studied using reed plants growing in brackish habitat water with an osmotic pressure (πM ) of 0.63 MPa. Roots bathed in this medium exuded a xylem sap with NaCl as the major osmolyte and did so even at higher salt concentration (πM up to 1.3 MPa). Exudation was stopped after a small increase of πM (0.26 MPa) using polyethylene glycol 600 as osmolyte. The endodermis of fine lateral roots was found to be the main barrier to radial solute diffusion on an apoplastic path. Apoplastic salt transfer was proven by rapid replacement of stelar Na(+) by Li(+) in an isomolar LiCl medium. Water fluxes did not exert a true solvent drag on NaCl. Xylem sap concentrations of NaCl in basal internodes of transpiring culms were more than five times higher than in medial and upper ones. It was concluded that the radial NaCl flux was mainly diffusion through the apoplast, and radial water transport, because of the resistance of the cell wall matrix to convective mass flow, was confined to the symplast. Radial salt permeation in roots reduced the water stress exerted by the brackish medium.
authors.affiliation: Institute of Biology, Humboldt-University, Invalidenstr. 42, 10115, Berlin, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Fritz
authors.forename: Rudolf
authors.initials: R
authors.lastname: Ehwald
mesh_terms: D001557:Benzenesulfonates / Q000378:metabolism; D001692:Biological Transport / Q000187:drug effects; D002473:Cell Wall / Q000187:drug effects / Q000378:metabolism; D006874:Hydrostatic Pressure; D007477:Ions; D007700:Kinetics; D018021:Lithium Chloride / Q000494:pharmacology; D009997:Osmotic Pressure / Q000187:drug effects; D053147:Plant Exudates / Q000378:metabolism; D018517:Plant Roots / Q000187:drug effects / Q000378:metabolism*; D006109:Poaceae / Q000187:drug effects / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D012965:Sodium Chloride / Q000378:metabolism* / Q000494:pharmacology; D014867:Water / Q000378:metabolism*
chemical_list: D001557:Benzenesulfonates; D007477:Ions; D053147:Plant Exudates; D014867:Water; D012965:Sodium Chloride; C086337:acid fuchsin; D012964:Sodium; D018021:Lithium Chloride
keywords: apoplasmic bypass; endodermis; root reflection coefficient; salinity; solvent drag; water relations; xylem transport