title: The cytosolic Na+ : K+ ratio does not explain salinity-induced growth impairment in barley: a dual-tracer study using 42K+ and 24Na+.
abstract: It has long been believed that maintenance of low Na+ : K+ ratios in the cytosol of plant cells is critical to the plant's ability to tolerate salinity stress. Direct measurements of such ratios, however, have been few. Here we apply the non-invasive technique of compartmental analysis, using the short-lived radiotracers 42K+ and 22Na+, in intact seedlings of barley (Hordeum vulgare L.), to evaluate unidirectional plasma membrane fluxes and cytosolic concentrations of K+ and Na+ in root tissues, under eight nutritional conditions varying in levels of salinity and K+ supply. We show that Na+ : K+ ratios in the cytosol of root cells adjust significantly across the conditions tested, and that these ratios are poor predictors of the plant's growth response to salinity. Our study further demonstrates that Na+ is subject to rapid and futile cycling at the plasma membrane at all levels of Na+ supply, independently of external K+, while K+ influx is reduced by Na+, from a similar baseline, and to a similar extent, at both low and high K+ supply. We compare our results to those of other groups, and conclude that the maintenance of the cytosolic Na+ : K+ ratio is not central to plant survival under NaCl stress. We offer alternative explanations for sodium sensitivity in relation to the primary acquisition mechanisms of Na+ and K+.
authors.affiliation: Department of Life Sciences, University of Toronto, 1265 Military Trail, Toronto, Ontario, Canada M1C 1A4. herbertk@utsc.utoronto.ca
authors.forename: Herbert J
authors.initials: HJ
authors.lastname: Kronzucker
authors.forename: Mark W
authors.initials: MW
authors.lastname: Szczerba
authors.forename: Maryam
authors.initials: M
authors.lastname: Moazami-Goudarzi
authors.forename: Dev T
authors.initials: DT
authors.lastname: Britto
mesh_terms: D003600:Cytosol / Q000378:metabolism*; D001467:Hordeum / Q000187:drug effects* / Q000254:growth & development*; D018517:Plant Roots / Q000187:drug effects; D018520:Plant Shoots / Q000187:drug effects; D011188:Potassium / Q000378:metabolism*; D011197:Potassium Radioisotopes; D011849:Radioactive Tracers; D036226:Seedlings / Q000187:drug effects; D012964:Sodium / Q000378:metabolism*; D012965:Sodium Chloride / Q000494:pharmacology*; D012979:Sodium Radioisotopes
chemical_list: D011197:Potassium Radioisotopes; D011849:Radioactive Tracers; D012979:Sodium Radioisotopes; D012965:Sodium Chloride; D012964:Sodium; D011188:Potassium