Plant Physiology 42:314-318 (1967)
© 1967 American Society of Plant Biologists
Articles
Sodium Absorption by Barley Roots: Role of the Dual Mechanisms of Alkali Cation Transport 1
D. W. Rains2 and
Emanuel Epstein
Department of Soils and Plant Nutrition, University of California, Davis, California 95616
Radioactively labeled Na+ absorbed by barley roots was sequestered in an intracellular compartment or compartments ("inner" spaces) in which it was only very slowly exchangeable with exogenous Na+. Absorption of this fraction proceeded at a constant rate for at least 1 hour.
When the rate of Na+ absorption was examined over the range of concentrations, 0.005 to 50 mM, the isotherm depicting the relation showed dual kinetics as follows. Over the range, 0.005 to 0.2 mM, a single Michaelis-Menten term describes the relation between the concentration of Na+ and the rate of its absorption. The mechanism of Na+ absorption operating over this range of concentrations, mechanism 1 of alkali cation transport, is severely inhibited in the presence of Ca2+ and virtually rendered inoperative for Na+ transport by the combined presence of Ca2+ and K+. The mechanism is equally effective in Na+ transport whether Cl– or F– is the anion, but is somewhat inhibited when the anion is SO42–.
Over the high range of concentrations, 0.5 to 50 mM Na+, a second, low-affinity mechanism of Na+ absorption comes into play. In the presence of Ca2+ and K+, this mechanism 2 is the only one to transport Na+ effectively, since Na+ absorption via mechanism 1 is virtually abolished under these conditions.
Anaerobic conditions, low temperature, and the uncoupler, 2,4-dinitrophenol, inhibit Na+ absorption both at low and high Na+ concentrations.
2 Present address: Kearney Foundation of Soil Science, University of California, Davis, California 95616.
1 Supported by grants from the National Science Foundation and the Office of Saline Water, United States Department of the Interior.
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