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Sodium Absorption by Barley Roots: Its Mediation by Mechanism 2 of Alkali Cation Transport

D. W. Rains and Emanuel Epstein
Plant Physiology
Vol. 42, No. 3 (Mar., 1967), pp. 319-323
Stable URL: http://www.jstor.org/stable/4260972
Page Count: 5
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Sodium Absorption by Barley Roots: Its Mediation by Mechanism 2 of Alkali Cation Transport
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Abstract

When barley roots absorb Na+ at concentrations ranging from 1 to 50 mM, in the presence of low concentrations of Ca2+ and K+, absorption of Na+ is mediated by carrier mechanism 2 of alkali cation transport, mechanism 1 being unavailable for Na+ transport under these conditions. The absorption isotherm depicting the rate of Na+ absorption as a function of the external Na+ concentration, over the 1 to 50 mM range of concentrations, shows several inflections. This stepwise response occurs whether $\text{C1}^{-}$ or SO4 2- is the counterion, but actual rates of Na+ absorption are lower in the latter case. When the concentration of Na+ is 50 mM, and the concentration of either K+ or Ca2+ is increased from nil to 50 mM, the rate of absorption of Na+ is diminished not as a smooth function of increasing concentrations of the interfering ions but stepwise. Similarly, when the concentration of K+ is 50 mM, and the concentration of either Na+ or Ca2+ is increased from nil to 50 mM, the rate of absorption of K+ is diminished not as a smooth function of increasing concentrations of the interfering ions but stepwise. Together, this evidence supports the previous conclusion to the effect that mechanism 2 of alkali cation transport possesses a spectrum of carrier sites with different ionic affinities. When both K+ and Na+ are presented at equivalent concentrations over the 1 to 50 mM range, mechanism 2 transports Na+ almost exclusively, and mechanism 1 K+ almost exclusively. These findings support previous conclusions to the effect that the active sites of mechanism 2 have higher affinity for Na+ than for K+, whereas the reverse is true for mechanism 1.

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