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 Cl− or SO42− 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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Selected References
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