Department of Soils and Plant Nutrition, University of California, Davis, California 95616.
Plant Physiol. 1967 Mar;42(3):319-23. doi: 10.1104/pp.42.3.319.
When barley roots absorb Na(+) at concentrations ranging from 1 to 50 mm, in the presence of low concentrations of Ca(2+) 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 SO(4) (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 Ca(2+) 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 Ca(2+) 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.
当大麦根在低浓度 Ca(2+)和 K(+)存在下吸收 1 至 50mm 的 Na(+)时，Na(+)的吸收是由碱阳离子转运载体机制 2 介导的，而在这些条件下机制 1 不能用于 Na(+)的转运。描绘 Na(+)吸收速率与外部 Na(+)浓度关系的吸收等温线，在 1 至 50mm 的浓度范围内，有几个拐点。这种逐步响应无论是 Cl(-)还是 SO(4) (2-)作为反离子都会发生，但在后一种情况下，Na(+)的实际吸收速率较低。当 Na(+)的浓度为 50mm，并且 K(+)或 Ca(2+)的浓度从 0 增加到 50mm 时，Na(+)的吸收速率不是作为干扰离子浓度增加的平滑函数而是逐步减小。同样，当 K(+)的浓度为 50mm，并且 Na(+)或 Ca(2+)的浓度从 0 增加到 50mm 时，K(+)的吸收速率不是作为干扰离子浓度增加的平滑函数而是逐步减小。总之，这些证据支持先前的结论，即碱阳离子转运载体机制 2 具有不同离子亲和力的载体位点谱。当 K(+)和 Na(+)在 1 至 50mm 的浓度范围内以等效浓度存在时，机制 2 几乎专门运输 Na(+)，而机制 1 几乎专门运输 K(+)。这些发现支持先前的结论，即机制 2 的活性位点对 Na(+)的亲和力高于对 K(+)的亲和力，而机制 1 则相反。
