Dual mechanisms of ion absorption in relation to long distance transport in plants.

The characteristics of ion transport to the shoots of young corn seedlings were studied with respect to the nature of the isotherm through a wide concentration range, the competitive influence of closely related ions upon the transport of a given ion, and the influence of the counter-ion. Both with respect to (36)Cl and (86)Rb transport, the characteristics of the process in every way resemble uptake by non-vacuolate root tips wherein the plasma membrane is the only membrane involved in absorption, and where system 1 - of the 2 systems which can be shown to participate in absorption by vacuolate tissue - is the only system operative. Net ion uptake by the roots per se was shown to display both the high affinity (system 1) and low affinity (system 2) mechanisms. It is concluded that the symplastic theory of ion movement to the xylem is valid, and that the contention that system 1 operates at the plasma membrane while system 2 functions at the tonoplast is strengthened.