Selective Inhibition of Absorption and Long Distance Transport in Relation to the Dual Mechanisms of Ion Absorption in Maize Seedlings

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Abstract
The influence of several uncouplers of oxidative phosphorylation and inhibitors of terminal electron transport was studied on absorption and long distance transport of both K and Cl at concentrations within each range of the dual isotherm typical of ion uptake by maize roots. At low concentrations in the range of system 1, the system considered to implement ion movement through the plasma membrane, root absorption and long distance transport are equally inhibited by a given inhibitor. In the high range of system 2, the system considered to mediate on passage through the tonoplast, long distance transport is markedly less sensitive to inhibitors than is absorption. Therefore, only system 1 is probably involved in the uptake of ions from the external solution into the symplast, and hence into the xylem. At high concentrations, entrance into the symplasm is deemed to be largely by diffusion and therefore less inhibitor sensitive. The plasma membrane system is more inhibitor sensitive than is the tonoplast system in absorption by the roots. The difference in sensitivity is probably real, and not the consequence of an inequality of inhibitor concentration in the vicinity of the plasma membrane and tonoplast respectively.