Efflux und Transport von Cl⁻ und Rb⁺ in Maiswurzeln. Wirkung von Außenkonzentration, Ca⁺⁺, EDTA und IES

Summary

The efflux of ³⁶Cl and ⁸⁶Rb and the fluxes of these ions into the xylem were investigated using the device shown in Fig. 1.

Efflux of ³⁶Cl is stimulated by external KCl while transport into the xylem is inhibited. Stimulation of the efflux appears to be stronger than inhibition of the transport.

The stimulation of the efflux of ³⁶Cl was also observed with roots of intact seedlings.

Assuming that the mode of transfer of Cl⁻ into the xylem (flux 3, Fig. 8) is diffusion exhibiting a linear isotherm (Luttge and Laties, 1966), these results suggest that the primary action of external salts is on the efflux across the plasma-lemma (Weigl, 1967, 1968). We were unable, however, to find a linear relationship between concentration and rate of chloride transport to the shoots of intact seedlings.

With respect to the mode of ion transfer to the xylem (Weigl and Lüttge, 1965; Luttge and Laties, 1966) we have to be aware of the following facts:

A linear isotherm cannot be taken to signify diffusive permeation (Torii and Laties, 1966; Luttge and Laties, 1966). If the Michaelis constant is extremely high relative to the ion concentration, the relationship between the ion concentration and the rate of a metabolic or mediated transport approaches linearity.

The isotherm of the transport into the xylem may primarily reflect the difference of two large fluxes (4 and 5; Fig. 8).

The transport data of Luttge and Laties (1966) need not be presented as a straight line (Fig. 6).

If at high external ion concentrations the ratio of the ion concentration in the exudation sap to the external ion concentration approaches unity, diffusive permeation into the stele is still not proved to be the mode of migration, since at high stelar ion concentration flux 6 tends to become equal to flux 3.

Considerations on radial ion transfer into the xylem depend on contemporary knowledge of the location of transport systems. Cl⁻-uptake into root tips (2 mm) from solutions of 1–10 mM KCl did not exhibit a linear isotherm. These results are unpublished since the discrepancy to the results of Torii and Laties (1966) may be due to a higher content of vacuoles in our root tips. We feel it unlikely, however, that a linear isotherm of Cl⁻-uptake into root tips is adequately explained by assuming that it is due to a lack of vacuoles while the sensibility to inhibitors is assumed to be due to the presence of vacuoles in root tips.

Transport of Cl⁻ into the xylem is susceptible to inhibitors of oxydative phosphorylation, suggesting that this process, even at high external ion concentrations, is dependent on metabolic energy in contrast to the passive efflux from the cortical cells across the plasmalemma into the environment of the root. The precise location of the metabolic step(s) on the pathway of ions from the environment of the root to the xylem is unknown.

The observed effects of Ca⁺⁺, EDTA and IAA may be considered in relation to the theory that auxin exerts its influence on growth by altering the diffusion potential across cell membranes (Brauner and Diemer, 1967). Growth is susceptible to the effect of Ca⁺⁺ and EDTA (Adamson, 1962; Setterfield, 1963; Thimann, 1963). Nevertheless, since IAA exerts no influence on ion fluxes in corn roots, it is not clear whether IAA really exerts its influence on growth by altering the diffusion potential across plant cell membranes. We might be dealing with occasional effects of secondary importance.