Cortical cell fluxes and transport to the stele in excised root segments of Allium cepa L. : I. Potassium, sodium and chloride.

From compartmental analysis of radioisotope elution measurements, concentrations and fluxes of K(+), Na(+) and Cl(-) were estimated for cortical cells in root segments of onion, Allium cepa L., relative to a complete nutrient solution. The transported fraction of the total efflux was estimated separately. With the Ussing-Teorell flux ratio equation as the criterion, it was concluded that all three ions were actively accumulated from the outside medium into the cytoplasm and that only Na(+) was actively accumulated into the vacuole. K(+) and Cl(-) moved passively, in both directions across the tonoplast. Failure to account for leakage from the stele via the segment cut ends resulted in an over-estimate of exchange across the tonoplast but did not alter the conclusions qualitatively. The consequences of changing the assumed value of the tonoplast electrical potential (from 0 to+10- mV), and the effects of different experimental procedures, were also assessed, and found not to affect the main conclusions significantly. Separate measurement of ions leaking from the segment ends revealed that Na(+) was transported almost exclusively in an acropetal direction in the stele. Cl(-) appeared at both ends of the segments in similar amounts and K(+) was transported mainly in the basipetal direction. The implications of these findings for the mechanism and site of ion selectivity are discussed.