Cortical cell fluxes and transport to the stele in excised root segments of Allium cepa L.

Summary From compartmental analysis of radioisotope elution measurements, concentrations and fluxes of Mg²⁺ were estimated for cortical cells in root segments of onion, Allium cepa L., relative to a complete nutrient solution containing 0.25 mM Mg²⁺. Five compartments for Mg²⁺ in the cortex were found and, in order of increasing rates of exchange, identified with the vacuoles and the cytoplasm of the cortical parenchyma, the Donnan free space, the water free space, and the superficial film of solution on the segments. With the Ussing-Teorell flux ratio equation as the criterion, it was concluded that Mg²⁺ entered the cytoplasm passively and was actively pumped back across the plasmalemma. Mg²⁺ concentration in the vacuole could be estimated only as lying between wide limits (1.3 to 14.3 eq ml^-1), but whatever the concentration within this range, it was concluded that Mg²⁺ was passively distributed across the tonoplast. Net flux was zero and the vacuolar concentration commensurate with this was found to be 6.6 eq ml^-1. The transported fraction of total efflux, appearing at the segment cut ends, was estimated separately. Magnesium was found to be transported almost exclusively in the basipetal direction.