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

From compartmental analysis of radioisotope elution measurements, concentrations and fluxes of Ca(2+) were estimated for cortical cells in root segments of onion, Allium cepa L., relative to a complete nutrient solution containing 1 mM Ca(2+). Five compartments for Ca(2+) in the cortex were revealed. These were identified, in order of increasing rates of exchange, with the vacuole and cytoplasm of the cortical parenchyma, the Donnan free space in the cell walls, the water free space in the tissue and the superficial film of solution on the segments. With the Ussing-Teorell flux ratio equation as the criterion, it was concluded that Ca(2+) entered the cytoplasm passively and was actively pumped back to the external solution. Ca(2+) concentration in the vacuole could only be estimated as lying between wide limits (1.0 to 7.5 μeq. ml(-1)), but even at the maximum concentration, it was concluded that entry was passive and content limited by an efflux pump across the tonoplast. Net flux was zero and the vacuolar concentration of Ca(2+) compatible with this was found to be 2.6 μeq. ml(-1). The transported fraction of the total efflux, appearing at the segment cut ends, was estimated separately. Calcium was found to be transported almost exclusively in the basipetal direction.