Cortical cell fluxes and transport to the stele in excised root segments of Allium cepa L. : IV. Calcium as affected by its external concentration.

From compartmental analysis of radioisotope elutin measurements, fluxes of Ca(2+) were estimated for cortical cells in root segments of onion, Allium cepa L., relative to complete nutrient solutions containing a range of calcium concentrations ([Ca0]) from 2 μeq l(-1) to 20 meq l(-1), increasing in 10-fold steps for Ca(2+). Except for the calcium counter-ion (usually NO 3 (-) , sometimes Cl(-) at the highest [Ca0]), the composition of the nutrient solution was other-wise the same at all calcium concentrations. Compartmental analysis indicated that the cytoplasm had a high content of exchangeable Ca(2+) but, in the light of evidence from animal studies, ionic activity of calcium in the cytoplasm was assumed to be no greater than 0.002 μeq ml(-1). With the Ussing-Teorell flux equation as the criterion, it was concluded that at all values of [Ca0] tested, Ca(2+) entered the cytoplasm passively and was actively pumped back into the external solution. Entry of calcium to the vacuole from the cytoplasm was active in all cases. The conclusions regarding the character of ion transport across the plasmalemma were the same as when the whole calcium content of the cytoplasm was taken to contribute to the ionic activity. However, the electrochemical activity gradient was very much steeper than formerly estimated. Calcium was transported to the stele in proportion to the calcium content of the cytoplasm and moved in the xylem almost exclusively in the basipetal direction.