The Radial and Longitudinal Path of Ion Movement in Roots

Abstract

AbstractThe existence of a barrier to lateral outward diffusion of ions from roots was demonstrated quantitatively and by autoradiography. Ions applied to the apical zone were excreted through the basal cut end with no lateral outward diffusion from the central zone. The ions moving through the conducting tissues showed no leakage to the external solution regardless whether the treatment roots came from seedlings grown under low or high salt conditions. However, when dinitrophenol (DNP) was also applied to the central zone in the external salt solution, leakage of apically applied calcium occurred from the conducting tissues. Autoradiographic studies with labeled calcium suggested that the endodermal layer was an effective barrier preventing the lateral outward diffusion of ions. The ions moved longitudinally through the stele. In the stelar tissues the autoradiographic studies failed to detect the presence of any radioactive calcium in the central duct and in the mature xylem vessels, although high concentrations of labeled ions were found in the living cells of the stele, particularly the xylem parenchyma. It is suggested that xylem parenchyma cells may be involved in the longitudinal transport of ions.