THE TRANSPORT OF CADMIUM AND LEAD IONS THROUGH ROOT TISSUES

To assess cadmium and lead mobility in root tissues, the lower part of the roots of two-day-old maize (Zea mays L.) seedlings were immersed for 120 h in cadmium and lead nitrate solutions of lethal concentratio n (10(-5) and 10(-4) M, respectively). The immersed root surface was p artly coated with vaseline. The upper part of the roots was exposed to the humid air over the solution. Lateral root development, the proces s most resistant to heavy metals, was employed as a toxicity index. Wh ile primary root growth was similarly inhibited by the two ions, lead, in contrast to cadmium, did not arrest lateral root initiation in the humid air or coleoptile growth. Lateral roots did not develop when ro ots were completely immersed in cadmium and lead solutions. Histochemi cal visualization of cadmium and lead with 4-(2-pyridylazo)-resorcinol confirmed that the higher mobility of the former in root tissues unde rlies its greater toxicity.