Distribution and Toxic Effects of Cadmium and Lead on Maize Roots

Two-day-old seedlings of maize (Zea mays L.) were incubated on Cd and Pb nitrate solutions at the concentrations that inhibited root growth approximately by 50% after two-day-long incubation (LC₅₀; 10^–4 and 10^–3 M, respectively) or completely terminated growth of the primary root after one-day-long incubation (LC; 5 × 10^–4 and 10^–2 M, respectively). Cd and Pb contents were measured using an anodic inversion voltammetric technique in a flow injection system and a histochemical method. At LC₅₀, Cd and Pb were discerned, by histochemical techniques, in all root apical tissues, whereas in the root hair zone, the heavy metals were primarily accumulated in the apoplast of the rhizodermis and cortex and to a lesser extent, in the vascular tissues and parenchyma cells surrounding the metaxylem vessels. Insignificant accumulation of Cd and Pb in the pericycle probably explains why root branching was tolerant to these agents. At LC, Cd and Pb were found in the apoplast of all root tissues, in accordance with the practically complete inhibition of root growth and branching. Irrespectively of Cd and Pb concentrations in the external solution, the metal contents in the root apex exceeded those in the basal region. Procion dyes were used to assess cell death inflicted by Cd and Pb. At LC, the root cap and meristematic cells perished, together with the rhizodermal cells and the outer cortical cells of the root apex, whereas only the rhizodermal cells in the root apical region died at LC₅₀. The evidence that Cd and Pb cross the endodermal barrier at LC presumes that, at lower metal concentrations, the Casparian strip and plasmalemma of the endodermis regulate the transport of these metals into the central cylinder. The authors conclude that the identical barriers control Cd and Pb transport in root tissues.