Abstract: Using a technique of insoluble salt precipitation, we investigated the permeability of endodermal Casparian bands (CBs) for Cu²⁺ in primary roots of soybean (Glycine max L. cv Nongda 5828). Insoluble brown crystals of Cu ferrocyanide, indicating passage of Cu²⁺, were observed after 200 μmol·L^–1 CuSO₄ was sucked into the root from its medium by use of a pump and subsequent perfusion of xylem with 400 μmol·L^–1 K₄[Fe(CN)₆]. Under suction, Cu²⁺ could cross the endodermis apoplastically in soybean, with brown salt precipitates in cell walls of xylem and in the region between CBs and xylem vessels. Regions where laterals penetrated the endodermis showed brown salt precipitates as well. When sides of CuSO₄ and K₄[Fe(CN)₆] application were reversed, Cu²⁺ could still cross the CBs and induce brown salt precipitates outside CBs and cell walls of exoderm cells. The results suggest that CBs are not perfect barriers to apoplastic ion flux.

Key words: apoplast, Casparian bands, Cu²⁺, primary root