Abstract <p>We have studied the ion-exchange properties of cell walls (CWs) isolated from sunflower plants grown in the absence of excess copper ions (control plants), and from the plants treated with 10 and 50 μM CuCl₂ for 24 h. Three types of cation-exchange groups are present in the CWs of both control and Cu-treated plants: carboxyl groups of polygalacturonic (PGA) and hydroxycinnamic (HCA) acids, and phenolic OH‑groups. pK_a values of these groups suggest that only carboxyl groups are able to bind cations in the physiological pH range. Sunflower shoot and root CWs do not differ in the content of both PGA and HCA carboxyl groups, but shoot CWs have significantly higher content of phenolic OH-groups. Treatment of plants with 10&#xa0;μM Cu results in an increase in the amount of phenolic OH-groups in the root CWs and PGA carboxyl groups the shoot CWs compared to the CWs of control plants. At 50 μM Cu the amount of phenolic OH-groups in the shoot CWs decreases 1.5-fold, while in the root CWs their content is the same as at 10 μM. A comparison of Cu content in the roots and the CWs isolated from them shows that the latter are capable of binding 100% of copper in the root at 10 and 50 μM Cu in the medium. This leads to preferential accumulation of Cu in sunflower roots and suggests that under the selected experimental conditions root apoplast is an effective barrier to the penetration of toxic Cu²⁺ ions into the symplast.</p>