Identifying the species of copper that are toxic to plant roots in alkaline nutrient solutions

Background and aims

The pH of the growth medium influences Cu speciation in solution, the negativity of plasma membrane (PM) surface potential, and hence the rhizotoxicity of Cu.

Methods

Solution culture experiments were conducted with wheat (Triticum aestivum L.) seedlings to examine the toxicity of various Cu species at pH values ranging from 4.50 to 8.25. The toxic species of Cu was identified, giving particular consideration to the electrical properties at the plant cell membrane and ion activities at the PM surface.

Results

The solution culture studies showed that at pH?<?6.60 (i.e., free Cu²⁺ >95 % of total Cu), the addition of cations (Ca²⁺ or H⁺) decreased the toxic effects of Cu by decreasing the negativity of the PM surface potential (and hence decreasing the activity of Cu²⁺ at the PM surface). For solutions with pH values from 7.50 to 8.25 (CuCO ⁰₃ >50 % of total Cu), an increase in pH significantly enhanced the toxicity of Cu, whilst the addition of Ca had negligible influence on toxicity.

Conclusions

Root growth in solution cultures was influenced primarily by the surface activities of free Cu²⁺ and CuCO ⁰₃ . Across all experiments, the data indicate that it was CuCO ⁰₃ , rather than CuOH⁺, that contributed Cu toxicity over pH?>?7.00. Although our data do not explore the mechanism of toxicity, we propose that CuCO ⁰₃ has an important role in Cu rhizotoxicity in alkaline growth media.