Influence of copper on root exudate patterns in some metallophytes and agricultural plants

A hydroponic experiment was carried out to determine the root exudation patterns in two Cu-metallophytes (Oenothera picensis and Imperata condensata) and two agricultural plants (Lupinus albus and Helianthus annuus). Plants were grown in nutrient solution at increasing Cu doses (0, 0.125, 0.25, 0.5, 1 and 2 mg Cu L−1), and plant growth, root elongation, Cu accumulation and root exudates were measured. All plants showed a decrease of over 60% in root elongation at the highest Cu supply level, being O. picensis the most sensitive specie and showing the highest shoot and root Cu concentrations (116 and 2657 μg Cu g−1, respectively), which were six fold higher than the other species. Differences in root exudation patterns of low molecular weight organic acids were found, with extremely high amounts of succinic acid exuded by O. picensis (1049 μmol g−1 h−1), and citric acid by I. condensata (164 μmol g−1 h−1). In metallophytes, the organic acid exudation was increased even with no root elongation, meanwhile agricultural plants exuded citric acid at constant levels. Exudation of phenolic compounds was highly species-dependent, with catechin mainly exuded by I. condensata, (2.62 μmol g−1 h−1) cinnamic acid by O. picensis (5.08 μmol g−1 h−1) and coumaric acid exclusively exuded by H. annuus (13.6 μmol g−1 h−1) at high Cu levels. These results indicated that differences in root exudation patterns among metallophytes and agricultural plants could affect their Cu tolerance. Particularly, the higher exudation rate showed by I. condensata can be an effective exclusion mechanism to tolerate high Cu concentrations, supporting its use in Cu phytostabilization programs.

► I. condensata exuded 4 fold more citric acid than agricultural plants at high Cu doses.
► O. picensis exuded high amounts of succinic acid, but Cu was highly concentrated into plants.
► I. condensata was the most Cu-tolerant metallophyte.
► Root exudation patterns could be useful for evaluating Cu tolerance in plants.