EDTA-induced heavy metal accumulation and phytotoxicity in cardoon plants

In most metal polluted soils, Pb usually appears together with other heavy metals (HM), such as zinc (Zn) and cadmium (Cd). In addition, in phytoextraction experiments, efficient Pb uptake is generally limited by its low phytoavailability. Then, as part of our studies on chelate-induced phytoextraction, cardoon (Cynara cardunculus L.) seedlings were hydroponically exposed to a range of ethylenediaminetetraacetic acid concentrations (EDTA: 0, 250, 500, and 750 μM) in the absence and presence of a mixture of heavy metals (300 μM Pb2+ + 150 μM Zn2+ + 50 μM Cd2+). Increasing EDTA concentrations in solution significantly increased shoot Pb accumulation and shoot/root ratios. On the contrary, above 250 μM, EDTA decreased root and shoot Zn and Cd accumulation. Our results suggest a non-selective apoplastic root uptake of metal–EDTA complexes. When supplied independently, both EDTA and the mixture of heavy metals negatively affected plant physiological parameters related to plant water transpiration, such as shoot water content, evapotranspiration, transpiration rate, and stomatal conductance. On the contrary, in the presence of heavy metals, increasing EDTA concentrations led to decreased metal phytotoxicity. The highest total amount of metals in shoots was obtained at 250 μM EDTA, a close to equimolar concentration to the metal with the lowest phytoavailability present in the solution (i.e., Pb). Our results indicate that proper management of the EDTA concentration can reduce metal phytotoxicity, maintain the free uptake of some metals and, at the same time, increase the uptake of metals with low phytoavailability.