Recent reports suggest that significant fractionation of stable metal isotopes occurs during biogeochemical cycling and that the uptake into higher plants is an important process. To test isotopic fractionation of copper (Cu) and zinc (Zn) during plant uptake and constrain its controls, we grew lettuce, tomato, rice and durum wheat under controlled conditions in nutrient solutions with variable metal speciation and iron (Fe) supply. The results show that the fractionation patterns of these two micronutrients are decoupled during the transport from nutrient solution to root. In roots, we found an enrichment of the heavier isotopes for Zn, in agreement with previous studies, but an enrichment of isotopically light Cu, suggesting a reduction of Cu(II) possibly at the surfaces of the root cell plasma membranes. This observation holds for both graminaceous and nongraminaceaous species and confirms that reduction is a predominant and ubiquitous mechanism for the acquisition of Cu into plants similar to the mechanism for the acquisition of iron (Fe) by the strategy I plant species. We propose two preliminary models of isotope fractionation processes of Cu and Zn in plants with different uptake strategies.

This work assessed in situ, copper (Cu) uptake and phytotoxicity for durum wheat (Triticum turgidum durum L.) cropped in a range of Cucontaminated, former vineyard soils (pH 4.2-7.8 and total Cu concentration 32-1,030 mg Cu kg −1 ) and identified the underlying soil chemical properties and related root-induced chemical changes in the rhizosphere. Copper concentrations in plants were significantly and positively correlated to soil Cu concentration (total and EDTA). In addition, Cu concentration in roots which was positively correlated to soil pH tended to be larger in calcareous soils than in non-calcareous soils. Symptoms of Cu phytotoxicity (interveinal chlorosis) were observed in some calcareous soils. Iron (Fe)-Cu antagonism was found in calcareous soils. Rhizosphere alkalisation in the most acidic soils was related to decreased CaCl 2 -extractable Cu. Conversely, waterextractable Cu increased in the rhizosphere of both non-calcareous and calcareous soils. This work suggests that plant Cu uptake and risks of Cu phytotoxicity in situ might be greater in calcareous soils due to interaction with Fe nutrition. Larger water extractability of Cu in the rhizosphere might relate to greater Cu uptake in plants exhibiting Cu phytotoxic symptoms.

The impact of a large rhizosphere alkalisation on copper (Cu) bioavailability to durum wheat (Triticum turgidum durum L.) initially exposed to a broad range of bulk soil pH (4.8-7.5) was studied. Plants were exposed to a Cu-contaminated soil treated with eight levels of lime (Ca(OH) 2 ) and supplied with NO 3 − or NH 4 + -NO 3 − . Nitrate-fed plants strongly increased their rhizosphere pH to about 6.9-7.6, whatever the initial pH. NH 4 + -NO 3 − -fed plants slightly acidified their rhizosphere down to 3.9. Free Cu 2+ concentration in the rhizosphere was 3 orders of magnitude larger for NH 4 + -NO 3 − than NO 3 − fed plants. Consequently, Cu bioavailability was 2.4-to 4.2-fold larger for NH 4 + −NO 3 − -fed plants which demonstrates the importance of rhizosphere alkalisation to restrict metal bioavailability in acidic soils. Copper bioavailability of NO 3 − -fed plants initially exposed to a broad range of bulk soil pH was insensitive to bulk soil pH, as rhizosphere pH was ultimately neutral in any case.