In vineyard soils, the repeated and massive use of Bordeaux mixture as fungicide to control vine downy mildew has led to increased copper (Cu) concentrations in soils, particularly in topsoils where Cu accumulation can reach 100 to 1000 mg Cu kg^-1 of soil. The vine itself does not seem to be sensitive, partly because its roots colonize mostly the deeper soil layers that are least contaminated. Nevertheless, bioavailability (and toxicity) of such accumulated Cu for other crops remains an open question because in certain Mediterranean regions such as the south of France many vineyards have been replaced by other cultures, particularly cereals such as durum wheat. These crop species are known for their efficiency to acquire iron (Fe) from calcareous soils via the release of chelators, the so-called phytosiderophores, by their roots (Römheld, 1987). Phytosiderophores are known to be released by root tips of cereals in response to Fe deficiency (Marschner et al, 1987). It has been shown that these Fe chelators released by roots of Fe deficient grasses could mobilize from calcareous soils not only Fe but also other metals such as Zn, Cu or Cd (Treeby et al, 1989; Zhang et al, 1991; Römheld and Awad, 2000; Chaignon et al, 2002). One may thus question if Fe deficiency and enhanced phytosiderophores release by cereals can lead to increased mobilization and acquisition of Cu by plants. The aim of the work was to study whether phytosiderophores secretion, in response to Fe deficiency, by roots of durum wheat cropped in Cu-contaminated, calcareous soils is involved in the mobilization and acquisition of Cu by the plant. Durum wheat (Triticum turgidum durum) has been cultivated in a pot experiment using two calcareous soils differing in their total Cu content (145 mg Cu kg^-1 and 398 mg Cu kg^-1). The cropping device used in this work is the one described by Chaignon and Hinsinger (2003). It enables an easy access to rhizosphere soil and thus allows the study of chemical changes occurring in the rhizosphere that could affect the bioavailability of Cu to plants. The culture experiment comprised two stages. In a first stage, plants were grown for four weeks in hydroponic conditions. During the last week of this first stage plants were separated into two groups and submitted to two Fe levels, (-Fe) 2 µM and (+Fe) 100 µM, in order to have two groups of plants differing in their levels of phytosiderophores release. For the second stage of the culture, pots with four weeks old wheat plants were transferred for 8 days onto small soil disks (3 g). Nor Cu neither Fe were supplied during this stage of the experiment so that soil Cu and soil Fe were the only sources for the plants. The amount of phytosiderophores in the root exudates was determined indirectly by the amount of Cu mobilized from a Cu-loaded resin. Concentrations of Cu in roots and shoots as well as Cu bound to root cell walls were determined in order to evaluate the acquisition of Cu by durum wheat in these two Cu-contaminated, calcareous soils. Cu fractionation in the rhizosphere of the two groups of plants (-Fe) and (+Fe) was studied with chemical extractions.

References

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Chaignon V, di Malta D and Hinsinger P, 2002. New Phytologist 154, 121-130.

Marschner H, Römheld V and Kissel M, 1987. Physiologia Plantarum 71, 157-162.

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Treeby M, Marschner H and Römheld V, 1989. Plant and Soil 114, 217-226.

Zhang FS, Römheld V and Marschner H, 1991. Plant and Soil 130, 173-178.