X-RAY NANOPROBE

98 ESRF

Fig. 81: a) Synchrotron µ-XRF distribution map of Cd-P-S in mature durum wheat grains sectioned transversally at the position of the root primordia. In (b), a more detailed map shows the distribution of Cd and P within the crease and the areas where µ-XANES spectra were collected: white arrows indicate typical positions of P and Cd hotspots, red arrows indicate typical positions of Cd hotspots and green arrows indicate typical Cd diluted areas. In (a), OP: outer pericarp, SE: starchy endosperm, al: aleurone layer, rp: root primordia, sl: scutellum. In (b), vb: vascular bundle area, ps: pigment strand, np: nucellar projection (either abaxial or adaxial).

Fig. 82: Cd LIII-edge µ-XANES spectra of Cd references and of various areas of the crease with their linear combination fits (dotted line). The fit criteria are expressed as normalised residual sums of squares Nss (Nss = S [μexp−μfit]2 / S [μexp]2 x 100).

Cd LIII-edge micro X-ray absorption near-edge structure (µ-XANES) spectra (Figure 82) recorded on different areas of the crease suggest various possible Cd ligands, probably corresponding to different forms of Cd transport or storage. Cadmium is nearly half bound by thiols in the Cd- enriched pigment strand, while in (Cd, S)-diluted areas of the vascular parenchyma this proportion increases to 63%. Assuming that the speciation of Cd in the vascular parenchyma mirrors that in the phloem vessels, this would mean that Cd is significantly associated with thiol-containing ligands in the phloem of durum wheat, in agreement with previous investigations on the Cd-hyperaccumulator Arabidopsis halleri [2]. Based on this result, one possible explanation for the strong accumulation of Cd in the maternal tissues of the crease is that transporters involved in metal loading in the nucellar projection