First, we grew plants from the Solanum family – Solanum nigrum, also known as black nightshade, that can accumulate cadmium; and S. melongena, known as eggplant, that is not tolerant towards Cd – in a controlled growth chamber. Plants were cultivated on a standard clayey loam soil doped with cadmium: 4 different Cd concentrations were investigated (0, 25, 50 and 100 mg Cd per kg of soil). For each soil concentration, 7 pots of two seedlings were prepared for S. nigrum and S. melongena, to have enough material to perform the SPECADIS project tasks. (i) have replicates to statistically investigate the amount of Cd in the plant parts; (ii) have samples for X-ray absorption measurements and (iii) have samples for Cd isotope composition measurements.
In parallel, the fellow applied for beamtime at several synchrotron facilities: the Swiss Light Source in Switzerland (obtained 15 8-hours shifts), the Diamond Light Source in the U.K. (12 shifts) and the Soleil Synchrotron in France (15 shifts).
For the X-ray absorption spectroscopy measurements, the samples were prepared frozen in liquid nitrogen, and kept at a temperature of 80°C, to prevent changes in Cd speciation between harvest and measurements.
During the 3 synchrotron sessions, cadmium speciation was determined in plant parts from S. nigrum (roots, stem, young and old leaves) and S. melongena (roots, stem, fresh and dead leaves) grown on the 25, 50 and 100 mg.kg-1 Cd-doped soil and on the soil used for their culture. We found that all the cadmium in the soils (pre and post culture) is adsorbed on iron oxyhydroxides such as goethite. Upon roots uptake, Cd undergoes a major change in speciation. For both plants, we found that sulphur plays a major role: in roots from S. nigrum and S. melongena, Cd is mainly bound to organic, sulphur-bearing thiol ligands. By comparing speciation results in fresh and dead leaves, we hypothesized that Cd is actively stored bound to S ligands in the vacuoles. Our results also show that, in S. nigrum leaves, a part of Cd is bound to inorganic sulphur. The SPECADIS results deepened our understanding of Cd detoxification mechanisms in Solanaceae.
Last, the fellow developed and implemented the measurements of cadmium stable isotopes on the multi-collector Inductively coupled plasma mass spectrometer from the CEREGE host department, and she measured the Cd isotope compositions of the plant samples previously analysed by X-ray absorption spectroscopy. The results were compared to the Cd speciation in the plant parts, and the potential of Cd stable isotopes as tracers of Cd speciation in the soil/plant system was assessed. Cadmium isotope compositions in the plant parts also provided information on the translocation of Cd from the roots to the leaves.
The work performed in SPECADIS has led to 1 published journal article, and two manuscripts in preparation. The results were presented at 2 international conferences, as well as 1 public outreach event. So far, 1 article and 1 dataset were deposited in open access repositories.
