Accumulation of Metals at the Soil-Root Interface: A Thermal Study.
Tanja Mimmo, Claudio Marzadori, Daniela Montecchio, and Carlo Gessa. Department of Agroenvironmental Sciences and Technologies, Alma Mater Studiorum – University of Bologna, Viale Fanin 40, Bologna, 40127, Italy
Pectates are acid polysaccharides largely found at the soil-root interface based on a L-1,4-a-galacturonan core with rhamnose residues interspersed and neutral side chains attached. They are secreted as highly esterified pectates from the symplast into the apoplast where demethylation takes place by pectin methylesterase. These demethylation reactions in the apoplast lead to pectates with free carboxylic groups playing thereby an important role in affecting the cation binding capacity, diffusion and indirectly thus the plant nutrition. In addition, the pectates forming a porous network of fibrils could act as direct channels for the movement of soil cations to the surface of root cells. Previous studies already showed how the pectate immobilization of toxic metals as aluminum altered the functionality of the soil-root interface reducing the phosphate diffusion and indicating that the root apoplast could play an important role in the expression of a possible mechanism of Al toxicity. The aim was to evaluate if the different degrees of esterification (DE) of pectates could influence the immobilization of toxic metals at the soil-root interface. Calcium-pectates (Ca-PG) with different degrees of esterification (0, 26 and 65 %) were formed in custom-made cells and used as a model of the soil-root interface. They were consequently treated with solutions of aluminum, cadmium and chromium at pH 4.50 at a concentration of 100 mM forming Ca-Cd-PG, Ca-Al-PG and Ca-Cr-PG networks. The metal sorptions were conducted for 24 hours under constant stirring at 25°C. Al, Cd, Cr and Ca concentrations were determined by ICP-OES. Thermogravimetric (TG) and differential thermal analysis (DTA) were carried out simultaneously in dynamic air using a TG-DTA92 instrument (SETARAM, France) analyzing about 5 mg of freeze-dried sample. The metal sorption showed no significant differences between the three DE's. However, the sorption of trivalent cations was higher than the one of divalent cations: Al > Cr > Cd. This might indicate that the metals interact with different coordination sites as the free carboxylic groups present on the pectate chains decrease with increasing DE. Basically, all the thermal profiles show three major exothermic peaks. The first around 340°C and the second one around 450°C could be considered the result of a partial decarboxylation reaction of protonated carboxylates, oxidation reactions of small fragments of the pectate ring and/or the opening of the pyranoid ring, respectively. The third exothermic peak around 580°C is probably caused by the decomposition of that part of the pectate chain coordinated to the metal. The thermal profiles varied with increasing DE which indicates structural modifications. The increasing presence of hydrophobic -CH3 groups could act as a shield and induce a tightening of the PG chains. On the whole, toxic metal sorption caused a decrease of the decomposition temperatures. For example, Ca-Cd-PG both at DE 0 and 65 %, was completely thermally decomposed at 470°C while Ca-PG had an exothermic peak still at 580°C. These results point out that metal coordination causes a weakening of the structure (Cd > Cr > Al) compared to the networks with calcium ions as the only reticulating ion. The DE would thus play a fundamental role in the mobilization/immobilization of toxic metals at the soil-root interface. Acknowledgement: This project was funded by MIUR (Ministero dell'Istruzione dell'Università e della Ricerca) PRIN 2004 (project code: 2004070459_003).