SPECTROSCOPIC INVESTIGATION OF ALUMINUM COMPLEXES IN PLANTS GROWTH MEDIA

Cytotoxicity of aluminum is a serious problem that can severely limit productivity in crops, particularly in acid soils. When the soil pH is lower than 4.5–5.0, Al3+ is solubilized in soil water and is absorbed by plant roots, inhibiting root elongation severely. Furthermore, the involvement of aluminum in biological systems has been emphasized in recent years, suggesting its potential interaction with cells. For example, the biotoxic effect of aluminum was correlated with the onset of Alzheimer's and other diseases. It has been observed that certain vegetal species that grow easily in acid soils using, for detoxify from aluminum, the production of organic acids capable of complexing it. To this purpose, plants make use of two strategies: the first is the exclusion of aluminum from roots (the complex formed in the soil cannot cross the cell membrane), the other provides a greater resistance to the effects of the aluminum through its complexation within the cell. In the present study, a tree species (olive tree, Olea Europea) and an herbaceous species (barley, Hordeum vulgare), were investigated through the analysis of the effects of increasing concentration of Al3+ ions in hydroponics growth conditions, analyzing the plants’ exudates. As expected, an increased concentration of Al3+ inhibited root elongation and proliferation. Moreover, a SEM observation of root tips evidenced an increasing number of cracks and fractures with increasing Al3+ concentration. The characterization of theses complexes is quite limited: the use of vibrational spectroscopic techniques (in particular Raman spectroscopy) offers the great advantage of operating satisfactorily also in an aqueous environment, allowing studies in systems similar to those occurring in nature and exploring the ppm concentration range through SERS analysis. We studied the Raman and IR spectra of three organic acids of agricultural interest (oxalic, tartaric and citric acids) together with their Na and Al salts, both in the solid state as well as in H2O and/or D2O solution. The results arising from the study of the differences between symmetric and asymmetric COO– stretching vibrations give interesting information on the type (monodentate, bidentate or bridging) and the strength of the bound involved in the complexing process. The structural data derived by spectroscopic methods are supported by computational methods, allowing the determination of complexing constants and stability