Resumen de Behavior of silene vulgaris (moench.) garcke against exposure to chromium. Evaluation of potential use in soil remediation
Chromium, due to its anticorrosive properties, is widely employed in metallurgical, chemical and refractory industries. Atmospheric emissions and improper disposal of liquid and solid wastes from these industries makes Cr pollution an increasing environmental concern. The two most common chemical species of Cr in the environment are Cr(VI) and Cr(III). Cr(VI) is highly oxidizing and soluble while Cr(III) is normally insoluble and between 10 and 100 times less toxic than Cr(VI).
Particularly, soil pollution with Cr is a serious environmental and human health hazard both by direct contact with Cr and by the possibility of Cr to be leached to the ground water or to be taken up by plant thus being transferred to the food chain. Moreover, soil pollution with Cr can lead to major economic losses. Therefore, there is a need of developing new technologies for Cr removal. Phytoremediation or phytotechnology could be a green and low cost alternative to this purpose. This technology is based on the ability of plants and their associated microorganisms for environmental clean-up.
A deep knowledge of metal-plant interactions and plant response in soil is required to the proper implementation of these technologies. Less information about Cr-plant interactions is available regarding other heavy metals and less species have been evaluated for their application in Cr polluted sites. The objective of this thesis has been to study the response of Silene vulgaris to Cr exposition in order to evaluate their potential to be used in Cr polluted sites. S. vulgaris is a facultative methalophyte, it is considered an interesting species in heavy metal phytoremediation but no information is available about its response to Cr. Hydroponics and hydroponics with perlite essays as well as a pot experiment have been carried out. Both cuttings from homogeneous genotypes and seedlings developed from seeds of the same population have been used as plant material.
In S. vulgaris Cr was mainly accumulated in roots and only translocated to shoots near toxicity levels which is the first protection mechanism to Cr. Plants accumulated more Cr in presence of Cr(VI) than of Cr(III) due to the different uptake mechanisms of each specie. Based on root growth, the most tolerant genotypes showed EC100 between 200 and 1200 µM and the less tolerant between 200 y 1000 µM. Seedlings developed from seeds showed lower tolerance levels, between 30 and 100 µM Cr(VI). S. vulgaris showed different tolerance mechanism to Cr. In one hand, the most tolerant genotypes increased their nutrient levels (especially Fe, Ca and Mg) to keep homeostasis balance and to synthesize antioxidant and essential metabolites. Exudation has also been identified as a possible tolerance mechanism to Cr as it is correlated with Cr accumulation in plant tissues but not with oxidative stress. Using HPLC-MS, quercetin and apiin have been identified as major polyphenols in S. vulgaris roots exudates. Concentrations of organics acids in both plant tissues and root exudates have been determined by ionic chromatography. Citric and malic acid showed the highest response to Cr exposition, their concentrations decreased in root exudates and increased in roots showing their role in Cr detoxification pathway in plant tissues.
With the aim of studying Cr distribution and speciation in plant tissues, micro synchrotron X-ray spectroscopy has been applied. Results from these studies showed the ability of S. vulgaris to reduce Cr(VI) to the less toxic Cr(III) as in plants growth in Cr(VI), all Cr found in plant tissues was Cr(III). Oxidation state maps showed Cr accumulated mainly in the roots tips and in roots cell wall. Once the retention capacity of roots was exceeded, Cr was transported through the steam xylem to the cell wall of leaf margins. µ-XANES analysis confirmed the role of organic acids in Cr accumulation and transport in the plant.
Starch granules have been identified in leaves of Cr(VI) treated plants using SEM and DRIFTS- FTIR. This could be attributed to an impairment between C assimilated in photosynthesis and C used in plant growth. This indicates that S. vulgaris plants were able to preserve photosynthetic function by Cr retention in roots and leaves margins but the energy invested in the managing of Cr excess made their biomass to decrease. DRIFTS- FTIR analysis confirmed the role of polysaccharides and proteins of the cell wall in Cr binding to avoid its presence in the cytoplasm. Moreover these data showed synthesis of aromatic compounds in leaves, possibly antioxidant molecules, and also the absence of membrane damage.
Last in a pot experiment, plants were grown for six months in soils polluted with Cr(III) or Cr(VI). Results from this study showed that S. vulgaris was able to growth in soils with different physic-chemical characteristics and moderate levels of Cr pollution. The development of S. vulgaris cover provided C to soil from root exudation. The C input increased soil pH and solubilised part of the Cr that was not available hence increasing plants absorption. Moreover it stimulated soil microbial activity, as shown in dehydrogenase activity, contributing to soil recovery.
The ability of S. vulgaris to reduce Cr(VI) to Cr(III), to accumulate Cr mainly in roots and to develop and provide C to Cr polluted soils, makes it and interesting species to be applied in the revegetation and recovery of Cr polluted sites.
