Accumulation and Distribution of Trivalent Chromium and Effects on Hybrid Willow (Salix matsudana Koidz × alba L.) Metabolism

Abstract

The metabolic response of plants to exogenous supply and bioaccumulation of trivalent chromium (Cr³⁺ ) was investigated. Pre-rooted young hybrid willows (Salix matsudana Koidz × alba L.) were exposed to hydroponic solution spiked with CrCl₃ at 24.0°C ± 1°C for 192 hours. Various physiologic parameters of the plants were monitored to determine toxicity from Cr exposure. The transpiration rate of willows exposed to 2.5 mg Cr/L was 49% higher than that of the untreated control plants, but it was decreased by 17% when exposed to 30.0 mg Cr/L. Significant decrease (≥20%) of soluble protein in young leaves of willows was detected in the treatment group with ≥7.5 mg Cr/L. The measured chlorophyll contents in leaves of treated plants varied with the dose of Cr, but a linear correlation could not be established. The contents of chlorophyll in leaves of willows exposed to ≥7.5 mg Cr/L were higher than that of the untreated plants but lower at 30.0 mg Cr/L. Superoxide dismutase activity (SOD) in leaves between the treated and untreated willows did not show any significant difference, but activities of both catalase (CAT) and peroxidase (POD) in leaf cells of all treated plants were higher than those in the untreated willows. The correlation between the concentration of Cr and CAT activity in leaf cells was the highest of all toxicity assays (R ² = 0.9096), indicating that CAT activity was most sensitive to the change in Cr³⁺ doses compared with the other selected parameters. Results from the Cr uptake study showed that significant removal of Cr from hydroponic solution was observed in the presence of hybrid willows without showing detectable phytotoxicity, even at high does of Cr. More than 90% of the applied Cr³⁺ was removed from the aqueous solution by willows at concentrations up to 7.5 mg Cr/L. Approximately 70% of the initial Cr was recovered in the plant materials. At the low-Cr³⁺ treatment (2.5 mg Cr/L), Cr accumulation by willow materials was the greatest (92%) in roots and the lowest (0.2%) in leaves, whereas the greatest (66%) was in stems and the lowest (0.1%) in leaves of willows exposed to 30.0 mg Cr/L. The correlation between applied Cr³⁺ (mg Cr/L) and Cr (μg Cr/g fresh weight [fw]) accumulated in plant materials was significant. The bioaccumulation kinetics of Cr by hybrid willows can be described by a typical saturation curve. Results also indicated that translocation of Cr from roots to shoots was possible. It is to conclude that hybrid willows have great potential as bioremediation technology in the removal of chromium (Cr³⁺) from contaminated effluents and sediments.