Hexavalent chromium induced stress and metabolic responses in hybrid willows

Metabolic responses to hexavalent chromium (Cr⁶⁺) stress and the uptake and translocation of Cr⁶⁺ were investigated using pre-rooted hybrid willows (Salix matsudana Koidz × Salix alba L.) exposed to hydroponic solution spiked with K₂CrO₄ at 24.0 ± 1°C for 192 h. Various physiological parameters of the plants were monitored to determine toxicity from Cr⁶⁺ exposure. At Cr⁶⁺ treatments of ≤2.1 mg Cr/l, the transpiration rate of plants was > 50% higher than that of the non-treated control plants. As Cr concentrations were increased further, a slight increase in the transpiration rate was also observed compared with the controls. Negligible difference in the chlorophyll contents in leaves between the treated and the non-treated control plants was measured, except for willows exposed to 1.05 mg Cr/l. The response of soluble proteins in leaves of willows to Cr treatments was remarkable. Cr-induced toxicity appeared in all treatments resulting in reduced activities of catalase (CAT) and peroxidase (POD) compared to the controls. Superoxide dismutases (SOD) activity in the leaf cells showed a positive increase after Cr exposure. Of all selected parameters, soluble proteins in leaves were the most sensitive to Cr⁶⁺ doses, showing a significant linear correlation negatively (R ² = 0.931). Uptake of Cr⁶⁺ by willows grown in flasks was found to increase linearly with the added Cr⁶⁺ (a zero order kinetics), as indicated by the high R ² (0.9322). Recovery of Cr in different parts of plant materials varied significantly with roots being the dominant site of Cr accumulation. Although the translocation to shoots was detected, the amount of Cr translocated to shoots was considerably small. The capacity of willows to assimilate Cr⁶⁺ was also evaluated using detached leaves and roots in sealed glass vessels in vivo. Uptake of Cr by roots was mediated possibly through an active transport mechanism, whereas the cuticle of leaves was the major obstacle to uptake Cr from the hydroponic solution. In addition, both cysteine and ascorbic acid showed a remarkable potential to reduce Cr⁶⁺ at a neutral pH. Results indicated that the added Cr did not cause deleterious effects on plant physiological functions over a 192-h period of exposure. Significant removal of Cr from the hydroponic solution was observed in the presence of hybrid willows. The data also suggest that phytoremediation of Cr⁶⁺ is possible and ecologically safe due to the minor translocation of Cr to aerial tissues.