Differences in uptake and translocation of hexavalent and trivalent chromium by two species of willows

Uptake and translocation of chromium (Cr) by two willow species was investigated. Intact pre-rooted weeping willows (Salix babylonica L.) and hankow willows (Salix matsudana Koidz) were grown hydroponically and spiked with hexavalent chromium [Cr (VI)] or trivalent chromium [Cr (III)] at 25.0 +/- 0.5 degrees C for 120 h. Removal of leaves was also performed as a treatment to quantify the effect of transpiration on uptake and translocation of either of the Cr species. Although the two willow species were able to eliminate Cr (VI) and Cr (III) from the hydroponic solution, significant differences in the removal rate for both chemical species were observed between the two willows (p < 0.05): faster removal rate for Cr (III) than Cr (VI) was detected in both willow species; hankow willows showed higher removal potential for both chemical species than weeping willows. Remarkable decreases in the removal rates for both Cr species were detected in the willows with leaves removed (p < 0.05). The results from the treatments spiked with Cr (VI) also revealed that Cr was more mobile in plant materials of hankow willows than that in weeping willows (p < 0.01), while higher translocation efficiency of Cr was observed in weeping willows than hankow willows for the Cr (III) treated (p < 0.01). However, a convincing decrease in the translocation efficiency due to the removal of leaves was only observed in the treatments spiked with Cr (VI) (p < 0.05). Substantial differences existed in the distribution of Cr species in plant materials after exposure of either of the chemical forms: roots and lower stems were the major sites for accumulation in weeping willows exposed to Cr (VI) and Cr (III), respectively; in contrast roots were the only sink in hankow willows exposed to both chemical species. The capacity of willows to assimilate both Cr species was also evaluated using detached leaves and roots of both willow species in sealed glass vessels in vivo. The results indicated that detached roots showed a more remarkable capacity to remove Cr (III) from the hydroponic solution than Cr (VI) (p < 0.01). Although detached leaves of both willow species were able to efficiently eliminate Cr (III), neither of them reduced the concentration of Cr (VI) in the solution. The results suggests that different mechanisms for uptake, assimilation and translocation of Cr (VI) and Cr (III) exist in different willow species and phytoremediation of Cr should consider this factor for the proposed target effectively.