Background, Aim and Scope:
Goal, Scope and Background. Due to its essentiality, deficiency and toxicity to living organisms and the extensive use in industrial activities, selenium (Se) has become an element of global environmental and health concern. Se removal from contaminated sites using physical, chemical and engineering techniques is quite complicated and expensive. The goal of this study is to investigate uptake and translocation of Se in common plant willows, and to provide quantitative information for risk assessment whether Se phytoremediation is ecologically safe.
Materials and Methods:
Material and Methods. Intact pre-rooted plants of hybrid willows (Salix matsudana Koidz × alba L.) and weeping willows (Salix babylonica L.) were grown hydroponically in growth chambers and treated with selenite or selenate at 24.0 ± 1 0C for 144 h. Detopping willows (leaf removal) were also used to quantify the effect of transpiration on volatilization of Se. At the end of the study, total Se in the hydroponic solution and in different parts of plant tissues was analyzed by hydride generation-atomic fluorescence spectrometry (HG-AFS). The capacity of willows to assimilate both chemical forms of Se was evaluated using detached leaves and roots in sealed glass vessels in vivo. Translocation efficiency of Se was estimated.
Results:
Results. Significant amounts of the applied selenite and selenate were eliminated from plant growth media by willows. Both willows showed a significantly higher removal rate for selenate than for selenite (p < 0.05). Substantial differences existed in the distribution of both chemical forms of Se in plant materials: lower stems were the major site for selenite accumulation and roots were the main sink for Se accumulation in selenate-supplied willows. Translocation efficiency for selenite was significantly higher than that for selenate in both willow species (p < 0.01). Compared to the intact trees, remarkable decrease in the removal rate of both chemical forms of Se was found for detopping willows (p < 0.01). Volatilization of Se by plant leaves was estimated to be approximate 10 % of the total applied selenite or selenate. Significant reduction (> 20 %) of selenate was observed in the sealed vessel with roots of willows, whereas trace amounts of selenite were eliminated from the hydroponic solution in the presence of roots. Leaves from none of them reduced the concentration of selenite and selenate in the solution.
Discussion:
Discussions. Due to the significant difference in the removal rate and the distribution in plant materials between the two chemical forms of Se, the conversion of selenate to selenite in hydroponic solution prior to uptake and within plant tissues was unlikely and independent uptake and translocation mechanisms largely existed. Uptake of selenate was mediated possibly through an active transport mechanism, whereas selenite uptake was possibly dependent on plant transpiration. Uptake velocities of selenite were linear (zero-order kinetics), while selenate removal processes were first-order kinetics. The cuticle of leaves was the major obstacle to extract both chemical forms of Se from the hydroponic solution. The phytovolatilization was a biological process for Se removal.
Conclusions:
Conclusions. Although faster removal rates of selenate than selenite from plant growth media was observed by both willow species, selenite in plant materials was more mobile than selenate. Significant decrease in removal rates of both chemical forms of Se was detected for the detopping willows due to the removal of leaves. Different extracting, assimilating and transporting pathways for selenite and selenate largely existed in willow trees.
Recommendations and
Perspectives:
Recommendations. Phytoremediation of Se is an attractive approach of treating Se contaminated environmental sites. More comprehensive investigation on the assimilation of Se in plant tissues will provide powerful evidence to explain the difference in uptake and translocation mechanisms between selenite and selenate in willows.
Perspectives. Willows show a great potential for uptake, assimilation and translocation of selenite and selenate. Phytotreatment of Se is an efficient technology for cleaning up contaminated environmental sites.
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