Shenker M, Fan T W, Crowley D E
Dep. of Environmental Sciences, University of California, Riverside 92521-0424, USA.
J Environ Qual. 2001 Nov-Dec;30(6):2091-8. doi: 10.2134/jeq2001.2091.
A constant anthropogenic release of cadmium to the environment has resulted in a continuous buildup of Cd in soils. Uptake and accumulation of Cd in plant tissue and in grains may lead to food chain transfer to humans. Application of synthetic chelates was suggested to increase metal mobilization and facilitate phytoextraction as a means for the remediation of metal-polluted soils. However, most of the chelate-extracted metal may be leached rather than mobilized to plant roots. In contrast to the synthetic chelates added to soils, plant-produced chelators called phytosiderophores (PS) are excreted directly to the rhizosphere. Previous studies have shown that PS facilitate uptake of Zn and Fe by graminaceous plants. In this study, a two-step PS mediation of Cd uptake was hypothesized: (i) extraction and chelation in the soil solution, and (ii) delivery of the chelated Cd to the uptake system of the plant. We examined Cd extraction by PS, the synthetic chelate HEDTA [N-(2-hydroxyethyl)-ethylenediaminetriacetic acid], and a fungal siderophore rhizoferrin from solid-phase Cd phosphate at pH 7.3 with and without Fe competition in the presence of Ca and Mg as additional competing metals. While rhizoferrin did not extract Cd, PS and HEDTA did extract Cd even in the presence of Fe. Yet, uptake of Cd by wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) plants was not significantly influenced by Fe stress, but instead was controlled primarily by Cd2+ activity in solution. These results suggest that even though Cd may be mobilized by PS, there is no significant uptake of the Cd-PS complex by the plant roots.
镉持续不断地人为释放到环境中，导致土壤中镉的不断积累。植物组织和谷物中镉的吸收与积累可能会导致食物链向人类的转移。有人建议施用合成螯合剂来增加金属的活化度，并促进植物提取，以此作为修复金属污染土壤的一种手段。然而，大部分经螯合剂提取的金属可能会被淋溶，而不是被转运到植物根部。与添加到土壤中的合成螯合剂不同，植物产生的螯合剂即植物铁载体（PS）会直接分泌到根际。此前的研究表明，PS能促进禾本科植物对锌和铁的吸收。在本研究中，我们假设了植物铁载体介导镉吸收的两步过程：（i）在土壤溶液中的提取和螯合，以及（ii）将螯合态镉输送到植物的吸收系统。我们研究了在pH 7.3的条件下，在有和没有铁竞争以及存在钙和镁作为额外竞争金属的情况下，植物铁载体、合成螯合剂HEDTA [N-(2-羟乙基)-乙二胺三乙酸] 和真菌铁载体根际铁素从固相磷酸镉中提取镉的情况。虽然根际铁素没有提取出镉，但即使在有铁存在的情况下，植物铁载体和HEDTA也能提取出镉。然而，小麦（Triticum aestivum L.）和大麦（Hordeum vulgare L.）植株对镉的吸收并没有受到铁胁迫的显著影响，而是主要受溶液中Cd2+活性的控制。这些结果表明，尽管镉可能会被植物铁载体活化，但植物根系对镉-植物铁载体复合物并没有显著吸收。
