Parker D R, Pedler J F, Ahnstrom Z A, Resketo M
Department of Environmental Sciences, University of California, Riverside 92521, USA.
Environ Toxicol Chem. 2001 Apr;20(4):899-906. doi: 10.1897/1551-5028(2001)020<0899:rtfiam>2.0.co;2.
Across a diverse spectrum of organisms, the absorption and toxicity of trace elements are usually correlated with the activity of the free metal ion, but reported exceptions to this generalization are increasing. For the first time, we tested the validity of the free-ion activity model (FIAM) in the case of terrestrial plants and organic acids that may be abundant in the soil solution and rhizosphere. Short-term (48-h) root elongation of wheat (Triticum aestitvum L.) in a simple medium (2 mM CaCl2, pH 6.0) was used to probe the toxicity of Cu and Zn in the presence of malonate, malate, and citrate. Precautions were taken to prevent biodegradation of the organic acids, and its absence was confirmed by ion chromatography. Copper speciation was verified using a Cu-selective ion electrode, and published stability constants were modified to improve agreement between measured and calculated Cu2+ activities. With additions of both malonate and malate, Cu toxicity was alleviated but not to the extent predicted by the FIAM; the Cu-ligand complexes seemingly contributed to the toxicity. No such departures were observed with citrate and Cu nor with any of the three ligands in combination with Zn. Thus, exceptions to the FIAM occur with higher plants as well as with aquatic biota but do not seem to occur in a predictable or systematic fashion with respect to metal or organic acid under investigation. Several possible explanations for the observed departures from the FIAM are discussed, including the possibility of accidental cotransport of metal and ligand into the cytoplasm.
在各种各样的生物体中，微量元素的吸收和毒性通常与游离金属离子的活性相关，但越来越多的报道表明这一普遍规律存在例外。我们首次在陆生植物以及土壤溶液和根际中可能大量存在的有机酸的情况下，测试了自由离子活性模型（FIAM）的有效性。在简单培养基（2 mM CaCl2，pH 6.0）中，利用小麦（Triticum aestitvum L.）的短期（48小时）根伸长来探究在丙二酸、苹果酸和柠檬酸存在下铜和锌的毒性。采取了预防措施以防止有机酸的生物降解，离子色谱法证实了其不存在。使用铜选择性离子电极验证了铜的形态，并修改了已发表的稳定常数，以提高测量和计算的Cu2+活性之间的一致性。添加丙二酸和苹果酸后，铜的毒性有所减轻，但未达到FIAM预测的程度；铜-配体络合物似乎导致了毒性。在柠檬酸与铜的组合以及三种配体与锌的任何组合中均未观察到这种偏差。因此，FIAM的例外情况在高等植物以及水生生物群中都有发生，但对于所研究的金属或有机酸而言，似乎并非以可预测或系统的方式出现。讨论了观察到的与FIAM偏差的几种可能解释，包括金属和配体意外共转运到细胞质中的可能性。
