Selenium (Se), a trace element essential for human and animal biological processes, is deficient in many agricultural soils. Some extremely rare plants can naturally accumulate extraordinarily high concentrations of Se. The native legume Neptunia amplexicaulis, endemic to a small area near Richmond and Hughenden in Central Queensland, Australia, is one of the strongest Se hyperaccumulators known on Earth, with foliar concentrations in excess of 4000 μg Se g-1 previously recorded. Here, we report on the Se distribution at a whole plant level using laboratory micro X-ray Fluorescence Microscopy (μXRF) and scanning electron microscopy (SEM-EDS), as well as on chemical forms of Se in various tissues using liquid chromatography-mass spectrometry (LC-MS) and synchrotron X-ray absorption spectroscopy (XAS). The results show that Se occurs in the forms of methyl-selenocysteine and seleno-methionine in the foliar tissues, with up to 13 600 μg Se g-1 total in young leaves. Selenium was found to accumulate primarily in the young leaves, flowers, pods and taproot, with lower concentrations present in the fine-roots and stem and the lowest present in the oldest leaves. Trichomes were not found to accumulate Se. We postulate that Se is (re)distributed in this plant via the phloem from older leaves to newer leaves, using the taproot as the main storage organ. High concentrations of Se in the nodes (pulvini) indicate this structure may play an important a role in Se (re)distribution. The overall pattern of Se distribution was similar in a non-Se tolerant closely related species (Neptunia gracilis), although the prevailing Se concentrations were substantially lower than in N. amplexicaulis.

中文翻译：

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来自澳大利亚昆士兰州中部的菊苣中硒的分布和化学形式。

硒（Se）是人类和动物生物学过程必不可少的微量元素，在许多农业土壤中都缺乏。一些极其稀有的植物会自然地积累极高浓度的硒。天然豆科植物龙脑Neptunia amplexicaulis流行于澳大利亚昆士兰州中部里士满和Hughenden附近的一小区域，是地球上已知的最强的Se超级蓄积物之一，其叶面浓度先前记录为超过4000μgSe g-1。在这里，我们使用实验室显微X射线荧光显微镜（μXRF）和扫描电子显微镜（SEM-EDS）报告了整个植物水平上硒的分布情况，并使用液相色谱-质谱法报告了各种组织中硒的化学形式光谱仪（LC-MS）和同步加速器X射线吸收光谱仪（XAS）。结果表明，Se以甲基硒代半胱氨酸和硒代蛋氨酸的形式存在于叶组织中，年轻叶片中的Se g-1总量高达13 600μg。硒主要在幼叶，花朵，豆荚和主根中富集，细根和茎中的含量较低，而最老叶中的含量最低。未发现毛cho积累硒。我们假设硒是通过韧皮部从老叶到新叶的（重新）分布在植物中，并以主根为主要贮藏器官。节点（pulvini）中的硒含量较高，表明该结构可能在硒（再）分布中起重要作用。在非耐硒的密切相关物种（Neptunia gracilis）中，硒分布的总体模式相似，