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The effect of different concentrations (80-640 µM) of sodium selenate and sodium selenite on lead absorption by Allium sativum (selenium absorber) and Pisum sativum (selenium nonaccumulator) roots treated with 10 µM Pb(NO3)2 was investigated. Lead alone and with either of the selenium compounds was supplied in aqueous solutions for 24 h. Selenite diminished lead content in the two plant species’ roots more effectively than selenate. The rhodizonate method showed the absence of lead in the presence of only 160 µM sodium selenite and only 640 µM sodium selenate. The effects of selenium compounds at 80 µM concentration on lead localization in meristematic cells were also investigated. While selenite in garlic cells reduced the number of electron-dense deposits in the cell wall and diminished their size in vacuoles, it increased their number in cytoplasm and plastids, and enlarged them in mitochondria. In pea cells it caused the disappearance of electron-dense sediments from the Golgi apparatus, endoplasmic reticulum, vacuoles and nucleus, or reduced their number in mitochondria, cytoplasm and plastids (lack of large deposits). On the basis of literature data we assume that selenium reduces the lead concentration in Allium sativum and Pisum sativum roots due to the formation of Pb-Se complex in the incubation medium.
Pisum sativum plants were treated for 3 days with an aqueous solution of 100 μM Pb(NO₃)₂ or with a mixture of lead nitrate and ethylenediaminetetraacetic acid (EDTA) or [S,S]-ethylenediaminedisuccinic acid (EDDS) at equimolar concentrations. Lead decline from the incubation media and its accumulation and localization at the morphological and ultrastructural levels as well as plant growth parameters (root growth, root and shoot dry weight) were estimated after 1 and 3 days of treatment. The tested chelators, especially EDTA, significantly diminished Pb uptake by plants as compared to the lead nitrate-treated material. Simultaneously, EDTA significantly enhanced Pb translocation from roots to shoots. In the presence of both chelates, plant growth parameters remained considerably higher than in the case of uncomplexed Pb. Considerable differences between the tested chelators were visible in Pb localization both at the morphological and ultrastructural level. In Pb+EDTA-treated roots, lead was mainly located in the apical parts, while in Pb+EDDS-exposed material Pb was evenly distributed along the whole root length. Transmission electron microscopy and EDS analysis revealed that in meristematic cells of the roots incubated in Pb+EDTA, large electron-dense lead deposits were located in vacuoles and small granules were rarely noticed in cell walls or cytoplasm, while after Pb+EDDS treatment metal deposits were restricted to the border between plasmalemma and cell wall. Such results imply different ways of transport of those complexed Pb forms.
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