In polluted areas, plants may be exposed to supra-optimal levels of the micronutrient molybdenum. The physiological basis of molybdenum phytotoxicity is poorly understood. Plants take up molybdenum as molybdate, which is a structural analogue of sulphate. Therefore, it is presumed that elevated molybdate concentrations may hamper the uptake and subsequent metabolism of sulphate, which may induce sulphur deficiency. In the current research, Chinese cabbage (Brassica pekinensis) seedlings were exposed to 50, 100, 150 and 200 μm Na2 MoO4 for 9 days. Leaf chlorosis and a decreased plant growth occurred at concentrations ≥100 μm. Root growth was more affected than shoot growth. At ≥100 μm Na2 MoO4 , the sulphate uptake rate and capacity were increased, although only when expressed on a root fresh weight basis. When expressed on a whole plant fresh weight basis, which corrects for the impact of molybdate on the shoot-to-root ratio, the sulphate uptake rate and capacity remained unaffected. Molybdate concentrations ≥100 μm altered the mineral nutrient composition of plant tissues, although the levels of sulphur metabolites (sulphate, water-soluble non-protein thiols and total sulphur) were not altered. Moreover, the levels of nitrogen metabolites (nitrate, amino acids, proteins and total nitrogen), which are generally strongly affected by sulphate deprivation, were not affected. The root water-soluble non-protein thiol content was increased, and the tissue nitrate levels decreased, only at 200 μm Na2 MoO4 . Evidently, molybdenum toxicity in Chinese cabbage was not due to the direct interference of molybdate with the uptake and subsequent metabolism of sulphate.

中文翻译：

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大白菜中的钼酸盐毒性不是硫代谢变化的直接结果。

在受污染的地区，植物可能会暴露在微量营养素钼的超最佳水平。钼植物毒性的生理基础了解甚少。植物吸收钼作为钼酸盐，这是硫酸盐的结构类似物。因此，据推测，钼酸盐浓度升高可能会阻碍硫酸盐的吸收和随后的代谢，从而可能导致硫缺乏。在当前的研究中，将大白菜（Brassica pekinensis）幼苗暴露于50、100、150和200μmNa2 MoO4中9天。在浓度≥100μm时发生叶片萎黄和植物生长减慢。根生长比苗生长受到更大的影响。在Na2 MoO4≥100μm时，虽然仅以根鲜重为基础表达，但硫酸盐的吸收速率和容量却增加了。当以整株鲜重为基础表示时（可校正钼酸盐对枝根比的影响），硫酸盐的吸收速率和容量保持不变。钼酸盐浓度≥100μm改变了植物组织的矿物质营养成分，尽管硫代谢物（硫酸盐，水溶性非蛋白硫醇和总硫）的水平没有改变。此外，通常受硫酸盐剥夺强烈影响的氮代谢物（硝酸盐，氨基酸，蛋白质和总氮）的水平没有受到影响。仅在200μmNa2 MoO4下，根系水溶性非蛋白质硫醇含量增加，组织硝酸盐含量降低。显然，