Molybdate Toxicity in Chinese Cabbage is not the direct Consequence of Changes in Sulfur Metabolism

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 sulfate. Therefore, it is presumed that elevated molybdate concentrations may hamper the uptake and subsequent metabolism of sulfate, which may induce sulfur deficiency. In the current research, Chinese cabbage (Brassica pekinensis) seedlings were exposed to 50, 100, 150 and 200 μM Na2MoO4 for nine days. Leaf chlorosis and a decreased plant growth occurred at concentrations ≥ 100 μM. Root growth was more affected than shoot growth. At ≥ 100 μM Na2MoO4, the sulfate uptake rate and capacity were increased, though 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 sulfate uptake rate and capacity remained unaffected. Molybdate concentrations ≥ 100 μM altered the mineral nutrient composition of plant tissues, although the levels of sulfur metabolites (sulfate, water‐soluble non‐protein thiols and total sulfur) were not altered. Moreover, the levels of nitrogen metabolites (nitrate, amino acids, proteins and total nitrogen), which are generally strongly affected by sulfate deprivation, were not affected. The root water‐soluble non‐protein thiol content was increased and the tissue nitrate levels decreased, only at 200 μM Na2MoO4. Evidently, molybdenum toxicity in Chinese cabbage was not due to the direct interference of molybdate with the uptake and subsequent metabolism of sulfate.