Effects of Excess Manganese on Production of Organic Acids in Mn-Tolerant and Mn-Sensitive Cultivars of Triticum aestivum L. (Wheat)

Abstract

Summary Tolerance to metals may be achieved via enhanced production of organic acids and subsequent formation of stable metal-organic acid complexes in the cytosol or vacuole. To determine the relationship between differential tolerance to Mn and organic acid content in Triticum aestivum L., a Mn-tolerant and a Mn-sensitive cultivar were grown in solution culture with 0, 250 or 500 μM Mn. Exposure to excess Mn reduced rates of growth and photosynthesis in the Mn-sensitive cultivar, while respiration declined after an initial stimulation. In the Mn-tolerant cultivar, growth was not affected by excess Mn, toxic effects on photosynthesis and respiration were less pronounced. Under these conditions, six carboxylate anions were detected in root and leaf tissues, five of which were present in sufficient amounts for quantification. In roots, concentrations of aconitate, α-ketoglutarate, citrate, malate, succinate did not differ between Mn-tolerant and Mn-sensitive cultivars. In leaves, growth in excess Mn was accompanied by increases in the concentrations of aconitate, α-ketoglutarate, succinate in the Mn-sensitive cultivar only. To examine the effects of Mn or organic acid synthesis, leaf slices from plants grown for 11 d in the presence or absence of Mn (500 μM) were incubated in [2- 14 C]sodium acetate. Growth in excess Mn increased concentrations of succinate in the leaf slices but decreased its specific radioactivity, indicating that its synthesis was not solely via acetyl CoA feeding into the citric-acid cycle. The specific radioactivity of malate also declined with Mn stress. Observed increases in organic acid content in the Mn-sensitive cultivar, the lack of similar increases in the Mn-tolerant cultivar, the general decline in specific radioactivity of labeled organic acids indicates that enhanced production of organic acids is not a primary mechanism of Mn-tolerance. Such effects more likely reflect responses to Mn toxicity.