The Role of Malate, Oxalate, and Mustard Oil Glucosides in the Evolution of Zinc‐Resistance in Herbage Plants

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Abstract
Malate, oxalate and mustard oils were analysed in zinc‐resistant and zinc‐sensitive ecotypes of Silene cucubalus Wib., Rumex acetosa L., Thlaspi alpestre L. and Agrostis tenuis Sibth. The effect of zinc on the activities of carbonic anhydrase and peroxidase in Siiene cucubalus was tested. Carbonic anhydrase of the zinc‐resistant ecotypes was stimulated by addition in vivo of high amounts of zinc. The high activity of peroxidase in the non‐zinc‐resistant ecotypes after the addition of zinc documented a poisoning of the sensitive plants by zinc. The amount of oxalate differed greatly between ecotypes. There was, however, no direct correlation between zinc‐resistance and oxalate. When applying zinc to the nutrient medium, the synthesis of oxalate was inhibited in zinc‐sensitive, but stimulated in zinc‐resistant ecotypes of Silene cucubalus and Rumex acetosa. In Thlaspi alpestre high concentrations of mustard oil glucosides were found. Zinc‐resistant plants produced twice as much glucosides as sensitive ones. A possible role of mustard oils in zinc‐resistance is discussed. In the content of malate there were great quantitative differences between zinc‐resistant and zinc‐sensitive plants. All zinc‐resistant ecotypes of all the species contained much higher concentrations in their green organs than the sensitive ones. It is assumed that malate is a major factor in the evolution of zinc‐resistance. Malate may act as a complexing agent for zinc within the plasma, whereas oxalate and mustard oils may function as “terminal acceptors” of large amounts of zinc. The role of malate may be extended by a special transport mechanism, by which zinc is eliminated from the plasma into the vacuole.

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