Manganese oxidation in the pea plant (Pisum sativum L.) grown under conditions of manganese toxicity

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Abstract
Pea plants (Pisum sativum) grown in water culture containing toxic concentrations of manganese sulphate accumulate compounds containing Mn of valency greater than 2 in the stems. Presence of these high-valency forms was demonstrated colorimetrically by the benzidine test, and the amount estimated spectrophotometrically as manganipyrophosphate and manometrically by oxidation of hydrazine. It is suggested that oxidation is due to peroxidase systems. Under toxic conditions of Mn supply the rate of oxidation of Mn2+ ions may exceed the rate of reduction of "higher-valency forms of Mn" by plant metabolites, thus leading to the observed accumulation of Mn higher oxides. In other plants examined under similar conditions the accumulation of "higher-valency forms of Mn" could not be demonstrated; possibly they were reduced as fast as they are formed. It is suggested that the function of Mn as an essential element in plant nutrition depends, to some extent, on such valency change.