Manganese Oxidation Capacity of Homogenates of Cowpea [ Vigna unguiculata (L.) Walp.] Leaves Differing in Manganese Tolerance

Abstract

Summary Dark-brown speckles on old leaves are manganese (Mn) toxicity symptoms in cowpea [Vigna unguiculata (L.) Walp)]. These speckles are deposits of brown substances found primarily in cell walls containing sensitivity Mn in an oxidized form, and their appearance is indicative of high Mn sensitivity of plant issue. In the present paper the hypothesis is tested that differences in leaf-tissue Mn tolerance owing to genotype, silicon (Si) concentration and leaf age are attributed to differences in Mn oxidation capacity of the leaf tissue. A Mn-sensitive and a Mn-tolerant cowpea genotype were precultured in complcomplete nutrient solution without and with Si (1400 μM) at optimum Mn supply (0.25 μM) for 17 days under controlled environmental conditions. Within 3 days of treatment with 10 μM Mn, Mn toxicity symptoms only appeared on older leaves of the Mn-sensitive genotype without Si supply. Homogenerates prepared from old and young leaves were incubated at 30 °C for 60 min either in the light or in the dark. Such homogenates had an extremely high potential to oxidize added and internal leaf Mn(II) in the light but not in the dark. Manganese oxidation could be described by enzyme kinetics with an apparent Km of approximately 0.5 mM Mn. Manganese oxidation capacity was higher in Mn-sensitive older leaves compared with Mn-tolerant younger leaves. However, no differences could be found between leaf tissues differing in Mn tolerance owing to genotype and Si status, which is not in aggrement with the working hypothesis. Therefore the results do not unequivocally support the hypothesis that high Mn leaf-tissue sensitivity is realted to high Mn(II) oxidation.