A Role for manganese in the Regulation of Soybean Nitrate Reductase Activity?

Abstract

The effect of manganese nutrient levels and light treatments on nitrate reductase activity (NRA) of soybean leaves was studied. Under dark conditions, soybean Mn-deficient leaves showed higher NRA than normal leaves, using the in vivo and in vitro assays. Under light, the differences in activities depended on the incubation media in the in vivo assay, whereas under the same conditions in the in vitro assay the differences observed between (- Mn)- and ( + Mn)-leaves were lower. In leaf homogenates, MV- and FMNH(2)-NR activities were also reduced in ( + Mn)-leaves in comparison to Mn-limiting leaves but DCPIP-NADH reductase was not affected. Apparently, differences in NRA could be localized on the final step of nitrate reduction, though reactivation by the addition of ferricyanide could not be obtained. Partial purification of NADH-NR-inhibitor from both, (- Mn)- and ( + Mn)-leaves, and activity assays showed lower inhibition rates of NRA by those fractions obtained from the Mn-deficient leaves. On the basis of the results obtained, an indirect role of manganese on the regulation of NRA is proposed.