Role of Nitrate and Nitrite in the Induction of Nitrite Reductase in Leaves of Barley Seedlings |
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| Authors: | Muhammad Aslam and Ray C Huffaker |
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| Affiliation: | Plant Growth Laboratory and Department of Agronomy and Range Science, University of California, Davis, California 95616 |
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| Abstract: | The role of NO3− and NO2− in the induction of nitrite reductase (NiR) activity in detached leaves of 8-day-old barley (Hordeum vulgare L.) seedlings was investigated. Barley leaves contained 6 to 8 micromoles NO2−/gram fresh weight × hour of endogenous NiR activity when grown in N-free solutions. Supply of both NO2− and NO3− induced the enzyme activity above the endogenous levels (5 and 10 times, respectively at 10 millimolar NO2− and NO3− over a 24 hour period). In NO3−-supplied leaves, NiR induction occurred at an ambient NO3− concentration of as low as 0.05 millimolar; however, no NiR induction was found in leaves supplied with NO2− until the ambient NO2− concentration was 0.5 millimolar. Nitrate accumulated in NO2−-fed leaves. The amount of NO3− accumulating in NO2−-fed leaves induced similar levels of NiR as did equivalent amounts of NO3− accumulating in NO3−-fed leaves. Induction of NiR in NO2−-fed leaves was not seen until NO3− was detectable (30 nanomoles/gram fresh weight) in the leaves. The internal concentrations of NO3−, irrespective of N source, were highly correlated with the levels of NiR induced. When the reduction of NO3− to NO2− was inhibited by WO42−, the induction of NiR was inhibited only partially. The results indicate that in barley leaves NiR is induced by NO3− directly, i.e. without being reduced to NO2−, and that absorbed NO2− induces the enzyme activity indirectly after being oxidized to NO3− within the leaf. |
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