Role of Nitrate and Nitrite in the Induction of Nitrite Reductase in Leaves of Barley Seedlings

摘要

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.