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In Vivo Assay of Nitrate Reductase in Cotton Leaf Discs

Effect of Oxygen and Ammonium ¹

^a Western Cotton Research Laboratory, United States Department of Agriculture, 4135 East Broadway, Phoenix, Arizona 85040

Factors affecting nitrate reduction by leaf discs of cotton (Gossypium hirsutum L.) were investigated. When incubated in 30 mM nitrate, discs reduced nitrate much more slowly under air or O₂ than under N₂. Inhibition by O₂ did not occur at nitrate levels of 100 mM or greater. Treatment with arsenate had little effect under N₂ but stimulated nitrate reduction under air. Similarly, ammonium inhibited nitrate reduction, with the inhibition being partially relieved by arsenate. Uptake of nitrate was unaffected by ammonium. The NAD/NADH ratio increased in response to both oxygen and ammonium. The effects of these treatments on nitrate reduction can be explained by competition with nitrate for NADH generated by glycolysis.