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Plasma-membrane-bound nitrate reductase (PM-NR) is located in roots and leaves of tobacco (Nicotiana tabacum L. cv. Samsun) and reduces nitrate with NADH as electron donor. When plasma membranes were prepared under specific protecting conditions, a PM-NR of roots was detected that accepts electrons from succinate to reduce nitrate. Comparison between the succinate dehydrogenase of mitochondria and the succinate-oxidising PM-NR of roots indicated that they are two different enzymes. Partial purification of the nitrate reductase forms by anion-exchange chromatography indicated that succinate and NADH supply electrons to the same plasma-membrane-bound protein.