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Nitrate Utilization and Dinitrogen Fixation (Acetylene Reduction) by Nitrate Reductase-Defecient Mutants of Pea¹

Three NO^–₃ reductase pea (Pisum sativum L.) mutants and the ‘Juneau’ control were evaluated for the ability to absorb and accumulate NO^–₃ from solution, and for growth and N₂ fixation (C₂H₂ reduction) in the presence and absence of NO^–₃. In short-term (20 h) hydroponic studies, the mutants absorbed NO^–₃ at rates similar to Juneau, but accumulated more NO^–₃ than Juneau in both shoots and roots. Seedlings of Juneau assimilated 63%, while seedlings of mutants assimilated <35% of absorbed NO^–₃. In long-term studies with plants grown in vermiculite in the presence of 15 mM NO^–₃, dry weights of mutants A300 and A317 were <50% of Juneau. However, growth of mutant A334, which had only 6 to 8% of the NO^–₃ reductase activity of Juneau, was equal to the control. Growth of mutant A300 was not as rapid as the other genotypes when N₂ fixation or NH⁺₄ was the only N source. All three mutants and Juneau exhibited good levels of C₂H₂ reduction activity and growth in the absence of mineral N. Also, the NO^–₃ reductase mutations did not lessen the inhibition of N₂ fixation by NO^–₃ and the cause of the inhibition remains unclear.

Key Words: Nitrate uptake • Nitrate reduction • Nitrate assimilation • Nitrogenase • Nodules • Nodulation • N₂ fixation • Pisum sativum L.

² Associate professor, Dep. of Agriculture, Western Illinois Univ., Macomb, IL 61455; and professor, Dep. of Agronomy & Soils, Washington State Univ., Pullman, WA 99164.

Received for publication June 5, 1986.