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

Abstract

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.