Can genotypes of soybean (Glycine max) selected for nitrate tolerance provide good “models” for studying the mechanism of nitrate inhibition of nitrogenase activity?

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In soybeans (Glycine max L. Merr.), high levels of soil nitrate inhibit N2 fixation, and nitrate‐tolerant symbioses have been identified within a chemically mutagenized line of cv. Bragg denoted nts382 and within the line K466, a genotype representative of a number of Korean soybean cultivars. The genotypes nts382 and K466 were examined to see if they could be used as a model system for studying the mechanism responsible for the short‐term (i.e. 3‐day) inhibition of specific nitrogenase activity, especially the mechanism behind the greater O2 limitation of nodule metabolism that is characteristic of nitrate inhibition of N2 fixation in soybean. In nts382, total nitrogenase activity (TNA = H2 production in Ar:O2) was inhibited to a lesser degree (48% of control) relative to Bragg (30% of control), and the nitrate‐treated symbioses showed less of an O2 limitation of nodule metabolism in nts382 than in Bragg. However, the relative proportion of O2 limitation to the total nitrate inhibition was similar (40 and 41%) in nts382 and Bragg, respectively. Therefore, the nts382 symbioses may be useful in elucidating the general mechanism for down‐regulation of nitrogenase activity in soybean, but would not be a useful model system for studying the control of O2‐limited metabolism following nitrate exposure. The effects of nitrate on TNA and on the degree of O2 limitation of nodule metabolism were the same in K466 and a reference cultivar Maple Arrow. Consequently, the tolerance of K466 to nitrate reported previously was attributed to the ability of this symbiosis to maintain nodule biomass in the presence of nitrate, not to any ability to maintain specific nitrogenase activity in the presence of nitrate.