Nitrate Inhibition of Nodulation and Nitrogen Fixation by Supernodulating Nitrate-Tolerant Symbiosis Mutants of Soybean
Eskew, David L. | Kapuya, Juma | Danso, Seth K. A.
Several soybean [Glycine max (L.) Merr.] mutants that form a greater number of nodules (supernodulation) and display a nitrate tolerant symbiosis (nts) phenotype when grown in sand with continuous additions of NO⁻₃ have recently been isolated. Greenhouse experiments were conducted to compare three of the nts mutants (nts 382, nts 1007, and nts 1116) to the parent cultivar Bragg, when grown in soil (3 kg) with a single addition of either 7.5, 150, 300, or 450 mg NO⁻₃−N (KNO₃) at planting. Estimated % N derived from the atmosphere (% Ndfa, ¹⁵N dilution) was significantly higher (P = 0.05) for nts 382 than for Bragg at the 7.5 mg NO⁻₃−N level at the V5 and R3 growth stages, but 150 mg NO⁻₃−N greatly reduced nodulation and % Ndfa in both of these genotypes. All four genotypes were compared at R7. Percent Ndfa was significantly higher for the nts mutants (72% average) than for Bragg (65%) at the 7.5 mg NO⁻₃−N level. Nodule number was reduced (53% average) for the nts mutants at the1 150 mg NO⁻₃−N level, but there was no reduction for Bragg. At the 300 mg NO⁻₃−N level, nts 1007 showed a greater tolerance, 49 % Ndfa, than nts 382 and nts 1116, 31 and 28 % Ndfa, respectively (Bragg not determined). At the 450 mg NO⁻₃−N level, nodulation was almost completely inhibited in all genotypes. Nitrate reductase activity was found in the shoots and roots of all genotypes. The results clearly indicate that the nts mutants were still sensitive to NO⁻₃ and suggest that these are not mutations in the NO⁻₃ response mechanism per se. Nitrogen fixation by nts 382 was greater than by Bragg early in growth, but only a slight advantage was maintained at physiological maturity. Contribution of the Joint FAO-IAEA Division.
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