Nitrate Inhibition of Root‐Nodule Symbiosis in Doubly Rooted Soybean Plants

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Split‐root experiments have demonstrated that NO−3 inhibits nodule formation on legumes primarily as a root‐localized effect rather than as a function of whole‐plant N nutrition. The degree to which the root‐localized effect is coupled with the N status of the shoot is not well understood. To investigate this, doubly rooted soybean [Glycine max (L.) Merr.] plants were produced by layering stems in adjacent pots to induce the formation of adventitious roots; thus, the effects of N added to the entire original root could be examined in terms of nodulation on both the original and the adventitious roots. When NO−3 level was sufficiently high to completely suppress nodulation on the original root, there was profuse nodulation and significant nitrogenase (C2H2 reduction) activity on the adventitious root. This indicated that even when mineral N supply to the root was abundant there was an N deficit in the shoot. The apparent poor coupling of N status of the root with that of the shoot was confirmed when doubly rooted plants accumulated more N than did singly rooted plants. Stem nodules of sesbania (Sesbania rostrata Brem. & Oberm.) are analogous to adventitious root nodules of doubly rooted soybean in that they form on root primordia not in contact with added N; the adverse effects of applied NO−3 on nodule development and function were more pronounced on sesbania stems than on soybean adventitious roots. Therefore, the coupling of root‐N status with shoot‐N status may be particularly loose in soybean with potentially important implications for growth in N‐rich soils.