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

Boyce Thompson Inst. for Plant Res., Cornell Univ., Ithaca, NY 14853

^* Corresponding author.

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.

Received for publication March 18, 1988.