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Previous research has shown that a significant limitation to the agricultural use of improved rhizobial inoculant strains is competition from the indigenous soil population. In this work, we sought to test whether chemical inhibitors of flavonoid-induced nod gene expression in Bradyrhizobium japonicum could be identified and utilized to affect interstrain competition for nodulation of soybeans. Approximately 1,000 structural and functional analogs of the known, natural inducers of nod gene expression were tested on six strains of B. japoniurt* containing a nodY-lacZ fusion. We successfully identified effective inhibitors of nodY expression. The addition of the inhibitor 7-hydroxy-5-methylflavone significantly inhibited nodulation by a sensitive strain and could be used to effectively manipulate the competition between strains for soybean nodulation. However, this work also uncovered significant limitations for the practical use of this methodology. For example, despite the almost universal induction response to the identified natural inducers, there was a wide variability among strains in their response to any specific inhibitor. Given this unexpected variability, the cost of registration of an agronomic chemical, and the potential for the development of resistant field populations, it is unlikely that chemical inhibitors can be successfully applied to a field situation.

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The search for common structural features within the great variety of organic compounds acting as PS II inhibitors revealed the existence of a vinylogous relationship. By applying hetero-analogy and the principle of vinylogy, a simple model is proposed which unifies the structures of the vast array of PS II inhibitors. In the present work, its predictive value has been tested and confirmed by the preparation of a variety of N-(benzylideneamino) and N-(phenylazo) heterocycles. In these bridged vinylogous ureas, carbamates, and amides, the aromatic and heterocyclic rings are connected via diatomic linkages capable of transmitting substituent conjugative effects. N-(Benzylideneamino) and N-(phenylazo) heterocycles derived from N-aminoimidazolidin-2-one, 4-amino-1,2,4-triazol-3(2H)-one, and 1-aminohydantoin are among the most active members. In general, herbicidal activity is increased with lipophilicity imparted by a meta--substituent on phenyl. meta-CF 3 (and OCF 3 ) derivatives which showed tolerance on cotton produced severe bleaching, inhibiting carotenoid biosynthesis at the phytoene desaturation stage.Fundamental problems in a given area of research can often be solved by finding analogies with problems previously solved in other areas of research. This rarely yields the whole solution, but it can provide useful insights and intuitions.For example, the principles of vinylogy and hetero-analogy which are almost as old as organic synthesis itself are familiar to chemists in the solution of certain types of synthetic problems. Since R. C. Fuson's review article (1) in 1935, examples of vinylogous relationships have increased