Where to now?

This paper is concerned with looking into the future and trying to discern the shape of the directions nitrogen fixation research will take. Accordingly, much of it may be proved incorrect or impracticable; this is the danger for anyone who makes forecasts. It seems clear that, although rapid progress is being made in our theoretical understanding of the nitrogen fixation process, little of that progress has yet been applied in a practical sense to improve crop production. Our future directions need to encompass this phase of application. One of the dilemmas is to decide how to use our techniques: to forge new nitrogen-fixing systems or associations, or to improve existing ones, or to pursue some combinations of the two. In the legume systems, there is still much slack in technology to be taken up across the world. Simple problems in production, such as widespread boron deficiency in Thailand, remain to be corrected. Some questions to be considered include the following: (i) The ability to manipulate expression of sym and nif genes exists; what are we going to do with it? (ii) Acid tolerance in legume bacteria remains a major problem. What conditions such tolerance, and how can it be recognized and exploited? (iii) Nitrogen fixation in legume nodules depends on dicarboxylate supplies from the plant, apparently because the legume controls what the nodule bacteroids receive. Would a greater supply of dicarboxylates improve nitrogen fixation? Would making other classes of substrates available to bacteroids in larger amounts have beneficial effects? (iv) ‘Alternative’ nitrogenases are now known; can they be used beneficially in existing or new systems?