Possibilities for the enhancement of biological nitrogen fixation

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Abstract
Modern developments in plant science and knowledge of nitrogen fixation have opened exciting medium and long term possibilities: (1) Extended exploitation of existing systems such as exotic legumes; non-leguminous shrubs; the newly discussed grass associations; wider use of ‘green manures’. (2) Augmentation of the effectiveness of existing systems by altering the complement of nitrogen fixation ( nif ) genes; altering the genetic control systems to avoid repression by fixed nitrogen; avoiding wastage of nitrogen fixing capacity in hydrogen evolution; altering the efficiency of uptake of essential metals such as Mo and Fe; altering the character of the plant partner in symbioses. (3) Constructing new nitrogen fixing systems by preparing somatic hybrids of desirable crop plants with natural nitrogen fixing plants; introducing nif into new commensal or symbiotic microbes. (4) Transfer of nitrogen fixing ability to plants by ( a ) constructing DNA viruses or transferable plasmids carrying nif genes as possible vectors; ( b ) associating nif genes with plant mitochondrial or chloroplast DNA; ( c ) seeking uptake of nitrogen fixing microbes as potential organelles. (5) Introducing nif genes into rumen microbes; use of nitrogen fixing microbes as animal fodder. All these projects require extensive basic research; the fact that evolution has not so far provided such systems may mean that unforeseen obstacles to their construction may now exist.