ABSTRACT

I. THE Rhizobium-LEGUME ASSOCIATION A. Dynamics of the Symbiosis Soil exists as a discontinuous matrix of organic ap~ Inorganic constituents that allow billions of organisms to coexist in soil. Rhizobia are one group of soil organisms with a unique feature: the ability to form a symbiotic association with members of the plant family, Fabaceae. The rhizobia are ubiquitous aerobic, heterotrophic, gram-negative, diazotrophic bacteria of the genus Rhizobium. The Fabaceae is one of the largest families of the angiosperms and is currently estimated to consist of approximately 750 genera and 19,000 species. Rhizobia have the ability to infect legume root hairs and cause the production of root nodules, which are subsequently inhabited by the rhizobia. Within the root nodules, rhizobia reduce or fix atmospheric nitrogen into ammonia, which is utilized by the plant. Simultaneously, plants undergo photosynthesis and reduce or fix carbon dioxide into simple sugars, which are transported to the roots and utilized by the rhizobia. Thus, both symbionts are capable of supplying a nutrient, which usually limits its partner's metabolism. The carbon cost of biological nitrogen fixation (BNF) has been estimated at 6.5 g C g-1 N (Phillips, 1980). However, 14C pulse-labeling studies have ,shown that legumes can compensate for this carbon cost by increased photosynthesis (Paul and Kucy, 1981). Nitrogen (N) is typically the nutrient that limits plant growth, and hence, BNF represents an attractive alternative to high-energy-input production agriculture when low inputs of N fertilizer are required and a sustainable agriculture is necessary.