New Developments in Nitrogen Fixation Research

Abstract

becoming available for plant growth and conversion to protein. Man has placed extreme demands upon the supply of available soil nitrogen for the production of agricultural crops. To satisfy this demand, processes for the production of synthetic nitrogenous fertilizers have been developed. But the increasing cost of the fossil fuels needed to produce these supplements is placing an even greater strain on the economy of food production. Grain and forage legumes present a partial solution to these problems. They have provided protein for human and animal nutrition since 7000 B.C. (Zohary and Hopf 1973). Among all agricultural crop plants, they possess the unique ability to reduce atmospheric nitrogen, thus placing a lesser demand upon nitrogen reserves in the soil. For more than 80 years it has been known that soil bacteria of the genus Rhizobium could infect the roots of legume plants, forming nodular structures. Within these root nodules, modified Rhizobium cells-called bacteroids (see Fig. 1)-use nutrients provided by the plant and, in return, fix atmospheric nitrogen into a form that can be assimilated by the plant. So efficient is this association that the bacterium is capable of supplying the plant with all of its nitrogen requirements. Thus, it provides the plant with a unique ecological ad-