Stem-Nodulating Legume-Rhizobium Symbiosis and Its Agronomic Use in Lowland Rice

Abstract

In recent years there has been a great surge of interest and activity in the field of biological nitrogen fixation (BNF). The legume-Rhizobium symbiosis is estimated to account for 40% of the world’s fixed nitrogen. Because the ultimate goal of BNF research is to increase crop yields on farmers’ fields, researchers’ attention must focus mainly on the understanding, improvement, and agronomic use of legume-Rhizobium symbiosis. Rightly so, since the increased cost of energy and the growing demand for N fertilizers are expected to cause dramatic increases in the price of this commodity in the years ahead. Rice (Oryza sativa L.) is the world’s most important food crop as more than 40% of the world’s population depend on rice as a major source of calories. Most of the rice in tropical developing countries is produced in lowland areas and by subsistence farmers whose resource base is so small and whose physical access to supply markets is so limited that they cannot take full advantage of technologies that are overdependent on purchased inputs. The BNF in legumes, used appropriately, reduces production constraints in low-input agricultural systems in the tropics. However, most traditionally used green manure (GM) species cannot tolerate prolonged periods of flooding that occur in most lowland rice-farming systems. A new and promising alternative may be the use of nitrogen-fixing, stem-nodulating legumes.