RHIZOBIUM – HOST LEGUME INTERACTIONS TO TARGET FOR HIGH NITROGEN FIXING COMBINATIONS

Abstract

Nitrogen (N) is one of the most important macronutrients that are required for plant survival, growth and agricultural productivity, but it is a significant plant yield-limiting factor. Interestingly, leguminous plants cooperate with soil-resident, nitrogen fixing bacteria called rhizobia and use the fixed nitrogen. The amount of fixed nitrogen is influenced by the host plant genotype, rhizobium species, soil environment, agronomic management and their interactions. Permitting symbiosis with only effective rhizobia strains is a crucial strategy for improving crop yields, reducing the need for synthetic fertilizers, and promoting soil health. Therefore, this thesis targets at evaluating rhizobial strains that exhibit high symbiotic effectiveness with a wide host range of leguminous crops. Fifteen strains were tested on five legume hosts; common bean, cowpea, groundnut, mung bean and soybean. The inoculated seedlings were grown in Modified Leonard Jars on sterile sand supplemented with N-free nutrient solution to screen broad spectrum or host-specific strains. Five test strains CWP01, M24, SB19, CB74 and SB24 were able to nodulate all the host legumes; hence, considered as broad-spectrum strains. Among which, SB19 was the best relatively on most hosts. On the other hand, common bean was the most promiscuous host nodulated by all the test strains while soybean was nodulated by few numbers of the strains. Symbiotic effectiveness (SE) of strains ranged from 39.17 to 108.42% with a significant variation (p< 0.05) in inducing nodulation and shoot biomass on both sympatric and allopatric hosts. Three strains performed better than N-fertilized control. Strain SB19 isolated from soybean root nodule had significantly (p< 0.05) higher Symbiotic effectiveness (108.42%) in inducing shoot dry weight than uninoculated control plants. Regarding nitrogen fixation, the highest amount was estimated from mung bean while the lowest was from common bean. The rhizobia strains CWP01, SB19 and M24 had wide host rang and high Symbiotic effectiveness than the Nfertilized controls, demanding further evaluation under field conditions and molecular characterization.