Isolation of highly effective, super nodulating and competitive Rhizobium strains of Egyptian clover (Trifolium alexandrinum L.)

Forty-eight root nodulating-clover Rhizobium strains isolated from 11 different Egyptian governorates were screened to select the high nitrogen fixing and well nodulated strains. Strains could be divided into three major groups based on variation of plant fresh weight between inoculated and non inoculated plants. The first group, high nitrogen fixing (47%), second moderate nitrogen fixing (33%) and third less nitrogen fixing (20%). The five effective strains for nodulating and nitrogen fixing were Rhiz 950, Rhiz 975, Rhiz 996, Rhiz 1017 and Rhiz1024. Inoculation with all the five representative strains resulted in duplicating the fresh weight of clover plants and NPK uptake than the control. These strains were mixed together with the same appropriate cell numbers (10^-8 cells ml^-1) and were tested to select the most competitive strains for nodulating the common and widely cultivated clover cv. Meskawy under neutral and alkaline pH using the REP and RAPD markers. Under normal conditions, results of REP profiles indicated that strains Rhiz 950 and Rhiz1017 were the most competitive Rhizobium strains with 35.71% nodule occupancy, while under alkaline stress and based on RAPD primers, the alkaline tolerant Rhiz 1017 strain occupied 60% of the nodules, while the other four examined strains had equal competitiveness (10%). These findings indicated that strain Rhiz 1017 can be used as effective and competitive clover rhizobial inoculants in Egyptian soils.