Screening for symbiotically effective and ecologically competitive chickpea rhizobial inoculants from Ethiopian soils

Abstract

Chickpea is one the most important pulse crops grown for food and crop rotation in order to improve soil fertility because it fixes atmospheric nitrogen with root nodule bacteria. However, effectiveness in nitrogen fixation depends upon the host variety, the efficient endosymbiont and environmental conditions. This study was initiated to isolate and characterize chickpea rhizobia for their symbiotic effectiveness adapted to local environmental conditions. A total of seventy root nodule bacteria were isolated from different sampling sites in central and northern Ethiopia of which only 52% were rhizobia and the remaining were endophytes and non-nodulating rhizobia. All but two of the isolates were fast growers with generation time (<3 h) with large mucoid and large watery colony texture, and colony size (1-5 mm) and were able to change BTB-YEMA medium to yellow indicating that they were fast growing Metarhizobium ciceri. The isolates utilized different carbohydrates, except dulcitol, aesculine and citrate, and many amino acids, but none of the isolates assimilated aspargine and arginine, and a few isolates were able to utilize glycine and thymine. The isolates showed a pattern of tolerance to high pH (>8.0), high salt (4-6%) and medium temperature (20-30°C), and were sensitive to low pH (pH 4-4.5) and high temperature (40-45°C). Most isolates were resistant to several antibiotics and heavy metals, but relatively sensitive to chloramphenicol, tetracycline (51%). Of the tested antibiotics and heavy metals, streptomycin sulfate and mercury were found to be the most potent against the isolates. The symbiotic effectiveness test showed that only 20% of the isolates were able to increase the shoot dry mass as much as 50-100%. Of the nitrogen-fertilized control plants, two isolates (isolates NSCPR13 and NSCPR14) were highly effective (80%-100%). Based on their symbiotic effectiveness and wide physiological diversity and tolerance, the two best isolates; NSCPR13 and NSCPR24 could be tested in the field as future candidates for commercial inoculation.

Key words/phrases: Antibiotic resistance, Eco-physiological diversity, Heavy metal resistance.