Efficacy of rhizobacteria for growth promotion in sorghum under greenhouse conditions and selected modes of action studies

Abstract

The screening of rhizobacteria for growth promotion of sorghum (Sorghum bicolour (L.) Moench) was conducted under greenhouse conditions for a total of 78 bacteria isolated from the rhizosphere of sorghum in Ethiopia, and 86 isolates from the rhizosphere and rhizoplane of grasses in South Africa. Three isolates from Ethiopia, all identified as Bacillus cereus, enhanced growth promotion by resulting in statistically significant increases in at least five parameters. Of these, B. cereus (KBE7-8) resulted in significant increase in shoot and root biomass. Among effective isolates from South Africa, B. cereus (NAS4-3) and Stenotrophomonas maltophilia (KBS9-B) showed significant increases in all the parameters measured. The isolates which resulted in significant growth promotion colonized the roots effectively with a count up to 108 cfu/g. In the study conducted to elucidate the possible modes of action by these effective isolates, indole 3-acetic acid-like substances were detected in culture filtrates of the isolates ranging from 4·2 μg/ml by Serratia marcescens (KBS9-R) to 22·8 μg/ml by B. cereus (KBS5-H) in the presence of 2 mg tryptophan/g nutrient broth solution. Higher rates of solubilization of tricalcium phosphate on Pikovskaya agar medium were shown by Chryseomonas luteola (KBS5-F), S. marcescens (KBS6-H) and B. cereus (KBE9-1). There is very limited knowledge of the use of rhizobacteria in agriculture in Ethiopia and South Africa. The current study therefore generates valuable information towards application of plant growth promoting rhizobacteria as alternatives to chemical fertilizers.