Rhizobial exopolysaccharides as supplement for enhancing nodulation and growth attributes of Cajanus cajan under multi-stress conditions: A study from lab to field

The present study aimed to enhance nodulation and productivity of Cajanus cajan (pigeon pea) by using rhizobia, and its metabolite, in stressed soil of Northern India where conditions of high alkalinity (pH 8.9), temperature (45 °C) and mineral N [33 ppm NO₃–N (nitrate) and 48 ppm NH₄–N (ammonium)] occur in combination. Farmers in the region excessively use NP fertilizers that disturbs soil ecology, and inhibits nodule formation in pigeon pea. Initial experiments were conducted in laboratory for shortlisting potent rhizobial strains. Initially survivability of different rhizobial strains was monitored under mono-stress conditions (one stress applied on bacterial strain at a time (temperature, pH or fertilizer) on yeast extract mannitol agar medium. Shortlisted strain IC3123 displayed survivability under diverse temperature and pH conditions, and fertilizer doses ranging from 30 °C to 45 °C, 4 to 12 pH and 25 % to 100 % diammonium phosphate concentrations, respectively. Dual-stress condition of pH 9 at 45 °C resulted in 1.2-fold increase in EPS production by the strain as compared to non-stressed conditions. Strain IC3123 showed much better growth profile in EPS amended medium in comparison to EPS excluded medium under multi-stress conditions, suggesting the protective role of EPS in stress tolerance. Plant growth promoting traits of IC3123 were also monitored in presence and absence of EPS supplementation that further confirmed the effectiveness of EPS in mitigating stress and enhancing plant growth promoting traits. Diverse sets of bioformulations were prepared using purified EPS alone (metabolite-based bioformulation), blend of IC3123 and EPS (blended formulation), and rhizobia cells (IC3123) alone (cell-based formulation). Field experiment was conducted with pigeon pea in naturally stressed farmer’s field in Northern India where all these stress conditions (pH 8.9, 45 °C, 33 ppm NO₃–N) were found to act simultaneously in combination. The experiment was conducted in set of two: with and without 100 % recommended dose of fertilizer in the form of di-ammonium phosphate. Amongst the strategically developed bioformulations, the blend of cell plus metabolite significantly enhanced germination, pod number, seed yield, and protein content by 1.14, 1.38, 1.31 and 1.37-fold, respectively in comparison to untreated set (control). No nodulation was observed in the control set, whereas nodule number per plant increased when seeds received treatment with blend. Thus, the present study propagates the application of blended formulations developed from rhizobia and EPS for augmenting nodulation and growth of pigeon pea crop in stressed habitat in comparison to pure cell alone, with results validated in field.