Screening of high efficiency symbiotic rhizobia to faba bean and a preliminary investigation on their promotion ability in Chengdu Plain

Objectives High efficient and widely adaptive faba bean rhizobia from Chengdu Plain were screened and preliminary evaluated for their growth promotion ability, to provide a reference for the development and application of the effective faba bean rhizobia inoculant in this region.

Methods A sand culture experiment was carried out using double-layer pots and low-N nutrition solution. The six tested rhizobia strains were isolated and screened from previous experiment, and matched well with the predominant faba bean cultivar Dabaidou in Chengdu Plain and Sichuan. In this experiment, their matching efficiency with faba bean cultivars of Chenghu-14 and Chenghu-15 were compared. Two cultivars of faba bean were sown in vermiculite of double-layer pots. Six strains were inoculated into each faba bean cultivar, and the plant biomass and nodule number were measured after 41 days of cultivation under light (22–25 ℃, 2800 lx, 14 h) and without inoculation as control (CK). The phosphate solubilization ability and IAA excretion ability of the 6 strains were determined by the conventional method. Two strains screened by matching test were tested in the field, the tested faba bean cultivar was Chenghu-15. The two microbial agents (the number of viable count was more than 5.0 × 10⁸ CFU/g and the carrier was peat) were mixed with faba bean, and without inoculation and sterilization of peat as control (CK). Plant height, nodule number and dry weight were measured in blooming stage (105 days of growing period). Yield was measured in harvest stage (200 days of growing period). N, P and K contents of plant samples were measured at two stages, and competitive nodulation ability of two strains was compared in field test using BOX Molecular Marker Method in blooming stage. Multilocus sequence analyses were applied to identify the phylogenetic position of this strain which was verified suitable for field application in Chengdu Plain by field experiments.

Results 1) The matching experiment indicated that 2 strains, SCAUf73 and SCAUf76, matched efficiently with 2 cultivars of faba bean in Chengdu Plain area were screened out. When inoculated with SCAUf76 and SCAUf73, the plant dry weight of faba bean were higher than those of CK. The plant dry weight increased significantly by 40.5%–61.6% compared with CK. 2) Field experiments showed that faba bean yield difference between SCAUf76 inoculated and CK was not significant, when inoculated with SCAUf73, the plant dry weight and total nitrogen content of faba bean were higher than those of CK. The fresh grain yield was increased significantly by 25.0% compared with CK, and significantly higher than those of SCAUf76 infected. Its nodule occupancy reached 33.3%. 3) Sequence analysis of multilocus genes showed that strain SCAUf73 might be a novel strain of Rhizobium. 4) Six strains could secrete IAA, and the maximum secretion amount was 21.0 mg/L happened in SCAUf76, indicated by promotion ability test. All strains did not show significant P solubilization ability.

Conclusions This study indicated that the strain SCAUf73 was the best one matched with main planting cultivars of faba bean in Chengdu Plain area. It had the ability of secreting IAA, and was highly efficient. Field experiments showed that faba bean yield and total nitrogen content were higher than those of CK, the tumour rate reached 33%.