Objective Walnut is an important non-wood product forest species. In this study, the effects of four endophytic bacteria on walnut seedling growth and their mechanism were clarified, and the endophytic bacteria with growth promotion effect were screened, so as to provide excellent strain resources for walnut seedling and green cultivation, and lay a foundation for further understanding the physiological and molecular mechanism of interaction between endophytic bacteria and walnut.

Method Four endophytic bacterial strains (B-N-11, B-N-12, HJ-N-4, W45-N-1) were isolated from walnut roots and leaves using tissue separation method. These strains were identified and their fundamental characteristics were systematically analyzed. Subsequently, a pot experiment was conducted to investigate the effects of the four endophytic bacteria on walnut seedlings from three dimensions: growth characteristics, mineral element absorption capacity, and rhizosphere soil nutrient status, aiming to screen endophytic bacteria with growth-promoting effects on walnut. Furthermore, correlation analysis and principal component analysis were employed to clarify the relationship between walnut growth characteristics and mineral elements, to identify evaluation indicators for the growth-promoting effects of endophytic bacteria on walnut seedlings, and to rank the growth-promoting effects of endophytic bacteria by calculating principal component scores.

Result (1)Among the 4 endophytic bacteria, strain B-N-11 belongs to Microbacterium, strain B-N-12 and strain W45-N-1 belong to Enterobacter, strain HJ-N-4 belongs to Bacillus. (2)Strain B-N-11 has the strongest ability of nitrogen fixation and potassium hydrolysis, strain W45-N-1 has the strongest ability of phosphorus dissolution and ACC deaminase production, strain B-N-12 has the strongest ability of IAA production. (3)B-N-11 bacteria solution treatment of walnut seedling stem diameter, aboveground biomass, leaf number, leaf area, and whole plant potassium, calcium, magnesium accumulation was the largest. HJ-N-4 bacteria solution treatment of walnut seedling root length, whole plant nitrogen accumulation was the largest. W45-N-1 bacteria solution treatment of walnut seedling underground biomass, total biomass, whole plant iron, manganese accumulation was the largest. (4)The growth characteristics of walnut seedlings were positively correlated with the accumulation of potassium, nitrogen, phosphorus, calcium, magnesium in whole walnut plant and the contents of iron, manganese, magnesium in soil. (5)The total biomass of walnut seedlings and the accumulation of calcium and magnesium in whole plant can be used as evaluation indexes to judge the effect of endophytic bacteria on walnut seedlings. The growth promotion effect of live bacteria solution was better than inactivated bacteria solution, culture medium and water, and strain B-N-11 was the best.

Conclusion The four endophytic bacterial strains enhanced walnut seedling growth by promoting mineral nutrient uptake, and strain B-N-11 had the strongest growth promoting effect, which could significantly increase potassium, calcium and magnesium accumulation of walnut seedlings. The results not only revealed the growth promotion mechanism of endophytic bacteria from the perspective of mineral nutrition, but also provided a new strategy for the use of microorganisms to regulate nutrient absorption and cultivate stress-resistant seedlings.