Abstract: Plant growth-promoting rhizobacteria (PGPR) are important species resources of microbial fertilizer, and they play important roles in agriculture production. In this study, the capability of hormone-secreting and antagonistic effect among experimental strains were evaluated, combinations of complex bacteria were constructed, and effect of complex bacteria on watermelon roots were studied. The results showed that the all twelve bacteria strains could secrete indoleacetic acid (IAA), the best one was Streptomyces CL05, whose yield was 117.30 mg·L^-1. Sphingomonas CL03, Lysobacter CL02 and Microbacterium LS01 could not product siderophores, Microbacterium LS01, Microbacterium HYL03 and Pantoea ananatis HYL01 could not secrete gibberellin. Furthermore, three groups of complex bacteria were generated based on the antagonistic experiments, which were named as R1, R2 and R3, respectively. In unsterilized soil, complex bacteria R2 significantly increased root length, root surface area, root volume, root number and root dry weight of watermelon seedlings by 79.0%, 46.8%, 57.1%, 162.7% and 49.1% compared with the control, respectively. In sterilized soil, complex bacteria R2_S increased root length, root surface area, root volume, root number and root dry weight of watermelon seedlings by 191.8%, 302.4%, 160.0%, 206.5% and 139.6% compared with the control, respectively, and decreased root mean diameter of watermelon seedlings by 17.5%. Interestingly, the promoting effect of R2 on watermelon roots was better in sterilized soil than in unsterilized soil. Overall, the study indicated that complex bacteria R2 could significantly promote the growth of watermelon roots, and the ability of gibberellins secretion in R2 (CL03, LS05, HYL02, WZW03) exceeded that in R1 and R3, which may be one of promoting mechanisms of complex bacteria R2.

Key words: watermelon root, complex bacteria, promoting effect, microbial fertilizer