Abstract: This study aimed to investigate the effects of the plant growth-promoting rhizobacteria Flavobacterium succinicans DSM4002 (FS) on plant growth and physiological characteristics of perennial ryegrass under different shade stress treatments (0, 20%, 40%, 60% and 80%). The combined effects of DSM4002 inoculation and various levels of shade stress on the plant morphology, malondialdehyde (MDA) content, superoxide anion free radical ($O_{2}^{·-}$) levels, antioxidant enzyme activities, and non-enzyme antioxidant contents, were assessed. It was found that shade stress significantly inhibited the growth of perennial ryegrass seedlings while FS treatment alleviated the growth inhibition caused by shade stress. MDA accumulation increased with increasing shade stress. However, FS inoculation significantly reduced MDA and $O_{2}^{·-}$levels under shade stress, compared with shade treatment alone. Treatments of 20% shade+FS, 40% shade+FS, 60% shade+FS, and 80% shade+FS resulted in decreases in $O_{2}^{·-}$ levels of 4.47%, 29.75%, 44.57% and 49.84%, respectively. Additionally, shade stress resulted in a change in antioxidant enzyme activities and non-enzyme antioxidant concentrations, while FS inoculation further improved the activities of superoxide dismutase, catalase, and glutathione reductase, and increased levels of reduced ascorbate and reduced glutathione. Peroxidase and ascorbate peroxidase activities were unchanged by FS. In conclusion, inoculation with FS stimulated the antioxidant defense mechanisms, preventing the accumulation of reactive oxygen species, thus enhancing shade tolerance in perennial ryegrass. The present study indicates that F. succinicans DSM4002 has a great potential to improve plant adaptation to stress.

Key words: Flavobacterium succinicans, rhizosphere bacteria, shading stress, perennial ryegrass