Isolation, Root Colonization and Evaluation of Some Plant Growth-promoting Rhizobacteria in Paddy Rice

In order to obtain promising rice growth-promoting microbial strains that can be used as substitutes for chemical fertilizers, 172 bacterial strains were isolated from rice roots grown in Korean and Russian soils. Out of them, the strains KR076, KR083, KR181 and RRj228 showed plant growth-promoting activities on maize seedlings. Bacillus megaterium KR076 and Bacillus sp. KR083 showed both nitrogen-fixing and plant growth-promoting activities, while Rhizobium sp. KR181 and Pseudomonas sp. RRj228 appeared to support only plant growth-promotion, but not N 2 fixation. Especially, RRj228 showed high growth promoting activity at low concentrations. Inoculation studies with KR083 and RRj228 revealed a high affinity to the Japonica rice variety such as Junambyeo than the Korean Tongil type variety such as Arumbyeo. Both KR083 and RRj228 strains showed rhizoplane and/or endophytic colonization in Japonica and Tongil types rice when soaked with the bacterial suspension of 1.1 × 10 5 cfu ml -1 for six and twelve hours. However, the total bacterial cell numbers were higher in the roots of Japonica variety than in the Tongil type. In inoculation trials with Daesanbyeo rice variety, the seedlings inoculated with KR181 and RRj228 at the rate of 2.0 × 10 6 cfu ml -1 showed yield increment of 35% and 33% ( p < 0.01), respectively, so that they contributed to the replacement of chemical fertilizer at half doses of N, P 2 O 5 , and K 2 O in pots. In Junambyeo rice seedlings, the strain RRj228, when inoculated with a cell suspension of 1.8 × 10 6 cfu ml -1 , promoted 3.4% higher yield at 70% dose than at a full dose level of N 110 kg ha -1 in field. These results suggest that the rhizobacteria KR181 and RRj228 are prospective strains for enhancing rice performance. Rice yield response Experiments on rice yield response to rhizobacterial inoculation were conducted under pot and field conditions. The rhizobacterial strains KR076, KR083, and RRj228 grown on PDA medium and the strain KR181 grown on YMA medium were used as inoculants. In pot trials, the rice variety Daesanbyeo was tested with single strain inoculation, while Mananbyeo was evaluated in coinoculation studies with RRj228 and KR076 or KR083. Inoculation was performed by dipping the roots of 30 days old rice seedlings in bacterial suspension of 2.0 × 10 6 cfu ml -1 for 12 hours. The inoculated seedlings were then transplanted to pots containing soil. Uninoculated plants treated with sterile medium before transplantation were served as control. Yield response of inoculated rice plants was determined under fertilization regime of N, P 2 O 5 and K 2 O at a half dose level, and compared it with yield response of uninoculated plants fertilized with a standard full dose of NPK at a level of N 110, P 2 O 5 45, and K 2 O 57 kg ha -1 . In field trials, growth response of the rice variety Junambyeo was evaluated using single strain inoculation with RRj228 at a 70% dose level of nitrogen, as well as by coinoculation with KR083/KR181 or KR083/RRj228 at a half dose level of nitrogen. In the former case, roots of the rice seedlings were dipped in a bacterial suspension of 1.8 × 10 6 cfu ml -1 for 12 hours, and then grown in nursery box for 30 days before transplanting them into the field. In the latter case, 30 day-old rice seedlings were treated with inoculum containing 1.0 × 10 6 cfu ml -1 like the former. Inoculated rice plants were transplanted to paddy soils of a fine silty loamy textured alluvial deposit, and the yield responses to inoculation were evaluated.