| Summary: | 碩士 === 國立中興大學 === 土壤環境科學系所 === 105 === Chemical fertilizers and pesticides are commonly used to maintain soil fertility, reduce plant pathogen and pest. But long-term and overdose usages of agrochemicals have lead to soil degradation and pollution. To achieve sustainable agriculture, biofertilizers and biopesticides which are environment friendly provide alternatives to promote plant growth directly or indirectly. The present study was undertaken to explore root associated bacteria from medicinal plant Platycodon grandiflorus. Samples were collected from rhizosphere and root interior and plated on nutrient agar, nitrogen-free agar or tricalcium phosphate agar. After isolation all isolates were subjected to 16S rDNA sequencing and phylogenetic analysis. Plant growth promoting traits regarding nitrogen fixation, phosphate solubilization or IAA production were determined. Besides, the plant constituent decomposing and carbon source utilizing abilities of selected isolates were also demonstrated. The superior isolates were used in seed bioassay and pot experiment to evaluate their performance on plant growth. Among 45 isolates obtained from rhizosphere, the dominant bacterial genera consist of Bacillus, Burkholderia, Microbacterium, Pseudomonas and Rhizobium. A total 25 isolates mainly affiliated with genera Burkholderia, Pantoea and Rhizobium were obtained from root interior. Some of these isolates showed higher phosphate solubilizing activity (> 300 μg ml-1) or IAA producing activity (> 50 μg ml-1). Considered for their plant growth promoting traits and biosafety level, six isolates namely Microbacterium sp. NAI1, Rhizobium sp. NAI3, NAI4, NAI5, NFI1 and Xenophilus sp. NFI5 were selected and further characterized. Seed bioassay demonstrated that strain NFI1 increase growth of epicotyl while strain NFI5 promote growth of radicle after 10 days of cultivation. Strain NAI1 showed prominent utilization of cellulose, pectin, starch and xylan, and a variety of carbohydrates were well utilized. As for Rhizobium spp., only cellulose was utilized by 4 isolates and no remarkable carbon utilization were revealed within 48 h of cultivation. Strain NFI5 showed pectin decomposing activity and several organic acids were weak utilized. In vermiculite containing flask, inoculation of strain NAI4 and NFI5 increased fresh weight of Platycodon grandiflorus after 50 days of cultivation. All these inoculants were obtained and confirmed by BOX-PCR fingerprinting after re-isolating from root interior. In pot experiment, treatment NPK + strain NAI4 or NPK + strain NFI5 increased fresh weight and dry weight of stems and leaves. The polyphenols content and antioxidative activities of Platycodon grandiflorus roots were the highest in treatment NPK + strain NAI4 after 4 months of cultivation. However, inoculation of strain NAI1 decreased biomass of roots while strain NFI1 decreased biomass of stems and leaves. From the present study it is proposed that carbon utilizing traits of bacteria should also be taken into considerations when developing superior bioinoculants.
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