Pine and spruce seedling growth and mycorrhizal infection after inoculation with plant growth promoting Pseudomonas strains

Abstract

Pine and spruce seeds were inoculated with antibiotic-resistant plant growth promoting fluorescent Pseudomonas strains Sm3-RN, Ss2-RN and Sw5-RN for evaluation of bacterial root colonization and seedling growth responses under greenhouse conditions. Mycorrhizal inoculum was introduced to seedling containers by placing 2 cc of forest floor soil around seeds at the time of sowing. Mycorrhizal roots were detected on 39% of pine and 30% of spruce seedlings treated with forest soil 13–15 weeks later. Most mycorrhizae were formed by Wilcoxina sp. (E-strain) (89% for spruce and 69% for pine); some Amphinema-like, Mycelium radicis atrovirens, Suillus-like, Thelephora-like, and Tuber-like mycorrhizae were also detected. In the absence of bacterial inoculum, spruce seedling biomass was positively correlated with the number of mycorrhizal root tips. This trend was obscured following inoculation with strains Ss2-RN and Sw5-RN. Pine seedling growth was weakly correlated with mycorrhizal root tip development only after treatment with strain Sm3-RN. Bacterial inoculation did not influence the mycorrhizal status of seedlings, but all three Pseudomonas strains stimulated biomass accumulation of spruce and pine seedlings, up to 19% ( P < 0.05). Non-mycorrhizal seedlings tended to support smaller root-associated Pseudomonas populations, but inoculant bacteria colonized seedlings with a minimum of 6.3 × 10 3 colony forming units per gram of rhizosphere soil regardless of their mycorrhizal status. For spruce, growth effects due to bacterial inoculation were similar in both mycorrhizal and non-mycorrhizal seedlings. However, statistically significant gains in pine biomass by inoculation with strains Sm3-RN and Ss2-RN occurred only in mycorrhizal seedlings, whereas strain Sw5-RN caused significant growth promotion only in non-mycorrhizal pine. Our results suggest that these fluorescent pseudomonad strains enhanced spruce seedling growth through mechanisms unrelated to increased mycorrhizal colonization, but growth promotion of pine by strains Sm3-RN and Ss2-RN was facilitated by an interaction with mycorrhizae.