Effects of plant growth-promoting rhizobacteria on loblolly and slash pine seedlings

Loblolly and slash pine seed were inoculated at sowing with 1 of 12 different strains of plant growth-promoting rhizobacteria (PGPR) in the greenhouse, Time to germination and seedling densities were determined at 21 days, and seedling biomass was measured at 12 wk after sowing. All bacterial strains significantly increased the speed of seedling emergence over nontreated pine seed. By 12 wk, however, no differences in stand densities were observed between bacteria-treated and nontreated seed for either pine species. Postemergence damping-off was reduced in loblolly pine when seed was treated with 3 of the 12 bacterial strains; however, postemergence damping-off on slash pine seedlings was not affected by rhizobacteria. Treatment with rhizobacteria had a significant positive and negative effect on seedling growth and biomass, which depended on tree species. Loblolly pine shoot and root lengths, as well as the above-and belowground biomass, were significantly reduced when seeds were treated with strains BS1 and BS2. In contrast, loblolly pine seeds treated with strains BS3, PM2, and INR7 significantly increased the below ground biomass of the seedling root systems. Slash pine seedlings had similar interactions with the bacterial strains. Strain BS1 significantly reduced shoot lengths compared with nontreated seeds, while strains 90-166, INR7, and SE49 increased shoot biomass. Slash pine root lengths and biomass were also reduced when treated with strains BS1 and BS2. Unlike loblolly pine, no bacterial strain increased slash pine root length or biomass. Th is study suggests th at the effects of rhizobacteria inoculation on seeding emergence and plant growth are independent and that the effects are species specific.