Endophytic colonization and field responses of hybrid spruce seedlings after inoculation with plant growth-promoting rhizobacteria

Bacterial colonization and growth responses of hybrid spruce (Picea glauca x engelmannii) seedlings after inoculation with plant growth-promoting rhiz obacteria (PGPR) were evaluated in controlled environment and field assays. Six antibiotic-resistant bacterial strains belonging to the genera Bacillu s and Pseudomonas were used as inocula. For controlled environment assays, surface sterilized seeds were inoculated with log 6 Bacillus or log 8 Pseud omonas colony-forming units (cfu) under gnotobiotic conditions for assessme nt of rhizosphere and internal root and stem tissue colonization. In the fi eld trial, one-year-old, container-grown hybrid spruce seedlings were inocu lated with each of the Bacillus or Pseudomonas strains 1-2 days before plan ting at nine field sites in British Columbia and Alberta, Canada. All six s trains colonized the rhizosphere with log 5.6-7.6 cfu g(-1) root tissue und er controlled environmental conditions. Two strains, Bacillus Pw-2R and Pse udomonas Sm3-RN, were also recovered from internal root and stem tissues wi th population sizes ranging from log 3.9 to log 5.0 cfu g(-1) plant tissue. In the field trial, PGPR survived the winter in the rhizosphere with popul ations of log 2 to log 5 cfu g(-1) root tissue 17 months after inoculation. In addition, spruce growth was significantly enhanced by bacterial inocula tion at certain sites. The most effective growth-promoting strains were the endophytes, Sm3-RN and Pw2-R, and the external root colonizing strain, Ss2 -RN. These three strains increased seedling dry weight up to 57% above noni noculated controls at five of the nine outplanting sites. Pseudomonas strai n Sw1 and the other two Bacillus strains produced mean spruce dry weight in creases at four of the sites. Seedling growth inhibition due to bacterial i noculation was detected at some sites. Our results confirm the short-term, sire specific effectiveness of PGPR for reforestation of conifer seedlings, and indicate that benefits of a single inoculation at planting can extend through the second year in the field. In addition, bacteria capable of colo nizing the seedling interior may be more effective PGPR for spruce than tho se restricted to the rhizosphere and the root surface. However, site specif ic seedling growth promotion may necessitate matching PGPR strains to outpl anting sites for effective growth promotion. (C) 2000 Elsevier Science B.V. All rights reserved.