Spruce growth response specificity after treatment with plant growth-promoting Pseudomonads

Naturally regenerating hybrid spruce seedlings (Picea glauca (Moench) Voss X Picea engelmannii Parry) were collected from sites near Mackenzie, Salmon Arm, and Williams Lake, British Columbia, Canada. Bacteria were isolated f rom roots and screened in greenhouse trials for their ability to enhance sp ruce growth. Three strains belonging to the genus Pseudomonas were selected for study based on their disparate geographic origins and their capacity t o consistently stimulate spruce seedling growth in screening trials. Factor ial experiments were performed in the greenhouse to evaluate the effectiven ess of these Pseudomonas strains with different spruce ecotypes. Factors te sted were spruce seed sources, Pseudomonas isolates, and forest floor soils originating from different sites, Three levels of each factor were studied : one spruce seedlot, one Pseudomonas isolate, and one forest floor type ea ch originated from a site at Mackenzie, Salmon Arm, and Williams Lake, Brit ish Columbia. Fourteen weeks after treatments were established, spruce biom ass accumulation was greatest when spruce ecotypes were inoculated with bac teria originating from the same geographical area as spruce seed. However, Pseudomonas strains originating from sites other than the seed collection a rea also stimulated seedling growth significantly, rendering the difference in growth promotion between bacterial treatments small and insignificant. In addition, spruce growth promotion was not enhanced when seed was treated with combinations of Pseudomonas strains and forest floor soils originatin g from the same forest ecosystem. We conclude that specificity between spru ce ecotypes and plant growth-promoting rhizobacteria strains can be detecte d under carefully controlled conditions, thereby supporting the hypothesis that growth-promoting bacteria may adapt to their plant hosts. However, the growth advantage accruing to seedlings treated with bacteria originating f rom the same ecosystem is small and suggests that it is not necessary to ma tch Pseudomonas strains with spruce ecotypes and soil types for effective s eedling-growth promotion.