Inoculation of canola (Brassica campestris) seeds with a nitrogen-fixing strain of Pseudomonas putida (GR12-2) drastically increased the root length of seedlings grown in sterile growth pouches. Seed inoculation with inactive bacteria did not affect root lengths. Root elongation capacity was retained by nonnitrogen-fixing mutants of strain GR12-2. On the other hand, two other wild-type pseudomonads that do not fix nitrogen also increased root elongation. The addition of mineral nitrogen to the growth solution at concentrations of 1 mM or higher significantly inhibited root elongation of either inoculated or noninoculated seedlings. On the other hand, the addition of phosphate to the growth solution at similar concentrations stimulated root elongation of inoculated and noninoculated seedlings. The combined effects of bacterial inoculation and addition of phosphate on root and shoot elongation and on root and shoot weight were additive. Seed inoculation with P. putida GR12-2 increased the uptake of labelled phosphorus (32P) by seedlings grown in growth pouches and also enhanced the shoot elongation of seedlings grown in sterile soil. The capacity of P. putida GR12-2 to enhance phosphate uptake and to promote plant growth under gnotobiotic conditions may open the door to a new direction in the development of plant growth promoting inoculants.

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Root-associated bacteria capable of reducing acetylene to ethylene (biological nitrogen fixation) were isolated from various native plants grown in the Canadian High Arctic. All the strains belonged to the genus Pseudomonas but varied in several physiological characteristics. The rates of acetylene reduction at 14 or 20°C were higher than at 25 or 9°C. Six strains reduced acetylene at 4°C. All the strains exhibited chemotaxis to L-asparagine in semisolid agar at 4 to 25°C. Eleven strains colonized roots of canola (Brassica campestris cv. Tobin) in field soil at population densities of log 4.3 to log 5.1 CFU/g of fresh root at 14°C and log 4.0 to log 5.2 CFU/g of fresh root at 25°C. Some of these nitrogen-fixing pseudomonad strains demonstrated a competitive advantage for root colonization over other rhizosphere bacteria at low temperatures. The combined capabilities of nitrogen fixation and root colonization by diazotrophic pseudomonads may be useful for the development of a biofertilizer inoculant for temperate and cold regions.

Abstract. Significant levels of nitrogenase activity (nitrogen fixation) were demonstrated in three species of Arctic legumes (Oxytropis maydelliana, O. arctobia and Astragalus alpinus) growing in high tundra at Sarcpa Lake, Melville Peninsula, N.W.T. Nitrogenase activity of intact plants was correlated with the number of nodules per plant, with field soil temperatures and limited by water shortage. Activity in freshly detached nodules showed a plateau of maximum activity between 10°C and 25°C and a near linear decline with temperature down to 0°C. Unusually, the segmented nodules of all three species are perennial in which growth and leghaemoglobin production resumes each spring from an overwintering apical meristem. Nodules are most numerous in the warmer soil stratum (2–10 cm. depth). Other studies indicate that the arctic rhizobia belong to a single cold‐adapted species which has co‐evolved with the legumes of tundra.

A series of greenhouse and field experiments were conducted to evaluate aqueous in-furrow spray techniques for inoculating crop plants with cell suspensions of rhizosphere-competent root-colonizing bacteria. Maximum root colonization of soybean or canola roots by strains of Serratia, Pseudomonas, and Bradyrhizobium occurred using log 8 colony-forming units (cfu)/mL in the spray in greenhouse conditions. Field experiments evaluating a dose reponse demonstrated that maximum soybean root colonization by strains of Serratia or Pseudomonas was achieved between log 7 and 8 cfu/mL, while root colonization by a Bacillus strain was not related to cell concentration in the spray suspension. Root colonization greater than log 4.5 cfu/g root fresh weight was achieved for most strains by in-furrow spray application of a suspension of log 8 cfu/mL at a rate of 10 mL/m for canola, soybean, and wheat, while root colonization of corn ranged from a maximum of log 3.4 to no recovery. In-furrow spray may be a useful method for inoculating plants with rhizosphere-competent bacteria for experimental purposes, thereby avoiding interactions of formulation. It may also have some value for commercial delivery of bacteria to agricultural crops when it is compatible with accepted agronomic practices. Key words: rhizosphere, plant growth promoting rhizobacteria, formulation, Pseudomonas, root colonization.