Bradyrhizobia and rhizobia are symbiotic bacterial partners forming nitrogen fixing nodules on legumes. These bacteria share characteristics with plant growth promoting rhizobacteria (PGPR). Nodule inducing bacteria, like other PGPR, are capable of colonizing the roots of non-legumes and produce phytohormones, siderophores and HeN. They also exhibit antagonistic effects towards many plant pathogenic fungi. The potential of nodule inducing bacteria to function as PGPR, was examined by using radish as a model plant. Three percent of the 266 strains tested were found to be cyanogens, while a majority (83%) produced siderophores. Fifty eight percent of the strains produced indole 3-acetic acid (IAA) and 54% solubilized phosphorus. Some of the bacterial species examined were found to have a deleterious effect while others were neutral or displayed a stimulatory effect on radishes. Bradyrizobium japonicum strain Soy 213 was found to have the highest stimulatory effect (60%), and an arctic strain (N44) was the most deleterious, causing a 44% reduction in radish dry matter yield. A second plant inoculation test, performed in growth cabinets, revealed that only strain Tal 629 of B. japonicum significantly increased (15%) the dry matter yield of radish. This indicates that specific bradyrhizobia have the potential to be used as PGPR on non-legumes.

Rhizobium leguminosarum bv. phaseoli strains P31 and R1, Serratia sp. strain 22b, Pseudomonas sp. strain 24 and Rhizopus sp. strain 68 were examined for their plant growth-promoting potential on lettuce and forage maize. All these phosphate solubilizing microorganisms (PSM) were isolated from Qu6bec soils. The plants were grown in field conditions in three sites having high to low amounts of available P. In site 1 (very fertile soil), strains R1 and 22b tended to increase the dry matter yield of lettuce shoots (p < 0.10). Lettuce inoculated with rhizobia R1 had a 6% higher P concentration (p < 0.10) than the uninoculated control. In site 2 (poorly fertile soil), the dry matter of lettuce shoots was significantly increased (p < 0.05) by inoculation with strain P31 and 24 plus 35 kg ha -1 P-superphosphate, or with strain 68 plus 70 kg ha -1 P-superphosphate. In site 3 (moderately fertile soil), the dry matter of maize shoots was significantly increased (p < 0.05) by inoculation with strain 24 plus 17.5 kg ha-l P-superphosphate, or with strain P31 plus 35 kg ha-I P-superphosphate. Inoculation with PSM did not affect lettuce P uptake in the less fertile soil in site 2. In site 3 with the moderately fertile soil, maize plants inoculated with strain R1 had 8% higher P concentration than the uninoculated control (p < 0.01), and 6% with strains P31 and 68 (t9 < 0.05). The results clearly demonstrate that rhizobia specifically selected for P solubilization function as plant growth promoting rhizobacteria with the nonlegumes lettuce and maize. The P solubilization effect seems to be the most important mechanism of plant growth promotion in moderately fertile and very fertile soils when P uptake was increased with rhizobia and other PSM.

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Phosphorus solubilizing microorganisms constituted 26 to 46% of the microbial population of the four Quebec soils studied. Among the microorganisms isolated 10 bacteria and 3 fungi were selected for their ability to form a large clarification zone on solid media containing different insoluble P salts. In a glasshouse assay, the inoculation of maize seeds with the 13 organisms selected did not significantly influence maize growth or P content, after 3 weeks of growth. In field trials, the Enterobacter sp. isolates 22a and 22c and the Pseudomonas sp. isolate 24 significantly increased (7–9%) plant height after 60 days of growth. After 108 days of growth, only isolate 22c caused a significant increase (23%) of shoots fresh mass. Field trials with Paris Island Cos lettuce seeds inoculated with the bacterial isolates 22a and 24 resulted in 14 and 18% increase in shoots fresh mass yields, respectively. Isolate 68 of Rhizopus sp. also increased lettuce yield by 11% over the control. The growth improvement observed in maize and lettuce probably did not result only from P solubilization activity but also, in part, from the ability of the four organisms inoculated to produce siderophores and auxins.Key words: microbial solubilization, phosphorus, corn, lettuce.