Growth promotion of maize and lettuce by phosphate-solubilizing Rhizobium leguminosarum biovar. phaseoli

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 Québec 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^-1 P-superphosphate, or with strain P31 plus 35 kg ha^-1 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 (p≤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.