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 for age maize. All these phosphate solubilizing microorganisms (PSM) were isolated from Quebec soils. The plants were grown in field conditions in three sites having high to low amounts of available P. In site 1 (v ery fertile soil), strains R1 and 22b tended to increase the dry matte r yield of lettuce shoots (p less than or equal to 0.10). Lettuce inoc ulated with rhizobia RI had a 6% higher P concentration (p less than o r equal to 0.10) than the uninoculated control. In site 2 (poorly fert ile soil), the dry matter of lettuce shoots was significantly increase d (p less than or equal to 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 matt er of maize shoots was significantly increased (p less than or equal t o 0.05) by inoculation with strain 24 plus 17.5 kg ha(-1) P-superphosp hate, or with strain P31 plus 35 kg ha(-1) P-superphosphate. Inoculati on with PSM did not affect lettuce P uptake in the less fertile soil i n site 2. In site 3 with the moderately fertile soil, maize plants ino culated with strain R1 had 8% higher P concentration than the uninocul ated control (p less than or equal to 0.01), and 6% with strains P31 a nd 68 (p less than or equal to 0.05). The results clearly demonstrate that rhizobia specifically selected for P solubilization function as p lant growth promoting rhizobacteria with the nonlegumes lettuce and ma ize. The P solubilization effect seems to be the most important mechan ism of plant growth promotion in moderately fertile and very fertile s oils when P uptake was increased with rhizobia and other PSM.