EFFECT OF PH ON COMPETITION FOR NODULE OCCUPANCY BY TYPE-I AND TYPE-II STRAINS OF RHIZOBIUM-LEGUMINOSARUM BV PHASEOLI

The effect of soil pH on the competitive abilities of two Rhizobium leuminosarum bv. phaseoli type I and one type II strains was examined in a nonsterile soil system. Phaseolus vulgaris seedlings, grown in unlimed (pH 5.2) or limed (pH 7.6) soil, were inoculated with a single-strain inoculum containing 1 x 10(6) cells mL(-1) of one of the three test strains or with a mixed inoculum(1:1, type I vs. type II) containing the type II strain CIAT 899 plus one type I strain (TAL 182 or CIAT 895). At harvest, nodule occupants were determined. In a separate experiment, a mixed suspension (1:1, type I vs. type II) of CIAT 899 paired with either TAL 182 or CIAT 895 was used to inoculate P. vulgaris seedlings grown in sterile, limed or unlimed soil. The numbers of each strain in the rhizosphere were monitored for 10 days following inoculation. The majority of nodules (> 60%) formed on plants grown in acidic soil were occupied by CIAT 899, the type II strain. This pattern of nodule occupancy changed in limed soil. When CIAT 899 was paired with TAL 182, the type I strain formed 78% of the nodules. The number of nodules formed by CIAT 899 and CIAT 895 (56% and 44%, respectively) were not significantly different. The observed patterns of nodule occupancy were not related to the relative numbers or specific growth rates of competing strains in the host rhizosphere prior to nodulation. The results indicate that soil pH can influence which symbiotype of R. leguminosarum bv. phaseoli will competitively nodulate P. vulgaris.