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 le uminosarum bv. phaseoli type I and one type II strains was examined in a nonsterile soil system. Phaseolus vulgaris seedlings, grown in unli med (pH 5.2) or limed (pH 7.6) soil, were inoculated with a single-str ain inoculum containing 1 x 10(6) cells mL(-1) of one of the three tes t 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 8 95). At harvest, nodule occupants were determined. In a separate exper iment, 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 see dlings grown in sterile, limed or unlimed soil. The numbers of each st rain in the rhizosphere were monitored for 10 days following inoculati on. The majority of nodules (> 60%) formed on plants grown in acidic s oil were occupied by CIAT 899, the type II strain. This pattern of nod ule 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 nodul es formed by CIAT 899 and CIAT 895 (56% and 44%, respectively) were no t significantly different. The observed patterns of nodule occupancy w ere not related to the relative numbers or specific growth rates of co mpeting strains in the host rhizosphere prior to nodulation. The resul ts indicate that soil pH can influence which symbiotype of R. legumino sarum bv. phaseoli will competitively nodulate P. vulgaris.