Autecology in Rhizospheres and Nodulating Behavior of Indigenous Rhizobium trifolii

Abstract

Indigenous serotype 1-01 of Rhizobium trifolii occupied significantly fewer nodules (6%) on plants of soil-grown noninoculated subterranean clover ( Trifolium subterraneum L.) cv. Woogenellup than on cv. Mt. Barker (36%) sampled at the flowering stage of growth. Occupancy by indigenous serotype 2-01, was not significantly different on the two cultivars (16 and 26%). Serotype-specific, fluorescent-antibody conjugates were synthesized and used to enumerate the indigenous serotypes in host (clovers) and nonhost (annual rye-grass, Lolium multiflorum L.) rhizospheres and in nonplanted soil. The form and concentration of Ca 2+ in the flocculating mixture and the presence of phosphate anions in the extracting solution were both critical for enumerating R. trifolii in Whobrey soil. The two serotypes were present in similar numbers in nonplanted soil (ca. 10 6 per g of soil) and each represented ca. 10% of the total R. trifolii population. Although host rhizospheres did not preferentially stimulate either serotype, the mean population densities of serotype 2-01 were significantly greater ( P = 0.05) than those of serotype 1-01 in clover rhizospheres on 8 of 14 samplings made between the time of seeding and the appearance of nodules (day 12). In this experiment, and in contrast to our earlier findings, serotype 1-01 occupied significantly fewer ( P ≤ 0.05) of the nodules (7 to 16%) on both cultivars than serotype 2-01 (51%) when sampled at 4 weeks. Differences between cultivars became apparent as the plants matured. There was a threefold increase (7 to 21%) in nodules occupied by serotype 1-01 on cv. Mt. Barker between 4 and 16 weeks. This was accompanied by increases in nodules coinhabited by both nonidentifiable occupants and either serotype 1-01 (0 to 20%) or 2-01 (11 to 51%). No increases in either of these parameters were observed on cv. Woogenellup.