Interstrain Competition between Representatives of Indigenous Serotypes of Rhizobium trifolii

Abstract

The symbiotic characteristics of Rhizobium trifolii strains 1-01 and 2-01 were evaluated both individually and in various combinations on two cultivars (Mt. Barker and Woogenellup) of subterranean clover (Trifolium subterraneum L.). Nodules were observed on day 8 independent of cultivar or strain. Cultivar differences were measured in nodulating efficiency by 1-01 since 54% of the primary nodules were formed on cv. Mt. Barker and only 15% were formed on cv. Woogenellup in the zone above, or 1 cm below, the root tip location at the time of inoculation. The percentage of nodules formed in this zone by 2-01 was similar on both cultivars (31 to 32%). When mixtures of strains 1-01 and 2-01 (230:1 and 1:20) were used to inoculate plants, >90% of the nodules on both cultivars were occupied by the more abundant strain in the inoculum regardless of sampling date (4 or 8 weeks). In contrast, large percentages of nodules on 4-week-old plants of both cultivars exposed to a 5:1 inoculum mixture were doubly occupied (64 and 74%). By week 8 these values had decreased significantly (P </= 0.01) and were accompanied by large increases in the percentage of nodules occupied by either strain 1-01 alone (1 to 65%) on cv. Mt. Barker or 2-01 alone (4 to 49%) on cv. Woogenellup. The superior (cv. Mt. Barker) and inferior (cv. Woogenellup) symbiotic performance of plants inoculated with the 5:1 mixture correlated more closely with the 8-week than the 4-week nodule occupancy data. Primary nodule occupancy by 1-01 and 2-01 was significantly influenced by changes in the inoculum ratios of 1-01/2-01 from 5.7:1 to 0.67:1 on cv. Mt. Barker and from 1.9:1 to 0.67:1 on cv. Woogenellup. Despite evidence for extensive proliferation of the inoculant strains on the rhizoplanes, no evidence was obtained for either interstrain antagonism or selective proliferation as a valid reason to explain the outcome of primary nodulation.