Competition for Nodule Occupancy Between Introduced Strains of Rhizobium leguminosarum Biovar trifolii and its Influence on Plant Production

Abstract

Interactions among white clover ( Trifolium repens L.) and selected Rhizobium leguminosarum biovar trifolii strains, and related clover/grass production, were studied in a field experiment during three growing seasons in Iceland. Weather conditions in this country are harsh and rhizobia are rare or absent in the soils. White clover and red fescue ( Festuca rubra L. ‘Leik’) were established in the field in May 1994, using a white clover breeding line (HoKv 9238) which originated from a breeding programme aimed at developing commercial cultivars for cold climates. Rhizobia were initially absent from the field site and clover was inoculated with one of three R. leguminosarum bv. trifolii strains, or a mixture of all three strains. White clover and grass yields were measured during three growing seasons after the establishment year. Nodule occupancy by the three Rhizobium strains was studied by ERIC-PCR DNA fingerprinting. In the mixed inoculum treatment, R. leguminosarum bv. trifolii strain 20-15 occupied 80–90% of the nodules tested, while strain 32-28 was present in up to 3% of the nodules tested. The dominance of strain 20-15 persisted and tended to increase throughout the 4 year experimental period. The introduced rhizobia persisted in high numbers in the soil 1 year after clover was removed and a non-host crop had been grown. ERIC-PCR was an efficient and reliable method by which to identify the Rhizobium strains from cells as well as directly from nodule suspensions. The selected breeding line of white clover persisted well under Icelandic conditions but produced low yield throughout the entire experimental period. Mixed strain inoculation had a positive effect, resulting in both higher white clover yields and higher total plant yields than on plots treated with single strain inoculum.