PHENOTYPIC CHARACTERIZATION OF SOYBEAN BRADYRHIZOBIA IN 2 SOILS OF NORTH-CAROLINA

Serotypic composition of nodules (480 per soil type) from five soybean cultivars grown on two (Dothan and Cape Fear) soils of the Atlantic C oastal Plain of North Carolina was characterized. Symbiotic N-2-fixati on efficiency, capacity for induction of foliar chlorosis symptomatic of rhizobitoxine production and antibiotic resistances of isolates pur ified from these nodules were also determined. While host plant cultiv ar had no significant effect on the serotype distribution, soil type h ad a large effect on the distribution and diversity of serotypes. Fort y-six serotypes were identified among nodules from the Cape Fear soil, but only serotype 46/76 (8%), 76 (11%), 94 (9%) and 122/124 (12%) occ urred in, more than 5% of the nodules. Thirty percent of nodule occupa nts were not identified with the eleven antisera used. Twenty-four ser otypes were identified among nodules from the Dothan soil. Of these se rotypes 31/94 (32%), 46/76 (16%), and 76 (23%) occurred in more than 1 5% of the nodules. Five percent of the nodule occupants were not ident ified. Major serotypes did not change, but their frequency changed whe n fields were sampled at different growth stages in the same season an d at the same growth stage in different seasons. Isolates serotyped as 31/94, 46/76, and most of the isolates serotyped as 76 generally exhi bited higher levels of resistance to streptomycin and erythromycin tha n isolates serotyped as 24, 94 and 122/124. Five percent of the isolat es from the Cape Fear soil (all serotyped as 31/94) and 18% of the iso lates from the Dothan soil (serotyped as 31/94 or 76) induced foliar c hlorosis when cultivar Brim was the host. Only 12-14% of the isolates from the two populations had N-2-fixation capacity equal to or greater than that of the efficient reference strain MN110. However, four impr oved soybean cultivars grown in the same fields and year that isolates were obtained did not exhibit a significant seed yield response to ap plication of 150 kg N ha(-1) when yields in the minus N treatment rang ed from 3.2 to 3.7 Mg ha(-1). A significant seed yield response by a n on-nodulated cultivar indicated that these soils were N limited. There fore, the symbiotic N-2-fixation capacity of these bradyrhizobial popu lations did not limit soybean seed yields despite the low percentage o f isolates with high N-2-fixation efficiency and the presence of isola tes with the capacity to induce leaf chlorosis symptomatic of rhizobit oxine production. Published by Elsevier Science Ltd.