Variation in nitrogen fixation and yield in landraces of bambara groundnut (Vigna subterranea L)

Field experiments were performed in 1993 and 1994 on soil free of Vigna subterranea-nodulating rhizobia to study the nodulation and nitrogen fixation of 23 indigenous bambara groundnut landraces (bunchy and spreading types) from Malawi. Inoculation with a mixture of two Bradyrhizobium strains (280A and 100M) resulted in abundant nodulation for most of the accessions tested. As estimated over the total experiment, there was no significant increase in the number and weight of nodules between 68 and 105 days after sowing (82-94 nodules per plant respectively), but nitrogenase activity in nodules increased in this period from 20 to 43 mu mol C2H4 g(-1) dry weight nodules h(-1). Landraces varied significantly in yield and N-harvest index, and the total amount of nitrogen fixed was not a clear guide to high pod and seed yields. Statistically significant correlations were found between harvest index per single plant and pod and seed yields of the plant. High-yielding landraces were found only among the accessions with a bunchy growth habit. The amount of symbiotic N measured in the shoots of 130 day old plants (landrace 3C(1)) was 1.2 g per plant, which was 80% of the total N accumulated in the plants.