EFFECTS OF BRADYRHIZOBIUM STRAIN AND HOST GENOTYPE, NODULE DRY-WEIGHTAND LEAF-AREA ON GROUNDNUT (ARACHIS-HYPOGAEA L SSP FASTIGIATA) YIELD

Effects of inoculating four Arachis hypogaea ssp. fastigiata cultivars with 17 Bradyrhizobium spp. strains were studied in a glasshouse expe riment using a sandy soil devoid of an indigenous Bradyrhizobium popul ation. Firstly, a wide range of parameters, indicative of symbiotic pe rformance, were assessed for their influence on seed yield, by correla tion and statistical analyses. It was found that nodule dry weight and leaf area were relevant parameters concerning seed yield. Secondly, t he effects of host and strain genotype on those parameters were descri bed. Variations in nodule dry weight did not have an effect on seed yi eld, except for cultivar Natal Common at lower nodule dry weight value s. Therefore, it was concluded that the quantity of nitrogen fixing ti ssue met the demand for combined nitrogen and did not limit seed yield . This conclusion was further supported by the observation that at low nodule numbers per plant the nodule size increased to generate suffic ient nitrogen fixing tissue. Leaf area, which comprises components for both photosynthetic capacity and plant development, was found to corr elate well with seed yield. An increase in leaf area resulted in signi ficant seed yield increases for all three spanish-type cultivars, but not for the valencia-type cultivar. Leaf area, thus, appeared as a fac tor limiting seed yield of spanish-type groundnuts. Cultivar performan ce concerning seed yield was significantly better for Natal Common com pared to the other three cultivars, while Natal Common had a significa ntly lower plant (biomass excluding seed) dry weight value. Inoculatio n with different strains of Bradyrhizobium resulted in significantly d ifferent nodule dry weight values, but hardly led to significant diffe rences in seed yield. This agreed with the finding that the amount of nitrogen fixing tissue appeared not to limit the availability of combi ned nitrogen. A large quantity of nitrogen was partitioned to the grou ndnut seeds: 62% to 76% of total accumulated nitrogen was located in t he seeds. This study showed that testing for symbiotic effectiveness i n the groundnut Bradyrhizobium symbiosis should include assessment of final (seed and biomass) yield, because parameters measured at stages prior to maturity, like nodulation parameters, may lead to flawed effe ctiveness ratings.