Effect of Bradyrhizobium strains on the yield of accessions of Bambara groundnut ( Vigna subteraenea (L) Verdc)

Abstract

A sizeable component of human diets consists of underutilised legumes. These legumes fix atmospheric nitrogen, giving smaller growers an economic advantage. Utilising the legumes in the appropriate way can help increase fertility of the soil and food security. A research was conducted to understand the growth, nutrient uptake, nitrogen fixation and yield of ten Bambara groundnut accessions in a screenhouse, field and laboratory at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria within 2018-2019 and 2019-2020 growing seasons. Ten Bambara groundnut accessions were randomly selected from IITA gene bank. B. japonicum strains (FA3, USDA110, IRJ2180A and RACA6) were coated separately to seeds of Bambara groundnut accessions before planting on the field and were applied as broth culture to seedlings of Bambara groundnut in the screenhouse. Nitrogen fertilizer was applied to seedlings of uninoculated Bambara groundnut accessions in both screenhouse and on the field, and an uninoculated control (no fertilizer and no inoculation). The screenhouse experiments were arranged in Completely Randomised Design (CRD), while the experiments on the field were arranged in Randomised Complete Block Design (RCBD). The research identified traits such as plant height, number of leaves, terminal leaf length, terminal leaf width, seed per pod, number of seeds per plot, seed width, and seed length that enhanced vegetative and reproductive stages of Bambara groundnut accessions when B. japonicum strains were inoculated in both screenhouse and on the field. Comparing TVSu-1698 to other Bambara groundnut accessions inoculated with B. japonicum strains, the result showed that higher mean values were recorded in the yield per plot (80.92g), yield per ha (2205.5kg), and the number of pods per plot (91.13) in both locations and seasons.

The B. japonicum strains inoculated to the accessions of Bambara groundnut fixed nitrogen and produced root nodules. Three Bambara groundnut accessions (TVSu-1606, TVSu-365 and TVSu- 1698) were identified as nitrogen-fixing accessions due to inoculation of B. japonicum strains. Inoculation of TVSu-1606 with RACA6 fixed nitrogen equivalent to 82.99 kg N ha-1 while inoculation of TVSu-365 with RACA6 fixed nitrogen equivalent to 81.10 kg N ha-1, followed by the inoculation of TVSu-1698 with IRJ2180A which fixed 79.69 kg N ha-1. B. japonicum strains inoculated to the accessions of Bambara groundnut, RACA6 and USDA110 strain, have stronger nitrogen-fixing capacity than other strains, with a mean value of 75.95 kg N ha-1 and 73.32 kg N ha-1 respectively. Nevertheless, the highest percentage of phosphorus uptake on the field was recorded in TVSu-787 at flowering, with a mean value of 0.21%, while the highest percentage of phosphorus at harvest was recorded in TVSu-378, with a mean value 0.15%. Also, the highest percentage of nitrogen uptake on the field was recorded in TVSu-1739 at flowering with a mean value of 2.09 %, and the highest percentage of nitrogen was recorded in TVSu-787 at harvest with a mean value of 1.75 %. In the screenhouse experiment, Bradyrhizobium spp. (B. diazoefficiens, B. japonicum) and Rhizobium spp were isolated from Bambara groundnut roots, and characterisation of isolates was done using the 16S rRNA gene. However, the nifH, Nod A, and Nod C gene analyses showed that Streptomyces bacillaris, Pseudomonas knackmussi, B. kanji, Sinorhizobium meliloti, Mesorhizobium spp., Bradyrhizobium spp., and Rhizobium spp. were isolated on the fields. The amount of nitrogen (derived from the atmosphere (Ndfa) in Bambara groundnut shoots revealed significant differences in soil status: the non-sterile soil in the screenhouse had the highest percentage of nitrogen derived from the atmosphere value (38.25%), while the sterile soil had the lowest mean value (36.57%). Additionally, higher significant differences in the fresh and dry shoot

weight at flowering and harvesting were recorded in the non-sterile soil than the sterile soil with a mean value of 4.44g and 1.23g (at flowering), 6.53g and 4.68g (at harvest) in the screenhouse. Significant differences between the B. japonicum strains and nitrogen fertilizer were equally observed. In comparison, the mean value recorded for nitrogen fertilizer applied to Bambara groundnut accessions at harvest were yield per plot (44.66g), and yield per ha (633.07kg ha-1). Compared to other B. japonicum strains inoculated to Bambara groundnut accessions, RACA6 strain exhibited higher significant differences in the growth trait of Bambara groundnut accessions on the field at various weeks’ intervals and yield component with a mean value of yield per plot (51.51g) and yield per ha (750. 72kg ha-1). Findings of the study demonstrated the significance of applying inoculation of B. japonicum strains as well as the application of the inorganic nitrogen fertilizer (Urea) used. Finally, the study provided evidence that inoculation of B. japonicum strains on Bambara groundnut accessions can help improve the growth and yield traits, nutrient uptake, nitrogen-fixing potential, nodulation and diversity of the bacteria nodulating the roots of Bambara groundnut.