Insights into nitrogen fixing traits and population structure analyses in cowpea (Vigna unguiculata L. Walp) accessions grown in Ghana

With legumes, symbiotic N 2 fixation can meet the species N demand and reduce the over-reliance on chemical fertilizers in tropical regions where N deficiency is a major factor limiting crop yields and increased agricultural sustainability. Therefore, to optimize the use of cowpea ( Vigna unguiculata L. Walp) germplasm in effective breeding, evaluation of genetic diversity and quantification of N 2 fixation are essential prerequisites. The aim of this study was to explore the level of diversity using SSR markers and N 2 -fixing traits in a set of cowpea germplasm grown in Ghana. We analysed 49 cowpea accessions collected from Northern Ghana using qualitative vegetative and N 2 fixation traits, and simple sequence repeat (SSR) markers. Experimental field results revealed considerable morpho-physiological variation for plant growth habits, grain yield and symbiotic performance between and among the cowpea accessions. Results from both the 15 N natural abundance and ureides in the xylem sap were able to descriminate between high and low levels of N 2 fixation in cowpea accessions. Five subpopulations were identified within accessions inferred from STRUCTURE 2.3.4. A general linear model was used to assess the association of SSR markers with N 2 -fixing traits. There were significant ( p  ≤ 0.05) links between SSR markers and symbiosis-related traits such as nodule number, nodule dry weight, shoot dry weight, N-fixed, N derived from air (Ndfa), and relative uried-N (RU-N).