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

With legumes, symbiotic N2 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 N2 fixation are essential prerequisites. The aim of this study was to explore the level of diversity using SSR markers and N2-fixing traits in a set of cowpea germplasm grown in Ghana. We analysed 49 cowpea accessions collected from Northern Ghana using qualitative vegetative and N2 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 15N natural abundance and ureides in the xylem sap were able to descriminate between high and low levels of N2 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 N2-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).