Thesis (DTech. degree in Agriculture.)--Tshwane University of Technology, 2010. / Low soil N fertility accounts for poor crop yields in Africa. Additionally, chemical fertilizers use is inadequate due to high cost and unavalability. Because crops grown on such nutrient-poor soils are micronutrient deficient, incorporating N2-fixing legumes into cropping systems can sustainably improve crop yields and micronutrient capture. Nodulated cowpea easily forms symbioses with native rhizobial species, and can potentially fix atmospheric N2 into ecosystems. These bio-fertilization/-fortification traits can increase crop yields and nutrient accumulation in edible plant organs. Field experiments conducted in Ghana and South Africa assessed N2 fixation in cowpea in 2005 and 2006 using the 15N natural abundance technique. The genotypes Vuli-1 and TVu11424 contributed 178 and 198 kg N ha-1 in 2005 and 2006, respectively at Wa. At Manga, IT84S-2246 and TVu11424 fixed 157 and 171 kg N.ha- 1 in 2005 and 2006, while Fahari and IT84S-2246 contributed 182 and 131 kg N ha-1 in 2005 and 2006 respectively, at Taung. The effect of genotype x environment interaction on plant growth and N2 fixation were evaluated at Wa and Manga in Ghana, and Taung in South Africa. Whole-plant dry matter was highest at Manga relatve to Taung. The G x E interaction was significant for biomass yield, symbiotic parameters and grain yield. Fahari produced 3.2 and 2.2- fold more biomass at Manga than at Wa and Taung respectively. All the 25 genotypes derived significantly higher levels of N nutrition from symbiotic fixation at Taung compared to Manga and Wa. vi The relationship between symbiotic N nutrition and photosynthetic carbon assimilation in young fully expanded leaves of cowpea was assessed in Ghana, in 2005 and 2006. Benpila, Glenda, IT86D-1951 in 2005, and Sanzie in 2006, showed high photosynthesis, greater stomatal conductance, high transpiration, and increased water-use efficiency. Symbiotic N contribution was greater in Mchanganyiko, Ngonji and IT86D-2075 in 2005. These data show that where symbiotic legumes derive a large proportion of their N from N2 fixation, photosynthetic C yield is increased as a result of elevated Rubisco Analysis of edible leaves and grain of cowpea genotypes revealed significant differences in mineral concentration. Cowpea genotype IT82D-889 exhibited high concentrations of P, K, Ca, Mg and S. Iron concentration in cowpea grain was 137 μg.g-1 in Soronko, while Zn was 65 μg.g-1 in Vuli-1. Leaf concentrations of Cu, Mn, B and Fe were consistently higher in the genotypes Apagbaala, IT84S-2246, Fahari and IT97K-499-39. Compared with spinach, cowpea leaves had 3-fold more Fe which can offer biological solution to micronutrient deficiency in Africa.