Symbiotic N2-fixation, carbon accumulation and water-use efficiency of 63 common bean (Phaseolus vulgaris L.) genotypes grown under field conditions at Malkerns, Eswatini using the 15N and 13C natural abundance technique

Common bean ( Phaseolus vulgaris L.) is consumed worldwide and is the third most important food legume after soybean and groundnut. The aim of this study was to evaluate plant growth, N 2- fixation, C accumulation and water use efficiency of 63 common bean genotypes grown in Malkerns, Eswatini under field conditions using the 15 N and 13 C natural abundance techniques. The data revealed marked (p ≤ 0.05) differences in shoot dry matter (SDM) yield, N concentration and content, δ 15 N, the proportion of N derived from symbiotic fixation (%Ndfa) and the N-fixed among the 63 common bean genotypes. Common bean plants’ %Ndfa values ranged between 19.6-88.4%, contributed 2.7-66.8 kg N-fixed ha -1 to the soil, while soil N-uptake ranged from 3.7-54.01 kg ha -1 . Out of the 63 test genotypes, 40 genotypes obtained more N from the soil than from symbiosis, implying that the N 2 -fixing efficacy of the microsymbionts nodulating common bean in Malkerns were low. The results further revealed shoot δ 13 C values ranging from -30.11‰ to -26.33‰, indicating differences in plant water use efficiency amongst different common bean genotypes. There was a significant positive correlation between δ 13 C and N content (r=0.25**), and also between N-fixed and shoot dry matter (r = 0.88***), indicating a functional link between N 2- fixation and photosynthesis. Taken together, the findings indicate that, increased N 2 -fixation is linked to improved photosynthetic activity, plant growth and water use efficiency in the bean plants.