Alphonsus K. Belane

Measurement of N2 fixation in 30 cowpea (Vigna unguiculata L. Walp.) genotypes under field conditions in Ghana, using the 15N natural abundance technique

In 2005 and 2006, 30 and 15 cowpea genotypes were respectively evaluated for plant growth and symbiotic performance at Manga in Northern Ghana, in order to identify N2-fixing potential of these cowpea genotypes as source of N for cropping systems. The results showed differences in biomass production by the 30 or 15 cowpea genotypes. In 2005, cultivars Fahari, Mchanganyiko, IT97K-499-39, IT93K-2045-29 and IT84S-2246 produced the most shoot biomass, while Apagbaala, Brown Eye, ITH98-46, Vita 7 and Iron Grey produced the least. Of the 15 genotypes tested in 2006, cv. TVu11424 produced the largest amount of biomass, and ITH98-46, the least. Isotopic analysis of15N in plant parts also revealed significant differences in δ15N of the cowpea genotypes studied. As a result, the percent N derived from fixation (% Ndfa) also differed among the cowpea genotypes tested in 2005, with only 5 out of the 30 cultivars obtaining over 50% of their N from symbiotic fixation. Whether expressed as mg N.plant−1 or kg N.ha−1, the levels of N2 fixation by the cowpea genotypes varied considerably during 2005 and 2006, with values of N contribution ranging from 14.1 kg N.ha−1 by cv. TVu1509 to 157.0 kg N.ha−1 by IT84S-2246 in 2005. The amounts of N-fixed in 2006 ranged from 16.7 kg N.ha−1 by cv. ITH98-46 to 171.2 kg N.ha−1 by TVu11424, clearly indicating genotypic differences in symbiotic N yield. Re-evaluating 15 out of the 30 cowpea genotypes for N2 fixation in 2006, revealed higher % Ndfa values (>50%) in all (15 cowpea genotypes) relative to those tested in 2005, indicating greater dependence on N2 fixation for their N nutrition even though, the actual amounts of fixed-N were lower in 2006. This was due, in part, to reduced plant biomass as a result of very late sampling in 2006, close to physiological maturity (72 DAP in 2006 vs. 46 DAP in 2005) when considerable leaf matter was lost. The amount of N-fixed in 2006 can therefore be considered as being under-estimated.

Nitrogen fixation and symbiosis-induced accumulation of mineral nutrients by cowpea (Vigna unguiculata L. Walp.)

Abstract. Little information currently exists on the relationship between rhizobial symbiosis and mineral accumulation in nodulated legumes. The aim of this study was to measure fixed nitrogen (N) in whole plants and in young fully expanded trifoliate leaves of cowpea genotypes, and to relate this to mineral accumulation in the leaves. The data revealed marked differences between high and low N2-fixing genotypes, with the former consistently showing greater %N, plant or leaf total N, and amount of N fixed compared with the latter. There was a 2.0–3.8-fold difference in amount of N fixed at whole-plant level between high- and low-fixing cowpea genotypes at Taung, South Africa, and 2.4–4.0-fold at Manga, Ghana. Furthermore, the genotypes with high N2 fixation consistently exhibited greater concentration and content of minerals (e.g. P, K, Mg, S, Na, Fe, Cu, Zn, Mn and B) in their trifoliate leaves, whereas those that recorded low N2 fixation accumulated lesser amounts of mineral nutrients in leaves. In a nodulation assay, we found that rhizobial isolates TUT53b2vu and TUT33b4vu, which exhibited higher symbiotic efficiency (measured here as nodule number, nodule fresh weight, and plant dry matter yield), also elicited greater mineral accumulation in cowpea shoots, while strains with low N2-fixing ability induced limited mineral accumulation. These results, together with a correlation analysis, show that, at least in nodulated cowpea, there is a strong relationship between N2-fixing efficiency and mineral accumulation, two traits that could be exploited in breeding programs for improved human nutrition and health.