B value of -1.759 for cowpea shoots was used in calculating %Ndfa [3]. At Wa, sorghum and maize crops planted at the same time and growing on an adjacent field (as monocrops)

were used as reference plants; they had an average δ15N value of +7.12‰. For Taung, an Eragrostis sp. and an unidentified herbaceous weed growing in the field with cowpea were analysed as the reference plants. Their average δ15N value of +5.03‰ was used to estimate %Ndfa in cowpea. While the cowpea plants were raised on ridges, the Eragrostis sp. and the herbaceous Rapamycin ic50 weed sampled as reference plants, were growing on the ploughed unridged area around the experimental plots. The amount of N-fixed was calculated as [16]: The amount of N-fixed in each cowpea shoot was divided by the plant’s nodule mass and age to obtain the specific nodule activity, expressed as μg N – fixed.mg nod DM-1.d-1 [17]. PLX3397 cost Nodule harvest and DNA extraction Two hundred and seventy (270) nodules were harvested from the 9 cowpea genotypes planted in Ghana, South Africa and Botswana for DNA extraction. The nodules harvested were generally representative of the total

nodule pool per plant, and were all effective in N2 fixation based on the pink internal colour (i.e. presence of AC220 leghaemoglobin). Total DNA (plant and microbial) was extracted from each of the 270 nodules, using the method described by [18]. To sterilise the nodules, they were

rehydrated in sterile distilled water, immersed in 3.3% w/v Ca(OCl)2 for 3 min, rinsed in sterile water, followed by soaking in 96% ethanol and rinsed twice in sterile distilled water. Each nodule (about 4 mg in weight) 4��8C was crushed in 100 μL TES/sucrose buffer (20 mM Tris-HCl, pH 8.0, 50 mM EDTA di-sodium, pH 8.0, 8% p/v) in a sterilised 1.5 mL Eppendorf tube (using a plastic pestle sterilised in absolute ethanol). Lyzozyme (4 mg/μL) was added to the crushed nodule macerate, vortexed for 20 s and incubated at 37°C for 15 min. A solution of GES (0.05 mM guanidine thiocyanate, 0.1 M EDTA di-sodium, pH 8.0, 1% N-Lauroylsarcosine sodium salt) was added to the lysed nodule homogenate, vortexed again for 20 s and incubated at 65°C for 15 min. The GES-cell lysate mixture was centrifuged at 10000 × g in a 3K15 Model Sigma centrifuge for 15 min at 4°C and the supernatant transferred into a new tube. Total DNA was pelleted by centrifuging at 4°C at 10000 × g for 15 min. The supernatant was discarded, and 0.5 mL 95% ethanol added to the pellet and centrifuged again at 4°C at 10000 × g for 15 min. This was repeated twice.