Nitrogenase activity, protein content and yield in soybean

Abstract Nitrogen (N) availability is a major parameter determining productivity in soybean. Accordingly, genetically improved N 2 fixation may increase soybean productivity. The aim of the investigations reported here was to analyze the relationships between nodulation parameters, nitrogenase activity and seed yield and seed protein content in two types of soils which were moderately rich in mineral N. Genotypic variation in nitrogenase activity obtained through divergent selection and based on acetylene reduction was studied in controlled environment chambers, under temperature and mineral N conditions that were favourable to the maximum expression of symbiosis. The seed yield of the genotypes was evaluated under normal density on two soils. In both soils, seed yields were significantly correlated with nitrogenase activity ( r =0.60** and r = 0.45**, significant at p = 0.01). The effect of genetic variation on nitrogenase activity on yield was greater in the soil that was poorer in N. The correlations between seed yield and nodule volume were more significant ( p ≤ 0.05) than between seed yield and nodule number or nodule weight per whole plant fresh weight. Nitrogenase activity improvement affected seed yield more than seed protein content. However, the correlation between seed protein content and seed yield was low, and simultaneous selection for the two traits should be effective. The nodulation parameters alone, although they were more readily measured than nitrogenase activity, were less efficient for yield prediction.