Growth patterns of common bean cultivars affect the 'B' value required to quantify biological N2 fixation using the 15N natural abundance technique

The 'B' value, the N-15 abundance of plants depending completely on biological nitrogen fixation (BNF) for growth, is required for quantifying BNF using the N-15 natural abundance technique. This study aimed to investigate how common bean (Phaseolus vulgaris L.) cultivars with different growth patterns could affect the 'B' value calculated for the plant shoot. Two experiments were conducted in N-free solution. Experiment I had a factorial design with three cultivars and two rhizobium inoculants. Plants were harvested at pre-flowering stage. Experiment II was also factorial with two cultivars and two times of sampling: full flowering and mid-pod filling. Total N and delta N-15 analyses of different plant parts were carried out to estimate the 'B' value. There were differences between cultivars for N accumulation and N-15 abundance, but no difference between rhizobium inoculants. Results revealed differences in 'B' value between cultivars of small and large seeds, the growth stage being also relevant. The 'B' values of -2.08 and -1.34aEuro degrees could be used for BNF estimates in small-seeded cultivars at full flowering and mid-pod filling stages, respectively, while such values should change, respectively, to -1.73 and -1.03aEuro degrees for large-seed cultivars. The mean 'B' value in shoots was -1.90 and -1.20aEuro degrees at full flowering and mid-pod filling stages, respectively. There are variations in 'B' value between cultivars of different cycle length and seed sizes, which are associated with plant architecture, thus a 'B' for each growth stage of common bean plants is required.