Symbiotic N2(C2H2) Fixation by Bean 1

The response of bean ( Phaseolus vulgaris L.) to N fertilization under field conditions indicates different Rhizobia ‐cultivar relationships or symbiotic N 2 ‐fixation limitations due to the cultivars. We have used the acetylene reduction method to determine the relative seasonal N 2 (C 2 H 2 ) fixation of several field‐grown bean cultivars. The relative nitrogenase activity was estimated from 8‐ ✕ 15‐cm soil cores taken around the main root of a decapitated plant. Activities rapidly increased from the three‐node vegetative (V3) to early pod‐filling (R3‐R4) growth stages, thereafter decreasing to zero at physiological maturity (R9). Accumulated daily activity totals showed a five‐ to sixfold difference in seasonal N 2 (C 2 H 2 ) fixation between cultivars, which was significantly related to the average nodule weight and to the plant dry weight near physiological maturity. However, cultivars with similar plant dry weights had a two‐ to threefold difference in relative N 2 (C 2 H 2 ) fixation. Seed yields and total‐N uptakes were also positively related. These observations indicate that it may be possible to increase both the symbiotic N 2 fixation and seed yields through plant breeding.