Symbiotic N2(C2H2) Fixation by Bean

The response of bean (Phaseolus vulgaris L.) to N fertilization under field conditions indicates different Rhizobia-cultivar relationships or symbiotic N?-fixation limitations due to the cultivars. We have used the acetylene reduction method to determine the relative seasonal N?(C?H?) fixation of several field-grown bean cultivars. The relative nitrogenase activity was estimated from 3- X 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?(C?H?) fixation between cultivars, which was significantly related to the average nodule weight and to the plant dry weight near physiological maturity. However, cultivars similar plant dry weights had a two- to threefold difference in relative N?(C?H?) fixation. Seed yields and total-N updates were also positively related. These observations indicate that it may be possible to increase both the symbiotic N? fixation and seed yields through plant breeding