Iniciação da nodulação em cultivares de feijoeiro

Abstract

The biological N2 fixation in common bean (Phaseolus vulgaris L.) is not efficient as in other grain legumes in part due to the asynchrony between the cotyledon abscission and the effective nitrogen fixation, which causes a nutritional stress early in the growth cycle. High levels of N in the soil can limit nodulation in common bean but appropriate yields under inoculation demand additional N fertilization. Five experiments were conducted in nutrient solution to evaluate the initiation of nodulation of bean cultivars and the effect of mineral N in this process. Additionally, two field experiments were conducted to evaluate the effect of time of N application on nodulation and grain yield of bean crop under inoculation with commercial rhizobia strains. In the experiments in nutrient solution, plants were harvested daily after the first nodule formation, and it were evaluated the acetylene reduction in the root system, plant dry mass and size of nodules by digital image analysis. In field experiments, the dry mass of shoots and nodules at full flowering and grain yield and yield components were evaluated. The initiation of nodulation occurred at 10 days after emergence (DAE), and of nitrogenase activity at 11 DAE, in the five bean cultivars. The early cultivars Radiante and Jalo Precoce showed nitrogenase activity lower than the cultivars Pontal and Valente, of normal cycle. The addition of N reduced nodulation in its initial phase in all cultivars, nullifying it at levels higher than 2 mmol of N. At 15 DAE, at the cotyledon fall, dry mass of shoots and roots of bean plants inoculated with rhizobia were lower as compared with inoculated plants receiving mineral N. In the field, the inoculation associated with N application covering treatments gave shoot dry mass at flowering and grain yield similar to those observed in the control N treatment. The results of the two field experiments suggest that the use of mineral N at planting can be replaced by rhizobia inoculation without reducing grain yield of common bean.