Early nitrogen supplementation stimulates the nodulation and growth of common bean plants inoculated with rhizobium

ABSTRACT

The initial development of the common bean plants (Phaseolus vulgaris L.) that rely on symbiotic nitrogen (N) can be limited when active N₂ fixation is delayed. Thus, adequate plant growth with rhizobium inoculation could require supplemental mineral N, which in turn may inhibit the symbiosis. Five experiments were performed using hydroponics to identify the initiation of nodulation and nitrogenase activity in common bean cultivars, and effects of mineral N addition on plant nodulation and growth. Three experiments evaluated the initial growth of five inoculated bean cultivars in the absence or presence of mineral N, while two experiments evaluated the effect of mineral N addition until the beginning of the reproductive stage. The first root nodules appeared 10 days after transplanting (DAT), while nitrogenase activity was initiated at 11 DAT. Large seed cultivars had lower levels of initial nodulation and nitrogenase activity than those of small seeds. Inoculated plants showed limited shoot growth that lasted until 21-25 DAT relative to the inoculated plants that received mineral N. Moreover, adding mineral N greatly reduced the nodule mass more than the nodule number, and caused an even greater reduction in nitrogenase activity. Mineral N that was applied until 15 DAT enhanced nodulation and nitrogenase activity without limiting shoot growth relative to the plants that received N throughout their growth. In contrast, plants that received N after 15 DAT had lower levels of nodule mass and nitrogenase activity than those of the plants that were only inoculated. Hence, these results indicate that symbiotic N was not sufficient for an adequate initial growth of the common beans. Therefore, some supplemental N is necessary, which should be added at sowing to stimulate plant growth without inhibiting further nodulation and N fixation.

Keywords:
Phaseolus vulgaris; rhizobia; nitrogen fixation; nitrogen fertilization; inoculation