Systemic and local effects of long-term application of nitrate on nodule growth and N2 fixation in soybean (Glycine max [L.] Merr.)

Abstract

The effect of nitrate placement on the nodule number, nodule growth, and N2 fixation activity in soybean plant was elucidated by using a two-layered pot system separating the upper roots grown in vermiculite medium from the lower roots cultured in hydroponics. Four treatments were imposed i.e. 0/0, 0/5, 5/0, and 5/5, with the 0 or 5 mM nitrate treatment in the upper pot / lower pot, respectively. The plants were harvested at the flowering stage and initial pod setting stage. Systemic and local inhibition on nodule number did not occur in the upper nodules in vermiculite. On the other hand, systemic inhibition on the nodule growth and N2 fixation activity in the upper pot was apparent at pod setting stage. In the lower pot where the nodules were in direct contact with 5 mM nitrate, inhibition on nodule number, nodule size, and N2 fixation activity were conspicuous. Systemic inhibition on the number and dry weight of lower nodules was also observed. Root growth was promoted with the root part in direct contact with 5 mM nitrate, irrespective of upper or lower roots. Nitrate accumulation was observed only in the part of roots and nodules in direct contact with 5 mM nitrate. The results indicated that systemic nitrate inhibition occurred both on N2 fixation activity and on nodule growth but not on the upper nodule number after longterm application of 5 mM nitrate. Milder inhibition on upper nodules compared with the lower nodules may be due to the rapid depletion of nitrate from the vermiculite medium compared with the culture solution in the lower pot.