Low concentrations of ammonium inhibit specific nodulation (nodule number g-1 root DW) in soybean (Glycine max (L.) Merr.)

摘要

A study was conducted to determine the immediate and residual effects of low concentrations of NH4+ on the nodulation and N2 fixation of soyabeans in sand culture. Soyabeans cv. Maple Ridge were exposed to 0.0, 0.5, 1.0 or 2.0 mM 15N-labelled (NH4)2SO4 for 28 days after inoculation (DAI) with Bradyrhizobium japonicum. From 29 to 56 DAI plants were grown on NH4+-free nutrient solution, and were harvested 7, 14, 21, 28 and 56 DAI. Root, shoot and nodule dry weight (DW), total N content, nodule countsand 15N enrichment of plant composites were calculated. Nitrogenase activity was measured by gas exchange at 28 DAI. The plants in the control treatment exhibited consistently lower relative growth rates during the first 28 DAI than those with NH4+ applied to the sand. Plant growth was less at 2.0 mM NH4+ compared to growth at 0.5 and 1.0 mM NH4+. At 28 DAI, plants exposed to 0.5 and 1.0 mM NH4+ exhibited significantly more nodules/plant and larger individual nodules than either the NH4+-free controlsor the 2.0 mM NH4+-supplied plants. However, at 28 DAI specific nodulation (nodule number g-1 root DW) and specific nitrogenase activity were on average approximately 286% and 60% higher in the control plants, respectively, compared with treated plants.In addition, at 28 DAI specific nodule DW (nodule DW g-1)> root DW) was 17, 44 and 53% higher in control plants than plants that had been exposed to 0.5, 1.0 and 2.0 mM NH4+, respectively. At 56 DAI, after an additional 4 weeks of NH4+-free nutrition, plants which had previously received 0.5 and 1.0 mM NH4+ application maintained the highest plant DW and N contents. It is concluded that NH4+ had a negative effect on the nodulation process in the soyabean/Bradyrhizobium symbiosis. Despite this negative effect on specific nodulation, 0.5 and 1.0 mM NH4+ resulted in higher whole plant nodulation and N2 fixation due to a compensating, positive, effect on overall plant growth. When NH4+ was removed from all treatments, plants moved towards a consistent level of nodule DW relative to root DW.