Localized and internal effect of nitrate on symbiotic dinitrogen fixation

Abstract

Alfalfa (Medicago sativa L.) is a deeply rooted perennial legume which, under field conditions, may be exposed to varying NO3− concentrations with depth. Our objective was to characterize the effect of localized (deep vs shallow) exposure of alfalfa root systems to NO3− on symbiotic N2 fixation and NO3−‐N uptake. Cuttings of a single alfalfa plant were grown in vertical split root systems in a controlled environment chamber. The split root system was a rigid acrylic tube (5 cm diam. by 60 cm long) filled with silica sand and divided into upper and lower sections at the 30‐cm depth by a 5‐mm‐thick wax layer. Roots penetrated the wax layer, but mixing of nutrient solutions between the sections was prevented. Nodulation was restricted to the upper section. The plants were subjected for 10 days to the following treatments: both sections of the split root system received nutrient solution containing either 0.5, 5.25, or 10 mM NO3−; the upper section received 0.5 mM NO3− while the lower section received 10 mM NO3−; or the upper section received 10 mM NO3− while the lower section received 0.5 mM NO3−. Increasing supply of NO3− in the nutrient solution to both sections resulted in higher NO3−‐N uptake, lower nodule mass and lower specific nitrogenase activity. Although NO3−‐N uptake did not differ, plants exposed to 10 mM NO3− for 10 days in the upper, nodulated section of the root system had a 20% lower nodule mass than plants exposed to the same NO3− concentration in the lower, non‐nodulated section of the root system. Specific nitrogenase activity was not different between these two treatments. Therefore, we conclude that: (1) nodule mass was dependent on two factors, the amount of NO3−‐N taken up and the concentration of NO3− within the nodulated root zone; and (2) specific nitrogenase activity was little affected by the concentration of NO3− surrounding the nodules, but was strongly inhibited by NO3−‐N taken up.