Development of the nodulated soybean plant after flooding of the root system with different sources of nitrogen

Abstract

Flooding leads to hypoxia, a stress to which symbiotic N2 fixation is especially sensitive. The response of fully nodulated soybean plants to a 21-day period of flooding was studied by measurements of growth parameters and xylem transport of organic nitrogenous components to the shoot, in the presence and absence of NO3- and NH4+ in the medium. Flooding was found to seriously impair N2 fixation, irrespective of the N source, as indicated by strongly reduced xylem ureide levels. In the absence of a source of N, growth was strongly reduced during flooding while accumulation of N in the shoot was virtually abolished. Flooding in the presence of 5 mM NO3- or NH4+ led to the accumulation of total N in the shoot but only NO3- promoted increases in total dry matter, plant height and leaf area above that found in the absence of N. The accumulation of N, however, was lower than that of the non-flooded control for both NO3- and NH4+. The increases in total dry matter, plant height and leaf area with NO3- was as high as those of the non-flooded control. These data clearly show the beneficial effects of NO3- during a prolonged period of flooding of the nodulated root system of soybean.