Efficiency of Nitrogen Assimilation by N2-Fixing and Nitrate-Grown Soybean Plants (Glycine max [L.] Merr.)

Publisher Website
Google Scholar
Abstract
Nodulated [with Rhizobium japonicum] and non-nodulated (not inoculated) soybeans (G. max [L.] Merr. cv. Wells) were grown in controlled environments with N2 or nonlimiting levels of NO3-, respectively, serving as sole source of N. The efficiency of the N2-fixing plants was compared with that of the nitrate-fixing plants was compared with that of the nitrate-supplied plants on the basis of plant age and plant size. Efficiency evaluations of the plants were expressed as the ratio of moles of C respired by the moles of C respired by the whole plant to the moles of N incorporated into plant material. Continuous 24-h CO2 exchange measurements on shoot and root systems made at the beginning of flowering (28 days after planting) indicated that N2-fixing plants respired 8.28 mol of C/mol of N, fixed from N, while nitrate-supplied plants respired only 4.99 mol of C/mol of nitrate reduced. Twenty-one-day-old nitrate-supplied plants were even more efficient, respiring only 3.18 mol of C/mol of nitrate reduced. The decreased efficiency of the N2-fixing plants was not due to plant size since, on a dry wt basis, the 28-day-old N2-fixing plants were intermediate between the 28- and 21-day-old nitrate-supplied plants. The calculated efficiencies were predominantly a reflection of root-system respiration. N2-fixing plants lost 25% of their daily net photosynthetic input of C through root-system respiration, compared with 16% for 28-day-old nitrate-supplied plants nitrate-suppled plants. Shoot respiration was similar for all 3 plant groups, varying between 7.9% and 9.0% of the apparent photosynthetate. The increased respiratory loss by the roots of the N2-fixing plants was not compensated for by increased net photosynthetic effectiveness. Canopy photosynthesis expressed on a leaf area basis was similar for 28-day-old N2-fixing plants (15.5 mg CO2 dm2/h) and 21-day-old nitrate-supplied plants (14.5 mgCO2 dm2/h). Both were similar in total canopy leaf area. The larger nitrate-supplied plants (28-day-old) had lower photosynthetic rates (12.5 mg CO2 dm2/h), presumably due to self-shading of the leaves. During the early stages of plant development, dependence solely on N2-fixation is an expensive process compared to nitrate reduction in nitrate-supplied plants, since the N2-fixing plants retained 8-12% less of their photosynthate as dry matter.