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

Abstract

Nodulated and non-nodulated (not inoculated) soybeans (Glyjine max IL.I Merr. cv Wells) were grown in controlled environments with N2 or nonlimiting levels of N03-, respectively, serving as sole source of nitrogen.The efficiency of the N2-fixing plants was compared with that of the nitrate-supplied plants on the basis of both plant age and plant size. Efficiency evaluations of the plants were expressed as the ratio of moles of carbon respired by the whole plant to the moles of nitrogen incorporated into plant material.Continuous To clarify the discrepancy in the efficiency values reported for soybeans, experiments were conducted to measure net carbon fixation and subsequent respiratory loss and incorporation into dry matter relative to the net nitrogen assimilated by whole soybean plants dependent on N2 fixation or nitrate reduction. A unique two-compartment plant chamber is described which permitted 24-h measurement of both shoot and root gas exchanges on the same plants without disruption of the normal diurnal rhythm or whole plant integrity. Seeds for germination were individually selected for soundness and uniformity. They were washed for 5 min in a solution of 5% (v/v) Clorox and immediately rinsed with a minimum of four changes of deionized H20 over a second 5-min period. Seeds were then soaked for 5 min in 0.1 mm CaSO4 (-0.5 ml seed-'). For the nodulated treatment, Rhizobium japonicum (USDA strain 31 1B7 1, grown axenically on slants and used at an approximate rate of 1 X 106 bacteria/seed) was included in the CaSO4 soak. The seeds were planted at a depth of 1.5 cm in moist sand which had previously been autoclaved to kill residual microorganisms. The CaSO4 soak solution (+ inoculum) was poured into the sand at time of planting. Germination pans were covered with foil and placed in the growth chamber under the conditions described. MATERIALS AND METHODS Plant

Effect of CO2Concentration, Temperature and N Fertilization on Biomass Production of Soybean Genotypes Differing in N Fixation Capacity

“…Whole plant energy costs for symbiotic N 2 fixation were found to be greater than for nitrate metabolism (Ryle et al, 1979;Finke et al, 1982). For example, Finke et al (1982) reported that 8.3 mol of C was respired per mol of N for N 2 fixation, while only 3.2 to 5.0 mol of C was respired per mol of N from nitrate assimilation, depending on the plant age. Thus, it is likely that a genotype with a higher capacity for N 2 fixation, such as Kanto 100, exhibits lower NUE than genotypes with a reduced capacity.…”

Nitrogen Utilization in the Supernodulating Soybean Variety “Sakukei 4” and Its Parental Varieties,“Enrei” and “Tamahomare”

“…The whole-plant energy cost for symbiotic N 2 fi xation is greater than that for NO 3 -assimilation (Yamaguchi, 1978;Rlye et al, 1979;Finke et al, 1982). Although Sakukei 4 had low N 2 fi xation activity per nodule weight and number, the respiratory loss of photosynthate should be greater because of the extremely large mass and number of nodules, and this greater loss presumably caused the lower NUED in Sakukei 4.…”

Genetic Gain × Management Interactions in Soybean: II. Nitrogen Utilization

“…Thus, soil supply of N was abundant (77% contribution to total yield) in Wisconsin during 2011 and additional N was not needed. Previous research has found the process of BNF to be more energetically expensive in comparison to N assimilation (Finke et al, 1982;Minchin et al, 1981;Ryle et al, 1979;Sinclair, 2004). Therefore, soybean grown in conditions with a large supply of N from soil, residual sources, and/or fertilizer will allow more photosynthate to be available for dry matter production and/or protein accumulation rather than expenditure on BNF.…”