Effect of CO2 enrichment on biomass production, photosynthesis, and sink activity in soybean [Glycine max] cv. bragg and its supernodulating mutant nts 1007

Soybean (Glycine max L. Merr.) cv. Bragg and its supernodulating mutant nts 1007 were grown in pots containing vermiculite with a N-free nutrient solution in order to examine the effect of elevated CO2 concentration (100+20 Pa CO2) on biomass production, photosynthesis, and biological nitrogen fixation. The whole plant weight increase in Bragg was higher than in the mutant at a high CO2 concentration. Apparent photosynthetic activities of the upper leaves in both Bragg and the mutant increased up to 14 d after treatment initiation by the CO2 enrichment and thereafter decreased to some extent. Both leaf area and leaf thickness of Bragg increased more than in nts 1007. With the elevated CO2 concentration, biological nitrogen fixation (BNF) also responded in the same manner as biomass production in both Bragg and nts 1007. The increase of BNF in Bragg was largely due to an increase in nodule weight. Starch contents in the leaves of both Bragg and the mutant increased significantly by CO2 enrichment, with a higher increase in Bragg than in its mutant. Sugar content in leaf differed only slightly in both Bragg and the mutant. N content in leaf decreased in both Bragg and its mutant, with the decrease being more pronounced in Bragg. However, in other plant parts (roots, stem, and petiole + pods), N content increased in the mutant while in Bragg, it decreased in the pod. N accumulation rate was higher in Bragg than in the mutant and increased more in Bragg than in the mutant by CO2 enrichment. The ureide content in leaf decreased in Bragg but increased in the mutant by elevated CO2 concentration. In the nodules,ureide content increased in both Bragg and the mutant by CO2 enrichment. Based on these results, it is suggested that in terms of biomass production and photosynthetic rate, Bragg responded more to elevated CO2 concentration than its mutant nts 1007