For protection and amendment of environment by plant, the response of soybean to elevated CO_2 concentration was examined in terms of translocation of photosynthate and nitrogenous assimilates. Soybean (Glycine max cv. Tamahomare or Greenhormer) was exposed to CO_2 concentration (35 and 100 Pa CO_2) for 17-21 days at the grain filling stages under controlled glasshouse conditions. In the experiment 1, nitrate concentration in the culture media was changed, and in the experiment 2, pod removal treatment was also applied.
Experiment 1, Effect of CO_2 concentration and nitrate levels on nitrate reduction, distribution of nitrogenous assimilate : (1) While CO_2 enrichment significantly increased reduced-N amount in the whole plant under the presence of nitrate in the culture media, such effect was not observed without application of nitrate. (2) Nitrate reductase activity in leaves increased after imposed to elevated CO_2 concentration. (3) More ^<15>N fed from a terminal leaflet of an upper leaf was exported by CO_2 enrichment. Experiment 2, Effect of CO_2 concentration and pod removal on partitioning of ^<13>C and ^<15>N : (1) The elevated CO_2 concentration led to enhancement of plant growth. (2) The elevated CO_2 concentration decreased export of both ^<13>C and ^<15>N, and the decrease was more pronounced in the latter. (3) While ^<15>N atom % excess and concentration in various amino acids in leaf petioles and stem was not affected by CO_2 enrichment, these values in asparagine as well as proline were increased by pod removal. This suggests that some nitogenous compounds are assimilated in petioles and stem when sink activity is limited. The current study suggest that CO_2 enrichment affects not only carbon metabolism but also nitrogen metabolism in plant.