Elevated CO2 concentrations increase leaf nitrate reduction by strengthening sink activity in soybean plants

An experiment was conducted to examine the effect of CO 2 enrichment on the nitrate uptake, nitrate reduction activity, and translocation of assimilated-N from leaves at varying levels of nitrogen nutrition in soybean using 15 N tracer technique. CO 2 enrichment significantly increased the plant biomass, apparent leaf photosynthesis, sugar and starch contents of leaves, and reduced-N contents of the plant organs only when the plants were grown at high levels of nitrogen. A high supply of nitrogen enhanced plant growth and increased the reduced-N content of the plant organs, but its effect on the carbohydrate contents and photosynthetic rate were not significant. However, the combination of high CO 2 and high nitrogen levels led to an additive effect on all these parameters. The nitrate reductase activity increased temporarily for a short period of time by CO 2 enrichment and high nitrogen levels. 15 N tracer studies indicated that the increase in the amount of reduced-N by CO 2 enrichment was derived from nitrate-N and not from fixed-N of the plant. To examine the translocation of reduced-N from the leaf in more detail, another experiment was conducted by feeding the plants with 15 NO 3 -N through a terminal leaflet of an upper trifoliated leaf under depodding and/or CO 2 enrichment conditions. The export rate of 15 N from the terminal leaflet to other plant parts decreased by depodding, but it increased by CO 2 enrichment. CO 2 enrichment increased the percentage of plant 15 N in the stem and / or pods. Depodding increased the percentage of plant 15 N in the leaf and stem. The results suggested that the increase in the leaf nitrate reduction activity by CO 2 enrichment was due to the increase of the translocation of reduced-N from leaves through the strengthening of the sink activity of pods and / or stem for reduced-N.