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

Abstract

An experiment was conducted to examine the effect of CO2 enrichment on the nitrate uptake, nitrate reduction activity, and translocation of assimilated-N from leaves at varying levels of nitrogen nutrition in soybean using 15N tracer technique. CO2 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 CO2 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 CO2 enrichment and high nitrogen levels. 15N tracer studies indicated that the increase in the amount of reduced-N by CO2 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 15NO3-N through a terminal leaflet of an upper trifoliated leaf under depodding and/or CO2 enrichment conditions. The export rate of 15N from the terminal leaflet to other plant parts decreased by depodding, but it increased by CO2 enrichment. CO2 enrichment increased the percentage of plant 15N in the stem and / or pods. Depodding increased the percentage of plant 15N in the leaf and stem. The results suggested that the increase in the leaf nitrate reduction activity by CO2 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.