Effect of pod removal on absorption and reduction of nitrate in soybean

The effect of the sink activity for nitrogen (N) on the uptake, translocation, and reduction of nitrate has not been full elucidated. An experiment was conducted to examine the effect of pod removal during the pod filling period on the accumulation of nitrate and reduced-N in soybean (Glycine max L. Merr. cv. Tamahomare) grown under field and hydroponic conditions. Pod removal led to a significant decrease in the leaf nitrate reductase activity 5010 d after treatment (DAT). In contrast partial defoliation increased the nitrate reductase activity at 5 DAT. These results indicate that the nitrate reductase activity is regulated by the e/sink activity of pods for N at the grain filling stage. The amount of total N (reduced-N plus nitrate-N) in a whole plant decreased by pod removal, suggesting that nitrate uptake is also controlled by the sink activity of pods. Pod removal increased the amount of nitrate-N and did not affect significantly the carbohydrate concentration in the leaves, indicating that nitrate and energy supply might not be involved in the inhibition of nitrate reductase activity. Pod removal decreased the amount of reduced-N significantly at 10 DAT, whereas it increased the amount of nitrate-N at 5 DAT. These results suggest that when the sink activity is reduced by depodding, nitrate reduction is more markedly impaired than nitrate uptake by roots, and its subsequent transport from roots to shoot. From these results, it is assumed that the uptake and reduction of nitrate are controlled by the sink activity of pods for N in a soybean cultivar