Rapid N transport to pods and seeds in N-deficient soybean plants

Non-nodulated soybean (Glycine max (L,) Merr,) plants were cultivated hydroponically under N-sufficient (5 mM NaNO3) or N-deficient (0.5 mM NaNO3) conditions. N-13- Or N-15-labelled nitrate was fed to the cut end of the stems, and the accumulation of nitrate-derived N in the pods, nodes and stems was compared. Real-time images of N-13 distribution in stems, petioles and pods were obtained using a Positron Emitting Tracer Imaging System for a period of 40 min. The results indicated that the radioactivity in the pods of N-deficient plants was about 10 times higher than that of N-sufficient plants, although radioactivity in the stems and nodes of N-deficient versus N-sufficient plants was not different. A similar result was obtained by supplying (NO3-)-N-15 to cut soybean shoots for 1 h, The fact that the N translocation into the pods from NO3- fed to the stem base was much faster in N-deficient plants may be due to the strong sink activity of the pods in N-deficient plants. Alternatively, the redistribution of N from the leaves to the pods via the phloem may be accelerated in N-deficient plants. The temporal accumulation of (NO3-)-N-13 in nodes was suggested in both N-sufficient and N-deficient plants. In one (NO3-)-N-13 pulse-chase experiment, radioactivity in the stem declined rapidly after transferring the shoot from the (NO3-)-N-13 solution to non-labelled NO3; in contrast, the radioactivity in the node declined minimally during the same time period.