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

Non-nodulated soybean (Glycine max (L,) Merr,) plants were cultivated hydro ponically under N-sufficient (5 mM NaNO3) or N-deficient (0.5 mM NaNO3) con ditions. 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 wa s compared. Real-time images of N-13 distribution in stems, petioles and po ds were obtained using a Positron Emitting Tracer Imaging System for a peri od 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 plant s, although radioactivity in the stems and nodes of N-deficient versus N-su fficient plants was not different. A similar result was obtained by supplyi ng (NO3-)-N-15 to cut soybean shoots for 1 h, The fact that the N transloca tion into the pods from NO3- fed to the stem base was much faster in N-defi cient plants may be due to the strong sink activity of the pods in N-defici ent plants. Alternatively, the redistribution of N from the leaves to the p ods via the phloem may be accelerated in N-deficient plants. The temporal a ccumulation of (NO3-)-N-13 in nodes was suggested in both N-sufficient and N-deficient plants. In one (NO3-)-N-13 pulse-chase experiment, radioactivit y 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 n ode declined minimally during the same time period.