Nitrate reduction in roots and shoots and exchange of reduced N between organs were quantitatively estimated in intact 13-d-old seedlings of two-row barley (Hordeum vulgareL. cv. Daisengold) using the15N-incorporation model (A. Gojon et al. (1986) Plant Physiol.82, 254–260), except that NH+4was replaced by NO-2. N-depleted seedlings were exposed to media containing both nitrate (1.8 mM) and nitrite (0.2 mM) under a light-dark cycle of 12:12 h at 20°C; the media contained different amounts of15N labeling. Experiments were started either immediately after the beginning (expt. 1) or immediately prior to the end (expt. 2) of the light period, and plants were sampled subsequently at each light-dark transition throughout 36 h. The plants effectively utilized15NO-3and accumulated it as reduced15N, predominantly in the shoots. Accumulation of reduced15N in both experiments was nearly the same at the end of the experiment but the accumulation pattern in roots and shoots during each 12-h period differed greatly depending on time and the light conditions. In expt. 1, the roots accounted for 31 (light), 58 (dark), and 9 (light) of nitrate reduction by the whole plants, while in expt. 2 the contributions of the root were 82 (dark), 20 (light), and 29 (dark), during each of the three 12-h periods. Xylem transport of nitrate drastically decreased in the dark, but that of reduced N rather increased. The downward translocation of reduced15N increased while nitrate reduction in the root decreased, whereas upward translocation decreased while nitrate reduction in the shoot increased. We conclude that the cycling of reduced N through the plant is important for N feeding of each organ, and that the transport system of reduced N by way of xylem and phloem, as well as nitrate reduction by root and shoot, can be modulated in response to the relative magnitude of reduced-N demands by the root and shoot, with the one or the other predominating under different circumstanc
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