Nitrate reduction in root and shoot and exchange of reduced nitrogen between organs in two-row barley seedlings under light-dark cycles

✍ Scribed by Yoshikiyo Oji; Yuto Otani; Yuki Hosomi; Nariaki Wakiuchi; Hitoichi Shiga

• Publisher: Springer-Verlag
• Year: 1989
• Tongue: English
• Weight: 1013 KB
• Volume: 179
• Category: Article
• ISSN: 0032-0935
• DOI: 10.1007/bf00391081

✦ Synopsis

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 vulgare L. cv. Daisengold) using the l SN-incorporation model (A. Gojon et al. (1986) Plant Physiol. 82, 254-260), except that NH2 was replaced by NO 2. N-depleted seedlings were exposed to media containing both nitrate (1.8 mM) and nitrite (0.2 raM) under a lightdark cycle of 12:12 h at 20 ~ C; the media contained different amounts of 15N labeling. Experiments were started either immediately after the beginning (expt. I) 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 utilized 15NO 3 and accumulated it as reduced ~SN, predominantly in the shoots. Accumulation of reduced 15N 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 reduced ~SN increased while nitrate reduction in the root decreased, whereas upward translocation de-Symbols: Anl=accumulation of reduced 15N from 15NO 3 in 14NO3-fed roots of divided root system; Ar=accumulation in root of reduced 15N from 15NO~; As=accumulation in shoot of reduced lSN from 15NO~; Rr=aSNO~ reduction in root; Rs = ~5NO3 reduction in shoot; Tp=translocation to root of shoot-reduced 15N from lsNO~-in phloem; Tx= translocation to shoot of root-reduced 1 s N from 15NO 3 in xylem creased 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 circumstances.