AN 15N INVESTIGATION INTO NITROGEN ASSIMILATION IN HYDROPONICALLY‐GROWN BARLEY (HORDEUM VULGARE L. CV. CLIPPER) IN RESPONSE TO NITRATE, AMMONIUM AND MIXED NITRATE AND AMMONIUM NUTRITION

SUMMARY Barley plants (Hordeum vulgare L. cv. Clipper) were grown in pH‐controlled, aerated solution culture with 2 mM inorganic 14N supplied as nitrate alone, ammonium alone or 1 :1 nitrate+ ammonium. At the 20th day of growth the 14N feeding solutions were replaced by 15N feeding solutions and the plants grown in these media for 4 and 8 h before harvesting. In the nitrate‐fed plants at both harvests most of the absorbed and assimilated 16N was found in the shoot. Xylem sap analyses showed that 66% of the 16N supply to the shoot was as nitrate indicating that the shoot was the main organ of nitrate assimilation. In the ammonium‐fed plants there was considerable accumulation of 15N ammonium in the root, where the assimilatory pools saturated quickly with 16N and the highest content of organic 16N was found (in the 4 h harvest). Xylem sap analysis showed that 93% of the 15N exported from root to shoot was as organic N (mainly glutamine) with very little ammonium present, indicating that the root was the major organ of ammonium assimilation. The high 15N shoot content at the 8 h harvest suggested however, that the shoot was the major destination of the root assimilated N. In the mixed‐feed plants, combined root and shoot assimilation was evident, 29 % of the 15N in the xylem sap being in the form of nitrate, indicating a suppression of nitrate ion absorption by ammonium ions. The highest rate of 15N incorporation into organic N, (24.8 μg h−1per plant) occurred in the mixed N‐fed plants (the largest and more robust plants), followed by the ammonium‐fed plants (24.8 μg h−1per plant) (the smallest, least robust plants) and the nitrate‐fed plants (5.6 μg h−1per plant). The lower N assimilation by the nitrate‐fed plants is ascribed mainly to decreased uptake of the nitrate ion in comparison to the ammonium ion. It appears that, for maximum growth and N assimilation of hydroponically‐grown barley, a mixed nitrate‐ammonium N source is required. Nitrate is poorly absorbed but appears to be necessary for the optimum growth of ammonium fed plants by increasing shoot N assimilation or by providing additional electron accepting potential to the roots. The situation could be markedly different in soil grown plants where the ammonium ion is not so readily available.