Nitrogen Assimilation in Barley (Hordeum vulgare L. cv. Mazurka) in Response to Nitrate and Ammonium Nutrition

Barley plants (Hordeum vulgare L. cv. Mazurka) were grown in aerated solution cultures with 2 mM or 8 mM inorganic nitrogen supplied as nitrate alone, ammonium alone or 1:1 nitrate+ammonium. Activities of the principal inorganic nitrogen assimilatory enzymes and nitrogen transport were measured. Activities of nitrate and nitrite reductases, glutamine synthetase and glutamate synthase were greater in leaves than in roots but glutamate dehydrogenase was most active in roots. Only nitrate and nitrite reductases changed notably (4–10 times) in response to the different nitrogen treatments. Nitrate reductase appeared to be rate-limiting for nitrate assimilation to glutamate in roots and also in leaves, where its total in vitro activity was closely related to nitrate flux in the xylem sap and was slightly in excess of that needed to reduce the transported nitrate. Xylem nitrate concentration was 13 times greater than that in the nutrient solution. Ammonium nitrogen was assimilated almost completely in the roots and the small amount released into the xylem sap was similar for the nitrate and the ammonium treatments. The presence of ammonium in the nutrient decreased both export of nitrate to the xylem and its accumulation in leaves and roots. Nitrate was stored in stem bases and was released to the xylem and thence to the leaves during nitrogen starvation. In these experiments, ammonium was assimilated principally in the roots and nitrate in the leaves. Any advantage of this division of function may depend partly on total conversion of inorganic nitrogen to amino acids when nitrate and ammonium are given in optimal concentrations.