Nitrate reduction in leaves and roots of young pedunculate oaks (Quercus robur) growing on different nitrate concentrations

Against the background of high rates of nitrogen (N) input into forest ecosystems and, in part, high nitrate (NO3-) concentrations in the soil solutions, NO3- reduction activity and N accumulation in leaves and roots of young pedunculate oaks (Quercus robur) were investigated. Seedlings with unrestricted root growth, and 2-year-old saplings with cut root-stocks, were grown hydroponically at different forms and concentrations of N. The nitrate reductase activity (NRA) of the leaves and fine roots was measured in vivo without addition (NRA(H2O)) or with addition of exogenous NO3- (NRA(H2O)) to the incubation assay. The amounts of reduced NO3- as calculated with the NRA(H2O) and NRA(KNO3) were compared with the uptake of (NO3-)-N-15 and with root-to-shoot translocation of NO3- as determined by NO3- concentrations of the xylem sap and transpiration rates. Compared with the NRA(H2O), the NRA(KNO3) was higher by a factor of approximately 10 in the current year's fine roots, and by a factor of about 60 in the leaves. In only one case did increased NO3- concentrations of the nutrient solution result in an increase in NRA. In some cases, NRA was diminished in the presence of ammonium (NH4+) in the root medium. The quantities of reduced NO3- as calculated on the basis of NRA(H2O), agreed with the amounts of N-15 accumulated in roots and leaves, and with the amounts of NO3- translocated from the roots to the shoots. The contribution of the leaves to the total plant's NO3- reduction as computed on the basis of NRA(H2O), was 1-17% in the seedlings, but up to 86% in the saplings; here, it correlated significantly with the leaf/root ratios on a fresh-weight basis. Compared with the leaf/root ratios, the form and concentration of the supplied N had a much lower impact on the share of the leaves in NO3- reduction; and did not affect the foliar N concentrations. In the leaves as well as in the roots, the concentrations of soluble NO3- were Very low (< 0.5 mg NO3--N g dry weight(-1)). The results show that young pedunculate oaks have a low affinity for NO3--N, even in the case of high NO3- supply. (C) 2000 Elsevier Science B.V. All rights reserved.