No abstract

The effects of different forms and concentrations of N in the rooting medium on the CO2/H2O gas exchange of leaves of the pedunculate oak (Quercus robur L.) were investigated. Two‐year‐old seedlings were grown in nutrient solutions containing low (1.8 mM) or high (4.8 mM) concentrations of NH4+, 3.6 mM NO3−, or both NH4+ and NO3− (1.8 mM + 1.8 mM). In various sets of plants subjected to these N treatments, the following parameters were determined: biomasses of leaves and fine roots, leaf area‐related net photosynthesis at light saturation (A) and leaf conductance (g), foliar concentrations of chlorophylls, N, Ca2+, Mg2+ and K+ and the ash alkalinity of the leaves (as a measure of the carboxylate content). In all treatments, the leaves were equally well supplied with nutrients. Oaks grown in high NH4+ concentrations produced significantly smaller leaf and root biomasses. Compared to oaks cultivated with both N forms or with low NH4+ concentration, oaks grown with high NH4+ supply showed lower values of A and g, but no significant differences in ash alkalinity and leaf area‐related chlorophyll concentrations. Oaks fed with NO3− as the only N form had an intermediate biomass production, but low values of A and g. The time courses of A in the different treatments closely followed the patterns of g. In all N treatments, the same linear relationship was found between A and g, indicating that, within a rather wide range, the variation in the form and amount of supplied N does not affect the instantaneous water use efficiency of young pedunculate oaks.