EFFECTS OF DIFFERENT FORMS OF NITROGEN SUPPLY ON THE GAS-EXCHANGE OF YOUNG PEDUNCULATE OAKS (QUERCUS-ROBUR L)

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 (Q uercus robur L.) were investigated. Two-year-old seedlings were grown in nutrient solutions containing low (1.8 mM) or high (4.8 mM) concent rations 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 follow ing parameters were determined: biomasses of Leaves and fine roots,lea f area-related net photosynthesis at light saturation (A) and leaf con ductance (g), foliar concentrations of chlorophylls, N, Ca2+, Mg2+ and K+ and the ash alkalinity of the leaves (as a measure of the carboxyl ate content). In all treatments, the leaves were equally well supplied with nutrients. Oaks grown in high NH4+ concentrations produced signi ficantly 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 differ ences in ash alkalinity and leaf area-related chlorophyll concentratio ns. 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 di fferent treatments closely followed the patterns of g. In all N treatm ents, the same linear relationship was found between A and g. indicati ng that, within a rather wide range, the variation in the form and amo unt of supplied N does not affect the instantaneous water use efficien cy of young pedunculate oaks.