Ammonium and nitrate uptake by Eucalyptus nitens

Low nitrogen availability commonly limits the growth of Eucalyptus miens (Deane and Maiden) Maiden plantations in south-eastern Australia. However, until this study was undertaken little was known of the nitrogen uptake and assimilation processes of temperate eucalypt species like E. nitens. In this study ammonium and nitrate uptake by young roots of solution cultured E. nitens were characterised under a variety of conditions using two different techniques: net uptake on a macro scale measured as depletion from solution, and net fluxes on a micro scale estimated from measurements of concentration gradients near root surfaces using microelectrodes. Datasets taken from the literature were used to both validate the use of the depletion method for nitrogen, and to find the most appropriate method to use for the estimation of kinetic parameters from depletion datasets. Ammonium and nitrate depletion from solution was characterised with respect to nitrogen source, pH, temperature, and N status. Ammonium uptake rates were consistently higher than nitrate uptake rates in all experiments. Uptake rates for both ammonium and nitrate were higher at pH 4 than at pH 6, and they were reduced to a similar extent with a decrease in temperature from 20°C to 10°C (Q10 values of 1.3 to 1.9). For ammonium uptake, there was evidence for rapid adaptation of uptake processes (within 24 hrs) to changing temperature. Nitrogen status, which was correlated with relative growth rate (RGR), had unclear effects on uptake characteristics and highlighted deficiencies of measuring uptake by long term (up to 10 hours) nutrient depletion. Ion selective electrodes were used in the MIFE (microelectrode ion flux estimation) technique to simultaneously measure ammonium, nitrate, and proton fluxes within the unstirred layer surrounding roots of E. nitens in solution culture. Measurements were within the region 20 to 50 mm from the tip of primary roots approximately 80 mm long. Within this region fluxes of ammonium, nitrate, and protons varied little, spatially or temporally. Under these conditions there was a consistent flux stoichiometry for ammonium: nitrate: and protons of 3.1: 1: -6.0. When ammonium and nitrate concentrations were both set at 100 µM there was no inhibition of nitrate uptake by ammonium, but at concentrations of ammonium and nitrate in the range 200-1000 µM there was an apparent suppression of nitrate uptake. There was evidence for two mechanisms of ammonium uptake. The mechanism at low concentrations (<200 µM) showed saturation kinetics with a K m of 15 µM whereas at high concentrations (200-1000µM) an approximately linear increase in uptake occurred. Excision of the shoot brought about a rapid and dramatic reduction in fluxes of ammonium, nitrate, and protons. The apparent preference shown here for ammonium over nitrate could be indicative of an adaptation by E. nitens to grow in cold, acidic, forest soils where ammonium is more readily available than nitrate. These results suggest that nitrogen uptake rates of E. nitens will be maximised if nitrogen is supplied predominantly in the ammonium form.