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Ammonium and nitrate uptake by Eucalyptus nitens


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Garnett, Trevor Paul 1996 , 'Ammonium and nitrate uptake by Eucalyptus nitens', PhD thesis, University of Tasmania.

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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.

Item Type: Thesis - PhD
Authors/Creators:Garnett, Trevor Paul
Keywords: Eucalyptus
Copyright Holders: The Author
Copyright Information:

Copyright 1996 [Published Date as found in Millennium record] the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1998. Includes bibliographical references. Contents: Estimating kinetic parameters for N uptake using the depletion method - Ammonium and nitrate uptake by roots of E. nitens - Nitrogen and proton fluxes at the surface of E. nitens roots

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