Please use this identifier to cite or link to this item: http://hdl.handle.net/2440/97381
Type: Thesis
Title: Nitrate and ammonium interactions in maize.
Author: George, Jessey
Issue Date: 2014
School/Discipline: School of Agriculture, Food and Wine
Abstract: Nitrogen (N) is one of the major mineral nutrients required by a plant for its growth and development. Nitrate (NO₃⁻) and ammonium (NH₄⁺) are the predominant forms of N available to plants in agricultural soils. Plants have the ability to absorb both these forms efficiently from the soil solutions. With soil solution concentrations of NH₄⁺ being much lower (on average 10%) than NO₃⁻, contribution of these small amounts of NH₄⁺ to the overall N budget of crop plants is often overlooked. This research focussed on the contribution of this NH₄⁺ in the nitrogen economy of maize plants. The study also investigated whether NH₄⁺ has any effect on uptake and utilization of other nutrients, and most importantly, NO₃⁻. Growth of maize inbred line B73 was increased when one-third of total nitrogen was supplied as NH₄⁺ with low NO₃⁻, but not for another inbred line Gaspe Flint. Further investigations on B73 found a 20% increase in plant growth when supplied with 10% NH₄⁺ along with sufficient NO₃⁻. Ammonium being a cheaper N source and the low energy and carbon skeleton requirement for its assimilation has contributed in increased shoot dry matter accumulation in these plants. A corresponding increase in total N, total free amino acids and sugars in the leaves of these plants were observed. A positive correlation was seen between transcript levels of putative high affinity NO₃⁻ and NH₄⁺ transporters. This together with an increased activity of N assimilatory enzymes suggested that small amounts of NH₄⁺ can increase the uptake and assimilation of N in these plants. 10% NH₄⁺ in the nutrient solution does not inhibit the NO₃⁻ uptake capacity in plants but when the concentration was increased to 50% there is a reduction in NO₃⁻ uptake capacity for plants growing in low N. This indicates that high concentration of NH₄⁺ limit the absorption of NO₃⁻ which is an important signalling molecule for various metabolic activities in plants. Reduction in NO₃⁻ uptake capacity of plants grown in 10% NH₄⁺ at sufficient N was correlated with higher total free amino acids in the roots, particularly glutamine and asparagine. This reduction in NO₃⁻ uptake capacity when grown in small amounts of NH₄⁺ is a long term effect caused by the products of N assimilation and could be reversed by moving plants to solely NO₃⁻ treatments. Higher concentrations of amino acids in the roots of these plants suggests that NH₄⁺ that enters the root gets first into the assimilatory pathway in the cytosol prior to the assimilation of NH₄⁺ formed by the reduction of NO₃⁻ in the plastids. This study showed that small amounts of NH4 + improve plant growth and lead to major changes in N uptake and assimilation processes. Based on these effects and the fact that plants in the field always have a small amount of N available as NH₄⁺, it is recommended that NH₄⁺ be added to the experimental nutrient solutions with maize and the effect be explored in other major plant species.
Advisor: Heuer, Sigrid
Garnett, Trevor Paul
Plett, Darren Craig
Tester, Mark Alfred
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2014
Keywords: nitrate; ammonium; flux capacity; assimilation; glutamine synthetase; nitrate transporter; ammonium transporter
Provenance: This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legals
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