Nitrate and ammonium interactions in maize.

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.