Plant ammonium tolerance has been associated with the capacity to accumulate large amounts of ammonium in the root vacuoles, to maintain carbohydrate synthesis and especially with the capacity of maintaining high levels of inorganic nitrogen assimilation in the roots. The tricarboxylic acid cycle (TCA) is considered a cornerstone in nitrogen metabolism, since it provides carbon skeletons for nitrogen assimilation. The hypothesis of this work was that the induction of anaplerotic routes of phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH) and malic enzyme (NAD-ME) would enhance tolerance to ammonium nutrition. An experiment was established with tomato plants (Agora Hybrid F1) grown under different ammonium concentrations. Growth parameters, metabolite contents and enzymatic activities related to nitrogen and carbon metabolism were determined. Unlike other tomato cultivars, tomato Agora Hybrid F1 proved to be tolerant to ammonium nutrition. Ammonium was assimilated as a biochemical detoxification mechanism, thus leading to the accumulation of Gln and Asn as free amino acids in both leaves and roots as an innocuous and transitory store of nitrogen, in addition to protein synthesis. When the concentration of ammonium in the nutrient solution was high, the cyclic operation of the TCA cycle seemed to be interrupted and would operate in two interconnected branches to provide α-ketoglutarate for ammonium assimilation: one branch supported by malate accumulation and by the induction of anaplerotic PEPC and NAD-ME in roots and MDH in leaves, and the other branch supported by stored citrate in the precedent dark period.

Ammonium is a paradoxical nutrient ion. Despite being a common intermediate in plant metabolism whose oxidation state eliminates the need for its reduction in the plant cell, as occurs with nitrate, it can also result in toxicity symptoms. Several authors have reported that carbon enrichment in the root zone enhances the synthesis of carbon skeletons and, accordingly, increases the capacity for ammonium assimilation. In this work, we examined the hypothesis that increasing the photosynthetic photon flux density is a way to increase plant ammonium tolerance. Wheat plants were grown in a hydroponic system with two different N sources (10mM nitrate or 10mM ammonium) and with two different light intensity conditions (300μmolphotonm−2s−1 and 700μmolphotonm−2s−1). The results show that, with respect to biomass yield, photosynthetic rate, shoot:root ratio and the root N isotopic signature, wheat behaves as a sensitive species to ammonium nutrition at the low light intensity, while at the high intensity, its tolerance is improved. This improvement is a consequence of a higher ammonium assimilation rate, as reflected by the higher amounts of amino acids and protein accumulated mainly in the roots, which was supported by higher tricarboxylic acid cycle activity. Glutamate dehydrogenase was a key root enzyme involved in the tolerance to ammonium, while glutamine synthetase activity was low and might not be enough for its assimilation.

Agricultural intensification has led to the use of very high inputs of nitrogen fertilizers into cultivated land. As a consequence of this, nitrous oxide (N2O) emissions have increased significantly. Nowadays, the challenge is to mitigate these emissions in order to reduce global warming. Addition of nitrification inhibitors (NI) to fertilizers can reduce the losses of N2O to the atmosphere, but field studies have shown that their efficiency varies depending greatly on the environmental conditions. Soil water content and temperature are key factors controlling N2O emissions from soils and they seem to be also key parameters responsible for the variation in nitrification inhibitors efficiency. We present a laboratory study aimed at evaluating the effectiveness of the nitrification inhibitor 3,4-dimethylpyrazol phosphate (DMPP) at three different temperatures (10, 15 and 20 degrees C) and three soil water contents (40%, 60% and 80% of WFPS) on N2O emissions following the application of 1.2 mg N kg(-1) dry soil (equivalent to 140 kg N ha(-1)). Also the CO2 and CH4 emissions were followed to see the possible side effects of DMPP on the overall microbial activities. Nitrogen was applied either as ammonium sulfate nitrate (ASN) or as ENTEC 26 (ASN + DMPP). The application of ENTEC 26 was effective reducing N2O losses up to the levels of an unfertilized control treatment in all conditions. Nevertheless, the percentage of reduction induced by DMPP in the ENTEC treatment with respect to the ASN varied from 3% to 45% depending on temperature and soil water content conditions. At 40% of WFPS, when nitrification is expected to be the main process producing N2O, the increase of N2O emissions in ASN together with temperature provoked an increase in DMPP efficiency reducing these emissions from 17% up to 42%. Contrarily, at 80% of WFPS, when denitrification is expected to be the main source of N2O, emissions after ASN application decreased with temperature, which induced a decrease from 45% to 23% in the efficiency of DMPP reducing N2O losses. Overall, the results obtained in this study suggest that DMPP performance regarding N2O emissions reduction would be the best in cold and wet conditions. Neither CO2 emissions nor CH4 emissions were affected by the use of DMPP at the different soil water contents and temperatures. (C) 2012 Elsevier Ltd. All rights reserved.

An experiment was carried out to determine how the late application of nitrogen (N) fertilizer affects the use of pre-anthesis carbon reserves during the grain-filling period of pot-grown wheat with no water shortage. Increasing doses (equivalent to 0, 140, and 180 kg N ha(-1)) of N fertilizer were applied, either in two amendments (stages GS20 and GS30) or in three amendments (stages GS20, GS30, and GS37, according to Zadoks scale). The management of fertilizer by combining an increased N rate with late N application was able to stimulate canopy development, to raise photosynthetic capacity and carbohydrate accumulation during the vegetative stages, and to increase grain yield. Based on the dynamics of carbohydrate accumulation in the ear, three phases were differentiated during the grain-filling period, whose temporary pattern remained stable regardless of the fertilizer management. The net remobilization of carbohydrates started 12 d after anthesis from the leaves and 28 d after anthesis from roots and stems. The increase of the N dose with late N application allowed on one hand a lower use of the pre-anthesis carbon reserves in favor of greater de novo photosynthesis during the grain-filling period, and on the other hand greater relative contribution of the leaf and ear C reserves to remobilization towards the grain. Further splitting the dose increased only the relative contribution of ear C reserves. The stem contribution seemed to be independent of N applied whereas the root contribution tended to diminish with late N application.