New Insights on Arabidopsis thaliana Root Adaption to Ammonium Nutrition by the Use of a Quantitative Proteomic Approach

Nitrogen Sources Reprogram Carbon and Nitrogen Metabolism to Promote Andrographolide Biosynthesis in Andrographis paniculata (Burm.f.) Nees Seedlings

Carbon (C) and nitrogen (N) metabolisms participate in N source-regulated secondary metabolism in medicinal plants, but the specific mechanisms involved remain to be investigated. By using nitrate (NN), ammonium (AN), urea (UN), and glycine (GN), respectively, as sole N sources, we found that N sources remarkably affected the contents of diterpenoid lactone components along with C and N metabolisms reprograming in Andrographis paniculata, as compared to NN, the other three N sources raised the levels of 14-deoxyandrographolide, andrographolide, dehydroandrographolide (except UN), and neoandrographolide (except AN) with a prominent accumulation of farnesyl pyrophosphate (FPP). These N sources also raised the photosynthetic rate and the levels of fructose and/or sucrose but reduced the activities of phosphofructokinase (PFK), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoenolpyruvate carboxylase (PEPC) and pyruvate dehydrogenase (PDH). Conversely, phosphoenolpyruvate carboxykinase (PEPCK) and malate enzyme (ME) activities were upregulated. Simultaneously, citrate, cis-aconitate and isocitrate levels declined, and N assimilation was inhibited. These results indicated that AN, UN and GN reduced the metabolic flow of carbohydrates from glycolysis into the TCA cycle and downstream N assimilation. Furthermore, they enhanced arginine and GABA metabolism, which increased C replenishment of the TCA cycle, and increased ethylene and salicylic acid (SA) levels. Thus, we proposed that the N sources reprogrammed C and N metabolism, attenuating the competition of N assimilation for C, and promoting the synthesis and accumulation of andrographolide through plant hormone signaling. To obtain a higher production of andrographolide in A. paniculata, AN fertilizer is recommended in its N management.

Integrated physiological, biochemical, and transcriptomics analyses reveal the underlying mechanisms of high nitrogen use efficiency of black sesame

Cultivating high nitrogen use efficient varieties is a sustainable solution to mitigating adverse effects on the environment caused by excessive nitrogen fertilizer application. However, in sesame, although immoderate nitrogen fertilizers are used to promote yield, the molecular basis of high nitrogen use efficiency (NUE) is largely unknown. Hence, this study aimed to identify high NUE black sesame variety and dissect the underlying physiological and molecular mechanisms. To achieve this, seventeen seedling traits of 30 black sesame varieties were evaluated under low nitrogen (LN) and high nitrogen (HN) conditions. Dry matter accumulation, root parameters, shoot nitrogen accumulation, and chlorophyll content are important factors for evaluating the NUE of sesame genotypes. The variety 17-156 was identified as the most efficient for N utilization. Comparative physiological and transcriptomics analyses revealed that 17-156 possesses a sophisticated nitrogen metabolizing machinery to uptake and assimilate higher quantities of inorganic nitrogen into amino acids and proteins, and simultaneously improving carbon metabolism and growth. Specifically, the total nitrogen and soluble protein contents significantly increased with the increase in nitrogen concentrations. Many important genes, including nitrate transporters (NPFs), amino acid metabolism-related (GS, GOGAT, GDH, etc.), phytohormone-related, and transcription factors, were significantly up-regulated in 17-156 under HN condition. In addition, 38 potential candidate genes were identified for future studies toward improving sesame's NUE. These findings offer valuable resources for deciphering the regulatory network of nitrogen metabolism and developing sesame cultivars with improved NUE.

Quantification of Glucose Metabolism and Nitrogen Utilization in Two Brassicaceae Species under Bicarbonate and Variable Ammonium Soil Conditions

In karst habitats under drought conditions, high bicarbonate (high pH), and an abundant nitrate soil environment, bicarbonate regulates the glycolysis (EMP) and pentose phosphate pathways (PPP), which distribute ATP and NADPH, affecting nitrate (NO3-) and ammonium (NH4+) utilization in plants. However, the relationship between EMP PPP and NO3-, and NH4+ utilization and their responses to bicarbonate and variable ammonium still remains elusive. In this study, we used Brassica napus (Bn, a non-karst-adaptable plant) and Orychophragmus violaceus (Ov, a karst-adaptable plant) as plant materials, employed a bidirectional nitrogen-isotope-tracing method, and performed the quantification of the contribution of EMP and PPP. We found that bicarbonate and ammonium inhibited glucose metabolism and nitrogen utilization in Bn under simulated karst habitats. On the other hand, it resulted in a shift from EMP to PPP to promote ammonium utilization in Ov under high ammonium stress in karst habitats. Compared with Bn, bicarbonate promoted glucose metabolism and nitrogen utilization in Ov at low ammonium levels, leading to an increase in photosynthesis, the PPP, carbon and nitrogen metabolizing enzyme activities, nitrate/ammonium utilization, and total inorganic nitrogen assimilation capacity. Moreover, bicarbonate significantly reduced the growth inhibition of Ov by high ammonium, resulting in an improved PPP, RCRUBP, and ammonium utilization to maintain growth. Quantifying the relationships between EMP, PPP, NO3-, and NH4+ utilization can aid the accurate analysis of carbon and nitrogen use efficiency changes in plant species. Therefore, it provides a new prospect to optimize the nitrate/ammonium utilization in plants and further reveals the differential responses of inorganic carbon and nitrogen (C-N) metabolism to bicarbonate and variable ammonium in karst habitats.

Epitranscriptome changes triggered by ammonium nutrition regulate the proteome response of maritime pine roots

Epitranscriptome constitutes a gene expression checkpoint in all living organisms. Nitrogen is an essential element for plant growth and development that influences gene expression at different levels such as epigenome, transcriptome, proteome, and metabolome. Therefore, our hypothesis is that changes in the epitranscriptome may regulate nitrogen metabolism. In this study, epitranscriptomic modifications caused by ammonium nutrition were monitored in maritime pine roots using Oxford Nanopore Technology. Transcriptomic responses mainly affected transcripts involved in nitrogen and carbon metabolism, defense, hormone synthesis/signaling, and translation. Global detection of epitranscriptomic marks was performed to evaluate this posttranscriptional mechanism in un/treated seedlings. Increased N⁶-methyladenosine (m⁶A) deposition in the 3'-UTR was observed in response to ammonium, which seems to be correlated with poly(A) lengths and changes in the relative abundance of the corresponding proteins. The results showed that m⁶A deposition and its dynamics seem to be important regulators of translation under ammonium nutrition. These findings suggest that protein translation is finely regulated through epitranscriptomic marks likely by changes in mRNA poly(A) length, transcript abundance and ribosome protein composition. An integration of multiomics data suggests that the epitranscriptome modulates responses to nutritional, developmental and environmental changes through buffering, filtering, and focusing the final products of gene expression.

Assessing optimal nitrate/ ammonium- ratios in baby-leaf lettuce to enhance the heat stress tolerance under elevated CO2 conditions

In recent years, the interest on baby-leaf lettuce has grown steadily, because it is richer in bioactive compounds than other traditional vegetables. However, the quality of lettuce is being increasingly affected by climate change. It is very rare for a climatic effect to occur in isolation. Even then, a large body of work has only focused on the effect of isolated heat stress, fertilization, and elevated CO2, on morphological, physiological and biochemical parameters. Thus, very few works have focused on how the combination of several of these factors can affect these parameters. For first time, the present work studied the combined effect derived from the application of two different levels of CO2 (400 and 1000 ppm of CO2), four different NO3-/ NH4+ ratios (100/0 (T-I), 100/0 before the short-term heat stress and finally without NO3- (T-II), 80/20 (T-III) and 50/50 (T-IV)), and a short-term heat stress (25 and 43°C), on some physiological and quality parameters (dry biomass, photosynthetic parameters, pigments content, lipid peroxidation and total soluble proteins content) of baby-leaf lettuce cv Derbi. Additionally, a comparison of that combined effect of all these parameters between inner and outer leaves was also performed. The results obtained indicated that the interaction between the nutrient solution containing a 50/50 NO3-/ NH4+ ratio and a high concentration of CO2 (1000 ppm) improved the biomass, photosynthesis, intercellular/external CO2 concentration ratio (ci/ca), stomatal conductance (gs), evapotranspiration (E) and lipid peroxidation, and protein content in this baby-leaf lettuce. The results obtained in this work lead us to conclude that this existing interaction between the NO3-/ NH4+ ratio and the elevated CO2 concentration may be considered as a new strategy for making baby-leaf lettuce more resistant to heat stress, in other words, stronger against the ever more frequent heat waves.

Quantitative trait loci mapping reveals important genomic regions controlling root architecture and shoot biomass under nitrogen, phosphorus, and potassium stress in rapeseed (Brassica napus L.)

Plants rely on root systems for nutrient uptake from soils. Marker-assisted selection helps breeders to select desirable root traits for effective nutrient uptake. Here, 12 root and biomass traits were investigated at the seedling stage under low nitrogen (LN), low phosphorus (LP), and low potassium (LK) conditions, respectively, in a recombinant inbred line (RIL) population, which was generated from Brassica napus L. Zhongshuang11 and 4D122 with significant differences in root traits and nutrient efficiency. Significant differences for all the investigated traits were observed among RILs, with high heritabilities (0.43-0.74) and high correlations between the different treatments. Quantitative trait loci (QTL) mapping identified 57, 27, and 36 loci, explaining 4.1-10.9, 4.6-10.8, and 4.9-17.4% phenotypic variances under LN, LP, and LK, respectively. Through QTL-meta analysis, these loci were integrated into 18 significant QTL clusters. Four major QTL clusters involved 25 QTLs that could be repeatedly detected and explained more than 10% phenotypic variances, including two NPK-common and two specific QTL clusters (K and NK-specific), indicating their critical role in cooperative nutrients uptake of N, P, and K. Moreover, 264 genes within the four major QTL clusters having high expressions in roots and SNP/InDel variations between two parents were identified as potential candidate genes. Thirty-eight of them have been reported to be associated with root growth and development and/or nutrient stress tolerance. These key loci and candidate genes lay the foundation for deeper dissection of the NPK starvation response mechanisms in B. napus.

Growth and nitrogen metabolism in Sophora japonica (L.) as affected by salinity under different nitrogen forms

Sophora japonica is a leguminous tree species native to China. To explore the nitrogen (N) source preference and its impact on stress tolerance, a hydroponic experiment was designed in which S. japonica seedlings were supplied with sole ammonium (NH₄⁺) or sole nitrate (NO₃^-) nutrition under 75 mM NaCl-induced salt stress. The growth and N metabolism performance were investigated. In the absence of NaCl, plants fed NH₄⁺ showed better root growth than those fed NO₃^-, but there was no difference in aerial part growth. Salinity inhibited the root growth of NH₄⁺-fed plants and the shoot growth of NO₃^--fed plants, while the total N accumulation was suppressed under either N form. Specifically, in NH₄⁺-fed plants, salinity significantly increased the net photosynthetic rate, root NH₄⁺ content and root antioxidant enzyme activities. Higher nitrate reductase (NR) activities but lower glutamate synthetase (GS) activities were observed in both leaves and roots. Leaf AMT1.1 and AMT2.1a in NH₄⁺-fed plants positively reacted to salt stress, whereas the expression of four AMTs was reduced or remained unchanged in roots. In contrast, salinity suppressed the net photosynthetic rate, antioxidant enzyme activities, and GS activity in the leaves of NO₃^--fed plants. Upregulation of NPF1.2, NPF2.11, NPF4.6 and NPF7.3, as well as unaltered NR activity, caused higher NO₃^- content in the leaves. Moreover, NR and glutamate synthase (GOGAT) activities together with the transcription of most NRTs were promoted by salinity in the roots of NO₃^--fed plants. Additionally, compared to those treated with NH₄⁺, in response to salinity, NO₃^--treated seedlings showed more intensive repression of the net photosynthetic rate, chlorophyll content, and both shoot and root growth. Overall, these results suggest that S. japonica plants grew better in NH₄⁺ medium than in NO₃^- medium, and the different N metabolism responses improved S. japonica tolerance to salinity with NH₄⁺ application. This study provides new insights for understanding the mechanism of salt tolerance, breeding resistant varieties of S. japonica, and developing scientific fertilization management strategies during the seedling cultivation period.

Coordination of root growth with root morphology, physiology and defense functions in response to root pruning in Platycladus orientalis

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A growth‒defense tradeoff following root pruning.

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Root growth lagged behind root physiology after root pruning.

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The growth–defense tradeoff was induced by indole-3-acetic acid.

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Proteomic analysis supported a growth–defense tradeoff.

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Root pruning altered the expression of genes at the protein and mRNA levels.

Introduction

Root pruning is commonly used to facilitate seedling transplantation for the restoration of degraded or damaged ecosystems. However, little is known about how root growth coordinates morphology, physiology and defense functions following root pruning.

Objectives

We aim to elucidate whether and how root growth trades off with defense functioning after pruning.

Methods

Seedlings of Platycladus orientalis, a tree species widely used in forest restoration, were subjected to root pruning treatment. A suite of root growth, morphological and physiological traits were measured after pruning in combination with proteomic analysis.

Results

Root growth was insensitive to pruning until at 504 h with a significant increase of 16.8%, whereas root physiology was activated rapidly after pruning. Key root morphological traits, such as root diameter, specific root length and root tissue density, showed no response to the pruning treatment. Plant defense syndromes such as reactive oxygen species-scavenging enzymes and defensive phytohormones such as jasmonic acid and abscisic acid, were recruited at six hours after pruning and recovered to the unpruned levels at 504 h. Compared with the controls, 271, 360 and 106 proteins were differentially expressed at 6, 72 and 504 h after root pruning, respectively. These proteins, associated with defense function, showed temporal patterns similar to the above defense syndromes.

Conclusion

Our results suggest a root growth-defense tradeoff following root pruning in P. orientalis. This tradeoff was potentially due to the significant increase of indole-3-acetic acid, the phytohormone stimulating root branching, which occurred soon after pruning. Together, these results provide a holistic understanding of how root growth is coordinated with root morphology, physiology, and defense in response to root pruning.

The Effect of Foliar Putrescine Application, Ammonium Exposure, and Heat Stress on Antioxidant Compounds in Cauliflower Waste

This work has been focused on the study of how we can affect the short heat stress on the bioactive compounds content. Some recent investigations have observed that management of nitrogen fertilization can alleviate short-term heat effects on plants. Additionally, the short-term heat stress can be also ameliorated by using putrescine, a polyamine, due to its crucial role in the adaptation of plants to heat stress Therefore, different NO₃⁻/NH₄⁺ ratios and a foliar putrescine treatment have been used in order to increase tolerance to thermal stress in order to take advantage of the more frequent and intense heat waves and make this crop more sustainable. So, other objective of this work is to make the cauliflower waste more attractive for nutraceutical and pharmaceutical preparations. Thus, the effect of a thermal stress combined with a 50:50 NO₃⁻/NH₄⁺ ratio in the nutrient solution, and the foliar application of 2.5 mM putrescine increased in the content of various sugars (inositol, glucose, and fructose), total phenolic compounds and polyamines, as well as in the antioxidant activity. The greatest accumulation of these compounds was observed in young leaves. Our results show from a physiological and agronomic point of view, that the foliar application of putrescine and the 50:50 NO₃⁻/NH₄⁺ treatment managed to alleviate the negative effects of the abiotic stress suffered at high temperature, yielding plants with higher antioxidant compounds content.

Ammonium supply induces differential metabolic adaptive responses in tomato according to leaf phenological stage

Nitrate (NO3-) and ammonium (NH4+) are the main inorganic nitrogen sources available to plants. However, exclusive ammonium nutrition may lead to stress characterized by growth inhibition, generally associated with a profound metabolic reprogramming. In this work, we investigated how metabolism adapts according to leaf position in the vertical axis of tomato (Solanum lycopersicum cv. M82) plants grown with NH4+, NO3-, or NH4NO3 supply. We dissected leaf biomass composition and metabolism through an integrative analysis of metabolites, ions, and enzyme activities. Under ammonium nutrition, carbon and nitrogen metabolism were more perturbed in mature leaves than in young ones, overall suggesting a trade-off between NH4+ accumulation and assimilation to preserve young leaves from ammonium stress. Moreover, NH4+-fed plants exhibited changes in carbon partitioning, accumulating sugars and starch at the expense of organic acids, compared with plants supplied with NO3-. We explain such reallocation by the action of the biochemical pH-stat as a mechanism to compensate the differential proton production that depends on the nitrogen source provided. This work also underlines that the regulation of leaf primary metabolism is dependent on both leaf phenological stage and the nitrogen source provided.

Nitrogen Uptake in Plants: The Plasma Membrane Root Transport Systems from a Physiological and Proteomic Perspective

Nitrogen nutrition in plants is a key determinant in crop productivity. The availability of nitrogen nutrients in the soil, both inorganic (nitrate and ammonium) and organic (urea and free amino acids), highly differs and influences plant physiology, growth, metabolism, and root morphology. Deciphering this multifaceted scenario is mandatory to improve the agricultural sustainability. In root cells, specific proteins located at the plasma membrane play key roles in the transport and sensing of nitrogen forms. This review outlines the current knowledge regarding the biochemical and physiological aspects behind the uptake of the individual nitrogen forms, their reciprocal interactions, the influences on root system architecture, and the relations with other proteins sustaining fundamental plasma membrane functionalities, such as aquaporins and H⁺-ATPase. This topic is explored starting from the information achieved in the model plant Arabidopsis and moving to crops in agricultural soils. Moreover, the main contributions provided by proteomics are described in order to highlight the goals and pitfalls of this approach and to get new hints for future studies.

Effects of Different Nitrogen Forms and Exogenous Application of Putrescine on Heat Stress of Cauliflower: Photosynthetic Gas Exchange, Mineral Concentration and Lipid Peroxidation

This study examines the effect of the exogenous application of polyamine putrescine together with the application of different ratios of nitrate/ammonium (NO₃⁻/NH₄⁺), on the physiology of cauliflower subjected to heat stress. The 50:50 NO₃⁻/NH₄⁺ ratio was the best ratio against heat stress. As a result of the joint application of these compounds, a higher photosynthetic rate, a higher accumulation of both photosynthesis-related compounds and pigments, total proteins, and a change in the status of nutrients were obtained. Particularly, the decrease in content of calcium, chloride and sulphate in plants under heat stress is ameliorated by the ammonium effect. Additionally, it is important to highlight that cauliflower waste contains a higher content of mineral nutrients than floret cauliflower. These effects were more marked in young leaves. Furthermore, a synergistic effect for coping with heat stress between the polyamine and the nutritional treatment was observed. For this, both the application of putrescine and the feeding of plants with a 50:50 NO₃⁻/NH₄⁺ ratio before heat stress is proposed for the first time as an agricultural practice for increasing the thermotolerance of cauliflower cv Moonshine. On the other hand, due to the lower lipid peroxidation rate obtained in cauliflower leaves, these plants could be used for health purposes as ointments or other nutraceutical products, making the cultivation of this kind of cruciferous more sustainable.

Arabidopsis thaliana transcription factors MYB28 and MYB29 shape ammonium stress responses by regulating Fe homeostasis

Although ammonium (NH₄⁺ ) is a key intermediate of plant nitrogen metabolism, high concentrations of NH₄⁺ in the soil provoke physiological disorders that lead to the development of stress symptoms. Ammonium nutrition was shown to induce the accumulation of glucosinolates (GSLs) in leaves of different Brassicaceae species. To further understand the link between ammonium nutrition and GSLs, we analysed the ammonium stress response of Arabidopsis mutants impaired in GSL metabolic pathway. We showed that the MYB28 and MYB29 double mutant (myb28myb29), which is almost deprived of aliphatic GSLs, is highly hypersensitive to ammonium nutrition. Moreover, we evidenced that the stress symptoms developed were not a consequence of the lack of aliphatic GSLs. Transcriptomic analysis highlighted the induction of an iron (Fe) deficiency response in myb28myb29 under ammonium nutrition. Consistently, ammonium-grown myb28myb29 plants showed altered Fe accumulation and homeostasis. Interestingly, we showed overall that growing Arabidopsis with increased Fe availability relieved ammonium stress symptoms and that this was associated with MYB28 and MYB29 expression. Taken together, our data indicated that the control of Fe homeostasis was crucial for the Arabidopsis response to ammonium nutrition and evidenced that MYB28 and MYB29 play a role in this control.

Combined Transcriptome and Proteome Analysis of Masson Pine (Pinus massoniana Lamb.) Seedling Root in Response to Nitrate and Ammonium Supplementations

Nitrogen (N) is an essential nutrient for plant growth and development. Plant species respond to N fluctuations and N sources, i.e., ammonium or nitrate, differently. Masson pine (Pinus massoniana Lamb.) is one of the pioneer plants in the southern forests of China. It shows better growth when grown in medium containing ammonium as compared to nitrate. In this study, we had grown masson pine seedlings in medium containing ammonium, nitrate, and a mixture of both, and performed comparative transcriptome and proteome analyses to observe the differential signatures. Our transcriptome and proteome resulted in the identification of 1593 and 71 differentially expressed genes and proteins, respectively. Overall, the masson pine roots had better performance when fed with a mixture of ammonium and nitrate. The transcriptomic and proteomics results combined with the root morphological responses suggest that when ammonium is supplied as a sole N-source to masson pine seedlings, the expression of ammonium transporters and other non-specific NH₄⁺-channels increased, resulting in higher NH₄⁺ concentrations. This stimulates lateral roots branching as evidenced from increased number of root tips. We discussed the root performance in association with ethylene responsive transcription factors, WRKYs, and MADS-box transcription factors. The differential analysis data suggest that the adaptability of roots to ammonium is possibly through the promotion of TCA cycle, owing to the higher expression of malate synthase and malate dehydrogenase. Masson pine seedlings managed the increased NH₄⁺ influx by rerouting N resources to asparagine production. Additionally, flavonoid biosynthesis and flavone and flavonol biosynthesis pathways were differentially regulated in response to increased ammonium influx. Finally, changes in the glutathione s-transferase genes suggested the role of glutathione cycle in scavenging the possible stress induced by excess NH₄⁺. These results demonstrate that masson pine shows increased growth when grown under ammonium by increased N assimilation. Furthermore, it can tolerate high NH₄⁺ content by involving asparagine biosynthesis and glutathione cycle.

RidA Proteins Protect against Metabolic Damage by Reactive Intermediates

The Rid (YjgF/YER057c/UK114) protein superfamily was first defined by sequence homology with available protein sequences from bacteria, archaea, and eukaryotes (L. Parsons, N. Bonander, E. Eisenstein, M. Gilson, et al., Biochemistry 42:80-89, 2003, https://doi.org/10.1021/bi020541w). The archetypal subfamily, RidA (reactive intermediate deaminase A), is found in all domains of life, with the vast majority of free-living organisms carrying at least one RidA homolog. In over 2 decades, close to 100 reports have implicated Rid family members in cellular processes in prokaryotes, yeast, plants, and mammals. Functional roles have been proposed for Rid enzymes in amino acid biosynthesis, plant root development and nutrient acquisition, cellular respiration, and carcinogenesis. Despite the wealth of literature and over a dozen high-resolution structures of different RidA enzymes, their biochemical function remained elusive for decades. The function of the RidA protein was elucidated in a bacterial model system despite (i) a minimal phenotype of ridA mutants, (ii) the enzyme catalyzing a reaction believed to occur spontaneously, and (iii) confusing literature on the pleiotropic effects of RidA homologs in prokaryotes and eukaryotes. Subsequent work provided the physiological framework to support the RidA paradigm in Salmonella enterica by linking the phenotypes of mutants lacking ridA to the accumulation of the reactive metabolite 2-aminoacrylate (2AA), which damaged metabolic enzymes. Conservation of enamine/imine deaminase activity of RidA enzymes from all domains raises the likelihood that, despite the diverse phenotypes, the consequences when RidA is absent are due to accumulated 2AA (or a similar reactive enamine) and the diversity of metabolic phenotypes can be attributed to differences in metabolic network architecture. The discovery of the RidA paradigm in S. enterica laid a foundation for assessing the role of Rid enzymes in diverse organisms and contributed fundamental lessons on metabolic network evolution and diversity in microbes. This review describes the studies that defined the conserved function of RidA, the paradigm of enamine stress in S. enterica, and emerging studies that explore how this paradigm differs in other organisms. We focus primarily on the RidA subfamily, while remarking on our current understanding of the other Rid subfamilies. Finally, we describe the current status of the field and pose questions that will drive future studies on this widely conserved protein family to provide fundamental new metabolic information.

Regulation of Ammonium Cellular Levels is An Important Adaptive Trait for the Euhalophytic Behavior of Salicornia europaea

Salinization of agricultural land is a devastating phenomenon which will affect future food security. Understanding how plants survive and thrive in response to salinity is therefore critical to potentiate tolerance traits in crop species. The halophyte Salicornia europaea has been used as model system for this purpose. High salinity causes NH4+ accumulation in plant tissues and consequent toxicity symptoms that may further exacerbate those caused by NaCl. In this experiment we exposed Salicornia plants to five concentrations of NaCl (0, 1, 10, 50 and 200 mM) in combination with two concentrations of NH4Cl (1 and 50 mM). We confirmed the euhalophytic behavior of Salicornia that grew better at 200 vs. 0 mM NaCl in terms of both fresh (+34%) and dry (+46%) weights. Addition of 50 mM NH4Cl to the growth medium caused a general growth reduction, which was likely caused by NH4+ accumulation and toxicity in roots and shoots. When plants were exposed to high NH4Cl, high salinity reduced roots NH4+ concentration (-50%) compared to 0 mM NaCl. This correlates with the activation of the NH4+ assimilation enzymes, glutamine synthetase and glutamate dehydrogenase, and the growth inhibition was partially recovered. We argue that NH4+ detoxification is an important trait under high salinity that may differentiate halophytes from glycophytes and we present a possible model for NH4+ detoxification in response to salinity.

Interactive effects of salinity and nitrogen forms on plant growth, photosynthesis and osmotic adjustment in maize

To enhance crop productivity and minimize the harmful effects of various environmental stresses, such as salinity and drought, farmers often use mineral fertilizers. However, inadequate or excessive fertilization can reduce plant growth and nutritive quality and contribute to soil degradation and environmental pollution. This study investigated the effects of salinity (0, 100 or 150 mM NaCl) and nitrogen form (sole NO₃^- or NH₄⁺, or combined NO₃^-:NH₄⁺ at 25:75 or 50:50) on growth, photosynthesis, and water and ion status of a commercial variety of maize (Zea mays SY Sincero). In the absence of NaCl, the media containing ammonium only or both nitrogen forms had higher aboveground growth rates than that containing nitrate only. Indeed, the maize growth, expressed as leaf dry matter, seen on NH₄⁺ in the absence of salinity, was nearly double the biomass compared to that with NO₃^-treatment. Irrespective of N form, the presence of NaCl severely reduced leaf and roots growth; the presence of ammonium in the nutrient solution diminished these negative effects. Compared to the NH₄⁺ only and combined treatments, the leaves of plants in the NO₃^--only medium showed signs of nitrogen deficiency (general chlorosis), which was more pronounced in the lower than upper leaves, indicating that nitrate is partly replaced by chloride during root uptake. NH₄⁺ favored maize growth more than NO₃^-, especially when exposed to saline conditions, and may improve the plant's capacity to osmotically adjust to salinity by accumulating inorganic solutes.