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Nitrogen Assimilation in Plants
Abstract
ABSTRACT
The fourth most abundant nutrient element in plants, nitrogen is a key constituent of nucleotides, proteins, and other vital metabolites of plants. It is an integral part of a plant regulating several growth processes. Inorganic nitrogen sources, nitrate (NO3 -) and ammonium (NH4 +), are the two forms, in which the majority of nitrogen is taken up from the soil. But soil ecosystems flourishing with various types of bacteria and microfauna pose a great challenge for nitrogen availability to plants. Consequently, optimizing the nitrogen levels in both natural and agricultural ecosystems is essential it is a limiting nutrient for plants. Thus, approximately 90 million metric tonnes of nitrogen fertilizers are applied to the soil each year worldwide. However, abundance of gaseous nitrogen in the atmosphere (78% by volume) in molecular form (N2) does not guarantee its accessibility to plants, as it has to be converted to biologically usable form with the assistance of microbial population to break the highly stable triple covalent bond of nitrogen. So, finally, inorganic nitrogen either taken from atmospheric (as N2) or soil pool (as NO3 -or NH4 +) has to undergo a series of complex biochemical reactions and processes to be converted to organic nitrogen in the form of amino acids or other nitrogenous metabolites. This process of incorporation of mineral nutrients (i.e., nitrogen into various organic forms such as proteins, nucleic acid, hormones, etc.) is termed nitrogen assimilation. In NO3 - assimilation, the nitrogen in NO3 - is converted to a higher-energy form in nitrite (NO2 -) by nitrate reductase enzyme in the cytosol then to a yet higher-energy form, ammonium (NH4 +), by nitrite reductase in chloroplasts and plastids, and finally into the amide nitrogen of glutamine. Plants such as legumes form symbiotic relationships with nitrogen-fixing bacteria to convert molecular nitrogen (N2) into ammonia (NH3), which however is protonated to form the ammonium ion (NH4 +) and subsequently into organic forms. The nitrate supply to the roots is proportional to the amount of absorbed nitrate that is translocated to the shoot, where it is assimilated.
So, we endeavor to elucidate how nitrogen traverses between its primary global pools (i.e., atmosphere, soil (and associated groundwater), and biomass), the biology and biochemistry of biological nitrogen fixation, NO3 – uptake & reduction (to NH4 +), numerous routes of assimilation of nitrogen as ammonium, and nitrate nitrogen along with a detailed view of synthesis of amino acids.
... Nitrogen is the most limiting nutrient for crop production worldwide, primarily due to soils' restricted capacity to provide sufficient available nitrogen. Consequently, humans have consistently supplemented soil nitrogen with inorganic fertilizers to boost crop yields [5]. ...
This study assessed the impact of nitrogen (N) and boron (B) fertilization on yield, nutrient composition, and nutrient uptake in Indian mustard (Brassica juncea) in Chitrakoot, Madhya Pradesh, India. A field trial featuring 13 treatments combined reduced nitrogen levels, nano-urea applications, and different boron doses, using a randomized block design with three replications. Treatment T12 [½ recommended nitrogen dose (RDN) + two nano-urea sprays + 1.25 kg B ha⁻¹] achieved the highest seed yield (1523.81 kg ha⁻¹), surpassing the control (100% NPK as per RDF) which yielded 958.73 kg ha⁻¹. T12 also had the highest nutrient concentrations 2.78% nitrogen, 0.482% phosphorus, and 0.74% potassium as well as the highest nutrient uptake, with nitrogen at 42.36 kg ha⁻¹, phosphorus at 7.34 kg ha⁻¹, and potassium at 11.28 kg ha⁻¹. These results indicate that combining nano-urea with boron, especially at elevated boron levels, can significantly improve yield and nutrient absorption in mustard, highlighting its potential for sustainable farming in semi-arid areas.
... In turn, the decrease in the level of NO 2 and the increase in the content of NH 4 + ions probably results from the increased activity of the NR enzyme (in the first case) and the NiR enzyme (in the second case) under the influence of the tested iodine compounds. It is worth mentioning that NH 4 + can be formed not only as a result of the NO 3 reduction process, but also when there are high rates of photorespiration due to glycine oxidation (Blasco et al., 2010;Kumari et al., 2022). From the point of view of health safety of potential consumers, the reduction of NO 3 and NO 2 content (obtained in these studies) is extremely important, because in the digestive system nitrates are converted into nitrites, and these in turn into carcinogenic N-nitrosamines (Anjana and Iqbal, 2007). ...
Iodine deficiency in the diet creates the need to search for innovative, more sustainable and more effective strategies for enriching food with this microelement. The adopted research hypothesis assumed that the use of organic forms of iodine for supplementation of lettuce (Lactuca sativa L.), compared to mineral iodine, has a more favorable effect not only on the concentration of iodine, but also on the yield and the content of other chemical components determining its nutritional and health-promoting value. Lettuce was planted in a nutrient film technique (NFT) hydroponic study in a greenhouse. The following application of iodine compounds (all in 5 µM molar mass equivalents) were tested in the studies: control (without of iodine application); potassium iodate (positive iodine control), 8-hydroxy-7-iodo-5-quinolinesulfonic acid, 5-chloro-7-iodo-8-quinolinol, 5,7-diiodo-8-quinolinol and 4-hydroxy-8-iodo-3-quinolinecarboxylic acid. In this work, it was shown for the first time that iodoquinolines can be 1) a source of iodine for plants; 2) they have a biostimulating effect on their yielding and 3) they increase the resistance of crops to stress (due to a significant increase in the level of polyphenolic compounds). Lettuce with the addition of 8-hydroxy-7-iodo-5-quinolinesulfonic acid was characterized by the highest content of iodine, which was 221.7 times higher than in control plants. The weight gain of the whole plant was particularly visible in the case of lettuce enriched with 5-chloro-7-iodo-8-quinolinol and amounted to 26.48% compared to the control. Lettuce biofortified with iodine in the form of iodoquinolines can successfully become part of a sustainable diet based on plant products, which has a low impact on the environment and contributes to the long-term good health of an individual or community. Reducing iodine deficiency through the use of organoiodine compounds can help achieve the sustainability goal of eliminating hidden hunger, improving nutritional status and promoting sustainable agriculture.
Potassium is an essential nutrient that influences key processes in plants, including osmotic regulation, photosynthesis, and nitrogen assimilation. This study investigated the drought tolerance of wheat (Triticum spp.) plants treated with sufficient potassium (SK, 1 mM) and low potassium (LK, 0.05 mM) under PEG-induced drought stress. Plant physiological development, canopy temperature, photosynthetic efficiency, antioxidant defense enzymes, and nitrogen assimilation enzymes were assessed. In non-drought conditions, LK increased canopy temperature and reduced dry matter yield and photosynthetic performance, with these effects becoming more pronounced under drought stress. SK-treated plants exhibited higher biomass, chlorophyll content, maximum quantum efficiency of photosystem II, and lower canopy temperatures, even under drought conditions. Furthermore, LK restricted the accumulation of key osmotic regulators, including proline, amino acids, and soluble sugars. Under drought stress, LK plants also showed increased hydrogen peroxide and superoxide anion levels, while SK plants had lower reactive oxygen species accumulation and higher antioxidant enzyme activities (catalase and superoxide dismutase). Additionally, LK resulted in reduced activity of nitrogen assimilation enzymes (nitrate reductase, NR, and nitrite reductase, NiR) under both normal and drought conditions. In contrast, SK-treated wheat seedlings maintained higher NR and NiR activities and higher soluble protein content during drought stress. These findings underscore the critical role of potassium management in enhancing wheat yield, particularly in water-scarce regions, as optimal potassium supply strengthens essential physiological and biochemical mechanisms that improve plant tolerance to drought stress.
In recent years, Morocco has grappled with a precipitation shortage, exacerbating the challenge of low assimilable phosphorus (P) availability in many Moroccan soils. This presents a dual stressor for crops, hindering plant growth, particularly for staple foods like legumes and cereals, crucial to the Moroccan diet. Our study aims to assess the physiological and biochemical responses of Triticum turgidum and Vicia faba intercrops to water and P limitation, and their combined effects within a greenhouse environment. Additionally, we seek to enhance wheat growth under these challenging conditions by implementing an intercropping system. For that purpose, a variety of (Vicia faba) (Aguadulce), and (Triticum turgidum durum) (Karim) were grown in a greenhouse under sole cropping and intercropping system. The combined stress of water and P limitation was maintained by irrigating at 40% of field capacity (FC) and using an agriculture soil deficient in assimilable P (11 mg kg-1), while control conditions were ensured by irrigating at 80% of (FC) and adding 100 mg kg-1 of P2O5 of soil. At the flowering stage. Based on the agro-physiological and biochemical parameters of both species, our findings revealed significant impacts of the combined stress on the biomass of both faba beans and wheat, regardless of the cropping system. With the most notable reduction of 76%, 67 %, and 75% in intercropped faba bean shoot dry weight (SDW), root dry weight (RDW), and nodules dry weight (NDW) respectively. While intercropping showed advantages for wheat with an increase of 94% and 130% for (SDW) and (RDW) respectively under control conditions compared to sole crops, intercropping with faba bean led to reduced levels of K+ and Na+ and glycine-betaine content in wheat plants, while enhancing nitrate reductase activity under the combined stress. Comparing the two cropping systems, our comprehensive analysis suggests that combined stress adversely affected nearly all parameters studied in both species, with intercropping favoring wheat growth at the expense of faba bean plants.
Nitrogen (N) fertilization affects grain quality in common buckwheat (Fagopyrum esculentum Moench). But the effects of N fertilizer on various buckwheat protein parameters are not fully understood. This study aimed to investigate the synthesis, accumulation, and quality of buckwheat protein under four N application rates in the Loess Plateau, China. Optimal N application (180 kg N ha−1) improved yield, agronomic traits, and N transport and increased protein yield and protein component accumulation. Prolamin and glutelin accumulation first increased and then decreased with increasing N application. The relationships between the contents of protein components and the amount of applied N generally followed quadratic functions. Nitrate reductase and glutamine synthetase activities first increased and then decreased with increasing N levels. Optimal N fertilizer increased the waterholding capacity and thermal stability of buckwheat protein and reduced its emulsification capacity, but negligibly changed its oil-absorption capacity. Hydrophobic amino acids and glutelin content were the main factors affecting protein quality.
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