Toxicity NH4+ in the plant ( Pampered little Musang King )
NH4+ is not only an important nitrogen source necessary for plant growth, but also a wide range of intermediate products existing in plant metabolism. However, when the external NH4+ concentration is too high, it can inhibit the growth of most higher plants. In this article, we discussed the susceptibility of different species to NH4+, and the symptoms of NH4+ poisoned plants, especially the mechanism of NH4+ poisoning and the methods to alleviate NH4+ poisoning. Different plants have different sensitivity to NH4+ poisoning. The growth of plants damaged by NH4+ poisoning is significantly inhibited. However, applying mixed nitrogen sources and increasing the K+ concentration in the culture conditions can reduce the degree of NH4+ poisoning.
NH4+ in plants can be directly absorbed from the soil through roots, and it is also the product of deamination of some organic compounds in plants during certain pathways such as protein degradation, photorespiration, and biosynthesis of lignin. High concentration of NH4+ can inhibit the growth of plants. In some land, the concentration of NH4+ can reach 40 mmol/Lt (720ppm). When NH4+ is the only nitrogen source, it can produce NH4+ toxicity to plants. In addition, in the soil using urea as fertilizer, due to the action of bacterial urease, urea is converted into NH4+ and CO2 in a large amount, which greatly increases the NH4+ concentration in the soil, which can cause NH4+ poisoning of plants. In addition, many abiotic stresses can also cause the hyperaccumulation of NH4+ in plants. If NH4+ cannot be assimilated effectively, it can also produce NH4+ poisoning symptoms. And recent studies have found that the accumulation of excessive NH4+ is considered a factor in the extinction of plant species in various ecosystems. Therefore, the in-depth understanding of the NH4+ toxicity mechanism and the exploration of methods to alleviate the NH4+ toxicity are of great significance.
1 Classification of NH4+ sensitive species and NH4+ resistant species
The phenomenon of NH4+ poisoning is very common, but different plant species have different thresholds for responding to it. In mature plants, the symptoms of ammonium poisoning usually appear first on the lower old leaves, while the young plants appear first on the new leaves. Domesticated plants that are sensitive to NH4+ ion toxicity include tomatoes, potatoes, barley, peas, castor, mustard, beets, strawberries and citrus. In the soil of many natural ecosystems, NH4+ has increasingly become the main form of N source. Wild-type herbaceous plants Arnica and thistle parsley and Cistanche cistanche are also particularly sensitive to NH4+ toxicity.
Existing plants that are more resistant to NH4+ toxicity include rice and onions among domesticated species. Wild plants include heather, Ulagrass, Proteas and some temperate quilt trees. Even species that are highly resistant to NH4+ toxicity, when a sufficient amount of NH4+ is applied to them, they can also show toxic properties. For example, under the treatment of excessive NH4+, rice showed a phenotype of yellowing leaves and inhibited growth, especially under low potassium conditions. The deposition of a high degree of NH4+ can cause a large number of deaths of the red spruce in the forest, which is considered to be highly resistant to NH4+. The resistance to NH4+ poisoning varies with the species and the growth period.
2 Symptoms of NH4+ poisoning
NH4+ poisoning can change the morphology of plants. The concentration of exogenous NH4+ that can produce NH4+ poisoning is generally 0.1~0.5mmol/L. Barley is a plant that is sensitive to NH4+. Studies on it have shown that NH4+ toxicity can cause yellowing of barley leaves and inhibit plant growth, especially root elongation. In addition, NH4+ poisoning can also cause other visible symptoms, such as lowering the root/leaf ratio and reducing yield. More importantly, NH4+ poisoning can also inhibit the germination and seedling establishment of seeds, and it can cause the extinction of certain seed plants in nature. The accumulation of excessive NH4+ in plants can also affect the absorption of certain nutrient elements and the balance of hormones, and can lead to a decrease in the concentration of soluble carbonic acid. At the same time, it can increase the concentration of amino acids. In addition, NH4+ stress can also induce a large amount of ROS (reactive oxygen species) in plants, including superoxide radicals O2- and H2O2. Although the absorption of inorganic cations is reduced under NH4+ poisoning conditions, the total amount of NH4+ absorbed is still very large, resulting in that the cation concentration in the plant is still higher than the anion concentration. At the same time, plants poisoned by NH4+ can acidify the environment around the root system. The possible reason for this phenomenon is to balance the charge disorder in the plant, and a large number of protons flow out from the plant. However, plants cultivated under nitrate nitrogen conditions can cause alkalization of the surrounding environment.
3 Mechanism of excessive NH4+ toxicity
Many researchers have speculated on the principle of the phenomenon that excessive NH4+ can be harmful to plants. Most people believe that a large amount of NH4+ in plants leads to a decrease in the absorption of K+, Mg2+ and Ca2+, thereby causing ion disorders in plants. The rapid assimilation of NH4+ in roots requires a large amount of carbon skeleton as a substrate to complete the assimilation of NH4+ into amino acids This process leads to insufficient carbon skeletons in the roots of plants, and eventually causes toxicity. The assimilation of NH4+ can also lead to the release of H+, and further cause decarboxylation reaction, which is a decrease in the concentration of carbonic acid in the cell. The decrease in the concentration of carbonic acid caused by the decarboxylation of the root further leads to the lack of the carbon skeleton in the root, which causes an increase in the absorption of anions to balance the intracellular charge. Some researchers believe that NH4+ toxicity is related to the hormone disorder in plants. The phenomenon of plant main root growth inhibition caused by excessive NH4+ is believed to be related to auxin transport or signal pathways, and ethylene production is found in the leaves of plants with higher NH4+ concentration. In addition, some researchers believe that NH4+ toxicity is related to the reduction of light sum rate. In addition, some researchers believe that NH4+ toxicity is related to the decrease of light sum rate. The results of recent research on barley show that under high NH4+ concentration, the growth of barley is inhibited due to the waste of energy caused by NH4+ passing through the plasma membrane. Excessive NH4+ in the cell is transported out of the cell by an unknown transporter at the expense of energy. The high respiration rate in the roots and the energy requirement for NH4+ excretion suggest that this may be the essential reason for the damage of high concentrations of exogenous NH4+ to plants. In plant cells, excessive accumulation of NH4+ results in excessive transport of NH4+ across the plasma membrane, thereby forming NH4+ toxicity. Many bacteria in the environment of low concentration of K+ and high concentration of NH4+ ions, due to the transport of excess NH4+ across the plasma membrane, resulting in enhanced respiration. The latest research found that GDP-mannose pyrophosphorylase (GMPase) is related to NH4+ sensitivity in Arabidopsis roots. In this study, the decrease in GMPase activity caused defects in the N-glycosylation process of proteins, which is considered to be an important molecular level downstream step involved in the phenomenon of NH4+ inhibiting root growth. N-glycan activation is necessary for the correct folding of proteins, and plays an important role in initiating protein folding, cellulose synthesis, cell wall stability and maintaining cell viability. GDP-mannose is very important for the correct N-glycosinification process of protein in Arabidopsis and the synthesis of ascorbic acid. cty1 is a mutant with nonsense mutations in GMPase. GMPase is a key enzyme in the synthesis of GDP-mannose. This mutant exhibits an embryonic lethal phenotype. vtc1 and hsn1 are homologous genes of GMPase. Under normal conditions, the phenotypes of these two mutant strains are not different from those of the wild type, but when in a high NH4+ environment, the root length of the mutant strain is obviously suppressed. The embryonic lethality shown in cty1 and the phenotype that the root lengths of vtc1 and hsn1 are sensitive to NH4+ are all caused by the defect of N-glucose activation. However, the question of whether the glycostimulation of protein is related to the process of NH4+ transport across the plasma membrane remains to be further studied.
4 Mitigation of NH4+ toxicity
The harm caused by NH4+ toxicity is great, but we can alleviate it by certain means. Increasing the pH of the solution can alleviate the degree of NH4+ toxicity to a certain extent. In addition, optimizing the light intensity can also alleviate the NH4+ toxicity symptoms of plants grown on the medium with NH4+ as the sole nitrogen source. Because NH4+ poisoning can affect the absorption of nutrient elements by plants, enhancing the concentration of nutrient elements in the environment, especially the concentration of nutrient cations, can also alleviate NH4+ poisoning to a certain extent. Either in the culture medium or in the field, increasing the K+ concentration can alleviate the NH4+ toxicity.
When NH4+ is used as the only nitrogen source, it can form obvious NH4+ poisoning, but when it is used as a nitrogen source with nitrate, the symptoms of NH4+ poisoning can be significantly improved. Moreover, the mixed nitrogen sources can form a kind of synergistic growth, and the total growth rate exceeds the growth of the two nitrogen sources alone. More interestingly, this kind of synergistic growth has also been observed in the growth of conifers. There are two possible explanations for this phenomenon: one is that when nitrogen sources are mixed, it can increase the synthesis of cytokinin; the other is that the absorption of nitrate by plants can cause the alkalinization of plant roots, thereby alleviating the plant roots caused by NH4+ poisoning. The acidification process of the roots.
For NH4+-sensitive species, excessive NH4+ has a serious inhibitory effect on their growth and yield. Many crops are sensitive to NH4+, and their yield is greatly affected by NH4+ poisoning. In addition, NH4+ poisoning has a significant impact on ecology. A more effective way to alleviate NH4+ toxicity is based on the understanding of NH4+ toxicity mechanism. The use of a mixed medium of nitrate and ammonium nitrogen and increasing the K+ concentration in the culture conditions to alleviate the NH4+ toxicity are successful application examples. However, because the symptoms of NH4+ poisoning are diverse, and it may be related to multiple metabolisms, the mechanism of the poisoning may not be single, which increases the difficulty of studying the NH4+ poisoning mechanism. Considering the degree of harm caused by NH4+ poisoning, more research is needed in the future to better reveal the mechanism of NH4+ poisoning.
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植物中NH4+的毒害 ( 小气鬼の小猫 )
植物体内的NH4+可以通过根系从土壤中直接吸收，也是某些途径如蛋白降解、光呼吸、和木质素的生物合成过程中，植物体内一些有机化合物的脱氨作用的产物。高浓度的NH4+能够对植物的生长产生抑制作用。在一些土地中，NH4+的浓度能够达到40 mmol/Lt (720ppm) 。NH4+作为唯一氮源时，能够对植物产生NH4+毒害。另外，在利用尿素作为肥料的土壤中，由于细菌脲酶的作用，将尿素大量的转化为NH4+和CO2，使土壤中NH4+浓度大幅度升高，从而能够造成植物的NH4+毒害。除此以外，许多非生物胁迫也能够引起植物体内的NH4+的超积累，如果不能有效的将NH4+同化，也能够产生NH4+毒害的症状。并且近来的研究发现，过量的NH4+的积累被认为是各类生态系统中植物物种灭绝的一个因素。因此，对于NH4+毒害机制的深入认识和对缓解NH4+毒害的方法的探索具有重要的意义.
植物体内过量的NH4+的积累还能够影响植物对某些营养元素的吸收和激素的平衡，并且能够导致溶解性碳酸浓度的下降，同时，它能够增加氨基酸的浓度。另外，NH4+胁迫也能够诱导植物体内产生大量的ROS (reactive oxygen species)，包括超氧自由基O2-和H2O2 。
请下载文件以对策 洗鹽 ( 鹽土的改良可利用排水、洗鹽方式排除過多鹽分 )