Ammonium toxicity

 

Toxicity NH4+ in the plant  ( Pampered little Musang King )

Abstract

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.

ammonium toxicity 1NH4+ 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.

ammonium toxicity 22 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.

ammonium toxicity 33 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.

nh4 toxicity4 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.

5 Outlook

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.

Please download “Salt Washing” for countermeasure  (the improvement of saline soil can use drainage or salt washing method to eliminate excessive salt)

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植物中NH4+的毒害 ( 小气鬼小猫 )

摘要

NH4+不但是植物生长必须的一类重要的氮源,而且也是存在于植物代谢中的一类广泛的中间产物。但是,当外界NH4+浓度过高的时候,能够对大多数的高等植物的生长产生抑制。在本文中,我们讨论了不同物种对NH4+的敏感性,和受NH4+毒害的植物表现出的症状,尤其是对NH4+毒害的机制和缓解NH4+毒害的手段做了较详细的总结论述。不同植物对NH4+毒害的敏感度是不同的,受NH4+毒害危害的植物的生长受到明显的抑制,然而,施用混合氮源和增加培养条件中K+浓度均能够降低NH4+毒害的危害程度.

ammonium toxicity 1

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.

1 NH4+敏感物种和NH4+抗性物种的分类
NH4+毒害的的现象是很常见的,但是不同的植物物种对它做出反应的阈值的不同的。在成熟植株中,銨毒害的症狀通常先出現在較低老葉上,而幼苗則是先出現在新葉上. 对NH4+离子毒性敏感的驯化植物中包括番茄、土豆、大麦、豌豆、蓖麻、芥末、甜菜、草莓和柑橘。在许多自然生态系统的土壤中,NH4+日益成为了主要的N源形式。野生型的草本植物山金车和蓟属荷兰芹、苦草肉苁蓉也都是对NH4+毒性尤其敏感的.

存在的对NH4+毒性抗性较强的植物包括驯化物种中的水稻和洋葱。野生植物包括石楠,乌拉草,山龙眼科的植物和一些温带被子树木。即使是对NH4+毒性抗性较高的物种,当对它们施用足够量的NH4+的时候,它们也能表现出中毒的性状。例如在过量NH4+的处理下,水稻表现出叶片变黄和生长受抑制的表型,尤其是在低钾的条件下。高度的NH4+的沉积能够引起森林中被认为是高度抗NH4+的物种红云衫的大量死亡。对NH4+毒害的抗性是随物种的不同,生长时期不同而变化的.

ammonium toxicity 2

2 NH4+毒害的症状
NH4+毒害能够使植物的形态发生改变。能够产生NH4+毒害的外源NH4+的浓度一般要在0.1~0.5mmol/L。大麦作为一种对NH4+敏感的植物,对其的研究显示,NH4+毒害能够造成大麦的叶片发黄,并且抑制植物的生长,尤其是抑制根的伸长。除此以外,NH4+毒害还能够造成其他一些可见的症状,如造成根/叶的比率下降、降低产量等。更为重要的是,NH4+毒害还能够对种子的萌发和立苗产生抑制作用,它能够造成自然界中某些种子植物的灭绝。

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 。

尽管在NH4+毒害条件下,降低了无机阳离子的吸收,但是吸收的NH4+的总量还是很大的,从而导致了植物体内的阳离子浓度仍然比阴离子浓度高。同时,受NH4+毒害的作用的植物能够将根系周围的环境酸化。这一现象可能的原因是为了平衡植物体内的电荷紊乱,大量的质子从植物中流出。然而,在硝态氮条件中培养的植株能够导致周围环境的碱化.

ammonium toxicity 3

3过量NH4+产生毒害的机制
许多研究者对过量的NH4+能够对植物产生毒害的现象的原理做了推测。大多数人认为,植物体内大量的NH4+导致对K+、Mg2+和Ca2+吸收的降低,从而引起植物体内离子的紊乱. 根中快速同化的NH4+,需要大量的碳骨架作为底物完成NH4+同化成氨基酸的过程,从而引起植物的根中碳骨架的不足,最终引起毒。NH4+的同化也能够导致H+的释放,并且进一步引起脱羧反应,是细胞中碳酸的浓度下降。根中脱羧导致的碳酸浓度的下降,进一步导致了根中碳骨架的缺乏,从而引起了增加吸收阴离子来平衡细胞内的电荷。

有研究者认为,NH4+毒害与植物体内激素的紊乱有关。过量的NH4+导致的植物主根生长受抑制的现象被认为是与生长素的转运或者信号途径有关的,并且在NH4+浓度较高的植物的叶中,发现有乙烯产生。

另外,有研究者认为NH4+毒害与光和速率的降低有关另外,有研究者认为NH4+毒害与光和速率的降低有关。

最近对大麦的研究结果显示,在高NH4+浓度下,生长受抑制的大麦,是由于NH4+穿过质膜导致的能量浪费造成的。细胞内过量的NH4+被一个未知的转运蛋白以消耗能量为代价转运出细胞。根中高的呼吸速率和排出NH4+对能量的需求暗示了,这可能是高浓度的外源NH4+对植物形成危害的本质原因。在植物细胞中,过量的积累NH4+,形成了对NH4+的跨质膜的过多的转运,从而形成了NH4+毒害。许多细菌在低浓度的K+和高浓度的NH4+离子的环境中,由于对过量的NH4+进行跨质膜转运,从而导致了呼吸作用的增强。

最新的研究发现GDP-甘露糖焦磷酸化酶(GMPase)与拟南芥根中的NH4+敏感有关。在这个研究中,GMPase活性的降低,造成了蛋白质的N-糖基化过程的缺陷,这被认为是参与NH4+抑制根生长现象的下游重要的分子水平的步骤。N-糖激化是蛋白正确折叠所必须的,并且对启动蛋白折叠、纤维素的合成、细胞壁的稳定性以及维持细胞的活力方面具有重要的作用。GDP-甘露糖是对于拟南芥中蛋白质的正确的N-糖激化的过程和抗坏血酸的合成是很重要的。cty1是GMPase发生无义突变的突变株,GMPase是GDP-甘露糖合成中的关键酶,这个突变株表现出了胚胎致死的表型。vtc1hsn1是GMPase的同源基因,在正常的条件下,这两个突变株表现出的表型与野生型没有差异,但是当处于高NH4+环境中,突变株的根长明显的受到抑制在cty1中表现出的胚胎致死,和vtc1hsn1的根长对NH4+敏感的表型,都是由于N-糖激化的缺陷造成的。然而,对于蛋白的糖激化是否与NH4+跨质膜转运的过程有关的问题还有待进一步的研究.

nh4 toxicity4 NH4+毒性的缓解
NH4+毒性造成的危害是很大的,但是我们可以通过一定的手段来缓解它。提高溶液中的pH能够在一定程度上缓解NH4+毒害的程度。另外优化光强度也能够缓解在NH4+为唯一氮源的培养基上生长的植物的NH4+毒害症状。由于NH4+毒害能够影响植物对营养元素的吸收,增强环境中的营养元素的浓度,尤其是营养阳离子的浓度也能够在一定程度上缓解NH4+毒害。无论是在培养基中还是在大田里提高K+的浓度,都能够起到缓解NH4+毒害的作用。

当NH4+作为唯一氮源供给的时候,能够形成明显的NH4+毒害,但是当与硝酸盐共同作为氮源的时候,NH4+毒害的症状能得到明显的改善。并且,混合的氮源能够形成一种协同生长,总的生长速率超过了这两种氮源单独存在时候的生长情况。更为有趣的是,在针叶树的生长中也观察到了这种协同生长的现象。对这个现象的可能解释有两个:一是当混合氮源的时候,能够提高细胞分裂素的合成;二是植物吸收硝酸根能够导致植物的根部碱化,从而缓解了由于NH4+毒害产生的植物根部的酸化程.

5 展望
对于NH4+敏感的物种来说,过量的NH4+对其生长和产量的抑制作用是很严重的,并且有许多农作物都属于对NH4+敏感的类型,它们的产量受NH4+毒害的影响也是很大的。另外,NH4+毒害对生态方面的影响也是很显著的。更为有效的缓解NH4+毒害的手段是基于对NH4+毒害机制的理解的基础上的。利用硝酸盐的和铵态氮的混合培养基和增加培养条件中K+浓度缓解NH4+毒害的手段就是成功的应用实例。但是,由于NH4+毒害产生的症状是多样性的,并且它可能与多种代谢相关,产生的毒害的机理也可能不是单一的,这就增加了对NH4+毒害机制研究的难度。考虑到NH4+毒害造成的危害的程度,将来还需要更多的研究,才能更好的揭示NH4+毒害的机理.

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 )

 

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