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(Joy, 1988)。高浓度的NH4+能够对植物的生长产生抑制作用(Gerendás et al., 1997)。在一些土地中，NH₄⁺的浓度能够达到40 mmol/Lt (Kronzucker et al., 2003)。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(Bremner and Krogmeier, 1989)。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(Lutts et al., 1999; Skopelitis et al., 2006)。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(Clark and Tilman, 2008)。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。对NH₄⁺离子毒性敏感的驯化植物中包括番茄(Magalhäes and Huber, 1989)、土豆(Cao and Tibbits, 1998)、大麦(Britto et al., 2001)、豌豆(Bligny et al., 1997)、蓖麻(van Beusichem et al., 1988)、芥末(Vollbrecht et al., 1989)、甜菜(Breteler, 1973)、草莓(Claussen and Lenz, 1999)和柑橘(Dou et al., 1999)。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(de Graaf et al., 1998)、苦草(Hauxwell et al., 2001)和肉苁蓉(Westwood and Foy, 1999)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(Sasakawa and Yamamoto, 1978)和洋葱(Abbes et al., 1995)。野生植物包括石楠(de Graaf et al., 1998)，乌拉草(Falkengren-Grerup, 1995)，山龙眼科的植物(Smirnoff et al., 1984)和一些温带被子树木(Garnett et al., 2001)。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。

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(Li et al., 2010)。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 (Wang et al., 2010)。

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(van Beusichem et al., 1988)。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(Goodchild and Givan, 1990)。

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(Roosta and Schjoerring, 2007)。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，最终引起毒害(Schjoerring et al., 2002)。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(Raven and Smith, 1976)。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(Barker, 1999; Walch-Liu et al., 2000)。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(Cao et al., 1993)，并且在NH₄⁺浓度较高的植物的叶中，发现有乙烯产生(You and Barker, 2004)。

另外，有研究者认为NH₄⁺毒害与光和速率的降低有关(Puritch and Barker, 1967; Gerendás et al., 1997)。
 
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(Britto et al., 2001)。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(Qin et al., 2008;Barth et al., 2010)。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(Lerouge et al., 1998)，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(Lukowitz et al., 2001; Koiwa et al., 2003; Hoeberichts et al., 2008; Kang et al., 2008)。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(Conklin et al., 1999; Lukowitz et al., 2001)。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(Nickle and Meinke, 1998; Lukowitz et al., 2001)。vtc1和hsn1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(Conklin et al., 1999; Qin et al., 2008)，但是当处于高NH₄⁺环境中，突变株的根长明显的受到抑制(Qin et al., 2008; Barth et al., 2010)。在cty1中表现出的胚胎致死，和vtc1和hsn1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。

4 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(Barker et al., 1967; Lips et al., 1990; Zhang et al., 1990; Feng and Barker, 1992; Barker, 1995)。

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(Gill and Reisenauer, 1993; Schortemeyer et al., 1997)。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(Heberer and Below, 1989)。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(Chen et al., 1998)；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度(Marschner, 1995)。

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。

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