环境科学研究  2018, Vol. 31 Issue (1): 117-122  DOI: 10.13198/j.issn.1001-6929.2017.03.04

引用本文  

钟爱文. 埃格草(Egeria densa)对铵氮胁迫的生长和生理响应[J]. 环境科学研究, 2018, 31(1): 117-122.
ZHONG Aiwen. Ammonium Nitrogen Stress on the Growth and Physiological Responses of Egeria densa[J]. Research of Environmental Sciences, 2018, 31(1): 117-122.

基金项目

国家自然科学基金项目(31560149);江西省自然科学基金项目(20151BAB204013)
Supported by National Natural Science Foundation of China (No.31560149); Natural Science Foundation of Jiangxi Province of China (No.20151BAB204013)

责任作者

作者简介

钟爱文(1978-), 女, 湖南新邵人, 助理研究员, 博士, 主要从事水生植物生理生态学研究, 16887634@qq.com

文章历史

收稿日期:2017-05-04
修订日期:2017-07-14
埃格草(Egeria densa)对铵氮胁迫的生长和生理响应
钟爱文     
江西省·中国科学院庐山植物园, 江西 庐山 332900
摘要:为了解富营养水体中NH4+-N胁迫对埃格草(Egeria densa)的影响,通过室外模拟试验,研究了埃格草在不同ρ(NH4+-N)(0、0.5、2.0 mg/L)下的RGR(relative growth rate,相对生长率)、R/S(root/shoot ratio,根冠比)、w(SC)(SC为可溶性糖,soluble sugar)、w(淀粉)、w(蔗糖)、w(FAA)(FAA为游离氨基酸,free amino acid)、w(NH4+-N)和w(NO3--N)的变化.结果表明:随着外源ρ(NH4+-N)的增加,埃格草的RGR和R/S呈降低的趋势,并且在ρ(NH4+-N)为2.0 mg/L时显著降低(RGR为P < 0.001,R/S为P < 0.05);埃格草中w(SC)和w(淀粉)在ρ(NH4+-N)为0.5和2.0 mg/L下有不同程度显著降低[w(SC)为P < 0.01和P < 0.001,w(淀粉)为P < 0.001和P < 0.05],w(蔗糖)在ρ(NH4+-N)为2.0 mg/L时显著降低(P < 0.001);w(FAA)和w(NH4+-N)有随外源ρ(NH4+-N)升高而升高的趋势,并且在ρ(NH4+-N)为2.0 mg/L时升高显著[w(FAA)为P < 0.01,w(NH4+-N)为P < 0.05];w(NO3--N)在ρ(NH4+-N)为0.5和2.0 mg/L下有不同程度显著降低(P < 0.01和P < 0.001).相关分析表明,w(SC)、w(淀粉)和w(蔗糖)之间呈显著正相关,三者与w(FAA)和w(NH4+-N)之间均呈显著负相关,而与w(NO3--N)呈显著正相关;w(FAA)和w(NH4+-N)呈显著正相关,而二者与w(NO3--N)均呈显著负相关.研究显示,NH4+-N影响埃格草的生长,导致C-N代谢的不平衡.
关键词埃格草    铵氮    胁迫    碳氮代谢    
Ammonium Nitrogen Stress on the Growth and Physiological Responses of Egeria densa
ZHONG Aiwen     
Lushan Botanical Garden, Jiangxi Province & the Chinese Academy of Sciences, Lushan 332900, China
Abstract: In order to understand the effects of ammonium nitrogen (NH4+-N) stress on Egeria densa in eutrophic water bodies, morphological and physiological variations such as relative growth rate (RGR), root/shoot ratio (R/S), soluble sugar (SC), starch, sucrose, free amino acid (FAA), NH4+-N, and nitrate nitrogen (NO3--N) of E. densa at different external NH4+-N concentrations of 0, 0.5 and 2.0 mg/L were investigated in an outdoor simulation experiment. The results showed that the RGR and R/S ratio of E. densa showed a tendency to decrease with increasing external NH4+-N concentration, but decreased significantly with 2.0 mg/L NH4+-N. The SC andstarch contents decreased significantly with 0.5 and 2.0 mg/L NH4+-N. The sucrose content decreased significantly with 2.0 mg/L NH4+-N. The FAA and NH4+-N contents tended to increase with increasing external NH4+-N concentration, but increased significantly with 2.0 mg/L NH4+-N. The NO3--N contents tended to decrease significantly with 0.5 and 2.0 mg/L NH4+-N. The SC, starch, and sucrose contents were positively correlated with each other, but were negatively correlated with FAA and NH4+-N and positively correlated with NO3--N. The FAA and NH4+-N were positively correlated with each other, but were negatively correlated with NO3--N. Our experimental results showed that external NH4+-N can affect the growth of E. densa and lead to an imbalance in carbon-nitrogen metabolism.
Keywords: Egeria densa    ammonium nitrogen    stress    carbon-nitrogen metabolism    

湖泊富营养化导致的沉水植物衰退是世界性生态问题.由工业污染、农业径流及生活污水引起的N和P过量积累已在世界范围内导致湖泊的富营养化和沉水植物的衰退[1-2],柱中高浓度的NH4+-N是其重要因素之一.由于很难将NH4+-N从水体环境中清除,因此富营养水体中NH4+-N能对水生植物造成胁迫[3-8].当NH4+-N与NO3--N共存时,沉水植物偏好优先利用NH4+-N,其吸收速率与底泥或湖水中的NH4+-N浓度呈正比例相关[9-11],并且当它作为唯一的氮源时一般对植物都是有害的[12-14].当前国际上对NH4+-N胁迫导致水生植物衰退的研究最为关注[15-16].

曹特等[4]在2007年的研究中发现,过量NH4+-N(ρ>0.56 mg/L)会导致苦草种群的退化,并扰乱植物体内可溶性碳水化合物与FAA(free amino acid, 游离氨基酸)的代谢平衡[17];其后在2009年的研究中发现,水柱中1 mg/L NH4+-N能导致沉水植物菹草(Potamogeton crispus)体内FAA的积累和SC(soluble sugar,可溶性糖)的消耗,并且在低光条件下,这种胁迫效应会显著加强[18].沉水植物的叶片对抑制NH4+-N的吸收缺少反馈机制,所以在高NH4+-N环境中它们就会倾向于吸收超过生长和维持生命所需要的N量[19].高NH4+-N对沉水植物的生长和生存有直接的抑制作用,甚至能引起很多水生植物的衰退[5-6, 20-21]. NH4+-N胁迫会诱导抗氧化系统的活化并产生过量的活性氧[7, 22]. NH4+-N增加也会干扰营养的吸收和激素的平衡,导致C和N储备物质的不平衡,并且对叶绿素的合成和光合作用有负面影响[23].为了阻止NH4+-N积累导致的毒性,许多水生植物会把它运出植物细胞以及/或者是合成FAA和胺化合物,这些过程需要消耗能量和碳水化合物[3-5, 15, 23-24],而这会使沉水植物中C-N代谢发生变化.

埃格草(Eaeria densa)是水鳖科埃格草属多年生的草本沉水植物,分布于阿根廷、巴拉圭及巴西等地,中国多有栽培.其叶色、株形美丽,是水族箱内的良好沉水型观赏植物.但其对NH4+-N的耐受性的研究较为鲜见,因此该研究展开埃格草对NH4+-N胁迫的生长和生理响应研究有助于理解其对富营养化环境的适应性,以期为水体中沉水植被修复时先锋物种的选择提供理论依据.

1 材料与方法 1.1 植物培养和处理

试验在庐山植物园鄱阳湖分园的水泥池进行.在对植物枝条进行NH4+-N处理之前,用从鄱阳湖最北端的鞋山湖区采回的底泥〔w(TP)为1.89 mg/g(以干质量计,余同),w(TN)为4.93 mg/g〕和自来水〔ρ(PO43--P)为0.015 mg/L,ρ(NO3--N)为1.35 mg/L,ρ(NH4+-N)未检测到〕在室外对植物进行2个月以上的预培养. 2015年5月,把108株形状大小一致的埃格草种植在9个玻璃缸(长×宽×高为50 cm×50 cm×80 cm)中,每个玻璃缸中均匀放置4个塑料桶(塑料桶中盛有15 cm厚的采自鞋山湖区的底泥),每个桶中种15株,总计共种植540株.所有的植物高度一致,均为15 cm左右,下部约5 cm插入底泥.玻璃缸中的自来水深75 cm.玻璃缸上方用一层黑色遮阳网遮去20%的日光.整个试验过程中,中午水面光照强度用Li-1400照度计(美国LICOR公司)测定为165~350 μmol/(m2·s),水温采用多参数水质检测仪MP6500(Alalis,中国)测定为22~32 ℃.

埃格草种植25 d后,每2 d在进行NH4+-N处理的玻璃缸中加入NH4HCO3,使ρ(NH4+-N)达到0.5和2.0 mg/L,对照组ρ(NH4+-N)为0 mg/L,每处理3个重复.处理21 d后开始采样,每个玻璃缸中每次采集一个塑料桶中的植物为一个样,植物样品先用自来水冲洗干净,其中一半样品用于称量植物鲜质量和植物根、茎鲜质量,用以测定植物的RGR(relative growth rate, 相对生长率)和R/S(root/shoot ratio, 根冠比);另一半样品在80 ℃下烘48 h并研磨成均匀粉末用于测定w(SC)、w(淀粉)、w(蔗糖)、w(FAA)、w(NH4+-N)和w(NO3--N).

1.2 生物化学分析

烘干样品测定前经80 ℃烘干0.5~1.0 h,研磨过100目(0.149 mm)筛,每个重复样品称量3个50 mg平行样.称量好的植物干样用80%乙醇(分析纯)提取,提取后的上清液用于测定w(SC)和w(蔗糖)[25]以及w(FAA)[26],上清液中w(NH4+-N)和w(NO3--N)的测定参照Hecht等[27]的方法;离心后试管内残渣经30%高氯酸提取2 h后10 000 r/min高速离心取上清液用于测定w(淀粉)[28].

1.3 统计分析

采用SPSS 23.0软件对所有数据进行统计分析.不同浓度外源NH4+-N处理对埃格草RGR、R/S、w(SC)、w(淀粉)、w(蔗糖)、w(FAA)、w(NH4+-N)和w(NO3--N)的影响采用单因素方差分析. C-N化合物之间相关性采用Spearman等级相关系数表征.

2 结果与讨论 2.1 NH4+-N对埃格草生长的影响

图 1可见,与对照组相比,在外源ρ(NH4+-N)为2.0 mg/L时埃格草的RGR极显著下降(P < 0.001),R/S显著下降(P < 0.05).

注:n=3;***表示P<0.001; *表示P<0.05. 图 1 不同NH4+-N处理21 d后埃格草RGR和R/S的变化 Fig.1 Changes of RGR and R/S of E. densa after 21 days treatments of different NH4+ availability
2.2 NH4+-N对埃格草C代谢的影响

图 2可见,与对照相比,在外源ρ(NH4+-N)为0.5和2.0 mg/L条件下,埃格草中w(SC)分别减少了29.7%(P < 0.01)和45.7%(P < 0.001);w(淀粉)分别减少了61.4%(P < 0.001)和51.5%(P < 0.05);w(蔗糖)分别减少了7.4%(P > 0.05)和40.4%(P < 0.001).

注:n=3;***表示P < 0.001; **表示P < 0.01; *表示P < 0.05. 图 2 不同NH4+-N处理21 d后埃格草w(SC)、w(淀粉)、w(蔗糖)的变化 Fig.2 Changes of w(SC), w(starch) and w(sucrose) of E. densa after 21 days treatments of different NH4+-N availability
2.3 NH4+-N对埃格草N代谢的影响

图 3可见,与对照相比,在外源ρ(NH4+-N)为0.5和2.0 mg/L条件下,埃格草中w(FAA)分别增加了48.8%(P > 0.05)和84.5%(P < 0.01);w(NH4+-N)分别增加了17.3%(P > 0.05)和114.1%(P < 0.05);w(NO3--N)分别减少了15.9%(P < 0.01)和25.7%(P < 0.001).

注:n=3;***表示P < 0.001; **表示P < 0.01; *表示P < 0.05. 图 3 不同NH4+-N处理21 d后埃格草w(FAA)、w(NH4+-N)和w(NO3--N)的变化 Fig.3 Changes of w(FAA), w(NH4+-N) and w(NO3--N) of E. densa after 21 days treatments of different NH4+-N availability
2.4 NH4+-N胁迫下埃格草C-N代谢生化指标相关分析

表 1可见,3种碳化合物含量〔w(SC)、w(淀粉)和w(蔗糖)〕之间呈显著正相关(P < 0.01),其与w(FAA)和w(NH4+-N)之间呈显著负相关(P < 0.01),与w(NO3--N)之间呈显著正相关(P < 0.01). w(FAA)和w(NH4+-N)之间呈显著正相关(P < 0.01),w(FAA)、w(NH4+-N)与w(NO3--N)之间呈显著负相关(P < 0.01).

表 1 埃格草中w(SC)、w(淀粉)、w(蔗糖)、w(FAA)、w(NH4+-N)和w(NO3--N)相关性分析 Table 1 Pearson correlation analysis of the relationship among w(SC), w(starch), w(sucrose), w(FAA), w(NH4+-N) and w(NO3--N) of E. densa
3 讨论 3.1 NH4+-N对埃格草生长的影响

研究[3-4, 15, 24]表明,沉水植物易受富营养水体中NH4+-N胁迫,NH4+-N胁迫严重抑制植物生长乃至导致植株死亡.该研究中外源ρ(NH4+-N)为2.0 mg/L时埃格草RGR显著降低,说明其生长受到了NH4+-N的胁迫. CAO等[4]指出,沉水植物出现严重胁迫甚至死亡的ρ(NH4+-N)临界值为0.56 mg/L,并认为是长江中下游湖泊水生植物大规模衰退的临界浓度.已有研究认为,植物同化大量的NH4+-N需要耗费很多碳水化合物[29],并因此影响到植物的生长[30].该研究中埃格草在外源ρ(NH4+-N)为2.0 mg/L时其R/S显著降低,说明NH4+-N在一定浓度范围内主要促进茎叶而非根的生长,可能的原因是植物叶片能方便地从水柱中吸收NH4+-N.其实沉水植物的根和叶都具有吸收N、P营养的能力[31],水柱中ρ(NH4+-N)为0.1 mg/L时,狐尾藻(Myriophyllum spicatum)的氮源多达90%来自叶片所吸收的NH4+-N[9]. CAO等[5]在试验中也观察到了R/S随着ρ(NH4+-N)的增加而降低.这可能是植物生物量分配形式和形态的可塑性,这种可塑性能增加植物适应外界环境的能力.

3.2 NH4+-N对埃格草C-N代谢的影响

埃格草中w(SC)、w(淀粉)和w(蔗糖)均随ρ(NH4+-N)的增加而显著降低,说明碳水化合物在高NH4+-N胁迫的环境中消耗很快,von Wiren等[32-33]提出了碳水化合物的消耗很可能是用于转化FAA的碳骨架的假说.埃格草中w(FAA)和w(NH4+-N)都随ρ(NH4+-N)的增加而显著上升,说明NH4+-N胁迫导致埃格草中FAA和NH4+-N的积累. NH4+-N是水生植物优先利用的无机氮源,当外界营养水平高时能大量被植物吸收[34].在该研究中,埃格草中w(FAA)与外源ρ(NH4+-N)呈显著正相关,反映了植物吸收外界NH4+-N并通过GS-GOGAT途径大量合成FAA[3-5, 35].不少植物在高ρ(NH4+-N)胁迫下会大量合成FAA以减轻NH4+-N的毒性,即使这一途径需要消耗能量和碳水化合物[36]. CAO等[3-5, 24]也发现,高ρ(NH4+-N)条件下w(SC)大幅下降而w(FAA)大幅上升,并认为该现象与消耗SC来转化氨基酸的机制有密切关系.这些研究表明,水生植物在外界高NH4+-N胁迫下的C-N代谢平衡呈现失调的趋势.

植物组织中C-N代谢的失衡表明水柱中过高的ρ(NH4+-N)对水生植物存在毒性[3-5, 15-16].研究发现,陆生植物体内C-N代谢的失衡与外界N的大量添加有关[37];在水生植物中也发现,水体富营养化包括N源的大幅增加导致沉水植物〔如苦草(Vallisneria natans)、菹草(Potamogeton crispus)金鱼藻(Ceratophyllum demersum)〕和挺水植物〔苔草(Carex rostrata)与芦苇(Phragmites communis)〕的C-N代谢的失衡[4, 15, 24, 35, 38-42]并伴有氧化胁迫的发生[3, 5, 22, 24].该研究结果表明,埃格草中w(NH4+-N)和w(FAA)一样都有不同程度的显著增幅,说明被植物吸收的NH4+-N除了合成FAA之外,还有一部分累积在植物体内.植物体内NH4+-N积累过多可使光合磷酸化解偶联,导致植物碳水化合物储备量下降[43],Britto等[44]研究发现,对NH4+-N较敏感的大麦需要消耗大量的能量用于维持NH4+-N的跨细胞膜无效循环(futile transmembrane NH4+ cycling).该研究中,w(NH4+-N)和w(FAA)都与w(NO3--N)之间呈显著负相关,外源NH4+-N的添加使得埃格草体内的FAA合成增加与NH4+-N积累是一方面的原因;另一方面,可能是外源NH4+-N的添加使得植物营养环境pH等发生改变,从而影响NH4+-N和NO3--N在埃格草中的相互转化[45].研究发现,作为一种诱导酶,在根和叶存在的硝酸还原酶强烈依赖于ρ(NO3--N)[46]ρ(NO3--N)能直接影响硝酸还原酶的表达[47]. GAO等[48]研究发现,长时间暴露在高ρ(NH4+-N)下的金鱼藻的硝酸还原酶的活性会受到影响,导致N代谢的紊乱,进而导致植物内部的物理伤害.该研究中外源NH4+-N的添加使得NH4+-N向NO3--N的亚硝化和硝化作用减弱或NO3--N向NH4+-N的还原作用增强,从而导致埃格草中NO3--N的减少.

4 结论

a) 埃格草中RGR、R/S和w(蔗糖)在ρ(NH4+-N)为2.0 mg/L时显著降低;w(SC)和w(淀粉)在ρ(NH4+-N)为0.5和2.0 mg/L时都有不同程度显著降低;w(FAA)和w(NH4+-N)都随外源ρ(NH4+-N)的升高而升高,并且在ρ(NH4+-N)为2.0 mg/L时升高显著;w(NO3--N)随外源ρ(NH4+-N)的升高而显著降低.

b) 埃格草中w(SC)、w(淀粉)和w(蔗糖)呈显著正相关,这3种碳化合物与两种氮化合物w(FAA)和w(NH4+-N)之间都是显著负相关,而与w(NO3--N)之间显著正相关. w(FAA)和w(NH4+-N)两者显著正相关,而与w(NO3--N)呈显著负相关. NH4+-N影响埃格草的生长,导致C-N代谢的不平衡.

致谢:

感谢山西农业大学张莉老师对英文摘要的修改.

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