葉欣怡,趙杏,王小鵬,鐘一銘,楊京平(浙江大學(xué)環(huán)境與資源學(xué)院,杭州310058)

土壤亞鐵、鎘對(duì)水稻2種抗氧化酶和植株富集鎘量的影響

葉欣怡,趙杏,王小鵬,鐘一銘,楊京平*
(浙江大學(xué)環(huán)境與資源學(xué)院,杭州310058)

摘要我國(guó)南方潴育及潛育型水稻土中積累有大量亞鐵,有研究表明土壤亞鐵會(huì)影響鎘在植株中的富集及鎘對(duì)植株的氧化性脅迫.試驗(yàn)設(shè)置4個(gè)土壤Fe(2+)水平(外源加入亞鐵分別為0 mg/kg、100 mg/kg、200 mg/kg和400 mg/kg)和2個(gè)Cd(2+)水平(外源加入鎘分別為0 mg/kg和5 mg/kg),研究不同亞鐵和鎘的共同作用對(duì)孕穗期水稻葉片脂質(zhì)過氧化、抗氧化酶活性以及對(duì)根膜和植株中重金屬累積量的影響.在無外源亞鐵添加的條件下,鎘使葉片中脂質(zhì)過氧化的監(jiān)測(cè)指標(biāo)丙二醛(malondialdehyde,MDA)含量下降了31.1%,同時(shí)抑制了超氧化物歧化酶(superoxide dismutase,SOD)和過氧化物酶(peroxidase,POD)活性.隨著土壤亞鐵量的增加,特別是在外源添加亞鐵400 mg/kg時(shí),鎘對(duì)水稻植株產(chǎn)生的上述影響得到了緩解.隨著土壤亞鐵量的升高,水稻的根膜、根和葉片對(duì)重金屬鎘的富集呈下降的趨勢(shì),且試驗(yàn)表明水稻根及葉片對(duì)鎘的積累與根際膜中含鎘量呈顯著正相關(guān).研究表明土壤中亞鐵量的增加會(huì)降低水稻中鎘的積累.

關(guān)鍵詞水稻;根膜;脂質(zhì)過氧化;超氧化物酶;過氧化物酶

浙江大學(xué)學(xué)報(bào)(農(nóng)業(yè)與生命科學(xué)版) 42(1):89～98,2016

Journal of Zhejiang University(Agric.&Life Sci.)

http://www.journals.zju.edu.cn/agr

E-mail:zdxbnsb@zju.edu.cn

第一作者聯(lián)系方式:葉欣怡(http://orcid.org/0000-0002-9767-5439),E-mail:546784861@qq.com

Effects of soil Fe2+and Cd2+on activities of antioxidant enzymes and Cd accumulation in rice plants.Journal of Zhejiang University(Agric.&Life Sci.),2016,42(1):89-98

YE Xinyi,ZHAO Xing,WANG Xiaopeng,ZHONG Yiming,YANG Jingping*(College of Environmental and Resource Sciences,Zhejiang University,Hangzhou 310058,China)

Summary Divalent heavy metal cation cadmium(Cd2+)causes phytotoxicity in plants.The high bioaccumulation index in plants and soil will also threat the human health through food chain.It has been shown that Cd will inhibit germination of seeds and exert a wide range of adverse effects on growth and metabolism of plants.It was reported that high concentration of Fe2+in paddy soil could exert some impacts on plant growth and Cd2+accumulation.Cadmium and ferrous with high concentration in soil,are known to cause oxidative damage to plants either directly or indirectly by triggering an increased level of production of reactive oxygen species(ROS).Plants have antioxidative enzymes,such as superoxide dismutase(SOD),catalase and peroxidase(POD),which can scavenge ROS to avoid oxidative damage.Malondialdehyde(MDA)level is used as an index of lipid peroxidation under stressful conditions in plants.Therefore activity of antioxidative enzymes(SOD and POD)and MDA level can be used to monitor the oxidative stress of plants.In anaerobic waterlogged environment,rice can form iron plaque ____around root surface and screen metals by adsorption and co-precipitation.Thus,the availability and balance of Fe/Cd in the rhizosphere of rice will be influenced.Many researchers investigated the impacts of ferrous and cadmium respectively on rice growth,while few efforts were made in interaction of these two metal ions,which exist in production conditions.

In order to investigate mechanisms of Fe and Cd stress on rice in real production condition,the pot experiment with 4 soil Fe2+levels(577,677,777 and 977 mg/kg)and 2 soil Cd2+concentrations(0.413 and 5.413 mg/kg) was conducted.The rice cultivar used in the experiment was Hang43.Shoots of rice collected at booting stage were ground with liquid nitrogen and were homogenized in phosphate buffer(p H 7.8).The extracted supernatant was used to assay SOD activity,POD activity and MDA level,by NBT method,guaiacol colorimetric method and the thiobarbituric acid method,respectively.Fe and Cd concentrations in shoots and roots were determined by flame atomic absorption spectrophotometer(FAAS)after digestion procedures.Fe and Cd contents in iron plaque on the roots were extracted using dithionite-citrate-bicarbonate(DCB)method,and were measured by AAS.

The experimental results showed that ferrous and cadmium in soil had interaction on the activities of lipid peroxidation,antioxidative enzymes and uptakes of these two metals.Cd2+remarkably decreased MDA levels by about 31.1%,meanwhile depressed activities of superoxide dismutase(SOD)and peroxidase(POD)enzyme.Whereas,those adverse effects were mediated by increasing Fe2+concentration in soil(especially at 977 mg/kg Fe2+).Variations of SOD activity and POD activity had similar trend in all the treatments,though POD activity was more stabilized,indicating SOD undertakes more protection responsibility under experimental conditions.Fe contents in iron plaque maintained 1-2 g/kg under all treatments,while Cd concentration in iron plaque declined quickly with the increase of soil ferrous level.Ferrous in soil stimulated the uptakes of Cd at a certain range but a relatively higher ferrous concentration in soil decreased the Cd contents in roots and shoots.

It can be concluded that iron plaque helps to screen Cd2+in the rhizosphere and inhibits the uptake of Cd2+in rice.Fe2+in soil can alleviate the stress of Cd2+by restraining the accumulation of Cd2+.

Key words rice;iron plaque;lipid peroxides;superoxide dismutase;peroxidase

鎘能經(jīng)食物鏈的生物富集和生物放大作用毒害高等動(dòng)物[1-2],因而受到廣泛關(guān)注.近年來的研究表明,植物非必需營(yíng)養(yǎng)元素鎘也具有植物毒性.鎘能抑制光合作用[3]、破壞呼吸系統(tǒng)和造成營(yíng)養(yǎng)代謝紊亂[2,4-7],從而影響植物生長(zhǎng),導(dǎo)致植物死亡.水稻是我國(guó)主要糧食之一,在稻田生態(tài)系統(tǒng)中,不僅土壤重金屬鎘會(huì)對(duì)水稻產(chǎn)生毒害作用,有時(shí)植物必需微量元素鐵也可能威脅水稻生長(zhǎng)[8 11].長(zhǎng)期淹水的稻田土壤因處于厭氧狀態(tài)而積累大量亞鐵,植物吸收過多亞鐵將會(huì)出現(xiàn)代謝失調(diào)、營(yíng)養(yǎng)紊亂[8]、光合作用能力降低[9-10]等一系列中毒現(xiàn)象,致使作物減產(chǎn)甚至死亡.鐵和鎘具有重疊的生理功能影響區(qū)[12],它們均在植物光合作用系統(tǒng)、氧化還原系統(tǒng)、營(yíng)養(yǎng)代謝系統(tǒng)發(fā)揮作用,因而在現(xiàn)實(shí)條件下它們對(duì)水稻生長(zhǎng)的影響可能存在交互作用[13].有研究表明,植株對(duì)鎘的吸收與環(huán)境中鐵的狀態(tài)和含量有關(guān)[6,14].

鎘及過量亞鐵造成的植物毒害主要是產(chǎn)生活性氧自由基(reactive oxygen species,ROS)[6,8,11].活性氧會(huì)造成植物氧化性損傷,破壞脂質(zhì)、蛋白質(zhì)、核酸等生物大分子[15].植物自身的抗氧化酶系統(tǒng),可以清除活性氧自由基(ROS)[2],從而減少氧化性損傷.其中,超氧化物歧化酶(superoxide dismutase,SOD)可以將超氧陰離子(.)歧化成H2O2和O2[2,16];過氧化物酶(peroxidase,POD)以愈創(chuàng)木酚為電子受體,利用H2O2氧化植物體內(nèi)有機(jī)物或無機(jī)物[2].丙二醛(malondialdehyde,MDA)是植物膜脂質(zhì)過氧化的產(chǎn)物[15],可代表生物膜的損傷程度,是檢驗(yàn)氧化脅迫重要指標(biāo)之一[2].研究表明,植株通過增加SOD、POD活性來抵御亞鐵毒害[10,17],但長(zhǎng)期或過量亞鐵脅迫也會(huì)使SOD、POD活性受到抑制[17-18].鎘也會(huì)對(duì)水稻抗氧化酶系統(tǒng)產(chǎn)生影響,但文獻(xiàn)所述的影響并不一致[2,7,19-20].如SHAH等[2]認(rèn)為鎘會(huì)造成水稻脂質(zhì)過氧化,刺激SOD和POD活性;CHIEN等[7]研究表明鎘刺激水稻葉片中POD活性,但抑制了SOD活性;邵國(guó)勝等[15]則認(rèn)為鎘能增加MDA含量,卻抑制了SOD和POD活性.鎘對(duì)植株抗氧化酶系統(tǒng)的影響可能與不同品系植株本身、生長(zhǎng)時(shí)期及處理?xiàng)l件相關(guān).

土壤在厭氧環(huán)境下,水稻根際釋放的氧氣和氧化劑可以將Fe2+氧化成Fe3+,鐵的氧化物或氫氧化物將附著于根表,形成根膜[5,21].根膜可以吸附、共沉淀多種元素[21-22],進(jìn)而影響這些元素在根際的含量和植物可用性,改變植株對(duì)它們的吸收遷移能力[23-24].研究表明,水稻根際膜能影響鎘的吸收遷移,可以減少[5,23]或增加[1,24]植株對(duì)鎘的吸收,這可能與植株品系、根際膜厚度和植株生長(zhǎng)時(shí)期等因素相關(guān)[5,23].

《全國(guó)土壤污染狀況調(diào)查公報(bào)》顯示我國(guó)土壤鎘污染較為嚴(yán)重,鎘米仍有可能成為糧食安全的隱患[25-26].調(diào)查表明我國(guó)南方水稻土中有大量亞鐵累積[27],在實(shí)際水稻種植條件下,土壤亞鐵會(huì)對(duì)鎘的毒害作用產(chǎn)生影響.以往研究多集中于2種金屬的單獨(dú)作用,雖然也有通過水培試驗(yàn)分析其交互作用[15-20,26],但水培條件與實(shí)際土壤狀況差異較大.因此,本試驗(yàn)將通過盆栽模擬水稻實(shí)際生長(zhǎng)環(huán)境,研究在南方土壤中亞鐵含量較高的條件下,以對(duì)金屬有一定富集量且抗氧化酶系統(tǒng)受植株衰老影響較小的孕穗期的水稻為研究對(duì)象,探討含大量亞鐵的南方潛育型土壤中鐵、鎘對(duì)水稻部分抗氧化酶活性,及鐵、鎘吸收的影響.從而為探索實(shí)際生產(chǎn)過程中水稻重金屬脅迫的機(jī)制提供一定的理論基礎(chǔ).

1　材料與方法

1.1供試材料

盆栽試驗(yàn)在浙江大學(xué)農(nóng)業(yè)試驗(yàn)站網(wǎng)室內(nèi)進(jìn)行.供試水稻品種為杭43(單季稻),水稻秧苗來自杭州市農(nóng)業(yè)科學(xué)研究院.供試土壤采自杭州市華家池稻田(長(zhǎng)期種植的水稻田),為0～20 cm耕層土壤,土壤基本理化性質(zhì)如表1.

1.2試驗(yàn)處理

將自然風(fēng)干并過2 mm篩的稻田土壤,按每桶7.5 kg干土裝入桶中.試驗(yàn)采用隨機(jī)分組的方式,設(shè)置4個(gè)土壤Fe2+水平F1、F2、F3和F4(外源加入亞鐵量分別為0 mg/kg、100 mg/kg、200 mg/kg和400 mg/kg);2個(gè)Cd2+水平C0,C1(外源加入總鎘量分別為0 mg/kg,5 mg/kg),共8個(gè)處理,每個(gè)處理3個(gè)重復(fù)(共24個(gè)桶).根據(jù)每種處理所要求的亞鐵及鎘含量,以硫酸亞鐵和硫酸鎘溶液的形式拌入土壤,混合均勻.將處理過的土重新裝入桶中,淹水(水面超過土面約1 cm)鈍化培養(yǎng)1個(gè)月.土壤添加亞鐵和鎘處理后每種處理土壤總亞鐵和鎘含量見表2.水稻種植為每桶3穴,每穴2株水稻(144株水稻).

每盆水稻施入尿素225 kg/hm2(以純氮計(jì))并分別以225 kg/hm2和75 kg/hm2的量施入磷肥(過磷酸鈣),鉀肥(氯化鉀).每盆土表面積為0.049 m2,土深為25 cm.每盆施入尿素、過磷酸鈣和氯化鉀的量分別為1.103 g、1.102 g和0.367 g.

1.3樣品采集及測(cè)定

1.3.1水稻生化指標(biāo)測(cè)定

在水稻孕穗期(移栽后51 d)采集水稻頂部3片完全展開葉片,在液氮環(huán)境下研磨,并用p H 7.8的磷酸緩沖液提取葉片中所需測(cè)定生化物質(zhì).SOD活性采用氮藍(lán)四唑(NBT)還原法[28];POD活性采用愈創(chuàng)木酚比色法[29];MDA含量用硫代巴比妥酸(TBA)比色反應(yīng)測(cè)定[28].

1.3.2水稻葉片葉綠素測(cè)定

用SPAD-502微型葉綠素計(jì)對(duì)孕穗期水稻頂部3片完全展開葉片進(jìn)行測(cè)量,每片葉子取3個(gè)點(diǎn)位(距葉片基部1/2處及其上下各約3 cm的位置)進(jìn)行測(cè)量并取平均值作為該張葉片的SPAD值.

1.3.3水稻根膜提取及金屬含量的測(cè)定

用DCB(dithionite citrate bicarbonate)法提取水稻根膜中的鐵和鎘[30-31].采集孕穗期水稻根,用自來水沖去泥土后用去離子水沖洗3次.將根放入150-m L錐形瓶中加入60 m L提取液(摩爾濃度0.3

mol/L檸檬酸鈉和0.1 mol/L碳酸氫鈉)和1.2 g硫代硫酸鈉,在27℃條件下振蕩2 h.將提取液轉(zhuǎn)移至100 m L容量瓶定容,用火焰原子吸收分光光度法(FAAS)測(cè)定其含鐵和鎘量.

1.3.4水稻植株中金屬含量的測(cè)定

采用微波消解-火焰原子吸收法測(cè)定,用萬分之一天平準(zhǔn)確稱取105℃殺青2 h并在80℃烘干至恒量的水稻葉片及提取過根膜的根0.2 g于聚四氟乙烯消解管中,加入5 m L HNO3、1 m L HF和1 m L H2O2.先在120℃下預(yù)消解20 min后進(jìn)行微波消解,之后在150℃下趕酸,剩余消解液定容至100-m L的容量瓶中,用火焰原子吸收分光光度法(FAAS)測(cè)定其含鐵和鎘量.

1.4數(shù)據(jù)處理

利用SPSS 20.0進(jìn)行雙因素方差分析,用鄧肯(Duncan)檢驗(yàn)分析各處理間差異性(顯著性水平α=0.05).

2　結(jié)果與分析

2.1鐵、鎘相互作用對(duì)水稻生理生化指標(biāo)的影響

在鐵、鎘共同處理下,表征水稻葉片葉綠素水平的SPAD值沒有顯著變化(圖1).通過雙因素方差分析,外源亞鐵和鎘對(duì)水稻葉片MDA含量存在交互作用.在無鎘處理(C0)下,水稻葉片中MDA含量隨著土壤亞鐵量的增加而呈減少的趨勢(shì),特別是在F3和F4處理下有顯著降低(圖1),大約是F1處理下MDA含量的45%.相關(guān)性分析表明MDA與葉片中含鐵量呈顯著負(fù)相關(guān)(表3).在F1處理下,有鎘處理(C1)顯著降低了葉片MDA含量,是無鎘處理(C0)下MDA含量的68.9%.這表明在一定亞鐵含量下(F1)鎘的存在會(huì)加劇植株?duì)I養(yǎng)代謝失調(diào).從整體來看,在有鎘處理(C1)下,水稻葉片中MDA含量在試驗(yàn)所設(shè)所有亞鐵含量條件下保持穩(wěn)定水平.

SOD與POD是植物減少活性氧(ROS)損傷的抗氧化酶系統(tǒng)[1].圖2所示,在F1和F2條件下,SOD活性在土壤高鎘處理(C1)時(shí)明顯低于無外源添加鎘(C0)的處理,鎘在這2個(gè)亞鐵處理下大約抑制了60% 的SOD活性.隨著外源亞鐵處理量的增加,在C1處理下SOD活性在不斷增強(qiáng),并逐漸與在C0處理下SOD活性水平相近似.這表明亞鐵量的提高能減少鎘對(duì)植株的影響.相關(guān)性分析也表明,SOD活性與葉片中含鎘量呈顯著負(fù)相關(guān),而與葉片中的含鐵量呈正相關(guān)(表3).在鎘污染的土壤中,亞鐵量的增加將會(huì)刺激SOD活性.在試驗(yàn)所設(shè)亞鐵條件下,鎘抑制了POD活性.在不同處理下,POD活性變化與SOD活性變化有著相似的趨勢(shì).相關(guān)性分析表明,兩者之間存在顯著正相關(guān).然而,POD活性變化幅度比SOD活性變化幅度小,表明在本試驗(yàn)的鎘和亞鐵條件下,SOD發(fā)揮了主要的保護(hù)作用,而POD處于較為次要的地位.在無外源添加鎘時(shí),F3、F4處理使得SOD、POD活性相對(duì)于F1、F2處理下均有所下降,這是由于亞鐵脅迫超過水稻自身抵御能力,產(chǎn)生過量的活性氧(ROS)消耗了這2種抗氧化酶[17,31].

2.2鐵、鎘相互作用對(duì)植株根膜、根及葉片中鐵、鎘含量的影響

在本試驗(yàn)條件下,根膜中鐵元素含量不因土壤中亞鐵或鎘量的變化而呈顯著性差異,其質(zhì)量分?jǐn)?shù)基本維持在1～2 g/kg(圖3).對(duì)于根膜中的含鎘量而言,在無鎘處理(C0)下,根膜中鎘的含量隨土壤亞鐵濃度呈現(xiàn)先降低后增加的趨勢(shì),在F3處理時(shí)達(dá)到最低,但整體而言變化幅度較小.土壤在高鎘(C1)條件下,根膜中含鎘量隨著土壤中亞鐵量的增加呈明顯減少的趨勢(shì),且在F4處理下其含量與F0處理下的含量接近.這可能是由于亞鐵離子的競(jìng)爭(zhēng)作用使得鎘向根際微區(qū)轉(zhuǎn)移能力降低,從而表現(xiàn)出隨著土壤亞鐵濃度的增加,土壤中的鎘會(huì)更少地吸附沉積在根膜中.

本試驗(yàn)所設(shè)土壤亞鐵水平(577～977 mg/kg)對(duì)根的含鐵量影響不顯著(圖4),根中含鐵量范圍維持在2.25～3.75 g/kg.這可能是在較高亞鐵條件下,根通過限制吸收,減少亞鐵積累的一種自我保護(hù)機(jī)制.土壤在低鎘(C0,外源加鎘0 mg/kg)條件下,根中所積累的鎘含量維持在一個(gè)穩(wěn)定較低的水平(約11.5 mg/kg),且不隨外源鐵變化而有所改變,這與根膜中鎘始終維持在一定水平有關(guān).但是,土壤在高鎘(C1,外源加鎘5 mg/kg)條件下,根對(duì)鎘的積累會(huì)隨土壤亞鐵量的變化而有波動(dòng),呈先增加后減少的趨勢(shì),在F2處理(土壤含F(xiàn)e2+量為677 mg/kg)時(shí)鎘達(dá)到最高值(102.4 mg/kg).這表明一定量的土壤亞鐵能刺激根對(duì)鎘的富集,但隨著土壤中亞鐵的升高又會(huì)阻礙根對(duì)鎘的吸收.

對(duì)水稻葉片中含鐵量分析發(fā)現(xiàn),葉片中含鐵量隨土壤亞鐵量的增加而增加,而外源土壤鎘的添加減少了鐵在葉片中的積累(圖5).葉片中的含鎘量與根中的含鎘量有著一致的變化趨勢(shì),即隨著含亞鐵量的增加呈先增加后減少的趨勢(shì),在F2處理(土壤含F(xiàn)e2+量為677 mg/kg)時(shí)鎘達(dá)峰值(12.1 mg/kg).相關(guān)性分析(表4)顯示,葉片中的含鎘量與根及根膜中含鎘量呈顯著正相關(guān),這反映了根膜中的鎘是植株吸收、累積鎘的主要來源.此外,正是由于亞鐵在根膜與鎘的競(jìng)爭(zhēng)減少了根膜的含鎘量,從而抑制了植株對(duì)鎘的積累,減緩了鎘對(duì)植株的影響.劉文菊等[32]的研究也表明,根際膜的存在可以抑制植株對(duì)鎘的吸收.同時(shí),本試驗(yàn)還發(fā)現(xiàn),根對(duì)鎘的富集遠(yuǎn)大于葉片,這與CHIEN等[7]的研究結(jié)果一致.

3　討論

前人研究表明,鐵是植物必需營(yíng)養(yǎng)元素,也是葉綠素形成的基礎(chǔ)元素,缺鐵會(huì)使水稻黃化[33],過量的亞鐵會(huì)使水稻積累大量氧自由基,氧化葉綠素[10],同樣導(dǎo)致葉綠素水平下降[9].此外,有報(bào)道稱鎘會(huì)使植物葉綠素水平減少[15,34].但本試驗(yàn)未能驗(yàn)證以上結(jié)果,這可能是因?yàn)镾PAD值作為間接指標(biāo)受到諸如氮素等營(yíng)養(yǎng)水平及植株自身?xiàng)l件等因素的綜合影響,所以并不能敏感的反映較高含量下外源亞鐵變化對(duì)植株的影響.同時(shí),在高亞鐵環(huán)境下,試驗(yàn)所設(shè)的含鎘量不足以影響葉綠素水平.有研究表明只有在高鎘處理下,水稻葉片SPAD值才會(huì)有所降低[15].

孕穗期是水稻由營(yíng)養(yǎng)生長(zhǎng)轉(zhuǎn)變?yōu)樯成L(zhǎng)過渡期[35].研究表明水稻葉片對(duì)鎘的富集呈現(xiàn)分蘗期＜孕穗期＜抽穗期的趨勢(shì)[36].水稻抗氧化酶系統(tǒng)受到逆境脅迫和植株衰老2種作用的影響,在生育晚期,植株衰老將成為主導(dǎo)因素[36],使得抗氧化酶系統(tǒng)對(duì)金屬脅迫將不再敏感.丙二醛(MDA)是植物膜脂質(zhì)過氧化的產(chǎn)物[15].多數(shù)實(shí)驗(yàn)研究表明,在過量亞鐵和鎘的脅迫下,誘發(fā)的活性氧自由基將會(huì)攻擊膜脂,使得水稻MDA含量增加[37 38].然而本實(shí)驗(yàn)結(jié)果顯示,水稻葉片中MDA含量隨亞鐵量增加呈下降趨勢(shì),這可能是在高亞鐵脅迫下植株機(jī)體本身出現(xiàn)了失調(diào)現(xiàn)象[39].在F1處理下,鎘使得MDA含量大幅減少,這可能是由于外源鎘的施入加劇了植株代謝失調(diào).隨著外源亞鐵量增加,鎘的施入對(duì)MDA量不再有顯著影響,這可能是高亞鐵減少了實(shí)驗(yàn)所設(shè)土壤含鎘量對(duì)水稻葉片MDA的不利影響.相關(guān)性分析表明,MDA含量與SOD和POD活性不相關(guān).在實(shí)驗(yàn)條件下,鎘抑制了SOD和POD活性,盡管這與邵勝國(guó)等[15]的研究結(jié)果相一致,但本實(shí)驗(yàn)表明土壤在不同亞鐵量下鎘對(duì)SOD、POD活性的抑制能力并不相同.這說明鎘對(duì)植株的氧化脅迫會(huì)受到土壤中亞鐵的影響,在相對(duì)較高的亞鐵量下(777～977 mg/kg)鎘對(duì)SOD、POD活性的抑制能力減弱.在不同處理下,POD比SOD活性變化幅度小,這表明本實(shí)驗(yàn)在鎘和亞鐵條件下,SOD發(fā)揮了主要的保護(hù)作用,這與王貴民等[17]的研究結(jié)果一致.值得注意的是,在無外源添加鎘時(shí),F3、F4處理使得SOD、POD活性有所下降,這可能是由于亞鐵脅迫超過水稻自身抵御能力,產(chǎn)生過量的活性氧(ROS)消耗了這2種抗氧化酶[17,31].

史錕等[40]研究表明受到根表鐵的富集點(diǎn)位所限,在土壤較高亞鐵量下,根表所能富集的鐵膠膜量趨于穩(wěn)定,這與本實(shí)驗(yàn)根膜中的含鐵量并不因土壤中亞鐵量變化而有顯著改變的結(jié)論一致.然而,根膜中的含鎘量卻隨著土壤中亞鐵量的增加而降低,這可能是由于亞鐵離子的競(jìng)爭(zhēng)作用使得鎘向根際微區(qū)轉(zhuǎn)移的能力降低,從而表現(xiàn)出隨著土壤亞鐵量的增加,土壤中的鎘會(huì)更少地吸附沉積在根膜中,這與史錕等[40]研究的水培下孕穗期水稻根膜中鐵、鎘含量結(jié)果相似.

本實(shí)驗(yàn)根中含鎘量隨亞鐵量增加呈先增加后減少的趨勢(shì),表明土壤在高鎘條件下,土壤亞鐵量增加在一定程度上刺激了鎘向根的富集,但隨著土壤亞鐵的進(jìn)一步提高,又會(huì)阻礙根對(duì)鎘的吸收富集.前人研究表明,水稻可以通過Fe2+跨膜運(yùn)輸?shù)鞍?Ⅰ)或借助Fe3+載體(Ⅱ)吸收鐵.Ⅱ方式是缺鐵條件下水稻吸收鐵的主要方式.本試驗(yàn)土壤在所設(shè)亞鐵條件下,水稻主要通過Ⅰ方式吸收鐵元素.Ⅰ方式所涉及的鐵載體蛋白也可以有效地運(yùn)輸鎘[6,14,33],外源亞鐵量從F1(577 mg/kg)增加到F2(677 mg/kg)時(shí),可能刺激水稻更多地通過Ⅰ方式吸收鐵,產(chǎn)生了更多親附鎘的鐵載體,從而使根富集了更多鎘.但是,隨著亞鐵量的升高,其與鎘的競(jìng)爭(zhēng)加強(qiáng),使得根對(duì)鎘的富集能力逐漸降低.同時(shí)高外源亞鐵也會(huì)抑制鐵載體的表達(dá)[6],從而減少植株對(duì)鐵和鎘的吸收.此外,根膜中鎘量的降低也可能是根中鎘量減少的原因之一.葉片對(duì)鎘的富集受到根和根膜富集鎘量的影響,隨著土壤亞鐵量的增加呈下降趨勢(shì),這也可能是隨著亞鐵量增加,鎘對(duì)植株MDA含量以及POD 和SOD活性的影響變得不顯著的原因之一.

4　結(jié)論

4.1在本試驗(yàn)條件下的土壤中亞鐵和鎘含量的增加對(duì)水稻葉片SPAD值沒有顯著的影響.

4.2在577 mg/kg的亞鐵條件下,鎘顯著降低了葉片中MDA含量,加劇了水稻營(yíng)養(yǎng)代謝失調(diào).但是隨著土壤亞鐵量的升高,鎘所產(chǎn)生的不利影響變得不再顯著.土壤亞鐵的增加減少了植株對(duì)鎘的吸收和富集,這也是高亞鐵能夠減少鎘對(duì)水稻葉片MDA含量以及SOD和POD活性的影響的部分原因.

4.3鎘的存在抑制了SOD和POD活性,但其抑制能力與土壤中含鐵量有關(guān).在試驗(yàn)所設(shè)較低亞鐵條件下鎘對(duì)2種抗氧化酶活性的抑制能力要高于試驗(yàn)所設(shè)較高亞鐵條件下的抑制能力.鎘也抑制了鐵在水稻葉片的富集,這表明鐵和鎘在水稻吸收和富集金屬過程中存在競(jìng)爭(zhēng)關(guān)系.

4.4試驗(yàn)表明水稻根及葉片積累富集鎘的量與根膜中含鎘量呈正相關(guān).控制鎘在植株根膜的積累可以作為減少植株吸收富集鎘的一個(gè)方式.隨著土壤含鐵量增加,鎘在根膜中含量減少.由于土壤高鐵量所產(chǎn)生根膜的屏蔽作用使得鎘向植株遷移的可能性降低.此外,土壤中含鐵量較高會(huì)阻礙高親和性鐵載體的表達(dá),從而減少了植物對(duì)鎘的吸收和積累.

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收稿日期(Received):2015-07-01;接受日期(Accepted):2015-09-10;網(wǎng)絡(luò)出版日期(Published online):2016-01-19 URL:http://www.cnki.net/kcms/detail/33.1247.S.20160119.1940.016.html

*通信作者(

Corresponding author):楊京平(http://orcid.org/0000-0002-3212-0184),E-mail:jpyang@zju.edu.cn

基金項(xiàng)目:國(guó)家自然科學(xué)基金(61174089).

DOI:10.3785/j.issn.1008-9209.2015.07.011

中圖分類號(hào)X 53;S 511

文獻(xiàn)標(biāo)志碼A