王成塵,田 穩(wěn),向 萍*,徐武美,管冬興,馬奇英

土壤-水稻/小麥重金屬吸收機制與安全調(diào)控

王成塵1,田 穩(wěn)1,向 萍1*,徐武美2,管冬興3,馬奇英3

(1.西南林業(yè)大學生態(tài)與環(huán)境學院,環(huán)境修復與健康研究院,云南 昆明 650224；2.云南師范大學能源與環(huán)境科學學院,云南 昆明 650500；3.浙江大學環(huán)境與資源學院,浙江 杭州 310058)

為掌握我國水稻和小麥作物的重金屬污染情況,收集了我國糧食主產(chǎn)區(qū)水稻/小麥-土壤系統(tǒng)中的砷(As)、鎘(Cd)、鉻(Cr)和鉛(Pb)含量,綜述了其吸收、轉運、積累機制和有效的修復措施.結果表明,我國糧食主產(chǎn)區(qū)水稻及小麥籽粒的Cd超標率達31.3%和22.2%,Pb的超標率達26.2%和32.1%,污染情況較為突出;降低As、Cd、Cr和Pb的生物有效性,控制其在水稻/小麥植株中的吸收,能夠有效減少籽粒中的積累;土壤-作物系統(tǒng)中重金屬的吸收可通過水肥管理、化學改良、植物修復、生物修復和遺傳學方法等有效調(diào)控,從而實現(xiàn)安全生產(chǎn).今后,應形成多學科交叉互融的全要素格局來完善我國的土壤污染研究,開發(fā)農(nóng)產(chǎn)品安全生產(chǎn)的污染土壤利用技術,從而更好地保障國家糧食安全生產(chǎn).

重金屬；水稻；小麥；轉運機制；積累；安全生產(chǎn)

水稻和小麥處于食物鏈的開端,需要從土壤中吸收必需和非必需元素,因此有毒金屬元素也會進入水稻和小麥,隨著食物鏈運輸傳遞最終進入人體[1].研究表明,作物中低濃度的重金屬也可能會損害人體健康[2].因此,研究農(nóng)作物(水稻和小麥)對重金屬As、Cd、Cr、Pb的吸收、遷移和積累具有重要的現(xiàn)實意義.近幾十年來,水稻/小麥的重金屬污染受到了廣泛關注,許多學者對土壤-水稻/小麥系統(tǒng)中的重金屬污染進行了大量研究[3-4].然而,大多數(shù)的研究都集中在小范圍內(nèi),針對全國尺度上的水稻/小麥重金屬污染研究鮮有報道.

糧食主產(chǎn)區(qū)的糧食作物重金屬含量很大程度上反映了我國的總體糧食重金屬污染水平.本文回顧了我國糧食主產(chǎn)區(qū)水稻/小麥籽粒中As、Cd、Cr、Pb污染的研究文章,分析計算其重金屬的濃度水平;同時,總結水稻/小麥中重金屬的吸收、積累機制和阻控措施,展望了未來研究前景,以期為全面了解中國糧食主產(chǎn)區(qū)水稻/小麥重金屬污染狀況,制定合理有效的防治策略提供參考.

1 研究區(qū)域和數(shù)據(jù)收集

我國糧食主產(chǎn)區(qū)是指擁有適宜種植糧食作物的地理區(qū)位、土壤條件、氣候、技術等,且種植比例大,糧食產(chǎn)量高,能夠滿足省內(nèi)糧食消費需求之余還能大量外調(diào)的特定糧食產(chǎn)區(qū)[5].從糧食種植的分布情況看,糧食主產(chǎn)區(qū)主要集中在東北地區(qū)、黃淮海地區(qū)以及長江中下游地區(qū),包括黑龍江、吉林、遼寧、山東、河北、內(nèi)蒙古、河南、江西、四川、湖南、湖北、江蘇和安徽省.

糧食主產(chǎn)區(qū)的水稻和小麥重金屬數(shù)據(jù)來自于Web of Science (WoS)核心合集和中國知網(wǎng) (CNKI),使用關鍵詞“水稻(rice)”、“小麥(wheat)”、“重金屬(heavy metals)”、“砷(As)”、“鎘(Cd)”、“鉻(Cr)”、“鉛(Pb)”、“中國(China)”等,時間設置為2000～2021年,選定的文獻應符合以下標準:1)糧食主產(chǎn)區(qū)范圍內(nèi)現(xiàn)場或當?shù)厮?小麥取樣,2)研究區(qū)域位置清晰或具有相關信息,3)水稻/小麥籽粒重金屬含量數(shù)據(jù)至少包含As、Cd、Cr、Pb其中一種,4)使用科學準確的測定重金屬含量的方法,如電感耦合等離子體質(zhì)譜法(ICP-MS)或原子吸收光譜法(AAS)等.經(jīng)過人工篩選文獻后共收集水稻籽粒樣品數(shù)據(jù)6078份,小麥籽粒樣品數(shù)據(jù)3024份,及相關土壤樣點數(shù)據(jù)10178份.

2 結果與討論

2.1 我國糧食主產(chǎn)區(qū)水稻和小麥籽粒的重金屬污染情況

2.1.1 水稻籽粒的重金屬污染與來源 據(jù)統(tǒng)計,水稻籽粒中的As含量在0.016～0.31mg/kg范圍內(nèi),與《食品安全國家標準-食品中污染物限量》(GB2762-2017)[6]谷物及其制品中的稻谷限值(￡0.2mg/kg)相比,超標率約10.3%;Cd含量處于0.0007～1.42mg/kg之間,約31.3%的水稻籽粒超過限值(￡0.2mg/kg);Cr含量在0.046～0.8mg/kg范圍內(nèi),所有水稻籽粒皆未超過限值(￡1mg/kg);Pb含量在0.006～1.35mg/kg范圍內(nèi),26.2%的水稻籽粒超過限值(￡0.2mg/kg)(圖1a).由此可知,我國糧食主產(chǎn)區(qū)水稻籽粒的Cd及Pb超標情況較為突出,其中,Cd含量超標的地區(qū)集中在長江流域,包括湖南、湖北和江西省,而Pb含量超標的地區(qū)集中在江蘇省(圖2).據(jù)湖南省湘潭市水稻籽粒Cd 超標的文獻報道,該地水稻Cd的主要來源包括灌溉水、化肥和大氣沉降,其中,大氣沉降輸入占總輸入通量的76.4%～ 98.3%,顯著高于灌溉水和化肥的輸入通量;此外,農(nóng)家肥是畜禽養(yǎng)殖區(qū)水稻Cd的重要來源[7];而湖北恩施的研究發(fā)現(xiàn),該地富硒頁巖的風化導致重金屬(Cd、Zn、Cu、As等)大量進入土壤,從而造成了重金屬復合污染,其中,Cd在土壤中具有極高的生物利用度,這與水稻籽粒中Cd的高積累相對應[8];江西的水稻籽粒Cd超標的一份文獻表明,稻漁綜合種養(yǎng)示范區(qū)的土壤Cd值與稻谷中的含量不具有顯著相關性,說明其污染源不僅限于土壤,可能還包括化學肥料和農(nóng)藥的使用[9];江蘇省水稻籽粒Pb超標地區(qū)的研究顯示,該區(qū)域Pb來源主要是汽車尾氣和肥料的應用[10].

圖1 全國糧食主產(chǎn)區(qū)水稻和小麥籽粒的重金屬含量

圖2 我國糧食主產(chǎn)區(qū)水稻和小麥籽粒中As、Cd、Cr、Pb的空間分布特征

為了對比本文水稻籽粒超標地區(qū)的水稻土Cd、Pb水平,本文同時收集了其他一些國家水稻土重金屬含量的相關研究,并列出了《土壤環(huán)境質(zhì)量農(nóng)用地土壤污染風險管控標準(試行)》(GB 15618-2018)[11](以下簡稱土壤管控標準)的Cd、Pb風險值(表1).與土壤管控標準相比,本研究中水稻土壤的Cd和Pb濃度均超出風險值.相較于前人的研究報道,除泰國外,湖南水稻土中的Cd含量遠遠超出其他國家和地區(qū),雖然這種差異可能是由于報告分布的不均勻,如在礦山或工業(yè)場所周邊的稻田中研究的文獻較多,但是該數(shù)據(jù)也應引起重點關注,有必要采取有效的治理和控制策略,減少水稻土壤中的Cd污染;而江蘇水稻土Pb含量和其他文獻相比處于中等水平,低于全國水稻土Pb平均值30.69mg/kg[12],但卻產(chǎn)出糧食主產(chǎn)區(qū)范圍內(nèi)Pb含量水平最高的水稻籽粒,可能是因為大氣沉積或農(nóng)藥化肥的過量施用[10,13],也可能與該區(qū)域種植的水稻品種有關.

表1 不同地區(qū)水稻/小麥土與水稻/小麥籽粒中Cd和Pb含量對比

注: 括號中的值代表平均數(shù).

2.1.2 小麥籽粒的重金屬污染與來源 根據(jù)文獻統(tǒng)計,小麥籽粒中的As含量在0.0003～0.97mg/kg范圍內(nèi),與《食品安全國家標準-食品中污染物限量》(GB2762-2017)谷物及其制品中的小麥限值(￡0.5mg/kg)相比,超標率約4.76%;Cd含量處于0.0018～0.9mg/kg之間,約22.2%的小麥籽粒超過限值(￡0.1mg/kg);Cr含量在0.053～3.1mg/kg范圍內(nèi),其中,12.5%的小麥籽粒超過限值(￡1mg/kg);Pb含量在0.22～3.64mg/kg范圍內(nèi),32.1%的小麥籽粒超過限值(￡0.2mg/kg)(圖1b).由此可知,我國糧食主產(chǎn)區(qū)小麥籽粒的Pb及Cd超標情況較為突出,其中,Cd含量超標的地區(qū)集中在江蘇和四川省,而Pb含量超標的地區(qū)集中在江蘇、湖北和安徽省(圖2).據(jù)報道,江蘇地區(qū)小麥籽粒Cd超標可能是因為小麥在有氧條件下更有利于對Cd的吸收,此外,不同的加工處理也會導致小麥中Cd含量的差異,由于麩皮中往往含有較高濃度的Cd,打磨過程可以有效地降低Cd含量,而該研究采集到的小麥未經(jīng)打磨[14];江蘇省小麥籽粒Pb超標的文獻表明,研究區(qū)域?qū)儆趥鹘y(tǒng)農(nóng)業(yè)生產(chǎn)區(qū),且在取樣時避開了工業(yè)區(qū)和交通要道,因而其重金屬源主要來自農(nóng)藥化肥投入[15];湖北大冶的研究結合土壤Pb形態(tài)分布和作物中Pb在殼部的富集情況,證實了該地土壤和作物主要受大氣沉降的污染[16];而安徽省的小麥籽粒Pb超標可能是因為該樣點的農(nóng)民大量使用了復合肥,在短時間內(nèi)不被作物吸收,因此導致局部土壤Pb含量增加,此外,該區(qū)域的煙花炮竹生產(chǎn)業(yè)也會造成土壤Pb含量的升高[17].

由表1可以看出,本研究中江蘇、四川、湖北和安徽省皆有部分小麥土壤的Cd濃度超出土壤管控標準,但是土壤Pb濃度都低于管控值.與其他國家的數(shù)據(jù)相比,四川、湖北的小麥土中的Cd含量處于較高水平,而江蘇的土壤Cd含量并不高,可能是由于在本文的文獻采集標準下,江蘇省的數(shù)據(jù)點較多,四川、湖北的數(shù)據(jù)點相對較少,且文獻多偏向污染區(qū)域的研究,因此產(chǎn)生了樣點濃度的差異.而江蘇、湖北和安徽省的小麥土Pb含量和其他文獻相比處于中等水平,但小麥籽粒中的Pb含量卻高于其他地區(qū),造成這一情況的原因很復雜,可能與土壤的pH值、有機質(zhì)含量、小麥品種、氣候條件、農(nóng)業(yè)活動和污水灌溉等相關[3].

2.2 土壤-水稻/小麥系統(tǒng)中的重金屬吸收與轉運機制

農(nóng)田土壤環(huán)境在一定程度上決定著農(nóng)產(chǎn)品的質(zhì)量和產(chǎn)量.土壤中的微量元素含量分布受成土母質(zhì)、土壤理化性質(zhì)、土壤類型、水分動態(tài)等共同作用影響.而水稻和小麥植株各部位微量金屬元素的攝取和積累量遵循根>莖/葉>穎殼>籽粒的大小順序,其總濃度取決于暴露水平.

2.2.1 水稻中的重金屬吸收與轉運機制 水稻對As的吸收積累隨生長周期而變化.在分蘗期,根、莖、葉的As含量迅速增加,在拔節(jié)期顯著降低,孕穗期和灌漿期再次微幅上升,成熟期達到最大;而稻穗中的As濃度在孕穗期最大,灌漿期急速下降、成熟期微幅上升卻低于孕穗期[27].水稻籽粒中As的形態(tài)以一甲基砷(MMAV)、二甲基砷(DMAV)和無機砷(iAs)為主,其中,iAs主要包括亞砷酸鹽As(III)和砷酸鹽As(V)[28](圖3a).據(jù)報道,水稻在淹水缺氧環(huán)境下具有高度吸收和易位As(III)的能力[29].As(III)在水稻體內(nèi)的運輸主要通過Nod26-like內(nèi)在蛋白(NIPs)、硅(Si)流入和流出轉運體OSLsi1/OSLsi2,從而有效地將As轉運到水稻植株的各個部位[30].而磷酸鹽吸收系統(tǒng)是As(V)進入水稻植株的主要途徑[31].研究表明,水稻植株能夠降低其根部的As(V),且將As(III)有效地上傳至木質(zhì)部汁液中[29].與木質(zhì)部途徑相比,韌皮部的一些功能也會對籽粒As含量產(chǎn)生較大影響[32].不同的水稻品種和水稻基因型在As積累方面存在顯著差異[33],然而,稻根中As的較高吸收和積累的實際機制尚不清楚,據(jù)學者推測,這可能是因為在水稻根際表面形成的鐵氧化物(鐵斑塊)吸附了As,從而限制了其進一步轉移到植株的地表組織中去[34].

與As相比,水稻中Cd的主要吸收轉運過程包含根系吸收、通過木質(zhì)部流動進行的根向莖/葉的遷移、節(jié)間維管傳遞和籽粒積累等(圖3b).在Cd吸收過程中,木質(zhì)部在Cd從根到莖的運輸中起著重要作用[35],而韌皮部則是運輸Cd到籽粒的主要途徑[13].在水稻植株中,Cd從根到莖的吸收、遷移和積累通常由ZIP(OsIRT1)轉運體推進[36].這些ZIP轉運體對水稻的生長過程至關重要,而水稻植株關鍵的微量金屬元素的積累能力與ZIP的表達水平有關.據(jù)報道,OsIRT轉運體在通過稻田根部的Cd攝取中發(fā)揮了重要作用[13],而NRAMP3和NRAMP4(金屬轉運蛋白)轉運體及其共同轉運體在Cd轉運、體內(nèi)平衡以及抵抗水稻植物Cd毒性作用中也起著至關重要的作用[37].

相較As和Cd,研究土壤-水稻系統(tǒng)中Cr吸收機制的文獻較少.存在于土壤中的Cr 主要為Cr(III)和Cr(VI),其中,Cr(VI)具有較高可溶性和毒性;而Cr(III)的毒性較小[38].水稻植株容易吸收并積累Cr(III)和Cr(VI),而水稻土壤-水稻系統(tǒng)中的Cr吸收機制仍然有待進一步深入研究.一般來說,植物通過特定的轉運蛋白吸收所需的微量元素,從而維持其生理代謝活動[39],但在吸收過程中,也會同時吸收有害金屬元素.研究發(fā)現(xiàn),植物系統(tǒng)中的Cr(III)吸收通常通過被動運輸機制發(fā)生,由于Cr(VI)與磷酸鹽(Pi)/硫酸鹽(ST)的結構相似,因此,水稻植株會主動吸收Cr(VI)[40](圖3c).

而Pb可通過多種途徑進入水稻植物,如質(zhì)子泵、共轉運體、反轉運體和離子通道等[13](圖3d).據(jù)研究報道,Pb2+在水稻根系的吸收并不均勻,其中,在新生細胞中吸收最強[41].同時,蒸騰作用對Pb2+通過木質(zhì)部從根細胞到地上部,及通過維管束從地上部到莖葉部的推動也起著重要作用;然而,Pb從植株其他部位向水稻籽粒的易位機制尚不清楚[13].此外,研究證實,植株吸收的大部分Pb被保留在水稻根細胞中;根細胞進一步限制了Pb的質(zhì)外體和共質(zhì)體運輸,阻礙了Pb向地上組織的運輸[42].因此,僅有一小部分被吸收的Pb被轉移到地上部分,并被重新分配到植株的不同部分.

圖3 水稻/小麥-土壤系統(tǒng)中As、Cd、Cr和Pb的吸收、易位及積累機制

2.2.2 小麥中的重金屬吸收與轉運機制 自然界中的As主要以As(III)、As(V)、MMA和DMA等4種形態(tài)被植株吸收[43](圖3e).不同品種的小麥莖稈和籽粒對As的積累能力差異較大.其中.As(V)是低pH值或好氧/氧化條件下As的主要形態(tài),它與磷酸根共用轉運蛋白,即As(V)的吸收是通過H2AsO4-或H2PO4-與2H+協(xié)同運輸并進入木質(zhì)部導管[44].As(V)在根細胞內(nèi)由砷酸還原酶(AR)轉化為亞砷酸鹽As(III)[45].As(III)是高pH值、厭氧/還原條件下的主要形態(tài),由于其吸附能力低,能迅速從土壤礦物脫附,并被Nod26-like內(nèi)在蛋白(NIPs)吸收[46],從而進入植物體內(nèi).

與As不同,土壤酸化提高了植物Cd的生物有效性,且根系分泌物會增加其溶解度[47].Cd可以通過質(zhì)外體和共質(zhì)體途徑在根、莖和葉中運輸[48](圖3f).此外,小麥中的鎘Cd還可以通過3個主要轉運體到達根細胞:(1)鋅鐵轉運蛋白(ZIP),如AtIRT1是一種在重金屬積累中發(fā)生中介的質(zhì)膜轉運體,對二價金屬具有廣泛的特異性[49],研究發(fā)現(xiàn),當AtIRT1轉運體位于根的外層時,它會從土壤中吸收Cd;而當TcZNT1/TcZIP4位于根中時,TcZNT1轉運體可介導高親和力的Zn轉運和低親和力的Cd攝取[50];(2)自然抗性相關巨噬細胞蛋白家族(NRAMP),如OsNRAMP1、OsNRAMP5和AtNRAMP6,其中,OsNRAMP1和OsNRAMP5鐵轉運蛋白也被稱為質(zhì)膜中的Cd2+內(nèi)流轉運蛋白[51];(3)低親和力的鈣轉運體,如TaLCT1[52].Cr在小麥的攝取和轉運機制尚未完全清楚.由于Cr是植物的非必要元素,因此植株本身不具有吸收Cr的特定機制,通常是其他離子的轉運載體參與其吸收過程[53].Cr(VI)的吸收轉運途徑是一種涉及必需離子載體(如硫酸鹽ST)的主動運輸過程[54],而Cr(III)的吸收途徑是被動轉運過程[55].據(jù)報道,根細胞攝入Cr(IV)后會立即還原為Cr(III)[56],Cr主要通過木質(zhì)部運輸傳遞,從而到達莖稈、葉片和籽粒[53](圖3g).一般的,重金屬總是大量積累在植物體的根中,少量存在于營養(yǎng)器官和生殖器官[57].據(jù)猜測,植物根Cr積累較高可能是因為Cr被固定在植株根細胞的液泡中,其毒性降低,從而維持自身的正常生理過程[58].

有關小麥Pb的分子吸收機制文獻較少.據(jù)報道,Pb通常以兩種方式進入小麥植株,一種是通過根從土壤中吸收Pb,第二種是通過葉片吸收大氣沉降中的Pb[59].從土壤中吸收的Pb會積累在根、莖、葉和種子等部位,其中,大部分留存于根中,主要分布在外根冠、覆蓋于根冠表面的粘液、根表皮細胞和內(nèi)皮層的細胞壁中,只有一小部分向上轉移到地上組織[60],很少有Pb能夠穿透根系內(nèi)皮層進入中柱部分,因此,內(nèi)皮層是Pb傳遞到新芽/莖的屏障[61](圖3h).

2.3 降低水稻/小麥重金屬污染的安全生產(chǎn)措施

現(xiàn)階段發(fā)現(xiàn)的與重金屬積累相關的因素包括土壤-植物重金屬吸收的動態(tài)過程、與吸收和易位相關的遺傳標記和作物收獲后的管理活動,而這些因素的有效運作取決于該作物的品種、生長環(huán)境和栽培管理的有機結合[22].為此,國內(nèi)外開展了一系列廣泛研究,表2和表3分別列舉了降低水稻/小麥重金屬污染的方法.主要包括:(1)農(nóng)業(yè)管理方法:水管理、外源添加物(化學改良)、養(yǎng)分管理和土壤改良劑;耕作方式管理;(2)生物修復法:植物修復和微生物修復;(3)遺傳學方法等.上述方法的選擇主要依賴于水稻/小麥的品種(類別)、環(huán)境條件、吸收機制和積累特征等.

2.3.1 農(nóng)業(yè)管理方法 農(nóng)業(yè)管理方法是通過改善土壤的物理化學性質(zhì)從而固定污染土壤中的重金屬.水管理方法的應用是基于土壤水分狀況能夠決定土壤pH值和氧化還原電位,并影響?zhàn)B分的溶解性和有效性[62].研究發(fā)現(xiàn),在營養(yǎng)期采取干濕交替的灌溉方法能夠降低秸稈和籽粒中的As濃度[63],且并不會大幅影響作物產(chǎn)量[64].而水稻在抽穗期前后分別采取干濕交替條件,能夠改變重金屬的有效性,有效降低植株對Cd的吸收[65].

外源添加物和養(yǎng)分管理屬于化學改良.由于As(III)與硅酸的化學性質(zhì)相似,二者從土壤到根細胞的吸收采用相同的轉運體系[66],這導致了水稻吸收和轉運過程中Si和As(III)的點位競爭,因此,施用Si可以有效降低As(III)含量.但是也有研究指出,由于Si和As在土壤表面的陰離子吸附存在位點競爭,因此土壤外部施用Si有時會增加土壤溶液中的As濃度,導致根細胞吸收更多的As[67].在水分虧缺條件下施用含P、Fe和/或Si的肥料能夠顯著提高糧食產(chǎn)量[68].

土壤改良劑通過沉淀、吸附、陽離子交換和表面絡合作用固定重金屬[69].生物炭具有很高的孔隙率和比表面積,適合從污染水體或土壤中吸附污染物,因此,眾多學者開展了大量生物炭運用于土壤改良、植物養(yǎng)分保留、重金屬鈍化(如As、Cd、Pb等)研究.

耕作方式的改變也能改善水稻和小麥植株的重金屬含量.例如,油菜等高積累作物間作可降低水稻Cd含量[70];減耕措施可確保土壤中有機質(zhì)含量的提高,從而增強Cd等重金屬的吸附和絡合作用[71].但是相對而言,間作種植不利于機械化操作,比較費工費時.

2.3.2 生物修復法 生物修復法是通過種植超積累植物、耐性微生物或引入富集動物(如蚯蚓)等,利用生物吸附、生物萃取或根系過濾等方式,將土壤重金屬轉移到生物體或降低重金屬的生物有效性,從而達到土壤修復改良的目的.

種植超積累植物能同時降低土壤中幾種重金屬含量,且生物量大、生長迅速、根系發(fā)達,即使在重金屬濃度較低時也有較高的積累速率.已發(fā)現(xiàn)多種超積累植物,如蜈蚣草具有很強的As富集特性,能夠有效去除土壤和土壤孔隙水中的As[72].此外,擬南芥、龍葵和夜蛾也被定義為Cd超積累植物.但是,使用超積累植物進行修復之后,應著重考量其植株的安全處置方式.

微生物在影響水稻和小麥生態(tài)系統(tǒng)中As轉化和生物有效性方面起著重要作用.它可以影響亞砷酸鹽氧化、呼吸、還原和甲基化等[73].例如,側孢短桿菌(AMF)通過加速As(III)的氧化,從而緩解As(III)在水稻中的毒性[74];接種AMF可促進水稻生長,降低Cd和Pb對地上部的潛在毒性[75].

生物修復技術一度被眾多學者認為是最具前景的土壤重金屬修復改良方法,但從目前的研究來看,微生物和動物修復依然局限于實驗室階段,在實際應用方面存在諸多痛點難點;反觀植物修復,雖然全球已發(fā)現(xiàn)700余種超積累植物,一些也已開展廣泛的實際應用操作,但是植物吸收/吸附土壤重金屬后的處置方法成為了植物修復研究的難題.因此,迫切需要對超積累植物回收的技術原理進行更系統(tǒng)、更深入的研究,以提高其回收效率和利用價值,避免二次污染.

2.3.3 遺傳學方法 與上述兩種方法相比,遺傳學方法是降低水稻/小麥籽粒重金屬積累的有效途徑之一.基因組編輯技術的最新進展已經(jīng)取代了傳統(tǒng)育種方法,開啟了一個新的作物改良時代.例如,水稻能夠降低As從根細胞向地上部吸收轉運,因此,具有As“抵御”機制的水稻品種(鐵斑的發(fā)育、根系孔隙度和徑向氧損失)與鐵斑結合更多,從而減少了As的吸收[76];通過轉基因技術加入Cd轉運蛋白已成功用于減少Cd污染土壤中的Cd積累[77];在Pb污染的土壤中,可盡量選擇在根中儲Pb能力強,而在植株的其他部位易位少的品種種植.因此,轉基因或非轉基因水稻(突變體)可以成為降低水稻Cd含量的潛在技術[13].

表2 降低水稻重金屬含量的方法

通過探索調(diào)控不同水稻和小麥性狀的基因和數(shù)量性狀基因座(QTLs),成功實現(xiàn)了雜交、選擇和雜交育種等常規(guī)技術[78].分子育種的方法主要是基于與基因/QTLs連鎖的分子標記的鑒定,以及隨后的標記輔助選擇(MAS)[79].然而,這種育種應用完全依賴于初級基因庫中的自然變異.隨著遺傳學的發(fā)展,基因組編輯技術又替代了常規(guī)育種的復雜操作,例如水稻ROS1基因與胞嘧啶DNA去甲基化和植物表觀遺傳改變有關.隨著鋅指核酸酶(ZFNs)、轉錄激活效應核酸酶(TALENs)和CRISPR相關核酸內(nèi)切酶(CRISPR/Cas)技術的發(fā)展,這些技術被諸多學者應用于修改谷類作物的特定基因/位點[79].其中,CRISPR/Cas9技術由于其廣泛接受性、高性價比以及靶向編輯效率高等優(yōu)勢被有效地應用于作物植株,尤其是谷類作物[80].研究推測,基因組編輯工具的重大改進有望消除轉基因技術的缺陷和擔憂,并有望取代轉基因的開發(fā)方法[78].因此,基因組編輯技術在水稻和小麥作物改良方面的應用,為培育高產(chǎn)、優(yōu)質(zhì)的新品種提供了更多可能性.

表3 降低小麥重金屬含量的方法

5 結論及展望

5.1 我國糧食主產(chǎn)區(qū)水稻籽粒的Cd及Pb含量相對較高,相較國家食品安全值超標情況較為突出;水稻土的Cd和Pb濃度均超出我國農(nóng)用地國家標準值,其中,湖南水稻土中的Cd超標情況最為明顯;小麥籽粒的Pb及Cd超標情況較為突出,江蘇、四川、湖北和安徽省皆有部分小麥土壤的Cd濃度超出土壤管控標準,但是土壤Pb濃度都低于我國農(nóng)用地土壤管控標準.未來,水稻/小麥生產(chǎn)的農(nóng)藝管理活動應分別針對污染和非污染區(qū)域因地制宜地提出建議,以高度適應具體地點的需要,減少對全國區(qū)域建議的依賴.水稻/小麥政策制定者需要將其關注范圍擴大到點源污染之外,并提出管理措施建議,綜合考慮多種污染物在毒性和生物有效性方面的潛在差異.

5.2 As、Cd、Cr和Pb等有毒和潛在有毒金屬不僅存在于土壤環(huán)境中,而且通過不同的轉運蛋白在水稻/小麥系統(tǒng)中吸收、轉運和積累.今后仍迫切需要在多方位、多層次、多學科地深入研究相關分子機制.為了更深入地闡明重金屬在作物中的轉運機制,穩(wěn)定同位素可以作為示蹤劑,以便更好地理解其吸收機制.另外,有必要進一步探討極端環(huán)境條件(如頻繁的洪水、酸雨和全球變暖)對水稻/小麥土-水系統(tǒng)中重金屬形態(tài)和遷移的影響.

5.3 綜述了水稻和小麥的農(nóng)業(yè)管理方法、生物修復法和遺傳學方法.今后可在農(nóng)藝實踐、土壤管理到基因操作等各個層面探索合理的安全生產(chǎn)措施,以確保作物的安全生產(chǎn).

5.4 迫切地需要農(nóng)業(yè)、環(huán)境和醫(yī)學領域的研究人員緊密合作,以全面評估這些重金屬在土壤-植物-人體系統(tǒng)中已發(fā)生的和潛在的健康影響;應從農(nóng)藝實踐、土壤管理到基因操作等各個層面探索更為合理有效的重金屬脅迫補救策略,以確保糧食作物的安全生產(chǎn).

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Mechanism of heavy metal uptake and transport in soil-rice/wheat system and regulation measures for safe production.

WANG Cheng-chen1, TIAN Wen1, XIANG Ping1*, XU Wu-mei2, GUAN Dong-xing3, Lena Q. MA3

(1.School of Ecology and Environment/Institute of Environmental Remediation and Human Health, Southwest Forestry University, Kunming 650224, China；2.School of Energy and Environment Science, Yunnan Normal University, Kunming 650500, China；3.College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310058, China)., 2022,42(2)：794～807

To master the heavy metal pollution in rice and wheat crops from main production regions in China, we collected the data from existing literature and analyzed the concentrations of arsenic (As), cadmium (Cd), chromium (Cr) and lead (Pb) in soil-rice/wheat system, summarized their underlying mechanisms of target heavy metals absorption, transport, and accumulation. In addition, effective remediation measures for safe production were also introduced. The results showed that 31.3% and 22.2% of Cd, 26.2% and 32.1% of Pb in rice and wheat grains are over the value of China National Standard (GB2762-2017). Reducing the bioavailability of As, Cd, Cr and Pb and controlling their absorption in the soil-rice/wheat system could effectively decrease their accumulation in grains. The heavy metals uptake in soils-crop systems can be effectively decreased by water and fertilizer management, chemical modification, phytoremediation, bioremediation and genetic methods to achieve safe production. In the future, we should form a multi-disciplinary and integrated all factor pattern to improve the soil pollution research in China. Furthermore, developing the contaminated soil utilization technology for the safe production of agricultural products, so as to better ensure the national food safety production.

heavy metals；rice；wheat；transport mechanism；accumulation；safety production

X131.3

A

1000-6923(2022)02-0794-14

王成塵(1988-),女,新疆石河子人,西南林業(yè)大學博士研究生,主要從事環(huán)境污染與人體健康研究.發(fā)表論文2篇.

2021-07-12

云南省創(chuàng)新團隊項目(202005AE160017);國家自然科學基金資助項目(41967026);國家林業(yè)和草原局林草科技創(chuàng)新青年拔尖人才項目(2020132613);云南省高層次人才引進計劃項目(YNQR-QNRC- 2018-049);云南省教育廳科學研究基金資助項目(2021Y231)

* 責任作者, 研究員, xiangping@swfu.edu.cn