鎘不同積累型作物品種對鎘吸收、轉運和積累特性的研究進展

2018-04-01 02:38:18于輝向言詞

生態(tài)科學 2018年2期

于輝,向言詞

湖南科技大學生科院,重金屬污染土壤生態(tài)修復與安全利用湖南省高校重點實驗室,湘潭 411201

1 前言

隨著現(xiàn)代工業(yè)的發(fā)展、工業(yè)“三廢”的排放以及礦藏的大量開采,農(nóng)田土壤重金屬污染問題日趨嚴重。根據(jù)環(huán)保部2014年4月發(fā)布的“土壤污染狀況調(diào)查公報”[1],耕地土壤點位超標率為19.4%,在無機污染物中鎘排在首位。鎘在土壤中具有高度移動性,容易被作物吸收,由此引起的農(nóng)產(chǎn)品污染事件屢有發(fā)生,也引起了社會的廣泛關注。近年來,利用品種間的差異,選育低鎘積累的作物品種成為了降低作物受鎘污染風險的有效策略之一[2],目前相關的研究在水稻(Oryza sativaL.)[3]、玉米(ZeamaysL.)[4]、辣椒(Capsicum annuumL.)[5]、花生(Arachis hypogaeaL.)[6]) 和甘薯(Ipomoea batatas(L.)Lam.)[7]等多種農(nóng)作物中已陸續(xù)開展。作物籽實(果實)中鎘的累積與鎘的轉運過程有關,包括根系對鎘的吸收、鎘從根向莖葉的木質(zhì)部轉運以及營養(yǎng)器官向生殖器官的韌皮部轉移。而鎘在植物體內(nèi)的遷移運輸與其在細胞內(nèi)的分布和結合形態(tài)密切相關[8-9]。對這些過程的了解是開展低鎘積累品種選育的前提。本文分析了鎘積累型不同的作物品種根系對鎘的吸收、鎘向地上部的運輸和分配及鎘在細胞內(nèi)的分布和結合形態(tài)的差異,探討引起品種間鎘積累差異的生理機制,旨在為鎘低積累作物品種的選育提供理論基礎。

2 根系吸收與作物鎘積累的關系

鎘進入植株體內(nèi)首先要通過根系。Laporte等[10]發(fā)現(xiàn)向日葵(Helianthus annuusL.)鎘含量與根吸收能力有關,高鎘品種根部鎘含量高于低鎘品種。Yan等[11]發(fā)現(xiàn)秈-粳雜交稻(Oryza sativaL.)籽粒鎘含量與根中鎘含量顯著正相關。Li等[12]對大蔥(Allium fistulosumL.)的研究中也有相似的結果。但在大豆(Glycine max)[13]、茄子(Solanum melongena)[14]和辣椒(Capsicum annuumL.)[15]等作物中卻發(fā)現(xiàn),低鎘品種根部鎘含量高于高鎘品種,籽實(果實)鎘含量與根吸收相關性不大,因此根系吸收和鎘積累型的關系因植物種類而異。即使是同種植物不同基因型也存在差異。如Yan等[11]的報道,4種基因型水稻35個品種中,6個秈-粳雜交稻粒鎘濃度與根的濃度成正相關,而秈稻、溫帶粳稻和熱帶粳稻則與鎘的轉運或地上部的再分配有關。趙云云等[16]發(fā)現(xiàn)供試的大豆中,部分品種籽粒鎘與根系鎘成正相關,而有些品種則相反。造成這些結果的原因可能與植物不同品種的根系形態(tài)和根際環(huán)境有關。

有研究表明,不同鎘積累特性的品種其根系形態(tài)存在差異。如吳啟堂等[17]發(fā)現(xiàn),水稻高鎘品種汕優(yōu)63的根干質(zhì)量、根長、根冠比和吸水量均高于低鎘品種野奧絲苗;Li等[18]報道,與非超積累生態(tài)型東南景天(Sedum alfredii)相比,超積累生態(tài)型具有細而多的分支根;Lu等[19]比較了不同花生品種,發(fā)現(xiàn)高鎘積累品種的細根較多;以上特性使得高鎘積累品種根部具有高的鎘含量。但硬質(zhì)小麥[20]、蕹菜[21]和辣椒[15]的低鎘品種根部鎘含量高于高鎘品種。對其根系形態(tài)比較顯示,硬質(zhì)小麥低鎘品種根表面積和根尖數(shù)目高于高鎘品種,而辣椒和蕹菜中則相反。進一步分析發(fā)現(xiàn),辣椒根長、根總表面積和根尖數(shù)目與鎘轉運系數(shù)成正相關,因此低鎘品種根部對鎘的轉運能力較低。但對蕹菜分析顯示,總根長、總表面積及總體積與鎘的轉運系數(shù)和遷移率無顯著相關,平均直徑與二者顯著負相關,故認為低鎘積累品種根平均直徑的增大減弱了鎘向地上部轉運的能力。Xin等[22]對比鎘積累型不同的甘薯,根部鎘積累無顯著差異,但低積累品種總根長和比根長短,根尖分泌的低分子量有機酸多,從而減弱了鎘從根向莖的轉移。

鎘積累型不同的生態(tài)型或品種,其根際土壤的特性亦存在差異,且與根際土壤重金屬的生物有效性及吸收積累特性的差異有關[23]。東南景天超積累生態(tài)型根際pH值低于非超積累生態(tài)型,其根際對鋅、鎘的活化吸收明顯高于非超積累生態(tài)型,根際酸化是重要機理[23-24]。在蕹菜中也發(fā)現(xiàn)品種的鎘積累與土壤pH呈顯著負相關[21,25],蕹菜低鎘品種根際較高含量的有機質(zhì)、較低的酸化和還原能力,導致其對土壤鎘的活化能力較低,從而降低了植株對鎘的吸收積累。

3 鎘的運輸對作物可食部位積累鎘的影響

鎘被根系吸收后通過木質(zhì)部運輸?shù)角o、葉和花等器官的各個組織,有學者認為此轉運過程是決定地上部鎘含量的關鍵因素[9]。Su等[9]報道,不同品種的花生(Arachis hypogaeaL.)其鎘濃度差異主要源于木質(zhì)部的轉運能力。Harris和Taylor[26]比較了鎘積累型不同的近等基因系硬質(zhì)小麥(Triticum aestivumL.)幼苗根系的吸收和轉運能力,經(jīng)過相同時間處理后,高鎘積累型木質(zhì)部汁液中鎘濃度是低鎘積累型的 1.7-1.9倍。Uraguchi等[8]對水稻低鎘品種Sasanishiki(秈稻)和高鎘品種Habataki(粳稻)鎘積累特性進行分析,發(fā)現(xiàn)盡管Sasanishiki根部鎘的累積量高于Habataki,但后者莖和籽粒中的鎘是前者的兩倍。進一步研究發(fā)現(xiàn),除這兩個品種外,其他69個不同基因型水稻木質(zhì)部汁液中的鎘與籽實鎘含量均成正相關,故認為鎘從根到莖的木質(zhì)部運輸能力是決定其積累型的主要因素。類似的現(xiàn)象在Arao等[27]和 Mori 等[14]對茄子(Solanum melongenaL.)的研究中也有發(fā)現(xiàn)。

鎘被轉運到地上部后,通過韌皮部轉移到籽實中,此部分的研究多集中在禾谷類作物。Tanaka等[28]報道,水稻籽實中91%—100%的鎘來源于韌皮部的轉移。Kato等[29]分析發(fā)現(xiàn),三個水稻品種籽粒鎘含量與木質(zhì)部汁液鎘無關,而與韌皮部中鎘的濃度正相關。鎘經(jīng)由韌皮部轉移到籽粒中一般認為有以下兩種途徑[11,30]:(1)鎘首先積累在營養(yǎng)器官,籽實形成期通過再分配經(jīng)韌皮部轉移進入籽粒。(2)根持續(xù)吸收鎘,在莖節(jié)通過木質(zhì)部-韌皮部的轉運直接進入籽粒。為了進一步了解籽實鎘積累的發(fā)生途徑和時間,部分學者開展了營養(yǎng)生長和生殖生長期作物吸收轉運鎘的研究。如Rodda等[31]對水稻水培,進行花前、花后和持續(xù)給鎘三個處理,發(fā)現(xiàn)籽實中的鎘60%是由開花前營養(yǎng)器官積累的鎘轉移而來,40%來自于灌漿期,由根部吸收,在莖桿基部、葉軸和花梗直接發(fā)生木質(zhì)部到韌皮部的快速轉移。Kashiwagi等[32]發(fā)現(xiàn)水稻籽實中的鎘主要來源于抽穗前的營養(yǎng)器官,尤其是旗葉中鎘的轉移。抽穗后根對鎘的吸收及向莖的轉運受限,此時根系吸收的鎘不會影響籽實中鎘含量。Yan等[11]對兩種鎘積累型水稻幼穗分化前、幼穗分化后、成熟早期和成熟中期分別給鎘兩周,成熟期收獲,也證實營養(yǎng)器官的轉移是籽實鎘積累的重要因素。進一步實驗發(fā)現(xiàn),葉是重要的鎘源器官,葉的衰老加速鎘釋放,相對應的農(nóng)藝性狀是高鎘品種Milyang23葉衰老速度顯著高于低鎘品種Chucheong,與Huang等[15]對辣椒的研究觀點相似。然而Fujimaki等[33]用無損成像方法探明,水稻中鎘進入莖稈和莖節(jié)部位后,被轉運到穗中而不是葉片。Tavarez等[34]、Harris和Taylor[26]用小麥的近等基因系為材料,在籽實形成期的不同階段收獲,發(fā)現(xiàn)灌漿期沒有發(fā)生從葉到籽粒的鎘轉移,根部持續(xù)從土壤吸收鎘,在莖桿中通過木質(zhì)部-韌皮部轉移進入籽粒,鎘積累型的差異主要源于根到莖的木質(zhì)部轉運。故而認為營養(yǎng)期葉中積累的鎘多存在于細胞中較穩(wěn)定的部位,不易被轉載到韌皮部進入?yún)R器官。Kubo等[35]以4個日本普通小麥為對象,分別在苗期、拔節(jié)期、開花期和灌漿期給鎘直到成熟,發(fā)現(xiàn)盡管籽粒中的鎘量主要來自營養(yǎng)器官的轉移,但品種積累型差異受到木質(zhì)部-韌皮部鎘直接運輸?shù)挠绊?與前人的報道又有不同。

綜合來看以上的研究報道并沒有達成一致的觀點,造成這種現(xiàn)象的原因可能與研究對象的不同以及實驗條件的差異有關。有報道顯示即使同一植物生長過程中器官之間對鎘的積累規(guī)律也可能發(fā)生變化,如水稻秀水11(低鎘品種)苗期葉中鎘含量高于秀水110(高鎘品種),而成熟期前者葉中鎘含量低于后者[36]。

4 鎘在細胞內(nèi)的分布和結合形態(tài)與鎘積累的關系

植物吸收鎘后,根據(jù)各自的應激策略將其區(qū)隔在細胞的不同部位,如通過與細胞壁的結合及液泡區(qū)室化等途徑阻滯鎘的遷移運輸。在對蕹菜(Iomoea aquaticaForsk.)[37-38]、菜心(Brassica parachinensisL.)[39]和水稻等[40]的研究中發(fā)現(xiàn),鎘大部分被固定在細胞壁,向其它部位的轉移被減弱。而在美洲商陸(Phytolacca americanaL)[41]和遏藍菜(Thlaspi caerulescens)[42]中,細胞中的鎘大部分儲存在可溶部分(液泡中)。苧麻(Bechmeria niveaGaud)根中48.2%—57.6%的鎘分布在細胞壁[43],而辣椒根中細胞壁只結合了一小部分鎘(8%—17%)[44],可溶部分的鎘比例很高。這可能是不同植物對鎘不同的應激反應。鎘的亞細胞分布是形成品種鎘積累型差異的一個重要因素。He等[45]發(fā)現(xiàn)鎘敏感性(高鎘)的水稻突變體比野生型的細胞器中鎘含量高。比較菜心[39]和蕹菜[38]的亞細胞分布,低鎘品種根部細胞壁結合鎘的濃度和比例高于高鎘品種,從而大部分鎘被滯留在根部。辣椒[44]和水稻[40]低鎘品種莖葉中細胞壁的鎘比例高于高鎘品種,降低了鎘進一步向果實(籽實)的轉移。鎘離子穿過細胞壁進入細胞后,可以不同的化學結合形態(tài)存在。在各種結合形態(tài)中,乙醇可提取態(tài)(與無機鹽結合的鎘)和水溶態(tài)鎘(與有機鹽結合的鎘)遷移活性最強,氯化鈉提取態(tài)次之(與蛋白質(zhì)和果膠酸鹽結合的鎘),醋酸(與磷酸鹽結合的鎘)和鹽酸(與草酸鹽結合的鎘)提取態(tài)遷移活性最弱。比較低鎘和高鎘水稻中鎘結合形態(tài)表明,高鎘品種中水溶態(tài)和乙醇提取態(tài)鎘的比例高于低鎘品種[40]。但在對不同品種煙草(Nicotiana tabacumL.)鎘化學形態(tài)分布研究中發(fā)現(xiàn),高鎘積累型根內(nèi)遷移性較弱的醋酸提取態(tài)分配比率高于低鎘積累型[46]。Zhou等[47]發(fā)現(xiàn)低鎘莧菜(Amaranthus mangostanusL.)莖中氯化鈉提取態(tài)鎘比例很低,認為與蛋白質(zhì)結合的鎘可能是造成鎘積累差異的主要因子,而菜心[39]低鎘品種中這部分比例則較高。辣椒[44]低鎘品種根中無機態(tài)鎘(乙醇提取態(tài))所占比例最大,研究者認為這部分鎘可能以螯合態(tài)的形式進入了細胞質(zhì),且區(qū)隔在液泡中的比例要高于高鎘品種,故可溶部分的鎘比例相應較高。莖葉中乙醇提取態(tài)和水溶態(tài)鎘較低,則減弱了鎘向果實的轉移。綜上,鎘在細胞內(nèi)的分布和結合形態(tài)與其在植物體內(nèi)的遷移運輸密切相關,不同植物及品種間存在差異。但以往此方面的研究主要集中在植物幼苗時期,而鎘的運輸和在籽實的積累是長期持續(xù)的過程,且不同的發(fā)育期對鎘的積累規(guī)律可能發(fā)生變化[36],因此單一生長期提供的數(shù)據(jù)缺乏系統(tǒng)性和全面性。

5 結論與展望

綜上所述,不同作物籽實(果實)積累鎘的差異受到多方面的影響。有些作物中鎘累積與根吸收能力成正相關,而有些則相關性不大,鎘含量主要取決于根向地上部的轉運能力或者地上部的再分配。鎘的亞細胞分布和結合形態(tài)影響著鎘的遷移運輸,與高鎘積累品種相比,低鎘作物品種細胞壁鎘和遷移活性較弱的鎘形態(tài)比例往往高于前者,由此降低了鎘的進一步轉移。

目前,在對低鎘積累作物品種的篩選和機制研究方面取得了一定的成效,但是,還有很多研究工作需要開展,主要包括以下方面:

(1)闡明根際環(huán)境對植物吸收積累鎘的綜合影響,鎘脅迫下不同積累型品種根際的動態(tài)調(diào)節(jié)過程,特別是根系分泌物、根際微生物對土壤重金屬鎘的形態(tài)轉化及生物有效性的影響,低積累鎘的作物品種根際微生物環(huán)境特點等。

(2)利用豐富的基因資源,篩選可食部位低鎘積累、非可食部位高鎘積累的作物品種,對鎘污染土壤進行邊修復邊利用。同時結合目前對鎘的認識與分子生物學方法,運用物理和化學誘變、分子標記輔助育種、耐性基因的分離與克隆技術等,培育出新品種。

(3)作物不同的發(fā)育期對鎘的積累規(guī)律會發(fā)生變化,而目前對于鎘在各生育時期植株體內(nèi)的動態(tài)變化過程了解不深,對重要農(nóng)作物鎘積累的途徑和再分配規(guī)律有待進一步深入研究,以保障我國主要農(nóng)產(chǎn)品的安全生產(chǎn)。

參考文獻

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