石陶然, 田 凱, 包環(huán)宇, 侯劭煒, 劉雪平, 吳福勇*

1.西北農(nóng)林科技大學(xué)資源環(huán)境學(xué)院, 陜西 楊凌 712100 2.農(nóng)業(yè)部西北植物營(yíng)養(yǎng)與農(nóng)業(yè)環(huán)境重點(diǎn)實(shí)驗(yàn)室, 陜西 楊凌 712100 3.河南城建學(xué)院市政與環(huán)境工程學(xué)院, 河南 平頂山 467036

多環(huán)芳烴在冬小麥體內(nèi)的吸收與轉(zhuǎn)運(yùn)及富集研究進(jìn)展

石陶然1,2, 田 凱1,2, 包環(huán)宇1,2, 侯劭煒1,2, 劉雪平3, 吳福勇1,2*

1.西北農(nóng)林科技大學(xué)資源環(huán)境學(xué)院, 陜西 楊凌 712100 2.農(nóng)業(yè)部西北植物營(yíng)養(yǎng)與農(nóng)業(yè)環(huán)境重點(diǎn)實(shí)驗(yàn)室, 陜西 楊凌 712100 3.河南城建學(xué)院市政與環(huán)境工程學(xué)院, 河南 平頂山 467036

多年來(lái)以煤炭為主的能源消費(fèi)結(jié)構(gòu)和經(jīng)濟(jì)社會(huì)持續(xù)發(fā)展，導(dǎo)致我國(guó)PAHs(多環(huán)芳烴)排放量居高不下，直接造成土壤和大氣PAHs嚴(yán)重污染. 為了探明PAHs在冬小麥體內(nèi)的積累過(guò)程和調(diào)控機(jī)制，在系統(tǒng)分析PAHs在冬小麥體內(nèi)的吸收、轉(zhuǎn)運(yùn)和富集的基礎(chǔ)上，重點(diǎn)闡述了冬小麥PAHs根系吸收和葉面吸收影響因素方面的最新研究進(jìn)展. 研究發(fā)現(xiàn)：①小麥根系對(duì)PAHs的吸收包括主動(dòng)吸收和被動(dòng)吸收兩種方式，其中主動(dòng)吸收是一個(gè)載體協(xié)助、消耗能量、PAHs與H+共運(yùn)的過(guò)程；被動(dòng)吸收除了在高等植物中普遍存在的簡(jiǎn)單擴(kuò)散外，水-甘油通道也參與了該過(guò)程. ②PAHs通過(guò)氣態(tài)、顆粒態(tài)沉降到小麥葉面角質(zhì)層或直接通過(guò)氣孔進(jìn)入葉片. ③影響PAHs根系和葉面吸收的主要因素包括PAHs理化性質(zhì)、植物生理狀況、環(huán)境因素等. ④小麥根系吸收的PAHs可以向地上部轉(zhuǎn)運(yùn)，并且與辛醇-水分配系數(shù)(KOW)、蒸騰速率、土壤中氮的形態(tài)和濃度有關(guān). 主要問(wèn)題：①對(duì)于小麥葉片吸收的PAHs向基運(yùn)輸機(jī)理有待進(jìn)一步研究. ②農(nóng)田生態(tài)系統(tǒng)中冬小麥往往遭受土壤及大氣雙重污染，根系吸收及葉面吸收分別對(duì)其體內(nèi)積累PAHs的貢獻(xiàn)尚不清楚. 因此，需關(guān)注韌皮部、木質(zhì)部在PAHs轉(zhuǎn)運(yùn)中所起的作用；利用同位素示蹤、雙光子激發(fā)顯微鏡等先進(jìn)技術(shù)觀察和跟蹤PAHs如何進(jìn)入小麥以及在小麥葉中的轉(zhuǎn)移和分布，闡明PAHs葉面吸收的微觀機(jī)理；注重大田試驗(yàn)研究，為揭示冬小麥對(duì)PAHs的吸收、積累及調(diào)控機(jī)理，同時(shí)也為有機(jī)污染地區(qū)生產(chǎn)安全農(nóng)產(chǎn)品提供重要依據(jù).

多環(huán)芳烴; 小麥; 根系吸收; 葉面吸收; 轉(zhuǎn)運(yùn)機(jī)理

PAHs(多環(huán)芳烴)是一類具有“三致”(致癌、致畸、致突變)效應(yīng)的持久性有機(jī)污染物，由于其在自然環(huán)境中無(wú)處不在、對(duì)人體危害大而備受關(guān)注. 我國(guó)是全球PAHs排放量最多的國(guó)家，2004年高達(dá)114 000 t，占全球PAHs排放總量的29%[1]，直接造成大氣、土壤等自然環(huán)境中PAHs含量升高. 華北平原空氣中ρ(PAHs)高達(dá)346 ng/m3[2]，分別為倫敦和芝加哥的8.9和4.9倍[3- 4]. 自然界90%以上的PAHs存在于表層土壤[5]. PAHs已經(jīng)成為我國(guó)農(nóng)田土壤中最為常見(jiàn)的污染物，無(wú)論是在東三省、京津地區(qū)，還是在長(zhǎng)三角、珠三角地區(qū)，均發(fā)現(xiàn)農(nóng)田遭受PAHs污染且呈持續(xù)惡化趨勢(shì)[6- 7]. 我國(guó)小麥種植面積約占農(nóng)作物種植總面積的1/4，華中、華北和西北地區(qū)既是小麥主產(chǎn)區(qū)也是煤炭生產(chǎn)和消費(fèi)大區(qū)，相當(dāng)面積的小麥種植區(qū)位于眾多的燃煤發(fā)電廠、煉焦廠、城市供熱廠等燃煤企業(yè)周邊區(qū)域. 調(diào)查結(jié)果[8- 12]顯示，濟(jì)南(7.6～495.2 μg/kg)、太原(161.5 μg/kg)和天津(177 μg/kg)小麥籽粒內(nèi)PAHs含量均遠(yuǎn)高于美國(guó)加利福尼亞和西班牙加泰羅尼亞小麥籽粒內(nèi)PAHs含量(10.3～27.9 μg/kg). 通常情況下，膳食攝入是人體暴露PAHs的主要途徑[8- 9]. 小麥?zhǔn)俏覈?guó)北方和中原地區(qū)大多數(shù)居民的主食，成人日均消耗250 g左右，其中河南省成年居民日均消耗量達(dá)376～547 g[13]. 太原居民膳食暴露PAHs致癌風(fēng)險(xiǎn)較高，通過(guò)小麥攝入的PAHs占膳食攝入總量的48.3%～53.5%[12]. 因此，進(jìn)行野外采樣和模型模擬探討PAHs在環(huán)境-植物系統(tǒng)中的富集規(guī)律，以及利用顯微鏡、光譜、熱重分析和元素分析等技術(shù)探明PAHs在植物體內(nèi)的行為，對(duì)于研究PAHs從土壤、大氣等自然環(huán)境到小麥的吸收、轉(zhuǎn)運(yùn)、積累過(guò)程的機(jī)理和影響因素，預(yù)測(cè)農(nóng)產(chǎn)品有機(jī)污染、降低農(nóng)作物污染風(fēng)險(xiǎn)、確保農(nóng)產(chǎn)品安全生產(chǎn)至關(guān)重要.

1 小麥對(duì)PAHs的吸收和積累

PAHs既可以通過(guò)土壤-根系也可以通過(guò)空氣-葉面進(jìn)入小麥體內(nèi)[14]，即根系從土壤中吸收PAHs進(jìn)入木質(zhì)部，隨蒸騰流向莖葉傳輸?shù)母滴眨籔AHs通過(guò)葉面角質(zhì)層或氣孔進(jìn)入小麥體內(nèi)的葉面吸收[15]. 有研究[16]認(rèn)為，與葉面吸收相比，根系對(duì)疏水性有機(jī)物(HOCs)的吸收是主要的；也有研究[17]認(rèn)為，由于PAHs易與土壤有機(jī)質(zhì)結(jié)合，并且根系吸收后很難轉(zhuǎn)運(yùn)至地上部分，因此植物暴露器官中的PAHs主要來(lái)自葉面吸收. 進(jìn)一步探討根系、葉片對(duì)HOCs的吸收以及在植物體內(nèi)的積累、轉(zhuǎn)移，對(duì)于治理PAHs污染土壤、確保食品安全、模擬潛在吸收量及進(jìn)行風(fēng)險(xiǎn)評(píng)估等至關(guān)重要[18].

1.1PAHs的根系吸收

小麥根系對(duì)PAHs的吸收包括主動(dòng)和被動(dòng)吸收兩種方式. 主動(dòng)吸收量約占吸收總量的40%，并且受PAHs/H+協(xié)同載體的影響[19]. PAHs與H+形成共運(yùn)對(duì)進(jìn)入細(xì)胞內(nèi)部后會(huì)導(dǎo)致細(xì)胞質(zhì)pH降低[19]，而細(xì)胞質(zhì)pH通常呈中性或偏堿性，這就存在一個(gè)pH變化和穩(wěn)定的過(guò)程. 進(jìn)一步研究細(xì)胞內(nèi)pH自我調(diào)控機(jī)制對(duì)農(nóng)產(chǎn)品安全保障和PAHs污染土壤修復(fù)具有重要意義. 由于大多數(shù)有機(jī)污染物均系人工合成，植物體內(nèi)沒(méi)有相應(yīng)的運(yùn)輸載體，因此大多數(shù)有機(jī)污染物進(jìn)入植物體主要是通過(guò)被動(dòng)運(yùn)輸. 被動(dòng)運(yùn)輸除了簡(jiǎn)單擴(kuò)散，還與水-甘油跨膜輸送蛋白通道有關(guān)[20]. 小麥根系對(duì)菲的吸收過(guò)程可分為快速吸收和慢速吸收兩個(gè)階段. 當(dāng)小麥根部浸到含有PAHs的營(yíng)養(yǎng)液中，快速吸收立即進(jìn)行，這一階段主要受吸收作用、擴(kuò)散作用和質(zhì)量流量的影響. 隨后是一個(gè)慢相循環(huán)階段，以載體為媒介且受新陳代謝的影響[19]. 與快速吸收階段的吸收速率相比，慢速吸收階段的吸收速率要低1個(gè)數(shù)量級(jí)[21]，但是對(duì)于其他種類PAHs是否具有與菲相同的吸收特征尚不明確. 用雙光子激發(fā)顯微鏡(TPEM)直接觀察PAHs被小麥根系的吸收、存儲(chǔ)和代謝，發(fā)現(xiàn)蒽和菲最初結(jié)合在根系表皮，隨后穿過(guò)表皮細(xì)胞到達(dá)皮層. PAHs呈放射狀進(jìn)入表皮，然而一旦接觸皮層細(xì)胞就變?yōu)榫徛臋M向運(yùn)輸[22]. 這可能與PAHs進(jìn)入表皮、皮層細(xì)胞的原生質(zhì)，并且滯留在原生質(zhì)中，然后通過(guò)胞間連絲進(jìn)入內(nèi)皮層、中柱和韌皮部有關(guān).

1.2PAHs的葉面吸收

圖1 疏水性有機(jī)污染物從大氣到葉角質(zhì)層的吸收過(guò)程[24]Fig.1 The uptake process of hydrophobic organic compounds from air onto leaf cuticles

空氣中的PAHs以氣態(tài)或顆粒態(tài)沉降至植物葉面，一部分結(jié)合在葉面角質(zhì)層的脂質(zhì)中，擴(kuò)散穿過(guò)脂質(zhì)層，最終被韌皮部運(yùn)輸至其他部位；另一部分向葉片內(nèi)部遷移，擴(kuò)散至細(xì)胞間隙，然后再分配到鄰近組織的液相或脂相中[23]. 此外，葉片表面還分布著許多氣孔，這些氣孔為有機(jī)污染物進(jìn)入植物體提供了另一個(gè)途徑[15]. 污染物從空氣到葉片吸收包括3個(gè)步驟[24](見(jiàn)圖1)：①污染物穿過(guò)大氣和葉之間的湍流帶；②污染物穿越邊界層；③污染物與葉片表面反應(yīng). 植物葉片角質(zhì)層對(duì)PAHs具有一定的屏障作用，PAHs在角質(zhì)層上會(huì)發(fā)生團(tuán)聚現(xiàn)象，表明角質(zhì)層對(duì)PAHs的吸附并非是均勻的[25]. 但經(jīng)過(guò)一段時(shí)間后PAHs可以滲透至角質(zhì)層，跨過(guò)細(xì)胞膜進(jìn)入葉肉細(xì)胞，累積在液泡組織內(nèi)[26]. Wild等[27]在觀察玉米葉片內(nèi)熒蒽在96 h的運(yùn)動(dòng)軌跡時(shí)發(fā)現(xiàn)，葉片中的熒蒽穿過(guò)上表皮蠟質(zhì)和角質(zhì)層后到達(dá)表皮細(xì)胞的細(xì)胞質(zhì)，在這一階段，熒蒽存在于葉片的5個(gè)部位，即上表皮蠟質(zhì)-薄擴(kuò)散層(約5 μm)、從上表層蠟質(zhì)穿過(guò)角質(zhì)層到達(dá)表皮細(xì)胞的細(xì)胞壁-厚擴(kuò)散帶(約28 μm)、上表皮細(xì)胞壁外表面、上表皮細(xì)胞壁內(nèi)表面及上表皮細(xì)胞的細(xì)胞質(zhì). PAHs在葉片中的存儲(chǔ)位置影響其在植物體內(nèi)的遷移轉(zhuǎn)化和最終歸趨. 如果滯留于表皮蠟質(zhì)和角質(zhì)層中，可能發(fā)生光降解、再揮發(fā)或從角質(zhì)層脫落[28]；如果進(jìn)入到表皮細(xì)胞壁或者細(xì)胞質(zhì)中，則易發(fā)生代謝作用[29]. 此外，Wild等[29]發(fā)現(xiàn)菲以氣態(tài)沉降進(jìn)入玉米葉片氣孔，出現(xiàn)在氣孔的保衛(wèi)細(xì)胞，但是在氣孔內(nèi)表面和氣孔下腔并沒(méi)有檢測(cè)到菲，因此推斷菲沒(méi)有通過(guò)氣孔而是被葉蠟所吸收. Barber等[30]認(rèn)為，當(dāng)角質(zhì)層較難穿透且氣孔密度較大時(shí)，氣孔吸收途徑相對(duì)重要；而當(dāng)角質(zhì)層極易穿透時(shí)，氣孔的作用幾乎為零. 目前，對(duì)于活體小麥葉片吸收PAHs的可視化實(shí)時(shí)追蹤研究相對(duì)較少. 小麥葉片對(duì)PAHs的吸收是角質(zhì)層途徑為主還是氣孔途徑為主，至今尚不清楚.

1.3籽粒對(duì)PAHs吸收

由于PAHs本身的疏水特性，Briggs等[31]認(rèn)為小麥籽粒內(nèi)的PAHs主要來(lái)源于大氣而不是土壤. 然而，DU等[32]通過(guò)野外Lymimeter試驗(yàn)追蹤14C標(biāo)記的菲在小麥體內(nèi)的積累發(fā)現(xiàn)，根系吸收的PAHs可以通過(guò)向頂運(yùn)輸進(jìn)入莖、葉、籽粒和穎殼. 有研究[33]發(fā)現(xiàn)，小麥籽粒內(nèi)積累的主要為2～4環(huán)PAHs，并且萘含量最高. 裸露的小麥籽粒對(duì)PAHs吸收速率大于有外殼的籽粒，可能是外殼阻礙了PAHs向籽粒的擴(kuò)散. 大田中的小麥籽粒從形成到收獲大致需要一個(gè)月，對(duì)于氣態(tài)PAHs的吸收處于動(dòng)力學(xué)限制階段且達(dá)不到平衡濃度[34]. 小麥籽粒PAHs濃度與接觸大氣的PAHs濃度有關(guān)，其關(guān)系受吸收時(shí)間和大田條件的影響[34]，但是目前關(guān)于二者之間的關(guān)系缺乏定量研究，需要?jiǎng)?chuàng)建合適的動(dòng)力學(xué)吸收模型，從而確定有效的途徑降低籽粒引起的PAHs膳食暴露風(fēng)險(xiǎn). 農(nóng)田中的小麥經(jīng)常遭受土壤及大氣雙重污染，因此葉面吸收及根系吸收PAHs分別對(duì)籽粒積累PAHs的貢獻(xiàn)是當(dāng)前亟待解決的科學(xué)問(wèn)題.

2 小麥對(duì)PAHs的吸收和積累的影響因素

2.1根系吸收PAHs的影響因素

2.1.1PAHs理化性質(zhì)

PAHs能否進(jìn)入植物根系，依賴于其KOW(辛醇-水分配系數(shù))、水溶解度(S)、亨利系數(shù)(H)、分子量(MW)等理化性質(zhì). 對(duì)于不同種類PAHs，根系對(duì)其吸收、轉(zhuǎn)運(yùn)速率不同[35]. Chiou等[36]提出了限制分配模型用于定量預(yù)測(cè)植物對(duì)有機(jī)物的積累：

Cpt=αpt[Cs/(fsomKsom)][fpw+fchKch+flipKOW]

(1)

式中：Cpt為植物體內(nèi)污染物的濃度，mg/kg；αpt為準(zhǔn)平衡因子；Cs為污染物在土壤中的濃度，mg/kg；fpw為植物中無(wú)機(jī)組分的含量，%；fch為植物中碳水化合物的含量，%；flip為植物中脂類的含量，%；fsom為土壤有機(jī)質(zhì)的質(zhì)量分?jǐn)?shù)，%；Kch為污染物在碳水化合物和水間的分配系數(shù)；Ksom為污染物在土壤有機(jī)質(zhì)和水間的分配系數(shù).

由式(1)可知，KOW是植物根系吸收有機(jī)污染物的主要限制因子. 有研究認(rèn)為小麥根系對(duì)PAHs的吸收量隨KOW的增加而增加[31]，也有研究認(rèn)為小麥根系PAHs富集系數(shù)與KOW沒(méi)有線性關(guān)系[14]，原因是大多數(shù)lgKOW>4的PAHs分配到根的表皮或土壤顆粒而不會(huì)被根系或木質(zhì)部吸收. 因此，KOW對(duì)小麥根系吸收PAHs的影響尚需深入系統(tǒng)研究. PAHs隨著分子量的增加其揮發(fā)性降低[37]. 植物根系對(duì)不同Mw的PAHs吸收轉(zhuǎn)運(yùn)能力不同. 小麥根系對(duì)4環(huán)PAHs吸收最多，其次是2環(huán)、3環(huán)、5～6環(huán)PAHs[14]. Wild等[22]觀察同分異構(gòu)體菲和蒽在小麥根系的遷移過(guò)程時(shí)發(fā)現(xiàn)，菲的吸收和遷移速率比蒽快. 這可能與二者不同的水溶性有關(guān)：在25 ℃下菲和蒽在水中的溶解度分別為1.65、0.075 mg/L[38].

然而，對(duì)于一些PAHs理化性質(zhì)精確的測(cè)量非常困難. 定量結(jié)構(gòu)-活性關(guān)系(quantitatives structure activity relationship QSAR)指化合物的分子結(jié)構(gòu)與其活性之間的關(guān)系，目前已從個(gè)別的、定性的描述方式發(fā)展到一般的、定量的數(shù)學(xué)模型表達(dá)[39]. 利用定量結(jié)構(gòu)-活性關(guān)系可以對(duì)PAHs理化性質(zhì)、環(huán)境歸趨和生物毒性進(jìn)行預(yù)測(cè)，彌補(bǔ)數(shù)據(jù)的缺失，降低昂貴的測(cè)試費(fèi)用. 目前對(duì)PAHs的定量結(jié)構(gòu)-活性關(guān)系研究主要集中在光解活性[40]、生物可降解性[41]和生物毒性[42]等方面，對(duì)其吸附-解吸等環(huán)境行為的研究相對(duì)缺乏. 預(yù)測(cè)的理化性質(zhì)參數(shù)代入多介質(zhì)逸度模型，可得到理想環(huán)境狀態(tài)下PAHs在多環(huán)境介質(zhì)中的分配歸趨，為了解PAHs在環(huán)境中的遷移轉(zhuǎn)化提供了簡(jiǎn)便的途徑.

2.1.2植物生理狀況

植物種類及其生理學(xué)特性(包括脂肪或水分含量及蒸騰速率等)都會(huì)影響植物對(duì)有機(jī)污染物的吸收[43]. 研究[44]表明，小麥根系對(duì)PAHs的吸收與根部脂肪含量、根表面積有關(guān). 吸附劑的極性也會(huì)顯著影響其對(duì)有機(jī)物的吸附能力，小麥根細(xì)胞的極性與PAHs吸收速率呈負(fù)相關(guān)[45]. 植物體主要由水、脂肪、碳水化合物、蛋白質(zhì)、纖維素等物質(zhì)構(gòu)成，這些成分對(duì)有機(jī)污染物的親和力不同[46]. 研究[47]表明，KOW<10的有機(jī)污染物，根系水吸收占主導(dǎo)作用(85%以上)；KOW=10的有機(jī)污染物，根系水和脂肪作用各占50%；KOW>1 000 的有機(jī)物，植物對(duì)有機(jī)污染物的吸收幾乎都來(lái)自根系脂肪對(duì)有機(jī)物的分配. 植物不同生育期由于生命代謝活動(dòng)強(qiáng)度不同，吸收污染物的能力也不同. 在不同生長(zhǎng)期小麥各組織器官低環(huán)、中高環(huán)PAHs分布有顯著差異[48]. 此外，不同的根系類型、根表面積、根系分泌物、菌根細(xì)菌等在種類和數(shù)量上的差異導(dǎo)致根際對(duì)PAHs的吸收、降解能力不同[49].

2.1.3土壤理化性質(zhì)

YANG等[50]發(fā)現(xiàn)，土壤DOM(可溶性有機(jī)質(zhì))不僅能明顯地促進(jìn)小麥對(duì)菲的吸收和富集，而且還能促進(jìn)根系吸收的菲向地上部轉(zhuǎn)運(yùn). 這可能是由于DOM改變了PAHs的理化性質(zhì)，如水溶解度和KOW，從而提高了PAHs的生物有效性，進(jìn)而促進(jìn)了植物對(duì)菲的吸收[50]. 但是小麥根系吸收PAHs與土壤有機(jī)碳(SOC)含量呈負(fù)相關(guān)(P<0.01)[14]. 低的pH促進(jìn)根吸收PAHs，研究[20]表明根系對(duì)菲的主動(dòng)吸收是以H+共運(yùn)方式進(jìn)行的. K+也會(huì)促進(jìn)小麥根系對(duì)菲的吸收，K+激活了質(zhì)膜H+-ATPase[44]. 植物對(duì)PAHs的吸收與土壤中PAHs的濃度和植物組分有關(guān)[51]，研究表明[14]小麥根中PAHs濃度與土壤中PAHs的濃度呈正相關(guān). 另外，陽(yáng)離子表面活性劑能顯著增強(qiáng)土壤對(duì)有機(jī)污染物的吸附[52]，而且能夠抑制作物吸收土壤中的PAHs[53]. 土壤粒徑組成影響其對(duì)有機(jī)污染物的吸附和利用能力. 粗砂和黏粒對(duì)芘的吸附能力較大，細(xì)砂和粉砂相對(duì)較小[54]. 此外，土壤中氮的濃度和形態(tài)也會(huì)影響小麥根系對(duì)PAHs的吸收[55].

2.2葉面吸收PAHs的影響因素

2.2.1PAHs理化性質(zhì)

植物葉片對(duì)PAHs的吸收和轉(zhuǎn)移與PAHs本身的理化性質(zhì)有關(guān)，如環(huán)數(shù)、分子量、水溶解度、辛醇-水分配系數(shù)、辛醇-氣分配系數(shù)、形態(tài)(氣態(tài)/顆粒態(tài))、亨利系數(shù)等. 氣態(tài)的PAHs容易在葉片角質(zhì)層擴(kuò)散而被吸收，而大部分顆粒態(tài)PAHs只是嵌入到角質(zhì)層，很容易被脫附[56]. 蒽和苯并[k]熒蒽混合噴施對(duì)生菜地上部分產(chǎn)生的積累效應(yīng)和單獨(dú)噴施有明顯差異[57]. 車前草葉內(nèi)部PAHs含量隨PAHs分子量的增加而減少[58]；然而也有研究[59]認(rèn)為，植物葉中PAHs以5、6環(huán)為主，根中以2～4環(huán)為主. 揮發(fā)性、半揮發(fā)性有機(jī)污染物在植物葉片和空氣之間的分配與其辛醇-氣分配系數(shù)密切相關(guān)[43].

2.2.2植物生理狀況

不同種類植物葉片性質(zhì)存在差異，如形態(tài)、葉面積、角質(zhì)層、葉片數(shù)量、氣孔大小及密度、葉毛長(zhǎng)度與密度、疏水性等[60]. 植物葉片攔截顆粒態(tài)PAHs與其葉向、葉面積、葉毛有關(guān)[24]. 研究表明，植物暴露器官中的PAHs主要來(lái)自葉的吸收[17]，影響葉片角質(zhì)層吸收PAHs的主要因素是暴露于大氣中的葉面積[61]. 此外，葉毛可以提高葉片清除和黏附大氣顆粒物的能力，因?yàn)樗鼈兙哂懈蟮谋砻娣e并且會(huì)在葉表面的邊緣形成相對(duì)靜態(tài)空氣[56]. 然而也有研究[62]表明，表面光滑的葉片比表面粗糙的葉片更易于吸附顆粒物. 因?yàn)槿~子越粗糙，防水性能就越好，所以對(duì)葉子表面顆粒物的黏附作用減弱[56]. 因此，葉毛對(duì)葉片吸收大氣顆粒物的影響還沒(méi)有統(tǒng)一的結(jié)果. 葉脂含量也是影響植物吸收和滯留大氣持久性有機(jī)污染物(POPs)的一個(gè)重要的因素[52]. 在這些因素中，哪些是影響PAHs穿過(guò)角質(zhì)層被小麥吸收的主要因素有待進(jìn)一步研究.

2.2.3環(huán)境因素

植物從空氣中吸收PAHs受溫度、空氣中污染物的濃度、暴露時(shí)間等影響. PAHs通過(guò)氣孔在葉片和大氣之間交換與大氣溫度有關(guān)[63]，因?yàn)闇囟扔绊憵饪椎拈_啟和閉合. 在溫度較低的秋冬季節(jié)，PAHs由大氣向植物遷移，而在溫度較高的夏季，部分PAHs又通過(guò)揮發(fā)作用回到大氣中[64]. 葉面對(duì)PAHs的吸收量還取決于葉面周圍大氣中PAHs的濃度[65]. 由于暴露時(shí)間的不同，老葉積累的PAHs量往往大于新葉[66].

3 PAHs在小麥體內(nèi)的分布和遷移

3.1向頂運(yùn)輸

植物根系吸收的有機(jī)污染物一部分固定在根的脂質(zhì)中，一部分則穿過(guò)根系不透水硬組織帶進(jìn)入內(nèi)表皮層到達(dá)管胞和導(dǎo)管組織，并通過(guò)木質(zhì)部隨蒸騰流向地上部分遷移，最終在莖葉中分布[43]. 小麥根系吸收的PAHs可以通過(guò)向頂運(yùn)輸進(jìn)入莖葉[14]. 有研究[67]認(rèn)為，蔬菜地上部大分子量PAHs主要來(lái)源于根系的轉(zhuǎn)運(yùn). 小麥地上部不同環(huán)數(shù)的PAHs的分布特征與大氣中的PAHs的分布有顯著的差異，表明地上部PAHs來(lái)自于根系的轉(zhuǎn)運(yùn)[14].

小麥對(duì)PAHs的向頂運(yùn)輸與PAHs的理化性質(zhì)有關(guān)，芴傾向于在小麥根部積累，而菲被轉(zhuǎn)運(yùn)到地上部分[68]. 有研究[14]表明小麥根吸收PAHs向地上部轉(zhuǎn)運(yùn)與lgKOW呈負(fù)相關(guān). lgKOW≤1的有機(jī)物易溶于水，可以在木質(zhì)部和韌皮部流動(dòng)；14的有機(jī)物可以在根部大量積累，但不能向頂運(yùn)輸. 因此，1

3.2向底運(yùn)輸

由于PAHs較高的疏水性和較低的溶解性，植物根系表皮吸收后難以運(yùn)輸?shù)礁膬?nèi)部或木質(zhì)部[71]，因此推斷植物體內(nèi)的PAHs可以通過(guò)地上部吸收后運(yùn)輸至根部. 在土壤菲濃度很低的情況下，三葉草和黑麥草根部檢測(cè)到高濃度的菲，很大程度上證明地上部吸收的菲被轉(zhuǎn)移至根部[72]. 然而也有研究[51]表明，植物(如大豆、空心菜等)地上部從大氣中積累的菲和芘沒(méi)有被運(yùn)輸?shù)礁? 用同位素示蹤法研究豌豆體內(nèi)熒蒽的轉(zhuǎn)運(yùn)發(fā)現(xiàn)：熒蒽通過(guò)韌皮部存在向基、向頂運(yùn)輸，尤其是在植物新生部位如莖尖及根尖積累[73]. 目前，小麥葉面吸收PAHs是否會(huì)向根部運(yùn)輸還沒(méi)有明確的結(jié)論，有待進(jìn)一步的研究.

4 結(jié)論

a) 目前，關(guān)于小麥對(duì)PAHs吸收和轉(zhuǎn)運(yùn)的模型在風(fēng)險(xiǎn)評(píng)價(jià)、植物修復(fù)等方面應(yīng)用得很多，然而，缺乏PAHs在小麥體內(nèi)轉(zhuǎn)運(yùn)過(guò)程的直接證據(jù). 研究韌皮部、木質(zhì)部運(yùn)輸動(dòng)力學(xué)對(duì)確定可食部分-小麥籽粒積累的PAHs主要來(lái)自根系吸收還是葉片吸收，從而采取有效的農(nóng)藝措施阻控PAHs進(jìn)入食物鏈?zhǔn)潜匾?

b) 小麥處于自然狀態(tài)下PAHs是如何進(jìn)入其葉片，如何在葉片中轉(zhuǎn)移和或與葉肉組織結(jié)合尚不清楚. 采用先進(jìn)技術(shù)探討小麥葉面吸收、轉(zhuǎn)運(yùn)PAHs的微觀機(jī)理，如利用同位素示蹤法、雙光子激發(fā)顯微鏡等技術(shù)跟蹤和觀察PAHs在小麥葉片的吸收和轉(zhuǎn)移，有望為宏觀調(diào)控植物吸收PAHs獲得安全農(nóng)產(chǎn)品提供科學(xué)依據(jù).

c) 大多數(shù)研究致力于室內(nèi)盆栽試驗(yàn)，因?yàn)槭茏匀画h(huán)境等不可控因素的影響小于田間試驗(yàn)而利于機(jī)理的研究，但是植物吸收PAHs的大田試驗(yàn)的經(jīng)驗(yàn)對(duì)于完全理解植物-環(huán)境體系的宏觀關(guān)系也是必要的. 探討影響PAHs在小麥體內(nèi)的吸收、轉(zhuǎn)運(yùn)和富集過(guò)程的因素，除了污染物性質(zhì)、植物特征和土壤性質(zhì)(尤其是土壤有機(jī)質(zhì))外，小麥生長(zhǎng)環(huán)境的污染史也需要考慮在內(nèi).

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Research Advances in Uptake, Translocation and Accumulation of Polycyclic Aromatic Hydrocarbons in Winter Wheat

SHI Taoran1,2, TIAN Kai1,2, BAO Huanyu1,2, HOU Shaowei1,2, LIU Xueping3, WU Fuyong1,2*

1.College of Natural Resources and Environment, Northwest A & F University, Yangling 712100, China 2.Key Laboratory of Plant Nutrition and the Agri-Environment in Northwest China, Ministry of Agriculture, Northwest A & F University, Yangling 712100, China 3.School of Municipal and Environment Engineering, Henan University of Urban Construction, Pingdingshan 467036, China

Due to continuous economic and social development and the predominant use of coal in energy consumption, emissions of PAHs in China has maintained a high level for years, which has resulted in serious PAHs contamination in atmospheric and soil environments. To evaluate the processes and mechanisms contributing to accumulation and regulation of PAHs in wheat, based on systematic analysis of characteristics and mechanisms of uptake, translocation and accumulation of PAHs in wheat, the present work focused on the effects of root and foliar uptake of PAHs. Previous studies have found that wheat root uptake of PAHs mainly includes active and passive processes, of which active process is a carrier-mediated, energy-consuming and H+-coupled symport process. Besides simple diffusion, which is especially prevalent for passive uptake in higher plants, PAHs could enter into roots via aquaglyceroporin. PAHs could enter into leaves by gas-phase and particle-phase deposition onto the waxy cuticle or via the stomata. Root and foliage uptake of PAHs are governed by the physicochemical properties of PAHs, plant species and environmental conditions. Acropetal translocation of PAHs by root is associated withKOW, transpiration rate, nitrogen form and concentration in soil. Current studies face some challenges. The mechanisms of PAHs translocating from leaves to root need further research. In addition, winter wheat always suffers from the double pollutions of soil and atmosphere under field conditions, and the role of root and foliar uptake of PAHs in the accumulation of PAHs in wheat has not been developed yet. Therefore, more efforts should be devoted to illustrating the effects of phloem and xylem in translocation of PAHs, using powerful techniques such as isotope trace and two-photon excitation microscopy to visualize and track how such compounds enter, move and distribute within wheat foliage, providing insight into PAHs foliar uptake. Paying more attention to field experiments to fully address root and foliar uptake of PAHs will provide logical proofs for revealing mechanisms of uptake, accumulation and regulation of PAHs in wheat and for the safety of agro-products growing in the PAHs-polluted areas.

polycyclic aromatic hydrocarbons; wheat; root uptake; foliar uptake; transport mechanism

2016-12-08

：2017-05-23

國(guó)家自然科學(xué)基金項(xiàng)目(41571456)；河南省高校科技創(chuàng)新人才支持計(jì)劃項(xiàng)目(14HASTIT048)

石陶然(1986-)，女，山西忻州人，shitaoran@126.com.

*責(zé)任作者，吳福勇(1973-)，男，河南方城人，教授，博士，博導(dǎo)，主要從事土壤植物修復(fù)、PAHs毒性機(jī)理、健康風(fēng)險(xiǎn)評(píng)價(jià)及食品質(zhì)量安全研究，wfy09@163.com

X56

：1001- 6929(2017)09- 1398- 08

ADOI：10.13198/j.issn.1001- 6929.2017.02.63

石陶然,田凱,包環(huán)宇,等.多環(huán)芳烴在冬小麥體內(nèi)的吸收與轉(zhuǎn)運(yùn)及富集研究進(jìn)展[J].環(huán)境科學(xué)研究,2017,30(9):1398- 1405.

SHI Taoran,TIAN Kai,BAO Huanyu,etal.Research advances in uptake, translocation and accumulation of polycyclic aromatic hydrocarbons in winter wheat[J].Research of Environmental Sciences,2017,30(9):1398- 1405.