礦質(zhì)營養(yǎng)元素阻控植物鎘積累：從機制到應(yīng)用

何小林，關(guān)美艷，范士凱，何虎，金崇偉*

鎘（Cd）是一種高毒性有害重金屬[1]，但在土壤中自然含量通常較低，不會對動植物造成毒害。然而，許多城市近郊的土壤因社會的飛速發(fā)展和工業(yè)廢水、廢氣、固體廢棄物的不合理排放而受到了不同程度的重金屬污染[2-4]。2014年《全國土壤污染狀況調(diào)查公報》顯示，我國土壤鎘污染的點位超標率達到7.0%[5]。土壤鎘污染還給農(nóng)產(chǎn)品生產(chǎn)帶來了嚴重的安全問題[6]。例如，湖南省因采礦和冶煉致使農(nóng)田土壤被鎘污染[7-8]，樂清的菜地受到鎘污染導致蔬菜中鎘含量超標[9]。植物根系生長和光合作用因土壤遭受嚴重鎘污染而受阻，植物體內(nèi)水分和營養(yǎng)元素代謝也會因鎘的毒害而失衡，從而使作物生長發(fā)育不良，其產(chǎn)量和品質(zhì)同時下降[10]。尤其令人擔憂的是，植物體內(nèi)能富集一定量的鎘，但是鎘毒害卻通常無明顯的肉眼可見癥狀，致使受污染農(nóng)作物中的鎘經(jīng)食物鏈富集作用，危害人類健康[11-12]。鎘會對腎、肝、骨骼以及神經(jīng)、血液等帶來嚴重危害[13]。20世紀70年代，日本居民因食用鎘米而得的“痛痛病”給日本造成了嚴重的傷害[14-15]。所以，如何確保我國耕地質(zhì)量，在中輕度鎘污染的土壤上實現(xiàn)農(nóng)業(yè)安全生產(chǎn)，保證糧食安全和人類健康，是農(nóng)業(yè)發(fā)展面臨的重大挑戰(zhàn)。

目前常用物理、化學等傳統(tǒng)方法來修復和治理重金屬污染的土壤。雖然它們的治理效果較為理想，所需周期也不長，但缺點是費用昂貴，不容易管理，易帶來二次污染[16]。而植物修復技術(shù)因其綠色、經(jīng)濟和環(huán)保而逐漸成為研究熱點。然而，由于超積累植物生物量小、生長緩慢，嚴重制約了鎘污染耕地的農(nóng)業(yè)生產(chǎn)[17]。近年來，許多學者嘗試利用以植物生理過程為原理的農(nóng)藝技術(shù)調(diào)控來阻控土壤中的鎘進入農(nóng)作物中，以達到保障農(nóng)產(chǎn)品質(zhì)量安全的目的。例如，近年來我們課題組研究發(fā)現(xiàn)，外源應(yīng)用脫落酸（abscisic acid,ABA）可顯著降低植物體內(nèi)的鎘積累并減少鎘對植物的毒害[18]。此外，近年來還有許多學者關(guān)注并研究了礦質(zhì)營養(yǎng)元素對植物鎘積累的影響及機制[19]。在農(nóng)業(yè)生產(chǎn)中，農(nóng)民通常依靠增加施肥量來獲得較高的作物產(chǎn)量。而許多養(yǎng)分元素和鎘的有效性以及植物對鎘的吸收之間存在密切的相互影響。因此，若能在施肥過程中正確處理礦質(zhì)元素和鎘之間的互作關(guān)系，將能夠有效阻控植物對鎘的吸收。本文主要根據(jù)近年來相關(guān)研究進展，綜述了各種植物營養(yǎng)元素對鎘吸收和耐性的影響機制，并探討了其可能在農(nóng)業(yè)生產(chǎn)中的應(yīng)用價值。

1 鐵

由于Cd2+與Fe2+的水合離子半徑高度相似，鐵營養(yǎng)與鎘在植物體內(nèi)的關(guān)系十分密切[20-21]。鐵營養(yǎng)不足會導致植物體中的鎘含量上升[22]；相反，增加外源供鐵可顯著降低植物的鎘含量[11,18,23]。在雙子葉植物中，F(xiàn)e2+吸收轉(zhuǎn)運體（iron-regulated transporter 1,IRT1）已被證明是根系吸收Cd2+的關(guān)鍵轉(zhuǎn)運蛋白之一[24]，我們在此基礎(chǔ)上發(fā)現(xiàn)，銨、脫落酸和增加外源供鐵處理主要是通過抑制IRT1表達從而阻控植物對鎘的吸收[11,18,25-26]。此外，雖然韌皮部卸載Fe2+的轉(zhuǎn)運體OPT3（oligopeptide transporter 3）不直接參與Cd2+的轉(zhuǎn)運，但OPT3突變后因減少了鐵在種子中的貯存而間接地增加了鎘的積累[27]。這些結(jié)果表明，Cd2+與Fe2+的吸收和轉(zhuǎn)運在雙子葉植物中均存在直接或間接的關(guān)系。在禾本科植物水稻中，盡管Mn2+轉(zhuǎn)運蛋白OsNRAMP5（natural resistance-associated macrophage protein 5）被認為是根系吸收Cd2+的主要途徑[28-31]，但研究發(fā)現(xiàn)水稻的鐵轉(zhuǎn)運蛋白OsIRT1和OsNRAMP1也參與根系的鎘吸收過程，過表達OsIRT1和OsNRAMP1均可使水稻的鎘含量顯著提高[32-33]。此外，在生理學層面也有研究發(fā)現(xiàn)，缺鐵脅迫會顯著促進水稻根系對Cd2+的吸收[34]，而增加鐵素營養(yǎng)的供應(yīng)則降低了水稻的鎘含量并緩解了鎘的毒害[35]。這些結(jié)果表明，水稻體內(nèi)Cd2+與Fe2+也存在密切的互作關(guān)系。由于Cd2+和Fe2+在化學性質(zhì)上的相似性，含鐵蛋白中的鐵極易被Cd2+所替換，從而導致植物發(fā)生生理性缺鐵[36]。對此，我們與其他研究小組在擬南芥和水稻中均發(fā)現(xiàn)鎘脅迫也能引起正常供鐵的植物出現(xiàn)缺鐵的癥狀，并誘導許多鐵吸收和鐵穩(wěn)態(tài)相關(guān)基因的表達[37-39]。這些結(jié)果表明，雖然缺鐵和鎘毒是2種截然不同的脅迫，但植物對這2種脅迫的響應(yīng)存在部分交叉重疊。植物缺鐵脅迫的響應(yīng)在增強根系鐵吸收和改善體內(nèi)鐵穩(wěn)態(tài)中均發(fā)揮著重要的作用[40]。理論上，也正是由于Fe2+和Cd2+在化學性質(zhì)上的相似性，植物體內(nèi)的鐵也可通過競爭作用阻止Cd2+吸收和轉(zhuǎn)運以及Cd2+對含鐵蛋白中鐵的替換作用。另外，供鐵水平與水稻鎘含量、毒害程度呈負相關(guān)的研究結(jié)果在生理水平上也支持了這一結(jié)論[35]。因此，我們認為“缺鐵脅迫響應(yīng)”和“鎘毒脅迫響應(yīng)”的交叉重疊過程極有可能是植物控制鎘吸收、轉(zhuǎn)運和耐性等過程的重要機制。

綜上，在農(nóng)業(yè)生產(chǎn)中若能有效改善根際土壤環(huán)境中鐵的有效性，比如，施用持久高效的鐵肥或利用促鐵吸收的微生物菌劑[41]，可起到防治作物缺鐵、減控作物鎘污染的目的。

2 磷

磷（P）元素對植物的生長發(fā)育極其重要[42]。磷既是各種重要化合物的組分，又是作物高產(chǎn)及保持品種優(yōu)良特性的重要保障[43]。人們普遍依靠長期增施磷肥以達到作物增產(chǎn)的目的，但與此同時也面臨著嚴重的問題。因為磷礦石中本身含有重金屬成分，所以施用磷肥極有可能會造成土壤重金屬（鎘、銅、錳、鎳、鉛和鋅）污染[44-45]。因此，因磷肥的大量施用可能帶來的鎘污染問題也倍受關(guān)注。大量研究發(fā)現(xiàn)，磷和鎘之間彼此影響的關(guān)系十分復雜[46-48]。例如：土壤溶液中的鎘離子可以被難溶性磷酸鹽直接吸附，同時磷酸根離子能增加溶液中陰離子含量，形成磷-鎘沉淀物使鎘鈍化[49]；此外，供磷增產(chǎn)顯著帶來的稀釋效應(yīng)可以使玉米和小麥莖葉及根系中鎘含量降低[50]；相反，也有研究發(fā)現(xiàn)，植物對鎘的吸收量因施用磷肥提高了鎘的溶解度及活性而上升[51]。因此，磷和鎘之間在植物生理過程和土壤化學過程中均存在極其復雜的交互關(guān)系。鑒于上述原因，需要合理地設(shè)計水培試驗以減少或避免根際土壤環(huán)境中磷和鎘之間的相互影響，來闡明磷和鎘在植物生理層面的相互關(guān)系。為此，有學者通過水培試驗發(fā)現(xiàn)水稻地上部鎘含量隨著磷濃度的增加而增加[52]；同樣也有水培試驗發(fā)現(xiàn)缺磷會降低植物對鎘的吸收[53]。這些結(jié)果表明，植物體內(nèi)磷營養(yǎng)狀況對鎘的積累存在負調(diào)控作用。ZHENG等[54]研究表明，植物體內(nèi)磷和鐵之間存在較強的拮抗作用，缺磷會顯著提升植物體內(nèi)鐵的可利用能力，而鐵轉(zhuǎn)運蛋白基因IRT1的表達受植物體內(nèi)鐵營養(yǎng)狀況的負調(diào)控。鑒于IRT1蛋白是植物吸收Cd的關(guān)鍵轉(zhuǎn)運蛋白之一，因此，我們推測磷可能通過調(diào)控IRT1的表達來影響植物對鎘的吸收和積累。

鑒于施用磷肥是最為頻繁的一項農(nóng)藝管理措施，所以有必要開展大量研究以系統(tǒng)闡明土壤環(huán)境中以及植物體內(nèi)的磷與鎘的關(guān)系，從而為合理進行磷素養(yǎng)分管理、降低作物鎘積累提供理論依據(jù)。

3 氮

銨態(tài)氮和硝態(tài)氮是植物從土壤環(huán)境中吸收利用的2種主要氮形態(tài)。植物獲取硝態(tài)氮時，會伴隨H+的獲取，使根際pH上升；植物吸收銨態(tài)氮時，會伴隨H+的釋放，導致根際pH降低[55]。由于土壤鎘的有效性受pH的影響極大[56]，多年來，許多學者一直認為以銨態(tài)氮作為氮源，可以通過酸化根際土壤來增加重金屬的生物有效性，從而促進植物對重金屬鎘的吸收。這一推測也得到了一些試驗證據(jù)的支撐。例如，施用銨態(tài)氮酸化土壤，增加土壤中活性鎘的含量，增強植物對鎘的獲取[19]。然而，國內(nèi)外也有許多研究得出了相反的結(jié)果，有研究發(fā)現(xiàn)供應(yīng)硝態(tài)氮反而會增加植物對鎘的吸收[57]。我們課題組在對小白菜盆栽試驗的研究中發(fā)現(xiàn)，之所以出現(xiàn)這樣截然不同的結(jié)果可能與土壤pH的緩沖性密切相關(guān)，即：當土壤pH緩沖性差時，與供應(yīng)銨態(tài)氮肥相比，供應(yīng)硝態(tài)氮處理使小白菜體內(nèi)的鎘含量顯著降低；而當土壤pH緩沖性較強時，與供應(yīng)銨態(tài)氮肥相比，供應(yīng)硝態(tài)氮反而使小白菜體內(nèi)的鎘含量顯著增加[58]。這一結(jié)果可能是在pH緩沖性強的土壤中，氮素形態(tài)對根際pH的影響較弱的緣故。同時，也說明氮素形態(tài)本身可能直接影響根系細胞對鎘的吸收。我們還以番茄植物為材料，采用2-嗎啉乙磺酸（2-morpholinoeethanesulfonic acid,MES）緩沖的水培試驗證實了這一推測，即供應(yīng)硝態(tài)氮能促進根系細胞對鎘的吸收[25]。此外，我們還對硝態(tài)氮影響植物對鎘吸收的機制進行了探究，結(jié)果發(fā)現(xiàn)，供應(yīng)硝態(tài)氮的番茄根系鐵轉(zhuǎn)運體基因LeIRT1的表達和鐵吸收量遠高于供銨植物。鑒于IRT1轉(zhuǎn)運體也是負責植物吸收鎘的主要途徑之一，我們以調(diào)控鐵吸收相關(guān)的FER番茄突變體為材料，發(fā)現(xiàn)供氮形態(tài)對LeIRT1的表達和對鎘的吸收都沒有影響[25]，這表明供應(yīng)硝態(tài)氮可以通過誘導根系細胞質(zhì)膜上的鐵吸收系統(tǒng)來促進鎘的吸收。

上述結(jié)果表明，合理的氮素養(yǎng)分管理可有效降低作物體內(nèi)的鎘積累，如在pH緩沖性較弱的土壤中施用硝態(tài)氮，在pH緩沖性較強的土壤中施用銨態(tài)氮，從而降低作物的鎘積累。然而在現(xiàn)實條件下，銨態(tài)氮和尿素均是我國當前主要的氮肥形態(tài)，因此需要探討在緩沖性弱的土壤中能更好地降低鎘積累的方法。在理論上，土壤中銨態(tài)氮的硝化反應(yīng)比植物吸收銨態(tài)氮所產(chǎn)生的酸化效應(yīng)明顯。通過盆栽試驗發(fā)現(xiàn)，在pH緩沖性較弱的土壤中，銨態(tài)氮和尿素的分次施肥或使用緩釋肥可減少施入土壤中銨的硝化比例，從而減輕硝化反應(yīng)產(chǎn)生的酸化效應(yīng)，同時也減少土壤中硝酸鹽的含量，減弱硝酸鹽促進植物根系細胞吸收鎘的作用，實現(xiàn)降低作物鎘含量的目標[59]。

既然外源硝酸鹽能調(diào)控植物對鎘的吸收，我們課題組還進一步研究了植物自身的硝酸鹽吸收系統(tǒng)對鎘的吸收是否也具有調(diào)控作用，結(jié)果發(fā)現(xiàn)鎘脅迫自身也可顯著抑制植物根系中NRT1.1（nitrate transporter 1.1）基因和NRT1.1蛋白的表達量，這一過程不僅抑制了根系對硝酸鹽的吸收，而且也可減少植物對鎘的吸收和減輕鎘毒害程度。因此，鎘脅迫引起的NRT1.1活性抑制是植物體內(nèi)一種重要的耐鎘毒機制[39]。為此，我們進一步研究了NRT1.1調(diào)控鎘吸收的內(nèi)在機制，發(fā)現(xiàn)NRT1.1活性受鎘抑制可減少根際中伴隨硝酸鹽吸收的質(zhì)子消耗[60]，說明根際pH變化并非是NRT1.1活性受抑導致鎘吸收降低的原因；通過轉(zhuǎn)錄組測序研究發(fā)現(xiàn)，NRT1.1功能缺失后，在植物根系中所有編碼二價陽離子轉(zhuǎn)運體的基因中，僅IRT1的表達顯著下調(diào)；運用IRT1和NRT1.1雙基因缺失手段，進一步揭示了IRT1活性下降是NRT1.1活性受抑制導致鎘吸收減少的一個重要過程，但不是唯一途徑。上述結(jié)果說明，利用生物技術(shù)手段調(diào)控作物自身的硝酸鹽吸收將是減控作物鎘污染的新策略[61]。

4 鈣

鈣在植物體內(nèi)具有十分重要的生理生化功能，密切關(guān)系到植物生長[62]。研究表明：鈣能增強植物抵抗環(huán)境脅迫的能力，如減輕水分脅迫、低溫脅迫、鹽脅迫等逆境對植物造成的傷害[63]；鈣也能增強植物對重金屬鎘脅迫的耐性，緩解鎘的毒害[64]，其機制主要有以下幾個方面。

4.1 抑制土壤鎘的有效性

pH值會改變土壤中鎘的活性。石灰施入土壤中使土壤pH值升高，不僅可以降低鎘的有效性，還可以增加土壤中的鈣含量，促進植物體內(nèi)礦質(zhì)營養(yǎng)元素的吸收。土壤中鎘的有效性隨土壤pH值的升高而下降[65]；在玉米地施用石灰后會使土壤鎘的濃度下降，減少玉米鎘含量[66]。因此，鈣可以通過抑制鎘的活性，緩解鎘毒。

4.2 降低鎘的運輸

為了滿足植物生長需求，植物營養(yǎng)元素一般經(jīng)主動運輸進入植物體內(nèi)。重金屬離子大多數(shù)通過離子通道進入植物體內(nèi)[67]。植物吸收重金屬鎘的其中一條途徑就是通過鈣離子通道，所以施鈣會減少通過鈣通道進入植物體內(nèi)鎘的含量[68]。另外，因為鈣離子和鎘離子的離子半徑相近和電荷相同，所以，鈣、鎘之間存在拮抗作用，兩者可以競爭植物根系上的吸收位點。研究表明，尤其在較高的鈣離子濃度下，鎘的吸收會顯著受到抑制[69-70]。如菜豆在幼苗期及成熟期處于鎘脅迫時，供鈣充足可以緩解鎘毒，缺鈣反而加劇鎘毒，說明鈣可以通過競爭作用減少植物對鎘的吸收與利用[71]。在土壤中含鈣礦物與鎘會生成復合物，減弱鎘的活性；同時鈣與鎘會發(fā)生競爭作用，減少細胞鎘積累量[72]。因此，鈣可以阻控鎘的運輸，緩解植物鎘毒。

4.3 增強植物的抗氧化脅迫能力、光合作用和呼吸作用

植物需要鈣參與調(diào)節(jié)抗氧化酶的活性，而且鈣能穩(wěn)定細胞膜的結(jié)構(gòu)和功能[73]。其具體作用可歸納為以下2個方面：1）鈣能降低植物細胞內(nèi)活性物質(zhì)的含量，使抗氧化系統(tǒng)中的保護酶及抗氧化物維持較高的水平；2）鈣能使植物細胞的結(jié)構(gòu)更加穩(wěn)固，使活性氧代謝保持平衡。因此，鈣能通過提高植物的抗氧化脅迫能力，增強其抵抗鎘毒的能力。

鈣是植物光合作用所必需的元素之一，同時能調(diào)控與光合作用密切相關(guān)的氣孔運動，是保障光合作用順利進行的關(guān)鍵因素[74]。研究表明，施用鈣肥后能顯著提高鎘處理下玉米的葉綠素含量和酶的活性，降低丙二醛含量，從而改善玉米的生長狀況[75]。蔡妙珍等[76]也認為，在鎘脅迫下施鈣可能是通過改善了細胞中酶的“生態(tài)環(huán)境”，提高了酶的活性，改善了植物的呼吸作用。

綜上所述，鈣可以緩解植物鎘脅迫，因此，通過施用鈣養(yǎng)分來阻控植物鎘積累是一種可行的農(nóng)藝措施。

5 其他礦質(zhì)元素

養(yǎng)分元素鋅（Zn）與植物的生理代謝、激素調(diào)節(jié)、抗逆性等功能都密切相關(guān)。我國耕地土壤缺鋅嚴重，施用鋅肥作物產(chǎn)量和品質(zhì)得到顯著提高。由于鋅離子和鎘離子都是二價離子，離子半徑相似，土壤中施用鋅肥可增強鋅與鎘的膜結(jié)合位點和運輸系統(tǒng)的競爭作用，從而限制鎘由韌皮部轉(zhuǎn)運到籽粒[77]。相關(guān)研究表明：在缺鋅土壤中，在一定范圍內(nèi)，隨著施用鋅肥量的增加，植物中鎘的積累量減少更加明顯[78]；土壤嚴重缺鋅時，施用鋅肥可減少小麥體內(nèi)鎘的含量[79]；施用鋅肥可通過減少萵苣和菠菜根部對鎘的吸收，阻止鎘向莖葉遷移[80]；運用同位素示蹤法研究鋅肥對小麥體內(nèi)鎘轉(zhuǎn)運的影響，發(fā)現(xiàn)在較高鋅濃度下，鎘的轉(zhuǎn)運受到顯著阻止[81]。因此，鋅可通過拮抗作用緩解植物鎘脅迫。

硅（Si）是土壤中含量第二豐富的元素，為大多數(shù)植物所必需的有益元素，對植物的生長發(fā)育起重要作用[82]。植物體內(nèi)鎘含量隨土壤中硅的增加而下降。例如：在一定的硅濃度條件下，施用硅肥能減少植物對鎘的吸收量[83]；增施硅肥會減少水稻體內(nèi)鎘的含量[84]。此外，增加細胞壁對鎘的截留量，阻止植物根系中的鎘向地上部轉(zhuǎn)移，也是一種硅緩解植物鎘毒的機制。比如，施用硅肥可以增加根的生物量，使根中鎘的截留量增加，減少鎘向地上部的遷移[85-86]。然而，在低鎘濃度下，施用硅肥能阻止鎘向地上部轉(zhuǎn)移；相反，在高鎘濃度下，施用硅肥反而促進鎘向地上部遷移[87]。另外，一些研究發(fā)現(xiàn)，在葉面施用硅肥可抑制鎘由水稻葉片向籽粒運輸，防治農(nóng)作物鎘污染[88-89]。除此之外，施用硅肥還可以增強植物抗生物和非生物脅迫。例如，施用硅肥可以提高植物體內(nèi)超氧化物歧化酶、抗壞血酸過氧化物酶、谷胱甘肽還原酶、脫氫抗壞血酸還原酶等的活性，提高植物對鎘脅迫的抵抗力[90-91]。然而，硅濃度過高會造成中毒，因此，應(yīng)適當利用硅肥才能達到理想效果。

硒（Se）是動植物體必需的有益元素[92-93]。施用硒肥能增強植物緩解鎘毒能力。例如：施用硒肥能減少植物對鎘的吸收量[94-95]；也能有效調(diào)節(jié)油菜和小麥體內(nèi)的鎘元素含量，改善植物體生理狀況，緩解鎘脅迫引起的氧化損傷[96-97]；葉面噴施硒肥可阻控水稻對鎘的吸收[98]。關(guān)于硒可以緩解植物鎘毒的可能機制包括：1）硒、鎘復合物的產(chǎn)生；2）硒可增強植物的抗氧化酶活性；3）硒可能參與植物新陳代謝，可以同其他元素相互作用，緩解鎘毒[99]。

錳（Mn）是植物必需的微量元素。由于錳離子和鎘離子都是二價離子，同時它們的離子半徑也相似，所以錳、鎘之間存在顯著的拮抗作用[100]。例如，增施錳肥可能通過競爭膜轉(zhuǎn)運蛋白減少植物對鎘的積累[101]。另外，錳還可以通過其他途徑緩解植物鎘毒。如施用錳肥可以通過促進植物的生長緩解植物鎘毒[102]；施用錳肥能阻止谷物中鎘的轉(zhuǎn)移和積累[103]；此外，施用錳肥也可以恢復鎘中毒的葉綠體結(jié)構(gòu)，改善植物光合作用，緩解植物鎘毒[104]。至于錳是否還可以通過其他途徑緩解植物鎘毒還需要進一步的研究。

6 小結(jié)

人體健康因鎘經(jīng)食物鏈的富集作用而受到嚴重威脅。因此，在中輕度鎘污染土壤上實現(xiàn)農(nóng)產(chǎn)品的安全生產(chǎn)，是一個亟待解決的現(xiàn)實問題。由表1可知養(yǎng)分元素對植物鎘吸收的影響及可能的原因。而且，研究認為，植物養(yǎng)分元素對植物的鎘積累和鎘毒害具有顯著影響。因此，在農(nóng)業(yè)生產(chǎn)中，利用養(yǎng)分元素與鎘之間的互作關(guān)系，合理地實施養(yǎng)分管理有望成為一種費用低、歷時短、效率高的阻控作物鎘積累的農(nóng)藝措施。但是，目前關(guān)于植物養(yǎng)分元素在鎘吸收、轉(zhuǎn)運和耐性中的作用及機制仍不完全清楚。所以，仍需對此進行大量的研究，以期為通過養(yǎng)分管理實現(xiàn)鎘污染土壤的作物安全生產(chǎn)提供全面的理論依據(jù)。

表1 不同礦質(zhì)營養(yǎng)元素對鎘吸收的影響及可能的原因Table 1 Effects of different mineral nutrient elements on Cd absorption and its possible reasons

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