水稻鎘積累影響因素與低鎘稻米生產(chǎn)策略

江南 顏旭1, 2 周延彪 周群豐 王凱 楊遠(yuǎn)柱

水稻鎘積累影響因素與低鎘稻米生產(chǎn)策略

江南1, 3, #顏旭1, 2, #周延彪1, 3周群豐1, 3王凱1, 3楊遠(yuǎn)柱1, 2, 3, 4,*

(1袁隆平農(nóng)業(yè)高科技股份有限公司 農(nóng)業(yè)農(nóng)村部南方水稻品種創(chuàng)制重點(diǎn)實(shí)驗(yàn)室/抗病蟲(chóng)水稻育種湖南省工程實(shí)驗(yàn)室，長(zhǎng)沙 410125；2華中農(nóng)業(yè)大學(xué) 植物科學(xué)技術(shù)學(xué)院，武漢 430070；3湖南亞華種業(yè)科學(xué)研究院，長(zhǎng)沙 410604；4湖南農(nóng)業(yè)大學(xué) 農(nóng)學(xué)院，長(zhǎng)沙 410128；#共同第一作者；*通信聯(lián)系人，E-mail: yzhuyah@163.com)

鎘是一種生物毒性極強(qiáng)且分布廣泛的重金屬元素。農(nóng)田中的鎘不僅影響作物的生長(zhǎng)發(fā)育，而且可通過(guò)食物鏈進(jìn)入人體內(nèi)，當(dāng)富集到一定程度會(huì)危害人體健康。水稻是我國(guó)重要的糧食作物之一，在保障我國(guó)糧食安全中的地位舉足輕重，但水稻同時(shí)也是對(duì)鎘吸收和積累最強(qiáng)的大宗谷類作物之一。近些年“鎘大米”事件在我國(guó)頻繁發(fā)生，稻米鎘污染已成為一個(gè)備受社會(huì)關(guān)注的嚴(yán)峻問(wèn)題，治理稻米鎘污染迫在眉睫。本文從低鎘水稻品種篩選與培育、優(yōu)化水分管理、調(diào)節(jié)土壤pH值、施用葉面阻隔劑等方面對(duì)稻米鎘污染治理技術(shù)的研究進(jìn)展進(jìn)行了綜述，旨在為技術(shù)的集成、推廣和應(yīng)用提供理論依據(jù)，同時(shí)為新技術(shù)的研發(fā)提供新的思路。

水稻；鎘積累；食品安全；品種；灌溉；pH

隨著現(xiàn)代工農(nóng)業(yè)的快速發(fā)展以及城市化進(jìn)程的推進(jìn)，土壤重金屬污染的問(wèn)題日趨嚴(yán)重，已成為當(dāng)前全球面臨的一個(gè)重要環(huán)境議題，而我國(guó)面臨的土壤重金屬污染形勢(shì)則尤為嚴(yán)峻[1]。2014年環(huán)境保護(hù)部和國(guó)土資源部共同發(fā)布《全國(guó)土壤污染狀況調(diào)查公報(bào)》，該公報(bào)指出我國(guó)耕地土壤點(diǎn)位超標(biāo)率達(dá)19.4%，遠(yuǎn)高于林地、草地和未利用地。土壤污染類型以無(wú)機(jī)型為主，其超標(biāo)點(diǎn)位數(shù)占全部超標(biāo)點(diǎn)位的82.8%。在鎘、汞、砷、銅、鉛、鉻、鋅、鎳8種主要無(wú)機(jī)重金屬污染物中，鎘(Cadmium, Cd)的點(diǎn)位超標(biāo)率達(dá)7.0%，遠(yuǎn)超其他7種重金屬。而早在2005年，有報(bào)道顯示，我國(guó)受鎘污染的耕地面積已達(dá)到1.33萬(wàn)hm2，涉及11個(gè)省份的25個(gè)地區(qū)，導(dǎo)致每年糧食減產(chǎn)1000多萬(wàn)t，受污染糧食達(dá)1200多萬(wàn)t，合計(jì)經(jīng)濟(jì)損失至少達(dá)200億元[2]。鎘是生物非必需元素，不參與生物體的結(jié)構(gòu)組成與代謝活動(dòng)，且具有較強(qiáng)潛伏性和毒性，世界衛(wèi)生組織(World Health Organization, WHO)將鎘列為重點(diǎn)研究的食品污染物[3]。國(guó)際癌癥研究機(jī)構(gòu)(International Agency for Research on Cancer, IARC)將鎘歸類為人類致癌物[4]。美國(guó)毒物和疾病登記署(Agency for Toxic Substances and Disease Registry, ATSDR)將鎘列為第7位危害人體健康的物質(zhì)[5]。水稻是全球三大糧食作物之一和半數(shù)以上人口的主糧，是世界糧食安全的基石，同時(shí)也是對(duì)鎘吸收最強(qiáng)的大宗谷類作物之一[6]。近些年，我國(guó)南方地區(qū)相繼發(fā)生的 “鎘大米”事件，使得包括稻米等農(nóng)產(chǎn)品鎘污染問(wèn)題受到社會(huì)的極大關(guān)注。近期的一項(xiàng)研究對(duì)我國(guó)20個(gè)省份的160份市場(chǎng)銷售稻米樣品進(jìn)行檢測(cè)，發(fā)現(xiàn)有10%的樣品鎘含量超過(guò)國(guó)家標(biāo)準(zhǔn)(0.2 mg/kg)，3.8%的樣品鎘含量甚至超過(guò)0.4 mg/kg[7]，可見(jiàn)，我國(guó)稻米鎘污染的管控與治理已十分緊迫。考慮到稻米鎘污染問(wèn)題的嚴(yán)重性，我國(guó)制定了《土壤環(huán)境質(zhì)量-農(nóng)用地土壤污染風(fēng)險(xiǎn)管控標(biāo)準(zhǔn)(試行)》(GB 15618－2018)和《食品中污染物限量》(GB2762－2012)，對(duì)農(nóng)田和稻米中的鎘含量進(jìn)行了明確規(guī)定。國(guó)內(nèi)外針對(duì)水稻鎘積累影響因素與低鎘稻米生產(chǎn)開(kāi)展了系列相關(guān)研究。湖南省農(nóng)業(yè)科學(xué)院、中國(guó)科學(xué)院亞熱帶農(nóng)業(yè)生態(tài)研究所、湖南農(nóng)業(yè)大學(xué)等單位聯(lián)合攻關(guān)，開(kāi)展稻米鎘污染控制技術(shù)的研究，構(gòu)建了VIP+n(V，Variety，鎘低積累水稻品種；I，Irrigation，優(yōu)化水分管理；P，土壤pH值，施用石灰等提高土壤pH值；n, 噴施葉面阻隔劑)的綜合控鎘技術(shù)(圖1)，可有效降低糙米中的鎘含量，保障中、輕度鎘污染土壤條件下的稻米生產(chǎn)安全[8-10]。本文圍繞VIP+n技術(shù)，針對(duì)鎘低積累水稻品種篩選與培育、田間水分管理、土壤pH值調(diào)節(jié)以及噴施葉面阻隔劑等研究進(jìn)展進(jìn)行了綜述，以期為該技術(shù)進(jìn)一步提升和推廣應(yīng)用提供參考信息。

圖1 VIP+n稻米鎘污染控制技術(shù)

Fig.1.VIP+n strategies for reducing cadmium accumulation in rice.

1 低鎘水稻品種篩選與培育

不同水稻基因型在鎘積累特性方面存在豐富的變異[11]，這為鎘低積累水稻品種的篩選、相關(guān)基因的發(fā)掘與功能研究以及遺傳改良奠定了基礎(chǔ)。在土壤鎘污染沒(méi)有得到根本治理的情況下，篩選、培育和推廣鎘低積累水稻品種是降低稻米鎘污染風(fēng)險(xiǎn)的有效技術(shù)手段之一。推廣鎘低積累水稻品種，不需要增加額外投入，也不改變現(xiàn)有種植結(jié)構(gòu)與方式，農(nóng)民容易接受，投入少，見(jiàn)效快。但對(duì)于何為“鎘低積累品種”，目前尚未有明確定義和標(biāo)準(zhǔn)[12]。從生產(chǎn)者的角度來(lái)看，鎘低積累水稻品種是在鎘污染土壤中能夠生產(chǎn)出稻米鎘含量達(dá)到國(guó)家食品安全標(biāo)準(zhǔn)的水稻品種；從遺傳學(xué)家的角度來(lái)看，鎘低積累水稻品種是在相同土壤環(huán)境條件下稻米鎘含量較低的水稻品種，只是一個(gè)相對(duì)的概念。由于稻米鎘積累受土壤環(huán)境和農(nóng)藝措施的綜合調(diào)控，目前還未發(fā)現(xiàn)有絕對(duì)的鎘低積累水稻資源或品種。綜合不同層面的認(rèn)識(shí)，對(duì)什么是鎘低積累水稻品種已形成了一些基本共識(shí)，即在中、輕度鎘污染土壤條件和正常栽培管理措施下種植，稻米鎘含量達(dá)到國(guó)家食品安全標(biāo)準(zhǔn)的水稻品種[12]。

1.1 鎘低積累水稻品種的篩選

利用當(dāng)?shù)氐闹髟运酒贩N就地篩選，可快速實(shí)現(xiàn)推廣和應(yīng)用。湖南是農(nóng)業(yè)大省，同時(shí)也是有色金屬之鄉(xiāng)，礦產(chǎn)品采選冶煉產(chǎn)業(yè)發(fā)達(dá)，造成農(nóng)田嚴(yán)重污染，稻田鎘污染尤為突出[13]。2014年，湖南省啟動(dòng)了長(zhǎng)株潭耕地重金屬污染修復(fù)試點(diǎn)及農(nóng)作物種植結(jié)構(gòu)調(diào)整試點(diǎn)項(xiàng)目，鎘低積累水稻品種篩選是其中的重要課題之一。通過(guò)多年、多點(diǎn)、多重復(fù)的大田與盆栽試驗(yàn)，從主栽品種中篩選出了49個(gè)稻米鎘積累較低的品種作為應(yīng)急性鎘低積累品種(表1)，其中湘早秈45號(hào)等常規(guī)稻品種6個(gè)，株兩優(yōu)189等兩系雜交稻品種27個(gè)，欣榮優(yōu)123等三系雜交稻品種16個(gè)(http://agri.hunan.gov.cn/agri/index.html)。上述品種在湖南省安全利用區(qū)域推廣面積達(dá)40萬(wàn)hm2[12]。此外，Duan等[14]通過(guò)兩年多點(diǎn)試驗(yàn)，從471個(gè)南方大面積推廣種植的水稻品種中篩選出8個(gè)鎘低積累的秈型雜交稻品種，它們?cè)谒性囼?yàn)點(diǎn)的糙米鎘含量均低于國(guó)家標(biāo)準(zhǔn)(<0.2 mg/kg)。這些品種具有高產(chǎn)和廣適等優(yōu)異特性，可快速在我國(guó)南方中、輕度鎘污染稻區(qū)應(yīng)用。

1.2 水稻鎘積累相關(guān)熱點(diǎn)基因的功能研究

土壤中的鎘被水稻根系吸收，經(jīng)木質(zhì)部加載向地上部運(yùn)輸，然后通過(guò)莖節(jié)完成定向轉(zhuǎn)運(yùn)與分配，最終通過(guò)葉片等器官的韌皮部轉(zhuǎn)移至谷粒中[15]。針對(duì)水稻鎘吸收、轉(zhuǎn)運(yùn)和積累的過(guò)程與分子機(jī)制，學(xué)者進(jìn)行了廣泛研究，一系列相關(guān)基因被克隆[15-16]，其中一些關(guān)鍵基因具有重大育種應(yīng)用價(jià)值。自然抗性相關(guān)巨噬細(xì)胞蛋白(natural resistance-associated macrophage protein, NRAMP)是一類高度保守的二價(jià)金屬離子轉(zhuǎn)運(yùn)蛋白家族，廣泛存在于各類生物中[17]。水稻基因組中有7個(gè)基因[18]，其中主要在根的表皮、外皮層、皮層外層以及木質(zhì)部周邊組織表達(dá)，其編碼的蛋白定位于細(xì)胞質(zhì)膜上，主要參與根系對(duì)錳(Manganese, Mn)和鎘的吸收[18-19]?；蚯贸捅磉_(dá)下調(diào)可顯著降低水稻對(duì)鎘和錳的吸收以及二者在稻米中的積累，但是植株在低錳的環(huán)境中生長(zhǎng)與產(chǎn)量會(huì)受到影響，可能由于材料遺傳背景、突變類型以及土壤條件等因素導(dǎo)致受影響程度不同[18-25]。近期Lü等[26]對(duì)1143份秈型雜交稻親本材料進(jìn)行重測(cè)序，發(fā)現(xiàn)紅蓮型不育系珞紅3A與珞紅4A在第7染色體基因位點(diǎn)存在一段408 kb的基因組片段缺失，表型鑒定顯示兩份材料葉片與根中鎘含量顯著低于對(duì)照品種華占，與華占背景的基因敲除突變體接近。近期的另一項(xiàng)研究發(fā)現(xiàn)，基因的過(guò)量表達(dá)增加了根對(duì)鎘和錳的吸收，但是通過(guò)破壞鎘徑向運(yùn)輸?shù)街兄⑦M(jìn)行木質(zhì)部裝載從而減少了鎘從根到莖的轉(zhuǎn)運(yùn)，最終使谷粒中鎘的濃度降低了49%～94%[27]。水稻NRAMP家族中的另一個(gè)成員OsNARMP1與OsNARMP5具有高度同源性，主要在中央維管束之外的其他根部細(xì)胞和葉肉細(xì)胞中表達(dá)。OsNARMP1同樣具備轉(zhuǎn)運(yùn)鎘和錳的能力，基因敲除會(huì)導(dǎo)致根部對(duì)鎘和錳的吸收以及二者在地上部與谷粒中的積累顯著下降，但是下降幅度不及敲除基因，而同時(shí)敲除兩個(gè)基因可進(jìn)一步降低對(duì)鎘和錳的吸收。突變體生長(zhǎng)同樣受到抑制，但受影響程度小于突變體以及雙突變體[28]。

Ueno等[29-30]利用地上部鎘高積累秈稻品種Anjana Dhan與鎘低積累粳稻品種日本晴構(gòu)建的遺傳群體在第7染色體定位并克隆到一個(gè)控制鎘積累的基因，它屬于P1B類型的重金屬ATP (Heavy Metal ATPase, HMA)酶基因家族成員，主要在根部表達(dá)[30]?；虺聊瑫?huì)促進(jìn)鎘從根部向地上部的轉(zhuǎn)運(yùn)，而過(guò)量表達(dá)則效果相反[30-31]。進(jìn)一步功能研究發(fā)現(xiàn)OsHMA3能夠?qū)㈡k轉(zhuǎn)運(yùn)至根部細(xì)胞的液泡中從而將其隔離[31-32]。第80位的氨基酸由精氨酸突變?yōu)榻M氨酸，是導(dǎo)致Anjana Dhan中OsHMA3蛋白功能喪失的原因[30]。在鎘高積累秈稻品種Cho-Ko-Koku和Jarjan中觀察到了相同的突變[31-32]。之后，又相繼發(fā)現(xiàn)了其他新的等位基因型[33-37]。

Yan等[38]對(duì)127份水稻材料進(jìn)行全基因組關(guān)聯(lián)分析，在水稻第3染色體鑒定到一個(gè)谷粒鎘積累相關(guān)基因，它屬于MFS家族(major facilitator superfamily)，主要在根細(xì)胞質(zhì)膜上表達(dá)。敲除基因可顯著降低植株對(duì)鎘的吸收和在谷粒中的積累，但會(huì)對(duì)產(chǎn)量造成影響。進(jìn)一步分析發(fā)現(xiàn)，基因在秈稻與粳稻之間出現(xiàn)明顯分化，粳稻主要是OsCd1等位基因型，秈稻中主要是OsCd1等位基因型。兩種等位基因型在表達(dá)水平和亞細(xì)胞定位方面未表現(xiàn)出明顯差異。將粳稻OsCd1等位基因型導(dǎo)入至秈稻中可顯著降低谷粒鎘含量，而對(duì)植株生長(zhǎng)和產(chǎn)量未產(chǎn)生顯著影響。

1.3 鎘低積累水稻新品種的培育

利用誘變技術(shù)結(jié)合表型篩選，是選育鎘低積累水稻品種的手段之一。Ishikawa等[20]采用碳離子束對(duì)日本高檔優(yōu)質(zhì)稻品種越光進(jìn)行輻射獲得了3個(gè)鎘低積累突變體，經(jīng)鑒定發(fā)現(xiàn)是由基因的突變導(dǎo)致了鎘積累量的顯著下降。其中兩個(gè)突變體與的生長(zhǎng)狀況、產(chǎn)量和品質(zhì)與野生型越光無(wú)明顯差異，而生長(zhǎng)和產(chǎn)量受到較大影響。進(jìn)行品種登記并重新命名為Kan 1(Koshihikari Kan 1)[39]。Cao等[40]對(duì)秈稻品種9311進(jìn)行EMS化學(xué)誘變獲得鎘低積累突變體，在不同田塊的谷粒鎘含量為0.02～0.13 mg/kg，遠(yuǎn)低于野生型(1.02～4.44 mg/kg)，進(jìn)一步研究發(fā)現(xiàn)是基因一個(gè)堿基的突變導(dǎo)致了鎘積累量的降低。相關(guān)QTL和基因的鑒定為通過(guò)分子標(biāo)記輔助選擇(marker-assisted selection, MAS)策略培育鎘低積累新品種奠定了基礎(chǔ)。我國(guó)在這一領(lǐng)域發(fā)展較快，取得了一系列進(jìn)展。隆平高科、湖南農(nóng)業(yè)大學(xué)、中國(guó)水稻研究所等單位利用高代回交結(jié)合MAS的策略，將鎘低積累等位基因轉(zhuǎn)育到9311、H819、創(chuàng)5S等雜交水稻親本中，均顯著降低了谷粒中鎘的積累[37, 41, 42]。Yan等[38]將粳稻日本晴中的OsCd1等位基因?qū)胫炼i稻9311中顯著降低谷粒鎘含量，而對(duì)植株生長(zhǎng)和產(chǎn)量未產(chǎn)生顯著影響。近些年以CRISPR/Cas9為代表的基因編輯技術(shù)發(fā)展日新月異，簡(jiǎn)單、精準(zhǔn)、高效等特點(diǎn)使其成為學(xué)者研究的熱點(diǎn)。Tang等[22]利用CRISPR/Cas9基因編輯手段對(duì)大面積推廣種植的雜交水稻品種隆兩優(yōu)華占雙親的基因進(jìn)行敲除，培育出稻米鎘積累量極低的改良隆兩優(yōu)華占(兩優(yōu)低鎘1號(hào))，其稻米鎘積累量比野生型隆兩優(yōu)華占降低98%以上(＜0.05 mg/kg)，且產(chǎn)量未受任何影響。由全國(guó)農(nóng)技推廣中心組織的兩優(yōu)低鎘1號(hào)多環(huán)境測(cè)試試驗(yàn)也表明，在長(zhǎng)江中下游稻區(qū)總鎘濃度0.2～2.43 mg/kg的土壤中種植，按當(dāng)?shù)亓?xí)慣栽培方式進(jìn)行田間管理，10個(gè)測(cè)試點(diǎn)中測(cè)試品種兩優(yōu)低鎘1號(hào)的稻米鎘含量均符合安全標(biāo)準(zhǔn)(≤0.2 mg/kg)，且產(chǎn)量和主要品質(zhì)性狀與隆兩優(yōu)華占相當(dāng)(未發(fā)表數(shù)據(jù))。之后又有多項(xiàng)研究在不同遺傳背景的水稻材料中靶向編輯基因，獲得了一系列鎘低積累株系[23, 24, 43]。

表1 湖南省篩選認(rèn)定的鎘低積累水稻品種

續(xù)表1

2 優(yōu)化水分管理

田間水分管理可改變土壤的Eh與pH值，影響鎘的形態(tài)和生物活性，進(jìn)而影響水稻對(duì)鎘的吸收和富集[44]。同其他稻米鎘污染治理措施相比，水分管理相對(duì)簡(jiǎn)單易行、可操作性強(qiáng)、成本較低。

2.1 水分管理對(duì)土壤Eh值的影響

土壤Eh值即土壤氧化還原電位值。土壤氧化還原反應(yīng)使得土壤物理、化學(xué)、生物環(huán)境發(fā)生變化并影響土壤中鎘的形態(tài)。淹水條件下，土壤Eh值低，處于還原狀態(tài)，土壤中的氧化組分包括NO? 3、SO2? 4、Fe3+和Mn3+/Mn4+，通過(guò)接受土壤微生物呼吸作用所釋放的電子，還原成NO?2、S2?、Fe2+和Mn2+, S2?與Cd2+形成CdS沉淀，降低了土壤中鎘的生物有效性[45]。另外，水稻可以通過(guò)葉片將大氣中的氧氣輸送到根系并釋放到根際，使大量的 Fe2+與Mn2+等氧化形成Fe-Mn氧化物，對(duì)土壤中的鎘吸附增加，從而降低了水稻根系對(duì)鎘的吸收[46-50]。相反，在排水條件下，有效態(tài)鎘被釋放，促進(jìn)了水稻對(duì)鎘的吸收[47, 49, 50]。

2.2 水分管理對(duì)土壤pH值的影響

土壤pH值是影響鎘形態(tài)、分布、轉(zhuǎn)化與生物有效性的另外一個(gè)重要因素[51]，在堿性和弱酸性土壤中，有效鎘的比例隨著pH值下降而增加[52]。當(dāng)土壤pH值升高時(shí)，土壤中的氧化物、礦物質(zhì)膠體、有機(jī)質(zhì)表面的負(fù)電荷增加，為游離態(tài)的鎘離子提供了更多吸附結(jié)合位點(diǎn)，土壤的吸附降低了生物有效態(tài)鎘的濃度，降低了鎘的遷移能力，從而減少了水稻對(duì)鎘的吸收和富集[49, 52]。酸性土壤淹水后，由于氧化物質(zhì)發(fā)生還原作用而消耗了大量的質(zhì)子和H2CO3-HCO3?反應(yīng)形成的緩沖作用，使得土壤pH值趨于中性[54-55]。因此，通過(guò)水分管理可調(diào)節(jié)土壤的pH值，從而控制水稻對(duì)鎘的吸收和積累。

2.3 水分管理具體措施

目前，已有一系列研究證實(shí)長(zhǎng)期淹水能有效降低稻米的鎘含量。張麗娜等[56]研究了不同水分管理方式對(duì)于水稻產(chǎn)量和稻米鎘積累的影響，全生育期淹水處理的水稻糙米鎘含量最低，而旱作栽培處理最高，全生育期淹水的水稻糙米鎘含量?jī)H為旱作栽培處理的37.6%。Arao等[57]研究了不同水分管理方式對(duì)稻米鎘和砷積累的影響，發(fā)現(xiàn)抽穗前后3周長(zhǎng)時(shí)間淹水對(duì)于降低稻米中的鎘濃度是最有效的，其中，全生育期淹水處理的稻米鎘濃度最低，而抽穗后淹水3周比抽穗前淹水3周更為有效。劉昭兵等[58]發(fā)現(xiàn)淹水時(shí)間顯著影響水稻鎘的吸收和累積，淹水時(shí)間越長(zhǎng)，糙米中的鎘含量越低，并且因生育期的不同會(huì)出現(xiàn)一定差異，分蘗盛期開(kāi)始淹水對(duì)抑制糙米鎘積累優(yōu)于灌漿期開(kāi)始淹水。楊小粉等[59]研究發(fā)現(xiàn)長(zhǎng)期淹水灌溉比濕潤(rùn)灌溉和階段性濕潤(rùn)灌溉能更有效降低糙米鎘含量。然而，長(zhǎng)期淹水會(huì)提高另外一種重金屬元素砷(Arsenic, As)在稻米中的積累[57, 60-62]。因此，在實(shí)際操作過(guò)程中，需根據(jù)土壤類型、理化性質(zhì)、重金屬組成與含量等具體情況，分析和權(quán)衡鎘和砷在稻米中積累的相對(duì)風(fēng)險(xiǎn)，合理地選擇水分管理方式。

3 調(diào)節(jié)土壤pH值

通過(guò)施撒石灰類等堿性物質(zhì)提高酸性土壤的pH值，可降低土壤中有效態(tài)鎘含量，進(jìn)而減少水稻對(duì)鎘的吸收[63]。在我國(guó)南方地區(qū)，通常在水稻分蘗期采用施加少量生石灰(CaO)于稻田表層土壤的方法，降低糙米中的鎘含量。但是，Wang等[64]研究發(fā)現(xiàn)單獨(dú)采取此措施對(duì)提高土壤pH值、降低糙米鎘含量作用有限，而且由于生石灰具有較強(qiáng)的腐蝕性，使用不當(dāng)會(huì)對(duì)作物的生長(zhǎng)造成影響。大田試驗(yàn)表明，在水稻種植前一次性施用7.5 t/hm2的CaCO3粉末，可將土壤pH值從5.5提升至6.5，在此田塊連續(xù)三季種植水稻的稻米鎘含量比未經(jīng)任何處理田塊低70%～80%，而施用CaCO3未對(duì)水稻產(chǎn)量、稻米中微量元素(鐵和鋅)以及無(wú)機(jī)砷的含量造成顯著影響[65, 66]。但是值得注意的是，農(nóng)田連續(xù)、大量的施用CaCO3和CaO，會(huì)引起土壤的鈣化、板結(jié)，顯著降低土壤肥效。因此，在施用石灰類堿性物質(zhì)時(shí)，需考慮頻率、用量、施用方法等。生物炭是由廢棄生物質(zhì)在完全或部分缺氧、低溫或相對(duì)低溫的條件下(＜700℃)熱分解所產(chǎn)生的一種高碳固體殘?jiān)黐67]。生物炭具有較大的比表面積和豐富的微孔結(jié)構(gòu)，因此具有較強(qiáng)的吸附能力。另外，由于生物炭含有灰分而呈堿性，而且其表面有機(jī)官能團(tuán)可吸收土壤中的H+，因此施加生物炭可提高土壤的pH值[68]。多點(diǎn)田間試驗(yàn)表明，在鎘污染稻田施用生物炭修復(fù)劑，可降低糙米中20%～90%的鎘[69]。此外，生物炭還具有促進(jìn)作物生長(zhǎng)和降低土壤溫室氣體排放等優(yōu)點(diǎn)[70-71]。此外，海泡石、坡縷石、膨潤(rùn)土、磷酸鹽等無(wú)機(jī)物也可通過(guò)提高土壤pH值，減少水稻等作物對(duì)于鎘等重金屬的吸收[72]。

4 葉面阻隔劑

葉面追肥在農(nóng)業(yè)生產(chǎn)上應(yīng)用有悠久的歷史，但是近些年發(fā)現(xiàn)葉面噴施營(yíng)養(yǎng)元素等可調(diào)控重金屬元素在作物植株內(nèi)的分配，減少向食用部分的轉(zhuǎn)移[73]。硅(Silicon, Si)是地殼第二豐富的元素，在植物中廣泛存在[74]。盡管不是大多數(shù)植物生長(zhǎng)發(fā)育的必需元素，但是大量證據(jù)顯示施用硅對(duì)植物生長(zhǎng)是有益的[75-76]。在水稻中，通過(guò)葉面施用硅不僅能促進(jìn)植株的生長(zhǎng)發(fā)育，提高產(chǎn)量和品質(zhì)以及對(duì)各種脅迫的抗性[77-78]，還可顯著降低鎘在稻米中的積累[79-81]。Shao等[82]發(fā)現(xiàn)施用硅可下調(diào)和基因的表達(dá)，進(jìn)而減少了對(duì)鎘的吸收和轉(zhuǎn)運(yùn)。硒(Selenium, Se)與硅類似，同樣是對(duì)植物生長(zhǎng)有益的元素，同時(shí)對(duì)減少水稻鎘吸收和積累[83-85]。硒處理可下調(diào)植株與基因的表達(dá)，同時(shí)上調(diào)基因的表達(dá)[86]。

5 展望

我國(guó)農(nóng)田土壤的重金屬污染問(wèn)題日益嚴(yán)重，稻米鎘超標(biāo)事件頻繁發(fā)生，嚴(yán)重威脅著人們的健康。盡管近些年來(lái)，國(guó)內(nèi)外學(xué)者在鎘低積累水稻品種的篩選與培育，農(nóng)藝措施降鎘，水稻鎘吸收、轉(zhuǎn)運(yùn)與籽粒鎘積累的分子機(jī)制等方面取得了一系列豐碩研究成果，但是仍然存在許多問(wèn)題，有待進(jìn)一步研究：1)鑒定的水稻鎘吸收、轉(zhuǎn)運(yùn)和積累相關(guān)的基因數(shù)量有限，真正具有育種應(yīng)用價(jià)值的基因更是屈指可數(shù)，有待進(jìn)一步發(fā)掘和研究；2)利用基因編輯手段敲除基因創(chuàng)制的“去鎘”水稻新品種，受制于轉(zhuǎn)基因作物法規(guī)監(jiān)管，目前難以產(chǎn)業(yè)化應(yīng)用，需創(chuàng)新誘變等育種技術(shù)的應(yīng)用，加快創(chuàng)制非轉(zhuǎn)基因“去鎘”水稻新種質(zhì)或品種；3)長(zhǎng)期淹水灌溉可顯著減輕稻米中的鎘積累，但是對(duì)于耐長(zhǎng)期淹灌、延遲收獲的水稻品種的應(yīng)用和基礎(chǔ)研究還很少；4)葉面阻隔劑降鎘的分子機(jī)理還不是非常清楚；5)對(duì)于中、低水平鎘污染稻田種植水稻可采取綜合措施實(shí)現(xiàn)稻米安全生產(chǎn)，但是對(duì)于重度污染田塊目前還缺少有效手段，需要加強(qiáng)高效、低成本的鎘污染土壤治理技術(shù)的研究。

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Factors Affecting Cadmium Accumulation in Rice and Strategies for Minimization

JIANG Nan1, 3, #, YAN Xu1, 2, #, ZHOU Yanbiao1, 3, ZHOU Qunfeng1, 3, WANG Kai1, 3, YANG Yuanzhu1, 2, 3, 4,*

(Key Laboratory of Southern Rice Innovation and Improvement,,.,.,,; College of Plant Science and Technology,,;,;College of Agronomy,,,;These authors contributed equally to this work;)

Cadmium (Cd) is an extremely toxic and widely distributed heavy metal.Cd in farmland has adverse impacts on crop growth and development, and threatens human health via the food chain.Rice, as a staple food crop in China, plays an important role in food security.However, rice tends to absorb and accumulate more Cd compared with other cereal crops.The ‘Cd-polluted rice’ events were frequently reported in recent years, which has made Cd pollution a serious public concern.Reducing the Cd accumulation in rice grains is urgent.In this review, we summarize the advances in screening and breeding for rice varieties with low Cd, water management, adjustment of soil pH and foliar dressing strategies.The article aims to lay a theoretical foundation for technological integration, extension and application, and development of new technologies.

rice;cadmium accumulation; food safety; variety; irrigation; pH

2020-09-18；

2021-01-15。

長(zhǎng)株潭國(guó)家自主創(chuàng)新示范區(qū)專項(xiàng)（2018XK2005）；湖南省科技創(chuàng)新計(jì)劃資助項(xiàng)目（2018NK1020）；湖南省科技人才專項(xiàng)（2019RS2054）。

10.16819/j.1001-7216.2021.200913