Research progress on the effects of nano-material-carbon dots on crop growth and development
ZHU Qi1 ,LIU Yang1,CUI Maoya’,GAO Pinglei’, CHEN Yinglong',DAI Qigen 1,2 (1JiangsuKyeddiCenteforoderucichfs/KboliloeetdUl(saliald),MinistrfJointInteseboceuefgUscDevelopment,Yangzhou University,Yangzhou 2250o9,China)
Abstract:Carbondots(CDs),asanovel class ofnanomaterials,have garnered increasing atention intheagriculturalfieldduetotheiradvantagessuchaslowcost,easeofsynthesis,lowtoxicity,biodegradability,highquantumyield, small ize,and excellentbiocompatibility.Inthis paper,wereviewedtheefectsofCDsonthegrowthanddevelopmentof cropsand elaborated on the mechanisms through aspects such as seed germination,root growth and development,mineral elementbsorption,photosyntheticeactions,carbonassimilation,ntioxidantcapacity,ndstress resistance,withtheim of providing theoreticalreferences forthesafeandsustainableimprovementofcropyield.Finally,wediscussedthechallnges faced by the potential applications of CDs in agricultural systems.
Key words: carbon dots;growth and development;photosynthesis;stress resistance
隨著人口增長和工業(yè)發(fā)展,到2050年,全球人口估計(jì)將達(dá)到 9.7×109 人,為滿足不斷增長人口對食物的需求,糧食產(chǎn)量至少需要提高 70%[1-3] 。而耕地面積不斷縮小、水資源短缺和極端天氣頻發(fā),給糧食生產(chǎn)和分配帶來了越來越大的壓力[46]。此外,耐逆作物品種培育進(jìn)展緩慢,轉(zhuǎn)基因作物的安全性受到公眾質(zhì)疑,而化學(xué)肥料和農(nóng)藥的使用對環(huán)境、農(nóng)產(chǎn)品以及人體的安全產(chǎn)生威脅[7-9],農(nóng)業(yè)需要提高生產(chǎn)力和多樣化,以應(yīng)對氣候變化和有限自然資源的限制。納米技術(shù)的迅速發(fā)展為多個領(lǐng)域帶來了重大進(jìn)步,并為農(nóng)業(yè)開辟了充滿希望的道路。
三維空間中至少有一維處于納米尺寸(1\~100nm)的材料被稱為納米材料,它具有獨(dú)特的理化性質(zhì),可以在原子或分子水平上運(yùn)行[1],被廣泛應(yīng)用于成像[11]、醫(yī)學(xué)[12-14]、能源[15-17]、食品[18-19]等領(lǐng)域。在其眾多應(yīng)用中,農(nóng)業(yè)領(lǐng)域的應(yīng)用正在被越來越多地探索,現(xiàn)已用于監(jiān)測作物生長狀況[20]、促進(jìn)作物生長[21-22]、提高肥料和農(nóng)藥的效率[23-24]以及防治病蟲害[25-26]。納米材料獨(dú)特的性質(zhì)使其可以通過不同于化學(xué)和基因工程的機(jī)制為可持續(xù)農(nóng)業(yè)做出貢獻(xiàn)。
碳點(diǎn)(CD)是一種新型的納米材料,根據(jù)其形成機(jī)理、微結(jié)構(gòu)和性能的不同,主要分為石墨烯量子點(diǎn)(GQD)、碳量子點(diǎn)(CQD)和碳化聚合物點(diǎn)(CPD)[27]。2004年科學(xué)家在制備單壁碳納米管時,意外地發(fā)現(xiàn)具有明亮熒光的碳納米粒子一—碳點(diǎn)。碳點(diǎn)是一種介于分子和凝聚態(tài)固體之間的新型體系或碳材料,融合了納米效應(yīng)、表面官能團(tuán)和碳元素的特性,因此具備光學(xué)、化學(xué)和生物學(xué)三種基本功能[28]。CD具有豐富的表面基團(tuán),可以與離子、有機(jī)分子、聚合物、DNA和蛋白質(zhì)結(jié)合,從而改變碳點(diǎn)的性質(zhì),并且這些功能化碳點(diǎn)可以滿足特定要求[29]與無機(jī)、金屬和氧化物基納米材料相比,具有低毒性(或無毒性)、可生物降解、優(yōu)異的可調(diào)光致發(fā)光(PL)、高量子產(chǎn)率(QY)、小尺寸、生物相容性好和低成本等特點(diǎn)[30-32]。自從2012 年 Qu 等[31]通過微波合成水溶性發(fā)光CD,揭示了其在水溶液中表現(xiàn)出穩(wěn)定的與激發(fā)波長相關(guān)的PL特性,以及對植物和動物的毒性為零或較低的特性以來,CD對各種作物生長的潛在影響引起了全世界研究人員的廣泛關(guān)注。通過不斷研究發(fā)現(xiàn),CD對作物的生長、光合作用和非生物/生物脅迫的抵抗力表現(xiàn)出優(yōu)異的性能。本文綜述CD對植物的生物學(xué)功能,并將討論關(guān)于CD研究的挑戰(zhàn)和機(jī)遇,以期為安全、可持續(xù)提高作物產(chǎn)量提供理論參考。
1 CD對作物種子發(fā)芽的影響
發(fā)芽是植物個體生長發(fā)育的開始,快速、整齊的種子發(fā)芽和幼苗生長對于農(nóng)業(yè)產(chǎn)量尤為重要[33]種子的發(fā)芽過程可以分為三個階段,分別是吸水、萌動和發(fā)芽[34]。CD 能夠通過影響種子呼吸與代謝、氧含量和水通道蛋白基因的表達(dá),改變種子休眠特性、增加種子對水和礦質(zhì)元素的吸收,從而促進(jìn)種子萌發(fā)、根伸長和幼苗生長。
Han等[35]通過水熱法制備二硝基苯酚鈉衍生碳點(diǎn)(SCD),將其應(yīng)用于棉花種子發(fā)芽試驗(yàn)中,通過對棉花芽苗自由水和結(jié)合水含量的分析,推測CD增強(qiáng)了芽苗的呼吸和代謝,從而解除了種子休眠,促進(jìn)了萌發(fā)。Li等[36通過電化學(xué)法合成不同含氧量的CD,探究碳納米顆粒對水稻生長的影響,發(fā)現(xiàn)CD可以滲透到種子細(xì)胞核中,進(jìn)而促進(jìn)種子萌發(fā)、根伸長、幼苗生長,此試驗(yàn)中獲得最佳CD質(zhì)量濃度為0.56mg/mL ,這些結(jié)果與之前關(guān)于其他碳點(diǎn)材料的報(bào)道類似[37-38];此外通過進(jìn)一步研究發(fā)現(xiàn)種子的含水量、根系發(fā)育和幼苗長度與CD中氧含量有關(guān),當(dāng)CD含氧量 29% 左右,能有效促進(jìn)水稻生長。同時在另一項(xiàng)研究中,Li等3通過室溫電化學(xué)方法利用碳棒合成了約 5nm 的納米顆粒,并將其加入培養(yǎng)基,用于擬南芥的培養(yǎng),其結(jié)果表明CD表面的羥基和羧基可為水分子提供大量的結(jié)合位點(diǎn),從而加速水分子進(jìn)入種子,促進(jìn)種子萌發(fā)。作物水通道蛋白(AQP)是作物生長發(fā)育過程中關(guān)鍵的吸水蛋白,可以介導(dǎo)其他小分子、氣體、營養(yǎng)物質(zhì)和金屬離子的轉(zhuǎn)運(yùn)[40]。Kou等[41]以 L -半胱氨酸和葡萄糖通過水熱法獲得碳點(diǎn),并將其加入水培營養(yǎng)液中,用于培養(yǎng)生菜和番茄,其結(jié)果表明CD的親水基團(tuán)(-COOH、-OH)可以通過增加水通道蛋白相關(guān)基因的表達(dá)來促進(jìn)種子發(fā)芽與作物生長,以及增加水和礦質(zhì)營養(yǎng)元素的吸收并顯著激活光合作用。Zhang等[42]研究發(fā)現(xiàn)CD短期處理,導(dǎo)致根中與植物激素生物合成、失活和信號轉(zhuǎn)導(dǎo)相關(guān)的一組基因的轉(zhuǎn)錄變化,促進(jìn)根和芽的生長,但對發(fā)芽率沒有太大影響。
種子萌發(fā)不僅受到水分的影響,還受到一系列酶和作物激素的調(diào)控。因此,研究人員應(yīng)更加關(guān)注相關(guān)酶和作物激素在分子水平上的變化,以揭示CD在種子萌發(fā)調(diào)節(jié)中的作用機(jī)制。這將有助于我們更好地理解CD的應(yīng)用潛力,并為進(jìn)一步開發(fā)CD在作物生長發(fā)育調(diào)控中的應(yīng)用提供理論依據(jù)。
2 CD對作物根系生長發(fā)育的影響
作物對水分和各種礦質(zhì)營養(yǎng)元素的吸收和傳導(dǎo)有質(zhì)外體途徑、共質(zhì)體途徑和跨細(xì)胞途徑3種[43]CD通過細(xì)胞間隙穿透種皮并積聚在子葉中以加速種子萌發(fā),根出現(xiàn)后,CD被吸附在根表面可通過質(zhì)外體和共質(zhì)體途徑被吸收并滲透到根維管束中,并在根壓和蒸騰拉力的作用下從根部轉(zhuǎn)移到莖葉[44],此外因?yàn)镃D小尺寸使其能夠跨越生物屏障,葉面噴施后也可擴(kuò)散到作物維管系統(tǒng)中發(fā)揮作用[45]
CD在適宜濃度范圍內(nèi)可以通過增強(qiáng)作物對養(yǎng)分的吸收從而促進(jìn)作物根系生長和增加作物生物量積累,進(jìn)而影響作物的營養(yǎng)品質(zhì)。Wang等[46]用不同濃度CD培養(yǎng)綠豆,其結(jié)果表明當(dāng)質(zhì)量濃度低于0.02mg/mL 時,隨著質(zhì)量濃度的增加,CD通過增強(qiáng)根系活力以及促進(jìn)水分吸收,提高豆芽的根長、莖長和生物量。Chen等[47]通過熱解法和水解法獲得功能性碳納米點(diǎn)(FCN),并將其作為擬南芥培養(yǎng)基中的植物生長調(diào)節(jié)劑,其結(jié)果表明FCN促進(jìn)幼苗主根生長,增加了幼苗生物量的積累,且植物的生理反應(yīng)與FCN的結(jié)構(gòu)特征之間存在很強(qiáng)的相關(guān)性,用官能團(tuán)最豐富且尺寸較小的原始FCN處理幼苗,觀察到植物生長增加最顯著。Kou等[41]使用 L -半胱氨酸和葡萄糖通過水熱法獲得的CD培育生菜和番茄,發(fā)現(xiàn)在 0.066mg/mL 至 0.132mg/mL 范圍內(nèi)顯著促進(jìn)根和下胚軸的伸長,這可能是因?yàn)镃D可以吸附帶正電荷的金屬離子,如 K+,Ca2+ 和 Mg2+ 等,并一起進(jìn)入作物體內(nèi)促進(jìn)作物根系發(fā)育和新陳代謝。Chakravarty等[48]在種植前將香菜和大蒜種子用0.2mg/mL 的石墨烯量子點(diǎn)處理 3h ,其結(jié)果表明石墨烯量子點(diǎn)作為植物生長調(diào)節(jié)劑提高了香菜和大蒜葉、根、芽、花和果實(shí)的生長速度。此外,CD表面的羧基還能降低根際環(huán)境的 ΔpH 值,通過化學(xué)吸附增強(qiáng)作物對養(yǎng)分的吸收能力,以及影響根際微生物環(huán)境,從而間接影響作物的生長[41,49]。
CD對作物根系生長發(fā)育的作用在一定濃度范圍內(nèi)表現(xiàn)出劑量效應(yīng)。相對較低的劑量下有助于根系生長發(fā)育,而較高的劑量會抑制根系生長發(fā)育。Chen等[50]通過整合生理學(xué)、代謝組學(xué)和轉(zhuǎn)錄組學(xué)技術(shù),系統(tǒng)地研究了CD對擬南芥毒性及其潛在的植物毒性機(jī)制,發(fā)現(xiàn)在 125mg/mL 至1 .000mg/L 范圍內(nèi)CD上調(diào)根和芽中與刺激反應(yīng)、UDP碳基轉(zhuǎn)移酶活性和細(xì)胞對磷酸鹽饑餓反應(yīng)相關(guān)基因的表達(dá),下調(diào)與葉綠體結(jié)構(gòu)和功能相關(guān)基因的表達(dá),影響植物代謝物的含量,導(dǎo)致植物產(chǎn)生氧化應(yīng)激,改變了擬南芥根和芽的滲透勢從而引起了作物毒性的反應(yīng),顯著抑制擬南芥幼苗的生長。相似地, Kou 等[4的研究結(jié)果表明當(dāng)CD質(zhì)量濃度大于 0.33mg/mL 時,會顯著抑制大白菜幼苗根系和下胚軸伸長,過量的CD可能會積聚在根部,對細(xì)胞內(nèi)穩(wěn)態(tài)產(chǎn)生負(fù)面影響以及CD可能通過產(chǎn)生活性氧導(dǎo)致脂質(zhì)過氧化,對蛋白質(zhì)和DNA造成損傷。
CD對作物根系形態(tài)、生理生化特性的影響及其在整個生長周期中的作用機(jī)制尚未完全明了。目前,關(guān)于CD調(diào)控作物其他器官的機(jī)制研究較為匱乏。亟需深入研究,以揭示CD的化學(xué)特性如何與植物生長發(fā)育過程中的生化過程相互作用。
3 CD對作物光合作用的影響
光合作用是作物吸收光能,同化 CO2 ,制造有機(jī)物質(zhì)并釋放氧氣的過程,分為光反應(yīng)和碳同化,對作物生長和生物質(zhì)積累來說至關(guān)重要[51]。目前,人們已經(jīng)對CD對光合作用中所有過程的作用進(jìn)行了研究。結(jié)果表明CD可以通過影響作物對光能的吸收、傳輸轉(zhuǎn)換效率,提高光能利用率和提高光合碳同化來提高光合效率。
3.1 光反應(yīng)
作物可以利用的太陽輻射的波長范圍相對較窄,主要集中在 400~500nm 和 650~800nm 的光譜[52],而CD具有優(yōu)異的光致發(fā)光性能,可以共價官能化,使其在可見光譜區(qū)具有很強(qiáng)的吸收和發(fā)射能力,發(fā)光范圍幾乎覆蓋了整個可見光譜區(qū),也可以延伸到近紅外光區(qū)[53-54]。同時,CD還可以發(fā)出作物需要的藍(lán)光或紅光以及將作物無法吸收的紫外線轉(zhuǎn)化為作物可以利用的藍(lán)紫光和紅光,提高葉片的光合速率[45]。Budak 等[55]的研究結(jié)果表明當(dāng)CD與葉綠體相互作用時,存在從CD到葉綠體的能量轉(zhuǎn)移,提高葉綠體對 360~420nm 波長光的吸收能力,并將其轉(zhuǎn)化為 680~720nm 波長的發(fā)射光。此外,葉綠素含量是衡量光合作用效率的重要指標(biāo),而由于CD的種類和濃度以及不同作物的光飽和點(diǎn)不同,CD對葉綠素含量的影響不同研究有不同的結(jié)果。Li等[56]研究發(fā)現(xiàn)將氮摻雜碳點(diǎn)(N-CD)引入葉綠體能夠提升光合作用效率,DCPIP和鐵氰化物的含量分別減少 52.48% 和 41.86% ,葉面噴施能上調(diào)水稻葉綠素合成基因和代謝相關(guān)酶基因的表達(dá),進(jìn)而導(dǎo)致葉綠素a和葉綠素b的含量增加。此外CD對葉綠素合成的調(diào)節(jié)作用與表面官能團(tuán),尤其是含氮官能團(tuán)密切相關(guān)[46]。在番茄[41]和生菜[57]的研究中也得出了類似的結(jié)論。但也有研究結(jié)果表明施用CD 不會增加葉綠素含量[36.37]
當(dāng)光照度超過光飽和點(diǎn)時,光合速率不再增加,電子轉(zhuǎn)移反應(yīng)和RuBisCO(1,5-二磷酸核酮糖羧化酶/加氧酶)酶活性成為限制因素[58]。CD具有優(yōu)異的光學(xué)性能和表面易于進(jìn)行功能修飾的特點(diǎn),既可作為電子供體又可作為電子受體為農(nóng)作物光合作用提供了有利的條件。Chandra等[59]首先報(bào)道了CD對電子傳遞的影響,CD和葉綠體的波長吸收范圍相同,其發(fā)射光譜與葉綠體的吸收光譜部分重疊,因此CD可以輕松地通過吸附在葉綠體表面上形成光捕獲復(fù)合物,將吸收的光電子轉(zhuǎn)移給葉綠體。CD也會接收光合色素多余的電子,再將電子傳遞至PSⅡ和PSI,增加對ATP和NADPH的需求,提高 CO2 同化速率,從而增強(qiáng)葉片的整體光合作用能力。同時CD能極大地提高PSⅡ反應(yīng)中心色素的熒光量子產(chǎn)率,增強(qiáng)能量轉(zhuǎn)換,并將多余的光作為熱量消散,以防止對PSI和相關(guān)光合作用機(jī)制造成損害,從而保護(hù)植物免受損害[57]。Tan等[60]的研究結(jié)果顯示,噴施CD后,水稻和玉米的光合電子傳遞最大速率分別提升 57.3% 和 65.5% ,Li等[43]發(fā)現(xiàn)噴施遠(yuǎn)紅外碳點(diǎn)后電子轉(zhuǎn)移速率比對照提升了 28% 。CD的摻雜修飾也可以增加電荷轉(zhuǎn)移量,在光照射下產(chǎn)生電子空穴,提高光電轉(zhuǎn)換效率。目前,CD的摻雜主要采用摻雜原子的方式(N、P、S、CI和B)[61-62]。其中,N摻雜到CD中,優(yōu)化了CD的結(jié)構(gòu),導(dǎo)致電子和空穴的高分離效率,提高了光反應(yīng)的光轉(zhuǎn)換效率,最終提高了光反應(yīng)速率[63]
3.2 碳同化
碳同化是作物光合作用的第2個階段,利用光反應(yīng)產(chǎn)生的同化力將 CO2 還原為糖類。RuBisCO酶是光合作用碳同化的關(guān)鍵酶,催化 CO2 同化和光呼吸碳氧化的第1步反應(yīng),其底物特異性差、羧化效率低,是光合作用碳同化反應(yīng)的限速酶[64-65]。研究結(jié)果表明CD可以增強(qiáng)其活性,從而提高光合效率[6]。Li等[36]首先以一系列具有不同氧含量的5nm的CD探索其對水稻生長的影響,發(fā)現(xiàn)CD處理后RuBisCO酶活性顯著增加(增加 42% );隨后Li等[39]在室溫下通過一步電化學(xué)方法直接從碳棒合成CD,所制備的CD也能有效增強(qiáng)RuBisCO酶活性,進(jìn)而促進(jìn)雙子葉作物(大豆、番茄、茄子等)生長,最終提高產(chǎn)量。同時,CD也能提高玉米、生菜和小球藻等的RuBisCO酶活性,提高其生物質(zhì)的積累[43,60,67]。此外在 H2O2 存在下,CD 可被辣根過氧化物酶 (HRP )降解為作物激素類似物(僅適用于石墨或部分石墨CD)和 CO2 。作物激素類似物可以促進(jìn)作物生長, CO2 通過光合作用的卡爾文循環(huán)轉(zhuǎn)化為碳水化合物,進(jìn)而提高作物的產(chǎn)量[39]。氣孔是調(diào)節(jié)作物與外界水、氣交換的重要通道,氣孔導(dǎo)度是氣孔開放程度的量度,氣孔可調(diào)節(jié)作物的光合作用和蒸騰作用[68]。CD可能促進(jìn)了水稻氣孔導(dǎo)度的提高,增強(qiáng)在葉綠體中的 CO2 擴(kuò)散,并增加葉肉吸收CO2 的能力,間接提高作物的光合作用[47]
光合作用是一個涉及多步驟的復(fù)雜生物化學(xué)過程,而CD在這一過程中的具體分子和細(xì)胞機(jī)制尚未得到充分研究;環(huán)境因素也可能調(diào)節(jié)碳點(diǎn)對光合作用的影響,當(dāng)前研究多在實(shí)驗(yàn)室控制環(huán)境下進(jìn)行,這可能與自然條件有所不同。因此,探究碳點(diǎn)在自然環(huán)境中的表現(xiàn)及其對光合作用的具體影響,對于全面評估其生態(tài)效應(yīng)具有重要意義。
4CD對作物抗逆性的影響
作物在其生命周期中面臨著極端溫度、鹽害、干旱、洪澇等多種非生物脅迫,也面臨著昆蟲、食草動物、真菌、細(xì)菌以及病毒等生物脅迫[69-72]。這些脅迫通常會引起滲透和氧化脅迫,造成細(xì)胞損傷,從而擾亂作物生長發(fā)育并降低作物產(chǎn)量和品質(zhì)[9,73] 。CD本身可以作為抗氧化劑清除非生物脅迫應(yīng)激中產(chǎn)生的過度自由基,還可以增強(qiáng)作物自身的抗氧化防御系統(tǒng)或誘導(dǎo)抗氧化相關(guān)酶基因的表達(dá),從而提高對于逆境的抵抗能力。
4.1 非生物脅迫
非生物脅迫,如干旱、高溫、寒冷、鹽害和強(qiáng)光,單獨(dú)或組合發(fā)生對作物存活、生長和發(fā)育產(chǎn)生負(fù)面影響,限制農(nóng)作物的產(chǎn)量[74-77]。當(dāng)作物遭受非生物脅迫時,會產(chǎn)生活性氮(RNS)、活性氧(ROS)、丙二醛(MDA)或丙酮醛(MG)等多種有害物質(zhì)。這些有害物質(zhì)會破壞作物細(xì)胞膜的結(jié)構(gòu),使細(xì)胞生理功能受損,引起脂質(zhì)過氧化反應(yīng),最終導(dǎo)致細(xì)胞死亡[78]CD表面易于功能化,富含羥基、羧基等官能團(tuán),是良好的電子受體和電子供體,又具有清除自由基的特性[4445,981]。Zhang 等[67]以小球藻為研究對象,其結(jié)果表明CD本身可以作為抗氧化劑,保護(hù)普通小球藻細(xì)胞免受氧化損傷,通過進(jìn)一步研究發(fā)現(xiàn)這主要由于CD表面羥基基團(tuán)和共軛 sp2 碳,可以穩(wěn)定自由電子。Li等以生物質(zhì)丹參為前驅(qū)體,采用一步水熱合成熒光CD,這些CD具有多種酶活性,可以清除DPPH ? 和·OH,對植物細(xì)胞內(nèi)ROS具有良好的清除作用,能有效緩解鹽脅迫下意大利生菜的氧化損傷。CD的氧空位可以可逆地與氧原子結(jié)合,以及硫醇、噻吩、羧基、酰胺和苯并噻唑等表面基團(tuán)的電子轉(zhuǎn)移和質(zhì)子轉(zhuǎn)移皆有助于CD的抗氧化[83]。此外具有豐富的官能團(tuán)的CD 通過化學(xué)基團(tuán)與金屬離子之間的靜電吸引和絡(luò)合,提高擬南芥的生物富集效率和對重金屬的耐受性來影響植物的生長和發(fā)育[84]。Ducic等[85]的研究結(jié)果也表明CD通過表面官能團(tuán)與 Cu2+ 形成絡(luò)合物,降低了玉米幼苗根部 Cu2+ 濃度,從而緩解了銅對根木質(zhì)部細(xì)胞壁多糖以及葉韌皮部和葉肉中多糖和蛋白質(zhì)的不利影響。Dong等[86]通過水熱合成方法合成的摻鈰碳量子點(diǎn),可誘導(dǎo)植物氣孔開放,從而增強(qiáng)植物的代謝效率和氯化鈉的排泄,有效緩解了鹽脅迫的影響。Li等[87]通過丹參衍生物制備CD,發(fā)現(xiàn)CD上的羥基和羧基通過環(huán)核苷酸和環(huán)核苷酸門控離子通道以及凝集素受體激酶促進(jìn) Ca2+ 活化,增強(qiáng)植物 Ca2+ 信號傳導(dǎo),避免ROS爆發(fā)引起的氧化損傷,從而使植物對各種環(huán)境脅迫具有高度適應(yīng)性,CD也可以通過調(diào)節(jié)離子( ΔNa+?K+ 和 Fe2+ )穩(wěn)態(tài),提高甘薯在營養(yǎng)缺乏情況下的適應(yīng)性。
CD除了自身可以清除活性氧,提高作物抗性,還可以增強(qiáng)作物自身的抗氧化防御系統(tǒng)或誘導(dǎo)抗氧化相關(guān)酶基因的表達(dá),從而提高作物對于逆境的抵抗能力[46]。 Su 等[88]用含 180mg/L CD 的水溶液培養(yǎng)花生植株幼苗,發(fā)現(xiàn)與未施用CD相比,施用CD的花生植株超氧化物歧化酶(SOD)、過氧化氫酶(CAT)和過氧化物酶(POD)活性較高,MDA含量較低,同時CD表面親水基團(tuán)可以使花生木質(zhì)部導(dǎo)管內(nèi)的微量營養(yǎng)元素被大量截留并緩慢釋放,進(jìn)一步保證花生所需養(yǎng)分的供應(yīng),提高花生的抗逆能力。Zhong等[89]的研究結(jié)果表明亞精胺碳點(diǎn)(Spd-CD)通過降低活性氧和脂質(zhì)過氧化,以及提高谷胱甘肽/氧化型谷胱甘肽、抗壞血酸/脫氫抗壞血酸比值以及其他抗氧化酶活性,增強(qiáng)番茄在熱應(yīng)激( 45°C )下的耐受性。Chen等[90]研究發(fā)現(xiàn),F(xiàn)CN能調(diào)節(jié)酶促和非酶促抗氧化系統(tǒng),有效減輕植物細(xì)胞的氧化損傷;增強(qiáng)根系活性,促進(jìn)水分吸收,進(jìn)而提升幼苗的細(xì)胞相對含水量,增強(qiáng)番茄對干旱脅迫的適應(yīng)性。Xiao等[9]用 Cd2+ 處理小麥時,添加CD( 50mg/L 和75mg/L )可以顯著降低根和葉中的鎘含量( 33.1%. )57.7% ),同時增加抗氧化應(yīng)激相關(guān)基因ATPSd的表達(dá),降低 APR 和SiR基因表達(dá),緩解 Cd2+ 對作物造成的非生物脅迫。Chandrakar等9也有類似的報(bào)道。在干旱脅迫下,施用CD能促進(jìn)玉米葉片中脯氨酸和脫落酸的合成和長距離運(yùn)輸?shù)礁?,從而?AQP 基因的表達(dá)上調(diào)2.3\~7.6倍,增加 K+/Na+ 比例,促進(jìn)干旱條件下作物根系吸水[29]。此外,作物還可以通過CD增強(qiáng)自身的抗氧化防御系統(tǒng)緩解UV- ?B[93] 、干旱和鹽害對作物的脅迫[94-95]以及延緩衰老[96]
CD可以促進(jìn)植物對土壤中氮素的吸收與利用,促進(jìn)作物生長進(jìn)而提高作物的抗逆性。Ji等的研究結(jié)果表明干旱條件下葉面噴施CD( 5mg/L 可以刺激根系分泌物(氨基酸、生長素和有機(jī)酸)的分泌,并豐富根際有益微生物群落(放線菌、子囊菌、酸桿菌和球囊菌等),促進(jìn)土壤中的氮活化。同時可以促進(jìn)氮代謝并增強(qiáng)氨基酸生物合成,使大豆地上部和根部的氮含量分別增加了 13.2% 和 30.5% 。Chen等[9]的研究結(jié)果表明FCN通過調(diào)節(jié)土壤 pH 、酶活性、有機(jī)碳和有機(jī)質(zhì)含量來誘導(dǎo)土壤細(xì)菌和真菌群落豐富度、多樣性和結(jié)構(gòu)的改變,提高營養(yǎng)利用率。Yang等[98]的研究結(jié)果表明,在玉米上施用CD可以提高碳水化合物向根部的運(yùn)輸速率,增加根系分泌物(有機(jī)酸、氨基酸等)的含量和根際微生物(變形菌、放線菌、子囊菌等)的豐度,促進(jìn)玉米對氮、磷的吸收,從而提高玉米的抗旱能力。同時,CD增強(qiáng)土壤中酶的活性,進(jìn)而促進(jìn)氮的吸收和代謝。Wang等[9]的研究結(jié)果表明CD可以與固氮酶結(jié)合,影響固氮酶的二級結(jié)構(gòu),改善生物催化過程中的電子傳遞,最終提高固氮酶的固氮活性;CD也可以增強(qiáng)脲酶的活性,上調(diào)豆血紅蛋白基因的相對表達(dá)水平和減弱硝酸還原酶的活性來增加 NH4+ -N向NO3- -N的轉(zhuǎn)化,從而促進(jìn)土壤的氮循環(huán),增強(qiáng)大豆對氮素的吸收從而促進(jìn)干旱條件下大豆生長。
4.2 生物脅迫
作物在生長過程中經(jīng)常受到各種病原微生物和害蟲的脅迫,導(dǎo)致植株生長遲緩、作物產(chǎn)量及品質(zhì)降低,嚴(yán)重威脅糧食安全[71-72]。CD 可以通過誘導(dǎo)相關(guān)基因表達(dá),增強(qiáng)作物抗病性和抗蟲性,這為其在抵抗作物生物脅迫中的應(yīng)用提供了可能性。Luo等[100]的研究結(jié)果表明葉面噴施 10mg/L 氮摻雜CD(N-CD)可抑制番茄青枯病,使病害嚴(yán)重程度降低71.19% ,其主要原因是增強(qiáng)了活性氧清除能力以及激活番茄中水楊酸和茉莉酸依賴性系統(tǒng)獲得性抗性(SAR),從而抑制體內(nèi)病原體生長。CD也可以進(jìn)入細(xì)胞核,通過嵌人結(jié)合的方式與DNA的大溝和小溝結(jié)合,使DNA結(jié)構(gòu)松散,誘導(dǎo)硫素基因(Os06g32600)過度表達(dá),增強(qiáng)抗病性[36]
鑒于研究方法、技術(shù)手段、成本及難度等因素,當(dāng)前關(guān)于CD對作物非生物脅迫作用的研究主要聚焦于干旱脅迫,而對其他非生物脅迫的關(guān)注相對較少。在生物脅迫方面,多數(shù)研究側(cè)重于利用人類病原體進(jìn)行體外實(shí)驗(yàn),以評價碳點(diǎn)的抗菌特性,而在作物上的研究較為有限。未來的研究應(yīng)深入探討碳點(diǎn)提高作物抵抗脅迫的具體響應(yīng)機(jī)制,以推動其在逆境下農(nóng)業(yè)中的實(shí)際應(yīng)用。
5 CD對作物品質(zhì)的影響
不同種類和濃度的CD對不同作物品質(zhì)的影響研究結(jié)果各異。 Hu 等[49利用營養(yǎng)液培養(yǎng)香菜幼苗7d,其中添加了不同濃度的CD,結(jié)果發(fā)現(xiàn)用40mg/L CD 處理的香菜葉中可溶性糖、可溶性蛋白質(zhì)和維生素C含量分別增加了 17.0%.27.1% 和26.0% ,并增加了鉀、鈣、鎂、磷、錳、鐵等礦質(zhì)營養(yǎng)元素的含量,顯著改善了香菜的營養(yǎng)品質(zhì)。此外Wang等[99]在土壤中施用CD (5mg/kg) ,通過提高氮生物有效性促進(jìn)大豆生長,大豆籽粒中碳水化合物和蛋白質(zhì)含量分別增加 28.2% 和 3.4% ,油酸、亞油酸、亞麻酸、棕櫚酸、硬脂酸5種主要脂肪酸含量分別增加 9.7%.8.2%.6.0%.5.7% 和 5.0% 。同時在對番茄和生菜的研究中也得到類似的結(jié)果[41,101]。但是,Li等[36]的研究結(jié)果則表明CD雖然能夠增加水稻產(chǎn)量,但水稻表觀直鏈淀粉含量和凝膠稠度(GC)顯著降低,稻米品質(zhì)總體下降。
6展望
本文綜述了CD對作物生長發(fā)育影響的最新研究進(jìn)展,顯示CD在提高作物產(chǎn)量方面的可觀潛力。大部分研究結(jié)果表明CD有利于作物的生長發(fā)育,包括具有親水表面基團(tuán)的CD在種子發(fā)芽過程中可以增加種子的水分含量并加速種子萌發(fā)和根發(fā)育;通過靜電吸附金屬離子,從而促進(jìn)作物對養(yǎng)分的吸收。適宜的CD也可以通過優(yōu)化光能的吸收、傳輸和轉(zhuǎn)換,提高光能的有效利用,提高光合碳同化效率,加速作物的光合作用,另外CD可以通過提高作物對活性氧的清除能力增強(qiáng)作物對生物脅迫和非生物脅迫的抵抗能力。但是CD在濃度較高時會使作物產(chǎn)生氧化損傷,抑制作物的生長。
盡管在農(nóng)業(yè)領(lǐng)域關(guān)于CD對作物生長發(fā)育影響的研究已經(jīng)取得了一定進(jìn)展,但仍存在許多需要解決的問題,例如缺乏大規(guī)模生產(chǎn)經(jīng)驗(yàn)、結(jié)構(gòu)和化學(xué)式不明確,以及大田試驗(yàn)中的研究較少等。然而,相對于其他納米材料,CD具有明顯優(yōu)勢,如前驅(qū)體來源廣泛、制備方法環(huán)保、生物相容性好、易于表面功能修飾等。因此,CD在農(nóng)業(yè)領(lǐng)域有著廣闊的應(yīng)用前景,未來的研究應(yīng)該重點(diǎn)關(guān)注以下幾個方面。
(1)尋找系統(tǒng)且可擴(kuò)展的合成方案來生產(chǎn)具有所需結(jié)構(gòu)(例如尺寸、形狀、結(jié)晶度、官能團(tuán)數(shù)量、缺陷類型和位置)的高質(zhì)量CD方法,且需制備更高產(chǎn)率的固態(tài)CD。
(2)推進(jìn)CD在實(shí)際生產(chǎn)中的應(yīng)用,促進(jìn)其市場化和產(chǎn)業(yè)化,為改善生態(tài)環(huán)境、保障作物生產(chǎn)可持續(xù)發(fā)展做出貢獻(xiàn)。
(3)改進(jìn)研究方法和技術(shù)手段,在非干旱環(huán)境中,對其他非生物脅迫和生物脅迫下CD的作用機(jī)制展開深人研究至關(guān)重要。這有助于加深對CD潛在應(yīng)用價值的認(rèn)識,對農(nóng)業(yè)和環(huán)境領(lǐng)域具有重要意義。
(4)跨學(xué)科結(jié)合方法,諸如運(yùn)用計(jì)算機(jī)軟件合成和模擬CD結(jié)構(gòu),研究其與作物細(xì)胞或細(xì)胞器的相互作用。同時,需關(guān)注相關(guān)酶以及作物激素在分子水平上的變化,從代謝組學(xué)、蛋白質(zhì)組學(xué)和基因組學(xué)等方面揭示CD對作物生長發(fā)育調(diào)控的具體分子機(jī)制。
總之,CD作為一種新興的納米材料,通過有效利用其特性,能夠顯著促進(jìn)作物的生長與發(fā)育,實(shí)現(xiàn)增產(chǎn)。同時,其在農(nóng)產(chǎn)品檢測[102-103]、農(nóng)業(yè)環(huán)境監(jiān)測[104-105]以及農(nóng)業(yè)廢棄物資源化利用[106-107]等方面也有應(yīng)用。隨著研究的深人和技術(shù)的進(jìn)步,未來CD在農(nóng)業(yè)領(lǐng)域的應(yīng)用將更加廣泛和深人,為實(shí)現(xiàn)農(nóng)業(yè)可持續(xù)發(fā)展提供強(qiáng)有力的技術(shù)支持。
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(責(zé)任編輯:蔣永忠)