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1 總述

干旱脅迫對(duì)植物光合效率產(chǎn)生負(fù)面影響，干擾氣孔功能，影響同化物質(zhì)的積累和運(yùn)輸[1,2,3,4,5]。植物受到干旱脅迫會(huì)激活各種機(jī)制避免缺水造成的負(fù)面影響[6,7]。缺水限制了植物碳代謝和光反應(yīng)產(chǎn)物的利用，使得大量吸收的光能不能被轉(zhuǎn)化為化學(xué)能，從而導(dǎo)致PSⅡ受到破壞[3,8,9,10]。此外水分限制同樣會(huì)影響植物葉綠素含量[11,12]。干旱脅迫下大麥植株光合效率的降低可能是由于氮、磷、鉀和鐵元素的缺乏所造成[13]，隨之而來(lái)會(huì)造成PSII蛋白脫磷酸化增加，LHCII蛋白（如b4和CP29）快速磷酸化[14]。

1.1 干旱脅迫對(duì)光系統(tǒng)PSII的影響

與PSII相比，PSⅠ對(duì)水分虧缺具有更高的耐受性，只有在極端干旱條件下才會(huì)出現(xiàn)負(fù)面效應(yīng)[15,16,17]。對(duì)幾種生態(tài)型椰子(Cocos nucifera L.)進(jìn)行的試驗(yàn)研究表明，干旱脅迫限制了光能的吸收和PSII的最大量子產(chǎn)率，降低了電子傳輸速度和羧化效率[18]。同樣，在進(jìn)行性干旱期間，桑樹(shù)(Morusindica L.)觀察到由于非活性RCs的增加、電子傳遞減少和能量耗散增強(qiáng)而導(dǎo)致的PSII活性降低[19]。在小麥[20,21,22]、橄欖[23]、葡萄[11]以及一些沙漠灌木的葉片中[24,25]也發(fā)現(xiàn)了PSII的最大量子產(chǎn)量下降。
在灌木中，還觀察到CO2同化減少和電子傳輸受到抑制[25]。二氧化碳同化減少可能導(dǎo)致PSII光化學(xué)活性與NADPH需求之間的不平衡。在這種情況下，活性氧(ROS)的產(chǎn)生增加，這可能是PSII對(duì)光破壞敏感性增加的原因[26]。在多數(shù)情況下，葉綠素?zé)晒鉁y(cè)量表明，通過(guò)調(diào)整光系統(tǒng)之間的能量分配和激活替代電子流，增強(qiáng)了對(duì)PSII和PSI光化學(xué)的保護(hù)[27,28]。

1.2 干旱脅迫和熱脅迫的關(guān)系

在自然界中，強(qiáng)烈的光照輻射伴隨著高溫和缺水，可能會(huì)發(fā)生慢性光抑制[16]。事實(shí)上，干旱和高溫是影響農(nóng)業(yè)地區(qū)作物生長(zhǎng)和產(chǎn)量的兩大非生物脅迫，眾所周知，它們一般同時(shí)發(fā)生。干旱和熱脅迫的聯(lián)合效應(yīng)與它們單獨(dú)作用時(shí)觀察到的不同，表明這兩種應(yīng)激源以不同的方式影響新陳代謝[29,30,31]。
González Cruz和Pastenes證明，與脅迫敏感大豆品種Arroz Tuscola相比，干旱脅迫下的抗逆性大豆品種Orfeo INIA具有更高的耐熱性。作者討論了葉黃素、脂類(lèi)和脂肪酸成分在提高大豆葉片耐高溫性中的可能作用。干旱脅迫下葉片與高溫的相互作用對(duì)PSII的影響已被廣泛研究，普遍表明干旱脅迫下使得葉片PSII的熱穩(wěn)定性增強(qiáng)[31,32,33]。
植物的干旱脅迫和熱脅迫之間存在拮抗效應(yīng)。事實(shí)上，可能是由于植物在脅迫環(huán)境下某些滲透調(diào)節(jié)物質(zhì)(如脯氨酸)的積累提高了植物對(duì)高溫的耐受性[34]。此外，如圖1所示，OJIP曲線(xiàn)中K峰消失表明干旱脅迫可能會(huì)增強(qiáng)PSII對(duì)熱脅迫的耐受能力[31]。

圖1.暗適應(yīng)條件下大麥OJIP曲線(xiàn)。大麥培育2周后，無(wú)水干旱處理2周。對(duì)照組和干旱處理組離體葉片45℃熱處理10min，適應(yīng)環(huán)境溫度5min后，測(cè)定葉綠素?zé)晒?strong>[31]。

2 干旱脅迫對(duì)植物OJIP曲線(xiàn)和JIP-test參數(shù)的影響

葉綠素?zé)晒釰IP-test方法用于檢測(cè)植物干旱脅迫，可獲取植物組織和器官在水分脅迫條件下光合作用過(guò)程的重要信息[4,35,36,37]。而目前，水分脅迫對(duì)植物光合機(jī)構(gòu)影響導(dǎo)致的熒光參數(shù)的變化尚未有統(tǒng)一定論[4,21,22,38]。

2.1 L&K峰

JIP-test方法可作為篩選耐旱性基因型作物品種的有效工具[19,39,40,41]。干旱脅迫可以直接或間接影響植物的光合活性，從而改變?nèi)~綠素?zé)晒鈩?dòng)力學(xué)曲線(xiàn)。OJIP曲線(xiàn)2~3ms的熒光上升階段與原初光化學(xué)反應(yīng)相關(guān)，L峰和K峰可作為評(píng)價(jià)植物耐旱潛力的有力工具[42]。L峰受PSII各組分間能量轉(zhuǎn)移的連通性影響[43]。K峰的出現(xiàn)與放氧復(fù)合體(OEC)的解離相關(guān)[44]。O-L-K-J-I-P熒光瞬態(tài)的測(cè)量和JIP-test可作為干旱脅迫出現(xiàn)前耐旱性和生理紊亂的潛在指標(biāo)。

2.2 性能指數(shù)PI(performance index)

性能指數(shù)PI是OJIP曲線(xiàn)中為人熟知的一個(gè)重要參數(shù)，是植物狀態(tài)和活性的定量參數(shù)。PI由三個(gè)獨(dú)立的表達(dá)式組成：?jiǎn)挝蝗~綠體活性反應(yīng)中心的數(shù)量，原初光化學(xué)反應(yīng)的有關(guān)的表達(dá)式和一個(gè)與電子傳遞相關(guān)的表達(dá)式[45]。因此，PI易受到天線(xiàn)色素活性、捕獲效率和電子傳遞效率發(fā)生的任何輕微變化的影響。PI對(duì)冬小麥的持續(xù)干旱脅迫敏感[46]。根據(jù)干旱脅迫下記錄的PI值評(píng)估的小麥基因型的耐旱性與糧食產(chǎn)量評(píng)定的結(jié)果高度一致[47]。

PI與干旱因子指數(shù)(DFI)密切相關(guān)，能夠顯示不同基因型植物對(duì)干旱反應(yīng)的巨大差異。DFI是指在任意干旱脅迫時(shí)間內(nèi)，干旱引起的PI相對(duì)降低量。Strauss等人于2006年即運(yùn)用相似定義CFI(Chill Factor Index)檢測(cè)不同大豆基因型的耐寒性。DFI還用于10個(gè)大麥品種(圖2)[42]和21個(gè)芝麻突變體種質(zhì)[48]在干旱脅迫下的特性鑒定。利用性能指數(shù)PI和OJIP曲線(xiàn)確定了埃及雙色大麥和高粱**耐性和最敏感的地方品種[49]。這些研究證明在PSII水平上區(qū)分耐旱品種和敏感品種是可能的。

圖2. 10個(gè)大麥品種在連續(xù)兩周干旱脅迫下干旱因子指數(shù)（DFI）與驅(qū)動(dòng)力（DF）的關(guān)系。每個(gè)基因型都由表中代碼表示[42]。

 

2.3 I~P相
干旱脅迫對(duì)植物光合系統(tǒng)產(chǎn)生許多影響。干旱脅迫下ABS/RC比率的增加[41,50]，這可能是由于某些PSII RCs失活或天線(xiàn)尺寸增加所致。RCs的失活是對(duì)光抑制敏感的一個(gè)指標(biāo)。這意味著在干旱時(shí)期，光化學(xué)活動(dòng)會(huì)降低，把吸收的多余的光通過(guò)熱耗散進(jìn)行消散。此外干旱脅迫會(huì)影響OJIP曲線(xiàn)中I~P相位的相對(duì)振幅。I~P相為快速葉綠素?zé)晒馍仙�的最慢階段(約30~200 ms)，與質(zhì)體籃素PC+和PSⅠ中P700+的還原相關(guān)[51,52]。I~P相似乎與通過(guò)820nm透射測(cè)量的PSⅠ反應(yīng)中心數(shù)量相關(guān)[53]。此外已證明，不同大麥品種I~P相振幅的變化與其耐旱性相關(guān)[53,54]。

2.4 延遲熒光

葉綠素?zé)晒釩hlF是在光合樣品由暗到光轉(zhuǎn)換后發(fā)射的，而延遲熒光則是由光到暗轉(zhuǎn)換期間檢測(cè)得到[55,56,57]。延遲熒光**由Strehler和Arnold于1951年報(bào)道，是由PSII所發(fā)射。DF被認(rèn)為反映了光誘導(dǎo)電荷分離后，還原的初級(jí)電子受體QA-與氧化的電子供體P680+的再?gòu)?fù)合。DF誘導(dǎo)曲線(xiàn)的形狀取決于樣品類(lèi)型及其生理狀態(tài)。同時(shí)測(cè)量葉綠素Chl a熒光(即時(shí)熒光，PF)、延遲熒光DF、在820nm處調(diào)制反射MR820和遠(yuǎn)紅光(735nm)反射RR的試驗(yàn)設(shè)備已開(kāi)發(fā)出來(lái)(Hansatech, M-PEA)，可獲得不同光合反應(yīng)的速率常數(shù)[56]。如圖3，由Golteev等于2013年提出的Σ方案解釋了光合電子傳遞中上述信號(hào)的來(lái)源[58]。如圖4，通過(guò)該技術(shù)使用M-PEA，Goltsev等于2012年發(fā)現(xiàn)干旱脅迫下QA-的再氧化受到抑制，由PSII至QA的電子傳遞量子產(chǎn)率下降同時(shí)OJIP曲線(xiàn)快相部分受到抑制[59]。

 

圖3. Σ方案解釋光合電子傳遞鏈中PF、DF和mr820信號(hào)來(lái)源[58]。 * 方框表示光合結(jié)構(gòu)構(gòu)件。綠色箭頭表示可以測(cè)量的物理信號(hào)，紅色箭頭表示根據(jù)這些信號(hào)重新計(jì)算的電子和能量流。信號(hào)：DF，延遲熒光；PF，即時(shí)熒光；MR，調(diào)制反射；RR，遠(yuǎn)紅光（735nm）反射。 * 電子流：TR，能量俘獲；E21，從PSII天線(xiàn)到PSI的能量遷移（溢出）；ED，來(lái)自?xún)?nèi)部供體的水或中間供體（ID）向PSII的電子供應(yīng)；RE，通過(guò)PSI到NADP的電子流；CE，環(huán)式電子流。 * RC1*和RC2*分別是PSI和PSII的反應(yīng)中心葉綠素，其他縮略語(yǔ)是光合光反應(yīng)的經(jīng)典Z方案的標(biāo)準(zhǔn)縮寫(xiě)。

圖4. JIP-test參數(shù)和延遲熒光參數(shù)I1/I2，該數(shù)據(jù)根據(jù)1184組不同含水量離體大豆葉片測(cè)量[59]。 * 雷達(dá)圖顯示了根據(jù)不同RWC的葉片計(jì)算出的參數(shù)。對(duì)于每個(gè)組，取50片相似RWC的葉片測(cè)量值的平均值，并標(biāo)準(zhǔn)化為100%RWC時(shí)的值。 * I1/I2是DF延遲熒光誘導(dǎo)曲線(xiàn)快速階段延遲熒光最大振幅的比值[60]。雷達(dá)圖生動(dòng)地表示了干旱對(duì)光合機(jī)械的影響。每一個(gè)干旱等級(jí)都由一個(gè)多邊形表示，其角點(diǎn)對(duì)應(yīng)于相對(duì)（相對(duì)于對(duì)照全水化葉的值）JIP參數(shù)，以及DF（I1/I2）誘導(dǎo)曲線(xiàn)上的兩個(gè)峰值的比值。這個(gè)比率I1/I2被發(fā)現(xiàn)與PSII中的電子流成反比[61]。光合機(jī)構(gòu)的功能狀態(tài)可以看作是一個(gè)幾何圖形，其形狀是干旱脅迫所特有的。它對(duì)不同的干旱程度很敏感，所選參數(shù)的雷達(dá)圖可直接用于RWC的經(jīng)驗(yàn)預(yù)測(cè)。

本文內(nèi)容源自《Emerging Technologies and Management of Crop Stress Tolerance A Sustainable Approach》Volume 2，Edited by Parvaiz Ahmad and Saiema Rasool. 

CHAPTER 15——Kalaji H M ,  Jajoo A ,  Oukarroum A , et al. The Use of Chlorophyll Fluorescence Kinetics Analysis to Study the Performance of Photosynthetic Machinery in Plants[J]. Emerging Technologies and Management of Crop Stress Tolerance, 2014:347-384.

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