張彩霞，符冠富，奉保華，陳婷婷，陶龍興

（中國(guó)水稻研究所/水稻生物學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室，杭州 310006）

由于溫室氣體排放增多，全球極端氣候頻發(fā)，農(nóng)作物產(chǎn)量及糧食安全受到嚴(yán)重威脅[1-4]。在某種惡劣條件下，單一脅迫可導(dǎo)致產(chǎn)量絕收[5]，加之耕地面積銳減，糧食安全問(wèn)題日趨嚴(yán)峻[6-7]。水稻是世界最重要的糧食作物之一[8-11]，為滿足人類對(duì)糧食日益增長(zhǎng)的需求，預(yù)計(jì)到2035年，水稻產(chǎn)量與2010年相比需增加25%[12]。鑒于糧食安全隱患加劇及需求的增加，通過(guò)有效生產(chǎn)方式提高作物產(chǎn)量迫在眉睫[13-14]。產(chǎn)量形成與干物質(zhì)積累、分配及轉(zhuǎn)運(yùn)密切相關(guān)，實(shí)質(zhì)上是“源庫(kù)流”協(xié)調(diào)統(tǒng)一的過(guò)程?！霸础笔侵圃旎蚬?yīng)光合產(chǎn)物的器官，“庫(kù)”是接受或積累光合產(chǎn)物的器官，“流”是指光合同化物從源到庫(kù)的運(yùn)輸[15]。

光合同化物主要以蔗糖的形式通過(guò)韌皮部進(jìn)行運(yùn)輸，即蔗糖裝載進(jìn)入小葉脈篩管分子后，經(jīng)長(zhǎng)距離運(yùn)輸進(jìn)入庫(kù)器官。適宜生長(zhǎng)條件下，光合作用形成的碳水化合物大部分以非結(jié)構(gòu)性碳水化合物形式暫時(shí)貯存在莖鞘中[16]，籽粒灌漿開(kāi)啟后，貯存于莖鞘中的同化物重新活化、裝載進(jìn)入韌皮部，最終運(yùn)向籽粒。莖稈貯藏同化物及其向籽粒轉(zhuǎn)運(yùn)能力可能是影響作物高產(chǎn)的重要途徑。因而，近年來(lái)作物單產(chǎn)難以提高甚至減產(chǎn)，極有可能與極端溫度等逆境下同化物轉(zhuǎn)運(yùn)受抑有關(guān)。目前對(duì)“源”和“庫(kù)”的研究較多，但對(duì)“流”的關(guān)注相對(duì)較少，尤其是逆境下同化物轉(zhuǎn)運(yùn)特征及響應(yīng)機(jī)理。因此，本文在綜述同化物運(yùn)輸和分配機(jī)理的基礎(chǔ)上，明確植株對(duì)逆境脅迫的響應(yīng)機(jī)制，重點(diǎn)分析同化物轉(zhuǎn)運(yùn)對(duì)逆境脅迫的響應(yīng)機(jī)制，以期為提高水稻的穩(wěn)產(chǎn)性和抗逆性提供理論參考，為未來(lái)糧食安全提供保障[17]。

1 水稻韌皮部同化物轉(zhuǎn)運(yùn)機(jī)理

1.1 同化物在葉片的裝載

同化物在葉片的裝載主要有兩種途徑，即質(zhì)外體與共質(zhì)體途徑。質(zhì)外體途徑需要大量轉(zhuǎn)運(yùn)體的參與，例如 SWEET蛋白及蔗糖轉(zhuǎn)運(yùn)蛋白（sucrose transporter，SUT）[18-20]。一般情況下，SWEET蛋白將蔗糖分子運(yùn)輸?shù)郊?xì)胞壁，再由蔗糖轉(zhuǎn)運(yùn)蛋白（sucrose transporter，SUT）轉(zhuǎn)運(yùn)至篩管-伴胞復(fù)合體[18-20]。在同化物裝載過(guò)程中，質(zhì)膜上的 SUT是重要載體，裝載的過(guò)程由質(zhì)子動(dòng)力勢(shì)驅(qū)動(dòng)[21]，主要負(fù)責(zé)蔗糖從“源”到“庫(kù)”的質(zhì)外體運(yùn)輸，并在蔗糖感應(yīng)、“源”器官裝載、韌皮部長(zhǎng)距離運(yùn)輸和“庫(kù)”器官卸載等過(guò)程中發(fā)揮重要作用[21]。目前水稻上已鑒定出 5個(gè) SUTs基因，即 OsSUT1、OsSUT2、OsSUT3、OsSUT4和OsSUT5，僅OsSUT1和OsSUT3可能在質(zhì)外體韌皮部裝載中發(fā)揮作用[22-25]。OsSUT2主要作用于蔗糖從液泡到胞質(zhì)的轉(zhuǎn)運(yùn)[26]，OsSUT3僅在花粉管中表達(dá)，在葉片裝載中的表達(dá)很少[27-28]，而OsSUT4是一類低親和性載體，主要與蔗糖在次生維管組織韌皮部中的裝載有關(guān)[29-31]。鑒于此，OsSUT1可能是質(zhì)外體韌皮部裝載中最為重要的蔗糖轉(zhuǎn)運(yùn)體，因?yàn)橐延醒芯勘砻?，OsSUT1蛋白占主導(dǎo)地位，其表達(dá)主要集中在葉片和葉鞘韌皮部篩管和伴胞中[32]。然而OsSUT1基因敲除后，水稻營(yíng)養(yǎng)生長(zhǎng)未發(fā)生顯著變化，但籽粒淀粉積累減少，結(jié)實(shí)率下降[33]，似表明OsSUT1蛋白表達(dá)受阻時(shí)，有另外的韌皮部裝載途徑代替或者彌補(bǔ)其功能，例如共質(zhì)體途徑[24]。

同化物的共質(zhì)體裝載是一個(gè)被動(dòng)的過(guò)程，無(wú)需消耗能量，但需要葉肉細(xì)胞和韌皮部間有較高的濃度梯度及細(xì)胞間有發(fā)達(dá)的胞間連絲[34-35]。由于水稻葉片中有發(fā)達(dá)的胞間連絲，共質(zhì)體裝載途徑可能起主要作用[36]。已有研究表明，蔗糖濃度與胞間連絲數(shù)量呈顯著正相關(guān)關(guān)系，絕大部分具有高等或中等胞間連絲數(shù)量的植物均具有較高的蔗糖濃度[37]，而水稻葉片蔗糖含量明顯高于擬南芥等質(zhì)外體裝載植物[38-41]。然而，還需要更多的生理和分子實(shí)驗(yàn)予以證明。近年來(lái)有學(xué)者提出一種特殊的共質(zhì)體裝載形式，即聚合物陷阱，通過(guò)消耗代謝能量將蔗糖轉(zhuǎn)化成棉子糖、水蘇四糖等低聚糖。低聚糖不能通過(guò)胞間連絲從篩管細(xì)胞擴(kuò)散回葉肉細(xì)胞，但可以通過(guò)大的胞間連絲轉(zhuǎn)入篩管細(xì)胞，從而維持韌皮部中高的糖濃度[19,36-37,42-43]?？傊?，植物同化物裝載機(jī)制有很強(qiáng)的可塑性，會(huì)隨著生長(zhǎng)發(fā)育、生物和非生物脅迫以及不同基因型而發(fā)生改變[44-45]，在特定環(huán)境下采取的裝載方式還需進(jìn)一步探討。

1.2 同化物在莖鞘韌皮部的運(yùn)輸

高等植物的韌皮部由一系列縱向排布的篩管分子、伴胞和薄壁細(xì)胞組成[46]。同化物在韌皮部中的運(yùn)輸速率不僅受到“源”端和“庫(kù)”端壓力梯度控制，還受到“源”中韌皮部裝載速率和“庫(kù)”中韌皮部卸載速率的調(diào)節(jié)。此外，篩管幾何體結(jié)構(gòu)也是影響同化物在韌皮部運(yùn)輸不可忽視的因素，尤其篩板孔形態(tài)特征。當(dāng)篩管受損有汁液外流時(shí)，P蛋白就被定位于篩板孔，封堵篩分子以阻止汁液的進(jìn)一步流失。長(zhǎng)效解決篩管受損的機(jī)制則是在篩孔處產(chǎn)生胼胝質(zhì)，即β-1，3-葡聚糖，由質(zhì)膜中的胼胝質(zhì)合酶合成并沉積于質(zhì)膜和細(xì)胞壁之間[47]。Mullendore等[48]研究表明，植物在傷害條件下胼胝質(zhì)幾分鐘內(nèi)可把小篩板孔堵塞。當(dāng)環(huán)境恢復(fù)正常后，胼胝質(zhì)可在胼胝質(zhì)水解酶介導(dǎo)下降解[47]。因此，篩管中的 P-蛋白及胼胝質(zhì)等物質(zhì)可堵塞篩板孔[49]，從而影響同化物運(yùn)輸。

植物營(yíng)養(yǎng)生長(zhǎng)階段，葉片等同化器官產(chǎn)生的光合同化物主要貯存于莖鞘中，通過(guò)質(zhì)外體或共質(zhì)體途徑卸載進(jìn)入細(xì)胞后，以葡萄糖和果糖形式貯存于液泡中，當(dāng)籽粒開(kāi)始灌漿后，重新合成蔗糖裝載進(jìn)入篩管運(yùn)往庫(kù)器官。從莖鞘薄壁細(xì)胞重新裝載回韌皮部的機(jī)理尚不清楚，既可能是共質(zhì)體途徑，也可能是質(zhì)外體途徑[43]。共質(zhì)體途徑同薄壁細(xì)胞與韌皮部篩管分子之間胞間連絲的數(shù)量、大小及是否通暢有關(guān)，而質(zhì)外體途徑主要受蔗糖轉(zhuǎn)運(yùn)體（SUTs）的影響[43]。Scofield等認(rèn)為[33,50-52]，OsSUT1參與葉鞘儲(chǔ)藏淀粉重新動(dòng)員的同化物轉(zhuǎn)運(yùn)，而SUT2作為蔗糖信號(hào)感應(yīng)器，在蔗糖轉(zhuǎn)運(yùn)等分子調(diào)控過(guò)程中發(fā)揮重要作用[53-54]。然而，有關(guān)莖鞘中同化物貯藏及轉(zhuǎn)運(yùn)的研究相對(duì)較少，還需要進(jìn)一步探討。

穗頸節(jié)間是“源”器官連接穗部的唯一通道，其結(jié)構(gòu)特征與結(jié)實(shí)率、籽粒充實(shí)度和產(chǎn)量密切相關(guān)[55]。研究表明，水稻穗頸節(jié)間維管束數(shù)量對(duì)莖鞘非結(jié)構(gòu)性碳化合物的轉(zhuǎn)運(yùn)具有顯著的正向直接效應(yīng)[56]。穗頸較粗、維管束數(shù)目多的品種，能夠保持“流”的通暢，有利于提高同化物向籽粒轉(zhuǎn)運(yùn)的效率，從而提高收獲指數(shù)[57]。于曉剛等[58]發(fā)現(xiàn)，穎果維管束可以通過(guò)調(diào)控同化物的運(yùn)輸與卸載影響穎果形成和稻米品質(zhì)。然而，有關(guān)穗頸節(jié)間特征與同化物轉(zhuǎn)運(yùn)關(guān)系的研究較少，有待進(jìn)一步深入研究。

1.3 同化物在穎果中的運(yùn)輸與卸載

韌皮部卸載是指韌皮部中同化物進(jìn)入生長(zhǎng)或貯藏組織的過(guò)程，包括同化物從篩管分子卸載和韌皮部后運(yùn)輸[59]。同化物通過(guò)葉、莖、穗軸、枝梗和小穗軸等維管系統(tǒng)向穎果輸送，最終通過(guò)穎花背部維管束進(jìn)入穎果。同化物在韌皮部中進(jìn)行長(zhǎng)距離運(yùn)輸后可通過(guò)質(zhì)外體或共質(zhì)體途徑卸出，這兩個(gè)途徑可單獨(dú)起作用，也可同時(shí)存在，不同發(fā)育階段可采取不同的卸載途徑[60-61]。在發(fā)育的種子中，母體與胚性組織間無(wú)胞間連絲，同化物必須經(jīng)質(zhì)外體途徑卸載。盡管水稻光合同化物從背部維管束卸載后進(jìn)入胚乳的途徑存在爭(zhēng)議，但有一點(diǎn)可以肯定，背部維管束和胚乳組織不直接接觸，二者之間無(wú)共質(zhì)體連通，但有珠心突起組織和質(zhì)外體空間，由背部維管束運(yùn)進(jìn)穎果的同化物必須經(jīng)過(guò)質(zhì)外體后才能進(jìn)入胚乳[62-63]。養(yǎng)分基本是按照小穗軸中央維管束—子房背部維管束—珠心突起—珠心層質(zhì)外體—胚乳的路線進(jìn)行運(yùn)輸?shù)腫64]。

在質(zhì)外體卸載中，蔗糖由載體介導(dǎo)跨膜卸出到質(zhì)外空間，一方面可被蔗糖轉(zhuǎn)化酶分解為葡萄糖和果糖，然后由己糖載體吸收進(jìn)入庫(kù)細(xì)胞；另一方面可由蔗糖載體介導(dǎo)直接吸收進(jìn)庫(kù)細(xì)胞[65]。期間涉及3個(gè)過(guò)程，首先是蔗糖在轉(zhuǎn)運(yùn)體（SUTs）的作用下跨膜進(jìn)入質(zhì)外體，然后蔗糖被細(xì)胞壁轉(zhuǎn)化酶（CINs）水解，最后在單糖轉(zhuǎn)運(yùn)體（MSTs）的作用下進(jìn)入細(xì)胞[66]，表明SUTs、CINs和MSTs共同協(xié)調(diào)蔗糖的卸載過(guò)程。一般情況下，蔗糖轉(zhuǎn)化酶活性在源（葉）中較低，有利于形成較高的蔗糖濃度以向貯藏器官轉(zhuǎn)運(yùn)，而在貯藏器官中較高，有利于蔗糖水解并轉(zhuǎn)化為淀粉。

已經(jīng)被鑒定出的5個(gè)蔗糖轉(zhuǎn)運(yùn)體中，OsSUT2、OsSUT3、OsSUT4和OsSUT5 mRNAs僅在開(kāi)花時(shí)表達(dá)且僅維持到花后5d，而OsSUT1授粉后立即表達(dá)，并且持續(xù)到開(kāi)花后25d[22,67]。此外，OsSUT1還參與了同化物跨過(guò)糊粉層運(yùn)輸?shù)桨l(fā)育中胚乳的過(guò)程[33,63,67]。研究表明，OsSUT1水稻突變體的營(yíng)養(yǎng)生長(zhǎng)過(guò)程和野生型幾乎沒(méi)有區(qū)別[33,68]，敲除基因OsSUT1后葉片碳水化合物積累無(wú)變化，但水稻籽粒淀粉積累減少，結(jié)實(shí)率下降[33]。雖然這5種OsSUTs的表達(dá)及功能有所不同，但OsSUTs之間可相互協(xié)調(diào)，共同維持生物體的整個(gè)生命活動(dòng)[22-25,26-31]。對(duì)于轉(zhuǎn)化酶基因，研究表明CINs過(guò)量表達(dá)株系籽粒淀粉含量及粒重顯著增加，目前已經(jīng)有7個(gè)CINs被鑒定出，其中OsCIN1、OsCIN2、OsCIN4和OsCIN7在未成熟種子中表達(dá)，但作用時(shí)期不一致，表明這4個(gè)CINs基因都可能參與蔗糖卸載[69]。水稻中已經(jīng)鑒定出 4個(gè)單糖轉(zhuǎn)運(yùn)體（MSTs），其中OsMST2、OsMST3和OsMST5屬于糖轉(zhuǎn)運(yùn)體[70-71]。盡管水稻轉(zhuǎn)運(yùn)體表達(dá)特征已有研究，但其在水稻中表達(dá)活性與同化物卸載和產(chǎn)量形成的關(guān)系報(bào)道還較少，其具體功能還有待深入探討。

另外還有研究表明，同化物從韌皮部的卸出與庫(kù)端蔗糖酶及質(zhì)膜H+-TP酶的活性密切相關(guān)[72]。同化物卸出到籽粒時(shí)，首先是質(zhì)膜 H+-TP酶促進(jìn)糖的逆化學(xué)勢(shì)共轉(zhuǎn)運(yùn)，然后被轉(zhuǎn)運(yùn)的蔗糖在蔗糖酶的作用下被水解，從而維持篩管末端質(zhì)外體空間較低的蔗糖濃度，防止其重新裝載，同時(shí)蔗糖水解使質(zhì)外體空間的水勢(shì)降低，并促進(jìn)篩管中糖和水的流出[72]。

2 水稻同化物轉(zhuǎn)運(yùn)對(duì)逆境脅迫的響應(yīng)

2.1 高溫?zé)岷?/h3>

2.1.1 高溫脅迫對(duì)水稻葉片裝載的影響

如前所述，水稻光合同化物葉片韌皮部裝載主要為共質(zhì)體裝載，葉肉細(xì)胞與篩管分子之間的胞間連絲數(shù)量、頻率、大小及生理活性均可能成為限制同化物運(yùn)輸?shù)囊蛩?。研究表明胞間連絲易受環(huán)境因素的影響，低溫脅迫4h可觀察到玉米葉片有胞間連絲關(guān)閉的現(xiàn)象，低溫處理28h，葉片維管束鞘和維管薄壁細(xì)胞接口處的胞間連絲因胼胝質(zhì)堵塞而關(guān)閉[73]。長(zhǎng)時(shí)間的高溫脅迫也可能會(huì)導(dǎo)致水稻葉片胞間連絲關(guān)閉，從而影響水稻同化物的裝載，但類似的研究仍未見(jiàn)報(bào)道。雖然目前的研究表明質(zhì)外體裝載不是水稻葉片主要的裝載方式，但在共質(zhì)體裝載受阻的情況下，質(zhì)外體裝載不失為一個(gè)很好的補(bǔ)充。這些蔗糖轉(zhuǎn)運(yùn)體基因（OsSUT1、OsSUT2、OsSUT3、OsSUT4和OsSUT5）表達(dá)可能會(huì)受到高溫的抑制從而影響水稻同化物的裝載。此外，裝載過(guò)程由質(zhì)子動(dòng)力勢(shì)所驅(qū)動(dòng)[21]，如果高溫導(dǎo)致植物體內(nèi)代謝紊亂，不能形成正常的質(zhì)子動(dòng)力循環(huán)，勢(shì)必引起裝載紊亂。

高溫導(dǎo)致水稻產(chǎn)量降低與籽粒充實(shí)度變差關(guān)系較大，因?yàn)橥镅b載、運(yùn)輸及卸載中任意一個(gè)環(huán)節(jié)受高溫逆境脅迫均可能導(dǎo)致蔗糖轉(zhuǎn)運(yùn)受阻，造成“流”的不暢。前期研究結(jié)果表明，高溫條件下，無(wú)論是耐熱性較強(qiáng)還是耐熱性較差的品種，劍葉光合能力并未發(fā)生顯著下降[74]，說(shuō)明其光合同化物的合成在高溫逆境中并未受顯著傷害，因?yàn)槿~片表面實(shí)際溫度只有35℃左右[74]。因此推測(cè)高溫對(duì)水稻同化物在葉片裝載的影響較小，“流”的不暢可能主要與同化產(chǎn)物在莖鞘韌皮部運(yùn)輸及籽粒卸載有關(guān)。但由于胞間連絲超微結(jié)構(gòu)及生理活性方面的研究還較薄弱，還有待進(jìn)一步驗(yàn)證。

2.1.2 高溫脅迫對(duì)同化物在莖鞘中運(yùn)輸?shù)挠绊?/h4>

目前，籽粒開(kāi)始灌漿后光合產(chǎn)物從莖鞘薄壁細(xì)胞重新裝載回韌皮部的機(jī)理還不清楚，但不論是共質(zhì)體還是質(zhì)外體途徑均會(huì)受高溫影響[43]。與葉片相比，高溫下莖鞘和籽粒的散熱能力較低，極易受高溫傷害，氣溫40℃時(shí)，莖鞘和籽粒溫度在38℃以上[74]。研究表明，植物受到高溫脅迫時(shí)，胼胝質(zhì)大量產(chǎn)生并在篩孔中沉積，幾分鐘內(nèi)可以將篩板孔堵塞[48]。因此，高溫逆境下篩板孔堵塞而引起“流”不暢的可能性較大，導(dǎo)致同化產(chǎn)物儲(chǔ)藏于莖鞘薄壁細(xì)胞中[43]。逆境解除后，胼胝質(zhì)在胼胝質(zhì)水解酶的作用下降解，同化物可從薄壁細(xì)胞重新裝載進(jìn)入韌皮部篩管分子，隨后運(yùn)輸至其它“庫(kù)”組織[43]。然而，高溫脅迫時(shí)間過(guò)長(zhǎng)，脅迫強(qiáng)度超過(guò)臨界值，胼胝質(zhì)氧化酶可能受高溫影響而導(dǎo)致活性降低甚至失活，阻礙胼胝質(zhì)的降解，導(dǎo)致胼胝質(zhì)始終沉積于篩板上，使莖鞘干物質(zhì)量和可溶性碳水化合物含量在高溫脅迫解除后仍呈增加趨勢(shì)[43]。另外，高溫下與蔗糖代謝、轉(zhuǎn)運(yùn)相關(guān)酶活性及相關(guān)基因表達(dá)受抑也可能是同化物轉(zhuǎn)運(yùn)受阻的主要原因。已有研究表明，高溫等逆境可顯著降低葉片及籽粒中蔗糖代謝及轉(zhuǎn)運(yùn)相關(guān)酶及基因的表達(dá)。然而，莖鞘中的這些酶及基因?qū)Ω邷仨憫?yīng)的研究仍未見(jiàn)報(bào)道[75]。雖然灌漿結(jié)實(shí)期莖鞘貯藏同化物向籽粒轉(zhuǎn)運(yùn)是提高作物產(chǎn)量的有效手段，但有關(guān)莖鞘韌皮部裝載與卸載方面的研究報(bào)道還較少，需要進(jìn)一步深入探討。

2.1.3 高溫脅迫對(duì)同化物在穎果中運(yùn)輸及卸載的影響

水稻韌皮部同化物進(jìn)入穎果需要以共質(zhì)體途徑經(jīng)過(guò)穎果背部韌皮部，維管束薄壁細(xì)胞及珠心突起，然后再以質(zhì)外體途徑進(jìn)入穎果背部糊粉層，最終進(jìn)入胚乳細(xì)胞。研究表明，水稻受精后6d穎果背部維管束才能發(fā)育完整，因而此期發(fā)生高溫脅迫不僅會(huì)影響穎果背部韌皮部、木質(zhì)部、維管束薄壁細(xì)胞間胞間連絲的發(fā)育，還會(huì)影響背部維管束及胞間連絲的生理活性，從而阻礙光合產(chǎn)物及其它營(yíng)養(yǎng)物質(zhì)進(jìn)入籽粒。更為嚴(yán)重的是高溫還會(huì)導(dǎo)致穎果背部維管束早衰，嚴(yán)重阻礙同化物運(yùn)輸。高溫不僅抑制背部維管束的發(fā)育，還可能影響穎果珠心及珠心突起的發(fā)育，不利于光合產(chǎn)物的卸載[62]。已有研究表明，珠心及珠心突起異常會(huì)導(dǎo)致灌漿速率降低，產(chǎn)量顯著下降[76]。在質(zhì)外體卸載階段，同化物受SUTS、CINs和MSTs轉(zhuǎn)運(yùn)體共同調(diào)控。若高溫抑制其活性及表達(dá)量，同化物卸載將受阻。研究表明，高溫脅迫可顯著降低籽粒中VIN活性，從而減少蔗糖向己糖的轉(zhuǎn)化及胚乳中淀粉的合成[77]。此外，無(wú)論高溫還是適溫環(huán)境中，耐熱性較強(qiáng)的植株幼嫩果實(shí)中 CWIN活性均顯著高于耐熱性差的品種[65,78-82]。

2.2 低溫脅迫

眾多非生物脅迫中，冷熱害已成為水稻面臨的主要?dú)庀鬄?zāi)害[83]，是制約糧食作物產(chǎn)量及地域分配的主要限制因素[84-85]。盡管全國(guó)大部分地區(qū)冷害的頻率和強(qiáng)度有所下降，但階段性和局地性的冷害仍有加重的趨勢(shì)[86]。植物的起源地與其耐受低溫冷害的能力關(guān)系密切[87-88]，水稻作為熱帶亞熱帶作物，對(duì)低溫較敏感，通常氣溫降到4℃植株便會(huì)死亡[89]。將培養(yǎng)3周的水稻植株置于6℃條件下低溫處理6h后葉片卷曲，恢復(fù)正常溫度 24h后葉片展開(kāi)[85]。研究表明，低溫脅迫4h葉片有胞間連絲關(guān)閉的現(xiàn)象，28h葉片維管束鞘和維管薄壁細(xì)胞接口處的胞間連絲因胼胝質(zhì)堵塞而關(guān)閉[90]。冷害脅迫韌皮部的瞬間堵塞可能是由于鈣依賴封閉蛋白暫時(shí)性堵塞篩管引起的[91]。前人通過(guò)對(duì)植物莖進(jìn)行局部低溫處理，觀察到葉片光合作用受抑，暗呼吸增強(qiáng)，韌皮部阻力增大，同化物轉(zhuǎn)運(yùn)受抑制[92]。通過(guò)對(duì)植物進(jìn)行局部低溫處理，研究結(jié)果表明低溫處理上部組織碳水化合物積累，下部組織碳水化合物含量減少；低溫處理部位上部韌皮部糖分側(cè)漏增加，同時(shí)向下運(yùn)輸?shù)奶妓衔餃p少，向根部運(yùn)輸?shù)奶妓衔锖繙p少[93]。灌漿過(guò)程中低溫逆境不僅導(dǎo)致植株凈光合生產(chǎn)能力下降，同時(shí)也抑制了碳水化合物向韌皮部轉(zhuǎn)運(yùn)[94]，導(dǎo)致稻谷的充實(shí)度變差及品質(zhì)變劣[95]。

2.3 干旱脅迫

水稻生長(zhǎng)季節(jié)發(fā)生干旱脅迫將嚴(yán)重影響產(chǎn)量的形成[96-100]，尤其生殖生長(zhǎng)期[101]。碳水化合物的轉(zhuǎn)運(yùn)和分配受水分影響，適度干旱脅迫能有效促進(jìn)莖鞘中儲(chǔ)藏的非結(jié)構(gòu)性碳水化合物向穗部轉(zhuǎn)運(yùn)[102]。正常條件下莖鞘同化物對(duì)產(chǎn)量的貢獻(xiàn)約為 20%，干旱脅迫下可提高至50%。該現(xiàn)象與氣孔關(guān)系密切[103]，因?yàn)楦珊迪職饪讓?dǎo)度降低，CO2吸收減少，作物光合產(chǎn)物生成受到限制，因而灌漿前莖鞘中貯藏的同化物成為籽粒灌漿的主要來(lái)源[104-106]。研究表明，ABA可加速灌漿進(jìn)程，調(diào)用積累在莖鞘中的 NSC，促進(jìn)積累同化物的再分配[107-108]。Travaglia等[109]研究表明，外源ABA可增加花期小麥碳水化合物積累并向籽粒轉(zhuǎn)運(yùn)，提高旱作小麥的產(chǎn)量。其原因主要是ABA提高細(xì)胞壁轉(zhuǎn)化酶活性[110]，促進(jìn)篩管分子運(yùn)來(lái)的蔗糖分解為己糖，從而實(shí)現(xiàn)通過(guò)調(diào)節(jié)細(xì)胞壁轉(zhuǎn)化酶的活性使“源”器官合成的同化物進(jìn)入細(xì)胞壁空間，顯著緩解源器官的壓力[110]。

2.4 氮元素

作為水稻生長(zhǎng)過(guò)程最重要的營(yíng)養(yǎng)之一，氮的供應(yīng)對(duì)水稻植株源庫(kù)流關(guān)系影響甚大。氮供應(yīng)不足時(shí)，葉面積指數(shù)過(guò)早下降，在籽粒灌漿時(shí)易產(chǎn)生源限制；長(zhǎng)期低氮，植株的生長(zhǎng)受到抑制，如分蘗發(fā)生停止，植株矮小。非結(jié)構(gòu)性碳水化合物在植物體內(nèi)積累是植物適應(yīng)低氮條件的一種重要機(jī)制[111]。蔗糖在莖鞘韌皮部薄壁細(xì)胞合成果聚糖，維持蔗糖在源端與臨時(shí)庫(kù)端的濃度梯度，在籽粒灌漿期重新將積累的同化物輸送至籽粒，從而補(bǔ)償由于光合作用減弱造成的同化物積累不足[111]。然而，低氮能促進(jìn)莖鞘碳、氮同化物的轉(zhuǎn)運(yùn)，與氮低效品種相比，氮高效品種抽穗后物質(zhì)轉(zhuǎn)運(yùn)能力更強(qiáng)，氮素轉(zhuǎn)運(yùn)和氮素利用率更高[112-113]。增施氮肥可明顯影響植株的蔗糖代謝，使蔗糖含量及合成能力提高，葉片碳氮同化物的轉(zhuǎn)運(yùn)量增加，但莖鞘碳氮同化物向籽粒的轉(zhuǎn)運(yùn)減弱[112]，主要是由于儲(chǔ)存的碳為氮代謝提供碳骨架，從而構(gòu)成很難再被釋放的蛋白質(zhì)或者氨基酸[114]。當(dāng)?shù)?yīng)過(guò)大時(shí)，莖稈生物量過(guò)量增加，葉片的正常衰老受到抑制，造成營(yíng)養(yǎng)器官過(guò)度生長(zhǎng)，莖稈伸長(zhǎng)，根冠比下降，植株易倒伏，導(dǎo)致產(chǎn)量下降[115]。

3 研究展望

綜上所述，無(wú)論正?；蚰婢硹l件下，同化物在葉片的裝載、莖鞘中的運(yùn)輸及穎果的卸載均為影響水稻產(chǎn)量及品質(zhì)形成的關(guān)鍵因素，尤其是同化物（蔗糖）在莖鞘中的儲(chǔ)藏及再分配，然而高溫等逆境條件下與糖轉(zhuǎn)運(yùn)蛋白相關(guān)的研究相對(duì)匱乏[116-119]。面對(duì)生態(tài)環(huán)境日益惡化及糧食需求急劇增加的困境[1-7]，加大對(duì)作物體內(nèi)同化物代謝及轉(zhuǎn)運(yùn)的研究，并進(jìn)行調(diào)控以實(shí)現(xiàn)逆境條件下穩(wěn)產(chǎn)甚至增產(chǎn)的意義重大。雖然近年來(lái)，越來(lái)越多研究者認(rèn)識(shí)到，“流”已經(jīng)成為大穗型水稻進(jìn)一步發(fā)展的限制因素，“流”的不暢不僅會(huì)限制產(chǎn)量潛力的發(fā)揮，還會(huì)影響稻米品質(zhì)。然而，以往的研究多集中在以維管束為主的承載“流”組織的解剖、亞種間數(shù)量差異及遺傳等方面，對(duì)流的質(zhì)量研究較少，即對(duì)維管束的流量面積和“流”的生理活性，如胼胝質(zhì)對(duì)流的影響、同化物的源端裝載與庫(kù)端卸出的酶和動(dòng)力的研究較少，尤其在高溫、低溫及干旱等脅迫條件下，同化物在韌皮部的裝載、運(yùn)輸及卸載特征更是少有報(bào)道。目前有關(guān)逆境脅迫對(duì)同化物轉(zhuǎn)運(yùn)影響的研究還有待完善，本文認(rèn)為最主要的原因是受研究手段及技術(shù)所限。因此，新技術(shù)的開(kāi)發(fā)應(yīng)用可為該研究領(lǐng)域提供新的發(fā)展機(jī)遇，例如正電子發(fā)射動(dòng)態(tài)放射性示蹤技術(shù)（dynamic radiotracer imaging techniques using positron emission tomography），預(yù)測(cè)該技術(shù)的成功應(yīng)用可彌補(bǔ)掃描電子顯微鏡、透射電子顯微鏡以及13C同位素示蹤等研究方法的不足，成為探索逆境脅迫下同化物轉(zhuǎn)運(yùn)的強(qiáng)有力手段[120-122]?？茖W(xué)技術(shù)的進(jìn)步，科研成果的日臻完善及科研水平的提高，可為該領(lǐng)域的研究提供更好的機(jī)遇，從而更好地造福人類。

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