樹木木質(zhì)部生長動態(tài)及其調(diào)節(jié)機制研究進(jìn)展

郭霞麗, 余碧云, 張邵康, 黎敬業(yè), 王婕, 黃建國*

樹木木質(zhì)部生長動態(tài)及其調(diào)節(jié)機制研究進(jìn)展

郭霞麗1,2,3, 余碧云1,2,3, 張邵康1,2, 黎敬業(yè)1,2,3, 王婕1,2,3, 黃建國1,2*

(1. 中國科學(xué)院華南植物園退化生態(tài)系統(tǒng)植被恢復(fù)與管理重點實驗室, 廣東省應(yīng)用植物學(xué)重點實驗室, 廣州 510650; 2. 中國科學(xué)院核心植物園植物生態(tài)學(xué)協(xié)同中心，廣州 510650；3. 中國科學(xué)院大學(xué), 北京 100049)

全球變化對樹木木質(zhì)部生長產(chǎn)生了深遠(yuǎn)影響，進(jìn)而影響了森林生態(tài)系統(tǒng)的固碳功能以及全球生態(tài)系統(tǒng)能量和物質(zhì)的循環(huán)過程。樹木木質(zhì)部生長動態(tài)主要包括形成層活動開始和結(jié)束的時間、生長季長度以及分裂速率等，其受到多種因素的共同調(diào)節(jié)，如植物激素、碳水化合物、氮素和氣象因子等。通過在精細(xì)的時間尺度上對比研究樹木木質(zhì)部生長動態(tài)，揭示木質(zhì)部形成的決定因子，可以加深對樹木生長生理機制的理解，從而提高其對氣候變化響應(yīng)的預(yù)測精度。對近年來在木質(zhì)部的形成動態(tài)及其調(diào)節(jié)機制方面取得的研究進(jìn)展進(jìn)行了綜述，并對未來的研究方向進(jìn)行了展望。

生長季長度；生長速率；植物激素；碳水化合物；氮素；氣象因子

樹木是森林生態(tài)系統(tǒng)的重要組成部分，可通過對大氣中二氧化碳的吸收和固定，從而減緩全球變化的進(jìn)程[1]。木質(zhì)部不僅為樹木生長提供機械支撐，還有傳輸水分和養(yǎng)分，抵御風(fēng)雪等功能，同時作為重要的碳匯器官，具有巨大的經(jīng)濟(jì)和社會效益。當(dāng)前氣候變化背景下，高溫和干旱事件頻發(fā), 導(dǎo)致木質(zhì)部的生長急劇衰退，進(jìn)而引發(fā)大面積森林死亡[2]。因此，深入探討木質(zhì)部生長動態(tài)及其調(diào)控機制可以幫助我們更好地預(yù)測森林生態(tài)系統(tǒng)對氣候變化的響應(yīng)以及適應(yīng)。

木質(zhì)部生長是一個復(fù)雜并具有生態(tài)彈性的過程。在寒帶和溫帶地區(qū)，隨著春季溫度的上升，樹干形成層打破休眠開始分裂活動，向外產(chǎn)生韌皮部，向內(nèi)產(chǎn)生木質(zhì)部。從形成層釋放出來的細(xì)胞經(jīng)過體積增大，細(xì)胞壁加厚，最終發(fā)育為成熟的木質(zhì)部細(xì)胞[3–4]。大量研究表明，生長季內(nèi)樹干木質(zhì)部的形成過程是一個“S”型曲線，即在生長季初期木質(zhì)部生長較慢；隨著持續(xù)升溫，木質(zhì)部進(jìn)入快速生長期，之后逐漸減緩并進(jìn)入冬季休眠期。木質(zhì)部生長動態(tài)一般分為時間和數(shù)量兩個既獨立又相互聯(lián)系的維度，包括形成層活動開始和結(jié)束的時間、生長季長度和木質(zhì)部的生長速率以及最終的木質(zhì)部總量等。木質(zhì)部形成層活動開始和結(jié)束的時間是樹木對環(huán)境適應(yīng)性的表現(xiàn)，體現(xiàn)了對資源的充分利用及對不利環(huán)境的躲避[5]。研究表明，氣候變暖已經(jīng)引起了生長季的開始期提前，結(jié)束期延后，從而導(dǎo)致生長季延長[6–9]。這種改變預(yù)期將提高木質(zhì)部生長量，從而使得森林生態(tài)系統(tǒng)的固碳作用進(jìn)一步加強，對林分生長和森林生產(chǎn)力產(chǎn)生深遠(yuǎn)影響[10]。

研究木質(zhì)部生長動態(tài)變化及調(diào)控機理對物種生存以及群落維持有重要意義，同時可以幫助我們預(yù)測未來的森林生產(chǎn)力和碳匯等，然而目前相關(guān)綜述較為缺乏。木質(zhì)部生長受到多種因子的共同調(diào)節(jié)，如植物激素、碳水化合物、氮素和氣象因子等。通過系統(tǒng)地梳理以上因子與木質(zhì)部生長之間的聯(lián)系，可以加深我們對樹木生理生態(tài)過程及生態(tài)系統(tǒng)過程和功能的了解。因此，本文將在前人綜述的基礎(chǔ)上，重點突出木質(zhì)部的生長動態(tài)及其調(diào)控機理,并為后續(xù)工作進(jìn)行展望，以期為全面理解樹木生長的生理機制提供一些新思路。

1 木質(zhì)部生長動態(tài)

有研究表明，木質(zhì)部生長季長度和生長速率共同決定木質(zhì)部生長量，即較長的生長季長度和較慢的生長速率或者較短的生長季長度和較快的生長速率均可產(chǎn)生相似的年木質(zhì)部生長量[11–13]。因此,準(zhǔn)確定量生長季長度與生長速率對木質(zhì)部生長量的相對貢獻(xiàn)，可以幫助我們深入理解木質(zhì)部的生長動態(tài)，從而有效預(yù)測未來的森林碳匯變化。普遍認(rèn)為，木質(zhì)部生長開始時間越早，生長季越長，則會產(chǎn)生較寬的年輪[14]。香脂冷杉()的生長季長度對木質(zhì)部生長量貢獻(xiàn)率達(dá)76%，遠(yuǎn)遠(yuǎn)高于生長速率對木質(zhì)部生長量的貢獻(xiàn)率[15]。Rossi等[16]通過分析北半球大范圍尺度的微樹芯數(shù)據(jù)，認(rèn)為生長季長度主要決定了木質(zhì)部生長量，并且生長季延長13%, 對應(yīng)的木質(zhì)部細(xì)胞數(shù)增長了109%[16], 這證明生長季的延長會導(dǎo)致木質(zhì)部生長量的不對稱增加，從而促進(jìn)森林生產(chǎn)力[16]。然而，通過監(jiān)測歐洲地區(qū)的挪威云杉()、樟子松()和歐洲冷杉()的木質(zhì)部生長動態(tài)和進(jìn)一步定量分析，結(jié)果表明，生長速率對木質(zhì)部生長量的貢獻(xiàn)率為75%，而生長季長度的貢獻(xiàn)率為25%[12,17]。同樣，生長速率對青藏高原祁連圓柏()木質(zhì)部的生長也起到?jīng)Q定性作用[18–19]。有研究表明，在青藏高原半干旱區(qū)，溫暖而又干燥的氣候條件導(dǎo)致的較長生長季不利于針葉樹木質(zhì)部形成，而溫度升高誘導(dǎo)的干旱可能通過降低木質(zhì)部生長速率來限制碳的固定[20]。這些結(jié)果表明生長季長度或者生長速率不能單獨決定木質(zhì)部生長，兩者之間的權(quán)衡關(guān)系共同決定了樹木生長對氣候變化的響應(yīng)[12]。

2 木質(zhì)部發(fā)育調(diào)控機制

2.1 植物激素

激素在植物體內(nèi)廣泛分布，通過直接或間接地促進(jìn)或減慢植物的代謝過程，進(jìn)而調(diào)節(jié)其生長和發(fā)育過程。生長素是第一個被發(fā)現(xiàn)的植物激素，其產(chǎn)生、運輸和代謝活動均對木質(zhì)部生長起著重要的調(diào)節(jié)作用[21]。一般認(rèn)為，在幼嫩的分生組織，如嫩芽中產(chǎn)生大量生長素。春季，生長素沿著樹干向下極性運輸，刺激樹干的形成層開始分裂活動，形成木質(zhì)部[22]。生長素促進(jìn)細(xì)胞生長的作用體現(xiàn)在兩方面：首先，生長素可使細(xì)胞壁疏松，增強其可塑性，從而促進(jìn)了細(xì)胞的縱向伸長；其次，生長素誘導(dǎo)蛋白質(zhì)等物質(zhì)的合成，從而增加了細(xì)胞原生質(zhì)體[23–24]。有研究表明，生長素含量在形成層區(qū)域最高，沿著增大期細(xì)胞、增厚期細(xì)胞和成熟期細(xì)胞區(qū)域依次降低[25]，其濃度梯度維持著形成層和木質(zhì)部細(xì)胞結(jié)構(gòu)穩(wěn)定性。另外，生長素對于木質(zhì)部生長的調(diào)節(jié)作用隨著季節(jié)變化而有所差異。在生長季早期，生長素水平和形成層細(xì)胞數(shù)呈現(xiàn)顯著正相關(guān)[26]，而在生長季晚期，即使生長素含量很高，形成層依然進(jìn)入休眠期，說明休眠期可能不是由生長素單獨控制[27]。生長素含量降低引發(fā)細(xì)胞壁較薄、管腔較大的早材向細(xì)胞壁較厚、管腔較小的晚材轉(zhuǎn)化[28]。而Uggla等[29]通過連續(xù)監(jiān)測生長素含量，認(rèn)為晚材開始形成時，生長素含量并沒有明顯變化。因此，相比于生長素含量變化，生長素本身可能提供了一種信號作用，從而決定木質(zhì)部的發(fā)育過程[27]。除了生長素，其他植物激素，如細(xì)胞分裂素、赤霉素、乙烯、脫落酸等也會共同調(diào)控形成層的分裂活動以及木質(zhì)部形成。與生長素的分布不同，細(xì)胞分裂素含量在韌皮部最高[30]，赤霉素含量在發(fā)育的木質(zhì)部中最高[31]。同時，各種植物激素之間通過相互作用, 共同調(diào)控木質(zhì)部細(xì)胞的生長。細(xì)胞分裂素和生長素具有協(xié)同作用，可以共同促進(jìn)形成層細(xì)胞分裂和木質(zhì)部細(xì)胞的發(fā)育[30,32]。在生長素的參與下，赤霉素調(diào)控纖維細(xì)胞的伸長過程[33]。

2.2 碳水化合物

木質(zhì)部的生長過程需要消耗的大量能量主要由碳水化合物提供[34]。植物體內(nèi)的碳水化合物分為結(jié)構(gòu)性碳水化合物和非結(jié)構(gòu)性碳水化合物。結(jié)構(gòu)性碳水化合物用于細(xì)胞壁構(gòu)成，如纖維素、半纖維素和木質(zhì)素等。非結(jié)構(gòu)性碳水化合物是葉片進(jìn)行光合作用之后的產(chǎn)物，主要為淀粉和可溶性糖，即葡萄糖、果糖、麥芽糖和蔗糖等，是植物用于新陳代謝的重要能量物質(zhì)[35]。Deslauriers等[36]首次研究了加拿大楊()和美洲黑楊()在生長季內(nèi)木質(zhì)部的產(chǎn)生和可利用性碳的關(guān)系，證明形成層內(nèi)的非結(jié)構(gòu)性碳含量和木質(zhì)部的形成過程正相關(guān)，即當(dāng)木質(zhì)部生長速率最大時非結(jié)構(gòu)性碳濃度較高，并且碳含量是限制木質(zhì)部活細(xì)胞新陳代謝的首要因子[36]。糖分既可以為細(xì)胞的分裂活動提供能量，同時也可以作為生長調(diào)節(jié)物質(zhì)，通過調(diào)控相關(guān)基因的表達(dá)，從而促進(jìn)細(xì)胞有絲分裂和細(xì)胞增殖，對樹木生長具有重要意義[37–39]。有研究表明，歐洲赤松()糖分含量的季節(jié)波動和形成層季節(jié)活動高度吻合[40]。而歐洲云杉()在糖分含量最高的時候，增厚期的細(xì)胞數(shù)和木質(zhì)部生長量也達(dá)到最大值[41]。

通常情況下，葉片光合作用產(chǎn)生的碳水化合物一部分直接用于樹木生長，一部分則會通過韌皮部向下運輸，儲存在木質(zhì)部中，以應(yīng)對極端氣候下由于光合作用不足導(dǎo)致的樹木碳饑餓[42]。通過深入了解木質(zhì)部生長的碳源機制，可以幫助我們了解樹木內(nèi)在的碳分配機制并預(yù)測樹木對極端天氣的響應(yīng)。有研究表明，木質(zhì)部的生長和碳的累積具有高度同步性。在生長季早期，木質(zhì)部生長和可溶性碳累積同步進(jìn)行。在生長季后期，木質(zhì)部生長逐步停止,可溶性碳含量達(dá)到最大值，為下一年的樹木生長做好能量儲備[43]。氣候條件，如溫度和光照可以通過直接影響光合作用，從而影響樹木生長的能量供應(yīng)，因此本研究從能量的角度上解釋了氣候條件對樹木生長的滯后效應(yīng)，即上一年的氣候可以顯著影響下一年的樹木生長。在干旱地區(qū)，夏季高溫導(dǎo)致木質(zhì)部生長速率下降甚至停止生長，因此非結(jié)構(gòu)性碳含量累積[44]。一旦有充足的水分，形成層可以通過存儲的碳水化合物提供能量，進(jìn)而重新開始分裂活動，形成一年內(nèi)木質(zhì)部生長的雙峰曲線。因此, 木質(zhì)部生長動態(tài)的靈活性很大程度上依賴于木質(zhì)部中存儲的非結(jié)構(gòu)性碳含量。

2.3 氮素

作為氨基酸和其他有機物質(zhì)構(gòu)成的重要原料,氮素是植物生長必需的大量元素，對于植物的生長和發(fā)育具有重要作用。氮添加可以通過增加葉片中Rubisco和葉綠素的濃度促進(jìn)光合作用，或者通過提高樹木對存儲碳水化合物的可利用性[45]，從而為木質(zhì)部形成提供關(guān)鍵能量。普遍認(rèn)為氮素是森林生態(tài)系統(tǒng)主要的生長限制因子，然而，由于近年來人類活動的加劇，大氣氮沉降大幅度增加，對森林生態(tài)系統(tǒng)造成了很大的影響。因此，深入研究氮素對木質(zhì)部發(fā)育動態(tài)的影響，可用于評估當(dāng)前氮沉降對樹木生長和森林生態(tài)系統(tǒng)的影響，預(yù)測全球氣候變化下森林生態(tài)系統(tǒng)的發(fā)展。目前全球已開展了大量模擬氮添加對木質(zhì)部生長影響的研究，然而由于氮添加方式、氮添加速率以及實驗?zāi)晗薜纫蛩氐牟煌?，相關(guān)研究未取得共識。在寒帶和溫帶森林中, 短期氮添加均未對香脂冷杉、黑云杉()、馬尾松()、楓香()木質(zhì)部形成動態(tài)產(chǎn)生顯著影響[46–50]。但在長期的氮沉降環(huán)境中，木質(zhì)部的形成是否受其影響仍需進(jìn)一步研究。Yu等[51]證實, 相比于林下氮添加, 林冠氮添加能夠顯著促進(jìn)麻櫟()木質(zhì)部生長，說明樹木冠層截留的氮素可以被有效利用[52]，之前傳統(tǒng)的林下氮添加可能低估了氮沉降對樹木生長的影響[53]。通過監(jiān)測中國亞熱帶氮添加對優(yōu)勢樹種木質(zhì)部解剖結(jié)構(gòu)的影響，發(fā)現(xiàn)林冠和林下施氮均顯著促進(jìn)木荷()的木質(zhì)部管胞增大，而對錐栗()則無顯著影響[54]，說明即使是在氮飽和的亞熱帶森林生態(tài)系統(tǒng)，適量的氮添加仍然可以對木質(zhì)部形成產(chǎn)生影響。

2.4 氣象因子

大量研究表明，氣象因子包括溫度、降雨和光周期對于木質(zhì)部形成具有重要的調(diào)節(jié)作用。普遍認(rèn)為溫度是調(diào)控樹木形成層活動的啟動因子[55]。一方面，形成層分裂和細(xì)胞增大涉及的一系列酶促反應(yīng)對溫度極其敏感；其次，溫度可以通過影響非結(jié)構(gòu)性碳的可利用性間接影響木質(zhì)部生長。通過分析青藏高原不同海拔梯度上祁連圓柏樹干木質(zhì)部的生長物候期，結(jié)果表明木質(zhì)部生長開始的時間與海拔引起的溫度變化相關(guān)，即海拔每降低100 m，木質(zhì)部開始生長的時間提前8.2 d，而木質(zhì)部生長結(jié)束的時間與海拔引起的溫度差異關(guān)系較弱[19]。通過大空間尺度范圍內(nèi)探索木質(zhì)部發(fā)育動態(tài)的一般規(guī)律及其機理，發(fā)現(xiàn)木質(zhì)部的起始生長受到冬、春季積溫的共同影響，進(jìn)一步揭示了溫度對木質(zhì)部形成的主導(dǎo)作用[56]。另外，在研究相對較少的亞熱帶地區(qū),同樣發(fā)現(xiàn)溫度對于調(diào)節(jié)馬尾松木質(zhì)部增大期和增厚期細(xì)胞具有顯著的促進(jìn)作用[57]。一般來講，對溫帶和寒帶地區(qū)的樹木，當(dāng)春季溫度達(dá)到低溫閾值(4℃~5℃)，木質(zhì)部才開始生長。對藏東南色季拉山史密斯杉樹()的研究表明，大氣最低溫是影響木質(zhì)部生長的主要氣候因素，而且限制木質(zhì)部分化開始的最低溫閾值為(0.7±0.4)℃[58]，遠(yuǎn)遠(yuǎn)低于之前報道的溫度閾值。

水分對于木質(zhì)部的生長發(fā)揮著重要作用。形成層細(xì)胞的分裂活動和細(xì)胞增大是受膨壓驅(qū)動的過程，需要充足的水分[59–60]。因此，在干旱地區(qū)，相比于溫度，降雨是調(diào)控形成層活動開始的關(guān)鍵因子[13]。通過模型預(yù)測，發(fā)現(xiàn)在合適的溫度下，連續(xù)12 d的累積降雨達(dá)(17.0±5.6) mm才能啟動祁連圓柏的木質(zhì)部生長[61]。對于熱帶常綠樹種來說，水分條件則決定了形成層活動的持續(xù)時間[62]。通過對不同水分虧缺下木質(zhì)部的發(fā)育動態(tài)進(jìn)行監(jiān)測，表明水分是調(diào)節(jié)分生組織形成層細(xì)胞分裂的首要因子[36]，碳次之，這解釋了全球氣候變暖所誘導(dǎo)的干旱抑制樹木生長及導(dǎo)致死亡率增加的生理機制。相比于溫度和降雨，光周期可以為植物生長提供穩(wěn)定的信號，進(jìn)而調(diào)控木質(zhì)部發(fā)育。通過分析北半球的樹木木質(zhì)部生長動態(tài)，認(rèn)為其最大生長速率發(fā)生在夏至日左右。樹木在環(huán)境適宜的情況下提前降低形成層分裂速率，可能是為了保證樹木在入冬之前完成所有的木質(zhì)化過程[63]。

3 總結(jié)和展望

樹木木質(zhì)部生長是重要的碳匯過程，通過深入了解其調(diào)節(jié)機制，可為預(yù)測森林生態(tài)系統(tǒng)碳匯變化及可持續(xù)森林經(jīng)營管理提供理論依據(jù)。然而，當(dāng)前在該領(lǐng)域方面仍存在一些問題，以期未來研究中能進(jìn)一步關(guān)注。首先，樹木生長同時受到多種因素的共同調(diào)節(jié)，并且各因素之間存在相互影響。例如低溫會通過限制碳水化合物的可利用性，從而對樹木生長產(chǎn)生不利影響[64]。較高的碳水化合物和生長季早期溫度可通過促進(jìn)生長素合成及運輸，進(jìn)而促進(jìn)形成層分裂[65–66]。而生長季晚期短日照引發(fā)的形成層對生長素的不敏感性，導(dǎo)致形成層進(jìn)入休眠[27]。這說明各個因子之間通過復(fù)雜的相互聯(lián)系，共同調(diào)節(jié)木質(zhì)部生長。因此，未來研究應(yīng)該更加關(guān)注植物激素、碳水化合物、氮素和氣象因子之間的相互作用，從而對樹木生長的調(diào)節(jié)機制有更加全面的認(rèn)識。另外，樹木作為一個有機整體，樹冠、樹干和根部的生長相互耦合，協(xié)調(diào)發(fā)展，同時監(jiān)測三者的動態(tài)生長過程，結(jié)合激素、碳水化合物及氮素含量的測定，通過定量分析和結(jié)構(gòu)方程等模型手段，有利于在整樹水平上深入理解樹木受到以上因素調(diào)節(jié)的時空差異性，從而進(jìn)一步探索樹木在不同器官內(nèi)的碳分配策略以及對全球變化的響應(yīng)。最后，由于全球數(shù)據(jù)分布的不均勻性，相比于寒帶及溫帶森林，熱帶及亞熱帶對于木質(zhì)部生長的相關(guān)研究相對較少。因此，亟需在低緯度地區(qū)盡快開展相關(guān)工作，從而有利于在全球尺度上評估樹木生長和森林生態(tài)系統(tǒng)對全球變化的響應(yīng)和適應(yīng)機制，為國家生態(tài)文明建設(shè)以及全球可持續(xù)發(fā)展服務(wù)。

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Research Progresses on Xylem Formation Dynamics and Its Regulation Mechanism

GUO Xia-li1,2,3, YU Bi-yun1,2,3, ZHANG Shao-kang1,2, LI Jing-ye1,2,3, WANG Jie1,2,3, HUANG Jian-guo1,2*

(1. Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, Guangdong Provincial Key Laboratory of Applied Botany, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China; 2. Center for Plant Ecology, Core Botanical Garden, Chinese Academy of Sciences,Guangzhou 510650, China; 3. University of Chinese Academy of Sciences, Beijing 100049, China)

Global changes impose a profound impact on the xylem formation, which in turn affects the carbon sequestration of forest ecosystems and fundamental services of global ecosystems. The xylem formation dynamic of tree is mainly characterized by the timing of the onset and the end of cambial activity, the length of the growing season, and the growth rate, etc., which are jointly regulated by various factors, such as phytohormone, carbohydrate, nitrogen and meteorological factors. By investigating the formation dynamics of xylem over a fine time scale, the determinants of xylem formation could be revealed, the understanding of physiological mechanism of tree growth would be deepen, and the prediction accuracy of the tree growth response to climate changes would further improve. The recent research progresses in the xylem formation dynamic and its regulation mechanism were reviewed, and the prospects for the future research were provided.

Length of growing season; Growth rate; Phytohormone; Carbohydrate; Nitrogen; Meteorological factor

10.11926/jtsb.4101

2019–05–29

2019–07–15

國家自然科學(xué)基金項目(41861124001, 31570584, 41661144007)；廣東自然科學(xué)基金項目(2016A030313152)資助

This work was supported by the National Natural Science Foundation of China (Grant No. 41861124001, 31570584, 41661144007), and the Natural Science Foundation in Guangdong (Grant No. 2016A030313152).

郭霞麗，主要從事樹木生理學(xué)和森林生態(tài)學(xué)研究。E-mail: guoxl@scbg.ac.cn

E-mail: huangjg@scbg.ac.cn