郭霞麗, 余碧云, 張邵康, 黎敬業(yè), 王婕, 黃建國(guó)*

樹木木質(zhì)部生長(zhǎng)動(dòng)態(tài)及其調(diào)節(jié)機(jī)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(guó)1,2*

(1. 中國(guó)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(diǎn)實(shí)驗(yàn)室, 廣東省應(yīng)用植物學(xué)重點(diǎn)實(shí)驗(yàn)室, 廣州 510650; 2. 中國(guó)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(guó)科學(xué)院大學(xué), 北京 100049)

全球變化對(duì)樹木木質(zhì)部生長(zhǎng)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(guò)程。樹木木質(zhì)部生長(zhǎng)動(dòng)態(tài)主要包括形成層活動(dòng)開始和結(jié)束的時(shí)間、生長(zhǎng)季長(zhǎng)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(guò)在精細(xì)的時(shí)間尺度上對(duì)比研究樹木木質(zhì)部生長(zhǎng)動(dòng)態(tài)，揭示木質(zhì)部形成的決定因子，可以加深對(duì)樹木生長(zhǎng)生理機(jī)制的理解，從而提高其對(duì)氣候變化響應(yīng)的預(yù)測(cè)精度。對(duì)近年來(lái)在木質(zhì)部的形成動(dòng)態(tài)及其調(diào)節(jié)機(jī)制方面取得的研究進(jìn)展進(jìn)行了綜述，并對(duì)未來(lái)的研究方向進(jìn)行了展望。

生長(zhǎng)季長(zhǎng)度；生長(zhǎng)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(guò)對(duì)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(zhǎng)提供機(jī)械支撐，還有傳輸水分和養(yǎng)分，抵御風(fēng)雪等功能，同時(shí)作為重要的碳匯器官，具有巨大的經(jīng)濟(jì)和社會(huì)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(zhǎng)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(zhǎng)動(dòng)態(tài)及其調(diào)控機(jī)制可以幫助我們更好地預(yù)測(cè)森林生態(tài)系統(tǒng)對(duì)氣候變化的響應(yīng)以及適應(yīng)。

木質(zhì)部生長(zhǎng)是一個(gè)復(fù)雜并具有生態(tài)彈性的過(guò)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(dòng)，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(lái)的細(xì)胞經(jīng)過(guò)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(zhǎng)季內(nèi)樹干木質(zhì)部的形成過(guò)程是一個(gè)“S”型曲線，即在生長(zhǎng)季初期木質(zhì)部生長(zhǎng)較慢；隨著持續(xù)升溫，木質(zhì)部進(jìn)入快速生長(zhǎng)期，之后逐漸減緩并進(jìn)入冬季休眠期。木質(zhì)部生長(zhǎng)動(dòng)態(tài)一般分為時(shí)間和數(shù)量?jī)蓚€(gè)既獨(dú)立又相互聯(lián)系的維度，包括形成層活動(dòng)開始和結(jié)束的時(shí)間、生長(zhǎng)季長(zhǎng)度和木質(zhì)部的生長(zhǎng)速率以及最終的木質(zhì)部總量等。木質(zhì)部形成層活動(dòng)開始和結(jié)束的時(shí)間是樹木對(duì)環(huán)境適應(yīng)性的表現(xiàn)，體現(xiàn)了對(duì)資源的充分利用及對(duì)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(zhǎng)季的開始期提前，結(jié)束期延后，從而導(dǎo)致生長(zhǎng)季延長(zhǎng)[6–9]。這種改變預(yù)期將提高木質(zhì)部生長(zhǎng)量，從而使得森林生態(tài)系統(tǒng)的固碳作用進(jìn)一步加強(qiáng)，對(duì)林分生長(zhǎng)和森林生產(chǎn)力產(chǎn)生深遠(yuǎn)影響[10]。

研究木質(zhì)部生長(zhǎng)動(dòng)態(tài)變化及調(diào)控機(jī)理對(duì)物種生存以及群落維持有重要意義，同時(shí)可以幫助我們預(yù)測(cè)未來(lái)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(zhǎng)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(guò)系統(tǒng)地梳理以上因子與木質(zhì)部生長(zhǎng)之間的聯(lián)系，可以加深我們對(duì)樹木生理生態(tài)過(guò)程及生態(tài)系統(tǒng)過(guò)程和功能的了解。因此，本文將在前人綜述的基礎(chǔ)上，重點(diǎn)突出木質(zhì)部的生長(zhǎng)動(dòng)態(tài)及其調(diào)控機(jī)理,并為后續(xù)工作進(jìn)行展望，以期為全面理解樹木生長(zhǎng)的生理機(jī)制提供一些新思路。

1 木質(zhì)部生長(zhǎng)動(dòng)態(tài)

有研究表明，木質(zhì)部生長(zhǎng)季長(zhǎng)度和生長(zhǎng)速率共同決定木質(zhì)部生長(zhǎng)量，即較長(zhǎng)的生長(zhǎng)季長(zhǎng)度和較慢的生長(zhǎng)速率或者較短的生長(zhǎng)季長(zhǎng)度和較快的生長(zhǎng)速率均可產(chǎn)生相似的年木質(zhì)部生長(zhǎng)量[11–13]。因此,準(zhǔn)確定量生長(zhǎng)季長(zhǎng)度與生長(zhǎng)速率對(duì)木質(zhì)部生長(zhǎng)量的相對(duì)貢獻(xiàn)，可以幫助我們深入理解木質(zhì)部的生長(zhǎng)動(dòng)態(tài)，從而有效預(yù)測(cè)未來(lái)的森林碳匯變化。普遍認(rèn)為，木質(zhì)部生長(zhǎng)開始時(shí)間越早，生長(zhǎng)季越長(zhǎng)，則會(huì)產(chǎn)生較寬的年輪[14]。香脂冷杉()的生長(zhǎng)季長(zhǎng)度對(duì)木質(zhì)部生長(zhǎng)量貢獻(xiàn)率達(dá)76%，遠(yuǎn)遠(yuǎn)高于生長(zhǎng)速率對(duì)木質(zhì)部生長(zhǎng)量的貢獻(xiàn)率[15]。Rossi等[16]通過(guò)分析北半球大范圍尺度的微樹芯數(shù)據(jù)，認(rèn)為生長(zhǎng)季長(zhǎng)度主要決定了木質(zhì)部生長(zhǎng)量，并且生長(zhǎng)季延長(zhǎng)13%, 對(duì)應(yīng)的木質(zhì)部細(xì)胞數(shù)增長(zhǎng)了109%[16], 這證明生長(zhǎng)季的延長(zhǎng)會(huì)導(dǎo)致木質(zhì)部生長(zhǎng)量的不對(duì)稱增加，從而促進(jìn)森林生產(chǎn)力[16]。然而，通過(guò)監(jiān)測(cè)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(zhǎng)動(dòng)態(tài)和進(jìn)一步定量分析，結(jié)果表明，生長(zhǎng)速率對(duì)木質(zhì)部生長(zhǎng)量的貢獻(xiàn)率為75%，而生長(zhǎng)季長(zhǎng)度的貢獻(xiàn)率為25%[12,17]。同樣，生長(zhǎng)速率對(duì)青藏高原祁連圓柏()木質(zhì)部的生長(zhǎng)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(zhǎng)生長(zhǎng)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(guò)降低木質(zhì)部生長(zhǎng)速率來(lái)限制碳的固定[20]。這些結(jié)果表明生長(zhǎng)季長(zhǎng)度或者生長(zhǎng)速率不能單獨(dú)決定木質(zhì)部生長(zhǎng)，兩者之間的權(quán)衡關(guān)系共同決定了樹木生長(zhǎng)對(duì)氣候變化的響應(yīng)[12]。

2 木質(zhì)部發(fā)育調(diào)控機(jī)制

2.1 植物激素

激素在植物體內(nèi)廣泛分布，通過(guò)直接或間接地促進(jìn)或減慢植物的代謝過(guò)程，進(jìn)而調(diào)節(jié)其生長(zhǎng)和發(fā)育過(guò)程。生長(zhǎng)素是第一個(gè)被發(fā)現(xiàn)的植物激素，其產(chǎn)生、運(yùn)輸和代謝活動(dòng)均對(duì)木質(zhì)部生長(zhǎng)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(zhǎng)素。春季，生長(zhǎng)素沿著樹干向下極性運(yùn)輸，刺激樹干的形成層開始分裂活動(dòng)，形成木質(zhì)部[22]。生長(zhǎng)素促進(jìn)細(xì)胞生長(zhǎng)的作用體現(xiàn)在兩方面：首先，生長(zhǎng)素可使細(xì)胞壁疏松，增強(qiáng)其可塑性，從而促進(jìn)了細(xì)胞的縱向伸長(zhǎng)；其次，生長(zhǎng)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(zhǎng)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(zhǎng)素對(duì)于木質(zhì)部生長(zhǎng)的調(diào)節(jié)作用隨著季節(jié)變化而有所差異。在生長(zhǎng)季早期，生長(zhǎng)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(zhǎng)季晚期，即使生長(zhǎng)素含量很高，形成層依然進(jìn)入休眠期，說(shuō)明休眠期可能不是由生長(zhǎng)素單獨(dú)控制[27]。生長(zhǎng)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(guò)連續(xù)監(jiān)測(cè)生長(zhǎng)素含量，認(rèn)為晚材開始形成時(shí)，生長(zhǎng)素含量并沒有明顯變化。因此，相比于生長(zhǎng)素含量變化，生長(zhǎng)素本身可能提供了一種信號(hào)作用，從而決定木質(zhì)部的發(fā)育過(guò)程[27]。除了生長(zhǎng)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(huì)共同調(diào)控形成層的分裂活動(dòng)以及木質(zhì)部形成。與生長(zhǎng)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(shí)，各種植物激素之間通過(guò)相互作用, 共同調(diào)控木質(zhì)部細(xì)胞的生長(zhǎng)。細(xì)胞分裂素和生長(zhǎng)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(zhǎng)素的參與下，赤霉素調(diào)控纖維細(xì)胞的伸長(zhǎng)過(guò)程[33]。

2.2 碳水化合物

木質(zhì)部的生長(zhǎng)過(guò)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(zhǎng)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(guò)程正相關(guān)，即當(dāng)木質(zhì)部生長(zhǎng)速率最大時(shí)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(dòng)提供能量，同時(shí)也可以作為生長(zhǎng)調(diào)節(jié)物質(zhì)，通過(guò)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(duì)樹木生長(zhǎng)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(dòng)和形成層季節(jié)活動(dòng)高度吻合[40]。而歐洲云杉()在糖分含量最高的時(shí)候，增厚期的細(xì)胞數(shù)和木質(zhì)部生長(zhǎng)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(zhǎng)，一部分則會(huì)通過(guò)韌皮部向下運(yùn)輸，儲(chǔ)存在木質(zhì)部中，以應(yīng)對(duì)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(guò)深入了解木質(zhì)部生長(zhǎng)的碳源機(jī)制，可以幫助我們了解樹木內(nèi)在的碳分配機(jī)制并預(yù)測(cè)樹木對(duì)極端天氣的響應(yīng)。有研究表明，木質(zhì)部的生長(zhǎng)和碳的累積具有高度同步性。在生長(zhǎng)季早期，木質(zhì)部生長(zhǎng)和可溶性碳累積同步進(jìn)行。在生長(zhǎng)季后期，木質(zhì)部生長(zhǎng)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(zhǎng)做好能量?jī)?chǔ)備[43]。氣候條件，如溫度和光照可以通過(guò)直接影響光合作用，從而影響樹木生長(zhǎng)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(duì)樹木生長(zhǎng)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(zhǎng)。在干旱地區(qū)，夏季高溫導(dǎo)致木質(zhì)部生長(zhǎng)速率下降甚至停止生長(zhǎng)，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(guò)存儲(chǔ)的碳水化合物提供能量，進(jìn)而重新開始分裂活動(dòng)，形成一年內(nèi)木質(zhì)部生長(zhǎng)的雙峰曲線。因此, 木質(zhì)部生長(zhǎng)動(dòng)態(tài)的靈活性很大程度上依賴于木質(zhì)部中存儲(chǔ)的非結(jié)構(gòu)性碳含量。

2.3 氮素

作為氨基酸和其他有機(jī)物質(zhì)構(gòu)成的重要原料,氮素是植物生長(zhǎng)必需的大量元素，對(duì)于植物的生長(zhǎng)和發(fā)育具有重要作用。氮添加可以通過(guò)增加葉片中Rubisco和葉綠素的濃度促進(jìn)光合作用，或者通過(guò)提高樹木對(duì)存儲(chǔ)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(zhǎng)限制因子，然而，由于近年來(lái)人類活動(dòng)的加劇，大氣氮沉降大幅度增加，對(duì)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(duì)木質(zhì)部發(fā)育動(dòng)態(tài)的影響，可用于評(píng)估當(dāng)前氮沉降對(duì)樹木生長(zhǎng)和森林生態(tài)系統(tǒng)的影響，預(yù)測(cè)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(duì)木質(zhì)部生長(zhǎng)影響的研究，然而由于氮添加方式、氮添加速率以及實(shí)驗(yàn)?zāi)晗薜纫蛩氐牟煌?，相關(guān)研究未取得共識(shí)。在寒帶和溫帶森林中, 短期氮添加均未對(duì)香脂冷杉、黑云杉()、馬尾松()、楓香()木質(zhì)部形成動(dòng)態(tài)產(chǎn)生顯著影響[46–50]。但在長(zhǎng)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(shí), 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(zhǎng)，說(shuō)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(duì)樹木生長(zhǎng)的影響[53]。通過(guò)監(jiān)測(cè)中國(guó)亞熱帶氮添加對(duì)優(yōu)勢(shì)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(duì)錐栗()則無(wú)顯著影響[54]，說(shuō)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(duì)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(duì)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(dòng)的啟動(dòng)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(duì)溫度極其敏感；其次，溫度可以通過(guò)影響非結(jié)構(gòu)性碳的可利用性間接影響木質(zhì)部生長(zhǎng)。通過(guò)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(zhǎng)物候期，結(jié)果表明木質(zhì)部生長(zhǎng)開始的時(shí)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(zhǎng)的時(shí)間提前8.2 d，而木質(zhì)部生長(zhǎng)結(jié)束的時(shí)間與海拔引起的溫度差異關(guān)系較弱[19]。通過(guò)大空間尺度范圍內(nèi)探索木質(zhì)部發(fā)育動(dòng)態(tài)的一般規(guī)律及其機(jī)理，發(fā)現(xiàn)木質(zhì)部的起始生長(zhǎng)受到冬、春季積溫的共同影響，進(jìn)一步揭示了溫度對(duì)木質(zhì)部形成的主導(dǎo)作用[56]。另外，在研究相對(duì)較少的亞熱帶地區(qū),同樣發(fā)現(xiàn)溫度對(duì)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(lái)講，對(duì)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(zhǎng)。對(duì)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(zhǎng)的主要?dú)夂蛞蛩?，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(bào)道的溫度閾值。

水分對(duì)于木質(zhì)部的生長(zhǎng)發(fā)揮著重要作用。形成層細(xì)胞的分裂活動(dòng)和細(xì)胞增大是受膨壓驅(qū)動(dòng)的過(guò)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(dòng)開始的關(guān)鍵因子[13]。通過(guò)模型預(yù)測(cè)，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(dòng)祁連圓柏的木質(zhì)部生長(zhǎng)[61]。對(duì)于熱帶常綠樹種來(lái)說(shuō)，水分條件則決定了形成層活動(dòng)的持續(xù)時(shí)間[62]。通過(guò)對(duì)不同水分虧缺下木質(zhì)部的發(fā)育動(dòng)態(tài)進(jìn)行監(jiān)測(cè)，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(zhǎng)及導(dǎo)致死亡率增加的生理機(jī)制。相比于溫度和降雨，光周期可以為植物生長(zhǎng)提供穩(wěn)定的信號(hào)，進(jìn)而調(diào)控木質(zhì)部發(fā)育。通過(guò)分析北半球的樹木木質(zhì)部生長(zhǎng)動(dòng)態(tài)，認(rèn)為其最大生長(zhǎng)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(guò)程[63]。

3 總結(jié)和展望

樹木木質(zhì)部生長(zhǎng)是重要的碳匯過(guò)程，通過(guò)深入了解其調(diào)節(jié)機(jī)制，可為預(yù)測(cè)森林生態(tài)系統(tǒng)碳匯變化及可持續(xù)森林經(jīng)營(yíng)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(wèn)題，以期未來(lái)研究中能進(jìn)一步關(guān)注。首先，樹木生長(zhǎng)同時(shí)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(huì)通過(guò)限制碳水化合物的可利用性，從而對(duì)樹木生長(zhǎng)產(chǎn)生不利影響[64]。較高的碳水化合物和生長(zhǎng)季早期溫度可通過(guò)促進(jìn)生長(zhǎng)素合成及運(yùn)輸，進(jìn)而促進(jìn)形成層分裂[65–66]。而生長(zhǎng)季晚期短日照引發(fā)的形成層對(duì)生長(zhǎng)素的不敏感性，導(dǎo)致形成層進(jìn)入休眠[27]。這說(shuō)明各個(gè)因子之間通過(guò)復(fù)雜的相互聯(lián)系，共同調(diào)節(jié)木質(zhì)部生長(zhǎng)。因此，未來(lái)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(duì)樹木生長(zhǎng)的調(diào)節(jié)機(jī)制有更加全面的認(rèn)識(shí)。另外，樹木作為一個(gè)有機(jī)整體，樹冠、樹干和根部的生長(zhǎng)相互耦合，協(xié)調(diào)發(fā)展，同時(shí)監(jiān)測(cè)三者的動(dòng)態(tài)生長(zhǎng)過(guò)程，結(jié)合激素、碳水化合物及氮素含量的測(cè)定，通過(guò)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(shí)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(duì)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(duì)于木質(zhì)部生長(zhǎng)的相關(guān)研究相對(duì)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(píng)估樹木生長(zhǎng)和森林生態(tài)系統(tǒng)對(duì)全球變化的響應(yīng)和適應(yīng)機(jī)制，為國(guó)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(guó)家自然科學(xué)基金項(xiàng)目(41861124001, 31570584, 41661144007)；廣東自然科學(xué)基金項(xiàng)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