基于FvCB模型的葉片光合生理對(duì)環(huán)境因子的響應(yīng)研究進(jìn)展

唐星林,曹永慧,顧連宏,周本智,*

1 中國(guó)林業(yè)科學(xué)研究院亞熱帶林業(yè)研究所,杭州 311400 2 國(guó)家林業(yè)局錢江源森林生態(tài)系統(tǒng)定位觀測(cè)研究站,杭州 311400 3 美國(guó)橡樹嶺國(guó)家實(shí)驗(yàn)室環(huán)境科學(xué)部,USA TN 37831

基于FvCB模型的葉片光合生理對(duì)環(huán)境因子的響應(yīng)研究進(jìn)展

唐星林1,2,曹永慧1,2,顧連宏3,周本智1,2,*

1 中國(guó)林業(yè)科學(xué)研究院亞熱帶林業(yè)研究所,杭州 311400 2 國(guó)家林業(yè)局錢江源森林生態(tài)系統(tǒng)定位觀測(cè)研究站,杭州 311400 3 美國(guó)橡樹嶺國(guó)家實(shí)驗(yàn)室環(huán)境科學(xué)部,USA TN 37831

為提高葉片光合速率并更好地理解葉片光合生理對(duì)環(huán)境因子變化的響應(yīng)機(jī)制,FvCB模型(C3植物光合生化模型)常用于分析不同環(huán)境條件下CO2響應(yīng)曲線并預(yù)測(cè)葉片活體內(nèi)光合系統(tǒng)的內(nèi)在變化狀況。系統(tǒng)介紹了FvCB模型的建立、發(fā)展過程和擬合方法等基本理論,綜述了該模型在葉片光合生理對(duì)光、CO2、水、溫度和N營(yíng)養(yǎng)等環(huán)境因子變化的響應(yīng)機(jī)制中的應(yīng)用研究。為進(jìn)一步完善FvCB模型并更好地理解葉片活體內(nèi)光合系統(tǒng)對(duì)環(huán)境因子變化的響應(yīng)機(jī)制,未來擬加強(qiáng)以下研究：1)羧化速率與光合電子傳遞速率之間的聯(lián)系；2)葉肉導(dǎo)度的具體組分及其對(duì)FvCB模型參數(shù)估計(jì)的影響；3)葉片氣孔導(dǎo)度和葉肉導(dǎo)度對(duì)環(huán)境因子變化的調(diào)控機(jī)制。

C3植物；光合作用；FvCB光合模型；光合生理；環(huán)境因子

1980年,Farquhar等根據(jù)Rubisco酶動(dòng)力學(xué)反應(yīng)和RuBP再生反應(yīng)化學(xué)計(jì)量學(xué),提出C3植物光合生化模型(簡(jiǎn)稱FvCB模型)[1]。它可以模擬葉片內(nèi)部的光合生化反應(yīng),并通過分析不同環(huán)境條件下的CO2響應(yīng)曲線獲得光合參數(shù),預(yù)測(cè)葉片活體光合系統(tǒng)的內(nèi)在變化狀況[2- 3]。FvCB模型已廣泛應(yīng)用于葉片光合生理對(duì)光照、溫度、水分、CO2濃度和N營(yíng)養(yǎng)等環(huán)境因子變化的響應(yīng)機(jī)制等方面的研究。它還有助于建立作物產(chǎn)量預(yù)測(cè)模型[4]、全球碳循環(huán)模型[5]、氣孔模型[6]、C4植物模型[7]等。最新的研究發(fā)現(xiàn)葉肉導(dǎo)度(gm)可以分為細(xì)胞壁導(dǎo)度、細(xì)胞膜導(dǎo)度、細(xì)胞質(zhì)導(dǎo)度、葉綠體膜導(dǎo)度、葉綠體基質(zhì)等組分,并進(jìn)一步改進(jìn)FvCB模型理論,但葉肉導(dǎo)度的具體路徑非常復(fù)雜。另外,大量研究表明環(huán)境因子可能通過控制水通道蛋白基因的表達(dá)來調(diào)控葉肉細(xì)胞內(nèi)CO2的轉(zhuǎn)運(yùn)過程,但該研究仍缺乏直接的實(shí)驗(yàn)證據(jù)。

國(guó)內(nèi)光合模型的相關(guān)研究多采用光合數(shù)學(xué)模型來描述光合速率與環(huán)境因子間的數(shù)量關(guān)系[8- 9],并反映光合速率等隨環(huán)境因子的變化情況,但卻難以反映葉片內(nèi)部的光合生化反應(yīng)狀況。而FvCB模型可以揭示不同環(huán)境條件下葉片活體內(nèi)部光合系統(tǒng)的變化狀況,是光合生理生態(tài)研究中的重要工具。盡管國(guó)外已經(jīng)開展了大量FvCB模型及其應(yīng)用相關(guān)研究,但國(guó)內(nèi)該領(lǐng)域的研究還相對(duì)較少,對(duì)該模型的基本理論及應(yīng)用還缺乏深入和系統(tǒng)的認(rèn)識(shí),僅有的一些研究也大多只局限于溫室茄子和光合生化模型模擬分析等[10]。本文將從FvCB模型的建立出發(fā),分析FvCB模型的理論和發(fā)展過程,探討該模型在植物光合作用和環(huán)境因子關(guān)系研究中的應(yīng)用,并提出FvCB模型及其應(yīng)用中存在的問題及可能的發(fā)展方向,以促進(jìn)國(guó)內(nèi)相關(guān)研究的深入開展。

1 C3植物FvCB模型理論

在碳反應(yīng)中,CO2、O2和核酮糖- 1,5-二磷酸(RuBP)等在Rubisco酶催化下發(fā)生羧化反應(yīng)和氧化反應(yīng)。當(dāng)CO2濃度較低時(shí),底物RuBP濃度過量,Rubisco酶催化活性達(dá)到最大,光合速率受Rubisco酶活性的限制,即Rubisco酶活性限制階段；隨著CO2濃度升高,底物RuBP再生速率小于其消耗速率,使RuBP濃度不足而限制光合速率,即RuBP再生速率限制階段[1]。由于前兩個(gè)限制階段不能解釋一定環(huán)境條件下葉片凈光合速率不隨CO2和O2濃度變化而改變的現(xiàn)象,Sharkey發(fā)現(xiàn)光合速率還可能受磷酸丙糖轉(zhuǎn)運(yùn)速率的限制(即TPU限制)[11]。隨著FvCB模型和CO2擴(kuò)散路徑等相關(guān)研究的深入,人們發(fā)現(xiàn)光合速率還受CO2從胞間向Rubisco羧化位點(diǎn)擴(kuò)散阻力的限制,即葉肉導(dǎo)度限制[12]。由于,溫度對(duì)光合生化反應(yīng)過程有直接影響,FvCB模型的溫度相關(guān)性研究也非常重要。在模型擬合方面,由于FvCB模型包括3個(gè)公式完全不同的子模型并且子模型間的轉(zhuǎn)換點(diǎn)無(wú)法確定,模型的擬合過程變得非常復(fù)雜。

1.1 Rubisco酶活性限制階段

Farquhar等認(rèn)為在高光強(qiáng)和低CO2下,光合作用受Rubisco酶活性大小的限制,并根據(jù)Rubisco酶動(dòng)力學(xué)理論和光合碳反應(yīng)的化學(xué)計(jì)量學(xué)提出Rubisco酶活性限制階段的子模型[1]。

首先是Rubisco酶動(dòng)力學(xué)理論。Laing提出當(dāng)RuBP濃度過量時(shí)羧化反應(yīng)和氧化反應(yīng)的Rubisco酶動(dòng)力學(xué)公式[13]

(1)

(2)

式中,Vc為羧化速率、Vo為氧化速率、Vcmax為最大羧化速率、Vomax為最大氧化速率、Kc為CO2米氏常數(shù)、Ko為O2米氏常數(shù)、Cc為Rubisco酶羧化位點(diǎn)CO2濃度、O為Rubisco酶羧化位點(diǎn)O2濃度。公式2除以公式1得Vo與Vc的比值(Φ)[13]

(3)

其次是碳反應(yīng)化學(xué)計(jì)量學(xué)。如圖1,碳反應(yīng)主要包括光合碳還原循環(huán)(PCR)和光呼吸循環(huán)(PCO)。在Rubisco酶的催化下,1mol CO2與1mol RuBP發(fā)生羧化反應(yīng)生成2mol 3-磷酸甘油酸(PGA),1mol O2和1mol RuBP發(fā)生氧化反應(yīng)生成1mol磷酸乙醇酸(PGIA)和1mol PGA,其中,1mol PGIA發(fā)生氧化反應(yīng)生成1mol甘氨酸(Gly)并釋放0.5mol CO2。由此,凈光合速率(A)[1]為：

A=Vc-0.5Vo-Rd

(4)

式中,Rd為光下暗呼吸速率。其中,Rd是指在光照條件下線粒體呼吸作用釋放的CO2,與PCO循環(huán)無(wú)關(guān)。

圖1 Rubisco酶羧化反應(yīng)和氧化反應(yīng)簡(jiǎn)圖[1]Fig.1 Simplified photosynthetic carbon reduction and photorespiratory carbon oxidationΦ： 氧化速率與羧化速率的比值 Ratio of Rubisco carboxylase rates to Rubisco oxygenase rates; RuBP: 核酮糖- 1,5-二磷酸 Ribulose- 1,5-bisphosphate; PGA: 3-磷酸甘油酸 3-phosphoglycerate; PGIA: 磷酸乙醇酸 Phosphoglycolate; Gly: 甘氨酸還原態(tài)的鐵氧還蛋白 Reduced ferredoxin

公式3代入公式4[11]得：

A=(1-0.5Φ)Vc-Rd

(5)

當(dāng)A和Rd均為0時(shí),葉綠體內(nèi)的CO2濃度被稱為缺乏暗呼吸的CO2補(bǔ)償點(diǎn)[14],記為Γ*,由公式5和公式3得：

Γ*=VomaxKcO/2VcmaxKo

(6)

公式6代入公式3得：

(7)

公式7代入公式5得：

(8)

當(dāng)光合作用受Rubisco酶活性限制時(shí),Vc由公式1給出,代入公式8得：

(9)

式中,Ac為Rubisco酶活性限制階段的凈光合速率。公式9為FvCB模型中Rubisco酶活性限制階段的子模型。

1.2 RuBP再生速率限制階段

如圖1,在碳反應(yīng)中, Gly和PGA被光反應(yīng)提供的同化力還原成RuBP,其中,NADPH和ATP等同化力是通過光合電子傳遞生成的。Farquhar等認(rèn)為在低光強(qiáng)和高CO2的條件下,光合作用會(huì)受RuBP再生速率限制,并根據(jù)RuBP再生過程中NADPH的需求量(由光反應(yīng)提供)建立Vc和光合電子傳遞速率(J)之間的聯(lián)系,提出RuBP再生速率限制階段的子模型[1]。

首先是Vc和NADPH消耗速率之間的聯(lián)系。在碳反應(yīng)中(如圖1),1mol RuBP發(fā)生羧化反應(yīng)生成2mol PGA；1mol RuBP發(fā)生氧化反應(yīng)生成1.5mol PGA。因此,PGA的生成速率(VPGA)[1]為：

VPGA=2Vc+1.5Vo

(10)

(11)

結(jié)合公式4、公式10和公式11得：

VNADPH=(2+2Φ)Vc

(12)

其次是J與NADPH生成速率之間的聯(lián)系。在光反應(yīng)中,1mol NADP+接受2mol e-和1mol H+生成1mol NADPH[1],因此,J為：

J=2VNADPH

(13)

由公式12和公式13得：

J=4+4ΦVc

(14)

公式7代入公式14得：

J=(4+8Γ*/Cc)Vc

(15)

由公式15變換得：

Vc=J/(4+8Γ*/Cc)

(16)

第三是J和最大電子傳遞速率(Jmax)之間的聯(lián)系。J主要由有效光輻射和植物特性決定。一般用非直角雙曲線函數(shù)來描述J和Jmax之間的關(guān)系[16]:

(17)

式中,σ為葉片吸收常數(shù)、I為入射光輻射、θ為曲率。參數(shù)σ主要受葉片對(duì)入射光輻射的吸收比例和葉片吸收的有效輻射在光系統(tǒng)I和光系統(tǒng)II之間分配比例的影響。Jmax由葉綠體類囊體膜上電子載體的組成成分決定。

當(dāng)光合作用受J限制時(shí),Vc由公式16給出,代入公式8得：

(18)

式中,Aj為RuBP再生速率限制階段的凈光合速率。公式18為FvCB模型RuBP再生速率限制階段的子模型。

1.3 磷酸丙糖轉(zhuǎn)運(yùn)限制

在碳反應(yīng)中,葉綠體內(nèi)生成的磷酸丙糖(TP)在葉綠體膜上的磷酸丙糖/無(wú)機(jī)磷轉(zhuǎn)運(yùn)蛋白的作用下與細(xì)胞質(zhì)內(nèi)的無(wú)機(jī)磷酸(Pi)交換,再在細(xì)胞質(zhì)內(nèi)合成蔗糖并釋放Pi。Sharkey發(fā)現(xiàn)在一定條件下,葉綠體內(nèi)TP的轉(zhuǎn)運(yùn)速率會(huì)小于其生成速率,同時(shí)Pi的轉(zhuǎn)運(yùn)速率也會(huì)小于葉綠體內(nèi)Pi的消耗速率；葉綠體內(nèi)TP的積累和Pi的不足會(huì)限制光合作用,即TPU限制[8]。Rubisco酶氧化反應(yīng)雖然會(huì)影響RuBP的再生速率,但不會(huì)影響TP的生成速率,又1mol TP含有3mol碳原子。因此,在TPU限制階段,TP的生成速率必須小于或等于1/3倍CO2固定速率,否則葉綠體內(nèi)自由Pi的下降會(huì)限制光合作用[11],凈光合速率為

Ap=3Tp-Rd

(19)

式中,Ap為TPU限制階段的凈光合速率、Tp為磷酸丙糖的最大轉(zhuǎn)運(yùn)速率。

在高CO2濃度、低光強(qiáng)和低溫條件下,凈光合速率反而會(huì)隨O2濃度增加而增大,隨CO2濃度增加而減少。由于公式19無(wú)法解釋該現(xiàn)象,Sharkey等認(rèn)為這些現(xiàn)象與碳反應(yīng)中甘油酸鹽的代謝過程有關(guān)[11,17]。光呼吸循環(huán)在細(xì)胞質(zhì)內(nèi)生成的甘油酸鹽最終會(huì)返回葉綠體內(nèi)。Harley和Sharkey發(fā)現(xiàn)在TPU階段,只有部分甘油酸鹽返回到葉綠體內(nèi)[18],并根據(jù)碳反應(yīng)中Pi的化學(xué)計(jì)量學(xué)進(jìn)一步完善TPU階段的子模型。根據(jù)碳反應(yīng)中Pi的化學(xué)計(jì)量學(xué),葉綠體內(nèi)Pi的凈消耗速率等于Vc/3-Vo/6[11]。當(dāng)光合作用受Pi濃度限制時(shí),Pi的凈消耗速率等于Pi的轉(zhuǎn)入速率,而Pi的轉(zhuǎn)入速率又等于Tp,即:

(20)

在光呼吸循環(huán)中,每個(gè)氧化反應(yīng)會(huì)消耗半個(gè)Pi并生成半個(gè)甘油酸鹽。假設(shè)有a倍的甘油酸鹽(0

(21)

公式4代入公式21得:

(22)

求解公式22,又Vc>0得:

(23)

當(dāng)光合作用受TP轉(zhuǎn)運(yùn)速率限制時(shí),Vc由公式23給出,代入公式8得:

(24)

公式24為FvCB光合模型TPU階段的子模型。當(dāng)a=0時(shí),公式24可簡(jiǎn)化為公式19。

1.4 葉肉導(dǎo)度

1980年,Farquhar等認(rèn)為葉肉細(xì)胞對(duì)CO2擴(kuò)散的阻力很小,在FvCB模型中可以忽略不計(jì),即Cc等于胞間CO2濃度(Ci)[1]。隨著研究的深入,人們發(fā)現(xiàn)葉肉細(xì)胞對(duì)CO2擴(kuò)散的阻力是光合作用的一個(gè)重要限制因子[9]。一般把CO2從胞間到Rubisco酶羧化位點(diǎn)擴(kuò)散的導(dǎo)度稱為葉肉導(dǎo)度,記為gm:

(25)

為估計(jì)參數(shù)gm,需結(jié)合公式25和FvCB模型的3個(gè)子模型推導(dǎo)出A關(guān)于Ci的函數(shù)表達(dá)式。根據(jù)公式25計(jì)算出Cc,再分別代入公式9、公式18和公式24得改進(jìn)后的FvCB模型。

首先是Rubisco酶限制階段。由公式25和公式9得Ac關(guān)于Ci的函數(shù)[19]:

(26)

求解公式26并取正解得：

(27)

其次是RuBP再生速率限制階段。由公式25和公式18得Aj關(guān)于Ci的函數(shù)[19]：

(28)

求解公式28并取正解得：

(29)

第三是TPU限制階段。由公式25和公式24得AP關(guān)于Ci的函數(shù)[20]:

(30)

b=3Tp-Rd+[Ci-1+3αΓ*]gm

c={3Ci-Γ*Tp-[Ci-1+3αΓ*]Rd}gm

(31)

公式27、公式29和公式31為加入gm參數(shù)后改進(jìn)的FvCB模型。

1.5 參數(shù)擬合

在模型擬合中,一般假定所有C3植物有相等的Rubisco酶動(dòng)力學(xué)常數(shù)(Kc和Ko),葉綠體內(nèi)O2濃度等于空氣O2濃度。由于FvCB模型本身存在超參數(shù)現(xiàn)象,一般把Γ*作為輸入常數(shù)[20]。根據(jù)公式27、公式29、公式31,FvCB模型擬合CO2響應(yīng)曲線可以獲得Vcmax、Jmax、Tp、a等階段特異性參數(shù)和gm、Rd等共同參數(shù),而擬合的關(guān)鍵點(diǎn)是3個(gè)子模型分界點(diǎn)Ci的確定。一般,把Rubisco酶活性限制階段到RuBP再生速率限制階段轉(zhuǎn)換點(diǎn)的Ci記為Ci_CJ；RuBP再生速率限制到TPU限制轉(zhuǎn)換點(diǎn)的Ci記為Ci_JP。根據(jù)分界點(diǎn)Ci確定方法的不同,可以把現(xiàn)有的擬合方案大致分為3類[17]。第1類方案認(rèn)為Ci_CJ在20—40Pa的范圍內(nèi)變化[21- 22],并且TPU限制階段在田間試驗(yàn)中很少出現(xiàn)。第2類方案利用CO2響應(yīng)曲線中的全部數(shù)據(jù)同時(shí)來擬合FvCB模型[23-24]。第3類方案認(rèn)為FvCB模型是變點(diǎn)模型,并采用特有的擬合方法進(jìn)行模型擬合[20]。第1類方案的擬合過程相對(duì)簡(jiǎn)單,但有很多潛在的問題。首先,共同參數(shù)的取值不好確定。對(duì)2個(gè)子模型分別進(jìn)行擬合,會(huì)獲得2組不相等的gm和Rd參數(shù),實(shí)踐中一般取平均值。其次,轉(zhuǎn)換點(diǎn)Ci_CJ的不確定性。大量研究表明Ci_CJ隨物種和環(huán)境條件的不同而不同[21,25],并且Ci_CJ的錯(cuò)誤會(huì)影響參數(shù)擬合的準(zhǔn)確性[21]。第2類方案雖然克服了人為確定Ci_CJ的缺點(diǎn),但其擬合獲得的最佳階段分配組合可能不符合Rubisco酶活性限制、RuBP再生速率限制和TPU限制的實(shí)際順序。第3類方案不僅克服3個(gè)階段人為劃分的缺點(diǎn),而且符合3個(gè)階段的實(shí)際順序,是較好的擬合方法,但是,其計(jì)算過程復(fù)雜,難以普遍使用。

1.6 參數(shù)的溫度相關(guān)性

根據(jù)FvCB模型的溫度相關(guān)性,不同溫度下Kc、Ko和Γ*等參數(shù)的取值不同。為此,一般把25℃下的Kc、Ko和Γ*等參數(shù)值作為標(biāo)準(zhǔn)(如表1),并根據(jù)參數(shù)的溫度相關(guān)性函數(shù)來計(jì)算某測(cè)量溫度下的參數(shù)值[26]。實(shí)踐中,一般用阿倫尼烏斯方程來建立參數(shù)與溫度間的函數(shù)關(guān)系:

PT=P(25℃)e{[T-25]E/[298R(273+T)]}

(32)

式中,T為測(cè)量溫度、P(T)為測(cè)量溫度T下的參數(shù)(Kc、Ko和Γ*)、P(25℃)為25℃下的參數(shù)、E為活化能、R為通用的氣體常數(shù)。

表1 25℃下的Kc、Ko、Γ*、E等參數(shù)[26]

經(jīng)過30多年的發(fā)展,FvCB模型已基本完善,并通過大量實(shí)驗(yàn)的驗(yàn)證。但在RuBP再生速率限制階段中,光合電子傳遞全為線性電子傳遞的假設(shè)及ATP需求的忽略[1]會(huì)影響FvCB模型及其參數(shù)估計(jì)的準(zhǔn)確性。由于gm的組分非常復(fù)雜,而FvCB模型把gm作為一個(gè)復(fù)合參數(shù)進(jìn)行估計(jì),這也會(huì)影響模型參數(shù)估計(jì)的準(zhǔn)確性。由于FvCB模型可以不進(jìn)行葉片離體實(shí)驗(yàn),而根據(jù)簡(jiǎn)單的氣體交換數(shù)據(jù)獲得葉片Vcmax、Jmax、Tp、gs、gm等光合生理生化信息[3],它在植物光合生理與環(huán)境因子相互關(guān)系的研究中有廣泛的應(yīng)用。接下來將對(duì)FvCB模型在葉片光合生理對(duì)環(huán)境因子響應(yīng)的應(yīng)用研究進(jìn)展進(jìn)行論述。

2 FvCB模型在葉片光合生理對(duì)環(huán)境因子響應(yīng)的應(yīng)用研究進(jìn)展

FvCB模型結(jié)合葉片Rubisco酶、細(xì)胞色素f(cyt f)和ATP合成酶等生理指標(biāo)可以揭示葉片活體光合系統(tǒng)對(duì)光照、CO2濃度、溫度、水分和N養(yǎng)分等環(huán)境因子變化的響應(yīng)機(jī)制,其中,Vcmax反映光合系統(tǒng)中Rubisco酶最大羧化能力、Jmax反映光合電子傳遞鏈的最大電子傳遞能力、Tp反映磷酸丙糖的合成能力、gs和gm反映CO2擴(kuò)散阻力對(duì)光合作用的限制。

2.1 光照

在不同環(huán)境光強(qiáng)下,葉片的形態(tài)和生化成分發(fā)生改變[27- 28],其光合機(jī)構(gòu)也會(huì)發(fā)生變化。大量研究表明陽(yáng)生葉片的細(xì)胞色素f(cyt f)、ATP合成酶、Rubisco酶等含量均大于陰生葉片[29-31],導(dǎo)致陽(yáng)生葉片Vcmax和Jmax參數(shù)均顯著大于陰生葉片[32-33],由此,陽(yáng)生葉片的光合能力顯著大于陰生葉片。Hanba等發(fā)現(xiàn)陽(yáng)生葉片的氣孔密度顯著大于陰生葉片[31],這可能導(dǎo)致陽(yáng)生葉片的gs顯著大于陰生葉片[32,34]。Piel等發(fā)現(xiàn)陽(yáng)生葉片gm有效路徑的長(zhǎng)度顯著小于陰生葉片[32]。另外,有研究表明槭樹和水青岡陽(yáng)生葉片中單位面積葉綠體暴露在胞間的面積(Sc)顯著大于陰生葉片[32,34]。這兩個(gè)因素可能導(dǎo)致陽(yáng)生葉片的gm顯著大于陰生葉片。

雖然瞬時(shí)光強(qiáng)對(duì)葉片Vcmax和Jmax參數(shù)沒有顯著影響,但對(duì)CO2擴(kuò)散阻力有顯著的影響。研究表明gs與瞬時(shí)光強(qiáng)呈正相關(guān)[35- 36]。有人認(rèn)為葉片光合系統(tǒng)與保衛(wèi)細(xì)胞之間可能存在信號(hào)傳遞[35- 36],使葉片可以通過改變氣孔的張開程度來平衡gs與光合速率的大小。有研究發(fā)現(xiàn)水稻[35]、桉樹[36]、煙草[22]和班克木[37]等葉片的gm隨測(cè)量光強(qiáng)增大而增加,但小麥和煙草葉片的gm在不同瞬時(shí)光強(qiáng)下保持穩(wěn)定不變[38-39]。Douthe等認(rèn)為不同物種光合特性的差異可能導(dǎo)致gm隨瞬時(shí)光強(qiáng)的響應(yīng)情況不同[36]。另外,水通道蛋白基因的表達(dá)速率隨光強(qiáng)的增加而加快[40- 41],而水通道蛋白的含量與CO2的跨膜轉(zhuǎn)運(yùn)過程直接相關(guān)[42- 43],所以,光強(qiáng)的瞬時(shí)變化可能通過調(diào)控水通道蛋白基因的表達(dá)來改變gm。

2.2CO2濃度

在長(zhǎng)期高CO2濃度下,植物葉片的結(jié)構(gòu)和成分會(huì)發(fā)生變化,從而影響其光合作用[44]。長(zhǎng)期高CO2濃度下生長(zhǎng)的植株葉片Rubisco酶含量及活性和葉片N含量均顯著小于正常CO2濃度下生長(zhǎng)的植株[45- 46]。而葉片N含量可以影響光合系統(tǒng)中Rubisco酶含量及活性、光捕獲組分、光合電子傳遞鏈組分的功能[47- 48],進(jìn)而影響葉片的光合能力,從而使得長(zhǎng)期高CO2濃度下生長(zhǎng)植株的葉片Vcmax[45]和Jmax[49- 50]等均顯著小于在正常CO2濃度下生長(zhǎng)的植株。長(zhǎng)期高CO2濃度下生長(zhǎng)的植株gs[44- 45]和gm[50]均小于在正常CO2濃度下生長(zhǎng)的植株。研究發(fā)現(xiàn)在長(zhǎng)期高CO2處理后,植株葉片的氣孔特性和表皮細(xì)胞密度發(fā)生改變[51- 52],這可能使得gs減小。Kürschner等發(fā)現(xiàn)長(zhǎng)期高CO2濃度下生長(zhǎng)的植株葉片厚度大于正常條件下生長(zhǎng)的植株[53],而葉片厚度的增大可能會(huì)增大gm的有效路徑,導(dǎo)致gm減小。

CO2濃度的短期變化不影響葉片的Vcmax和Jmax參數(shù),但對(duì)CO2擴(kuò)散阻力有顯著影響。Flexas等發(fā)現(xiàn)gs與CO2濃度的短期變化呈負(fù)相關(guān)關(guān)系[22],對(duì)此,一般有3種解釋：1)Hedrish等發(fā)現(xiàn)葉片質(zhì)外體中的pH值和膜電位[54]會(huì)隨CO2濃度的增加而發(fā)生改變,并伴隨著氣孔關(guān)閉,從而導(dǎo)致gs變小。2)CO2濃度的大小會(huì)影響葉肉細(xì)胞中蘋果酸的釋放,蘋果酸又可以調(diào)控保衛(wèi)細(xì)胞質(zhì)膜中陰離子的釋放代謝,從而調(diào)控氣孔行為[55]。3)CO2濃度變化還可能通過ATP調(diào)節(jié)機(jī)制[56]對(duì)gs進(jìn)行調(diào)控。大量研究表明gm與CO2濃度呈負(fù)相關(guān)關(guān)系[22,36,57],但也有部分研究表明gm與CO2濃度不相關(guān)[58- 59]。對(duì)此,一般有2種解釋：1)由于水通道蛋白基因的表達(dá)速率受CO2濃度變化的影響[60],gm對(duì)CO2濃度短期變化的快速響應(yīng)可能受水通道蛋白的調(diào)節(jié)。2)Sharkey發(fā)現(xiàn)葉綠體的變形可能會(huì)減小葉肉導(dǎo)度[61]。而且Tholen認(rèn)為葉綠體的移動(dòng)對(duì)gm有顯著的影響[62]。由此可知,葉綠體的行為可能與不同CO2濃度下gm的快速調(diào)控有關(guān)。

2.3 溫度

FvCB模型常用于研究溫度(不對(duì)植物產(chǎn)生損傷)對(duì)葉片光合系統(tǒng)內(nèi)在變化狀況的影響。有研究發(fā)現(xiàn)Rubisco酶羧化能力、光合電子傳遞能力和CO2擴(kuò)散過程均隨溫度的變化而改變。由于Rubisco酶及其激活酶活性均隨溫度的增加而增大(10—40℃),Kc、Ko、Vcmax會(huì)隨溫度的增加而增大[63- 64]。Bernacchi等用指數(shù)函數(shù)來描述Vcmax的溫度相關(guān)性[64]。光系統(tǒng)II(PSII)電子傳遞速率、光系統(tǒng)I(PSI)和光系統(tǒng)II(PSII)間的電子傳遞速率(質(zhì)體醌PQ和質(zhì)體藍(lán)素PC)[65- 66]和循環(huán)電子傳遞速率[63,67]均隨溫度的增加而增大,從而導(dǎo)致Jmax隨溫度的增加而增大[68]。溫度不僅影響葉片的光合能力,而且影響CO2的擴(kuò)散阻力。盡管gs與溫度變化不相關(guān)[69- 70],但gm會(huì)隨溫度的增加而增大[35,70]。Evans等認(rèn)為Sc、細(xì)胞壁厚度(Tcell_wall)、細(xì)胞液和葉綠體膜厚度等細(xì)胞結(jié)構(gòu)特點(diǎn)會(huì)影響gm[71],而von Caemmerer等發(fā)現(xiàn)CO2的質(zhì)膜滲透性和CO2擴(kuò)散的液相路徑長(zhǎng)度也受溫度的影響[70]。由此可以推測(cè)溫度可能通過改變?nèi)~肉細(xì)胞結(jié)構(gòu)來調(diào)控gm。另外,Kuwagata等發(fā)現(xiàn)水通道蛋白基因的表達(dá)速度會(huì)隨溫度的增加而增大[72],所以,溫度還可能通過控制水通道蛋白基因的表達(dá)速率來調(diào)控gm。

2.4 干旱或鹽脅迫

干旱和鹽脅迫可以直接導(dǎo)致植物缺水[73],進(jìn)而影響植物光合作用。有研究表明干旱或鹽脅迫盡管對(duì)Vcmax和Jmax參數(shù)沒有顯著影響[74- 75],但對(duì)CO2的擴(kuò)散阻力有顯著影響[76]。在干旱或鹽脅迫條件下,植株為減少蒸騰作用,葉片氣孔會(huì)關(guān)閉。另外,在鹽脅迫條件下,鹽離子會(huì)在葉片保衛(wèi)細(xì)胞內(nèi)積累進(jìn)而干擾氣孔功能[77- 78],導(dǎo)致氣孔關(guān)閉。因此,在干旱或鹽脅迫條件下植株葉片gs顯著小于在正常條件下生長(zhǎng)的植株[79- 80]。大量研究表明在干旱或鹽脅迫條件下生長(zhǎng)的植株葉片gm顯著小于在正常條件下生長(zhǎng)植株的葉片[74,79-80]。長(zhǎng)期干旱或鹽脅迫顯著減少了表皮細(xì)胞和葉肉細(xì)胞的斷面面積、寬度和半徑[81-82],從而使gm減小。另外,由于水通道蛋白基因的表達(dá)速率受干旱或鹽脅迫的影響[83],干旱或鹽脅迫可能通過控制水通道蛋白基因的表達(dá)來調(diào)控gm。

2.5 葉片N含量

由于葉肉細(xì)胞光合系統(tǒng)中的Rubisco酶、光捕獲組分(葉綠素和相關(guān)蛋白)和cty f等均含有大量的N元素[84],葉片N含量對(duì)光合作用有顯著影響。FvCB模型常用于研究葉片N含量對(duì)葉片光合系統(tǒng)內(nèi)在變化狀況的影響。大量研究表明葉片Rubisco酶含量與葉片N含量呈正相關(guān)[35,85],而葉片Rubisco酶含量及活性決定Vcmax的大小,從而使得葉片Rubisco酶的羧化能力與葉片N含量呈正相關(guān)[47- 48]。Nakano等發(fā)現(xiàn)葉綠素和Cyt f等含量均與葉片N含量呈正相關(guān)[47],從而使得Jmax與葉片N含量呈正相關(guān)[84,86]。葉片N含量不僅影響光合能力,還影響CO2的擴(kuò)散阻力。有研究表明葉片N含量與gs呈正相關(guān)[35]。雖然已知gs與氣孔特點(diǎn)(大小和密度)、氣孔張開程度有關(guān),但氣孔對(duì)葉片N含量變化的具體響應(yīng)機(jī)制還不清楚。大量研究表明葉片N含量與gm呈正相關(guān)[35,87]。大量研究表明Tcell_wall、單位葉面積葉肉細(xì)胞接觸胞間間隙的面積(Sm)、Sc等細(xì)胞結(jié)構(gòu)特點(diǎn)與gm直接相關(guān)[71,88]。Xiong等研究發(fā)現(xiàn)Sc會(huì)隨葉N含量的增加而增大[35]。Yong還發(fā)現(xiàn)葉綠體的尺寸與葉片N含量呈正相關(guān)[89]。由此可知,葉片N含量可能通過改變?nèi)~片結(jié)構(gòu)來調(diào)控gm。另外,由于葉片N含量的增加可以促進(jìn)水通道蛋白的基因表達(dá)[90],不同葉片N含量還可能通過控制水通道蛋白的表達(dá)來調(diào)控gm。

目前關(guān)于植物光合生理與環(huán)境因子的關(guān)系已有大量研究,但這些研究多停留在單因子水平,更沒有考慮相互作用的生物因素與環(huán)境因素協(xié)同作用對(duì)植物光合生理的影響。同時(shí),盡管人們已經(jīng)提出多種假設(shè)來解釋光合生理反應(yīng)隨環(huán)境因子變化的響應(yīng)機(jī)制,但尚缺乏直接的實(shí)驗(yàn)證據(jù)。因此,結(jié)合植物生理分子實(shí)驗(yàn)與FvCB模型進(jìn)行綜合分析是研究不同環(huán)境因子下植物光合生理響應(yīng)機(jī)制的有效途徑。

3 研究展望

FvCB模型光合參數(shù)的準(zhǔn)確估計(jì)不僅有利于正確理解植物光合生理對(duì)環(huán)境變化的響應(yīng)機(jī)理,而且可以更精確地估計(jì)作物產(chǎn)量和全球氣候變暖情況[91]。光合電子傳遞、碳反應(yīng)ATP需求和葉肉細(xì)胞內(nèi)CO2的具體擴(kuò)散路徑等方面的假設(shè)影響FvCB模型理論及參數(shù)估計(jì)的準(zhǔn)確性,從而制約相關(guān)領(lǐng)域的研究。此外,盡管科學(xué)家已經(jīng)開展了大量植物光合作用對(duì)環(huán)境條件變化響應(yīng)等方面的研究,但是,在光合作用對(duì)環(huán)境因子變化的響應(yīng)機(jī)制的研究中仍然存在很多問題。為此,未來需加強(qiáng)以下幾個(gè)方面的研究。

1)羧化速率與光合電子傳遞速率間的聯(lián)系

羧化速率與光合電子傳遞速率間的聯(lián)系直接影響到RuBP再生速率限制階段的子模型,進(jìn)而影響Jmax、Vcmax、Tp、Rd、gm等參數(shù)估計(jì)的準(zhǔn)確性。在RuBP再生速率限制階段,Farquhar等忽略了假電子傳遞、循環(huán)電子傳遞以及碳反應(yīng)的ATP需求,并根據(jù)碳反應(yīng)的NADPH消耗速率與J相等來獲得RuBP再生速率限制階段的子模型[1]。盡管人們已經(jīng)開展J和NADPH/ATP生成速率等相關(guān)的研究[92],但J與ATP生成速率之間的關(guān)系比較復(fù)雜,目前仍未研究清楚[2]。因此,未來應(yīng)該加強(qiáng)光合電子傳遞、NADPH/ATP代謝化學(xué)計(jì)量學(xué)等方面研究,以正確地建立Vc與J間的聯(lián)系。

2)葉肉細(xì)胞內(nèi)CO2的擴(kuò)散阻力

Sun等發(fā)現(xiàn)gm對(duì)Vcmax、Jmax、Tp等的參數(shù)估計(jì)有很大的影響[93]。目前,人們認(rèn)為細(xì)胞壁、細(xì)胞膜、細(xì)胞質(zhì)、葉綠體膜和葉綠體基質(zhì)均對(duì)CO2的擴(kuò)散有限制作用。并且,線粒體呼吸作用和光呼吸釋放的CO2有一部分會(huì)被光合作用重新固定。這部分CO2需通過線粒體膜、細(xì)胞質(zhì)、葉綠體膜和葉綠體基質(zhì)最終到達(dá)Rubisco酶羧化位點(diǎn)[94],這使得CO2的擴(kuò)散路徑變得非常復(fù)雜。目前,有研究已經(jīng)把gm區(qū)分為細(xì)胞壁阻力和葉綠體膜阻力[12,95],但gm各組分的估計(jì)還有待進(jìn)一步的研究。因此,未來可以結(jié)合細(xì)胞顯微結(jié)構(gòu)觀察、葉肉細(xì)胞內(nèi)CO2擴(kuò)散同位素跟蹤技術(shù)和FvCB模型對(duì)gm各組分的參數(shù)估計(jì)進(jìn)行研究。

3)gs和gm對(duì)環(huán)境因子變化的具體調(diào)控機(jī)制

大量研究表明gs[34,36,70,79]和gm[31,35,36,39]隨環(huán)境因子的變化而改變。盡管人們已經(jīng)提出多種與氣孔相關(guān)的調(diào)控機(jī)制,但其具體調(diào)控機(jī)制還不清楚,未來的研究可結(jié)合氣孔調(diào)控相關(guān)生理指標(biāo)測(cè)量、光合機(jī)構(gòu)信號(hào)傳導(dǎo)和FvCB模型等3方面的實(shí)驗(yàn)對(duì)氣孔的調(diào)控機(jī)制進(jìn)行深入研究。目前,人們已經(jīng)提出細(xì)胞結(jié)構(gòu)特點(diǎn)、水通道蛋白和葉綠體行為等幾種假說來解釋gm的調(diào)控機(jī)制,但缺乏直接的實(shí)驗(yàn)證據(jù)。因此,未來的研究要重點(diǎn)研究不同環(huán)境條件下葉片細(xì)胞結(jié)構(gòu)、水通道蛋白代謝和葉綠體行為的改變情況,并結(jié)合FvCB模型來研究gm的調(diào)控機(jī)制。而gm的相關(guān)研究需多領(lǐng)域的科學(xué)家共同參與

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Advancesinphoto-physiologicalresponsesofleavestoenvironmentalfactorsbasedontheFvCBmodel

TANG Xinglin1,2, CAO Yonghui1,2, GU Lianhong3, ZHOU Benzhi1,2,*

1ResearchInstituteofSubtropicalForestry,ChineseAcademyofForestry,Hangzhou311400,China2QianjiangyuanForestEcosystemResearchStation,StateForestryAdministration,Hangzhou311400,China3EnvironmentalSciencesDivision,OakRidgeNationalLaboratory,OakRidge,TN37831,USA

Biochemical models of leaf photosynthesis are invaluable tools for exploring the photo-physiological responses to environmental factors and identify potential targets to improve the efficiency of CO2fixation. The FvCB model can be used to fit CO2response curves developed under different environmental conditions and predict underlying photosynthetic biochemistry. However, to do this successfully it is important to improve chloroplast electron transport modeling, and gain a better understanding of internal CO2diffusion limitations and elucidate the mechanisms of stomatal (gs) and mesophyll (gm) conductance responses to environmental factors. The FvCB model and its application in determining the photo-physiological responses to environmental factors, such as light, CO2, water, temperature, and N nutrition have been reviewed in this paper. To improve the veracity of the parameter estimations and reveal the mechanism of photo-physiological responses to environmental factors, the following studies should be emphasized in the future: 1) the relationship between the carboxylation rate of Rubisco and chloroplast electron transport rate; 2) the CO2diffusion limitations in mesophyll cells and its effect on parameter estimations; and 3) the regulation ofgsandgmresponses to different environmental conditions.

C3plants; photosynthesis; FvCB model; photosynthetic physiology; environmental factors

國(guó)家林業(yè)局948項(xiàng)目(2014- 4- 57)；浙江省自然科學(xué)基金項(xiàng)目(LY13C160002)；中央級(jí)公益性科研院所基本科研業(yè)務(wù)費(fèi)專項(xiàng)資金(RISF2013002)資助

2016- 07- 16; < class="emphasis_bold">網(wǎng)絡(luò)出版日期

日期:2017- 05- 27

*通訊作者Corresponding author.E-mail: benzhi_zhou@126.com

10.5846/stxb201607161450

唐星林,曹永慧,顧連宏,周本智.基于FvCB模型的葉片光合生理對(duì)環(huán)境因子的響應(yīng)研究進(jìn)展.生態(tài)學(xué)報(bào),2017,37(19):6633- 6645.

Tang X L, Cao Y H, Gu L H, Zhou B Z.Advances in photo-physiological responses of leaves to environmental factors based on the FvCB model.Acta Ecologica Sinica,2017,37(19):6633- 6645.