高靜，陳吉玉，譚先明，吳雨珊，楊文鈺，楊峰

光強(qiáng)對(duì)苗期大豆葉片水力導(dǎo)度及葉脈性狀的影響

高靜，陳吉玉，譚先明，吳雨珊，楊文鈺，楊峰

四川農(nóng)業(yè)大學(xué)農(nóng)學(xué)院/農(nóng)業(yè)農(nóng)村部西南作物生理生態(tài)與耕作重點(diǎn)實(shí)驗(yàn)室/四川省作物帶狀復(fù)合種植工程技術(shù)研究中心，成都 611130

【目的】探究光強(qiáng)對(duì)苗期大豆葉片水力導(dǎo)度、光合特性和葉水勢(shì)的影響，分析葉脈性狀對(duì)不同生長光強(qiáng)的適應(yīng)機(jī)制，為提高大豆光能利用提供理論支撐。【方法】選用強(qiáng)耐蔭型的大豆品種南豆12和弱耐蔭型的大豆品種桂夏7為試驗(yàn)材料，在人工氣候室進(jìn)行盆栽試驗(yàn)，設(shè)置高光強(qiáng)（（424.47±12.32）μmol·m-2·s-1，HL）、中光強(qiáng)（（162.52±20.31）μmol·m-2·s-1，ML）和低光強(qiáng)（（93.93±9.87）μmol·m-2·s-1，LL）處理。在處理20 d后研究不同生長光強(qiáng)對(duì)苗期大豆葉片水力導(dǎo)度、光合參數(shù)、葉片水勢(shì)及葉脈性狀的影響?！窘Y(jié)果】相對(duì)于高光強(qiáng)處理，低光強(qiáng)處理下南豆12和桂夏7的葉片水力導(dǎo)度顯著降低，南豆12的葉片水力導(dǎo)度在3個(gè)處理下均顯著高于桂夏7。與高光強(qiáng)處理相比，在中、低光強(qiáng)處理下南豆12的葉片水力導(dǎo)度分別降低7.56%和21.24%，氣孔導(dǎo)度分別降低43.96%和58.89%，凈光合速率分別降低29.44%和46.49%。同樣，桂夏7的葉片水力導(dǎo)度分別降低42.16%和23.71%，氣孔導(dǎo)度分別降低54.55%和45.79%，凈光合速率分別降低37.03%和42.06%。南豆12和桂夏7的葉片水勢(shì)在處理間均無顯著差異。大豆的葉片水力導(dǎo)度與氣孔導(dǎo)度在3個(gè)光強(qiáng)處理下均達(dá)到極顯著正相關(guān)（＜0.01），隨著光強(qiáng)的降低，葉片水力導(dǎo)度與凈光合速率呈顯著正相關(guān)（＜0.05），與氣孔導(dǎo)度呈極顯著正相關(guān)（＜0.01）。對(duì)于葉脈結(jié)構(gòu)，與高光強(qiáng)相比，中、低光強(qiáng)處理下兩個(gè)大豆品種的小葉脈密度以及主葉脈和小葉脈的木質(zhì)部導(dǎo)管面積均顯著降低，且南豆12的小葉脈密度和主葉脈木質(zhì)部導(dǎo)管面積在中、低光強(qiáng)處理下均顯著高于桂夏7。南豆12的主葉脈密度在處理間無顯著變化，桂夏7的主葉脈密度在中、低光強(qiáng)處理下較高光強(qiáng)顯著降低11.4%和15.0%。光強(qiáng)降低顯著增長了葉脈到氣孔的距離。南豆12在中、低光強(qiáng)處理下葉脈到氣孔的距離較高光強(qiáng)增長21.33%和60.01%，桂夏7葉脈到氣孔的距離增長31.50%和53.59%。相關(guān)性分析表明，大豆葉片水力導(dǎo)度與小葉脈密度、主葉脈和小葉脈的木質(zhì)部導(dǎo)管面積呈顯著正相關(guān)（＜0.05），與葉脈到氣孔的距離呈極顯著負(fù)相關(guān)（＜0.01）?！窘Y(jié)論】光強(qiáng)會(huì)通過調(diào)控大豆葉脈結(jié)構(gòu)影響葉片水力導(dǎo)度，弱光降低大豆葉片水力導(dǎo)度，但葉片水力導(dǎo)度和氣孔導(dǎo)度保持協(xié)調(diào)，維持葉片水分供需平衡。弱光下具有較高的葉脈密度能夠縮短水分運(yùn)輸?shù)木嚯x，保證較好的葉片水分供應(yīng)能力，從而有利于CO2的擴(kuò)散和光合作用，這是耐蔭型大豆適應(yīng)弱光環(huán)境的又一策略。

大豆；葉片水力導(dǎo)度；氣孔導(dǎo)度；光強(qiáng)；葉脈

0 引言

【研究意義】大豆是我國重要的糧、油、飼兼用作物，營養(yǎng)價(jià)值高。近年來大豆消費(fèi)呈剛性增長，但大豆種植面積和產(chǎn)量已連續(xù)多年下降，供需嚴(yán)重失衡[1]。在農(nóng)業(yè)生產(chǎn)中常采用密植或間套作提高大豆種植面積，但不同種植模式直接影響大豆冠層光照強(qiáng)度。弱光下大豆的形態(tài)建成和光合能力均會(huì)受到抑制，極大地限制了產(chǎn)量增加和品質(zhì)提升[2]。光合作用是干物質(zhì)積累和形態(tài)建成的基礎(chǔ)生理過程[3]。在C3植物中，CO2的擴(kuò)散已被證明是非生物脅迫下降低光合速率主要的限制因子[4-6]。葉片水力導(dǎo)度（leaf）是指液態(tài)水通過葉片運(yùn)輸?shù)男?，是光合速率的重要制約因素。當(dāng)葉片張開氣孔以吸收CO2進(jìn)行光合作用時(shí)，不可避免地引起水分的散失導(dǎo)致葉肉細(xì)胞的干燥。植物面臨著平衡最大限度吸收CO2和最小限度散失水分的挑戰(zhàn)。水分由葉脈運(yùn)輸澆灌容易干燥的葉肉細(xì)胞，較好的水分供給能力能夠保持氣孔最大程度的開放，確保胞間CO2的穩(wěn)定供應(yīng)[7]。這表明氣孔保持開放進(jìn)行光合作用的能力取決于葉片中水分供給能力，也就是leaf。氣孔導(dǎo)度（s）和leaf的協(xié)同對(duì)維持葉片水分的供需平衡，保證葉片組織進(jìn)行正常的生理生化功能具有重要意義[8]?！厩叭搜芯窟M(jìn)展】光強(qiáng)已被證明會(huì)影響大豆光合特性和leaf。弱光會(huì)導(dǎo)致大豆凈光合速率和氣孔導(dǎo)度不同程度的降低、改變柵欄細(xì)胞排列和形狀、增加類囊體的垛疊程度從而增加光捕獲[9-10]。由黑暗到光亮的轉(zhuǎn)變?cè)黾恿撕颂?、向日葵和水稻的leaf[11-13]。但是紅橡樹和銀杏的leaf對(duì)光強(qiáng)的變化不敏感[13-14]。在杉樹中，冠層上部的葉片比下部的葉片具有更高的leaf和s[15]。與喜蔭樹種相比，喜陽樹種具有較高的leaf[16-17]。leaf和光合作用的關(guān)系也是近幾年的研究熱點(diǎn)。在高溫、鉀肥和干旱處理下均觀察到leaf和s之間良好的相關(guān)性[18-20]。葉脈性狀與植物的光合碳固定、水分運(yùn)輸直接或間接相關(guān)，是植物適應(yīng)逆境脅迫的關(guān)鍵因素[21-22]。已有的研究揭示了高溫和干旱下葉脈結(jié)構(gòu)的適應(yīng)機(jī)制。李曉鵬[23]研究表明，棗的葉脈密度隨溫度升高而增加。為了適應(yīng)高溫下的高蒸騰速率，葉脈系統(tǒng)必須增加水分運(yùn)輸?shù)牧颗c速率，因此要匹配較高的葉脈密度。在干旱脅迫下水稻葉片的葉脈密度顯著增加[24]，較高的葉脈密度能夠在干旱造成木質(zhì)部栓塞時(shí)，通過在栓塞周圍運(yùn)輸水以保證單位面積內(nèi)的水分充足和高光合效率?！颈狙芯壳腥朦c(diǎn)】以往研究中對(duì)于大豆光合特性對(duì)光強(qiáng)的響應(yīng)主要集中在光反應(yīng)，但圍繞光合作用中CO2擴(kuò)散過程對(duì)弱光的響應(yīng)相對(duì)欠缺。關(guān)于leaf和s相關(guān)關(guān)系的研究主要集中在飽和光條件下，而leaf和s在不同光強(qiáng)下是否協(xié)同變化尚不明確。此外，大豆葉脈性狀對(duì)不同光強(qiáng)適應(yīng)的研究也鮮見報(bào)道?！緮M解決的關(guān)鍵問題】以大豆為研究對(duì)象，闡明光強(qiáng)對(duì)大豆葉片水力導(dǎo)度、葉片水勢(shì)及光合特性的影響。探討leaf和s在不同光強(qiáng)處理下的協(xié)調(diào)關(guān)系，并從影響leaf的葉脈性狀出發(fā)闡明光強(qiáng)對(duì)光合作用的調(diào)控機(jī)理。研究大豆葉片水分運(yùn)輸能力對(duì)光強(qiáng)的響應(yīng)，為提高光合能力提供理論依據(jù)。

1 材料與方法

試驗(yàn)于2022年在四川農(nóng)業(yè)大學(xué)完成。

1.1 試驗(yàn)材料

根據(jù)武曉玲等[25]不同耐蔭型大豆品種篩選的結(jié)果，選擇強(qiáng)耐蔭型的南豆12（四川省南充市農(nóng)業(yè)科學(xué)院）和弱耐蔭型的桂夏7（廣西農(nóng)業(yè)科學(xué)院）作為試驗(yàn)材料。

1.2 試驗(yàn)設(shè)計(jì)

試驗(yàn)在四川農(nóng)業(yè)大學(xué)成都校區(qū)人工氣候室中進(jìn)行。氣候室相對(duì)濕度保持在55%，白天溫度25 ℃，晚上22 ℃，光周期設(shè)置為12 h光照/12 h黑暗。挑選均勻飽滿的大豆種子種植于PINDSTRUP營養(yǎng)土﹕蛭石體積比3﹕1的基質(zhì)中，每盆1株，每個(gè)處理10盆。出苗后2 d開始對(duì)大豆幼苗進(jìn)行光強(qiáng)處理。通過使用具有不同透光性能的黑色遮陽網(wǎng)，參照陳吉玉等[26]的研究設(shè)置以下處理：高光強(qiáng)（HL）、中光強(qiáng)（ML）和低光強(qiáng)（LL），光環(huán)境數(shù)據(jù)見表1。光強(qiáng)處理20 d后，選擇每個(gè)處理大豆的倒數(shù)第二片完全展開葉進(jìn)行相關(guān)指標(biāo)的測定。

1.3 項(xiàng)目測定與方法

1.3.2 光合參數(shù) 上午9：00—11：00，選擇每個(gè)處理下長勢(shì)一致的6株大豆進(jìn)行測試。使用Li-Cor公司的Li-6400光合儀，在葉室內(nèi)，光強(qiáng)設(shè)置為500 μmol·m-2·s-1，CO2濃度調(diào)節(jié)至400 μmol CO2·mol-1。測量過程中溫度設(shè)置為25 ℃。達(dá)到穩(wěn)定狀態(tài)后，記錄凈光合速率（n）、s、胞間CO2濃度（i）和[18]。取3次測量的平均值作為最終測量值。

1.3.3 氣孔密度 在光合參數(shù)測量完成后，從每片葉子的頂部、中部和底部分別切下1 cm2的樣品，并在固定液中固定保存24 h。所有樣品均在當(dāng)天的同一時(shí)間采集。葉片樣品分別在5種梯度乙醇溶液（50%、70%、85%、95%和100%）中各脫色30 min，然后置于5% NaOH（w/v）水溶液中2 d。葉片樣品用蒸餾水清洗后在三氯乙醛水合物中固定和清除，直到葉子半透明。最后，使用1%亞甲基藍(lán)溶液染色后置于載玻片上[28]。利用尼康公司的TI-E型倒置熒光顯微鏡在10倍放大倍率下拍攝每個(gè)樣品3張圖像。用ImageJ軟件測定氣孔密度。

1.3.4 葉脈密度 將一級(jí)、二級(jí)和三級(jí)葉脈歸為主葉脈，四級(jí)及以上歸為小葉脈[29]。使用尼康公司D5600相機(jī)拍攝完全展開的大豆葉片，以量化其面積和主葉脈長度。小葉脈用尼康公司的TI-E型倒置熒光顯微鏡在10倍放大倍率下觀察和拍照[30]。采用ImageJ軟件分別測定小葉脈密度和主葉脈密度。

1.3.6 葉脈解剖結(jié)構(gòu) 用手術(shù)刀沿大豆葉片主葉脈中部取樣（0.5 cm×2 cm左右），固定液固定24 h，固定后的葉脈組織用梯度酒精進(jìn)行脫水。將組織于包埋機(jī)內(nèi)進(jìn)行包埋，隨后用石蠟切片機(jī)切片。石蠟切片用環(huán)保型脫蠟透明液脫蠟后用甲苯胺藍(lán)染色液進(jìn)行染色，切片入干凈的二甲苯透明5 min，中性樹膠封片。在尼康公司Eclipse E100顯微鏡下觀察，采用NikonS-U3成像系統(tǒng)獲取圖片。使用ImageJ軟件分別測量小葉脈木質(zhì)部導(dǎo)管面積（Xminor）和主葉脈木質(zhì)部導(dǎo)管面積（Xmajor）。

1.4 數(shù)據(jù)處理與分析

采用Microsoft Excel 2016軟件整理數(shù)據(jù)并作圖，SPSS 25.0軟件進(jìn)行方差分析和相關(guān)性分析。

2 結(jié)果

2.1 光強(qiáng)對(duì)大豆葉片水力導(dǎo)度及葉片水勢(shì)的影響

隨著光強(qiáng)降低，兩個(gè)大豆品種的leaf具有相同的變化趨勢(shì)。在中、低光強(qiáng)處理下，南豆12和桂夏7的leaf較高光強(qiáng)處理分別降低7.56%和21.24%（中光強(qiáng)和低光強(qiáng)，下同）、42.16%和23.71%。在相同處理下，南豆12的leaf均高于桂夏7。與高光強(qiáng)相比，中、低光強(qiáng)處理對(duì)兩個(gè)大豆品種的Ψleaf影響均不顯著。在高、低光強(qiáng)處理下兩個(gè)品種間Ψleaf無顯著差異，在中光強(qiáng)處理下桂夏7的Ψleaf顯著低于南豆12（圖1）。

柱上不同小寫字母表示各處理間差異顯著（P＜0.05）Different lowercases on the bars represent significantly different atp＜0.05 level

2.2 光照對(duì)大豆葉片光合特性的影響

由表2可知，弱光降低了大豆葉片s、n和。與高光強(qiáng)相比，中、低光強(qiáng)處理下南豆12的s顯著降低43.96%和58.89%，桂夏7顯著降低54.55%和45.79%；南豆12葉片的n顯著降低29.44%和46.49%，桂夏7顯著下降37.03%和42.06%；南豆12的顯著降低16.95%和27.56%，桂夏7顯著下降30.99%和31.44%。在相同光強(qiáng)處理下，南豆12和桂夏7的n無顯著差異，但在高、中光強(qiáng)處理下南豆12的s均顯著高于桂夏7。南豆12和桂夏7的i在處理間無顯著差異。在中、低光強(qiáng)處理下，南豆12的i均顯著高于桂夏7，在中光強(qiáng)處理下，南豆12的顯著高于桂夏7。隨著光強(qiáng)的降低，兩個(gè)大豆品種的leaf/s均顯著增加。在中、低光強(qiáng)處理下，兩大豆品種的leaf/s較高光強(qiáng)增加112.38%和93.86%、27.58%和57.70%。

表2 光強(qiáng)對(duì)大豆葉片光合特性的影響

同列數(shù)據(jù)后不同小寫字母表示在0.05水平差異顯著。表3同

Different lowercases after the data in the same column indicate significant differences at 0.05 level. The same as Table 3

2.3 大豆葉片水力導(dǎo)度與氣孔導(dǎo)度、凈光合速率的相關(guān)性分析

3個(gè)光強(qiáng)處理下s與leaf均呈極顯著正相關(guān)，且相關(guān)性較強(qiáng)，相關(guān)性系數(shù)分別為0.96、0.93和0.99，因此leaf的變化能夠部分解釋s在不同光強(qiáng)下發(fā)生變化的原因。在3個(gè)光強(qiáng)處理下，leaf與n之間均無顯著的相關(guān)關(guān)系（圖2-A）。Pearson相關(guān)性分析表明，在光強(qiáng)處理間leaf與s呈極顯著正相關(guān)，與n呈顯著正相關(guān)，相關(guān)性系數(shù)分別為0.83和0.44（圖2-B）。

2.4 光強(qiáng)對(duì)大豆葉脈性狀和氣孔密度的影響

由表3可知，南豆12的VLAmajor在處理間無顯著差異，桂夏7的VLAmajor在中、低光強(qiáng)處理下較高光強(qiáng)顯著降低11.4%和15.0%。與高光強(qiáng)相比，中、低光強(qiáng)處理下大豆小葉脈變細(xì)、變少（圖3-A），且隨著光強(qiáng)的降低，VLAminor降低的幅度增加。南豆12在中、低光強(qiáng)下的VLAminor較高光強(qiáng)分別降低34.56%和38.88%，桂夏7在中、低光強(qiáng)下的VLAminor較高光強(qiáng)分別降低26.42%和37.09%（表3）。由圖3-B可知，弱光下大豆氣孔密度均降低，且隨著光強(qiáng)的降低變化幅度增加。與高光強(qiáng)相比，中、低光強(qiáng)處理下南豆12的氣孔密度分別降低38.29%和49.11%，桂夏7分別顯著降低33.35%和50.49%。在3個(gè)光強(qiáng)處理下，南豆12的氣孔密度均高于桂夏7，且在高、低光強(qiáng)處理下達(dá)到顯著水平（表3）。由圖3-C可知，弱光處理增長了Dm。與高光強(qiáng)相比，南豆12在中、低光強(qiáng)處理下Dm分別增長21.33%和60.01%，桂夏7的Dm分別增長31.50%和53.59%。光強(qiáng)降低減少了大豆葉片木質(zhì)部導(dǎo)管大?。▓D3-D）。南豆12在中、低光強(qiáng)處理下的Xmajor、Xminor較高光強(qiáng)分別顯著降低18.33%和32.29%、14.12%和27.09%。桂夏7在中、低光強(qiáng)處理下的Xmajor、Xminor較高光強(qiáng)分別顯著降低19.36%和34.67%、25.77%和42.79%。

表3 光強(qiáng)對(duì)大豆葉脈結(jié)構(gòu)和氣孔密度的影響

A：不同光強(qiáng)處理下葉片水力導(dǎo)度和光合參數(shù)相關(guān)性分析the correlation analysis of leaf hydraulic conductivity and photosynthetic parameters under different light intensity treatments；B：處理間葉片水力導(dǎo)度和光合參數(shù)的相關(guān)性分析The correlation analysis of leaf hydraulic conductivity and photosynthetic parameters among treatments

2.5 大豆葉片水力導(dǎo)度和葉脈性狀的相關(guān)性分析

leaf與VLAminor呈顯著正相關(guān)，與Dm極顯著負(fù)相關(guān)，且相關(guān)性較強(qiáng)，相關(guān)性系數(shù)分別為0.73和0.80。leaf與Xmajor、Xminor呈顯著正相關(guān)，相關(guān)性系數(shù)分別為0.68和0.69。這表明隨著光強(qiáng)的降低，Xmajor、Xminor和VLAminor的變化均會(huì)顯著影響leaf。VLAminor與Dm呈極顯著負(fù)相關(guān)，相關(guān)性系數(shù)達(dá)到了0.91。大豆氣孔密度與leaf呈顯著正相關(guān)。此外，大豆氣孔密度與VLAminor極顯著正相關(guān)，與Dm極顯著負(fù)相關(guān)，相關(guān)性系數(shù)分別為0.97和0.93（表4）。

A：葉脈密度Leaf vein density；B：氣孔密度Stomatal density；C：小葉脈解剖結(jié)構(gòu)，V、sto和Dm分別代表葉脈、氣孔、葉脈到氣孔的距離Minor leaf vein anatomy, V, sto and Dm represent leaf veins, stomata and distance from leaf veins to stomata, respectively；D：主葉脈解剖結(jié)構(gòu)，Xy代表木質(zhì)部導(dǎo)管Major leaf vein anatomy, Xy represents xylem conduit

表4 大豆葉片水力導(dǎo)度與葉脈性狀的相關(guān)性分析

*：在0.05水平上差異顯著significant difference at 0.05 level；**：在0.01水平上差異顯著significant difference at 0.01 level；***：在0.001水平上差異顯著significant difference at 0.001 level

3 討論

3.1 光強(qiáng)對(duì)大豆葉片水力導(dǎo)度的影響

葉片是植物水力系統(tǒng)的瓶頸，大約貢獻(xiàn)了整個(gè)植株中30%—98%的水分運(yùn)輸阻力[31]。與根系和莖稈相比，葉片更容易受到環(huán)境的影響，因此是限制氣體交換和干物質(zhì)形成的根本因素[32]。與高光強(qiáng)相比，低光強(qiáng)處理顯著降低了兩個(gè)大豆品種的leaf，但中光強(qiáng)處理下南豆12的leaf變化不顯著（圖1）。Raimondo等[33]指出橄欖樹的leaf不受冠層光強(qiáng)影響，leaf對(duì)光強(qiáng)缺乏差異可能是由于陰生葉冠層的光強(qiáng)仍較高（約1 000 μmol·m-2·s-1）。因此，只有在較低光強(qiáng)下才可能引起leaf的顯著變化。與高光強(qiáng)相比，中光強(qiáng)下南豆12和桂夏7的leaf分別減少7.56%和42.16%（圖1），可見不同大豆品種的leaf對(duì)光強(qiáng)的響應(yīng)具有不同的可塑性。南豆12的leaf在3個(gè)光強(qiáng)下均顯著高于桂夏7，因此南豆12在苗期具有更高的耐旱性，桂夏7的光合作用則更容易受到葉片干燥的影響。這種差異可能與兩個(gè)品種的種植區(qū)域相關(guān)，南豆12適宜在四川夏季套作種植，正逢夏季干旱[34]，而桂夏7適宜在廣西作夏大豆種植，種植區(qū)域氣候濕熱，雨水豐沛[35]。

3.2 大豆葉片水力導(dǎo)度和氣孔導(dǎo)度協(xié)調(diào)變化以適應(yīng)不同的生長光強(qiáng)

弱光對(duì)大豆光合特性有很大影響[9,36]。與前人的研究結(jié)果相同，低光強(qiáng)處理顯著降低了大豆葉片的n、和s（表2）。leaf和s之間的聯(lián)系是近年來的研究熱點(diǎn)。在較高的生長光強(qiáng)下，夏威夷半邊蓮的leaf顯著增加，而s無顯著變化[37]。而在白樺樹冠層上部的陽生葉到冠層下部的陰生葉中，leaf和s協(xié)同變化[38]。本研究結(jié)果與Sellin等[38]的結(jié)果一致，leaf和s在不同光強(qiáng)處理下均呈極顯著相關(guān)，而且隨著光強(qiáng)的降低，leaf和s緊密協(xié)調(diào)（圖2），因此leaf是決定大豆CO2擴(kuò)散的重要限制因子。而本研究結(jié)果與Scoffoni等[37]結(jié)果的差異可能與物種分布有關(guān)。夏威夷半邊蓮來自干旱地區(qū)，相比于s，leaf具有更高的可塑性讓植物能夠在不關(guān)閉氣孔的情況下耐受短暫的土壤干旱[15]。leaf/s代表了葉片水分供應(yīng)與需求的比值。與高光強(qiáng)相比，中、低光強(qiáng)處理下兩個(gè)大豆的leaf/s顯著增加，表明弱光下大豆葉片對(duì)水分需求的減少與水分供應(yīng)的減少相匹配。氣孔是leaf和s的共同影響因素，氣孔保持開放的能力取決于葉片中穩(wěn)定的水分狀態(tài)，當(dāng)Ψleaf降至一定的閾值后，就會(huì)引起氣孔關(guān)閉，防止水分進(jìn)一步散失[39]。Sellin等[40]報(bào)道陰生葉的Ψleaf低于陽生葉。在本研究中，Ψleaf在不同生長光強(qiáng)下無顯著差異（圖1），表明弱光并沒有通過影響大豆葉片的水分供應(yīng)情況來降低葉片水勢(shì)進(jìn)而影響葉片氣孔的張開程度。

3.3 大豆葉脈性狀對(duì)不同光強(qiáng)的適應(yīng)機(jī)制

leaf對(duì)光強(qiáng)的長期適應(yīng)主要由于葉片結(jié)構(gòu)變化[29]。對(duì)雙子葉植物大豆而言，水分從葉柄進(jìn)入葉片后，首先通過主脈依次向下一級(jí)葉脈運(yùn)輸，并通過維管束鞘向葉肉細(xì)胞擴(kuò)散[41]。由于木質(zhì)部導(dǎo)管的存在，葉脈被直觀地認(rèn)為是leaf的決定因素[21]。一個(gè)高效、流暢的木質(zhì)部導(dǎo)管對(duì)葉片水分運(yùn)輸非常重要[42]。同一物種的陽生葉比陰生葉具有更大的木質(zhì)部導(dǎo)管[15]，這與本研究結(jié)果一致（表3）。此外，相關(guān)性分析表明木質(zhì)部導(dǎo)管大小與leaf顯著正相關(guān)，leaf隨著Xminor、Xmajor的減少而減少（表4）。North等通過Hagen-Poiseuille方程模擬計(jì)算葉脈內(nèi)水力導(dǎo)度，在該方程中，葉脈內(nèi)水力導(dǎo)度與木質(zhì)部導(dǎo)管直徑的四次方成正比[43]。因此木質(zhì)部導(dǎo)管的微小減少會(huì)被放大，并影響葉片整體水分輸送。

葉脈結(jié)構(gòu)的等級(jí)特征導(dǎo)致不同級(jí)別的葉脈承擔(dān)不同功能。主脈支撐和最大化鋪開葉片，小葉脈以其顯著的高密度為水轉(zhuǎn)移至葉肉提供更大的接觸面積[44]。在低光強(qiáng)處理下，大豆葉片維管組織發(fā)育不良，小葉脈變少、變細(xì)（圖3-A），這與李盛藍(lán)[30]、韓玲等[45]的研究結(jié)果一致。本研究中光強(qiáng)對(duì)葉脈密度的影響主要?dú)w因于小葉脈的變化，隨著光強(qiáng)的降低南豆12和桂夏7的VLAminor顯著減少，而南豆12的VLAmajor無顯著變化（表3）。葉脈和氣孔是葉片水分供給和散失的基礎(chǔ)結(jié)構(gòu)，二者的數(shù)量和形態(tài)深刻影響葉片水分利用率和碳同化[46]。本研究結(jié)果表明隨著光強(qiáng)的降低大豆葉脈密度和氣孔密度呈極顯著正相關(guān)（表4）。葉脈密度和氣孔密度的協(xié)同變化體現(xiàn)了氣孔蒸騰對(duì)水分需求和葉脈結(jié)構(gòu)對(duì)水分供給的相互匹配[47]。因此大豆葉脈密度與氣孔密度隨著光環(huán)境的不同協(xié)同變化，以適應(yīng)不同的光強(qiáng)環(huán)境。

在喜蔭和喜陽樹種中，leaf與葉脈密度正相關(guān)[15]。本研究結(jié)果與之一致，VLAminor與leaf呈顯著正相關(guān)（表4）。Xiong等[48]研究發(fā)現(xiàn)，在11個(gè)栽培和野生水稻品種中，leaf與葉脈密度不相關(guān)；North等[43]報(bào)道在低輻照度下生長的鳳梨葉脈密度降低，但leaf與葉脈密度不相關(guān)。本研究結(jié)果與這些報(bào)道矛盾的原因可能是水稻和鳳梨均屬于單子葉植物，單子葉植物氣孔總是聚集在平行脈兩側(cè)，這意味著葉脈密度的降低對(duì)葉片水分運(yùn)輸，尤其是葉脈外水分運(yùn)輸?shù)挠绊懣梢院雎圆挥?jì)。與葉脈木質(zhì)部的水力阻力相比，葉肉組織對(duì)水流的阻力非常大[15]，Brodribb等[21]采集了43種植物（包括苔蘚植物和被子植物），證明Dm與葉片水分運(yùn)輸阻力之間的強(qiáng)相關(guān)性，該水力阻力主要由葉脈密度所驅(qū)動(dòng)。因此，水分從葉脈末端流經(jīng)葉肉組織至氣孔蒸發(fā)這段距離，深刻影響著leaf。這與本研究中Dm與leaf呈顯著負(fù)相關(guān)吻合，兩者的相關(guān)性由VLAminor驅(qū)動(dòng)（表4）。因此，弱光下大豆VLAminor的降低不僅減少了水分轉(zhuǎn)移至葉肉的接觸面積，而且增加了水分從葉脈到表皮蒸發(fā)部位的輸送距離，從而增加了葉片水分運(yùn)輸?shù)淖枇?，?dǎo)致leaf降低。前人對(duì)弱光下不同大豆品種光合能力評(píng)價(jià)的結(jié)果表明南豆12具有較強(qiáng)的耐蔭性，屬于強(qiáng)光合能力品種[25,49]。在本研究中，中、低光強(qiáng)處理下南豆12葉脈密度顯著高于桂夏7，而且南豆12的leaf、s和i均顯著高于桂夏7，較高的葉脈密度能夠保證較好的葉片水分供應(yīng)能力，有利于CO2的擴(kuò)散和維持較高的i，從而提高光合速率。因此增加葉脈密度，縮短葉脈到氣孔的水分運(yùn)輸距離是耐蔭型大豆適應(yīng)弱光環(huán)境的又一策略。

4 結(jié)論

隨著光強(qiáng)降低，大豆leaf顯著減少，而Ψleaf在不同光強(qiáng)處理下無顯著差異。leaf和s在不同光強(qiáng)處理下呈極顯著正相關(guān)。弱光引起大豆葉脈性狀的變化，葉脈密度顯著降低，葉脈到氣孔的距離顯著增加，木質(zhì)部導(dǎo)管面積減小，從而導(dǎo)致leaf降低。因此，大豆葉脈性狀會(huì)調(diào)整以適應(yīng)不同光強(qiáng)環(huán)境下的葉片水分需求，弱光下對(duì)水分供應(yīng)的減少與水分需求的減少相匹配。

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Effect of Light Intensity on Leaf Hydraulic Conductivity and Vein Traits of Soybean at Seedling Stage

GAO Jing, CHEN JiYu, TAN XianMing, WU YuShan, YANG WenYu, YANG Feng

College of Agronomy, Sichuan Agricultural University/Key Laboratory of Crop Ecophysiology and Farming System in Southwest China, Ministry of Agriculture and Rural Affairs/Sichuan Engineering Research Center for Crop Strip Intercropping System, Chengdu 611130

【Objective】The objective of this study is to explore the effects of light intensity on leaf hydraulic conductivity, photosynthetic traits, and water potential in soybean seedlings, analyze the adaptive mechanisms of leaf vein traits in response to varying light intensities, and to provide theoretical support for enhancing future light energy utilization in soybean.【Method】Two soybean varieties, Nandou 12 (shade-tolerant) and Guixia 7 (shade-sensitive), were cultivated and placed in growth chambers. The plants were exposed to varying light conditions, including high light intensity (HL) at (424.47±12.32) μmol·m-2·s-1, medium light intensity (ML) at (162.52±20.31) μmol·m-2·s-1, and low light intensity (LL) at (93.93±9.87) μmol·m-2·s-1. After a 20-day treatment period, the impacts of different light intensities on hydraulic conductivity, photosynthetic parameters, leaf water potential, and leaf vein traits in the seedling leaves of soybean were examined.【Result】Compared with HL treatment, the leaf hydraulic conductivity of Nandou 12 and Guixia 7 under LL treatment was significantly decreased, and the leaf hydraulic conductivity of Nandou 12 under the three treatments was significantly higher than that of Guixia 7 under the three treatments. Compared with HL treatment, the leaf hydraulic conductivity of Nandu 12 under ML and LL treatments decreased by 7.56% and 21.24%, stomatal conductance decreased by 43.96% and 58.89%, and net photosynthetic rate decreased by 29.44% and 46.49%, respectively. Similarly, the leaf hydraulic conductivity of Guixia 7 under the ML and LL treatments decreased by 42.16% and 23.71%, stomatal conductance decreased by 54.55% and 45.79%, and net photosynthetic rate decreased by 37.03% and 42.06%, respectively. Additionally, no statistically significant differences were observed in the leaf water potential of both soybean varieties across the various treatments. Notably, leaf hydraulic conductivity and stomatal conductance of soybean exhibited a highly significant positive correlation (＜0.01) under the three light intensity treatments. As the light intensity decreased, a positive correlation was observed between leaf hydraulic conductivity and net photosynthetic rate (＜0.05) as well as stomatal conductance (＜0.01). Conversely, there was a noticeable decrease in the minor leaf vein density and the area of xylem conduits in major and minor veins under the ML and LL treatments for both soybean varieties. In the case of the minor leaf vein density and the area of xylem conduits in major veins, Nandou 12 exhibited significantly higher values than Guixia 7 under the ML and LL treatments. The major leaf vein density of Nandou 12 remained relatively stable across treatments, while that of Guixia 7 experienced a significant reduction of 11.4% and 15.0% under the ML and LL treatments compared to the HL treatment. Furthermore, a decrease in light intensity had a notable effect on increasing the distance between leaf veins and stomata. Specifically, under the ML and LL treatments, the distance from veins to stomata increased by 21.33% and 60.01% for Nandou 12 and by 31.50% and 53.59% for Guixia 7 in comparison to the HL treatment. The correlation analyses revealed significant positive correlations (＜0.05) between the hydraulic conductivity of soybean leaves and the density of minor leaf veins, the area of xylem conduits in major and minor veins. Conversely, a significant negative correlation (＜0.01) was observed between hydraulic conductivity and the distance from veins to stomata.【Conclusion】Light intensity exerts an influence on the leaf hydraulic conductivity by modulating the leaf vein structure of soybean. Under low light conditions, there is a reduction in leaf hydraulic conductivity in soybean; however, the coordination between leaf hydraulic conductivity and stomatal conductance is maintained to establish equilibrium between leaf water supply and demand as light intensity diminishes. The presence of a higher vein density under low light serves to abbreviate the distance required for water transport, thereby enhancing leaf water supply capacity. Consequently, this facilitates CO2diffusion and photosynthesis, representing an additional strategy employed by shade-tolerant soybean to acclimate to low-light environments.

soybean; leaf hydraulic conductivity; stomatal conductance; light intensity; leaf vein

10.3864/j.issn.0578-1752.2023.22.005

2023-03-27；

2023-05-04

國家重點(diǎn)研發(fā)計(jì)劃（2022YFD2300902）、國家自然科學(xué)基金（32071963）

高靜，E-mail：1787913440@qq.com。通信作者楊峰，E-mail：f.yang@sicau.edu.cn

（責(zé)任編輯 岳梅）