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在气候变化和城市扩张影响下,降水分布不均、极端高温及城市热岛效应使城市绿化树种生长受到干旱胁迫的抑制作用[1]。而城市植被为人类提供固碳释氧、移除空气污染物、降温、增湿、涵养水源、消减洪峰等,具有多重生态功能[2],是维持与改善城市环境的重要生态屏障。据2018年北京市水资源公报统计,2018年全市总用水量为39.3亿m3。其中,环境用水13.4亿m3,占总量34%。在北京市控制用水规模和拓展绿色发展空间规划背景下,如何用较少水养护好高质量城市森林景观是目前城市绿地发展中急需解决的问题。因此,选取评价植物应对干旱生境的敏感要素尤为重要。
植物水分利用效率(WUE)表征植物每单位水蒸发所固定碳量[3],是评价植物生长适宜程度的综合指标之一[4]。气体交换法测得植物瞬时碳水耗散即瞬时水分利用效率(WUEi),但其不能代替田间WUE表征植物中长期生理性状与水分利用改变[5]。根据叶片碳同位素丰度(δ13Cleaf)可推算植物叶片成熟至采样时刻的平均水分利用效率(WUEL)[6-7]。在土壤干旱及其伴随出现的环境空气湿度降低背景下,植物通过关闭气孔来减弱蒸腾与光合作用,但由于其光合速率未减小前,其气孔导度已经降低,造成蒸腾速率迅速减小,因而WUEi 升高[8-9];与此同时,气孔关闭会限制气孔间隙与环境大气碳交换,胞间CO2浓度(Ci)减小及其与环境CO2浓度(Ca)比值同时减小,最终显著提高δ13Cleaf[10]和WUEL[11-12]。也有研究观测到WUE随水分胁迫逐渐加剧而下降[13]。
干旱胁迫影响叶绿体光化学特性,使光合生理过程受到抑制。不同干旱胁迫处理下光合器的损伤程度有所不同。轻度干旱下植物通过调节气孔开闭调整水分散失平衡,不会损伤或者产生可逆损伤,从而适应干旱逆境;中度干旱胁迫对叶绿体光合器产生部分损伤,但在短期内难以恢复;重度干旱下植物胁迫由气孔限制转变为非气孔限制,其中植物内部光合反应过程及其键酶发生较大程度损伤,光合速率下降,即使复水仍不能恢复至正常生长[14-15],这些光系统内部结构与功能变异均可通过光合、荧光参数进行表征[16]。植物遭受干旱胁迫后其基础荧光(也称为初始荧光,Fo)增加,最大荧光(Fm)和PSII原初光能转换效率降低(Fv/Fm)[17],最终造成植物最大净光合速率(Pn)、蒸腾速率(Tr)以及气孔导度(gs)显著下降[18],表明植物光合性能减弱,PSII反应中心光合电子传递率下降,PSII潜在活性受抑。
WUE对干旱敏感度存在种间差异,这是由于不同植物在应对干旱胁迫时采用的水分调节策略有所不同,一类为冒险型植物,气孔对环境变化不敏感,干旱胁迫状态下气孔不会立刻关闭,具有非等水调节行为;另一类为保守型植物,随环境变化调节气孔开闭,具有等水调节行为[19-20]。不同树种间耗水特性及其调节机制存在较大差异[21-22],但针对落叶与常绿树种WUE应对干旱响应差异没有获得一致性结论[23]。因此,本研究选取北京地区典型绿化落叶树种—银杏(Ginkgo biloba)、栾树(Koelreuteria paniculata)和国槐(Sophora japonica),常绿树种—侧柏(Platycladus orientalis)、油松(Pinus tabuliformis)和白皮松(Pinus bungeana)盆栽幼树为研究对象,设置不同土壤水分处理组,利用气体交换法与碳稳定同位素法分别测定不同处理组盆栽幼树WUEi与WUEL及其与土壤含水、植物叶水势、光合、荧光参数间的相互关系,探讨不同水分胁迫下典型绿化树种WUE差异,解析WUE与其他生理生态因子间的相关性,寻找各树种WUE峰值出现的土壤水分阈值区间,为北京市水资源短缺背景下,筛选低耗水、高水分利用的城市绿化树种,构建绿化植被配置及其相应的水资源管理,为城市规划提供理论依据。
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本试验在北京市林业果树科学研究院资源苗圃旱棚内完成。北京市林业果树科学研究院(39°59′ N,116°13′ E)位于北京香山脚下,海拔高度约80 m,属典型暖温带半湿润大陆性气候区,近60年来年均气温12.23 ℃,年降雨量580 mm (1958—2008年)[24],6—8月降水量占全年降水的70%以上,全年降雨分配不均,植物在生长季前期易遭受干旱胁迫影响其生长、发育。因此,本研究利用旱棚进行截雨控水,模拟生长季初期季节性干旱对植物影响。冬季将旱棚卷帘落下,保障苗木过冬;春、夏、秋季将旱棚卷帘全部拉起,保证旱棚内外气温差不超过2 ℃。
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本试验选择北京市6种典型绿化树种——银杏、栾树、国槐、侧柏、油松和白皮松为试验试材。于2012年,在保证盆栽土壤状况基本一致的条件下,每个树种选取20棵株高、地径、生长状况基本一致的1年生苗木,盆栽基质选自本地土壤,保证盆栽土壤密度为1.21 g/cm3,移植苗木装盆(高50 cm,内径50 cm),1盆1棵苗木,入旱棚缓苗培养。每年进行苗木常规管护,提高苗木成活率。
至2018年7月,各盆栽苗木林龄均达到5年,每个树种各选择5棵生长状况基本一致的苗木,进行不同土壤水分梯度处理试验,即根据盆栽土壤田间持水量(field capacity, FC),设置3组盆栽土壤水分胁迫梯度,即轻度干旱(slight drought, SLD,土壤体积含水量为50% ~ 70% FC)、中度干旱(moderate drought, MD, 30% ~ 50% FC)和重度干旱(extreme drought, ED, 低于30% FC)。同步设置对照处理组(CK,90% ~ 100% FC)与上述3种干旱处理组进行差异分析。利用土壤温湿盐度测定仪WET-2 K1(Delta T, UK),每日下午19:00连续观测每个处理盆栽土壤体积含水量(SWC),并于每天17:00—19:00及时补充水分。
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2018年7—8月,选取上述经过3个土壤水分胁迫处理3周的盆栽幼树,分别于每月上、中、下旬的典型晴天观测以下各个参数:
(1)光合参数
采用手持式CI-340超轻型便携式光合测定仪(CID, USA)测量不同土壤水分胁迫处理组与对照组盆栽幼树净光合速率(Pn,μmol/(m2·s))、蒸腾速率(Tr,mmol/(m2·s))、气孔导度(gs,mol/(m2·s))等光合生理参数。于典型晴朗无云9:00—11:00观测日,选取盆栽幼树顶层1/3树冠向阳处6片成熟、无病虫害、色度、大小相近的叶片,避开阔叶树种叶脉进行上述光合参数测定。为了准确确定针叶树种光合测定面积,将CI-340光合测定仪叶室所覆盖的针叶区域进行标记,带回室内扫描标记区域针叶叶面积(Epson Perfection V700 Photo扫描仪,Seiko Epson Corporation,Japan),带入CI-340光合测定计算程序中,替换原始面积为扫描叶面积,获得光合相关参数。同时可获得植物瞬时水分利用效率WUEi值(公式1):
$$ {\mathrm{W}\mathrm{U}\mathrm{E}}_{\mathrm{i}}={P}_{\mathrm{n}}/{T}_{\mathrm{r}} $$ (1) 式中:Pn为盆栽幼树09:00—11:00净光合速率平均值;Tr为盆栽幼树09:00—11:00蒸腾速率平均值。
(2)叶水势
于典型晴天09:00—11:00,选取盆栽幼树顶层1/3树冠向阳处成熟、无病虫害、色度、大小相近的叶片,采用PSYPRO水势仪原位测定处理组与对照组盆栽幼树叶片水势(leaf water potential,LWP)。
(3)叶绿素荧光参数
于典型晴天09:00—11:00,选取处理组与对照组盆栽幼树顶层1/3树冠向阳处成熟、无病虫害、色度、大小相近叶片,将叶片暗适应15 min,然后暴露于饱和脉冲光(3 500 µmol/(m2·s))下,利用连续激发式植物效率仪Handy PEA(Hansatech, U.K.),测定不同处理组盆栽幼树叶片荧光特征参数,包括初始荧光(Fo)、最大荧光(Fm)、可变荧光(Fv)、PSⅡ潜在活性(Fv/Fo)、PSⅡ原初光能转换效率(也称最大光化学效率,Fv/Fm)等。
(4)叶片δ13Cleaf测定
采集完成上述光合、荧光参数测定的处理组与对照组盆栽幼树叶片,每个树种采集4 ~ 6片叶片,带回实验室105 ℃杀青,后在70 ℃烘箱中烘干48 h,粉碎研磨后过80目筛。利用元素分析仪(Thermo Scientific Corporation,USA)高温煅烧为CO2,将CO2通入质谱仪(Delta V Advantage,Thermo Scientific Corporation,USA)中,检测CO2中的δ13C值,精度为0.1‰。盆栽幼树叶片中的δ13Cleaf值以标准物质PDB(pee dee belemnite)为标准,计算公式如下:
$$ {{\delta }^{13}{\rm{C}}}_{\mathrm{leaf}}=\left(\frac{{R}_{\mathrm{sample}}}{{R}_{\mathrm{standard}}}-1\right)\times 1\;000 $$ (2) 式中:δ13Cleaf表示植物叶片13C/12C与标准样品偏离的千分率,Rsample和Rstandard分别表示样品和标准物质PDB的13C/12C。
(5)植物水分利用效率
运用Farquhar 等的线性模型求盆栽幼树平均水分利用效率(WUEL)[12]。叶同位素分辨率
$\varDelta $ ,可以通过以下公式求算:$$ \varDelta =\left(\delta {}^{13}{{C}}_{\mathrm{a}}-\delta {}^{13}{\rm{C}}_{\mathrm{l}\mathrm{e}\mathrm{a}\mathrm{f}}\right)\bigg/\left(1+\delta {}^{13}{\rm{C}}_{\mathrm{l}\mathrm{e}\mathrm{a}\mathrm{f}}\right) $$ (3) 式中:δ13Ca和δ13Cleaf分别为环境CO2碳同位素组成和盆栽叶片碳同位素组成。
$$ {{C}}_{\mathrm{i}}:{{C}}_{\mathrm{a}}=\left(\varDelta -\mathrm{a}\right)/\left(\mathrm{b}-\mathrm{a}\right) $$ (4) 式中:Ci是胞间CO2浓度;Ca是环境CO2浓度,本文取378.2 μmol/mol[25];a是环境CO2在静止空气中的扩散所造成的同位素分馏系数(4‰);b是经由1,5-二磷酸核酮糖羧化酶(Rubisco)在叶绿体固定CO2的过程和其内部扩散所产生的分馏系数(30‰);基于气体交换法所求算的植物瞬时水分利用效率WUEi值可以表示为:
$$ {\mathrm{WUE}}_{\mathrm{i}}={P}_{\mathrm{n}}:{T}_{\mathrm{r}}=\left({{C}}_{\mathrm{a}}-{{C}}_{\mathrm{i}}\right)/1.6\Delta e $$ (5) 通过公式(3) ~公式(5)求算WUEL为:
$$\begin{split} &{\mathrm{WUE}}_{\mathrm{L}}=\frac{{P}_{\mathrm{n}}}{{T}_{\mathrm{r}}}=\left(1-\text{φ}\right)\left({{C}}_{\mathrm{a}}-{C}_{\mathrm{i}}\right)/1.6\Delta e=\\ &{{C}}_{\mathrm{a}}\left(1-{\text{φ}}\right)\left(1-\frac{{\delta }^{13}{{C}}_{\mathrm{a}}-{\delta }^{13}{\mathrm{C}}_{\mathrm{leaf}}}{\mathrm{b}-\mathrm{a}}\right)/1.6\Delta e \end{split} $$ (6) 式中:φ为盆栽树种整个生长期叶片和其他器官夜间呼吸消耗掉的碳比率,取φ=0.3[26];δ13Ca取−8‰;1.6为空气中水蒸气与CO2扩散率的比率;WUEL为盆栽幼树平均水分利用效率;Δe为盆栽幼树叶片胞间水蒸汽压与环境水蒸汽压差值,由取样日的日间(08:00 —18:00)平均气象参数(如环境温度T、环境湿度RH等)(EnviroMonitor无线网络环境测量系统,北京米特技术发展有限公司)计算获得:
$$ \Delta e={e}_{\mathrm{lf}}-{e}_{\mathrm{atm}}=0.611\times {\mathrm{e}}^{17.502T/\left(240.97+T\right)}\times \left(1-\mathrm{RH}\right) $$ (7) 式中:elf和eatm为叶片内部与环境水蒸汽压。
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采用SPSS 18.0 分别对经历3种土壤水分胁迫处理后的不同树种盆栽幼树的生理生态参数、WUEi、δ13Cleaf和WUEL进行方差齐性检验,获得数据均符合正态分布且方差齐性,进而进行ANOVA方差分析,比较不同树种水分利用效率及其生理生态指标应对干旱的响应差异。采用SPSS 18.0中的Pearson相关分析盆栽幼树各光合、荧光参数、δ13Cleaf分别与WUEi、WUEL两参数间的相关性。
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本研究中需要控制4种土壤水分梯度,即轻度干旱(SLD,50% ~ 70% FC)、中度干旱(MD,30% ~ 50% FC)和重度干旱(ED,低于30% FC)以及同步设置对照处理组(CK,90% ~ 100% FC)。利用环刀法测定盆栽土壤密度范围为0.98 ~ 1.09 g/cm3,土壤田间持水量为(24.0 ± 0.3)%。由图1,各盆栽树种4种土壤体积含水量均分别控制在CK、SLD、MD和ED范围内,且种内4种水分梯度差异显著(P < 0.05)。
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水分胁迫对6种盆栽幼树叶水势(LWP)的影响如图2所示,盆栽幼树从良好水分条件到遭受轻度干旱胁迫影响,盆栽幼树应激迅速消耗水分,大幅降低机体叶水势,因此SLD组落叶、常绿盆栽幼树LWP下降幅度最大(−0.15 ~ −2.92 MPa,P < 0.05)。盆栽幼树在重度干旱处理下一段时间后,体内生理代谢与水分调节能力较SLD、MD时期相对减弱,与SLD、MD相比,ED下各盆栽落叶幼树LWP有所增加,常绿幼树则增长至略大于对照组LWP。在盆栽幼树从土壤田间持水至重度土壤水分胁迫过程中,利用盆栽幼树CK处理下LWP值与上述过程中LWP最小值的差值求算每种盆栽幼树LWP的下降率,获得各盆栽幼树LWP的下降率排序为银杏(14.64%)> 国槐(8.03%)> 侧柏(3.67%)> 栾树(3.03%)> 白皮松(1.09%)> 油松(1.02%)。
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如图3所示,3种落叶树种及侧柏、油松盆栽幼树净光合速率(Pn)均随着水分胁迫的增加而明显减小(P < 0.05),其中银杏和栾树盆栽幼树Pn在SLD显著下降,国槐和侧柏则在MD显著下降,油松Pn在ED下显著减小。而随着土壤水分胁迫加剧白皮松盆栽幼树Pn无显著变化。比较土壤田间持水条件(CK)下各盆栽幼树光合能力发现,国槐Pn最大,白皮松Pn最小,两者差异显著(P < 0.05)。随土壤水分逐渐减小,国槐Pn降幅最大(85.3%),而白皮松光合能力无显著变化。由图4可知,银杏、栾树、国槐和侧柏盆栽幼树气孔导度(gs)均沿土壤水分胁迫加剧逐渐减小,且落叶树种gs降幅较大(0.085 ~ 0.269 mol/(m2·s)),最低值为0.011 ~ 0.018 mol/(m2·s)。油松盆栽幼树在SLD、MD处理下gs与CK比较无显著差异。与CK相比,水分胁迫条件下的白皮松gs均显著下降,稳定于0.028 ~ 0.033 mol/(m2·s)范围内。
图 3 土壤水分胁迫对北京地区绿化树种光合能力的影响
Figure 3. Effects of water deficits on photosynthetic capacities in typical greening tree species of Beijing
图 4 土壤水分胁迫对北京市绿化树种气孔导度的影响
Figure 4. Effects of water deficits on stomatal conductance in typical greening tree species of Beijing
不同土壤水分亏缺对6种绿化树种幼树PSⅡ原初光能转换效率(Fv/Fm)均无显著影响(P > 0.05,图5)。落叶树种PSⅡ潜在活性(Fv/Fo)随水分胁迫加剧逐渐减小,但与CK差异不显著(P > 0.05)。常绿树种油松Fv/Fo从MD开始逐渐增大,但增幅不显著(13.58% ~ 20.13%,P > 0.05);而侧柏、白皮松Fv/Fo分别从SLD和MD处理开始随着水分胁迫加剧显著降低(P < 0.05),降幅分别为7.97% ~ 16.82%和10.89% ~ 21.26%。
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不同土壤水分胁迫下北京地区绿化树种WUEi变化趋势如图6所示。土壤水分胁迫处理后银杏、栾树、国槐和侧柏WUEi显著低于对应CK水平(P < 0.05),在SLD、MD和ED条件下银杏、国槐、栾树和侧柏WUEi间无显著差异。干旱处理后的油松WUEi比CK水平有所降低,但无显著差异(P > 0.05)。不同土壤水分条件下白皮松WUEi变化不明显(P > 0.05)。
图 6 不同土壤水分胁迫下北京地区绿化树木瞬时水分利用效率
Figure 6. Instantaneous water use efficiencies in typical greening tree species of Beijing under different soil water stresses
根据叶片δ13Cleaf(表1)与环境变量计算出盆栽幼树不同土壤水分亏缺条件下的平均水分利用效率(WUEL)如图7所示,田间持水条件下(CK)侧柏、油松WUEL高于落叶树种(P < 0.05)。SLD处理下,侧柏WUEL显著高于其他幼树,白皮松与栾树WUEL间无显著差异,而在SLD条件下国槐幼树WUEL最低。MD与ED下,常绿树种WUEL均显著高于落叶树种(P < 0.05),且在ED背景下3种落叶树种间WUEL存在显著差别,排序为国槐 > 栾树 > 银杏(P < 0.05)。
表 1 不同土壤水分胁迫下北京地区绿化树木叶片δ13Cleaf比较
Table 1. Comparison in leaf δ13Cleaf in greening tree species of Beijing under varied soil water stresses
物种 Species CK SD SLD SD MD SD ED SD 银杏
G. biloba−29.352bB 0.217 −27.787aC 0.109 −27.930bB 0.138 −29.584cD 0.253 栾树
K. paniculata−28.828cB 0.570 −27.412aBC 0.464 −27.757abB 0.251 −28.572bcC 0.507 国槐
S. japonica−29.784bB 0.444 −28.940bE 0.187 −27.817aB 0.209 −27.894abB 0.486 侧柏
P. orientalis−26.887abA 0.101 −26.496aA 0.248 −26.462aA 0.049 −27.290bAB 0.121 油松
P. tabuliformis−27.503aA 0.454 −27.994bD 0.368 −26.438aA 0.739 −27.352aAB 0.183 白皮松
P. bungeana−28.353bB 0.746 −27.083aB 0.440 −26.664aAB 0.090 −26.640aA 0.204 注:δ13Cleaf为植物叶片碳-13丰度值;同行不同小写字母表示不同水分胁迫处理间差异显著(P < 0.05);同列不同大写字母表示物种间差异显著(P < 0.05)。
Notes: δ13Cleaf is Cabon-13 isotope abundance in plant leaf. Different lowercase letters in the same row mean significant differences at P < 0.05 level among the treatments of soil water stresses. Different capital letters in the same column mean significant differences at P < 0.05 level among tree species.图 7 不同土壤水分胁迫下北京地区绿化树木平均水分利用效率
Figure 7. Mean water use efficiencies in typical greening tree species of Beijing under different soil water stresses
随着土壤水分胁迫加剧,银杏和栾树均在SLD达到最大WUEL(1.27 mg/g和1.35 mg/g),随后WUEL逐渐降低。在30% ~ 70% FC下,侧柏与油松WUEL均随着土壤水分的降低逐渐增加,而在ED条件下两树种WUEL有所降低。土壤越干旱,国槐、白皮松WUEL越高,ED条件下分别比CK增加了44.19%和30.35% (P < 0.05)。
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针对上述北京市6种常见绿化树种光合、荧光指标与叶碳同位素丰度、WUEi和WUEL之间的相关性分析表明(表2和表3),落叶树种LWP与Pn、Fv/Fo间呈显著正相关。Pn与gs呈正相关,但与WUEi、WUEL均不相关。gs与δ13Cleaf呈正相关(P < 0.01),其与WUEi呈负相关(P < 0.01)。荧光参数Fv/Fm和Fv/Fo均与落叶树种WUEi正相关,而落叶树种δ13Cleaf分别与WUEi呈负相关(P < 0.01),与WUEL呈极显著正相关。
表 2 落叶幼树生理生态指标与水分利用效率相关分析
Table 2. Pearson correlations between eco-physiological parameters and WUEs in different deciduous tree species
项目 Item LWP Pn gs Fv/Fm Fv/Fo δ13Cleaf WUEi WUEL LWP 1 Pn 0.239* gs 0.166 0.538** Fv/Fm 0.172 −0.006 −0.006 Fv/Fo 0.210* 0.024 −0.063 0.913** δ13Cleaf −0.098 0.135 0.383** −0.117 −0.192 WUEi 0.072 0.153 −0.365** 0.250* 0.393* −0.335** WUEL −0.21 −0.009 −0.06 −0.226 −0.176 0.821** 0.205 1 注:LWP. 09:00—11:00叶水势; Pn. 净光合速率; gs. 气孔导度; Fv/Fm. PSII原初光能转换效率; Fv/Fo. PSⅡ潜在活性; δ13Cleaf. 植物叶片碳-13丰度值; WUEi. 瞬时水分利用效率; WUEL. 平均水分利用效率. “*”、“**”分别表示相关性显著(P < 0.05)和极显著(P < 0.01)。下同。
Notes: LWP, leaf potential at 09:00−11:00; Pn, net photosynthetic rate; gs, stomatal conductance; Fv/Fm, primary light energy conversion of PSII; Fv/Fo, potential activity of PSII; δ13Cleaf, Cabon-13 isotope abundance in plant leaf;;WUEi, instantaneous water use efficiency;WUEL, mean water use efficiency. Significant correlations at P <0.05 level marked with an asterisk and those at P <0.01 level marked with two asterisks. The same below.表 3 常绿幼树生理生态指标与水分利用效率相关分析
Table 3. Pearson correlations between eco-physiological parameters and WUEs in different evergreen tree species
项目 Item LWP Pn gs Fv/Fm Fv/Fo δ13Cleaf WUEi WUEL LWP 1 Pn −0.79 gs 0.062 0.155 Fv/Fm −0.067 −0.009 0.062 Fv/Fo −0.58 0.012 0.098 0.951** δ13Cleaf 0.185 0.256 0.162 −0.025 0.023 WUEi −0.116 0.425** −0.007 −0.165 −0.174 0.164 WUEL 0.121 0.321* 0.192 −0.089 −0.008 0.528** 0.203 1 常绿树种LWP与光合、荧光参数不相关,其Pn与WUEi、WUEL呈显著正相关,与gs关联度不显著。gs与δ13Cleaf、WUEi无显著相关性。常绿树种δ13Cleaf与WUEi关联度不高,但其与WUEL呈极显著正相关。
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植物应对干旱胁迫最重要的生理表现为光合作用受抑,这将直接减少植物生物量积累[27]。在本研究中,银杏、栾树、国槐、侧柏和油松幼树净光合速率在经历轻度、中度与重度干旱处理后均显著减小,一般认为是水分胁迫下植物气孔开度减小或关闭,光合底物CO2亏缺,合成产物过程受抑制所导致的[28-29]。而常绿树种白皮松经历水分胁迫后其Pn变化不显著。同时发现,所有研究树种的气孔导度均随土壤水分的减小而显著减小,其中落叶树种gs降幅较大(0.085 ~ 0.269 mol/(m2·s))。虽然常绿树种在CK水平gs均小于落叶树种,但其经历不同干旱强度处理后gs降幅比落叶树种要小。这是由于以油松和白皮松为代表的常绿树种可控制叶片气孔导度相对恒定,保证光合与蒸腾作用正常运行,最终其水分利用效率不易受到土壤水分条件变动而发生变化[30],有利于生存于环境水分受限区域。而随土壤水分胁迫加剧,落叶树种银杏、栾树、国槐会及时调控气孔开度,调节叶水势,减小过度蒸腾耗水,适宜生长在水分良好环境中[31]。
在干旱早期阶段,植物CO2同化率和饱和光量子通量密度出现下降趋势,是由于气孔与非气孔因素共同作用所产生[32],若干旱胁迫继续加剧,则上述光合参数的降低则由非气孔因素所决定[33]。由图5所示,虽然土壤水分胁迫对6种绿化树种PSII原初光能转换效率(Fv/Fm)并无显著影响,但是所有落叶树种和侧柏Fv/Fm均随着干旱梯度的增强出现不同程度的下降,而油松与白皮松Fv/Fm却在轻度水分胁迫下比CK水平提高了0.9%和1.1%,这与其Pn变化趋势正好相吻合,即水分胁迫所导致的气孔或非气孔调节幅度存在种间差异性[34]。结合上述不同干旱处理下盆栽幼树gs变化趋势可知(图4),银杏、栾树、国槐随土壤水分亏缺加重,植株根系至叶片水分运输受阻,其气孔感知干旱信号较敏感,迅速关闭气孔维持LWP以发挥水分运输功能,此时银杏、栾树、国槐Pn随气孔关闭、CO2来源减少而降低,发生气孔等水调节行为[35]。在土壤田间持水条件下油松、白皮松gs略低于落叶树种,但在干旱胁迫处理后油松、白皮松gs并无显著下降,继续维持一定气孔开度,以维持较高光合速率,这些植物具有气孔的非等水调节行为[35]。
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目前,利用气体交换法获得的Pn和Tr计算瞬时水分利用效率WUEi和使用稳定同位素技术求算的WUEL成为植物水分利用效率研究的主要方法。WUEi能够反映特定条件下植株利用单位水分合成碳产出的即时特征[36]。由此,影响Pn与Tr的梯度因子,如作用于Pn的叶内外CO2浓度比值(Ci/Ca),以及作用于Tr的叶内外水汽差(∆e),将成为提高植株WUEi的重要途径。而光合与蒸腾作用之间的平衡差值导致WUEi存在物种差异[6]。这种平衡关系受气孔行为(gs)调控。如表2和表3所示,植物gs与WUEi呈负相关,这是由于干旱过程中gs对Tr影响作用要强于Pn所造成[37]。如图6所示,银杏、栾树和国槐在遭受水分胁迫梯度处理后WUEi下降明显(最大下降幅度分别为60.43%、69.14%和81.41%),表明落叶树种对土壤水分波动影响较敏感;而白皮松在30% ~ 50%FC条件下WUEi超过CK组值。已有研究表明短期水分胁迫下,植物通过减小气孔开度,减小蒸腾速率以增加WUE[38]。本研究结果也印证了Fitter[39]提出植物在适度土壤水分亏缺下才能达到其最优水分利用效率的观点。
本研究表明(表2和3),落叶和常绿植物叶片干物质δ13Cleaf(13C/12C)与其平均水分利用效率WUEL呈极显著正相关关系,已有研究证实植物WUEL变异主要是由植物组织δ13C大小所决定的[14]。在本研究中这种相关性体现于物种特异性,即同一水分处理下常绿树种δ13Cleaf大于落叶树种,具有较高WUE,这是由于落叶树种叶肉细胞对碳需求较大,则扩散至叶内部CO2浓度较大,因此其具有较高∆13C和较小δ13C;而常绿树种gs和Pn较低[40]。落叶幼树gs与Pn显著正相关,而在常绿幼树中没有体现这两种参数的关联性。由图7可知,在干旱梯度作用下不同树种δ13Cleaf峰值出现的土壤水分含量适应范围有所不同。这与常绿、落叶树种水分利用策略不同有关,即不同植物种在干旱生境下,通过改变自身水分利用效率以适应土壤水分状况的竞争能力和生态适应性是不同的[41]。
由本研究结果可知,同一物种同一水分处理下植物的WUEi与WUEL值有所不同。在土壤田间持水量背景下,本研究中针阔叶树种WUEi波动范围为4.35 ~ 17.20 mg/g,且在SLD下针阔叶盆栽幼树WUEi均低于对应对照水平,其他处理中WUEi随土壤干旱加剧趋于平缓;而CK下针阔叶树种WUEL波动范围为0.86 ~ 1.46 mg/g,远小于CK下各盆栽树种WUEi,这是由于由叶片Pn与Tr推算出的WUEi可反映植物叶片瞬时水分与碳合成平衡功能,忽略了夜间呼吸、防御等其他生理生化行为消耗量[42]。WUEi能够及时反映盆栽幼树应对土壤水分等环境条件的瞬时响应策略和微观有效耗水量,强调气孔在植物水分调控策略中的重要作用[43],对于保守型植物在遭受干旱时,由于气孔关闭与胞间CO2浓度瞬时变化,WUEi会瞬时改变。而WUEL则是盆栽幼树自叶片成熟、具有光合碳同化能力至采样时刻长期适应生境条件的水碳综合参数,表示盆栽幼树一段生活史中的光合同化量、耗水量和水分利用效率。虽然某种植物在水分胁迫下表现为WUEi减小,但观测其WUEL可知一定程度干旱胁迫可能会对其生长起促进作用[44-45],即WUEi与WUEL在不同土壤水分梯度上的差异大小,综合反映不同时间尺度上外界环境条件与植物协同调节自身水分生理的复合结果。因此,需要根据不同尺度植物水分利用机制,开展多时间尺度植物水分利用特征及其影响机制研究,阐明植物抗旱与耐旱机理,以揭示陆地植被应对全球气候变化与自身适应生存对策的核心理论,旨在筛选出低耗水、高水分利用的园林绿化树种,建设高水分利用的城市森林景观,全面实现节水林业与绿色林业。
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随干旱程度加剧,银杏、栾树、国槐和侧柏光合参数逐步减小。油松光合能力在重度干旱下才显著减小,白皮松未出现显著变化。不同树种WUEi对干旱的敏感度存在差异,即各落叶树种和侧柏WUEi分别在轻度与中度干旱下显著降低,而油松、白皮松WUEi分别在中度干旱作用下小幅升高后有所降低。不同树种WUEL随水分胁迫梯度的变化趋势不同。除国槐外,落叶树种WUEL均在轻度胁迫时达到该水分处理阶段最大值后下降;而常绿树种则在SWC介于30% ~ 70%FC范围内WUEL达到峰值,在SWC < 30%FC下白皮松WUEL比对照水平显著增加了30.35%。Pearson相关分析发现气孔导度影响落叶树种光合、蒸腾过程,对WUEi影响较大;而常绿树种气孔导度随水分梯度变异不敏感,对WUEi影响较小。
综上,常绿树种油松、白皮松比阔叶树种银杏、栾树适应干旱能力强。植物应对水分胁迫是一种复杂、综合性状,除本文所提及的光合、荧光及叶片碳稳定同位素丰度、水分利用效率等指标外,还需针对植物内部水分结构、形态指标及生化指标进行补充观测,以求更加全面的评价城市绿化树种的抗旱性与节水性,旨在为构建高水资源利用效率的城市园林绿化系统提供理论依据。
Water use efficiency and its influencing factors of typical greening tree species in Beijing region
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摘要:
目的 在北京地区绿化率需求提升和水资源短缺背景下,城市绿化与城市生产、生活用水矛盾日益尖锐。因此,探寻城市绿化树种对干旱生境的响应机制,筛选低耗水、高水分利用的园林绿化树种,成为北京高质量城市森林景观建设的重要需求。 方法 该研究以北京地区落叶树种银杏、栾树、国槐和常绿树种侧柏、油松和白皮松盆栽幼树为研究对象,基于碳稳定同位素技术与树种生理参数观测分析3种干旱胁迫处理下(50% ~ 70%土壤田间持水量(FC)(轻度干旱SLD);30% ~ 50% FC(中度干旱MD);低于30% FC(重度干旱ED))树种瞬时水分利用效率(WUEi)和平均水分利用效率(WUEL)差异。 结果 (1) 与对照处理(90% ~ 100% FC,CK)相比,银杏、栾树、国槐和侧柏的光合能力、WUEi在土壤体积含水量(SWC)小于70%条件下显著减小(P < 0.05),而油松、白皮松WUEi在水分胁迫作用下无显著变化。(2) MD与ED下,常绿树种WUEL均显著高于落叶树种(P < 0.05)。在ED处理下3种落叶树种间WUEL存在显著差异,排序为国槐 > 栾树 > 银杏(P < 0.05)。银杏、栾树在SLD、侧柏、油松在MD时分别达到各自WUEL峰值后下降;而在土壤逐渐干旱处理下,国槐、白皮松WUEL逐渐增大,且在ED处理下分别比CK增加了44.19%和30.35%(P < 0.05)。(3) 比较不同树种光合荧光参数分别与WUEi、WUEL之间的相关关系发现,落叶树种银杏、栾树和国槐的气孔导度(gs)影响其光合、蒸腾过程(P < 0.01),对WUEi影响较大;而常绿树种侧柏、油松和白皮松gs对土壤水分变异不敏感(P > 0.05),对WUEi影响较小。在MD和ED水分胁迫条件下常绿树种WUEL均高于落叶树种。 结论 北京典型常绿树种比落叶树种更能优化光合性能,提高自身水分利用效率以适应干旱生境。 Abstract:Objective Due to the water scarcity and the improvement in urban greening, the tension has grown from the struggles for water between industries, urban life, and urban afforestation in Beijing. Thus, it is the top priority to explore the response mechanism of urban greening tree species to arid habitats and to screen the low-water consumption and high-water utilization landscaping tree species for high-quality urban forest landscape. Method Typical greening tree species including Ginkgo biloba, Koelreuteria paniculata, Sophora japonica, Platycladus orientalis, Pinus tabuliformis, and Pinus bungeana in Beijing were selected and subjected to three groups of soil water stresses such as slight drought (SLD, 50%−70% field capacity (FC)), moderate drought (MD, 30%−50% FC) and extreme drought (ED, lower than 30% FC). The interspecific differences in the instantaneous water use efficiency (WUEi) and mean water use efficiency (WUEL) of these tree species under water stresses and their relationship to eco-physiological factors were investigated based on the stable isotope technology and the observations on tree physiological traits. Result The photosynthetic capacities and WUEi in G. biloba, K. paniculata, S. japonica and P. orientalis decreased significantly compared with control (CK, 90%−100% FC) when potted soil volumetric water content (SWC) was less than 70% (P < 0.05), whereas there was no significant difference in WUEi of P. tabuliformis and P. bungeana subjected to continuous drought (P > 0.05). The WUELs of evergreen tree species were significantly higher than those of deciduous tree species under MD and ED (P < 0.05). The WUELs of three deciduous tree species differed in response to extreme drought (P < 0.05), and the sequence of WUELs in three deciduous tree species was S. japonica > K. paniculata > G. biloba. The WUELs of G. biloba and K. paniculata in SLD, and P. orientalis and P. tabuliformis in MD reached their respective peaks and then decreased along with the increases in soil water stress; while those of S. japonica and P. bungeana increased with potted soil drying, and were 44.19% and 30.35% higher than control, respectively at severe drought condition (P < 0.05). Comparing the correlations between photosynthetic parameters, WUEi and WUEL of different tree species, it was found that the stomatal conductance (gs) of G. biloba, K. paniculata and S. japonica significantly affected its photosynthesis and transpiration processes (P < 0.01) and hence exerted a strong influence on WUEi, while those in evergreen tree species such as P. orientalis, P. tabuliformis, and P. bungeana were insensitive to the variation in soil moisture and had little impact on WUEi (P > 0.05). The WUEL of evergreen tree species was higher than that of deciduous ones under moderate and extreme water stress. Conclusion Therefore, considering the limited water resources of Beijing, evergreen tree species have stronger abilities to conserve water content and optimize photosynthetic performance to improve their water use efficiency, to adapt to arid habitats than those of deciduous trees in Beijing. -
Key words:
- Beijing /
- drought stress /
- greening tree species /
- water use efficiency /
- carbon stable isotope
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图 1 6种绿化树种盆栽不同干旱胁迫下土壤体积含水量
CK、SLD、MD、ED分别为对照处理组(CK,90%~100% FC)、轻度干旱(SLD,50%~70% FC)、中度干旱(MD,30%~50% FC)和重度干旱(ED,低于30% FC)处理。FC为盆栽土壤田间持水量。小写字母表示不同水分胁迫处理间差异显著( P <0.05)。下同。CK, SLD, MD, ED are the control group (CK, 90%−100% FC) and three groups of soil water stresses as slight drought (SLD, 50%−70% FC), moderate drought (MD, 30%−50% FC) and extreme drought (ED, lower than 30% FC). FC is potted soil field capacity. Different small letters mean significant differences at P <0.05 level among the treatments of soil water stresses. The same as below.
Figure 1. Variations in soil volumetric water content of potting soil of six tree species
表 1 不同土壤水分胁迫下北京地区绿化树木叶片δ13Cleaf比较
Table 1. Comparison in leaf δ13Cleaf in greening tree species of Beijing under varied soil water stresses
物种 Species CK SD SLD SD MD SD ED SD 银杏
G. biloba−29.352bB 0.217 −27.787aC 0.109 −27.930bB 0.138 −29.584cD 0.253 栾树
K. paniculata−28.828cB 0.570 −27.412aBC 0.464 −27.757abB 0.251 −28.572bcC 0.507 国槐
S. japonica−29.784bB 0.444 −28.940bE 0.187 −27.817aB 0.209 −27.894abB 0.486 侧柏
P. orientalis−26.887abA 0.101 −26.496aA 0.248 −26.462aA 0.049 −27.290bAB 0.121 油松
P. tabuliformis−27.503aA 0.454 −27.994bD 0.368 −26.438aA 0.739 −27.352aAB 0.183 白皮松
P. bungeana−28.353bB 0.746 −27.083aB 0.440 −26.664aAB 0.090 −26.640aA 0.204 注:δ13Cleaf为植物叶片碳-13丰度值;同行不同小写字母表示不同水分胁迫处理间差异显著(P < 0.05);同列不同大写字母表示物种间差异显著(P < 0.05)。
Notes: δ13Cleaf is Cabon-13 isotope abundance in plant leaf. Different lowercase letters in the same row mean significant differences at P < 0.05 level among the treatments of soil water stresses. Different capital letters in the same column mean significant differences at P < 0.05 level among tree species.表 2 落叶幼树生理生态指标与水分利用效率相关分析
Table 2. Pearson correlations between eco-physiological parameters and WUEs in different deciduous tree species
项目 Item LWP Pn gs Fv/Fm Fv/Fo δ13Cleaf WUEi WUEL LWP 1 Pn 0.239* gs 0.166 0.538** Fv/Fm 0.172 −0.006 −0.006 Fv/Fo 0.210* 0.024 −0.063 0.913** δ13Cleaf −0.098 0.135 0.383** −0.117 −0.192 WUEi 0.072 0.153 −0.365** 0.250* 0.393* −0.335** WUEL −0.21 −0.009 −0.06 −0.226 −0.176 0.821** 0.205 1 注:LWP. 09:00—11:00叶水势; Pn. 净光合速率; gs. 气孔导度; Fv/Fm. PSII原初光能转换效率; Fv/Fo. PSⅡ潜在活性; δ13Cleaf. 植物叶片碳-13丰度值; WUEi. 瞬时水分利用效率; WUEL. 平均水分利用效率. “*”、“**”分别表示相关性显著(P < 0.05)和极显著(P < 0.01)。下同。
Notes: LWP, leaf potential at 09:00−11:00; Pn, net photosynthetic rate; gs, stomatal conductance; Fv/Fm, primary light energy conversion of PSII; Fv/Fo, potential activity of PSII; δ13Cleaf, Cabon-13 isotope abundance in plant leaf;;WUEi, instantaneous water use efficiency;WUEL, mean water use efficiency. Significant correlations at P <0.05 level marked with an asterisk and those at P <0.01 level marked with two asterisks. The same below.表 3 常绿幼树生理生态指标与水分利用效率相关分析
Table 3. Pearson correlations between eco-physiological parameters and WUEs in different evergreen tree species
项目 Item LWP Pn gs Fv/Fm Fv/Fo δ13Cleaf WUEi WUEL LWP 1 Pn −0.79 gs 0.062 0.155 Fv/Fm −0.067 −0.009 0.062 Fv/Fo −0.58 0.012 0.098 0.951** δ13Cleaf 0.185 0.256 0.162 −0.025 0.023 WUEi −0.116 0.425** −0.007 −0.165 −0.174 0.164 WUEL 0.121 0.321* 0.192 −0.089 −0.008 0.528** 0.203 1 -
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