三种植被恢复树种的冠层气孔导度特征及其对环境因子的敏感性

应用Granier热消散探针,长期监测华南地区荷木、大叶相思和柠檬桉林不同径级样树的树干液流,结合同步观测的气象数据,求算冠层气孔导度(g_c),并分析其对环境因子的响应方式及敏感性.结果表明: 不同季节荷木林日间平均g_c显著高于大叶相思和柠檬桉(P<0.05)（除3月外）.在干季和湿季,g_c与光合有效辐射(PAR)呈现对数正相关关系(P<0.001),湿季g_c对PAR响应比干季更敏感.g_c与水汽压亏缺(VPD)在干湿季均呈现对数负相关关系(P<0.001),同样在湿季表现出更高的敏感性.湿季g_c与VPD的偏相关系数高于干季,VPD对气孔行为的调控作用在湿季更为明显.随着土壤含水量的降低,g_c对VPD的敏感性下降,荷木和柠檬桉林下降的幅度大于大叶相思林,荷木和柠檬桉林下降的幅度相当.通过综合分析g_c对环境因子(PAR和VPD)的敏感性及其对土壤含水量变化的响应规律,发现乡土树种荷木作为植被恢复树种比外来引种的大叶相思和柠檬桉更为适宜.     

华北山区侧柏冠层气孔导度特征及其对环境因子的响应

冠层气孔导度(g_s)是衡量冠层-大气界面水汽通量的重要生物学常数,研究其特征及对环境因子的响应,能为开展森林冠层水汽交换过程的机理性研究提供理论依据.于2014年利用SF-L热扩散式探针测定了侧柏的树干液流密度(J_s),同步监测光合有效辐射(PAR)、饱和水汽压差(VPD)、气温(T)等环境因子,计算侧柏的冠层气孔导度特征并分析其对各环境因子的响应.结果表明: 侧柏液流密度的日变化总体呈双峰曲线,生长季高于非生长季,且胸径越大液流密度越大;冠层气孔导度日变化与单位叶面积冠层蒸腾(E_L)趋势相近,均呈双峰曲线,生长季的冠层气孔导度和蒸腾较非生长季略高.侧柏冠层气孔导度与空气温度呈抛物线关系,在10 ℃左右冠层气孔导度达到峰谷;光合有效辐射以400 μmol·m^-2·s^-1为界,小于该阈值两者呈正相关关系,大于该阈值则冠层气孔导度受其影响较小;与饱和水汽压差呈负对数函数关系,随饱和水汽压差增大而逐渐降低.较高的空气温度和光合有效辐射、较低的饱和水汽压差有利于侧柏形成较大的冠层气孔导度,进而促进冠层蒸腾.     

北京山区油松和元宝槭冠层气孔导度特征及其环境响应

叶片气孔是植物进行水汽交换的通道, 影响着植物的蒸腾和光合作用。然而叶片气孔行为受环境条件和树种类型的影响, 不同树种冠层气孔导度对环境因子响应的差异性, 以及在生长季不同时期叶片气孔对冠层蒸腾的调节作用是否会发生改变, 仍不清楚。该研究目的是通过探究各环境因子对不同树种冠层气孔导度的相对贡献率以及叶片气孔对冠层蒸腾的调节作用, 为深入了解植物水分利用状况和山区森林经营提供参考依据。于2018年生长季以北京八达岭国家森林公园内的58年生油松(Pinus tabuliformis)和39年生元宝槭(Acer truncatum)为研究对象, 利用热扩散技术对其树干液流进行连续监测, 并同步监测环境因子。利用彭曼公式计算冠层气孔导度(G_s)。主要结果: (1)油松和元宝槭日间G_s在日、月时间尺度上存在明显差异。5-7月油松和元宝槭日动态G_s均随饱和水汽压差(VPD)和太阳辐射(GR)的增加呈上升趋势, 上升持续时间比8月和9月长; 在月尺度上, 随着VPD、GR的降低和土壤湿度(VWC)的升高, G_s从5月到9月整体上升。(2)利用增强回归树法分析得到VWC和VPD对G_s的贡献率最大, 其次是GR、气温和风速。VWC和VPD对油松G_s的贡献率分别为66.4%和17.4%, 对元宝槭G_s的贡献率分别为54.8%和21.0%。(3)油松和元宝槭的dG_s/dlnVPD值与参考冠层气孔导度之间的斜率均显著高于0.6, 气孔调节作用相对较强。综上所述, 气孔对环境因子的响应在树种以及生长季不同时期之间存在差异, 为防止水分过度散失, 两树种在不同土壤水分条件下均通过严格的气孔调节控制蒸腾量。     

整树水力导度协同冠层气孔导度调节森林蒸腾

冠层气孔导度决定森林的蒸腾效率,它对驱动水汽移动的水汽应力的响应受树木水力结构的影响,并随水汽压亏缺上升和水力导度下降而降低,维持水势在最低阈值之上,避免出现水力灾变,调控冠层蒸腾。由于叶形和树冠结构的特点,部分脱耦联反映了湿润地区阔叶林冠层与大气的水汽交换特征,单纯以气孔导度的变化难以完整描述水分通量的调节规律,因而,需要考虑冠层气孔导度与水力导度协同控制冠层蒸腾的潜在机理。通过整合叶片气孔气体交换、树干液流、冠层微气象和其他环境因子的野外观测值,估测不同时间尺度的森林冠层气孔导度与大气的脱耦联系数和变异范围,以基于树干液流的冠层蒸腾,结合叶片/土壤水势梯度计算的水力导度,分析水力导度影响冠层气孔导度响应水汽压亏缺的敏感性,可以揭示和阐明水力导度和冠层气孔导度联合调节森林蒸腾的机理,对准确估测全球变化背景下森林对水资源利用的潜在生态效应有明显的理论意义。     

北京市5种园林树木蒸腾作用模拟研究

通过对北京市园林5种常用乔木, 国槐(Sophora japonica)、银杏(Ginkgo biloba)、白蜡(Fraxinus chinensis)、杜仲(Eucommia ulmoides)和臭椿( Ailanthus altissima)等植物蒸腾作用与周围环境气象因子(温度、湿度、太阳有效辐射)及植株叶面积指数相关关系的研究, 利用Javis公式计算冠层气孔阻力, 同时采用PM公式计算冠层蒸腾速率和植株日蒸腾量, 并分析不同乔木的冠层气孔导度对环境主要驱动因子的响应规律。结果表明: 5种被观测乔木中, 国槐耗水量最大, 白蜡耗水量最小, 植株蒸腾量大小依次为国槐>银杏>杜仲>臭椿>白蜡(P<0.01)。植物叶片气孔导度及蒸腾量与环境驱动因子太阳辐射及水气压亏缺的相关关系表明, 在土壤水分条件较好时, 国槐长势优于其它4种乔木, 但是其对水分的利用不够经济, 在干旱的情况下不能有效节水。     

控水条件下侧柏冠层气孔导度对土壤水的响应

建立了不同控水条件下(无降水、一半降水、自然降水和二倍降水)的侧柏样地,于2016年8月—2017年8月监测了样地土壤含水量(SWC)、降水量、液流密度(J_s)、叶面积指数(LAI)和水汽压亏缺(VPD)等因子,分析SWC对侧柏冠层气孔导度(g_s)的影响。结果表明: 一半、自然和二倍降水样地的SWC与降水量呈正相关,SWC变化范围分别为4.9%～16.0%、7.2%～22.9%、7.4%～29.6%,无降水样地的SWC在8—10月下降50%;7月的日g_s在14:00达到峰值(166.64 mmol·m^-2·s^-1),显著高于其他月份,且出现双峰现象, 1月的日g_s在12:00达到峰值(54.1 mmol·m^-2·s^-1);3个降水条件下,侧柏g_s与SWC呈负二次相关关系,且g_s达到峰值,对应的SWC分别为8.5%、12.5%和18.5%,均趋近于年平均SWC。不同控水样地内侧柏g_s对VPD的敏感性(δ)/参比冠层气孔导度(g_sref)均≥0.6,表明不同控水条件下土壤水分状况较适合侧柏蒸腾用水的需求。当SWC在3.7%～7.5%时,δ和g_sref值迅速增大,说明气孔调节能力更好,植物气孔对VPD的响应更敏感;当SWC上升到11%时,SWC变化对g_sref和g_s对VPD响应敏感性的影响不显著。可能存在侧柏产生适应状态的SWC阈值,植物体在自身的生命活动中关闭或减小气孔开度,降低叶片水势以适应过高的VPD,保护植物不会引起过度蒸腾,从而对蒸腾的调控更加有效。     

北京山区侧柏林冠层-大气蒸腾导度模拟及环境因子响应

蒸腾导度模型是衡量冠层-大气界面水汽输出的重要阻力模型,研究其特征及对环境因子的响应,为揭示森林冠层-大气界面水汽输出阻力机制提供理论依据。以首都圈森林生态系统定位观测研究站侧柏林为研究对象,采用TDP热探针法测定侧柏林树干液流密度,同步监测光合有效辐射、饱和水汽压差、气温、风速等主要环境因子,分析冠层导度和空气动力学导度的动态变化,构建冠层-大气蒸腾导度模型并模拟,明确冠层-大气蒸腾导度对各环境因子的响应关系。结果表明:蒸腾导度季节变化表现为非生长季与冠层导度趋势一致,生长季与空气动力学导度趋势一致,全年均为单峰趋势。冬季蒸腾导度与冠层导度保持较稳定差值(45 mol m^(-2 )s^-1左右),其他季节蒸腾导度与冠层导度、空气动力学导度的最大差值,均在各季节冠层导度、空气动力学导度的峰值水平。全年日均蒸腾导度冬季最大(86.92 mol m^(-2 )s^-1),其他季节较小且稳定(40—50 mol m^(-2 )s^-1之间)。在非生长季各环境因子对蒸腾导度的影响与对冠层导度的影响基本一致,温度为主要影响因子(r=-0.198),其他环境因子影响较小(r<0.1);在生长季中风速为主要影响因子(r=0.488),光合有效辐射(r=0.228)和饱和水汽压差(r=-0.299)的影响明显升高,温度的影响降低(r=0.114)。蒸腾导度模型较好的模拟了冠层-大气界面侧柏蒸腾不同季节的变化规律,阐明了各环境因子和冠层导度、空气动力学导度对蒸腾导度的影响机制,证实在生长季应重视空气动力学导度对蒸腾的影响。     

宁夏河东沙区侧柏冠层气孔导度对环境因子的响应及其模拟

为了解林木冠层气孔导度对环境因子的响应规律,进一步阐释冠层气孔导度调控森林蒸腾的机理,本研究以宁夏河东沙区典型防护林树种侧柏(Platycladus orientalis)为对象,利用Granier热扩散技术,在生长旺季(7—9月)连续监测树干液流,并同步观测小气候;利用Penman-Monteith方程计算获得连续的冠层气孔导度(gc),并采用Jarvis-Stewart气孔模型对侧柏冠层气孔导度变化过程进行模拟。结果表明:环境因子与gc呈极显著相关关系(P0.01),对侧柏gc的影响从大到小依次为大气温度(Ta)太阳辐射(Rs)饱和水气压亏缺(VPD)空气湿度(RH)。典型晴天下,gc与VPD、Ta的关系曲线上升阶段和下降阶段路径不同,呈现"磁滞回环"的特性,且不同时段表现出不同的响应规律。Jarvis-Stewart模型较好地模拟了侧柏冠层气孔导度对环境变量Rs、VPD、Ta的响应过程,采用交叉验证法对模型进行验证和参数率定,可以解释84.4%的变量。     

华南荷木林冠层气孔导度对水汽压亏缺的响应

冠层气孔导度(Gs)是量化气孔在冠层尺度水平上表现的参数,能够表征森林冠层表面水汽和能量交换的动态.本研究利用Granier树干液流测定系统,连续监测华南地区荷木林的树干液流,通过尺度转换和扩展获得冠层蒸腾速率,结合微气象观测值,以Pen-man-Monteith公式计算了Gs,并比较不同土壤水分条件下Gs对水汽压亏缺的响应.结果显示,Gs与气孔气体交换方法实测的叶片气孔导度(gs)日变化相似,单位转换数值大小与实测gs数量级一致.Gs对水汽压亏缺的响应在干季和湿季有明显差别:(1)在土壤水分充足的湿季(土壤含水量θ ＞33％),Gs对水汽压亏缺的响应更敏感(偏相关系数-0.316),而在干季(θ＜23％)则对光合有效辐射的响应更敏感(偏相关系数0.885).(2)荷木林冠层-大气脱耦联系数(Ω)在湿季接近l,干季则较湿季小,说明湿季叶片的界面层较厚,水汽压亏缺对Gs影响较小,而光合有效辐射是控制Gs的主要环境因子.     

开放式空气CO2浓度增高对水稻冠层微气候的影响

利用位于江苏省无锡市安镇的我国唯一的农田开放式空气CO₂浓度增高(FACE)系统平台,于2001年8月26日至10月13日(水稻抽穗至成熟期)进行水稻作物冠层微气候连续观测,以研究FACE对水稻冠层微气候特征的影响.结果表明,FACE降低了水稻叶片的气孔导度,FACE与对照水稻叶片气孔导度的差异上层叶片大于下层叶片,生长前期大于生长后期.FACE使白天水稻冠层和叶片温度升高,这种差异生长前期大于生长后期;但FACE对夜间水稻冠层温度的影响不明显.在水稻旺盛生长的抽穗开花期,晴天正午前后FACE水稻冠层温度比对照高1.2℃;从开花至成熟期,FACE水稻冠层白天平均温度比对照高0.43℃.FACE对冠层空气温度也有影响,白天水稻冠层空气温度FACE高于对照,这种差异随太阳辐射增强而增大且冠层中部大于冠层顶部;冠层中部空气温度FACE与对照的差异(T_face-T_ambient)日最大值在0.47～1.2℃之间,而冠层顶部的T_face-T_ambient日最大值在0.37～0.8℃之间.夜间水稻冠层空气温度FACE与对照差别不大,变化在±0.3℃之内.而FACE对水稻冠层空气湿度无显著影响,表明FACE使水稻叶片气孔导度降低,从而削弱了植株的蒸腾降温作用,导致水稻冠层温度和冠层空气温度升高,改变了整个水稻冠层的温度环     

Estimating transpiration from 6-year-old Eucalyptus grandis trees: development of a canopy conductance model and comparison with independent sap flux measurements

Daily patterns of stomatal conductance (g_s), xylem pressure potential (P) and canopy microclimatic variables were recorded on 11 sample days as part of a one-year study of the water use of Eucalyptus grandis Hill ex Maiden in the eastern Transvaal, South Africa. Measured g_s was found to be largely controlled by quantum flux density (Q) and ambient vapour pressure deficit (D). Canopy conductance (g_c) was determined for hourly intervals using g_s measurements and leaf areas in four different canopy levels. A simple model was constructed to allow the prediction of g_c and transpiration from Q, D and season of year. The model was used to estimate transpiration rates from 10 trees in a later study of similarly-aged E. grandis trees, in which sap flow in each tree was measured using the heat pulse velocity (HPV) technique. Five of the trees were monitored on a summer day and five on a winter day. Correspondence between HPV sap flow and modelled transpiration was good for the summertime comparisons, but measured winter-time sap flow rates were underestimated by the model, especially under conditions of high sap flow. The discrepancy is believed to result from having insufficient data from the conductance study to describe the response of g_s to relatively high D in winter. Marked variation in transpiration per unit leaf area indicates that a relatively large number of trees must be sampled for the HPV technique to be used to obtain a mean rate for an entire stand in winter.     

Late-stage canopy tree species with extremely low δ13C and high stomatal sensitivity to seasonal soil drought in the tropical rainforest of French Guiana

We assessed the daily time‐courses of CO₂ assimilation rate (A), leaf transpiration rate (E), stomatal conductance for water vapour (g_s), leaf water potential ( Ψ _w) and tree transpiration in a wet and a dry season for three late‐stage canopy rainforest tree species in French Guiana differing in leaf carbon isotope composition ( δ ¹³C). The lower sunlit leaf δ ¹³C values found in Virola surinamensis ( ? 29·9‰) and in Diplotropis purpurea ( ? 30·9‰), two light‐demanding species, as compared to Eperua falcata ( ? 28·6‰), a shade‐semi‐tolerant species, were clearly associated with higher maximum g_s values of sunlit leaves in the two former species. These two species were also characterized by a high sensitivity of g_s, sap flow density (Ju) and canopy conductance (g_c) to seasonal soil drought, allowing maintenance of high midday Ψ _w values in the dry season. The data for Diplotropis provided an original picture of increasing midday Ψ _w with increasing soil drought. In Virola, stomata were extremely sensitive to seasonal soil drought, leading to a dramatic decrease in leaf and tree transpiration in the dry season, whereas midday Ψ _w remained close to ? 0·3 MPa. The mechanisms underlying such an extremely high sensitivity of stomata to soil drought remain unknown. In Eperua, g_s of sunlit leaves was non‐responsive to seasonal drought, whereas Ju and g_c were lower in the dry season. This suggests a higher stomatal sensitivity to seasonal drought in shaded leaves than in sunlit ones in this species.     

桂花树冠层气孔导度模型的优化及其参数分析

冠层气孔导度(g_c)是许多陆面过程模型中的重要参数,提高对冠层气孔导度的模拟精度非常重要。以环境因子阶乘的Jarvis形式的模型是气孔导度模型中的典型代表,但研究中不同的环境因子有不同的响应方程和参数。研究认为不同的响应方程有不同的模拟效果,并通过比较各环境因子的不同响应方程组合的模型的模拟效果来确定最优的g_c模型。以桂花树为例,测定了树干液流、茎水势和微气象环境,用Penman-Monteith(PM)方程反推计算冠层气孔导度并检验不同方程组合的16种模型。模型的参数用DiffeRential Evolution Adaptive Metropolis(DREAM)模型优化。结果表明这种方法能够有效地找到各环境因子最优的响应方程,从而最优化g_c模型。优化的g_c模型很好地模拟了桂花树冠层气孔导度的变化,尤其是对干旱的响应,模拟值与PM计算值的相关系数和均方根误差分别为0.803和0.000623 m/s。同时也证明了模型中温度函数f(T)1的现象并非个例,由于温度(T)和水汽压亏缺(D)常是高度相关的,建议在以后的g_c模型研究中应把T和D看成一个影响因子,但f(T)1的这种现象是否具有全球性还有待进一步研究证实。     

Apparent responses of stomata to transpiration and humidity in a hybrid poplar canopy

It has been proposed that the stomatal response to humidity relies on sensing of the transpiration rate itself rather than relative humidity or the saturation deficit per se. We used independent measurements of stomatal conductance (g_s), transpiration (E), and leaf-to-air vapour pressure difference (V) in a hybrid poplar canopy to evaluate relationships between g_s and E and between g_s and V. Relationships between E, V and total vapour phase conductance or crown conductance (g_c) were also assessed. Conductance measurements were made on exposed and partially shaded branches over a wide range of incident solar radiation. In exposed branches, g_s appeared to decline linearly with increasing E and increasing V at both high and low irradiance. However, in a partially shaded branch, a bimodal relationship between g_s and E was observed in which g_s continued to decrease after E had reached a maximum value and begun to decrease. The relationship between g_s and V for this branch was linear. Plots of g_c against E always yielded bimodal or somewhat variable relationships, whereas plots of g_c against V were invariably linear. It was not possible to derive a unique relationship between conductance and E or V because prevailing radiation partially determined the operating range for conductance. Normalization of data by radiation served to linearize responses observed within the same day or type of day, but even after normalization, data collected on partly cloudy days could not be used to predict stomatal behaviour on clear days and vice versa. An additional unidentified factor was thus also involved in determining operating ranges of conductance on days with different overall radiation regimes. We suggest that the simplest mechanism to account for the observed humidity responses is stomatal sensing of the epidermal or cuticular transpiration rate rather than the bulk leaf or stomatal transpiration rate.     

Regulation of water flux through trunks, branches, and leaves in trees of a lowland tropical forest

We studied regulation of whole-tree water use in individuals of five diverse canopy tree species growing in a Panamanian seasonal forest. A construction crane equipped with a gondola was used to access the upper crowns and points along the branches and trunks of the study trees for making concurrent measurements of sap flow at the whole-tree and branch levels, and vapor phase conductances and water status at the leaf level. These measurements were integrated to assess physiological regulation of water use from the whole-tree to the single-leaf scale. Whole-tree water use ranged from 379 kg day⁻¹ in a 35 m-tall Anacardium excelsum tree to 46 kg day⁻¹ in an 18 m-tall Cecropia longipes tree. The dependence of whole-tree and branch sap velocity and sap flow on sapwood area was essentially identical in the five trees studied. However, large differences in transpiration per unit leaf area (E) among individuals and among branches on the same individual were observed. These differences were substantially reduced when E was normalized by the corresponding branch leaf area:sapwood area ratio (LA/SA). Variation in stomatal conductance (g _s) and crown conductance (g _c), a total vapor phase conductance that includes stomatal and boundary layer components, was closely associated with variation in the leaf area-specific total hydraulic conductance of the soil/leaf pathway (G _t). Vapor phase conductance in all five trees responded similarly to variation in G _t. Large diurnal variations in G _t were associated with diurnal variation in exchange of water between the transpiration stream and internal stem storage compartments. Differences in stomatal regulation of transpiration on a leaf area basis appeared to be governed largely by tree size and hydraulic architectural features rather than physiological differences in the responsiveness of stomata. We suggest that reliance on measurements gathered at a single scale or inadequate range of scale may result in misleading conclusions concerning physiological differences in regulation of transpiration. Received: 1 October 1997 / Accepted: 6 March 1998     

Whole-Plant Water use and Canopy Conductance of Cassava Under Iimited Available Soil Water and Varying Evaporative Demand

Cassava (Manihot esculenta Crantz), a perennial woody shrub, is known to be highly productive under favourable conditions and produce reasonably well under adverse conditions where other crops fail. Using constant heat sap flow sensors, sap flow density (F _d ) of cassava was monitored for 10 days in December 2002. Sap flow was highly correlated (R ² =0.72, P<0.05) to incoming solar radiation (R _s) than to other climatic factors. Using cross-correlation analysis, no time shift was detected between F _d and solar radiation, whereas vapour pressure deficit (VPD) lags F _d by 110 min. Solar radiation and VPD together explained 83% of diurnal variation in sap flow. Whole-plant transpiration ranged from 0.8 to 1.2 mm day⁻¹ and daily canopy conductance (g _c), computed based on the inverted Penman–Monteith model, varied between 0.7 and 2.1 mm s⁻¹ (mean = 1.4 ± 0.5 mm s⁻¹). For the measurement period, characterized by high evaporative demand coupled with low available soil water, transpiration accounted for 21% of the available energy and was only able to meet 24% of the atmospheric water demand. Average decoupling factor (Ω) of 0.05±0.02 estimated suggested that a 10% change in g _c may lead to more than 9% change in transpiration which further supports the notion that stomata play significant role in regulating cassava water use compared to other known mechanisms. Beyond light saturation (R _s >300 W m⁻²) and at higher VPD (>1.0 kPa), wind effects on the canopy transpiration under water stress condition were low, while VPD explains 94% of the observed variance in daily canopy conductance.     

Control of transpiration in an irrigated Eucalyptus globulus Labill. plantation

Stomatal conductance and transpiration were measured concurrently in an irrigated Eucalyptus globulus Labill. plantation. Canopy stomatal conductance, canopy boundary layer conductance and the dimensionless decoupling coefficient (Ω) were calculated (a) summing the conductance of three canopy layers (g_c) and (b) weighting the contribution of foliage according to the amount of radiation received (g_c′). Canopy transpiration was then calculated from g_c and g_c′ for Ω = 1 (E_eq), Ω = 0 (E_imp) and by weighting E_eq and E_imp using Ω (E_Ω). E_eq, E_imp and E_Ω were compared to transpiration estimated from measurements of heat pulse velocity. The mean value of Ω was 0·63. Transpiration calculated using g_c and assuming perfect coupling (12·5 ± 0·9 mmol m^?2 s^?1) significantly overestimated measured values (8·7 ± 0·8 mmol m^?2 s^?1). Good estimates of canopy transpiration were obtained either (a) calculating E_Ω separately for the individual canopy layers or (b) treating the canopy as a single layer and using g_c′ in a calculation of E_imp (Ω = 0). The latter approach only required measurement of stomatal conductance at a single canopy position but would be unsuitable for use in combined models of canopy transpiration and assimilation. It should however, be suitable for estimating transpiration in forests regardless of the degree of coupling.     

Drought acclimation of two deciduous tree species of different layers in a temperate forest canopy

Water-use strategies of Populus tremula and Tilia cordata, and the role of abscisic acid in these strategies, were analysed. P. tremula dominated in the overstorey and T. cordata in the lower layer of the tree canopy of the temperate deciduous forest canopy. Shoot water potential (), bulk-leaf abscisic acid concentration ([ABA]_leaf), abscisic acid concentration in xylem sap ([ABA]_xyl), and rate of stomatal closure following the supply of exogenous ABA (v) decreased acropetally through the whole tree canopy, and foliar water content per area (w), concentration of the leaf osmoticum (c), maximum leaf-specific hydraulic conductance of shoot (L), stomatal conductance (g_s), and the threshold dose per leaf area of the exogenous ABA (d_a) required to reduce stomatal conductance increased acropetally through the tree canopy (from the base of the foliage of T. cordata to the top of the foliage of P. tremula) in non-stressed trees. The threshold dose per leaf dry mass of the exogenous ABA (d_w) required to reduce stomatal conductance, was similar through the tree canopy. After a drought period (3 weeks), the , w, L, g_s, d_a and d_w had decreased, and c and v had increased in both species. Yet, the effect of the drought period was more pronounced on L, g_s, d_a, d_w and v in T. cordata, and on , w and c in P. tremula. It was concluded that the water use of the species of the lower canopy layer—T. cordata, is more conservative than that of the species of the overstorey, P. tremula. [ABA]_leaf had not been significantly changed in these trees, and [ABA]_xyl had increased during the drought period only in P. tremula. The relations between [ABA]_leaf, [ABA]_xyl and the stomatal conductance, the osmotic adjustment and the shoot hydraulic conductance are also discussed.     

镉胁迫下硒对罗汉果组培苗光合特性的影响

实验以罗汉果组培苗为材料,室内栽培在内装市售营养土的塑料盆中,以0、10、50、100、200mg·kg-1浓度镉离子和1mg·kg-1浓度硒处理,培养20d后分析罗汉果幼苗的相关光合生理指标。结果表明:低浓度Cd2+对叶片叶绿素含量、光合速率(Pn)、蒸腾速率(Tr)、气孔导度(Gs)影响不大或稍有上升,但高浓度镉离子处理植株叶片的叶绿素含量、光合速率(Pn)、蒸腾速率(Tr)、气孔导度(Gs)明显下降;随Cd2+处理浓度的增加,叶片胞间CO2浓度(Ci)呈现上升趋势;加硒则延缓叶绿素下降,促进光合速率(Pn)、蒸腾速率(Tr)、气孔导度(Gs)上升,降低叶片胞间CO2浓度(Ci)。表明高浓度镉离子的毒害导致罗汉果组培苗叶片光合性能受到伤害,从而影响罗汉果幼苗生长。镉硒混合处理反映出硒对镉的毒害有缓解作用。