黄土高原苹果园蒸腾导度大气驱动规律比较

植物蒸腾导度是表征土壤-植物-大气连续体（SPAC）中植物-大气间水汽传导过程、反映植物水分调控能力的一类重要变量,常见有冠层导度（G_c）、冠层气孔导度（G_s）与叶片气孔导度（g_s）,明确三者在反映冠层蒸腾过程时的异同或关联性对于理解植物水分利用机制具有重要意义。本研究基于对黄土高原果园苹果树生长季内树干液流（J_s）及环境因子的连续观测,计算了G_c、G_s及脱耦联系数（Ω）等变量,并与短期连续观测的叶片气孔导度（g_s）比较,分析了G_c、G_s和g_s在反映冠层蒸腾特征方面的异同及其关系。结果表明,日变化过程中G_s、g_s呈\"单峰\"型曲线,而G_c则呈\"先增后减,午后抬升\"的\"双峰\"型曲线。g_s与G_s存在较紧密的线性关系（R²=0.80）,但与G_c的线性关系较弱（R²=0.02）。G_c、G_s均随大气水汽压亏缺（VPD）的变化呈现确定的规律,其中,上边界函数呈递减的对数函数关系,平均值则符合先增后减的Log-Normal函数关系（R²>0.95）,拐点对应的VPD值分别为1.33和1.16 kPa。在一日内,G_s对VPD变化的响应过程与g_s对VPDL （基于叶片温度计算的水汽压亏缺）变化的响应过程总体一致,其一致性高于G_c对VPD变化的响应。整个生长季（4-10月）中果树的Ω平均值为0.12,随着Ω递减,G_c与G_s的线性相关性愈趋紧密,其斜率呈递增趋势,G_c越来越趋近于G_s。研究结果表明,在北方地区,基于树干液流的监测能较准确的推导整株并估算林分的冠层蒸腾导度。与实测g_s的变化过程比较,G_s比G_c具有更高的一致性,G_s可以作为描述苹果树水分利用过程响应大气驱动的更为恰当的变量。     

Relationships Between Nitrogen Content and Net Gas Exchange Components of a Cotton Leaf During Ontogeny

Relationships between leaf nitrogen (N) content and leaf gas exchange components of a single cotton (Gossypium hirsutum L.) leaf subtending the fruit during ontogeny were investigated under field conditions. A 20-d old leaf exhibited the highest physiological activity characterized by net photosynthetic (PN) and transpiration (E) rates, stomatal conductances to CO2 exchange (gsCO2) and water vapor transfer (gsH2O), and nitrogen (N) content. With the advent of leaf senescence, the gas exchange rates declined as exhibited by the 30-, 40-, and 60-d old leaves. Regression analysis indicated close relationships between gsCO2 and PN, and gsH2O and E as the leaves advanced towards senescence. Both PN and gsCO2 were related to N as they declined with leaf age. Thus, the declines in PN were associated with stomatal closure and removal of N during leaf ontogeny.     

A coupled model of stomatal conductance, photosynthesis and transpiration

A model that couples stomatal conductance, photosynthesis, leaf energy balance and transport of water through the soil–plant–atmosphere continuum is presented. Stomatal conductance in the model depends on light, temperature and intercellular CO₂ concentration via photosynthesis and on leaf water potential, which in turn is a function of soil water potential, the rate of water flow through the soil and plant, and on xylem hydraulic resistance. Water transport from soil to roots is simulated through solution of Richards’ equation. The model captures the observed hysteresis in diurnal variations in stomatal conductance, assimilation rate and transpiration for plant canopies. Hysteresis arises because atmospheric demand for water from the leaves typically peaks in mid‐afternoon and because of uneven distribution of soil matric potentials with distance from the roots. Potentials at the root surfaces are lower than in the bulk soil, and once soil water supply starts to limit transpiration, root potentials are substantially less negative in the morning than in the afternoon. This leads to higher stomatal conductances, CO₂ assimilation and transpiration in the morning compared to later in the day. Stomatal conductance is sensitive to soil and plant hydraulic properties and to root length density only after approximately 10 d of soil drying, when supply of water by the soil to the roots becomes limiting. High atmospheric demand causes transpiration rates, LE, to decline at a slightly higher soil water content, θ_s, than at low atmospheric demand, but all curves of LE versus θ_s fall on the same line when soil water supply limits transpiration. Stomatal conductance cannot be modelled in isolation, but must be fully coupled with models of photosynthesis/respiration and the transport of water from soil, through roots, stems and leaves to the atmosphere.     

Photosynthesis and water-use characteristics in Indian mangroves

Photosynthesis and water efflux were measured in different PAR and stomatal conductance in members of Avicenniaceae and Rhizophoraceae. Trend of leaf temperature with irradiance and its effect on photosynthesis were also estimated. In most of the studied species, photosynthesis and stomatal conductance followed similar trends with increase in irradiance. The rate of net photosynthesis and stomatal conductance were higher in members of Avicenniaceae than in Rhizophoraceae. In Avicenniaceae, the optimum PAR for maximum photosynthesis ranged between 1340–1685 (μmol m^-2s^-1, which was also higher than that of Rhizophoraceae (840-1557 μmol m^-2s^-1). Almost in all the studied taxa, transpiration and stomatal conductance followed similar trends and reached the maximal peaks at the same PAR value. The range of breakeven leaf temperature was almost the same in both the families (34-36°C in Avicenniaceae and 33.5-36.3°C in Rhizophoraceae), beyond which assimilation rate declined.     

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。     

植物光合生产力与冠层蒸散模拟研究进展

植物的光合与蒸腾的模拟已经从经验模型发展到过程模型的时代。概括地论述叶片和冠层尺度上,植物生理生态的基本过程,分析近年来几个有代表性的模型在模拟光合作用,蒸腾作用时,对这些听参数化处理的方法,即在叶片水平上,以Ｆａｒｑｕｈａｒ的叶片光合作用的生化模型,Ｂａｌｌ－Ｂｅｒｒｙ的气孔导度模型等为基础。     

叶片生长进程中气孔发育对叶温调节的影响

鉴于气孔发育影响气孔导度和蒸腾速率,推测气孔发育可能影响叶温调节。为验证这一假设并阐述相关规律,在控光和控温条件下研究了冬青卫矛和华北紫丁香气孔发育、气孔导度、蒸腾速率及其与叶温的关系。结果表明,伴随冬青卫矛、华北紫丁香叶片生长气孔逐渐增大,但气孔密度下降;在此过程中,气孔导度和蒸腾速率逐步提高,而叶片温度降低;尽管冬青卫矛和华北紫丁香叶片的气孔密度和大小差异很小,但华北紫丁香近轴侧和远轴侧均有气孔分布,而冬青卫矛则只有远轴侧分布气孔,且相同条件下华北紫丁香的气孔导度和蒸腾速率高、叶温低。因此,气孔发育能够促进气孔导度和蒸腾速率提高,有助于降低叶温;近轴侧气孔可能更有利于蒸腾降温。     

Photosynthesis and water-use efficiency of some mangroves from Sundarbans, India

We studied seasonal fluctuations in the rates of photosynthesis, transpiration, PAR, and stomatal conductance for 16 species of true mangroves from the Sundarbans region of West Bengal. Soil salinity and pH were also measured. Leaf temperatures were almost always higher than the ambient temperature. We observed considerable seasonal (summer vs winter) as well as interspecific variations in photosynthesis, with the highest rates occurring inHeritiera fomes (13.21 pmol m^-2s^-1) andAvicennia marina (11.8 mol m^-2s^-1), and the lowest inNypa fruticans (1.56 mol m^-2s^-1) andCeriops decandra (2.32 pmol m^-2s^-1), in many species, an abrupt rise in leaf temperature retarded the photosyn-thetic process. In winter, the rate of transpiration and stomatal conductance reached their maxima inA. marina (4.83 mmol ra^-2s^-1 and 124.23 m mol m^-2s^-1, respectively) and their mimima inExcoecaria agallocha (1.85 mmol m^-2s^-1 and 49.19 mmol m^-2s^-1, respectively). In contrast, the maximum summer readings were recorded in E.agallocha (6.07 mmol m^-2s^-1 and 192.74 mmol m^-2s^-1 respectively).     

Transpiration of trees and forest stands: short and long-term monitoring using sapflow methods

We show that sapflow is a useful tool for studies of water fluxes in forest ecosystems, because (i) it gives access to the spatial variability within a forest stand, (ii) it can be used even on steep slopes, and (iii) when combined with eddy correlation measurements over forests, it allows separation of individual tree transpiration from the total water loss of the stand. Moreover, sapflow techniques are quite easy to implement. Four sapflow techniques currently coexist, all based on heat diffusion in the xylem. We found a good agreement between three of these techniques. Most results presented here were obtained using the radial flow meter (Granier 1985). Tree sapflow is computed as sap flux density times sapwood area. To scale up from trees to a stand, measurements have to be made on a representative sample of trees. Thus, a number of trees in each circumference class is selected according to the fraction of sapwood they represent in the total sapwood area of the stand. The variability of sap flux density among trees is usually low (CV. 10–15%) in close stands of temperate coniferous or deciduous forests, but is much higher (35–50%) in a tropical rain forest. It also increases after thinning or during a dry spell. A set of 5–10 sapflow sensors usually provides an accurate estimate of stand transpiration. Transpiration measured on two dense spruce stands in the Vosges mountains (France) and one Scot's pine plantation in the Rhine valley (Germany) showed that maximum rate was related to stand LAI and to local climate. Preliminary results comparing the sapflow of a stand of Pinus banksiana to the transpiration of large branches, as part of the BOREAS programme in Saskachewan, Canada showed a similar trend. For modelling purposes, tree canopy conductance (g_c) was calculated from Penman-Monteith equation. In most experiments, calculated canopy conductance was dependent on global radiation (positive effect) and on vapour pressure deficit (negative effect) in the absence of other limiting factors. A comparison of the vapour pressure deficit response curves of g_c for several tree species and sites showed only small differences among spruce, oak and pine forests when including understorey. Tropical rainforests exhibited a similar behaviour.     

Comparative ecophysiology of leaf and canopy photosynthesis

Leaves and herbaceous leaf canopies photosynthesize efficiently although the distribution of light, the ultimate resource of photosynthesis, is very biased in these systems. As has been suggested in theoretical studies, if a photosynthetic system is organized such that every photosynthetic apparatus photosynthesizes in concert, the system as a whole has the sharpest light response curve and is most adaptive. This condition can be approached by (i) homogenization of the light environment and (ii) acclimation of the photosynthetic properties of leaves or chloroplasts to their local light environments. This review examines these two factors in the herbaceous leaf canopy and in the leaf. Changes in the inclination of leaves in the canopy and differentiation of mesophyll into palisade and spongy tissue contribute to the moderation of the light gradient. Leaf and chloroplast movements in the upper parts of these systems under high irradiances also moderate light gradients. Moreover, acclimation of leaves and chloroplasts to the local light environment is substantial. These factors increase the efficiency of photosynthesis considerably. However, the systems appear to be less efficient than the theoretical optimum. When the systems are optically dense, the light gradients may be too great for leaves or chloroplasts to acclimate. The loss of photosynthetic production attributed to the imperfect adjustment of photosynthetic apparatus to the local light environment is most apparent when the photosynthesis of the system is in the transition between the light-limited and light-saturated phases. Although acclimation of the photosynthetic apparatus and moderation of light gradients are imperfect, these markedly raise the efficiency of photosynthesis. Thus more mechanistic studies on these adaptive attributes are needed. The causes and consequences of imperfect adjustment should also be investigated.     

苹果三维树冠的净光合速率分布模拟

构建三维树冠光合模型可模拟出叶片净光合速率(P_n)、气孔导度(G_s)和光能利用效率(LUE)在树冠内的三维分布。以17年生纺锤形\"富士\"苹果树(Malus domestica Borkh. cv. ‘Fuji’)为试材,通过实测确定三维树冠内叶片和辐射分布,根据不同部位叶片最大光合速率经验公式模拟叶片P_n 在三维树冠空间内分布,并据2007-2009年测定数据拟合相关模型参数。模拟表明,苹果树冠叶片P_n 和辐射的三维分布相似,在树冠上部P_n 三维分布比较平缓,然后随辐射的减少而迅速降低。高辐射条件下(PAR=1500 μmol·m^-2·s^-1),从树冠上部3 m处降到到1 m,平均相对辐射从71.18%降到8.05%,减少了89%,叶片平均P_n从15.05 μmol·m^-2·s^-1降到1.92 μmol·m^-2·s^-1,减少了87%。单位体积小室内的总净光合速率大小主要取决于叶面积密度,部分取决于P_n。G_s三维分布与P_n相似,而LUE分布与辐射相反,中下部高,上部低。根据光合机理模型、树冠内辐射和叶面积三维分布可模拟出苹果三维树冠内叶片的P_n、G_s和LUE分布,该模型参数少,可方便用于其它果树三维光合模型构建和果树整形修剪研究。     

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。     

Leaf nitrogen, photosynthesis, conductance and transpiration: scaling from leaves to canopies

A model is presented which solves simultaneously for leaf-scale stomatal conductance, CO₂ assimilation and the energy balance as a function of leaf position within canopies of well-watered vegetation. Fluxes and conductances were calculated separately for sunlit and shaded leaves. A linear dependence of photosynthetic capacity on leaf nitrogen content was assumed, while leaf nitrogen content and light intensity were assumed to decrease exponentially within canopies. Separate extinction coefficients were used for diffuse and direct beam radiation. An efficient Gaussian integration technique was used to compute fluxes and mean conductances for the canopy. The multilayer model synthesizes current knowledge of radiation penetration, leaf physiology and the physics of evaporation and provides insights into the response of whole canopies to multiple, interacting factors. The model was also used to explore sources of variation in the slopes of two simple parametric models (nitrogen- and light-use efficiency), and to set bounds on the magnitudes of the parameters. For canopies low in total N, daily assimilation rates are ～10% lower when leaf N is distributed uniformly than when the same total N is distributed according to the exponentially decreasing profile of absorbed radiation. However, gains are negligible for plants with high N concentrations. Canopy conductance, G_c should be calculated as G_c=Aσ(f_slg_sl+f_shg_sh), where Δ is leaf area index, f_si and f_sh are the fractions of sunlit and shaded leaves at each level, and g_si and g_sh are the corresponding stomatal conductances. Simple addition of conductances without this weighting causes errors in transpiration calculated using the ‘big-leaf’ version of the Penman-Monteith equation. Partitioning of available energy between sensible and latent heat is very responsive to the parameter describing the sensitivity of stomata to the atmospheric humidity deficit. This parameter also affects canopy conductance, but has a relatively small impact on canopy assimilation. Simple parametric models are useful for extrapolating understanding from small to large scales, but the complexity of real ecosystems is thus subsumed in unexplained variations in parameter values. Simulations with the multilayer model show that both nitrogen- and radiation-use efficiencies depend on plant nutritional status and the diffuse component of incident radiation, causing a 2- to 3-fold variation in these efficiencies.     

Photosynthesis affects following night leaf conductance in Vicia faba

Night-time stomatal opening in C₃ plants may result in significant water loss when no carbon gain is possible. The objective of this study was to determine if endogenous patterns of night-time stomatal opening, as reflected in leaf conductance, in Vicia faba are affected by photosynthetic conditions the previous day. Reducing photosynthesis with low light or low CO₂ resulted in reduced night-time stomatal opening the following night, irrespective of the effects on daytime stomatal conductance. Likewise, increasing photosynthesis with enriched CO₂ levels resulted in increased night-time stomatal opening the following night. Reduced night-time stomatal opening was not the result of an inability to regulate stomatal aperture as leaves with reduced night-time stomatal opening were capable of greater night-time opening when exposed to low CO₂. After acclimating plants to long or short days, it was found that night-time leaf conductance was greater in plants acclimated to short days, and associated with greater leaf starch and nitrate accumulation, both of which may affect night-time guard cell osmotic potential. Direct measurement of guard cell contents during endogenous night-time stomatal opening will help identify the mechanism of the effect of daytime photosynthesis on subsequent night-time stomatal regulation.