叶肉细胞导度研究进展

叶肉细胞导度指叶片叶肉细胞内部的CO2扩散能力,在植物生理生态及全球气候变化和陆地生态系统相互关系的研究中具有重要作用。系统介绍了叶肉细胞导度的发现、发展过程及其研究进展、几种目前国际上常用的叶肉细胞导度测度方法的原理、计算过程;强调了叶肉细胞导度作为光合作用扩散过程一部分的重要意义,明确了叶肉细胞导度的定义及分布范围。并探讨了不同方法的优缺点及注意事项。总结分析了叶肉细胞导度对不同环境因子(温度、水分及环境中CO2和O3浓度等)的响应,从不同角度对叶肉细胞导度的生态学意义进行了简单的概括。对叶肉细胞导度的未来研究进行了展望。     

耦合叶肉导度的陆面过程模型最大叶肉导度参数的敏感性分析

陆面过程模型添加叶肉导度能有效改善模型模拟的CO₂施肥效应精度,但叶肉导度模拟受最大叶肉导度参数取值的影响,优化模型中最大叶肉导度参数是改进陆面过程模型叶肉导度和CO₂施肥效应模拟的重要途径。以EALCO（Ecological Assimilation of Land and Climate Observations）模型为例添加叶肉导度,通过人为改变最大叶肉导度值的取值,分析模型输出结果对最大叶肉导度的响应,揭示最大叶肉导度参数在模型中的敏感性,并与已有研究结果或观测数据比较,探讨耦合叶肉导度的陆面过程模型最大叶肉导度参数优化的途径。模拟试验以美国哈佛森林典型温带落叶阔叶林生态监测站（US-Ha1 site,Harvard Forest Environmental Monitoring site）数据为驱动。结果显示：（1）随最大叶肉导度增加,总初级生产力（GPP, Gross Primary Production）模拟精度增加,但最大叶肉导度取值大于1.0 mol m^-2 s^-1后模拟精度改善有限,最大叶肉导度小于1.0 mol m^-2 s^-1时GPP模拟精度对最大叶肉导度变化响应敏感;（2）证实了叶肉导度与气孔导度之间存在明显线性关系,最大叶肉导度取值的变化能明显影响这种线性关系的斜率。当最大叶肉导度取值从0.5 mol m^-2 s^-1增加到1.2 mol m^-2 s^-1时,气孔导度与叶肉导度的比值从0.75左右降至0.36,这个结果表明,通过明确某一植被功能型叶肉导度与气孔导度比值,可以间接确定模型最大叶肉导度的合理取值范围;（3）证实了陆面过程模型添加叶肉导度能改进CO₂施肥效应模拟精度,最大叶肉导度值能影响施肥效应模拟结果,当最大叶肉导度高于0.57 mol m^-2 s^-1后,随最大叶肉导度增加,模拟GPP随大气CO₂浓度增加的增长率呈下降趋势;（4）在月尺度上叶肉导度模拟对最大叶肉导度值的敏感性随不同生长季而不同,在生长盛期的7、8月份最大叶肉导度对叶肉导度模拟结果影响最大,其次是5、6、9月份等生长次盛期,其他月份的影响较小。     

红树林植物叶片形态和解剖特征对叶肉导度、叶片导水率的影响

叶肉导度和叶片导水率是影响光合作用的两个重要过程,叶肉导度通过影响从气孔下腔到Rubisco酶位点的二氧化碳浓度梯度直接影响光合作用,而叶片导水率则通过影响水分供应或气孔行为来影响光合作用,然而对这两个生理过程之间的协同性研究较少。本研究选择9种红树林植物为研究对象,探讨盐生环境下植物叶肉导度和叶片导水率的协同性及其与叶片解剖结构特征之间的相关性。结果表明,9种红树林植物叶片导水率(0.78~5.83 mmol·m~(-2)·s~(-1)·MPa-1)、叶肉导度(0.06~0.36 mol·m~(-2)·s~(-1))、最大光合速率(7.23~23.71μmol·m~(-2)·s~(-1))等特征的差别较大;叶肉导度与最大光合速率呈显著正相关,而与比叶重无显著相关性,其原因是由于比叶重与叶片厚度、叶片密度不存在相关性;叶脉密度与气孔密度呈较强的相关性,说明红树林植物叶片水分运输与散失相关的叶片结构之间存在协同关系;叶片导水率不受叶脉密度影响,并且与叶肉导度、最大光合速率也不存在相关性,这很可能与红树林植物叶片的肉质化、有发达的储水组织有关,体现了红树林植物叶片结构和功能的特殊性。     

氮磷亏缺对玉米根系水流导度的影响

在人工气候室水培条件下,从单根和整株根系两个层次研究了N、P营养与玉米(Zea mays L.)根系水流导度(root hydraulic conductivity,Lpr)间的关系。结果表明：表型抗旱的杂交种F1代户单4号和母本天四的单根水导和整株根系水导均高于不抗旱的父本478,其中天四的单根水导最高,而户单4号的整株根系水导最高。N、P亏缺均使玉米单根水导和整株根系水导降低,但与N亏块相比,P亏缺的植株具有较高的整株根系水导和较低的单根水导。整株根系的水导更能反映植物根系的输水性能。     

植物气孔导度对CO2响应模型的构建

构建一个普适性的植物叶片气孔导度(g_s)对CO₂浓度响应(g_s-C_a)的模型, 对定量研究植物叶片g_s对CO₂浓度的响应变化尤为必要。该研究运用便携式光合仪(LI-6400)测量了大豆(Glycine max)和小麦(Triticum aestivum)光合作用对CO₂的响应曲线(A_n-C_a), 在比较传统的Michaelis-Menten模型(M-M模型)和叶子飘构建的CO₂响应模型拟合大豆和小麦A_n-C_a效果的基础上, 构建了g_s-C_a响应新模型。然后用新构建的模型拟合大豆和小麦的g_s-C_a曲线, 并将拟合结果与传统模型的拟合结果, 以及与其对应的观测数据进行比较, 以判断所构建模型是否合理。结果显示: 叶子飘构建的A_n-C_a模型可较好地拟合大豆和小麦的A_n-C_a曲线, 确定系数(R²)均高达0.999。M-M模型拟合大豆和小麦的A_n-C_a曲线时的R²虽然也较高, 但在较高CO₂浓度时的拟合曲线偏离观测曲线。因此, 基于叶子飘的A_n-C_a模型构建g_s-C_a模型更为可行。新构建的g_s-C_a模型可较好地拟合大豆和小麦的g_s-C_a曲线, R²分别为0.995和0.994, 而且还可以直接给出最大气孔导度(g_s-max)、最小气孔导度(g_s-min), 以及与g_s-min相对应的CO₂浓度值(C_s-min)。拟合得到大豆和小麦的g_s-max分别为0.686和0.481 mol·m^-2·s^-1, 与其对应的观测值(分别为0.666和0.471 mol·m^-2·s^-1)之间均不存在显著差异; 同样, 拟合得到的大豆和小麦的g_s-min分别为0.271和0.297 mol·m^-2·s^-1, 与其对应的观测值(分别为0.279和0.293 mol·m^-2·s^-1)之间也均不存在显著差异; 此外, 新构建的g_s-C_a模型给出大豆和小麦的C_s-min值分别为741.45和1 112.43 μmol·mol ^-1, 与其对应的观测值(732.78和1 200.34 μmol·mol ^-1)也不存在显著差异。由此可见, 该研究新构建的g_s-C_a模型可作为定量研究植物叶片气孔导度对CO₂浓度变化的有效数学工具。     

羊草叶片气孔导度对环境因子的响应模拟

准确定量描述植物气孔对环境的响应是了解植物光合作用机理、预测植物生产力及其大气－植被－土壤系统中水分和热量交换的关键。利用松嫩平原盐碱化草地羊草光合生理特征的野外观测数据,分析了羊草叶片气孔导度对环境因子的反应,结果表明：羊草叶片气孔导度对环境因子变化敏感,尤其对瞬时光合有效辐射（ＰＡＲ）、叶片与空气间的水汽压亏损（ＶＰＤ）和空气温度（Ｔａ）反应十分明显。依据野外实测资料对国际上两类代表性气孔导度     

胡杨叶片气孔导度特征及其对环境因子的响应

依据2005年对极端干旱区荒漠河岸林胡杨的观测资料,对胡杨气孔运动进行了分析研究以揭示胡杨的水分利用特征与抗旱机理。结果表明:(1)胡杨叶片气孔导度日变化呈现为周期波动曲线,其波动周期为2 h,傍晚(20:00)波动消失;净光合速率和蒸腾速率与气孔导度的波动相对应而呈现同步周期波动。(2)胡杨的阳生叶气孔导度高于阴生叶,且不同季节气孔导度值不同,阳生叶气孔导度的季节变幅大于阴生叶。(3)胡杨气孔导度与气温、相对湿度和叶水势有显著相关关系,当CO2浓度较小时,胡杨气孔导度随CO2浓度的增加而增加,当CO2浓度达到一定值后气孔导度不再增加,反而随CO2浓度的增加大幅度降低。(4)胡杨适应极端干旱区生境的气孔调节机制为反馈式反应,即由于叶水势降低导致气孔导度减小,从而减少蒸腾耗水,达到节约用水、适应干旱的目的,表明胡杨的水分利用效率随气孔限制值的增大而减小,二者呈显著负相关。     

光合作用-蒸腾作用-气孔导度的耦合模型及C_3植物叶片对环境因子的生理响应(英文)

以叶片的气体传输过程为基础,将蒸腾作用包括在以往光合作用气孔导度的耦合模型中,建立了光合作用蒸腾作用气孔导度的耦合模型。该模型可以模拟边界层导度对生理过程的影响。模拟了Ｃ３植物叶片对环境因子,如光照、温度、湿度、边界层导度和ＣＯ２浓度等的生理响应（光合作用、蒸腾作用、气孔导度）以及Ｃｉ和水分利用效率的变化。在环境因子变化于较大范围的情况下,模拟结果符合许多实验结论。     

秸秆覆盖与施磷对西南冬小麦叶肉导度和CO2同化效率的影响

为探究西南麦区冬小麦旗叶叶肉导度(g_m)与CO₂同化对土壤速效磷缺乏的响应。试验于2020-2022年在四川仁寿试验站进行秸秆覆盖和磷素水平的二因素裂区试验,以秸秆覆盖(SM)和不覆盖(NSM)为主区;三个磷水平0(P0)kg/hm²、75(P75)kg/hm²和120(P120)kg/hm²为裂区,分析小麦旗叶光响应曲线、叶肉导度和CO₂同化对旗叶磷素水平的响应。结果表明:与NSM相比,SM下旗叶比叶重(LMA)、单位面积磷含量(PA)、单位质量磷含量(PM)和净光合速率(P_n)分别提高4.1%、16.9%、12.2%和6.9%,且随着施磷量的增加,旗叶的LMA、PA、PM和P_n呈增加趋势。秸秆覆盖与施磷显著提高旗叶最大净光合速率(P_nmax)和表观量子效率(AQY),增加旗叶的光合潜能。施磷显著提高旗叶光系统II实际光化学效率(ΦPSII),最大羧化效率(V_cmax)、核酮糖-1,5-二磷酸羧化酶(Rubisco)活力、胞间CO₂浓度(C_c)、叶绿体CO₂浓度(C_c)、气孔导度(g_s)和g_m,P75和P120较P0分别提高16.8%-23.8%、27.9%-35.3%、30.9%-61.7%、1.4%-3.6%、4.8%-13.1%、9.0%-10.2%和16.0%-16.9%,P75和P120无显著差异。秸秆覆盖与施磷降低旗叶的气孔限制(L_s)和叶肉限制(L_m),生化限制(L_b)是光合速率的主要限制因子。综上所述,秸秆覆盖配施75kg/hm²磷肥可提高西南冬小麦旗叶磷素含量、光系统II实际光化学效率、叶肉导度和CO₂同化效率,降低气孔限制和叶肉限制,从而提高旗叶净光合速率。     

Structural Adaptation of the Leaf Mesophyll to Shading

Structural characteristics of the mesophyll were studied in five boreal grass species experiencing a wide range of light and water supply conditions. Quantitative indices of the palisade and spongy mesophyll tissues (cell and chloroplast sizes, the number of chloroplasts per cell, the total cell and chloroplast surface area per unit leaf surface area) were determined in leaves of each of the species. The cell surface area and the cell volume in spongy mesophyll were determined with a novel method based on stereological analysis of cell projections. An important role of spongy parenchyma in the photosynthetic apparatus was demonstrated. In leaves of the species studied, the spongy parenchyma constituted about 50% of the total volume and 40% of the total surface area of mesophyll cells. The proportion of the palisade to spongy mesophyll tissues varied with plant species and growth conditions. In a xerophyte Genista tinctoria, the total cell volume, cell abundance, and the total surface area of cells and chloroplasts were 30–40% larger in the palisade than in the spongy mesophyll. In contrast, in a shade-loving species Veronica chamaedris, the spongy mesophyll was 1.5–2 times more developed than the palisade mesophyll. In mesophyte species grown under high light conditions, the cell abundance and the total cell surface area were 10–20% greater in the palisade mesophyll than in the spongy parenchyma. In shaded habitats, these indices were similar in the palisade and spongy mesophyll or were 10–20% lower in the palisade mesophyll. In mesophytes, CO₂ conductance of the spongy mesophyll accounted for about 50% of the total mesophyll conductance, as calculated from the structural characteristics, with the mesophyll CO₂ conductance increasing with leaf shading.     

The chloroplast avoidance response decreases internal conductance to CO2 diffusion in Arabidopsis thaliana leaves

The relationship between chloroplast arrangement and diffusion of CO(2) from substomatal cavities to the chloroplast stroma was investigated in Arabidopsis thaliana. Chloroplast position was manipulated by varying the amount of blue light and by cytochalasin D (CytD) treatment. We also investigated two chloroplast positioning mutants. Chloroplast arrangement was assessed by the surface area of chloroplasts adjacent to intercellular airspaces (S(c)). Although it has been previously shown that long-term acclimation to high light is linked with a large S(c), we found that the short-term chloroplast avoidance response reduces S(c). This effect was not apparent in the blue-light-insensitive phot2 mutant, which did not show the avoidance response. As expected, the smaller S(c) induced by the avoidance response was coupled to a similar decrease in internal conductance. This reduction in internal conductance resulted in an increased limitation of the rate of photosynthesis. The limiting effect of S(c) on internal conductance and photosynthesis was also shown in chup1, a mutant with a constant small S(c) as the result of an unusual chloroplast arrangement. We conclude that chloroplast movements in A. thaliana can rapidly alter leaf morphological parameters, and this has significant consequences for the diffusion of CO(2) through the mesophyll.     

Mesophyll conductance to CO2 in Arabidopsis thaliana

The close rosette growth form, short petioles and small leaves of Arabidopsis thaliana make measurements with commercial gas exchange cuvettes difficult. This difficulty can be overcome by growing A. thaliana plants in 'ice-cream cone-like' soil pots. This design permitted simultaneous gas exchange and chlorophyll fluorescence measurements from which the first estimates of mesophyll conductance to CO(2) (g(m)) in Arabidopsis were obtained and used to determine photosynthetic limitations during plant ageing from c. 30-45 d. Estimations of g(m) showed maximum values of 0.2 mol CO(2) m(-2) s(-1) bar(-1), lower than expected for a thin-leaved annual species. The parameterization of the response of net photosynthesis (A(N)) to chloroplast CO(2) concentrations (C(c)) yielded estimations of the maximum velocity of carboxylation (V(c,max_Cc)) which were also lower than those reported for other annual species. As A. thaliana plants aged from 30 to 45 d, there was a 40% decline of A(N) that was entirely the result of increased diffusional limitations to CO(2) transfer, with g(m) being the largest. The results suggest that in A. thaliana A(N) is limited by low g(m) and low capacity for carboxylation. Decreased g(m) is the main factor involved in early age-induced photosynthetic decline.     

Cell wall composition strongly influences mesophyll conductance in gymnosperms

Cell wall thickness is widely recognized as one of the main determinants of mesophyll conductance to CO₂ (g_m). However, little is known about the components that regulate effective CO₂ diffusivity in the cell wall (i.e. the ratio between actual porosity and tortuosity, the other two biophysical diffusion properties of cell walls). The aim of this study was to assess, at the interspecific level, potential relationships between cell wall composition, cell wall thickness (T_cw) and g_m. Gymnosperms constitute an ideal group to deepen these relationships, as they present, on average, the thickest cell walls within spermatophytes. We characterized the foliar gas exchange, the morphoanatomical traits related with g_m, the leaf fraction constituted by cell walls and three main components of primary cell walls (hemicelluloses, cellulose and pectins) in seven gymnosperm species. We found that, although the relatively low g_m of gymnosperms was mainly determined by their elevated T_cw, g_m was also strongly correlated with cell wall composition, which presumably sets the final effective CO₂ diffusivity. The data presented here suggest that (i) differences in g_m are strongly correlated to the pectins to hemicelluloses and cellulose ratio in gymnosperms, and (ii) variations in cell wall composition may modify effective CO₂ diffusivity in the cell wall to compensate the negative impact of thickened walls. We speculate that higher relative pectin content allows higher g_m because pectins increase cell wall hydrophilicity and CO₂ molecules cross the wall dissolved in water.     

The Simulation of Leaf Conductance Responses to Changes in Environmental Water Status

A simulation model of stomatal response to change of environmental water status was set up based on the works on the mechanism of stomatal movement. The variations of leaf conductance, water potential and turgot pressure in guard cells, subsidiary cells and the other cells or tissues in leaf with leaf-air vapour pressure difference and soil water potential have been calculated by our model. The calculated results fit very well with the data from experiments. The different patterns of leaf transpiration variation with the difference between leaf-air and vapour pressure can be explained quantitatively.     

Variations in leaf anatomical characteristics drive the decrease of mesophyll conductance in poplar under elevated ozone

Decline in mesophyll conductance (g_m) plays a key role in limiting photosynthesis in plants exposed to elevated ozone (O₃). Leaf anatomical traits are known to influence g_m, but the potential effects of O₃-induced changes in leaf anatomy on g_m have not yet been clarified. Here, two poplar clones were exposed to elevated O₃. The effects of O₃ on the photosynthetic capacity and anatomical characteristics were assessed to investigate the leaf anatomical properties that potentially affect g_m. We also conducted global meta-analysis to explore the general response patterns of g_m and leaf anatomy to O₃ exposure. We found that the O₃-induced reduction in g_m was critical in limiting leaf photosynthesis. Changes in liquid-phase conductance rather than gas-phase conductance drive the decline in g_m under elevated O_3, and this effect was associated with thicker cell walls and smaller chloroplast sizes. The effects of O₃ on palisade and spongy mesophyll cell traits and their contributions to g_m were highly genotype-dependent. Our results suggest that, while anatomical adjustments under elevated O₃ may contribute to defense against O₃ stress, they also cause declines in g_m and photosynthesis. These results provide the first evidence of anatomical constraints on g_m under elevated O₃.     

Leaf responses to drought stress in Mediterranean accessions of Solanum lycopersicum: anatomical adaptations in relation to gas exchange parameters

In a previous study, important acclimation to water stress was observed in the Ramellet tomato cultivar (TR) from the Balearic Islands, related to an increase in the water‐use efficiency through modifications in both stomatal (g_s) and mesophyll conductances (g_m). In the present work, the comparison of physiological and morphological traits between TR accessions grown with and without water stress confirmed that variability in the photosynthetic capacity was mostly explained by differences in the diffusion of CO₂ through stomata and leaf mesophyll. Maximization of g_m under both treatments was mainly achieved through adjustments in the mesophyll thickness and porosity and the surface area of chloroplasts exposed to intercellular airspace (S_c). In addition, the lower g_m/S_c ratio for a given porosity in drought‐acclimated plants suggests that the decrease in g_m was due to an increased cell wall thickness. Stomatal conductance was also affected by drought‐associated changes in the morphological properties of stomata, in an accession and treatment‐dependent manner. The results confirm the presence of advantageous physiological traits in the response to drought stress in Mediterranean accessions of tomato, and relate them to particular changes in the leaf anatomical properties, suggesting specific adaptive processes operating at the leaf anatomical level.     

Leaf anatomical properties in relation to differences in mesophyll conductance to CO2 and photosynthesis in two related Mediterranean Abies species

Abies alba and Abies pinsapo are closely related species with the same ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) large subunit (rbcL) but contrasting hydraulic traits and mesophyll structure occurring in the Iberian Peninsula under contrasting conditions. As photosynthesis and hydraulic capacities often co‐scale, we hypothesize that these species differ in mesophyll conductance to CO₂ (g_m). g_m and key anatomical traits were measured in both species. Drought‐adapted population of A. pinsapo has higher photosynthesis than the more mesic population of A. alba, in agreement with its higher hydraulic capacity. However, A. alba exhibits the largest stomatal conductance (g_s), and so water use efficiency (WUE) is much higher in A. pinsapo. The differences in photosynthesis were explained by differences in g_m, indicating a correlation between hydraulic capacity and g_m. We report a case where g_m is the main factor limiting photosynthesis in one species (A. alba) when compared with the other one (A. pinsapo). The results also highlight the discrepancy between g_m estimates based on anatomical measurements and those based on gas exchange methods, probably due to the very large resistance exerted by cell walls and the stroma in both species. Thus, the cell wall and chloroplast properties in relation to CO₂ diffusion constitute a near‐future research priority.     

Mesophyll cell properties for some C3 and C4 species with high photosynthetic rates

Leaves of twelve C₃ species and six C₄ species were examined to understand better the relationship between mesophyll cell properties and the generally high photosynthetic rates of these plants. The CO₂ diffusion conductance expressed per unit mesophyll cell surface area (g_CO2^cell) cell was determined using measurements of the net rate of CO₂ uptake, water vapor conductance, and the ratio of mesophyll cell surface area to leaf surface area (A^mes/A). A^mes/A averaged 31 for the C₃ species and 16 for the C₄ species. For the C₃ species g_CO2^cell ranged from 0.12 to 0.32 mm s^-1, and for the C₄ species it ranged from 0.55 to 1.5 mm s^-1, exceeding a previously predicted maximum of 0.5 mm s^-1. Although the C₃ species Cammissonia claviformis did not have the highest g_CO2^cell, the combination of the highest A^mes and highest stomatal conductance resulted in this species having the greatest maximum rate of CO₂ uptake in low oxygen, 93 μmol m^-2 s^-1 (147 mg dm^-2 h^-1). The high g_CO2^cell of the C₄ species Amaranthus retroflexus (1.5 mm s^-1) was in part attributable to its thin cell wall (72 nm thick).