Differential coordination of stomatal conductance,mesophyll conductance,and leaf hydraulic conductance in response to changing light across species

Stomatal conductance (g_s) and mesophyll conductance (g_m) represent major constraints to photosynthetic rate (A), and these traits are expected to coordinate with leaf hydraulic conductance (K_leaf) across species, under both steady‐state and dynamic conditions. However, empirical information about their coordination is scarce. In this study, K_leaf, gas exchange, stomatal kinetics, and leaf anatomy in 10 species including ferns, gymnosperms, and angiosperms were investigated to elucidate the correlation of H₂O and CO₂ diffusion inside leaves under varying light conditions. Gas exchange, K_leaf, and anatomical traits varied widely across species. Under light‐saturated conditions, the A, g_s, g_m, and K_leaf were strongly correlated across species. However, the response patterns of A, g_s, g_m, and K_leaf to varying light intensities were highly species dependent. Moreover, stomatal opening upon light exposure of dark‐adapted leaves in the studied ferns and gymnosperms was generally faster than in the angiosperms; however, stomatal closing in light‐adapted leaves after darkening was faster in angiosperms. The present results show that there is a large variability in the coordination of leaf hydraulic and gas exchange parameters across terrestrial plant species, as well as in their responses to changing light.     

The light response of mesophyll conductance is controlled by structure across leaf profiles

Mesophyll conductance to CO₂ (g_m) may respond to light either through regulated dynamic mechanisms or due to anatomical and structural factors. At low light, some layers of cells in the leaf cross‐section approach photocompensation and contribute minimally to bulk leaf photosynthesis and little to whole leaf g_m (g_m,leaf). Thus, the bulk g_m,leaf will appear to respond to light despite being based upon cells having an anatomically fixed mesophyll conductance. Such behaviour was observed in species with contrasting leaf structure using the variable J or stable isotope method of measuring g_m,leaf. A species with bifacial structure, Arbutus × ‘Marina’, and an isobilateral species, Triticum durum L., had contrasting responses of g_m,leaf upon varying adaxial or abaxial illumination. Anatomical observations, when coupled with the proposed model of g_m,leaf to photosynthetic photon flux density (PPFD) response, successfully represented the observed gas exchange data. The theoretical and observed evidence that g_m,leaf apparently responds to light has large implications for how g_m,leaf values are interpreted, particularly limitation analyses, and indicates the importance of measuring g_m under full light saturation. Responses of g_m,leaf to the environment should be treated as an emergent property of a distributed 3D structure, and not solely a leaf area‐based phenomenon.     

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.     

Recent developments in mesophyll conductance in C3, C4, and crassulacean acid metabolism plants

The conductance of carbon dioxide (CO₂) from the substomatal cavities to the initial sites of CO₂ fixation (g_m) can significantly reduce the availability of CO₂ for photosynthesis. There have been many recent reviews on: (i) the importance of g_m for accurately modelling net rates of CO₂ assimilation, (ii) on how leaf biochemical and anatomical factors influence g_m, (iii) the technical limitation of estimating g_m, which cannot be directly measured, and (iv) how g_m responds to long‐ and short‐term changes in growth and measurement environmental conditions. Therefore, this review will highlight these previous publications but will attempt not to repeat what has already been published. We will instead initially focus on the recent developments on the two‐resistance model of g_m that describe the potential of photorespiratory and respiratory CO₂ released within the mitochondria to diffuse directly into both the chloroplast and the cytosol. Subsequently, we summarize recent developments in the three‐dimensional (3‐D) reaction‐diffusion models and 3‐D image analysis that are providing new insights into how the complex structure and organization of the leaf influences g_m. Finally, because most of the reviews and literature on g_m have traditionally focused on C₃ plants we review in the final sections some of the recent developments, current understanding and measurement techniques of g_m in C₄ and crassulacean acid metabolism (CAM) plants. These plants have both specialized leaf anatomy and either a spatially or temporally separated CO₂ concentrating mechanisms (C₄ and CAM, respectively) that influence how we interpret and estimate g_m compared with a C₃ plants.     

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。     

Effects of elevated ozone and nitrogen addition on leaf nitrogen metabolism in poplar

臭氧和氮添加对杨树叶片氮代谢的影响臭氧(O₃)污染和氮(N)沉降/施肥都能同时影响植物的生长。然而,几乎没有研究探究O₃和N添加对植物叶片N代谢过程的复合影响。本研究在开顶式气室(OTC)中对杨树进行了为期95 d的熏蒸实验,包括两个O₃水平(NF,环境O₃水平;NF60,NF + 60 ppb O₃)和4个N处理(N0,没有N添加;N50,N0 + 50 kg N ha⁻¹ yr⁻¹;N100,N0 + 100 kg N ha⁻¹ yr⁻¹;N200,N0 + 200 kg N ha⁻¹ yr⁻¹)。测定了与叶片N代谢相关的一些指标,包括叶片N代谢酶的活性、总叶片N浓度、NO₃⁻-N浓度、NH₄⁺-N浓度、总氨基酸浓度(TAA)、总可溶性糖的浓度(TSP)。研究结果表明,相对于NF,在8月份NF60处理显著刺激了硝酸还原酶(NR)的活性,使其升高了47.2%。当平均所有的N处理和两次取样时间时,NF60处理下谷氨酰胺酶(GS)的活性比NF处理下的高57.3%。但是O₃处理并没有显著影响TSP浓度,并且在8月也没有降低TAA的浓度。相对N0,高的N添加处理(N200)显著增加了杨树叶片的饱和光合速率(A_sat) 24%,并且分 别在8和9月增加了总叶片N浓度70.3%和43.3%。但是在8月份,N200处理下光合N利用效率比N0的低26.1%。这表明N添加导致的A_sat和叶片总的N浓度的升高是不匹配的,高N处理下,叶片中一些剩余的N没有被用于优化植物碳的同化。同时,也发现高N添加显著刺激了叶片N代谢过程,叶片中的NO₃⁻-N浓度、NH₄⁺-N浓度、TAA浓度、NR和GS活性都显著升高。然而,O₃和N添加对杨树叶片所有N代谢相关的指标都没有交互影响。这些结果将有助于更好地了解在高O₃污染和N沉降/施肥下植物的N代谢过程以及生物地球化学循环过程。     

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。     

Development of leaf thickness for Plectranthus parviflorus – Influence of photosynthetically active radiation

Photosynthetically active radiation (PhAR) is apparently the environmental factor having the greatest influence on leaf thickness for Plectranthus parviflorus Henckel (Labiatae). A four-fold increase in leaf thickness from 280 to 1170 μm occurred as the PhAR was raised from 1.3 to 32.5 mol m⁻² day⁻¹. Compared to a constant PhAR of 2.5 mol m⁻² day⁻¹, a PhAR of 32.5 mol m⁻² day⁻¹ for one week during the first week (with return to 2.5 mol m⁻² day⁻¹ during the second and third weeks) led to an increase in final leaf thickness by 323 μm (to 802 μm). When increased PhAR was applied during the second week the increase in final thickness over the control was 217 μm, and when increased PhAR was applied during the third week it was 99 μm. However, leaf thickness was not simply responding to total daily PhAR, since a leaf 450 μm thick could occur at a low instantaneous PhAR for a long daytime (total daily PhAR of 1.5 mol m⁻² day⁻¹) and at a high PhAR for a short daytime (4.5 mol m⁻² day⁻¹). Total daily CO₂ uptake (net photosynthesis) was approximately the same in the two cases, suggesting that this is an important factor underlying the differences in leaf thickness. Leaf thickness is physiologically important, since thicker leaves tend to have greater mesophyll surface area per unit leaf area ( A ^mes/ A ) and hence higher photosynthetic rates.     

Temperature responses of photosynthesis and leaf hydraulic conductance in rice and wheat

Studies on the temperature (T) responses of photosynthesis and leaf hydraulic conductance (K_leaf) are important to plant gas exchange. In this study, the temperature responses of photosynthesis and K_leaf were studied in Shanyou 63 (Oryza sativa) and Yannong 19 (Triticum aestivum). Leaf water potential (Ψ_leaf) was insensitive to T in Shanyou 63, while it significantly decreased with T in Yannong 19. The differential Ψ_leaf − T relationship partially accounted for the differing g_m–T relationships, where g_m was less sensitive to T in Yannong 19 than in Shanyou 63. With different g_m–T and Ψ_leaf–T relationships, the temperature responses of photosynthetic limitations were surprisingly similar between the two lines, and the photosynthetic rate was highly correlated with g_m. With the increasing T, K_leaf increased in Shanyou 63 while it decreased in Yannong 19. The different K_leaf–T relationships were related to different Ψ_leaf–T relationships. When excluding the effects of water viscosity and Ψ_leaf, K_leaf was insensitive to T in both lines. g_m and K_leaf were generally not coordinated across different temperatures. This study highlights the importance of Ψ_leaf on leaf carbon and water exchanges, and the mechanisms for the g_m–T and K_leaf–T relationships were discussed.     

The importance of mesophyll conductance in regulating forest ecosystem productivity during drought periods

Water availability is the most limiting factor to global plant productivity, yet photosynthetic responses to seasonal drought cycles are poorly understood, with conflicting reports on which limiting process is the most important during drought. We address the problem using a model‐data synthesis approach to look at canopy level fluxes, integrating twenty years of half hour data gathered by the FLUXNET network across six Mediterranean sites. The measured canopy level, water and carbon fluxes were used, together with an inverse canopy ecophysiological model, to estimate the bulk canopy conductance, bulk mesophyll conductance, and the canopy scale carbon pools in both the intercellular spaces and at the site of carboxylation in the chloroplasts. Thus the roles of stomatal and mesophyll conductance in the regulation of internal carbon pools and photosynthesis could be separated. A quantitative limitation analysis allowed for the relative seasonal responses of stomatal, mesophyll, and biochemical limitations to be gauged. The concentration of carbon in the chloroplast was shown to be a potentially more reliable estimator of assimilation rates than the intercellular carbon concentration. Both stomatal conductance limitations and mesophyll conductance limitations were observed to regulate the response of photosynthesis to water stress in each of the six species studied. The results suggest that mesophyll conductance could bridge the gap between conflicting reports on plant responses to soil water stress, and that the inclusion of mesophyll conductance in biosphere–atmosphere transfer models may improve their performance, in particular their ability to accurately capture the response of terrestrial vegetation productivity to drought.     

Slow development of leaf photosynthesis in an evergreen broad-leaved tree, Castanopsis sieboldii: relationships between leaf anatomical characteristics and photosynthetic rate

Changes in net photosynthetic rate on a leaf area basis and anatomical properties during leaf development were studied in an evergreen broad‐leaved tree, Castanopsis sieboldii and an annual herb, Phaseolus vulgaris. In C. sieboldii, surface area of mesophyll cells facing the intercellular air spaces on a leaf area basis (S_mes) was already considerable at the time of full leaf area expansion (FLE). However, surface area of chloroplasts facing the intercellular air spaces on a leaf area basis (S_c), and chlorophyll and Rubisco contents on a leaf area basis increased to attain their maximal values 15–40 d after FLE. In contrast, in P. vulgaris, chloroplast number on a leaf area basis, S_c and S_mes at 10 d before FLE were two to three times greater than the steady‐state levels attained at around FLE. In C. sieboldii, the internal CO₂ transfer conductance (g_i) slightly increased for 10 d after FLE but then decreased toward the later stages. Limitation of photosynthesis by g_i was only about 10% at FLE, but then increased to about 30% at around 40 d after FLE. The large limitation after FLE by g_i was probably due to the decrease in CO₂ concentration in the chloroplast caused by the increases in thickness of mesophyll cell walls and in Rubisco content per chloroplast surface area. These results clearly showed that: (1) in C. sieboldii, chloroplast development proceeded more slowly than mesophyll cell expansion and continued well after FLE, whereas in P. vulgaris these processes proceeded synchronously and were completed by FLE; (2) after FLE, photosynthesis in leaves of C. sieboldii was markedly limited by g_i. From these results, it is suggested that, in the evergreen broad‐leaved trees, mechanical protection of mesophyll cells has priority over the efficient CO₂ transfer and quick construction of the chloroplasts.     

C4 grasses adapted to low precipitation habitats show traits related to greater mesophyll conductance and lower leaf hydraulic conductance

In habitats with low water availability, a fundamental challenge for plants will be to maximize photosynthetic C-gain while minimizing transpirational water-loss. This trade-off between C-gain and water-loss can in part be achieved through the coordination of leaf-level photosynthetic and hydraulic traits. To test the relationship of photosynthetic C-gain and transpirational water-loss, we grew, under common growth conditions, 18 C₄ grasses adapted to habitats with different mean annual precipitation (MAP) and measured leaf-level structural and anatomical traits associated with mesophyll conductance (g_m) and leaf hydraulic conductance (K_leaf). The C₄ grasses adapted to lower MAP showed greater mesophyll surface area exposed to intercellular air spaces (S_mes) and adaxial stomatal density (SD_ada) which supported greater g_m. These grasses also showed greater leaf thickness and vein-to-epidermis distance, which may lead to lower K_leaf. Additionally, grasses with greater g_m and lower K_leaf also showed greater photosynthetic rates (A_net) and leaf-level water-use efficiency (WUE). In summary, we identify a suite of leaf-level traits that appear important for adaptation of C₄ grasses to habitats with low MAP and may be useful to identify C₄ species showing greater A_net and WUE in drier conditions.     

Effects of low level O3 exposure on leaf diffusive conductance and water-use efficiency in hybrid poplar

Abstract Young, amphistomatous hybrid poplar (Populus deltoides x trichocarpa) plants were exposed daily to either background (0.025 cm³ m^-3) or elevated (0.125 cm³ m^-3) concentrations of O₃. Levels of abaxial and adaxial leaf conductance were affected interactively by pollutant treatment, leaf age, and photon fluence rate. Consequently, conductance in O₃-treated leaves was sometimes higher and sometimes lower than in comparable control leaves, depending on leaf age or level of photon fluence rate. For example, at low photon fluence rate or in the dark, conductance was greater in O₃-treated than in control plants, while at high photon fluence rate that relationship was reversed. Exposure to O₃ also reduced the water-use efficiency and range of leaf conductance of individual leaves, and altered the relationship between the conductances of the two leaf surfaces (the ratio of abaxial to adaxial leaf conductance was increased). Furthermore, O₃ treatment resulted in diminished stomatal control of water loss; excised O₃-treated leaves had higher conductances and wilted sooner than excised control leaves of identical ages. Overall, the data indicate that exposure to O₃ resulted in impaired stomatal function.     

Anatomical variation of mesophyll conductance under potassium deficiency has a vital role in determining leaf photosynthesis

Leaves exposed to potassium (K) deficiency usually present decreased mesophyll conductance (g_m) and photosynthesis (A). The relative contributions of leaf anatomical traits in determining g_m have been quantified; however, anatomical variabilities related to low g_m under K starvation remain imperfectly known. A one‐dimensional model was used to quantify anatomical controls of the entire CO₂ diffusion pathway resistance within a leaf on two Brassica napus L. cultivars in response to K deficiency. Leaf photosynthesis of both cultivars was significantly decreased under K deficiency in parallel with down‐regulated g_m. The mesophyll conductance limitation contributed to more than one‐half of A decline. The decreased internal air space in K‐starved leaves was associated with the increase of gas‐phase resistance. Potassium deficiency reduced liquid‐phase conductance by decreasing the exposed surface area of chloroplasts per unit leaf area (S_c/S), and enlarging the resistance of the cytoplasm that can be interpreted by the increasing distance of chloroplast from cell wall, and between adjacent chloroplasts. Additionally, the discrepancies of A between two cultivars were in part because of g_m variations, ascribing to an altered S_c/S. These results emphasize the important role of K on the regulation of g_m by enhancing S_c/S and reducing cytoplasm resistance.     

Heterogeneity in bean leaf mesophyll tissue and ion flux profiles: leaf electrophysiological characteristics correlate with the anatomical structure

It has been suggested for some time that the architectural properties of leaf venation are related to leaf functions; however, experimental evidence is scant and, when present, mainly investigates water or carbohydrate transport patterns. Transport of inorganic nutrients in relationship to leaf anatomical structure remains, to a large extent, an unexplored area in plant physiology. In this study, we correlated ion flux profiles with the anatomical structure of bean (Vicia faba L.) leaf mesophyll tissue using a non-invasive ion flux measuring technique (microelectrode ion flux estimation) and scanning electron microscopy. Quasi-periodic patterns of net H+ and K+ flux distributions were found when the mesophyll surface was scanned along the longitudinal axis with 0.1-0.2 mm increments. These patterns showed a high correlation with anatomical features of the mesophyll tissue (i.e. the distribution of vascular bundles). The observed flux profiles were not time-dependent, showed qualitative similarity in both light and dark conditions, and resulted in heterogeneous plant physiological responses. The possible physiological role of the observed findings, specifically in relation to stomatal 'patchiness' and phloem loading mechanisms, is discussed.     

Tobacco aquaporin NtAQP1 is involved in mesophyll conductance to CO2 in vivo

Leaf mesophyll conductance to CO(2) (g(m)) has been recognized to be finite and variable, rapidly adapting to environmental conditions. The physiological basis for fast changes in g(m) is poorly understood, but current reports suggest the involvement of protein-facilitated CO(2) diffusion across cell membranes. A good candidate for this could be the Nicotiana tabacum L. aquaporin NtAQP1, which was shown to increase membrane permeability to CO(2) in Xenopus oocytes. The objective of the present work was to evaluate its effect on the in vivo mesophyll conductance to CO(2), using plants either deficient in or overexpressing NtAQP1. Antisense plants deficient in NtAQP1 (AS) and NtAQP1 overexpressing tobacco plants (O) were compared with their respective wild-type (WT) genotypes (CAS and CO). Plants grown under optimum conditions showed different photosynthetic rates at saturating light, with a decrease of 13% in AS and an increase of 20% in O, compared with their respective controls. CO(2) response curves of photosynthesis also showed significant differences among genotypes. However, in vitro analysis demonstrated that these differences could not be attributed to alterations in Rubisco activity or ribulose-1,5-bisphosphate content. Analyses of chlorophyll fluorescence and on-line (13)C discrimination indicated that the observed differences in net photosynthesis (A(N)) among genotypes were due to different leaf mesophyll conductances to CO(2), which was estimated to be 30% lower in AS and 20% higher in O compared with their respective WT. These results provide evidence for the in vivo involvement of aquaporin NtAQP1 in mesophyll conductance to CO(2).     

臭氧和气溶胶复合污染对杨树叶片光合作用的影响

由于经济的快速发展, 中国大部分地区正面临着严峻的复合型大气污染, 其中臭氧和气溶胶是两种主要污染物。已有的研究表明臭氧对叶片的氧化性伤害能够抑制光合作用, 而气溶胶可通过增加散射辐射比例或缓解高温抑制促进光合作用。但复合污染下, 臭氧和气溶胶如何共同调控叶片光合作用, 仍缺乏研究。该研究利用北京及周边地区之间的污染梯度, 选择加杨(Populus × canadensis)作为实验对象, 于2012-2013年生长季期间对叶片光合速率进行连续观测, 并同时监测臭氧浓度(AOT40)、气溶胶光学厚度(AOD)、空气温度和冠层内外光合有效辐射(PAR)等环境因子, 以期探讨大气复合污染下臭氧和气溶胶变化对植物叶片光合作用的影响及相关机制。结果表明: (1)臭氧浓度与空气温度、气溶胶浓度之间均呈显著正相关关系, 但气溶胶浓度与空气温度没有显著相关关系; (2)臭氧浓度增加显著抑制了阳生叶片的光合作用, 但气溶胶浓度上升促进了阳生叶片的光合作用; 臭氧浓度升高对阴生叶片光合作用的影响较小, 但气溶胶浓度上升促进了阴生叶片的光合作用; (3)标准化后的结果显示, 臭氧对阳生叶片光合作用的影响最大, 此时气溶胶的促进作用一定程度上补偿了臭氧浓度上升所带来的抑制效应。对于阴生叶片光合作用而言, 气溶胶则是最重要的影响因素。该研究发现复合污染下阴生叶和阳生叶光合响应不同, 这表明冠层结构可能通过影响阴生叶和阳生叶的比例, 从而对植物生长产生不同影响。该研究对理解大气复合污染如何影响光合作用提供了的机理支持, 同时也表明, 为了维持生态系统生产力及功能, 需要同时控制气溶胶和臭氧污染。     

Rapid variations of mesophyll conductance in response to changes in CO2 concentration around leaves

The effects of short-term (minutes) variations of CO2 concentration on mesophyll conductance to CO2 (gm) were evaluated in six different C3 species by simultaneous measurements of gas exchange, chlorophyll fluorescence, online carbon isotope discrimination and a novel curve-fitting method. Depending on the species, gm varied from five- to ninefold, along the range of sub-stomatal CO2 concentrations typically used in photosynthesis CO2-response curves (AN)-Ci curves; where AN is the net photosynthetic flux and Ci is the CO2 concentrations in the sub-stomatal cavity), that is, 50 to 1500 micromol CO2 mol(-1) air. Although the pattern was species-dependent, gm strongly declined at high Ci, where photosynthesis was not limited by CO2, but by regeneration of ribulose-1,5-bisphosphate or triose phosphate utilization. Moreover, these changes on gm were found to be totally independent of the velocity and direction of the Ci changes. The response of gm to Ci resembled that of stomatal conductance (gs), but kinetic experiments suggested that the response of gm was actually faster than that of gs. Transgenic tobacco plants differing in the amounts of aquaporin NtAQP1 showed different slopes of the gm-Ci response, suggesting a possible role for aquaporins in mediating CO2 responsiveness of gm. The importance of these findings is discussed in terms of their effects on parameterization of AN-Ci curves.     

Variability in mesophyll conductance between barley genotypes,and effects on transpiration efficiency and carbon isotope discrimination

Leaf internal, or mesophyll, conductance to CO₂ (g_m ) is a significant and variable limitation of photosynthesis that also affects leaf transpiration efficiency (TE). Genotypic variation in g_m and the effect of g_m on TE were assessed in six barley genotypes (four Hordeum vulgare and two H. bulbosum). Significant variation in g_m was found between genotypes, and was correlated with photosynthetic rate. The genotype with the highest g_m also had the highest TE and the lowest carbon isotope discrimination as recorded in leaf tissue (Δ_p). These results suggest g_m has unexplored potential to provide TE improvement within crop breeding programmes.