Mesophyll conductance: internal insights of leaf carbon exchange

Leaf anatomy does not explain the large variability of mesophyll conductance across C3 crop species

Increasing mesophyll conductance of CO₂ (g_m ) is a strategy to improve photosynthesis in C₃ crops. However, the relative importance of different anatomical traits in determining g_m in crops is unclear. Mesophyll conductance measurements were performed on 10 crops using the online carbon isotope discrimination method and the 'variable J' method in parallel. The influences of crucial leaf anatomical traits on g_m were evaluated using a one-dimensional anatomical CO₂ diffusion model. The g_m values measured using two independent methods were compatible, although significant differences were observed in their absolute values. Quantitative analysis showed that cell wall thickness and chloroplast stroma thickness are the most important elements along the diffusion pathway. Unexpectedly, the large variability of g_m across crops was not associated with any investigated leaf anatomical traits except chloroplast thickness. The g_m values estimated using the anatomical model differed remarkably from the values measured in vivo in most species. However, when the species-specific effective porosity of the cell wall and the species-specific facilitation effect of CO₂ diffusion across the membrane and chloroplast stoma were taken into account, the model could output g_m values very similar to those measured in vivo. These results indicate that g_m variation across crops is probably also driven by the effective porosity of the cell wall and effects of facilitation of CO₂ transport across the membrane and chloroplast stroma in addition to the thicknesses of the elements.

When vegetation indicates reproduction: The affinity between leaf morphology and flowering commitment in the lily meristem

At the reproductive stage, lily plants bear two morphological types of mature leaves, one at the lower and one at the upper part of the stem. At the vegetative stage, all the leaves are similar to each other and to the reproductive plant's lower leaves. This heterophylly has not yet been explored. In this study, we show that it is not a result of the plant's age but rather an outcome of floral induction. The induction appears as an on-going process, during which the meristem still produces leaves but progressively becomes committed to reproduction. This intermediate period lasts until the ultimate switch to flower primordia occurs. The leaves produced during floral induction, termed here as "inductive," appear at the upper part of the stem. Besides their typical higher stomata density, these leaves have a poly-layered palisade mesophyll, whose cells exhibit a unique morphology and contain more chlorophyll than leaves of vegetative plants. These leaves display higher carbon assimilation, soluble sugar production, and chloroplast-lipid accumulation. Accordingly, genes associated with stomata, chloroplast, and photosynthesis are upregulated in these leaves. Our results were obtained when floral induction was achieved either by vernalization or photoperiod signals, ruling out a mere environmental effect. We suggest that lily plants prepare themselves for the high-energy-demanding bloom by producing leaves with enhanced photosynthetic capacity, leading to an increase in soluble sugars. These novel findings introduce an adjacent affinity between photosynthesis and flowering and provide a nondestructive tool for identifying the plant's developmental stage-vegetative or reproductive.

Contrasting controls on tree ring isotope variation for Amazon floodplain and terra firme trees

Isotopes in tropical trees rings can improve our understanding of tree responses to climate. We assessed how climate and growing conditions affect tree-ring oxygen and carbon isotopes (δ18OTR and δ13CTR) in four Amazon trees. We analysed within-ring isotope variation for two terra firme (non-flooded) and two floodplain trees growing at sites with varying seasonality. We find distinct intra-annual patterns of δ18OTR and δ13CTR driven mostly by seasonal variation in weather and source water δ18O. Seasonal variation in isotopes was lowest for the tree growing under the wettest conditions. Tree ring cellulose isotope models based on existing theory reproduced well observed within-ring variation with possible contributions of both stomatal and mesophyll conductance to variation in δ13CTR. Climate analysis reveal that terra firme δ18OTR signals were related to basin-wide precipitation, indicating a source water δ18O influence, while floodplain trees recorded leaf enrichment effects related to local climate. Thus, intrinsically different processes (source water vs leaf enrichment) affect δ18OTR in the two different species analysed. These differences are likely a result of both species-specific traits and of the contrasting growing conditions in the floodplains and terra firme environments. Simultaneous analysis of δ13CTR and δ18OTR supports this interpretation as it shows strongly similar intra-annual patterns for both isotopes in the floodplain trees arising from a common control by leaf stomatal conductance, while terra firme trees showed less covariation between the two isotopes. Our results are interesting from a plant physiological perspective and have implications for climate reconstructions as trees record intrinsically different processes.

Leaf anatomy does not explain apparent short-term responses of mesophyll conductance to light and CO2 in tobacco

Mesophyll conductance to CO2 (gm ), a key photosynthetic trait, is strongly constrained by leaf anatomy. Leaf anatomical parameters such as cell wall thickness and chloroplast area exposed to the mesophyll intercellular airspace have been demonstrated to determine gm in species with diverging phylogeny, leaf structure and ontogeny. However, the potential implication of leaf anatomy, especially chloroplast movement, on the short-term response of gm to rapid changes (i.e. seconds to minutes) under different environmental conditions (CO2 , light or temperature) has not been examined. The aim of this study was to determine whether the observed rapid variations of gm in response to variations of light and CO2 could be explained by changes in any leaf anatomical arrangements. When compared to high light and ambient CO2 , the values of gm estimated by chlorophyll fluorescence decreased under high CO2 and increased at low CO2 , while it decreased with decreasing light. Nevertheless, no changes in anatomical parameters, including chloroplast distribution, were found. Hence, the gm estimated by analytical models based on anatomical parameters was constant under varying light and CO2 . Considering this discrepancy between anatomy and chlorophyll fluorescence estimates, it is concluded that apparent fast gm variations should be due to artefacts in its estimation and/or to changes in the biochemical components acting on diffusional properties of the leaf (e.g. aquaporins and carbonic anhydrase).

Components of mesophyll resistance and their environmental responses: A theoretical modelling analysis

Mesophyll resistance (r_m ), stomatal resistance, and biochemical limitations are recognized as three critical factors limiting leaf photosynthesis. Contrary to the expectation of being a constant, r_m not only varies with light and CO₂ conditions but also shows different responses among species. To elucidate the mechanistic basis of these responses, we derived an analytical model of r_m , which incorporates various anatomical and biochemical factors including permeabilities of cell wall and chloroplast envelope to CO₂ and HCO₃^- , carbonic anhydrase activities in cytosol and stroma, Rubisco activities, and relative location of mitochondria and chloroplast. The robustness of this model was confirmed by comparing the predicted r_m and its components to numerical models developed at cell and leaf levels, which incorporate detailed 3-dimensional cell and leaf anatomies, CO₂ hydration and diffusion processes from intercellular air space to stroma, and CO₂ fixation by Rubisco. A combination of these model analyses shows that the varying r_m is influenced by four biochemical factors: (a) nonuniform photosynthesis status across the leaf, (b) photorespiration and respiration, (c) bicarbonate leakage on the chloroplast envelope, and (d) hydration activity in cytosol and stroma. This study provides a theoretical framework to study components of r_m and their responses to environmental perturbations.

Coupled response of stomatal and mesophyll conductance to light enhances photosynthesis of shade leaves under sunflecks

Light gradients within tree canopies play a major role in the distribution of plant resources that define the photosynthetic capacity of sun and shade leaves. However, the biochemical and diffusional constraints on gas exchange in sun and shade leaves in response to light remain poorly quantified, but critical for predicting canopy carbon and water exchange. To investigate the CO₂ diffusion pathway of sun and shade leaves, leaf gas exchange was coupled with concurrent measurements of carbon isotope discrimination to measure net leaf photosynthesis (A_n ), stomatal conductance (g_s ) and mesophyll conductance (g_m ) in Eucalyptus tereticornis trees grown in climate controlled whole-tree chambers. Compared to sun leaves, shade leaves had lower A_n , g_m , leaf nitrogen and photosynthetic capacity (A_max ) but g_s was similar. When light intensity was temporarily increased for shade leaves to match that of sun leaves, both g_s and g_m increased, and A_n increased to values greater than sun leaves. We show that dynamic physiological responses of shade leaves to altered light environments have implications for up-scaling leaf level measurements and predicting whole canopy carbon gain. Despite exhibiting reduced photosynthetic capacity, the rapid up-regulation of g_m with increased light enables shade leaves to respond quickly to sunflecks.

Introducing an algal carbon-concentrating mechanism into higher plants: location and incorporation of key components

Many eukaryotic green algae possess biophysical carbon-concentrating mechanisms (CCMs) that enhance photosynthetic efficiency and thus permit high growth rates at low CO2 concentrations. They are thus an attractive option for improving productivity in higher plants. In this study, the intracellular locations of ten CCM components in the unicellular green alga Chlamydomonas reinhardtii were confirmed. When expressed in tobacco, all of these components except chloroplastic carbonic anhydrases CAH3 and CAH6 had the same intracellular locations as in Chlamydomonas. CAH6 could be directed to the chloroplast by fusion to an Arabidopsis chloroplast transit peptide. Similarly, the putative inorganic carbon (Ci) transporter LCI1 was directed to the chloroplast from its native location on the plasma membrane. CCP1 and CCP2 proteins, putative Ci transporters previously reported to be in the chloroplast envelope, localized to mitochondria in both Chlamydomonas and tobacco, suggesting that the algal CCM model requires expansion to include a role for mitochondria. For the Ci transporters LCIA and HLA3, membrane location and Ci transport capacity were confirmed by heterologous expression and H(14) CO3 (-) uptake assays in Xenopus oocytes. Both were expressed in Arabidopsis resulting in growth comparable with that of wild-type plants. We conclude that CCM components from Chlamydomonas can be expressed both transiently (in tobacco) and stably (in Arabidopsis) and retargeted to appropriate locations in higher plant cells. As expression of individual Ci transporters did not enhance Arabidopsis growth, stacking of further CCM components will probably be required to achieve a significant increase in photosynthetic efficiency in this species.

Three-dimensional microscale modelling of CO2 transport and light propagation in tomato leaves enlightens photosynthesis

We present a combined three-dimensional (3-D) model of light propagation, CO2 diffusion and photosynthesis in tomato (Solanum lycopersicum L.) leaves. The model incorporates a geometrical representation of the actual leaf microstructure that we obtained with synchrotron radiation X-ray laminography, and was evaluated using measurements of gas exchange and leaf optical properties. The combination of the 3-D microstructure of leaf tissue and chloroplast movement induced by changes in light intensity affects the simulated CO2 transport within the leaf. The model predicts extensive reassimilation of CO2 produced by respiration and photorespiration. Simulations also suggest that carbonic anhydrase could enhance photosynthesis at low CO2 levels but had little impact on photosynthesis at high CO2 levels. The model confirms that scaling of photosynthetic capacity with absorbed light would improve efficiency of CO2 fixation in the leaf, especially at low light intensity.

Dynamic photosynthesis in different environmental conditions

Incident irradiance on plant leaves often fluctuates, causing dynamic photosynthesis. Whereas steady-state photosynthetic responses to environmental factors have been extensively studied, knowledge of dynamic modulation of photosynthesis remains scarce and scattered. This review addresses this discrepancy by summarizing available data and identifying the research questions necessary to advance our understanding of interactions between environmental factors and dynamic behaviour of photosynthesis using a mechanistic framework. Firstly, dynamic photosynthesis is separated into sub-processes related to proton and electron transport, non-photochemical quenching, control of metabolite flux through the Calvin cycle (activation states of Rubisco and RuBP regeneration, and post-illumination metabolite turnover), and control of CO₂ supply to Rubisco (stomatal and mesophyll conductance changes). Secondly, the modulation of dynamic photosynthesis and its sub-processes by environmental factors is described. Increases in ambient CO₂ concentration and temperature (up to ~35°C) enhance rates of photosynthetic induction and decrease its loss, facilitating more efficient dynamic photosynthesis. Depending on the sensitivity of stomatal conductance, dynamic photosynthesis may additionally be modulated by air humidity. Major knowledge gaps exist regarding environmental modulation of loss of photosynthetic induction, dynamic changes in mesophyll conductance, and the extent of limitations imposed by stomatal conductance for different species and environmental conditions. The study of mutants or genetic transformants for specific processes under various environmental conditions could provide significant progress in understanding the control of dynamic photosynthesis.

The arc mutants of Arabidopsis with fewer large chloroplasts have a lower mesophyll conductance

Photosynthetic cells of most land plant lineages have numerous small chloroplasts even though most algae, and even the early diverging land plant group the hornworts, tend to have one or a few large chloroplasts. One constraint that small chloroplasts could improve is the resistance to CO2 diffusion from the atmosphere to the chloroplast stroma. We examined the mesophyll conductance (inverse of the diffusion resistance) of mutant Arabidopsis thaliana plants with one or only a few large chloroplasts per cell. The accumulation and replication of chloroplasts (arc) mutants of A. thaliana were studied by model fitting to gas exchange data and (13)CO2 discrimination during carbon fixation. The two methods generally agreed, but the value of the CO2 compensation point of Rubisco (Γ *) used in the model had a large impact on the estimated photosynthetic parameters, including mesophyll conductance. We found that having only a few large chloroplasts per cell resulted in a 25-50 % reduction in the mesophyll conductance at ambient CO2.

Temperature responses of mesophyll conductance differ greatly between species

The temperature responses of mesophyll conductance (gm ) were investigated for nine species using carbon isotope techniques combining tunable diode laser spectroscopy and gas exchange measurements. Species included the evergreen trees Eucalyptus pauciflora and Quercus engelmannii; the tropical evergreen tree Lophostemon confertus; as well as the herbaceous species Nicotiana tabacum, Oryza sativa, Triticum aestivum, Gossypium hirsutum, Glycine max and Arabidopsis thaliana. Responses varied from a two- to threefold increase in mesophyll conductance between 15 and 40 °C observed for N. tabacum, G. hirsutum, G. max and E. pauciflora to almost no change in L. confertus and T. aestivum. To account for the different temperature responses between species, we suggest that there must be variation in both the activation energy for membrane permeability and the effective pathlength for liquid phase diffusion. Stomatal conductance was relatively independent of increases in leaf temperature and concomitant increases in leaf to air vapour pressure difference. Two exceptions were Eucalyptus and Gossypium, where stomatal conductance increased with temperature up to 35 °C despite increasing leaf to air vapour pressure. For a given species, temperature responses of stomatal and mesophyll conductance were independent of one another.

Peach water relations, gas exchange, growth and shoot mortality under water deficit in semi-arid weather conditions

In this study the sensitivity of peach tree (Prunus persica L.) to three water stress levels from mid-pit hardening until harvest was assessed. Seasonal patterns of shoot and fruit growth, gas exchange (leaf photosynthesis, stomatal conductance and transpiration) as well as carbon (C) storage/mobilization were evaluated in relation to plant water status. A simple C balance model was also developed to investigate sink-source relationship in relation to plant water status at the tree level. The C source was estimated through the leaf area dynamics and leaf photosynthesis rate along the season. The C sink was estimated for maintenance respiration and growth of shoots and fruits. Water stress significantly reduced gas exchange, and fruit, and shoot growth, but increased fruit dry matter concentration. Growth was more affected by water deficit than photosynthesis, and shoot growth was more sensitive to water deficit than fruit growth. Reduction of shoot growth was associated with a decrease of shoot elongation, emergence, and high shoot mortality. Water scarcity affected tree C assimilation due to two interacting factors: (i) reduction in leaf photosynthesis (-23% and -50% under moderate (MS) and severe (SS) water stress compared to low (LS) stress during growth season) and (ii) reduction in total leaf area (-57% and -79% under MS and SS compared to LS at harvest). Our field data analysis suggested a Ψ_stem threshold of -1.5 MPa below which daily net C gain became negative, i.e. C assimilation became lower than C needed for respiration and growth. Negative C balance under MS and SS associated with decline of trunk carbohydrate reserves – may have led to drought-induced vegetative mortality.

Synchrotron X-ray computed laminography of the three-dimensional anatomy of tomato leaves

Synchrotron radiation computed laminography (SR-CL) is presented as an imaging method for analyzing the three-dimensional (3D) anatomy of leaves. The SR-CL method was used to provide 3D images of 1-mm² samples of intact leaves at a pixel resolution of 750 nm. The method allowed visualization and quantitative analysis of palisade and spongy mesophyll cells, and showed local venation patterns, aspects of xylem vascular structure and stomata. The method failed to image subcellular organelles such as chloroplasts. We constructed 3D computer models of leaves that can provide a basis for calculating gas exchange, light penetration and water and solute transport. The leaf anatomy of two different tomato genotypes grown in saturating light conditions was compared by 3D analysis. Differences were found in calculated values of tissue porosity, cell number density, cell area to volume ratio and cell volume and cell shape distributions of palisade and spongy cell layers. In contrast, the exposed cell area to leaf area ratio in mesophyll, a descriptor that correlates to the maximum rate of photosynthesis in saturated light conditions, was no different between spongy and palisade cells or between genotypes. The use of 3D image processing avoids many of the limitations of anatomical analysis with two-dimensional sections.

Morphological and anatomical determinants of mesophyll conductance in wild relatives of tomato (Solanum sect. Lycopersicon, sect. Lycopersicoides; Solanaceae)

Natural selection on photosynthetic performance is a primary factor determining leaf phenotypes. The complex CO2 diffusion path from substomatal cavities to the chloroplasts - the mesophyll conductance (g(m)) - limits photosynthetic rate in many species and hence shapes variation in leaf morphology and anatomy. Among sclerophyllous and succulent taxa, structural investment in leaves, measured as the leaf dry mass per area (LMA), has been implicated in decreased gm . However, in herbaceous taxa with high g(m), it is less certain how LMA impacts CO2 diffusion and whether it significantly affects photosynthetic performance. We addressed these questions in the context of understanding the ecophysiological significance of leaf trait variation in wild tomatoes, a closely related group of herbaceous perennials. Although g(m) was high in wild tomatoes, variation in g(m) significantly affected photosynthesis. Even in these tender-leaved herbaceous species, greater LMA led to reduced g(m). This relationship between g(m) and LMA is partially mediated by cell packing and leaf thickness, although amphistomy (equal distribution of stomata on both sides of the leaf) mitigates the effect of leaf thickness. Understanding the costs of increased LMA will inform future work on the adaptive significance of leaf trait variation across ecological gradients in wild tomatoes and other systems.

A system dynamics model integrating physiology and biochemical regulation predicts extent of crassulacean acid metabolism (CAM) phases

A system dynamics (SD) approach was taken to model crassulacean acid metabolism (CAM) expression from measured biochemical and physiological constants. SD emphasizes state-dependent feedback interaction to describe the emergent properties of a complex system. These mechanisms maintain biological systems with homeostatic limits on a temporal basis. Previous empirical studies on CAM have correlated biological constants (e.g. enzyme kinetic parameters) with expression over the CAM diel cycle. The SD model integrates these constants within the architecture of the CAM 'system'. This allowed quantitative causal connections to be established between biological inputs and the four distinct phases of CAM delineated by gas exchange and malic acid accumulation traits. Regulation at flow junctions (e.g. stomatal and mesophyll conductance, and malic acid transport across the tonoplast) that are subject to feedback control (e.g. stomatal aperture, malic acid inhibition of phosphoenolpyruvate carboxylase, and enzyme kinetics) was simulated. Simulated expression for the leaf-succulent Kalanchoë daigremontiana and more succulent tissues of Agave tequilana showed strong correlation with measured gas exchange and malic acid accumulation (R(2)  = 0.912 and 0.937, respectively, for K. daigremontiana and R(2)  = 0.928 and 0.942, respectively, for A. tequilana). Sensitivity analyses were conducted to quantitatively identify determinants of diel CO2 uptake. The transition in CAM expression from low to high volume/area tissues (elimination of phase II-IV carbon-uptake signatures) was achieved largely by the manipulation three input parameters.

Leaf mesophyll conductance and leaf hydraulic conductance: an introduction to their measurement and coordination

Two highly contrasting variables summarizing the efficiency of transport of materials within the leaf are recognized as playing central roles in determining gas exchange and plant performance. This paper summarizes current approaches for the measurement of mesophyll conductance to CO2 (g m) and leaf hydraulic conductance (K leaf) and addresses the physiological integration of these parameters. First, the most common methods to determine g m and K leaf are summarized. Next, novel data compilation is analysed, which indicates that, across diverse species, g m is strongly linked with gas exchange parameters such as net CO2 assimilation (A area) and stomatal conductance (g s), and with K leaf, independently of leaf vein length per leaf area. Based on their parallel responses to a number of environmental variables, this review proposes that g m is linked to the outside-xylem but not to the xylem component of K leaf. Further, a mechanistic hypothesis is proposed to explain the interactions among all these and other physiological parameters. Finally, the possibility of estimating g m based on this hypothesis was tested using a regression analysis and a neurofuzzy logic approach. These approaches enabled the estimation of g m of given species from K leaf and leaf mass per area, providing a higher predictive power than from either parameter alone. The possibility of estimating g m from measured K leaf or vice-versa would result in a rapid increase in available data. Studies in which g m, K leaf, and leaf mass per area are simultaneously determined are needed in order to confirm and strengthen predictive and explanatory models for these parameters and importantly improve resolution of the integrated hydraulic-stomatal-photosynthetic system.