Estimating mesophyll conductance to CO2: methodology, potential errors, and recommendations

A guide to photosynthetic gas exchange measurements: Fundamental principles, best practice and potential pitfalls

Gas exchange measurements enable mechanistic insights into the processes that underpin carbon and water fluxes in plant leaves which in turn inform understanding of related processes at a range of scales from individual cells to entire ecosytems. Given the importance of photosynthesis for the global climate discussion it is important to (a) foster a basic understanding of the fundamental principles underpinning the experimental methods used by the broad community, and (b) ensure best practice and correct data interpretation within the research community. In this review, we outline the biochemical and biophysical parameters of photosynthesis that can be investigated with gas exchange measurements and we provide step-by-step guidance on how to reliably measure them. We advise on best practices for using gas exchange equipment and highlight potential pitfalls in experimental design and data interpretation. The Supporting Information contains exemplary data sets, experimental protocols and data-modelling routines. This review is a community effort to equip both the experimental researcher and the data modeller with a solid understanding of the theoretical basis of gas-exchange measurements, the rationale behind different experimental protocols and the approaches to data interpretation.

The interplay of short-term mesophyll and stomatal conductance responses under variable environmental conditions

Understanding the short-term responses of mesophyll conductance (gm) and stomatal conductance (gsc) to environmental changes remains a challenging yet central aspect of plant physiology. This review synthesises our current knowledge of these short-term responses, which underpin CO2 diffusion within leaves. Recent methodological advances in measuring gm using online isotopic discrimination and chlorophyll fluorescence have improved our confidence in detecting short-term gm responses, but results need to be carefully evaluated. Environmental factors like vapour pressure deficit and CO2 concentration indirectly impact gm through gsc changes, highlighting some of the complex interactions between the two parameters. Evidence suggests that short-term responses of gm are not, or at least not fully, mechanistically linked to changes in gsc, cautioning against using gsc as a reliable proxy for gm. The overarching challenge lies in unravelling the mechanistic basis of short-term gm responses, which will contribute to the development of accurate models bridging laboratory insights with broader ecological implications. Addressing these gaps in understanding is crucial for refining predictions of gm behaviour under changing environmental conditions.

The response of mesophyll conductance to short-term CO2 variation is related to stomatal conductance

The response of mesophyll conductance (gm) to CO2 plays a key role in photosynthesis and ecosystem carbon cycles under climate change. Despite numerous studies, there is still debate about how gm responds to short-term CO2 variations. Here we used multiple methods and looked at the relationship between stomatal conductance to CO2 (gsc) and gm to address this aspect. We measured chlorophyll fluorescence parameters and online carbon isotope discrimination (Δ) at different CO2 mole fractions in sunflower (Helianthus annuus L.), cowpea (Vigna unguiculata L.), and wheat (Triticum aestivum L.) leaves. The variable J and Δ based methods showed that gm decreased with an increase in CO2 mole fraction, and so did stomatal conductance. There were linear relationships between gm and gsc across CO2 mole fractions. gm obtained from A-Ci curve fitting method was higher than that from the variable J method and was not representative of gm under the growth CO2 concentration. gm could be estimated by empirical models analogous to the Ball-Berry model and the USO model for stomatal conductance. Our results suggest that gm and gsc respond in a coordinated manner to short-term variations in CO2, providing new insight into the role of gm in photosynthesis modelling.

The long and tortuous path towards improving photosynthesis by engineering elevated mesophyll conductance

The growing demand for global food production is likely to be a defining issue facing humanity over the next 50 years. To tackle this challenge, there is a desire to bioengineer crops with higher photosynthetic efficiencies, to increase yields. Recently, there has been a growing interest in engineering leaves with higher mesophyll conductance (gm), which would allow CO2 to move more efficiently from the substomatal cavities to the chloroplast stroma. However, if crop yield gains are to be realised through this approach, it is essential that the methodological limitations associated with estimating gm are fully appreciated. In this review, we summarise these limitations, and outline the uncertainties and assumptions that can affect the final estimation of gm. Furthermore, we critically assess the predicted quantitative effect that elevating gm will have on assimilation rates in crop species. We highlight the need for more theoretical modelling to determine whether altering gm is truly a viable route to improve crop performance. Finally, we offer suggestions to guide future research on gm, which will help mitigate the uncertainty inherently associated with estimating this parameter.

Reduced mesophyll conductance under chronic O3 exposure in poplar reflects thicker cell walls and increased subcellular diffusion pathway lengths according to the anatomical model

Ozone (O3) is one of the most harmful and widespread air pollutants, affecting crop yield and plant health worldwide. There is evidence that O3 reduces the major limiting factor of photosynthesis, namely CO2 mesophyll conductance (gm), but there is little quantitative information of O3-caused changes in key leaf anatomical traits and their impact on gm. We exposed two O3-responsive clones of the economically important tree species Populus × canadensis Moench to 120 ppb O3 for 21 days. An anatomical diffusion model within the leaf was used to analyse the entire CO2 diffusion pathway from substomatal cavities to carboxylation sites and determine the importance of each structural and subcellular component as a limiting factor. gm decreased substantially under O3 and was found to be the most important limitation of photosynthesis. This decrease was mostly driven by an increased cell wall thickness and length of subcellular diffusion pathway caused by altered interchloroplast spacing and chloroplast positioning. By contrast, the prominent leaf integrative trait leaf dry mass per area was neither affected nor related to gm under O3. The observed relationship between gm and anatomy, however, was clone-dependent, suggesting that mechanisms regulating gm may differ considerably between closely related plant lines. Our results confirm the need for further studies on factors constraining gm under stress conditions.

Effect of light-induced changes in leaf anatomy on intercellular and cellular components of mesophyll resistance for CO2 in Fagus sylvatica

Mesophyll resistance for CO2 diffusion (rm) is one of the main limitations for photosynthesis and plant growth. Breeding new varieties with lower rm requires knowledge of its distinct components. We tested new method for estimating the relative drawdowns of CO2 concentration (c) across hypostomatous leaves of Fagus sylvatica. This technique yields values of the ratio of the internal CO2 concentrations at the adaxial and abaxial leaf side, cd/cb, the drawdown in the intercellular air space (IAS), and intracellular drawdown between IAS and chloroplast stroma, cc/cbd. The method is based on carbon isotope composition of leaf dry matter and epicuticular wax isolated from upper and lower leaf sides. We investigated leaves from tree-canopy profile to analyse the effects of light and leaf anatomy on the drawdowns and partitioning of rm into its inter- (rIAS) and intracellular (rliq) components. Validity of the new method was tested by independent measurements of rm using conventional isotopic and gas exchange techniques. 73% of investigated leaves had adaxial epicuticular wax enriched in 13C compared to abaxial wax (by 0.50‰ on average), yielding 0.98 and 0.70 for average of cd/cb and cc/cbd, respectively. The rIAS to rliq proportion were 5.5:94.5% in sun-exposed and 14.8:85.2% in shaded leaves. cc dropped to less than half of the atmospheric value in the sunlit and to about two-thirds of it in shaded leaves. This method shows that rIAS is minor but not negligible part of rm and reflects leaf anatomy traits, i.e. leaf mass per area and thickness.

GasanalyzeR: advancing reproducible research using a new R package for photosynthesis data workflows

The analysis of photosynthetic traits has become an integral part of plant (eco-)physiology. Many of these characteristics are not directly measured, but calculated from combinations of several, more direct, measurements. The calculations of such derived variables are based on underlying physical models and may use additional constants or assumed values. Commercially available gas-exchange instruments typically report such derived variables, but the available implementations use different definitions and assumptions. Moreover, no software is currently available to allow a fully scripted and reproducible workflow that includes importing data, pre-processing and recalculating derived quantities. The R package gasanalyzer aims to address these issues by providing methods to import data from different instruments, by translating photosynthetic variables to a standardized nomenclature, and by optionally recalculating derived quantities using standardized equations. In addition, the package facilitates performing sensitivity analyses on variables or assumptions used in the calculations to allow researchers to better assess the robustness of the results. The use of the package and how to perform sensitivity analyses are demonstrated using three different examples.

Intraspecific differences in the photosynthetic responses to chloroplast CO2 and photon flux density at different leaf temperatures of four grapevine cultivars grown in common outdoor conditions

Comparative measurements of four Vitis vinifera cultivars were undertaken to assess assimilation tolerance to the high growth temperatures currently pervading Australian and other wine growing regions. The cultivars, cvs. Chardonnay, Merlot, Semillon, and Shiraz, were all grown in common growth conditions, and an hypothesis promulgated genotypic variation in assimilation and in the leaf temperature dependency. Assimilation responses to varying light intensity and to varying chloroplast CO2 at a range of leaf temperatures (15-45°C) were measured in leaves of each cultivar in mid-summer. Light response curves revealed marked genotype differences in maximum assimilation, but temperature effects also varied. Semillon leaves were most sensitive to temperature, with marked and steep differences in assimilation at different temperatures while Chardonnay and Merlot were least sensitive, with relatively flat responses. There were also marked cultivar differences in response to CO2 and significant effects of leaf temperature. CO2-saturated assimilation varied markedly, with Semillon and Merlot leaves most responsive to temperature, although there were differences in optimum temperatures and maximum rates. Chardonnay leaves remained least tolerant, with lowest rates of assimilation across most temperatures. Assimilation at 45°C also separated the cultivars and two cultivars had higher rates than at 15°C while Chardonnay and Merlot leaves had higher rates at 15°C. There were no cultivar differences in the temperature dependency of Ribulose 1,5-bisphosphate (RuBP) carboxylation, but Semillon had a much steeper temperature dependency on RuBP regeneration than the other cultivars. All these responses confirmed the hypothesis and concluded the high-temperature tolerance of Semillon and Shiraz and the poor adaptability of Chardonnay and possibly Merlot to perform in the current high-temperature growth conditions.

Combined leaf gas-exchange system for model assessment

Leaf gas-exchange measurements are useful in assessing plant environmental responses. However, uncertainties in the leaf gas-exchange model potentially limit its application. The main challenge in the model-dependent calculations is to detect violations of assumptions. Here, we developed a system that integrates into one instrument the direct measurement of leaf intercellular CO2 concentration and the standard open-flow (OF) and novel open-diffusion (OD) systems for flux measurement. In the OD system, a gas-permeable membrane between the leaf ambient air and outside air creates CO2 and H2O differentials, rather than the air flow in the OF chamber. We measured hypostomatous and amphistomatous leaves of several species with different photosynthetic capacities [sunflower (Helianthus annuus), grape (Vitis vinifera), lemon (Citrus limon), and cherry (Prunus avium)]. The CO2 and H2O differentials in the OD system strictly depend on the flux measured by the OF system. The lower permeability of the membrane resulted in a larger differential per flux, indicating that the OD system can increase the resolution for a small flux. An analysis of the conductance model along with observations suggested that cuticle and leaf intercellular conductances and the unsaturation of leaf humidity contributed to discrepancies between the direct measurement and standard calculation. The combined system developed here provides an opportunity to address these overlooked concepts in leaf gas exchange.

Restricted internal diffusion weakens transpiration-photosynthesis coupling during heatwaves: Evidence from leaf carbonyl sulphide exchange

Increasingly frequent and intense heatwaves threaten ecosystem health in a warming climate. However, plant responses to heatwaves are poorly understood. A key uncertainty concerns the intensification of transpiration when heatwaves suppress photosynthesis, known as transpiration-photosynthesis decoupling. Field observations of such decoupling are scarce, and the underlying physiological mechanisms remain elusive. Here, we use carbonyl sulphide (COS) as a leaf gas exchange tracer to examine potential mechanisms leading to transpiration-photosynthesis decoupling on a coast live oak in a southern California woodland in spring 2013. We found that heatwaves suppressed both photosynthesis and leaf COS uptake but increased transpiration or sustained it at non-heatwave levels throughout the day. Despite statistically significant decoupling between transpiration and photosynthesis, stomatal sensitivity to environmental factors did not change during heatwaves. Instead, midday photosynthesis during heatwaves was restricted by internal diffusion, as indicated by the lower internal conductance to COS. Thus, increased evaporative demand and nonstomatal limitation to photosynthesis act jointly to decouple transpiration from photosynthesis without altering stomatal sensitivity. Decoupling offered limited potential cooling benefits, questioning its effectiveness for leaf thermoregulation in xeric ecosystems. We suggest that adding COS to leaf and ecosystem flux measurements helps elucidate diverse physiological mechanisms underlying transpiration-photosynthesis decoupling.

The importance of species-specific and temperature-sensitive parameterisation of A/Ci models: A case study using cotton (Gossypium hirsutum L.) and the automated 'OptiFitACi' R-package

Leaf gas exchange measurements are an important tool for inferring a plant's photosynthetic biochemistry. In most cases, the responses of photosynthetic CO2 assimilation to variable intercellular CO2 concentrations (A/Ci response curves) are used to model the maximum (potential) rate of carboxylation by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, Vcmax) and the rate of photosynthetic electron transport at a given incident photosynthetically active radiation flux density (PAR; JPAR). The standard Farquhar-von Caemmerer-Berry model is often used with default parameters of Rubisco kinetic values and mesophyll conductance to CO2 (gm) derived from tobacco that may be inapplicable across species. To study the significance of using such parameters for other species, here we measured the temperature responses of key in vitro Rubisco catalytic properties and gm in cotton (Gossypium hirsutum cv. Sicot 71) and derived Vcmax and J2000 (JPAR at 2000 µmol m-2 s-1 PAR) from cotton A/Ci curves incrementally measured at 15°C-40°C using cotton and other species-specific sets of input parameters with our new automated fitting R package 'OptiFitACi'. Notably, parameterisation by a set of tobacco parameters produced unrealistic J2000:Vcmax ratio of <1 at 25°C, two- to three-fold higher estimates of Vcmax above 15°C, up to 2.3-fold higher estimates of J2000 and more variable estimates of Vcmax and J2000, for our cotton data compared to model parameterisation with cotton-derived values. We determined that errors arise when using a gm,25 of 2.3 mol m-2 s-1 MPa-1 or less and Rubisco CO2-affinities in 21% O2 (KC 21%O2) at 25°C outside the range of 46-63 Pa to model A/Ci responses in cotton. We show how the A/Ci modelling capabilities of 'OptiFitACi' serves as a robust, user-friendly, and flexible extension of 'plantecophys' by providing simplified temperature-sensitivity and species-specificity parameterisation capabilities to reduce variability when modelling Vcmax and J2000.

Greater mesophyll conductance and leaf photosynthesis in the field through modified cell wall porosity and thickness via AtCGR3 expression in tobacco

Mesophyll conductance (gm) describes the ease with which CO2 passes from the sub-stomatal cavities of the leaf to the primary carboxylase of photosynthesis, Rubisco. Increasing gm is suggested as a means to engineer increases in photosynthesis by increasing [CO2] at Rubisco, inhibiting oxygenation and accelerating carboxylation. Here, tobacco was transgenically up-regulated with Arabidopsis Cotton Golgi-related 3 (CGR3), a gene controlling methylesterification of pectin, as a strategy to increase CO2 diffusion across the cell wall and thereby increase gm. Across three independent events in tobacco strongly expressing AtCGR3, mesophyll cell wall thickness was decreased by 7%-13%, wall porosity increased by 75% and gm measured by carbon isotope discrimination increased by 28%. Importantly, field-grown plants showed an average 8% increase in leaf photosynthetic CO2 uptake. Up-regulating CGR3 provides a new strategy for increasing gm in dicotyledonous crops, leading to higher CO2 assimilation and a potential means to sustainable crop yield improvement.

Harnessing photosynthetic C18O16O discrimination dynamics under leaf water nonsteady state to estimate mesophyll conductance: a new, regression-based method

Mesophyll conductance (gm) is a crucial plant trait that can significantly limit photosynthesis. Measurement of photosynthetic C18O16O discrimination (Δ18O) has proved to be the only viable means of resolving gm in both C3 and C4 plants. However, the currently available methods to exploit Δ18O for gm estimation are error prone due to their inadequacy in constraining the degree of oxygen isotope exchange (θ) during mesophyll CO2 hydration. Here, we capitalized on experimental manipulation of leaf water isotopic dynamics to establish a novel, nonsteady state, regression-based approach for simultaneous determination of gm and θ from online Δ18O measurements. We demonstrated the methodological and theoretical robustness of this new Δ18O-gm estimation approach and showed through measurements on several C3 and C4 species that this approach can serve as a benchmark method against which to identify previously-unrecognized biases of the existing Δ18O-gm methods. Our results highlight the unique value of this nonsteady state-based approach for contributing to ongoing efforts toward quantitative understanding of mesophyll conductance for crop yield improvement and carbon cycle modeling.

Functional traits of fossil plants

A minuscule fraction of the Earth's paleobiological diversity is preserved in the geological record as fossils. What plant remnants have withstood taphonomic filtering, fragmentation, and alteration in their journey to become part of the fossil record provide unique information on how plants functioned in paleo-ecosystems through their traits. Plant traits are measurable morphological, anatomical, physiological, biochemical, or phenological characteristics that potentially affect their environment and fitness. Here, we review the rich literature of paleobotany, through the lens of contemporary trait-based ecology, to evaluate which well-established extant plant traits hold the greatest promise for application to fossils. In particular, we focus on fossil plant functional traits, those measurable properties of leaf, stem, reproductive, or whole plant fossils that offer insights into the functioning of the plant when alive. The limitations of a trait-based approach in paleobotany are considerable. However, in our critical assessment of over 30 extant traits we present an initial, semi-quantitative ranking of 26 paleo-functional traits based on taphonomic and methodological criteria on the potential of those traits to impact Earth system processes, and for that impact to be quantifiable. We demonstrate how valuable inferences on paleo-ecosystem processes (pollination biology, herbivory), past nutrient cycles, paleobiogeography, paleo-demography (life history), and Earth system history can be derived through the application of paleo-functional traits to fossil plants.

Shining Light into a "Black Box": Essential Rationale Underlying Multiphase Flash Methodology

The world we live in is very fragile. Sustainable food production is increasingly under intense pressure due to changing environmental conditions on many levels. Understanding the complexities of how to optimize food production under increasingly deleterious environmental conditions is dependent upon accurate and detailed analyses of plant productivity from the molecular-to-the-remote scales. One method that can link many of these scales has been around for decades, namely, pulse amplitude modulation (PAM) chlorophyll a fluorescence. This technique is used to measure an assortment of important parameters based on chlorophyll a fluorescence. One of the parameters measured by this method is termed the steady state maximum fluorescence yield ( Φ Fm ' ). This parameter, while extremely informative when used to quantify an assortment of processes of intense scientific interest, is nonetheless subject to intrinsic underestimation. A clever approach has evolved over several decades to more accurately estimate Φ Fm ' . The underlying rationale of the methodology requires a thorough and nuanced explanation, which is lacking in the literature. Herein, we systematically develop the essential rationale for accurately measuring Φ Fm ' based on the latest evolution of this approach, called multiphase flash (MPF) methodology.

Using Carbon Stable Isotopes to Study C3 and C4 Photosynthesis: Models and Calculations

Stable carbon isotopes are a powerful tool to study photosynthesis. Initial applications consisted of determining isotope ratios of plant biomass using mass spectrometry. Subsequently, theoretical models relating C isotope values to gas exchange characteristics were introduced and tested against instantaneous online measurements of 13C photosynthetic discrimination. Beginning in the twenty-first century, laser absorption spectroscopes with sufficient precision for determining isotope mixing ratios became commercially available. This has allowed collection of large data sets at lower cost and with unprecedented temporal resolution. More data and accompanying knowledge have permitted refinement of 13C discrimination model equations, but often at the expense of increased model complexity and difficult parametrization. This chapter describes instantaneous online measurements of 13C photosynthetic discrimination, provides recommendations for experimental setup, and presents a thorough compilation of equations available to researchers. We update our previous 2018 version of this chapter by including recently improved descriptions of (photo)respiratory processes and associated fractionations. We discuss the capabilities and limitations of the diverse 13C discrimination model equations and provide guidance for selecting the model complexity needed for different applications.

Exploring through the use of physiological and isotopic techniques the potential of a PGPR-based biofertilizer to improve nitrogen fertilization practices efficiency in strawberry cultivation

The use of microorganisms as a biofertilizer in strawberry has focused mainly on pathogen biocontrol, which has led to the underestimation of the potential of microorganisms for the improvement of nutritional efficiency in this crop. A study was established to investigate the impact of a plant growth-promoting rhizobacteria (PGPR) based biofertilizer integrated by self-compatible stress tolerant strains with multiple PGP properties, including atmospheric nitrogen fixation, on strawberry (Fragaria × ananassa cv. Rociera) tolerance to N deficiency in terms of growth and physiological performance. After 40 days of nitrogen fertilization shortage, inoculated plants were able to maintain root development and fertility structures (i.e. fruits and flowers) at a level similar to plants properly fertilized. In addition, inoculation lessened the negative impact of nitrogen deficiency on leaves' dry weight and relative water content. This effect was mediated by a higher root/shoot ratio, which would have allowed them to explore larger volumes of soil for the acquisition of water. Moreover, inoculation was able to buffer up to 50% of the reduction in carbon assimilation capacity, due to its positive effect on the diffusion efficiency of CO2 and the biochemical capacity of photosynthesis, as well as on the activity of photosystem II light harvesting. Furthermore, the higher leaf C/N ratio and the maintained δ15N values close to control plants were related to positive bacterial effects at the level of the plant nutritional balance. Despite these positive effects, the application of the bacterial inoculum was unable to completely counteract the restriction of fertilization, being necessary to apply a certain amount of synthetic fertilizer for the strawberry nutrition. However, according to our results, the complementary effect of this PGPR-based biofertilizer could provide a higher efficiency in environmental and economic yields on this crop.

Functional responses of two Mediterranean pine species in an ozone Free-Air Controlled Exposure (FACE) experiment

Effects of the phytotoxic and widespread ozone (O3) pollution may be species specific, but knowledge on Mediterranean conifer responses to long-term realistic exposure is still limited. We examined responses regarding to photosynthesis, needle biochemical stress markers and carbon and nitrogen (N) isotopes of two Mediterranean pine species (Pinus halepensis Mill. and Pinus pinea L.). Seedlings were grown in a Free-Air Controlled Exposure experiment with three levels of O3 (ambient air, AA [38.7 p.p.b. as daily average]; 1.5 × AA and 2.0 × AA) during the growing season (May-October 2019). In P. halepensis, O3 caused a significant decrease in the photosynthetic rate, which was mainly due to a reduction of both stomatal and mesophyll diffusion conductance to CO2. Isotopic analyses indicated a cumulative or memory effect of O3 exposure on this species, as the negative effects were highlighted only in the late growing season in association with a reduced biochemical defense capacity. On the other hand, there was no clear effect of O3 on photosynthesis in P. pinea. However, this species showed enhanced N allocation to leaves to compensate for reduced photosynthetic N- use efficiency. We conclude that functional responses to O3 are different between the two species determining that P. halepensis with thin needles was relatively sensitive to O3, while P. pinea with thicker needles was more resistant due to a potentially low O3 load per unit mass of mesophyll cells, which may affect species-specific resilience in O3-polluted Mediterranean pine forests.

Leaf physiological and morphological constraints of water-use efficiency in C3 plants

The increasing evaporative demand due to climate change will significantly affect the balance of carbon assimilation and water losses of plants worldwide. The development of crop varieties with improved water-use efficiency (WUE) will be critical for adapting agricultural strategies under predicted future climates. This review aims to summarize the most important leaf morpho-physiological constraints of WUE in C3 plants and identify gaps in knowledge. From the carbon gain side of the WUE, the discussed parameters are mesophyll conductance, carboxylation efficiency and respiratory losses. The traits and parameters affecting the waterside of WUE balance discussed in this review are stomatal size and density, stomatal control and residual water losses (cuticular and bark conductance), nocturnal conductance and leaf hydraulic conductance. In addition, we discussed the impact of leaf anatomy and crown architecture on both the carbon gain and water loss components of WUE. There are multiple possible targets for future development in understanding sources of WUE variability in plants. We identified residual water losses and respiratory carbon losses as the greatest knowledge gaps of whole-plant WUE assessments. Moreover, the impact of trichomes, leaf hydraulic conductance and canopy structure on plants' WUE is still not well understood. The development of a multi-trait approach is urgently needed for a better understanding of WUE dynamics and optimization.

Assessing the CO2 concentration at the surface of photosynthetic mesophyll cells

We present a robust estimation of the CO₂ concentration at the surface of photosynthetic mesophyll cells (c_w ), applicable under reasonable assumptions of assimilation distribution within the leaf. We used Capsicum annuum, Helianthus annuus and Gossypium hirsutumas model plants for our experiments. We introduce calculations to estimate c_w using independent adaxial and abaxial gas exchange measurements, and accounting for the mesophyll airspace resistances. The c_w was lower than adaxial and abaxial estimated intercellular CO₂ concentrations (c_i ). Differences between c_w and the c_i of each surface were usually larger than 10 μmol mol^-1 . Differences between adaxial and abaxial c_i ranged from a few μmol mol^-1 to almost 50 μmol mol^-1 , where the largest differences were found at high air saturation deficits (ASD). Differences between adaxial and abaxial c_i and the c_i estimated by mixing both fluxes ranged from -30 to +20 μmol mol^-1 , where the largest differences were found under high ASD or high ambient CO₂ concentrations. Accounting for c_w improves the information that can be extracted from gas exchange experiments, allowing a more detailed description of the CO₂ and water vapor gradients within the leaf.

Nutrient availability regulates Deschampsia antarctica photosynthetic and stress tolerance performance in Antarctica

Deschampsia antarctica is one of the only two native vascular plants in Antarctica, mostly located in the ice-free areas of the Peninsula's coast and adjacent islands. This region is characterized by a short growing season, frequent extreme climatic events, and soils with reduced nutrient availability. However, it is unknown whether its photosynthetic and stress tolerance mechanisms are affected by the availability of nutrients to deal with this particular environment. We studied the photosynthetic, primary metabolic, and stress tolerance performance of D. antarctica plants growing on three close sites (<500 m) with contrasting soil nutrient conditions. Plants from all sites showed similar photosynthetic rates, but mesophyll conductance and photobiochemistry were more limiting (~25%) in plants growing on low-nutrient availability soils. Additionally, these plants showed higher stress levels and larger investments in photoprotection and carbon pools, most probably driven by the need to stabilize proteins and membranes, and remodel cell walls. In contrast, when nutrients were readily available, plants shifted their carbon investment towards amino acids related to osmoprotection, growth, antioxidants, and polyamines, leading to vigorous plants without appreciable levels of stress. Taken together, these findings demonstrate that D. antarctica displays differential physiological performances to cope with adverse conditions depending on resource availability, allowing it to maximize stress tolerance without jeopardizing photosynthetic capacity.

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.

The ratio of electron transport to assimilation (ETR/AN): underutilized but essential for assessing both equipment's proper performance and plant status

ETR/A_N ratios should be in the range 7.5-10.5 for non-stressed C₃ plants. Ratios extremely out of this range can be reflecting both uncontrolled plant status and technical mistakes during measurements. We urge users to explicitly refer to this ratio in future studies as a proof for internal data quality control. For the last few decades, the use of infra-red gas-exchange analysers (IRGAs) coupled with chlorophyll fluorometers that allow for measurements of net CO₂ assimilation rate and estimates of electron transport rate over the same leaf area has been popularized. The evaluation of data from both instruments in an integrative manner can result in additional valuable information, such as the estimation of the light respiration, mesophyll conductance and the partitioning of the flux of electrons into carboxylation, oxygenation and alternative processes, among others. In this review, an additional and more 'straight' use of the combination of chlorophyll fluorescence and gas exchange-derived parameters is presented, namely using the direct ratio between two fully independently estimated parameters, electron transport rate (ETR)-determined by the fluorometer-and net CO₂ assimilation rate (A_N)-determined by the IRGA, i.e., the ETR/A_N ratio, as a tool for fast detection of incongruencies in the data and potential technical problems associated with them, while checking for the study plant's status. To illustrate this application, a compilation of 75 studies that reported both parameters for a total of 178 species under varying physiological status is presented. Values of ETR/A_N between 7.5 and 10.5 were most frequently found for non-stressed C₃ plants. C₄ species showed an average ETR/A_N ratio of 4.7. The observed ratios were larger for species with high leaf mass per area and for plants subjected to stressful factors like drought or nutritional deficit. Knowing the expected ETR/A_N ratio projects this ratio as a routinary and rapid check point for guaranteeing both the correct performance of equipment and the optimal/stress status of studied plants. All known errors associated with the under- or overestimation of ETR or A_N are summarized in a checklist that aims to be routinely used by any IRGA/fluorometer user to strength the validity of their data.

Mesophyll conductance response to short-term changes in pCO2 is related to leaf anatomy and biochemistry in diverse C4 grasses

Mesophyll CO₂ conductance (g_m ) in C₃ species responds to short-term (minutes) changes in environment potentially due to changes in leaf anatomical and biochemical properties and measurement artefacts. Compared with C₃ species, there is less information on g_m responses to short-term changes in environmental conditions such as partial pressure of CO₂ (pCO₂ ) across diverse C₄ species and the potential determinants of these responses. Using 16 C₄ grasses we investigated the response of g_m to short-term changes in pCO₂ and its relationship with leaf anatomy and biochemistry. In general, g_m increased as pCO₂ decreased (statistically significant increase in 12 species), with percentage increases in g_m ranging from +13% to +250%. Greater increase in g_m at low pCO₂ was observed in species exhibiting relatively thinner mesophyll cell walls along with greater mesophyll surface area exposed to intercellular air spaces, leaf N, photosynthetic capacity and activities of phosphoenolpyruvate carboxylase and Rubisco. Species with greater CO₂ responses of g_m were also able to maintain their leaf water-use efficiencies (TE_i ) under low CO₂ . Our study advances understanding of CO₂ response of g_m in diverse C₄ species, identifies the key leaf traits related to this response and has implications for improving C₄ photosynthetic models and TE_i through modification of g_m .

Contrasting anatomical and biochemical controls on mesophyll conductance across plant functional types

Mesophyll conductance (g_m ) limits photosynthesis by restricting CO₂ diffusion between the substomatal cavities and chloroplasts. Although it is known that g_m is determined by both leaf anatomical and biochemical traits, their relative contribution across plant functional types (PFTs) is still unclear. We compiled a dataset of g_m measurements and concomitant leaf traits in unstressed plants comprising 563 studies and 617 species from all major PFTs. We investigated to what extent g_m limits photosynthesis across PFTs, how g_m relates to structural, anatomical, biochemical, and physiological leaf properties, and whether these relationships differ among PFTs. We found that g_m imposes a significant limitation to photosynthesis in all C₃ PFTs, ranging from 10-30% in most herbaceous annuals to 25-50% in woody evergreens. Anatomical leaf traits explained a significant proportion of the variation in g_m (R²  > 0.3) in all PFTs except annual herbs, in which g_m is more strongly related to biochemical factors associated with leaf nitrogen and potassium content. Our results underline the need to elucidate mechanisms underlying the global variability of g_m . We emphasise the underestimated potential of g_m for improving photosynthesis in crops and identify modifications in leaf biochemistry as the most promising pathway for increasing g_m in these species.

The evolution of diffusive and biochemical capacities for photosynthesis was predominantly shaped by [CO2] with a smaller contribution from [O2]

The atmospheric concentration of carbon dioxide ([CO₂]) and oxygen ([O₂]) directly influence rates of photosynthesis (P_N) and photorespiration (R_PR) through the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO). Levels of [CO₂] and [O₂] have varied over Earth history affecting rates of both CO₂ uptake and loss, alongside associated transpirative water-loss. The availability of CO₂ has likely acted as a stronger selective pressure than [O₂] due to the greater specificity of RubisCO for CO₂. The role of [O₂], and the interaction of [O₂] and [CO₂], in plant evolutionary history is less understood. We exposed twelve phylogenetically diverse species to combinations of sub-ambient, ambient and super-ambient [O₂] and [CO₂] to examine the biochemical and diffusive components of P_N and the possible role of [O₂] as a selective pressure. Photosynthesis, photosynthetic capacity and stomatal, mesophyll and total conductance to CO₂ were higher in the derived eudicot and monocot angiosperms than the more basal ferns, gymnosperms and basal angiosperms which originated in atmospheres characterised by higher CO₂:O₂ ratios. The ratio of R_PR:P_N was lower in the monocots, consistent with greater carboxylation capacity and higher stomatal and mesophyll conductance making easier CO₂ delivery to chloroplasts. The effect of [O₂] and [CO₂] on P_N/R_PR was less evident in more derived species with a higher conductance to CO₂. The effect of [O₂] was less apparent at high [CO₂], suggesting that atmospheric [O₂] may only have exerted a strong selective pressure on plant photosynthetic processes during periods characterised by low atmospheric CO₂:O₂ ratios. Current rising [CO₂] will predominantly enhance P_N rates in species with low diffusive conductance to CO₂.

The response of mesophyll conductance to ozone-induced oxidative stress is genotype-dependent in poplar

The CO2 diffusion conductance within the leaf mesophyll (gm) is considered a major limiting factor of photosynthesis. However, the effects of the major secondary air pollutant ozone (O3) on gm have been poorly investigated. Eight genotypes of the economically important tree species Populus × canadensis Moench were exposed to 120 ppb O3 for 21 d. gm showed a genotype-dependent response to O3-induced oxidative stress and was a major limiting factor of net assimilation rate (Anet), ahead of stomatal conductance to CO2 (gsc) and of the maximum carboxylation capacity of the Rubisco enzyme (Vcmax) in half of the tested genotypes. Increased leaf dry mass per area (LMA) and decreased chlorophyll content were linked to the observed gm decrease, but this relationship did not entirely explain the different genotypic gm responses. Moreover, the oxidative stress defence metabolites ascorbate and glutathione were not related to O3 tolerance of gm. However, malondialdehyde probably mitigated the observed gm decrease in some genotypes due to its oxidative stress signalling function. The large variation of gm suggests different regulation mechanisms amongst poplar genotypes under oxidative stress.

Overestimated gains in water-use efficiency by global forests

Increases in terrestrial water-use efficiency (WUE) have been reported in many studies, pointing to potential changes in physiological forcing of global carbon and hydrological cycles. However, gains in WUE are of uncertain magnitude over longer (i.e. >10 years) periods of time largely owing to difficulties in accounting for structural and physiological acclimation. ¹³ C signatures (i.e. δ¹³ C) of plant organic matter have long been used to estimate WUE at temporal scales ranging from days to centuries. Mesophyll conductance is a key uncertainty in estimated WUE owing to its influence on diffusion of CO₂ to sites of carboxylation. Here we apply new knowledge of mesophyll conductance to 464 δ¹³ C chronologies in tree-rings of 143 species spanning global biomes. Adjusted for mesophyll conductance, gains in WUE during the 20th century (0.15 ppm year^-1 ) were considerably smaller than those estimated from conventional modelling (0.26 ppm year^-1 ). Across the globe, mean sensitivity of WUE to atmospheric CO₂ was 0.15 ppm ppm^-1 . Ratios of internal-to-atmospheric CO₂ (on a mole fraction basis; c_i /c_a ) in leaves were mostly constant over time but differed among biomes and plant taxa-highlighting the significance of both plant structure and physiology. Together with synchronized responses in stomatal and mesophyll conductance, our results suggest that ratios of chloroplastic-to-atmospheric CO₂ (c_c /c_a ) are constrained over time. We conclude that forest WUE may have not increased as much as previously suggested and that projections of future climate forcing via CO₂ fertilization may need to be adjusted accordingly.

Limitations to photosynthesis in bryophytes: certainties and uncertainties regarding methodology

Bryophytes are the group of land plants with the lowest photosynthetic rates, which was considered to be a consequence of their higher anatomical CO2 diffusional limitation compared with tracheophytes. However, the most recent studies assessing limitations due to biochemistry and mesophyll conductance in bryophytes reveal discrepancies based on the methodology used. In this study, we compared data calculated from two different methodologies for estimating mesophyll conductance: variable J and the curve-fitting method. Although correlated, mesophyll conductance estimated by the curve-fitting method was on average 4-fold higher than the conductance obtained by the variable J method; a large enough difference to account for the scale of differences previously shown between the biochemical and diffusional limitations to photosynthesis. Biochemical limitations were predominant when the curve-fitting method was used. We also demonstrated that variations in bryophyte relative water content during measurements can also introduce errors in the estimation of mesophyll conductance, especially for samples which are overly desiccated. Furthermore, total chlorophyll concentration and soluble proteins were significantly lower in bryophytes than in tracheophytes, and the percentage of proteins quantified as Rubisco was also significantly lower in bryophytes (<6.3% in all studied species) than in angiosperms (>16% in all non-stressed cases). Photosynthetic rates normalized by Rubisco were not significantly different between bryophytes and angiosperms. Our data suggest that the biochemical limitation to photosynthesis in bryophytes is more relevant than so far assumed.

Drought exerts a greater influence than growth temperature on the temperature response of leaf day respiration in wheat (Triticum aestivum)

We assessed how the temperature response of leaf day respiration (R_d ) in wheat responded to contrasting water regimes and growth temperatures. In Experiment 1, well-watered and drought-stressed conditions were imposed on two genotypes; in Experiment 2, the two water regimes combined with high (HT), medium (MT) and low (LT) growth temperatures were imposed on one of the genotypes. R_d was estimated from simultaneous gas exchange and chlorophyll fluorescence measurements at six leaf temperatures (T_leaf ) for each treatment, using the Yin method for nonphotorespiratory conditions and the nonrectangular hyperbolic fitting method for photorespiratory conditions. The two genotypes responded similarly to growth and measurement conditions. Estimates of R_d for nonphotorespiratory conditions were generally higher than those for photorespiratory conditions, but their responses to T_leaf were similar. Under well-watered conditions, R_d and its sensitivity to T_leaf slightly acclimated to LT, but did not acclimate to HT. Temperature sensitivities of R_d were considerably suppressed by drought, and the suppression varied among growth temperatures. Thus, it is necessary to quantify interactions between drought and growth temperature for reliably modelling R_d under climate change. Our study also demonstrated that the Kok method, one of the currently popular methods for estimating R_d , underestimated R_d significantly.

Higher CO2 Assimilation in Selected Rice Recombinant Inbred Lines Is Driven by Higher CO2 Diffusion and Light Use Efficiency Related to Leaf Anatomy and Mesophyll Cell Density

Leaf anatomy determining the light distribution within the leaf and exerting influence on CO₂ diffusion is considered to have dramatic potential for photosynthesis performance increase. In this study, we observed that two rice recombinant inbred lines, H138 and H217 (RILF₁₁ plants from Sasanishiki × IRAT10), have higher net CO₂ assimilation (An) than their parent Sasanishiki due mainly to the improvement of leaf anatomy. Our results showed that An positively correlated with anatomy traits' mesophyll cell number per cross-sectional area (NO_.mescell/A_cros) and mesophyll area (A_mes). NO._mescell/A_cros exert direct and indirect effects on An. Compared to Sasanishiki flag leaves, IRAT10, H138, and H217 have higher mesophyll cell numbers. Simultaneously, higher chlorophyll content and expression of genes encoding the light-harvesting protein of PSII and PSI (Lhcb1, 2, 3 and Lhca1, 2, 3) were recorded in IRAT10, H138, and H217, which facilitates light use efficiency. Higher electron transport rate and RuBP concentration were recorded in IRAT10, H138, and H217 flag leaves. Retinoblastoma-related gene (OsRBR1), exerting effects on mesophyll cell density, can be used to modify leaf anatomy for improving leaf photosynthesis. Additionally, higher stomatal conductance and mesophyll conductance were also recorded in H138 and H217 than in Sasanishiki. Furthermore, we modeled mesophyll conductance through anatomical traits, and the results revealed that chloroplast thickness was the dominant factor restricting CO₂ diffusion within mesophyll cells rather than cell wall thickness. Higher RuBP content accompanied by higher CO₂ concentration within the carboxylation set in H138 and H217 flag leaves contributed to higher CO₂ assimilation.

Mesophyll conductance is unaffected by expression of Arabidopsis PIP1 aquaporins in the plasmalemma of Nicotiana

In plants with C3 photosynthesis, increasing the diffusion conductance for CO2 from the substomatal cavity to chloroplast stroma (mesophyll conductance) can improve the efficiencies of both CO2 assimilation and photosynthetic water use. In the diffusion pathway from substomatal cavity to chloroplast stroma, the plasmalemma and chloroplast envelope membranes impose a considerable barrier to CO2 diffusion, limiting photosynthetic efficiency. In an attempt to improve membrane permeability to CO2, and increase photosynthesis in tobacco, we generated transgenic lines in Nicotiana tabacum L. cv Petite Havana carrying either the Arabidopsis PIP1;2 (AtPIP1;2) or PIP1;4 (AtPIP1;4) gene driven by the constitutive dual 2x35S CMV promoter. From a collection of independent T0 transgenics, two T2 lines from each gene were characterized, with western blots confirming increased total aquaporin protein abundance in the AtPIP1;2 tobacco lines. Transient expression of AtPIP1;2-mGFP6 and AtPIP1;4-mGFP6 fusions in Nicotiana benthamiana identified that both AtPIP1;2 and AtPIP1;4 localize to the plasmalemma. Despite achieving ectopic production and correct localization, gas exchange measurements combined with carbon isotope discrimination measurements detected no increase in mesophyll conductance or CO2 assimilation rate in the tobacco lines expressing AtPIP. We discuss the complexities associated with trying to enhance gm through modified aquaporin activity.

Mining for allelic gold: finding genetic variation in photosynthetic traits in crops and wild relatives

Improvement of photosynthetic traits in crops to increase yield potential and crop resilience has recently become a major breeding target. Synthetic biology and genetic technologies offer unparalleled opportunities to create new genetics for photosynthetic traits driven by existing fundamental knowledge. However, large 'gene bank' collections of germplasm comprising historical collections of crop species and their relatives offer a wealth of opportunities to find novel allelic variation in the key steps of photosynthesis, to identify new mechanisms and to accelerate genetic progress in crop breeding programmes. Here we explore the available genetic resources in food and fibre crops, strategies to selectively target allelic variation in genes underpinning key photosynthetic processes, and deployment of this variation via gene editing in modern elite material.

Fresh perspectives on an established technique: Pulsed amplitude modulation chlorophyll a fluorescence

Pulsed amplitude modulation (PAM) chlorophyll a fluorescence provides information about photosynthetic energy transduction. When reliably measured, chlorophyll a fluorescence provides detailed information about critical in vivo photosynthetic processes. Such information has recently provided novel and critical insights into how the yield potential of crops can be improved and it is being used to understand remotely sensed fluorescence, which is termed solar-induced fluorescence and will be solely measured by a satellite scheduled to be launched this year. While PAM chlorophyll a fluorometers measure fluorescence intensity per se, herein we articulate the axiomatic criteria by which instrumentally detected intensities can be assumed to assess fluorescence yield, a phenomenon quite different than fluorescence intensity and one that provides critical insight about how solar energy is variably partitioned into the biosphere. An integrated mathematical, phenomenological, and practical discussion of many useful chlorophyll a fluorescence parameters is presented. We draw attention to, and provide examples of, potential uncertainties that can result from incorrect methodological practices and potentially problematic instrumental design features. Fundamentals of fluorescence measurements are discussed, including the major assumptions underlying the signals and the methodological caveats about taking measurements during both dark- and light-adapted conditions. Key fluorescence parameters are discussed in the context of recent applications under environmental stress. Nuanced information that can be gleaned from intra-comparisons of fluorescence-derived parameters and intercomparisons of fluorescence-derived parameters with those based on other techniques is elucidated.

Limits to photosynthesis: seasonal shifts in supply and demand for CO2 in Scots pine

Boreal forests undergo a strong seasonal photosynthetic cycle; however, the underlying processes remain incompletely characterized. Here, we present a novel analysis of the seasonal diffusional and biochemical limits to photosynthesis (A_net ) relative to temperature and light limitations in high-latitude mature Pinus sylvestris, including a high-resolution analysis of the seasonality of mesophyll conductance (g_m ) and its effect on the estimation of carboxylation capacity ( VCmax ). We used a custom-built gas-exchange system coupled to a carbon isotope analyser to obtain continuous measurements for the estimation of the relevant shoot gas-exchange parameters and quantified the biochemical and diffusional controls alongside the environmental controls over A_net . The seasonality of A_net was strongly dependent on VCmax and the diffusional limitations. Stomatal limitation was low in spring and autumn but increased to 31% in June. By contrast, mesophyll limitation was nearly constant (19%). We found that VCmax limited A_net in the spring, whereas daily temperatures and the gradual reduction of light availability limited A_net in the autumn, despite relatively high VCmax . We describe for the first time the role of mesophyll conductance in connection with seasonal trends in net photosynthesis of P. sylvestris, revealing a strong coordination between g_m and A_net , but not between g_m and stomatal conductance.

Comparisons of photosynthetic and anatomical traits between wild and domesticated cotton

Mesophyll conductance (gm) is a crucial leaf trait contributing to the photosynthetic rate (AN). Plant domestication typically leads to an enhancement of AN that is often associated with profound anatomical modifications, but it is unclear which of these structural alterations influence gm. We analyzed the implication of domestication on leaf anatomy and its effect on gm in 26 wild and 31 domesticated cotton genotypes (Gossypium sp.) grown under field conditions. We found that domesticated genotypes had higher AN but similar gm to wild genotypes. Consistent with this, domestication did not translate into significant differences in the fraction of mesophyll occupied by intercellular air spaces (fias) or mesophyll and chloroplast surface area exposed to intercellular air space (Sm/S and Sc/S, respectively). However, leaves of domesticated genotypes were significantly thicker, with larger but fewer mesophyll cells with thinner cell walls. Moreover, domesticated genotypes had higher cell wall conductance (gcw) but smaller cytoplasmic conductance (gcyt) than wild genotypes. It appears that domestication in cotton has not generally led to significant improvement in gm, in part because their thinner mesophyll cell walls (increasing gcw) compensate for their lower gcyt, itself due to larger distance between plasmalemma and chloroplast envelopes.

Scaling Up from Leaf to Whole-Plant Level for Water Use Efficiency Estimates Based on Stomatal and Mesophyll Behaviour in Platycladus orientalis

Prediction of whole-plant short-term water use efficiency (WUEs,P) is essential to indicate plant performance and facilitate comparison across different temporal and spatial scales. In this study, an isotope model was scaled up from the leaf to the whole-plant level, in order to simulate the variation in WUEs,P in response to different CO2 concentrations (Ca; 400, 600, and 800 μmol·mol−1) and soil water content (SWC; 35–100% of field capacity). For WUEs,P modelling, leaf gas exchange information, plant respiration, and “unproductive” water loss were taken into account. Specifically, in shaping the expression of the WUEs,P, we emphasized the role of both stomatal (gsw) and mesophyll conductance (gm). Simulations were compared with the measured results to check the model’s applicability. The verification showed that estimates of gsw from the coupled photosynthesis (Pn,L)-gsw model accounting for the effect of soil water stress slightly outperformed the model neglecting the soil water status effect. The established coupled Pn,L-gm model also proved more effective in estimating gm than the previously proposed model. Introducing the two diffusion control functions into the whole-plant model, the developed model for WUEs,P effectively captured its response pattern to different Ca and SWC conditions. Overall, this study confirmed that the accurate estimation of WUEs,P requires an improved predictive accuracy of gsw and gm. These results have important implications for predicting how plants respond to climate change.

Species-specific variation of photosynthesis and mesophyll conductance to ozone and drought in three Mediterranean oaks

Mesophyll conductance (g_mCO2 ) is one of the most important components in plant photosynthesis. Tropospheric ozone (O₃ ) and drought impair physiological processes, causing damage to photosynthetic systems. However, the combined effects of O₃ and drought on g_mCO2 are still largely unclear. We investigated leaf gas exchange during mid-summer in three Mediterranean oaks exposed to O₃ (ambient [35.2 nmol mol^-1 as daily mean]; 1.4 × ambient) and water treatments (WW [well-watered] and WD [water-deficit]). We also examined if leaf traits (leaf mass per area [LMA], foliar abscisic acid concentration [ABA]) could influence the diffusion of CO₂ inside a leaf. The combination of O₃ and WD significantly decreased net photosynthetic rate (P_N ) regardless of the species. The reduction of photosynthesis was associated with a decrease in g_mCO2 and stomatal conductance (g_sCO2 ) in evergreen Quercus ilex, while the two deciduous oaks (Q. pubescens, Q. robur) also showed a reduction of the maximum rate of carboxylation (V_cmax ) and maximum electron transport rate (J_max ) with decreased diffusive conductance parameters. The reduction of g_mCO2 was correlated with increased [ABA] in the three oaks, whereas there was a negative correlation between g_mCO2 with LMA in Q. pubescens. Interestingly, two deciduous oaks showed a weak or no significant correlation between g_sCO2 and ABA under high O₃ and WD due to impaired stomatal physiological behaviour, indicating that the reduction of P_N was related to g_mCO2 rather than g_sCO2 . The results suggest that g_mCO2 plays an important role in plant carbon gain under concurrent increases in the severity of drought and O₃ pollution.

Mesophyll conductance in two cultivars of wheat grown in glacial to super-elevated CO2 concentrations

Mesophyll conductance (gm) is an important factor limiting photosynthesis. However, gm response to long-term growth in variable [CO2] is not well understood, particularly in crop plants. Here, we grew two cultivars of wheat (Halberd and Cranbrook), known to differ in gm under current environmental conditions, in four [CO2] treatments: glacial (206 μmol mol-1), pre-industrial (344 μmol mol-1), current ambient (489 μmol mol-1), and super-elevated (1085 μmol mol-1), and two water treatments (well-watered and moderate water limitation), to develop an evolutionary and future climate perspective on gm control of photosynthesis and water-use efficiency (WUE). In the two wheat genotypes, gm increased with rising [CO2] from glacial to ambient [CO2], but declined at super-elevated [CO2]. The responses of gm to different growth [CO2] also depend on water stress; however, the specific mechanism of gm response to [CO2] remains unclear. Although gm and gm/gsc (mesophyll conductance/stomatal conductance) were strongly associated with the variability of photosynthetic rates (A) and WUE, we found that plants with higher gm may increase A without increasing gsc, which increased WUE. These results may be useful to inform plant breeding programmes and cultivar selection for Australian wheat under future environmental conditions.

Evolution of a biochemical model of steady-state photosynthesis

On the occasion of the 40th anniversary of the publication of the landmark model by Farquhar, von Caemmerer & Berry on steady-state C₃ photosynthesis (known as the "FvCB model"), we review three major further developments of the model. These include: (1) limitation by triose phosphate utilization, (2) alternative electron transport pathways, and (3) photorespiration-associated nitrogen and C₁ metabolisms. We discussed the relation of the third extension with the two other extensions, and some equivalent extensions to model C₄ photosynthesis. In addition, the FvCB model has been coupled with CO₂ -diffusion models. We review how these extensions and integration have broadened the use of the FvCB model in understanding photosynthesis, especially with regard to bioenergetic stoichiometries associated with photosynthetic quantum yields. Based on the new insights, we present caveats in applying the FvCB model. Further research needs are highlighted.

Acclimation of mesophyll conductance and anatomy to light during leaf aging in Arabidopsis thaliana

Mesophyll conductance (g_m ), a key limitation to photosynthesis, is strongly driven by leaf anatomy, which is in turn influenced by environmental growth conditions and ontogeny. However, studies examining the combined environment × age effect on both leaf anatomy and photosynthesis are scarce, and none have been carried out in short-lived plants. Here, we studied the variation of photosynthesis and leaf anatomy in the model species Arabidopsis thaliana (Col-0) grown under three different light intensities at two different leaf ages. We found that light × age interaction was significant for photosynthesis but not for anatomical characteristics. Increasing growth light intensities resulted in increases in leaf mass per area, thickness, number of palisade cell layers, and chloroplast area lining to intercellular airspace. Low and moderate-but not high-light intensity had a significant effect on all photosynthetic characteristics. Leaf aging was associated with increases in cell wall thickness (T_cw ) in all light treatments and in increases in leaf thickness in plants grown under low and moderate light intensities. However, g_m did not vary with leaf aging, and photosynthesis only decreased with leaf age under moderate and high light, suggesting a compensatory effect between increased T_cw and decreased chloroplast thickness on the total CO₂ diffusion resistance.

Leaf scale quantification of the effect of photosynthetic gas exchange on Δ47 of CO2

The clumped isotope composition (Δ₄₇, the anomaly of the mass 47 isotopologue relative to the abundance expected from a random isotope distribution) of CO₂ has been suggested as an additional tracer for gross CO₂ fluxes. However, the effect of photosynthetic gas exchange on Δ₄₇ has not been directly determined and two indirect/conceptual studies reported contradicting results. In this study, we quantify the effect of photosynthetic gas exchange on Δ₄₇ of CO₂ using leaf cuvette experiments with one C₄ and two C₃ plants. The experimental results are supported by calculations with a leaf cuvette model. Our results demonstrate the important roles of the Δ₄₇ value of CO₂ entering the leaf, kinetic fractionation as CO₂ diffuses into, and out of the leaf and CO₂–H₂O isotope exchange with leaf water. We experimentally confirm the previously suggested dependence of Δ₄₇ of CO₂ in the air surrounding a leaf on the stomatal conductance and back-diffusion flux. Gas exchange can enrich or deplete the Δ₄₇ of CO₂ depending on the Δ₄₇ of CO₂ entering the leaf and the fraction of CO₂ exchanged with leaf water and diffused back to the atmosphere, but under typical ambient conditions, it will lead to a decrease in Δ₄₇.

Photosynthesis: a multiscopic view

A recurring analogy for photosynthesis research is the fable of the blind men and the elephant. Photosynthesis has many complex working parts, which has driven the need to study each of them individually, with an inherent understanding that a more complete picture will require systematic integration of these views. However, unlike the blind men, who are limited to using their hands, researchers have developed over the past decades a repertoire of methods for studying these components, many of which capitalize on unique features intrinsic to each. More recent concerns about food security and clean, renewable energy have increased support for applied photosynthesis research, with the idea of either improving photosynthetic performance as a desired trait in select species or using photosynthetic measurements as a phenotyping tool in breeding efforts or for high precision crop management. In this review, we spotlight the migration of approaches for studying photosynthesis from the laboratory into field environments, highlight some recent advances and speculate on areas where further development would be fruitful, with an eye towards how applied photosynthesis research can have impacts at local and global scales.

A meta-analysis of mesophyll conductance to CO2 in relation to major abiotic stresses in poplar species

Mesophyll conductance (gm) determines the diffusion of CO2 from the substomatal cavities to the site of carboxylation in the chloroplasts and represents a critical component of the diffusive limitation of photosynthesis. In this study, we evaluated the average effect sizes of different environmental constraints on gm in Populus spp., a forest tree model. We collected raw data of 815 A-Ci response curves from 26 datasets to estimate gm, using a single curve-fitting method to alleviate method-related bias. We performed a meta-analysis to assess the effects of different abiotic stresses on gm. We found a significant increase in gm from the bottom to the top of the canopy that was concomitant with the increase of maximum rate of carboxylation and light-saturated photosynthetic rate (Amax). gm was positively associated with increases in soil moisture and nutrient availability, but was insensitive to increasing soil copper concentration and did not vary with atmospheric CO2 concentration. Our results showed that gm was strongly related to Amax and to a lesser extent to stomatal conductance (gs). Moreover, a negative exponential relationship was obtained between gm and specific leaf area, which may be used to scale-up gm within the canopy.

Reduced photosynthetic thermal acclimation capacity under elevated ozone in poplar (Populus tremula) saplings

The sensitivity of photosynthesis to temperature has been identified as a key uncertainty for projecting the magnitude of the terrestrial carbon cycle response to future climate change. Although thermal acclimation of photosynthesis under rising temperature has been reported in many tree species, whether tropospheric ozone (O₃ ) affects the acclimation capacity remains unknown. In this study, temperature responses of photosynthesis (light-saturated rate of photosynthesis (A_sat ), maximum rates of RuBP carboxylation (V_cmax ), and electron transport (J_max ) and dark respiration (R_dark ) of Populus tremula exposed to ambient O₃ (AO₃ , maximum of 30 ppb) or elevated O₃ (EO₃ , maximum of 110 ppb) and ambient or elevated temperature (ambient +5°C) were investigated in solardomes. We found that the optimum temperature of A_sat (T_optA ) significantly increased in response to warming. However, the thermal acclimation capacity was reduced by O₃ exposure, as indicated by decreased T_optA , and temperature optima of V_cmax (T_optV ) and J_max (T_optJ ) under EO₃ . Changes in both stomatal conductance (g_s ) and photosynthetic capacity (V_cmax and J_max ) contributed to the shift of T_optA by warming and EO₃ . Neither R_dark measured at 25°C ( R dark 25 ) nor the temperature response of R_dark was affected by warming, EO₃ , or their combination. The responses of A_sat , V_cmax , and J_max to warming and EO₃ were closely correlated with changes in leaf nitrogen (N) content and N use efficiency. Overall, warming stimulated growth (leaf biomass and tree height), whereas EO₃ reduced growth (leaf and woody biomass). The findings indicate that thermal acclimation of A_sat may be overestimated if the impact of O₃ pollution is not taken into account.

Role of Hydraulic Signal and ABA in Decrease of Leaf Stomatal and Mesophyll Conductance in Soil Drought-Stressed Tomato

Drought reduces leaf stomatal conductance (g_s) and mesophyll conductance (g_m). Both hydraulic signals and chemical signals (mainly abscisic acid, ABA) are involved in regulating g_s. However, it remains unclear what role the endogenous ABA plays in g_m under decreasing soil moisture. In this study, the responses of g_s and g_m to ABA were investigated under progressive soil drying conditions and their impacts on net photosynthesis (A_n) and intrinsic water use efficiency (WUE_i) were also analyzed. Experimental tomato plants were cultivated in pots in an environment-controlled greenhouse. Reductions of g_s and g_m induced a 68-78% decline of A_n under drought conditions. While soil water potential (Ψ_soil) was over -1.01 MPa, g_s reduced as leaf water potential (Ψ_leaf) decreased, but ABA and g_m kept unchanged, which indicating g_s was more sensitive to drought than g_m. During Ψ_soil reduction from -1.01 to -1.44 MPa, Ψ_leaf still kept decreasing, and both g_s and g_m decreased concurrently following to the sustained increases of ABA content in shoot sap. The g_m was positively correlated to g_s during a drying process. Compared to g_s or g_m, WUE_i was strongly correlated with g_m/g_s. WUE_i improved within Ψ_soil range between -0.83 and -1.15 MPa. In summary, g_s showed a higher sensitivity to drought than g_m. Under moderate and severe drought at Ψ_soil ≤ -1.01 MPa, furthermore from hydraulic signals, ABA was also involved in this co-ordination reductions of g_s and g_m and thereby regulated A_n and WUE_i.

Changes in Leaf-Level Nitrogen Partitioning and Mesophyll Conductance Deliver Increased Photosynthesis for Lolium perenne Leaves Engineered to Accumulate Lipid Carbon Sinks

Diacylglycerol acyl-transferase (DGAT) and cysteine oleosin (CO) expression confers a novel carbon sink (of encapsulated lipid droplets) in leaves of Lolium perenne and has been shown to increase photosynthesis and biomass. However, the physiological mechanism by which DGAT + CO increases photosynthesis remains unresolved. To evaluate the relationship between sink strength and photosynthesis, we examined fatty acids (FA), water-soluble carbohydrates (WSC), gas exchange parameters and leaf nitrogen for multiple DGAT + CO lines varying in transgene accumulation. To identify the physiological traits which deliver increased photosynthesis, we assessed two important determinants of photosynthetic efficiency, CO₂ conductance from atmosphere to chloroplast, and nitrogen partitioning between different photosynthetic and non-photosynthetic pools. We found that DGAT + CO accumulation increased FA at the expense of WSC in leaves of L. perenne and for those lines with a significant reduction in WSC, we also observed an increase in photosynthesis and photosynthetic nitrogen use efficiency. DGAT + CO L. perenne displayed no change in rubisco content or V_cmax but did exhibit a significant increase in specific leaf area (SLA), stomatal and mesophyll conductance, and leaf nitrogen allocated to photosynthetic electron transport. Collectively, we showed that increased carbon demand via DGAT+CO lipid sink accumulation can induce leaf-level changes in L. perenne which deliver increased rates of photosynthesis and growth. Carbon sinks engineered within photosynthetic cells provide a promising new strategy for increasing photosynthesis and crop productivity.

Whole-tree mesophyll conductance reconciles isotopic and gas-exchange estimates of water-use efficiency

Photosynthetic water-use efficiency (WUE) describes the link between terrestrial carbon (C) and water cycles. Estimates of intrinsic WUE (iWUE) from gas exchange and C isotopic composition (δ¹³ C) differ due to an internal conductance in the leaf mesophyll (g_m ) that is variable and seldom computed. We present the first direct estimates of whole-tree g_m , together with iWUE from whole-tree gas exchange and δ¹³ C of the phloem (δ¹³ C_ph ). We measured gas exchange, online ¹³ C-discrimination, and δ¹³ C_ph monthly throughout spring, summer, and autumn in Eucalyptus tereticornis grown in large whole-tree chambers. Six trees were grown at ambient temperatures and six at a 3°C warmer air temperature; a late-summer drought was also imposed. Drought reduced whole-tree g_m . Warming had few direct effects, but amplified drought-induced reductions in whole-tree g_m . Whole-tree g_m was similar to leaf g_m for these same trees. iWUE estimates from δ¹³ C_ph agreed with iWUE from gas exchange, but only after incorporating g_m . δ¹³ C_ph was also correlated with whole-tree ¹³ C-discrimination, but offset by -2.5 ± 0.7‰, presumably due to post-photosynthetic fractionations. We conclude that δ¹³ C_ph is a good proxy for whole-tree iWUE, with the caveats that post-photosynthetic fractionations and intrinsic variability of g_m should be incorporated to provide reliable estimates of this trait in response to abiotic stress.