Artefactual responses of mesophyll conductance to CO2 and irradiance estimated with the variable J and online isotope discrimination methods

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 operation of PEPCK increases light harvesting plasticity in C4 NAD-ME and NADP-ME photosynthetic subtypes: A theoretical study

The repeated emergence of NADP-malic enzyme (ME), NAD-ME and phosphoenolpyruvate carboxykinase (PEPCK) subtypes of C4 photosynthesis are iconic examples of convergent evolution, which suggests that these biochemistries do not randomly assemble, but are instead specific adaptations resulting from unknown evolutionary drivers. Theoretical studies that are based on the classic biochemical understanding have repeatedly proposed light-use efficiency as a possible benefit of the PEPCK subtype. However, quantum yield measurements do not support this idea. We explore this inconsistency here via an analytical model that features explicit descriptions across a seamless gradient between C4 biochemistries to analyse light harvesting and dark photosynthetic metabolism. Our simulations show that the NADP-ME subtype, operated by the most productive crops, is the most efficient. The NAD-ME subtype has lower efficiency, but has greater light harvesting plasticity (the capacity to assimilate CO2 in the broadest combination of light intensity and spectral qualities). In both NADP-ME and NAD-ME backgrounds, increasing PEPCK activity corresponds to greater light harvesting plasticity but likely imposed a reduction in photosynthetic efficiency. We draw the first mechanistic links between light harvesting and C4 subtypes, providing the theoretical basis for future investigation.

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.

Addressing the Relationship between Leaf Nitrogen and Carbon Isotope Discrimination from the Three Levels of Community, Population and Individual

The carbon, nitrogen and water cycles of terrestrial ecosystems are important biogeochemical cycles. Addressing the relationship of leaf nitrogen (N) and carbon isotope discrimination (Δ) will enhance the understanding of the links between these three cycles in plant leaves because Δ can reflect time-integrated leaf-level water-use efficiency (WUE) over the period when the leaf material is produced. Previous studies have paid considerable attention to the relationship. However, these studies have not effectively eliminated the interference of environmental factors, inter-species, and inter-individual differences in this relationship, so new research is necessary. To minimize these interferences, the present work explored the relationship at the three levels of community, population, and plant individual. Three patterns of positive, negative and no relationship were observed across communities, populations, and individuals, which is dependent on environmental conditions, species, and plant individuals. The results strongly suggested that there is no general pattern for the relationship between leaf N and Δ. Furthermore, the results indicated that there is often no coupling between leaf-level long-term WUE and leaf N in the metabolic process of carbon, N and water in leaves. The main reason for the lack of this relationship is that most plants do not invest large amounts of nitrogen into photosynthesis. In addition, the present study also observed that, for most plant species, leaf N was not related to photosynthetic rate, and that variations in photosynthetic rates are mainly driven by stomatal conductance.

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.

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 .

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.

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.

Mesophyll conductance exerts a significant limitation on photosynthesis during light induction

Past studies have established mesophyll diffusion conductance to CO₂ (g_m ) as a variable and significant limitation to plant photosynthesis under steady-state conditions. However, the role of g_m in influencing photosynthesis (A) during the transient period of light induction is largely unknown. We combined gas exchange measurements with laser-enabled carbon isotope discrimination measurements to assess g_m during photosynthetic induction, using Arabidopsis as the measurement species. Our measurements revealed three key findings: (1) we found that the rate at which g_m approached steady state during induction was not necessarily faster than the induction rate of the carboxylation process, contradictory to what has been suggested in previous studies; (2) g_m displayed a strong and consistent coordination with A under both induction and steady-state settings, hinting that the mechanism driving g_m -A coupling does not require physiological stability as a prerequisite; and (3) photosynthetic limitation analysis of our data revealed that when integrated over the entire induction period, the relative limitation of A imposed by g_m can be as high as > 35%. The present study provides the first demonstration of the important role of g_m in limiting CO₂ assimilation during photosynthetic induction, thereby pointing to a need for more research attention to be devoted to g_m in future induction studies.

Leaf photosynthetic plasticity does not predict biomass responses to growth irradiance in rice

Phenotypic plasticity, the capacity of an organism to generate alternative phenotypes in response to different environments, is a particularly important characteristic to enable sessile plants to adapt to rapid changes in their surroundings. Leaf anatomical and physiological traits exhibit plasticity in response to growth irradiances, but it is relatively unclear if the plasticity varies among genotypes for a species. Equally importantly, empirical results on how leaf-level plasticity influences whole-plant growth are largely absent. We conducted an integrated investigation into the light-introduced plasticity by measuring 48 traits involving plant growth, leaf anatomy, leaf biochemistry, and leaf physiology of five rice genotypes grown under two irradiances. More than half of the estimated traits were significantly affected by growth light intensities, and the sizes of the cumulative effect of growth light ranged from -25.04% (stomatal conductance at high measurement light) to 135.2% (tiller number). Growth irradiance levels dramatically shifted the relationship between photosynthetic rate and stomatal conductance. However, the relationship between photosynthetic rate and mesophyll conductance was rarely influenced by growth light levels. Importantly, the present study highlights the significant variation in trait plasticity across rice genotypes and that the light-introduced biomass changes were rarely predicted by leaf photosynthetic plasticity. Our findings imply that the genotypes with high productivity at the low growth light conditions do not necessarily have high productivity under high light conditions.

Mesophyll conductance variability of rice aquaporin knockout lines at different growth stages and growing environments

The plasma membrane subfamily of aquaporins [plasma membrane intrinsic proteins (PIPs)], which facilitates the CO₂ diffusion across membranes, is proposed to play an important role in mesophyll conductance to CO₂ (g_m ), a major limiting factor of photosynthesis. However, recent studies implied no causal relationship between g_m and PIPs because they failed to repeat the previous observed differences in g_m between PIP knockout lines and their wild-type. The contrasting results on the role of PIPs in g_m were interpreted as the different growth conditions among studies, which has never been tested. Here, we assessed whether the differences in g_m between wild-type and PIP knockout lines, Ospip1;1, Ospip1;2 and Ospip2;1, varied with growth condition (field versus pot condition) and growth stages in rice. Under field conditions, no differences were observed in plant performance, photosynthetic rate (A) and g_m between PIP knockout lines and the wild-type. However, in agreement with previous studies, two out of three knockout lines showed significant declines in tiller number, plant height, A and g_m under pot conditions. Moreover, we found that the differences in A and g_m between PIP knockout lines and the wild-type varied with the growth stage of the plants. Our results showed that the differences in g_m between PIP knockout lines and wild-type were depending on the growth environments and stage of the plants, and further efforts are required to reveal the underlying mechanisms.

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.

Cell wall thickness and composition are involved in photosynthetic limitation

The key role of cell walls in setting mesophyll conductance to CO2 (gm) and, consequently, photosynthesis is reviewed. First, the theoretical properties of cell walls that can affect gm are presented. Then, we focus on cell wall thickness (Tcw) reviewing empirical evidence showing that Tcw varies strongly among species and phylogenetic groups in a way that correlates with gm and photosynthesis; that is, the thicker the mesophyll cell walls, the lower the gm and photosynthesis. Potential interplays of gm, Tcw, dehydration tolerance, and hydraulic properties of leaves are also discussed. Dynamic variations of Tcw in response to the environment and their implications in the regulation of photosynthesis are discussed, and recent evidence suggesting an influence of cell wall composition on gm is presented. We then propose a hypothetical mechanism for the influence of cell walls on photosynthesis, combining the effects of thickness and composition, particularly pectins. Finally, we discuss the prospects for using biotechnology for enhancing photosynthesis by altering cell wall-related genes.

Estimating uncertainty: A Bayesian approach to modelling photosynthesis in C3 leaves

The Farquhar-von Caemmerer-Berry (FvCB) model is extensively used to model photosynthesis from gas exchange measurements. Since its publication, many methods have been developed to measure, or more accurately estimate, parameters of this model. Here, we have created a tool that uses Bayesian statistics to fit photosynthetic parameters using concurrent gas exchange and chlorophyll fluorescence measurements whilst evaluating the reliability of the parameter estimation. We have tested this tool on synthetic data and experimental data from rice leaves. Our results indicate that reliable parameter estimation can be achieved whilst only keeping one parameter, K_m , that is, Michaelis constant for CO₂ by Rubisco, prefixed. Additionally, we show that including detailed low CO₂ measurements at low light levels increases reliability and suggests this as a new standard measurement protocol. By providing an estimated distribution of parameter values, the tool can be used to evaluate the quality of data from gas exchange and chlorophyll fluorescence measurement protocols. Compared to earlier model fitting methods, the use of a Bayesian statistics-based tool minimizes human interaction during fitting, reducing the subjectivity which is essential to most existing tools. A user friendly, interactive Bayesian tool script is provided.

Stomatal, mesophyll conductance, and biochemical limitations to photosynthesis during induction

The dynamics of leaf photosynthesis in fluctuating light affects carbon gain by plants. Mesophyll conductance (gm) limits CO2 assimilation rate (A) under the steady state, but the extent of this limitation under non-steady-state conditions is unknown. In the present study, we aimed to characterize the dynamics of gm and the limitations to A imposed by gas diffusional and biochemical processes under fluctuating light. The induction responses of A, stomatal conductance (gs), gm, and the maximum rate of RuBP carboxylation (Vcmax) or electron transport (J) were investigated in Arabidopsis (Arabidopsis thaliana (L.)) and tobacco (Nicotiana tabacum L.). We first characterized gm induction after a change from darkness to light. Each limitation to A imposed by gm, gs and Vcmax or J was significant during induction, indicating that gas diffusional and biochemical processes limit photosynthesis. Initially, gs imposed the greatest limitation to A, showing the slowest response under high light after long and short periods of darkness, assuming RuBP-carboxylation limitation. However, if RuBP-regeneration limitation was assumed, then J imposed the greatest limitation. gm did not vary much following short interruptions to light. The limitation to A imposed by gm was the smallest of all the limitations for most of the induction phase. This suggests that altering induction kinetics of mesophyll conductance would have little impact on A following a change in light. To enhance the carbon gain by plants under naturally dynamic light environments, attention should therefore be focused on faster stomatal opening or activation of electron transport.

The maximum carboxylation rate of Rubisco affects CO2 refixation in temperate broadleaved forest trees

Mesophyll resistance to CO₂ diffusion (r_m) and the maximum carboxylation rate of Rubisco (V_cmax) affect photosynthetic rates, and can potentially also influence the percentage of respiratory and photorespiratory CO₂ being refixated (P_r) by mesophyll cells. Here we investigated how various leaf anatomical traits (e.g. leaf mass per area [LMA] and leaf dry matter content [LDMC]) influenced r_m in leaves of mature forest trees. We further explored how r_m and V_cmax in turn affected P_r, and if these traits varied among species and leaves along a light gradient. Photosynthetic CO₂ response of leaves grown in high-, medium-, and low-light environments was measured, from Pinus sylvestris [Scots pine], Picea abies [Norway spruce], Quercus robur [English oak], and Betula pendula [Silver birch] in southern Sweden. A modified version of the Farquhar-von Caemmerer-Berry model was fitted to the leaf gas exchange data to estimate V_cmax, r_m and P_r. We found that of all leaf traits measured, only LMA for Q. robur was significantly higher in leaves from high-light environments. When comparing species, both r_m and LMA were significantly higher in the conifers, and r_m had a negative correlation with V_cmax. We found that P_r was similar between different species and functional groups, with an average of 73.2% (and SD of ±10.4) across all species. There was a strong, positive correlation between P_r and V_cmax in broadleaves, and we hypothesise that this effect might derive from a higher CO₂ drawdown near Rubisco in leaves with high V_cmax.

Variability in the chloroplast area lining the intercellular airspace and cell walls drives mesophyll conductance in gymnosperms

The photosynthetic efficiency of plants in different environments is controlled by stomata, hydraulics, biochemistry, and mesophyll conductance (gm). Recently, gm was demonstrated to be the key limitation of photosynthesis in gymnosperms. Values of gm across gymnosperms varied over 20-fold, but this variation was poorly explained by robust structure-bound integrated traits such as leaf dry mass per area. Understanding how the component structural traits control gm is central for identifying the determinants of variability in gm across plant functional and phylogenetic groups. Here, we investigated the structural traits responsible for gm in 65 diverse gymnosperms. Although the integrated morphological traits, shape, and anatomical characteristics varied widely across species, the distinguishing features of all gymnosperms were thick mesophyll cell walls and low chloroplast area exposed to intercellular airspace (Sc/S) compared with angiosperms. Sc/S and cell wall thickness were the fundamental traits driving variations in gm across gymnosperm species. Chloroplast thickness was the strongest limitation of gm among liquid-phase components. The variation in leaf dry mass per area was not correlated with the key ultrastructural traits determining gm. Thus, given the absence of correlating integrated easy-to-measure traits, detailed knowledge of underlying component traits controlling gm across plant taxa is necessary to understand the photosynthetic limitations across ecosystems.

Leaf mass area determines water use efficiency through its influence on carbon gain in rice mutants

Saving water and enhancing rice productivity are consensually the most important research goals globally. While increasing canopy cover would enhance growth rates by higher photosynthetic carbon gain, an accompanied increase in transpiration would have a negative impact on saving water as well as for sustainability under water-limited conditions. Increased water use efficiency (WUE) by virtue of higher carbon assimilatory capacity can significantly circumvent this trade-off. Here, we report leaf mass area (LMA) has an important canopy architecture trait which when combined with superior carboxylation efficiency (CE) would achieve higher water productivity in rice. A set of 130 ethyl methanesulfonate induced mutants of an upland cultivar Nagina-22 (N22), was screened for leaf morphological traits leading to the identification of mutants differing in LMA. The wild-type, N22, along with a selected low-LMA (380-4-3) and two high-LMA mutants (392-9-1 and 457-1-3), all with comparable total leaf area, were raised under well-watered (100% Field Capacity (FC)) and water-limited (60% FC) conditions. Low Δ¹³ C and a higher RuBisCO content in high-LMA mutants indicated higher carboxylation efficiency, leading to increased carbon gain. Single parent backcross populations developed by crossing high and the low-LMA mutants with N22, separately, were screened for LMA, Δ¹³ C and growth traits. Comparison of dry matter accumulation per unit leaf area among the progenies differing in LMA and Δ¹³ C reiterated the association of LMA with CE. Results illustrated that high-LMA when combined with higher CE (low Δ¹³ C) lead to increased WUE and growth rates.

Using photorespiratory oxygen response to analyse leaf mesophyll resistance

Classical approaches to estimate mesophyll conductance ignore differences in resistance components for CO2 from intercellular air spaces (IAS) and CO2 from photorespiration (F) and respiration (Rd). Consequently, mesophyll conductance apparently becomes sensitive to (photo)respiration relative to net photosynthesis, (F + Rd)/A. This sensitivity depends on several hard-to-measure anatomical properties of mesophyll cells. We developed a method to estimate the parameter m (0 ≤ m ≤ 1) that lumps these anatomical properties, using gas exchange and chlorophyll fluorescence measurements where (F + Rd)/A ratios vary. This method was applied to tomato and rice leaves measured at five O2 levels. The estimated m was 0.3 for tomato but 0.0 for rice, suggesting that classical approaches implying m = 0 work well for rice. The mesophyll conductance taking the m factor into account still responded to irradiance, CO2, and O2 levels, similar to response patterns of stomatal conductance to these variables. Largely due to different m values, the fraction of (photo)respired CO2 being refixed within mesophyll cells was lower in tomato than in rice. But that was compensated for by the higher fraction via IAS, making the total re-fixation similar for both species. These results, agreeing with CO2 compensation point estimates, support our method of effectively analysing mesophyll resistance.

Mesophyll conductance: the leaf corridors for photosynthesis

Besides stomata, the photosynthetic CO2 pathway also involves the transport of CO2 from the sub-stomatal air spaces inside to the carboxylation sites in the chloroplast stroma, where Rubisco is located. This pathway is far to be a simple and direct way, formed by series of consecutive barriers that the CO2 should cross to be finally assimilated in photosynthesis, known as the mesophyll conductance (gm). Therefore, the gm reflects the pathway through different air, water and biophysical barriers within the leaf tissues and cell structures. Currently, it is known that gm can impose the same level of limitation (or even higher depending of the conditions) to photosynthesis than the wider known stomata or biochemistry. In this mini-review, we are focused on each of the gm determinants to summarize the current knowledge on the mechanisms driving gm from anatomical to metabolic and biochemical perspectives. Special attention deserve the latest studies demonstrating the importance of the molecular mechanisms driving anatomical traits as cell wall and the chloroplast surface exposed to the mesophyll airspaces (Sc/S) that significantly constrain gm. However, even considering these recent discoveries, still is poorly understood the mechanisms about signaling pathways linking the environment a/biotic stressors with gm responses. Thus, considering the main role of gm as a major driver of the CO2 availability at the carboxylation sites, future studies into these aspects will help us to understand photosynthesis responses in a global change framework.

Revisiting carbon isotope discrimination in C3 plants shows respiration rules when photosynthesis is low

Stable isotopes are commonly used to study the diffusion of CO₂ within photosynthetic plant tissues. The standard method used to interpret the observed preference for the lighter carbon isotope in C₃ photosynthesis involves the model of Farquhar et al., which relates carbon isotope discrimination to physical and biochemical processes within the leaf. However, under many conditions the model returns unreasonable results for mesophyll conductance to CO₂ diffusion (g_m), especially when rates of photosynthesis are low. Here, we re-derive the carbon isotope discrimination model using modified assumptions related to the isotope effect of mitochondrial respiration. In particular, we treat the carbon pool associated with respiration as separate from the pool of primary assimilates. We experimentally test the model by comparing g_m values measured with different CO₂ source gases varying in their isotopic composition, and show that our new model returns matching g_m values that are much more reasonable than those obtained with the previous model. We use our results to discuss CO₂ diffusion properties within the mesophyll.

Maximizing leaf carbon gain in varying saline conditions: An optimization model with dynamic mesophyll conductance

While the adverse effects of elevated salinity levels on leaf gas exchange in many crops are not in dispute, representing such effects on leaf photosynthetic rates (A) continues to draw research attention. Here, an optimization model for stomatal conductance (g_c ) that maximizes A while accounting for mesophyll conductance (g_m ) was used to interpret new leaf gas exchange measurements collected for five irrigation water salinity levels. A function between chloroplastic CO₂ concentration (c_c ) and intercellular CO₂ concentration (c_i ) modified by salinity stress to estimate g_m was proposed. Results showed that with increased salinity, the estimated g_m and maximum photosynthetic capacity were both reduced, whereas the marginal water use efficiency λ increased linearly. Adjustments of g_m , λ and photosynthetic capacity were shown to be consistent with a large corpus of drought-stress experiments. The inferred model parameters were then used to evaluate the combined effects of elevated salinity and atmospheric CO₂ concentration (c_a ) on leaf gas exchange. For a given salinity level, increasing c_a increased A linearly, but these increases were accompanied by mild reductions in g_c and transpiration. The c_a level needed to ameliorate A reductions due to increased salinity is also discussed using the aforementioned model calculations.

Linking water relations and hydraulics with photosynthesis

For land plants, water is the principal governor of growth. Photosynthetic performance is highly dependent on the stable and suitable water status of leaves, which is balanced by the water transport capacity, the water loss rate as well as the water capacitance of the plant. This review discusses the links between leaf water status and photosynthesis, specifically focussing on the coordination of CO₂ and water transport within leaves, and the potential role of leaf capacitance and elasticity on CO₂ and water transport.

The role of leaf water potential in the temperature response of mesophyll conductance

Variation in temperature (T) is usually accompanied by changes in leaf water potential (Ψ_leaf ), which may influence mesophyll conductance (g_m ). However, the effects of Ψ_leaf on g_m have not yet been considered in models of the g_m response to temperature. Temperature responses of g_m and Ψ_leaf and the response of g_m to Ψ_leaf were studied in rice (Oryza sativa) and wheat (Triticum aestivum), and then an empirical model of Ψ_leaf was incorporated into an existing g_m -T model. In wheat, Ψ_leaf was dramatically decreased with increasing T, whereas in rice Ψ_leaf was less sensitive or insensitive to T. Without taking Ψ_leaf into account, g_m for wheat showed no response to T. However, at a given Ψ_leaf , g_m was significantly higher at high temperature compared with low. After incorporating the function of Ψ_leaf into the g_m -T model, we suggest that the g_m -T relationship can be influenced by the activation and deactivation energy for membrane permeability, Ψ_leaf gradient between temperatures, and the sensitivity of g_m to Ψ_leaf , below a threshold (Ψ_leaf,0 ). The data presented here suggest that Ψ_leaf plays an important role in the g_m -T relationship and should be considered in future studies related to the temperature response of g_m and photosynthesis.

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

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

Photosynthesis and photosynthetic efficiencies along the terrestrial plant's phylogeny: lessons for improving crop photosynthesis

Photosynthesis is the basis of all life on Earth. Surprisingly, until very recently, data on photosynthesis, photosynthetic efficiencies, and photosynthesis limitations in terrestrial land plants other than spermatophytes were very scarce. Here we provide an updated data compilation showing that maximum photosynthesis rates (expressed either on an area or dry mass basis) progressively scale along the land plant's phylogeny, from lowest values in bryophytes to largest in angiosperms. Unexpectedly, both photosynthetic water (WUE) and nitrogen (PNUE) use efficiencies also scale positively through the phylogeny, for which it has been commonly reported that these two efficiencies tend to trade-off between them when comparing different genotypes or a single species subject to different environmental conditions. After providing experimental evidence that these observed trends are mostly due to an increased mesophyll conductance to CO₂ - associated with specific anatomical changes - along the phylogeny, we discuss how these findings on a large phylogenetic scale can provide useful information to address potential photosynthetic improvements in crops in the near future.

Mesophyll conductance in land surface models: effects on photosynthesis and transpiration

The CO₂ transfer conductance within plant leaves (mesophyll conductance, g_m ) is currently not considered explicitly in most land surface models (LSMs), but instead treated implicitly as an intrinsic property of the photosynthetic machinery. Here, we review approaches to overcome this model deficiency by explicitly accounting for g_m , which comprises the re-adjustment of photosynthetic parameters and a model describing the variation of g_m in dependence of environmental conditions. An explicit representation of g_m causes changes in the response of photosynthesis to environmental factors, foremost leaf temperature, and ambient CO₂ concentration, which are most pronounced when g_m is small. These changes in leaf-level photosynthesis translate into a stronger climate and CO₂ response of gross primary productivity (GPP) and transpiration at the global scale. The results from two independent studies show consistent latitudinal patterns of these effects with biggest differences in GPP in the boreal zone (up to ~15%). Transpiration and evapotranspiration show spatially similar, but attenuated, changes compared with GPP. These changes are indirect effects of g_m caused by the assumed strong coupling between stomatal conductance and photosynthesis in current LSMs. Key uncertainties in these simulations are the variation of g_m with light and the robustness of its temperature response across plant types and growth conditions. Future research activities focusing on the response of g_m to environmental factors and its relation to other plant traits have the potential to improve the representation of photosynthesis in LSMs and to better understand its present and future role in the Earth system.

Photosynthetic efficiency and mesophyll conductance are unaffected in Arabidopsis thaliana aquaporin knock-out lines

Improving photosynthetic efficiency is widely regarded as a major route to achieving much-needed yield gains in crop plants. In plants with C3 photosynthesis, increasing the diffusion conductance for CO2 transfer from substomatal cavity to chloroplast stroma (gm) could help to improve the efficiencies of CO2 assimilation and photosynthetic water use in parallel. The diffusion pathway from substomatal cavity to chloroplast traverses cell wall, plasma membrane, cytosol, chloroplast envelope membranes, and chloroplast stroma. Specific membrane intrinsic proteins of the aquaporin family can facilitate CO2 diffusion across membranes. Some of these aquaporins, such as PIP1;2 in Arabidopsis thaliana, have been suggested to exert control over gm and the magnitude of the CO2 assimilation flux, but the evidence for a direct physiological role of aquaporins in determining gm is limited. Here, we estimated gm with four different methods under a range of light intensities and CO2 concentrations in two previously characterized pip1;2 knock-out lines as well as pip1;3 and pip2;6 knock-out lines, which have not been previously evaluated for a role in gm. This study presents the most in-depth analysis of gm in Arabidopsis aquaporin knock-out mutants to date. Surprisingly, all methods failed to show any significant differences between the pip1;2, pip1;3, or pip2;6 mutants and the Col-0 control.

Exploring natural variation of photosynthesis in a site-specific manner: evolution, progress, and prospects

Site-specific changes of photosynthesis, a relatively new concept, can be used to improve the productivity of critical food crops to mitigate the foreseen food crisis. Global food security is threatened by an increasing population and the effects of climate change. Large yield improvements were achieved in major cereal crops between the 1950s and 1980s through the Green Revolution. However, we are currently experiencing a significant decline in yield progress. Of the many approaches to improved cereal yields, exploitation of the mode of photosynthesis has been intensely studied. Even though the C₄ pathway is considered the most efficient, mainly because of the carbon concentrating mechanisms around the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, which minimize photorespiration, much is still unknown about the specific gene regulation of this mode of photosynthesis. Most of the critical cereal crops, including wheat and rice, are categorized as C₃ plants based on the photosynthesis of major photosynthetic organs. However, recent findings raise the possibility of different modes of photosynthesis occurring at different sites in the same plant and/or in plants grown in different habitats. That is, it seems possible that efficient photosynthetic traits may be expressed in specific organs, even though the major photosynthetic pathway is C₃. Knowledge of site-specific differences in photosynthesis, coupled with site-specific regulation of gene expression, may therefore hold a potential to enhance the yields of economically important C₃ crops.

Chloride as a macronutrient increases water-use efficiency by anatomically driven reduced stomatal conductance and increased mesophyll diffusion to CO2

Chloride (Cl- ) has been recently described as a beneficial macronutrient, playing specific roles in promoting plant growth and water-use efficiency (WUE). However, it is still unclear how Cl- could be beneficial, especially in comparison with nitrate (NO3- ), an essential source of nitrogen that shares with Cl- similar physical and osmotic properties, as well as common transport mechanisms. In tobacco plants, macronutrient levels of Cl- specifically reduce stomatal conductance (gs ) without a concomitant reduction in the net photosynthesis rate (AN ). As stomata-mediated water loss through transpiration is inherent in the need of C3 plants to capture CO2 , simultaneous increase in photosynthesis and WUE is of great relevance to achieve a sustainable increase in C3 crop productivity. Our results showed that Cl- -mediated stimulation of larger leaf cells leads to a reduction in stomatal density, which in turn reduces gs and water consumption. Conversely, Cl- improves mesophyll diffusion conductance to CO2 (gm ) and photosynthetic performance due to a higher surface area of chloroplasts exposed to the intercellular airspace of mesophyll cells, possibly as a consequence of the stimulation of chloroplast biogenesis. A key finding of this study is the simultaneous improvement of AN and WUE due to macronutrient Cl- nutrition. This work identifies relevant and specific functions in which Cl- participates as a beneficial macronutrient for higher plants, uncovering a sustainable approach to improve crop yield.

Sun-induced Chl fluorescence and its importance for biophysical modeling of photosynthesis based on light reactions

Recent progress in observing sun-induced Chl fluorescence (SIF) provides an unprecedented opportunity to advance photosynthesis research in natural environments. However, we still lack an analytical framework to guide SIF studies and integration with the well-developed active fluorescence approaches. Here, we derive a set of coupled fundamental equations to describe the dynamics of SIF and its relationship with C₃ and C₄ photosynthesis. These equations show that, although SIF is dynamically as complex as photosynthesis, the measured SIF simplifies photosynthetic modeling from the perspective of light reactions by integrating over the dynamic complexities of photosynthesis. Specifically, the measured SIF contains direct information about the actual electron transport from photosystem II to photosystem I, giving a quantifiable link between light and dark reactions. With much-reduced requirements on inputs and parameters, the light-reactions-centric, SIF-based biophysical model complements the traditional, dark-reactions-centric biochemical model of photosynthesis. The SIF-photosynthesis relationship, however, is nonlinear. This is because photosynthesis saturates at high light whereas SIF has a stronger tendency to keep increasing, as fluorescence quantum yield has a relatively muted sensitivity to light levels. Successful applications of the SIF-based model of photosynthesis will depend on a predictive understanding of several previously underexplored physiological and biophysical processes. Advances can be facilitated by coordinated efforts in plant physiology, remote sensing, and eddy covariance flux observations.

The temperature response of mesophyll conductance, and its component conductances, varies between species and genotypes

The temperature response of mesophyll conductance to CO₂ diffusion (g_m) has been shown to vary considerably between species but remains poorly understood. Here, we tested the hypothesis that increases in chloroplast surface area with increasing temperature, due to the formation of chloroplast protrusions, caused observed positive responses of g_m to temperature. We found no evidence of chloroplast protrusions. Using simultaneous measurements of carbon and oxygen isotope discrimination during photosynthesis to separate total g_m (g_m13) into cell wall and plasma membrane conductance (g_m18) and chloroplast membrane conductance (g_cm) components, we explored the temperature response in genotypes of soybean and barley, and sunflower plants grown at differing CO₂ concentrations. Differences in the temperature sensitivity of g_m18 were found between genotypes and between plants grown at differing CO₂ concentration but did not relate to measured anatomical features such as chloroplast surface area or cell wall thickness. The closest fit of modelled g_m13 to estimated values was found when cell wall thickness was allowed to decline at higher temperatures and transpiration rates, but it remains to be tested if this decline is realistic. The temperature response of g_cm (calculated from the difference between 1/g_m13 and 1/g_m18) varied between barley genotypes, and was best fitted by an optimal response in sunflower. Taken together, these results indicate that g_m is a highly complex trait with unpredictable sensitivity to temperature that varies between species, between genotypes within a single species, with growth environment, between replicate leaves, and even with age for an individual leaf.

Critical review: incorporating the arrangement of mitochondria and chloroplasts into models of photosynthesis and carbon isotope discrimination

The arrangement of mitochondria and chloroplasts, together with the relative resistances of cell wall and chloroplast, determine the path of diffusion out of the leaf for (photo)respired CO₂. Traditional photosynthesis models have assumed a tight arrangement of chloroplasts packed together against the cell wall with mitochondria located behind the chloroplasts, deep inside the cytosol. Accordingly, all (photo)respired CO₂ must cross the chloroplast before diffusing out of the leaf. Different arrangements have recently been considered, where all or part of the (photo)respired CO₂ diffuses through the cytosol without ever entering the chloroplast. Assumptions about the path for the (photo)respiratory flux are particularly relevant for the calculation of mesophyll conductance (g_m). If (photo)respired CO₂ can diffuse elsewhere besides the chloroplast, apparent g_m is no longer a mere physical resistance but a flux-weighted variable sensitive to the ratio of (photo)respiration to net CO₂ assimilation. We discuss existing photosynthesis models in conjunction with their treatment of the (photo)respiratory flux and present new equations applicable to the generalized case where (photo)respired CO₂ can diffuse both into the chloroplast and through the cytosol. Additionally, we present a new generalized Δ¹³C model that incorporates this dual diffusion pathway. We assess how assumptions about the fate of (photo)respired CO₂ affect the interpretation of photosynthetic data and the challenges it poses for the application of different models.

Drought-introduced variability of mesophyll conductance in Gossypium and its relationship with leaf anatomy

Mesophyll conductance (g_m ) is one of the major determinants of photosynthetic rate, for which it has an impact on crop yield. However, the regulatory mechanisms behind the decline in g_m of cotton (Gossypium. spp) by drought are unclear. An upland cotton (Gossypium hirsutum) genotype and a pima cotton (Gossypium barbadense) genotype were used to determine the gas exchange parameters, leaf anatomical structure as well as aquaporin and carbonic anhydrase gene expression under well-watered and drought treatment conditions. In this study, the decrease of net photosynthetic rate (A_N ) under drought conditions was related to a decline in g_m and in stomatal conductance (g_s ). g_m and g_s coordinate with each other to ensure optimum state of CO₂ diffusion and achieve the balance of water and CO₂ demand in the process of photosynthesis. Meanwhile, mesophyll limitations to photosynthesis are equally important to the stomatal limitations. Considering g_m , its decline in cotton leaves under drought was mostly regulated by the chloroplast surface area exposed to leaf intercellular air spaces per leaf area (S_c /S) and might also be regulated by the expression of leaf CARBONIC ANHYDRASE (CA1). Meanwhile, cotton leaves can minimize the decrease in g_m under drought by maintaining cell wall thickness (T_cw ). Our results indicated that modification of chloroplasts might be a target trait in future attempts to improve cotton drought tolerance.

Anatomical constraints to nonstomatal diffusion conductance and photosynthesis in lycophytes and bryophytes

Photosynthesis in bryophytes and lycophytes has received less attention than terrestrial plant groups. In particular, few studies have addressed the nonstomatal diffusion conductance to CO₂ g_nsd of these plant groups. Their lower photosynthetic rate per leaf mass area at any given nitrogen concentration compared with vascular plants suggested a stronger limitation by CO₂ diffusion. We hypothesized that bryophyte and lycophyte photosynthesis is largely limited by low g_nsd . Here, we studied CO₂ diffusion inside the photosynthetic tissues and its relationships with photosynthesis and anatomical parameters in bryophyte and lycophyte species in Antarctica, Australia, Estonia, Hawaii and Spain. On average, lycophytes and, specially, bryophytes had the lowest photosynthetic rates and nonstomatal diffusion conductance reported for terrestrial plants. These low values are related to their very thick cell walls and their low exposure of chloroplasts to cell perimeter. We conclude that the reason why bryophytes lie at the lower end of the leaf economics spectrum is their strong nonstomatal diffusion conductance limitation to photosynthesis, which is driven by their specific anatomical characteristics.

Effects of mesophyll conductance on vegetation responses to elevated CO2 concentrations in a land surface model

Mesophyll conductance (gm ) is known to affect plant photosynthesis. However, gm is rarely explicitly considered in land surface models (LSMs), with the consequence that its role in ecosystem and large-scale carbon and water fluxes is poorly understood. In particular, the different magnitudes of gm across plant functional types (PFTs) are expected to cause spatially divergent vegetation responses to elevated CO2 concentrations. Here, an extensive literature compilation of gm across major vegetation types is used to parameterize an empirical model of gm in the LSM JSBACH and to adjust photosynthetic parameters based on simulated An  - Ci curves. We demonstrate that an explicit representation of gm changes the response of photosynthesis to environmental factors, which cannot be entirely compensated by adjusting photosynthetic parameters. These altered responses lead to changes in the photosynthetic sensitivity to atmospheric CO2 concentrations which depend both on the magnitude of gm and the climatic conditions, particularly temperature. We then conducted simulations under ambient and elevated (ambient + 200 μmol/mol) CO2 concentrations for contrasting ecosystems and for historical and anticipated future climate conditions (representative concentration pathways; RCPs) globally. The gm -explicit simulations using the RCP8.5 scenario resulted in significantly higher increases in gross primary productivity (GPP) in high latitudes (+10% to + 25%), intermediate increases in temperate regions (+5% to + 15%), and slightly lower to moderately higher responses in tropical regions (-2% to +5%), which summed up to moderate GPP increases globally. Similar patterns were found for transpiration, but with a lower magnitude. Our results suggest that the effect of an explicit representation of gm is most important for simulated carbon and water fluxes in the boreal zone, where a cold climate coincides with evergreen vegetation.

Effects of instantaneous and growth CO2 levels and abscisic acid on stomatal and mesophyll conductances

C₃ photosynthesis is often limited by CO₂ diffusivity or stomatal (g_s ) and mesophyll (g_m ) conductances. To characterize effects of stomatal closure induced by either high CO₂ or abscisic acid (ABA) application on g_m , we examined g_s and g_m in the wild type (Col-0) and ost1 and slac1-2 mutants of Arabidopsis thaliana grown at 390 or 780 μmol mol^-1 CO₂ . Stomata of these mutants were reported to be insensitive to both high CO₂ and ABA. When the ambient CO₂ increased instantaneously, g_m decreased in all these plants, whereas g_s in ost1 and slac1-2 was unchanged. Therefore, the decrease in g_m in response to high CO₂ occurred irrespective of the responses of g_s . g_m was mainly determined by the instantaneous CO₂ concentration during the measurement and not markedly by the CO₂ concentration during the growth. Exogenous application of ABA to Col-0 caused the decrease in the intercellular CO₂ concentration (C_i ). With the decrease in C_i , g_m did not increase but decreased, indicating that the response of g_m to CO₂ and that to ABA are differently regulated and that ABA content in the leaves plays an important role in the regulation of g_m .

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).

The response of mesophyll conductance to short- and long-term environmental conditions in chickpea genotypes

. Mesophyll conductance (g m) has been shown to vary between genotypes of a number of species and with growth environments, including nitrogen availability, but understanding of g m variability in legumes is limited. We might expect g m in legumes to respond differently to limited nitrogen availability, due to their ability to fix atmospheric N2. Using online stable carbon isotope discrimination method, we quantified genetic variability in g m under ideal conditions, investigated g m response to N source (N2-fixation or inorganic N) and determined the effects of N source and water availability on the rapid response of g m to photosynthetic photon flux density (PPFD) and radiation wavelength in three genotypes of chickpea (Cicer arietinum). Genotypes varied 2-fold in g m under non-limiting environments. N-fed plants had higher g m than N2-fixing plants in one genotype, while g m in the other two genotypes was unaffected. g m response to PPFD was altered by N source in one of three genotypes, in which the g m response to PPFD was statistically significant in N-fed plants but not in N2-fixing plants. There was no clear effect of moderate water stress on the g m response to PPFD and radiation wavelength. Genotypes of a single legume species differ in the sensitivity of g m to both long- and short-term environmental conditions, precluding utility in crop breeding programmes.

Mesophyll conductance in cotton bracts: anatomically determined internal CO2 diffusion constraints on photosynthesis

Mesophyll conductance (gm) has been shown to affect photosynthetic capacity and thus the estimates of terrestrial carbon balance. While there have been some attempts to model gm at the leaf and larger scales, the potential contribution of gm to the photosynthesis of non-leaf green organs has not been studied. Here, we investigated the influence of gm on photosynthesis of cotton bracts and how it in turn is influenced by anatomical structures, by comparing leaf palisade and spongy mesophyll with bract tissue. Our results showed that photosynthetic capacity in bracts is much lower than in leaves, and that gm is a limiting factor for bract photosynthesis to a similar extent to stomatal conductance. Bract and the spongy tissue of leaves have lower mesophyll conductance than leaf palisade tissue due to the greater volume fraction of intercellular air spaces, smaller chloroplasts, lower surface area of mesophyll cells and chloroplasts exposed to leaf intercellular air spaces and, perhaps, lower membrane permeability. Comparing bracts with leaf spongy tissue, although bracts have a larger cell wall thickness, they have a similar gm estimated from anatomical characteristics, likely due to the cumulative compensatory effects of subtle differences in each subcellular component, especially chloroplast traits. These results provide the first evidence for anatomical constraints on gm and photosynthesis in non-leaf green organs.

Leaf hydraulic vulnerability triggers the decline in stomatal and mesophyll conductance during drought in rice

Understanding the physiological responses of crops to drought is important for ensuring sustained crop productivity under climate change, which is expected to exacerbate the frequency and intensity of periods of drought. Drought responses involve multiple traits, and the correlations between these traits are poorly understood. Using a variety of techniques, we estimated the changes in gas exchange, leaf hydraulic conductance, and leaf turgor in rice (Oryza sativa) in response to both short- and long-term soil drought. We performed a photosynthetic limitation analysis to quantify the contributions of each limiting factor to the resultant overall decrease in photosynthesis during drought. Biomass, leaf area, and leaf width significantly decreased during the 2-week drought treatment, but leaf mass per area and leaf vein density increased. Light-saturated photosynthetic rate declined dramatically during soil drought, mainly due to the decrease in stomatal conductance (gs) and mesophyll conductance (gm). Stomatal modeling suggested that the decline in leaf hydraulic conductance explained most of the decrease in stomatal closure during the drought treatment, and may also trigger the drought-related decrease of stomatal conductance and mesophyll conductance. The results of this study provide insight into the regulation of carbon assimilation under drought conditions.

Determination of leaf respiration in the light: comparison between an isotopic disequilibrium method and the Laisk method

Quantification of leaf respiration is important for understanding plant physiology and ecosystem biogeochemical processes. Leaf respiration continues in the light (R_L ) but supposedly at a lower rate than in the dark (R_Dk ). However, there is no method for direct measurement of R_L and the available methods require nonphysiological measurement conditions. A method based on isotopic disequilibrium quantified R_L (R_L13C ) and mesophyll conductance of young and old fully expanded leaves of six species. R_L13C was compared to R_L determined by the Laisk method (R_L Laisk ) on the very same leaves with a minimum time lag. R_L 13C and R_L Laisk were generally lower than R_Dk , and were not significantly affected by leaf ageing. R_L Laisk and R_L 13C were positively correlated (r²  = 0.35), and both were positively correlated with R_Dk (r²  ≥ 0.6). R_L Laisk was systematically lower than R_L 13C by 0.4 μmol m^-2  s^-1 . Using A/C_c instead of A/C_i curves, a higher photocompensation point Γ* (by 5 μmol mol^-1 ) was found but no influence on R_L Laisk estimates was observed. The results imply that the Laisk method underestimates actual R_L significantly, probably related to the measurement condition of low CO₂ and irradiance. The isotopic disequilibrium method is useful for assessing responses of R_L to irradiance and CO₂ , improving our mechanistic understanding of R_L .

Mesophyll conductance in Zea mays responds transiently to CO2 availability: implications for transpiration efficiency in C4 crops

Mesophyll conductance (g_m ) describes the movement of CO₂ from the intercellular air spaces below the stomata to the site of initial carboxylation in the mesophyll. In contrast with C₃ -g_m , little is currently known about the intraspecific variation in C₄ -g_m or its responsiveness to environmental stimuli. To address these questions, g_m was measured on five maize (Zea mays) lines in response to CO₂ , employing three different estimates of g_m . Each of the methods indicated a significant response of g_m to CO₂ . Estimates of g_m were similar between methods at ambient and higher CO₂ , but diverged significantly at low partial pressures of CO₂ . These differences are probably driven by incomplete chemical and isotopic equilibrium between CO₂ and bicarbonate under these conditions. Carbonic anhydrase and phosphoenolpyruvate carboxylase in vitro activity varied significantly despite similar values of g_m and leaf anatomical traits. These results provide strong support for a CO₂ response of g_m in Z. mays, and indicate that g_m in maize is probably driven by anatomical constraints rather than by biochemical limitations. The CO₂ response of g_m indicates a potential role for facilitated diffusion in C₄ -g_m . These results also suggest that water-use efficiency could be enhanced in C₄ species by targeting g_m .

The response of mesophyll conductance to short-term variation in CO2 in the C4 plants Setaria viridis and Zea mays

Mesophyll conductance (gm) limits rates of C3 photosynthesis but little is known about its role in C4 photosynthesis. If gm were to limit C4 photosynthesis, it would likely be at low CO2 concentrations (pCO2). However, data on C4-gm across ranges of pCO2 are scarce. We describe the response of C4-gm to short-term variation in pCO2, at three temperatures in Setaria viridis, and at 25 °C in Zea mays. Additionally, we quantified the effect of finite gm calculations of leakiness (ϕ) and the potential limitations to photosynthesis imposed by stomata, mesophyll, and carbonic anhydrase (CA) across pCO2. In both species, gm increased with decreasing pCO2. Including a finite gm resulted in either no change or increased ϕ compared with values calculated with infinite gm depending on whether the observed 13C discrimination was high (Setaria) or low (Zea). Post-transitional regulation of the maximal PEP carboxylation rate and PEP regeneration limitation could influence estimates of gm and ϕ. At pCO2 below ambient, the photosynthetic rate was limited by CO2 availability. In this case, the limitation imposed by the mesophyll was similar or slightly lower than stomata limitation. At very low pCO2, CA further constrained photosynthesis. High gm could increase CO2 assimilation at low pCO2 and improve photosynthetic efficiency under situations when CO2 is limited, such as drought.

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

Stomatal conductance (g_s ) and mesophyll conductance (g_m ) represent major constraints to photosynthetic rate (A), and these traits are expected to coordinate with leaf hydraulic conductance (K_leaf ) across species, under both steady-state and dynamic conditions. However, empirical information about their coordination is scarce. In this study, K_leaf , gas exchange, stomatal kinetics, and leaf anatomy in 10 species including ferns, gymnosperms, and angiosperms were investigated to elucidate the correlation of H₂ O and CO₂ diffusion inside leaves under varying light conditions. Gas exchange, K_leaf , and anatomical traits varied widely across species. Under light-saturated conditions, the A, g_s , g_m , and K_leaf were strongly correlated across species. However, the response patterns of A, g_s , g_m , and K_leaf to varying light intensities were highly species dependent. Moreover, stomatal opening upon light exposure of dark-adapted leaves in the studied ferns and gymnosperms was generally faster than in the angiosperms; however, stomatal closing in light-adapted leaves after darkening was faster in angiosperms. The present results show that there is a large variability in the coordination of leaf hydraulic and gas exchange parameters across terrestrial plant species, as well as in their responses to changing light.

Phenotypic Trait Identification Using a Multimodel Bayesian Method: A Case Study Using Photosynthesis in Brassica rapa Genotypes

Agronomists have used statistical crop models to predict yield on a genotype-by-genotype basis. Mechanistic models, based on fundamental physiological processes common across plant taxa, will ultimately enable yield prediction applicable to diverse genotypes and crops. Here, genotypic information is combined with multiple mechanistically based models to characterize photosynthetic trait differentiation among genotypes of Brassica rapa. Infrared leaf gas exchange and chlorophyll fluorescence observations are analyzed using Bayesian methods. Three advantages of Bayesian approaches are employed: a hierarchical model structure, the testing of parameter estimates with posterior predictive checks and a multimodel complexity analysis. In all, eight models of photosynthesis are compared for fit to data and penalized for complexity using deviance information criteria (DIC) at the genotype scale. The multimodel evaluation improves the credibility of trait estimates using posterior distributions. Traits with important implications for yield in crops, including maximum rate of carboxylation (V_cmax ) and maximum rate of electron transport (J_max ) show genotypic differentiation. B. rapa shows phenotypic diversity in causal traits with the potential for genetic enhancement of photosynthesis. This multimodel screening represents a statistically rigorous method for characterizing genotypic differences in traits with clear biophysical consequences to growth and productivity within large crop breeding populations with application across plant processes.

Using Stable Carbon 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, tunable diode laser spectroscopes with sufficient precision for determining isotope mixing ratios became commercially available. This has allowed collection of large data sets, at low cost and with unprecedented temporal resolution. With more data and accompanying knowledge, it has become apparent that there is a need for increased complexity in models and calculations. This chapter describes instantaneous online measurements of ¹³C photosynthetic discrimination, provides recommendations for experimental setup, and presents a thorough compilation of equations needed for different applications.