Assessing the CO2 concentration at the surface of photosynthetic mesophyll cells

Recent insights into air space formation in plant shoots

An adequate supply of oxygen and carbon dioxide is essential for plant survival. Although plant cell walls are somewhat porous, their hydrated nature hampers gas diffusion. Furthermore, the cuticular wax coating the epidermal layer of aerial tissues strongly inhibits gas exchange. Because plants lack specialized systems that bind and transport gases, gases must be directly delivered to the target cells. This necessitates the establishment of effective gas transport pathways connecting stomata to the target cells. However, our understanding of this process remains fragmented. Recent studies have shed light on the mechanisms underlying air space formation in various model and non-model plant species. This review aims to consolidate these findings, to provide a comprehensive overview of our current understanding of air space formation, and to outline potential avenues for future research that will address remaining gaps in knowledge.

Unsaturation of Leaf Air Spaces Sheds New Light on the Role of Aquaporins

Aquaporins have long been known to facilitate water transport across membranes inside leaves. However, their role in regulating the water potential (ψ) difference between the cytosol and cell wall has been questioned, as the ψ of the cytosol and cell wall would be in equilibrium under the assumption of water‐vapour‐saturated leaf intercellular air spaces. Recent advances suggest that intercellular air spaces are unsaturated at high vapour pressure deficit (VPD) and that aquaporins that up‐regulate water transport might simultaneously down‐regulate CO2 transport in a competitive manner. Therefore, the currently assumed mechanisms of CO2 and water transport in the mesophyll under varying VPD must be re‐examined. We incorporated the competitive aquaporin hypothesis into the leaf gas exchange pathways with unsaturated intercellular air spaces. We show that the putative competitive control of CO2‐ and water‐facilitating aquaporins is fully effective only when there is a large ψ gradient between the cytosol and cell wall at high VPD. In this context, the down‐regulation of water‐facilitating aquaporins and the up‐regulation of CO2‐facilitating aquaporins could protect the cytosol from drying and maintain the CO2 supply for photosynthesis, respectively. While it remains unclear whether unsaturation drives aquaporin activity or vice versa, we identify research challenges that need to be addressed.

Dry inside: progressive unsaturation within leaves with increasing vapour pressure deficit affects estimation of key leaf gas exchange parameters

Climate change not only leads to higher air temperatures but also increases the vapour pressure deficit (VPD) of the air. Understanding the direct effect of VPD on leaf gas exchange is crucial for precise modelling of stomatal functioning. We conducted combined leaf gas exchange and online isotope discrimination measurements on four common European tree species across a VPD range of 0.8-3.6 kPa, while maintaining constant temperatures without soil water limitation. In addition to applying the standard assumption of saturated vapour pressure inside leaves (ei), we inferred ei from oxygen isotope discrimination of CO2 and water vapour. ei desaturated progressively with increasing VPD, consistently across species, resulting in an intercellular relative humidity as low as 0.73 ± 0.11 at the highest tested VPD. Assuming saturation of ei overestimated the extent of reductions in stomatal conductance and CO2 mole fraction inside leaves in response to increasing VPD compared with calculations that accounted for unsaturation. In addition, a significant decrease in mesophyll conductance with increasing VPD only occurred when the unsaturation of ei was considered. We suggest that the possibility of unsaturated ei should not be overlooked in measurements related to leaf gas exchange and in stomatal models, especially at high VPD.

Unsaturation in the air spaces of leaves and its implications

Modern plant physiological theory stipulates that the resistance to water movement from plants to the atmosphere is overwhelmingly dominated by stomata. This conception necessitates a corollary assumption-that the air spaces in leaves must be nearly saturated with water vapour; that is, with a relative humidity that does not decline materially below unity. As this idea became progressively engrained in scientific discourse and textbooks over the last century, observations inconsistent with this corollary assumption were occasionally reported. Yet, evidence of unsaturation gained little traction, with acceptance of the prevailing framework motivated by three considerations: (1) leaf water potentials measured by either thermocouple psychrometry or the Scholander pressure chamber are largely consistent with the framework; (2) being able to assume near saturation of intercellular air spaces was transformational to leaf gas exchange analysis; and (3) there has been no obvious mechanism to explain a variable, liquid-phase resistance in the leaf mesophyll. Here, we review the evidence that refutes the assumption of universal, near saturation of air spaces in leaves. Refining the prevailing paradigm with respect to this assumption provides opportunities for identifying and developing mechanisms for increased plant productivity in the face of increasing evaporative demand imposed by global climate change.

Mesophyll airspace unsaturation drives C4 plant success under vapor pressure deficit stress

A fundamental assumption in plant science posits that leaf air spaces remain vapor saturated, leading to the predominant view that stomata alone control leaf water loss. This concept has been pivotal in photosynthesis and water-use efficiency research. However, recent evidence has refuted this longstanding assumption by providing evidence of unsaturation in the leaf air space of C3 plants under relatively mild vapor pressure deficit (VPD) stress. This phenomenon represents a nonstomatal mechanism restricting water loss from the mesophyll. The potential ubiquity and physiological implications of this phenomenon, its driving mechanisms in different plant species and habitats, and its interaction with other ecological adaptations have. In this context, C4 plants spark particular interest for their importance as crops, bundle sheath cells' unique anatomical characteristics and specialized functions, and notably higher water-use efficiency relative to C3 plants. Here, we confirm reduced relative humidities in the substomatal cavity of the C4 plants maize, sorghum, and proso millet down to 80% under mild VPD stress. We demonstrate the critical role of nonstomatal control in these plants, indicating that the role of the CO2 concentration mechanism in CO2 management at a high VPD may have been overestimated. Our findings offer a mechanistic reconciliation between discrepancies in CO2 and VPD responses reported in C4 species. They also reveal that nonstomatal control is integral to maintaining an advantageous microclimate of relatively higher CO2 concentrations in the mesophyll air space of C4 plants for carbon fixation, proving vital when these plants face VPD stress.

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 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.

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.

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.

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.

Amphistomy increases leaf photosynthesis more in coastal than montane plants of Hawaiian 'ilima (Sida fallax)

PREMISE

The adaptive significance of amphistomy (stomata on both upper and lower leaf surfaces) is unresolved. A widespread association between amphistomy and open, sunny habitats suggests the adaptive benefit of amphistomy may be greatest in these contexts, but this hypothesis has not been tested experimentally. Understanding amphistomy informs its potential as a target for crop improvement and paleoenvironment reconstruction.

METHODS

We developed a method to quantify "amphistomy advantage" (AA $\text{AA}$ ) as the log-ratio of photosynthesis in an amphistomatous leaf to that of the same leaf but with gas exchange blocked through the upper surface (pseudohypostomy). Humidity modulated stomatal conductance and thus enabled comparing photosynthesis at the same total stomatal conductance. We estimatedAA $\text{AA}$ and leaf traits in six coastal (open, sunny) and six montane (closed, shaded) populations of the indigenous Hawaiian species 'ilima (Sida fallax).

RESULTS

Coastal 'ilima leaves benefit 4.04 times more from amphistomy than montane leaves. Evidence was equivocal with respect to two hypotheses: (1) that coastal leaves benefit more because they are thicker and have lower CO2 conductance through the internal airspace and (2) that they benefit more because they have similar conductance on each surface, as opposed to most conductance being through the lower surface.

CONCLUSIONS

This is the first direct experimental evidence that amphistomy increases photosynthesis, consistent with the hypothesis that parallel pathways through upper and lower mesophyll increase CO2 supply to chloroplasts. The prevalence of amphistomatous leaves in open, sunny habitats can partially be explained by the increased benefit of amphistomy in "sun" leaves, but the mechanistic basis remains uncertain.

Simultaneous and Independent Abaxial and Adaxial Gas Exchange Measurements

Stomata can be distributed exclusively on the abaxial or adaxial leaf surface, but they are most commonly found on both leaf surfaces. Variations in stomatal arrangement, patterning, and the impact on photosynthesis can be measured using an infrared gas exchange system. However, when using standard gas exchange techniques, both surfaces are measured together and averaged to provide leaf-level values. Employing an innovative gas exchange apparatus with two infrared gas analyzers, separate gaseous flux from both leaf surfaces can be quantified simultaneously and independently. Here, we provide examples of typical measurements that can be performed using a "split chamber" gas exchange system.

Tools for Measuring Photosynthesis at Different Scales

Measurements of in vivo photosynthesis are powerful tools that probe the largest fluxes of carbon and energy in an illuminated leaf, but often the specific techniques used are so varied and specialized that it is difficult for researchers outside the field to select and perform the most useful assays for their research questions. The goal of this chapter is to provide a broad overview of the current tools available for the study of photosynthesis, both in vivo and in vitro, so as to provide a foundation for selecting appropriate techniques, many of which are presented in detail in subsequent chapters. This chapter will also organize current methods into a comparative framework and provide examples of how they have been applied to research questions of broad agronomical, ecological, or biological importance. This chapter closes with an argument that the future of in vivo measurements of photosynthesis lies in the ability to use multiple methods simultaneously and discusses the benefits of this approach to currently open physiological questions. This chapter, combined with the relevant methods chapters, could serve as a laboratory course in methods in photosynthesis research or as part of a more comprehensive laboratory course in general plant physiology methods.

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.

CO2 mesophyll conductance regulated by light: a review

MAIN CONCLUSION

Light quality and intensity regulate plant mesophyll conductance, which has played an essential role in photosynthesis by controlling leaf structural and biochemical properties. Mesophyll conductance (g_m), a crucial physiological factor influencing the photosynthetic rate of leaves, is used to describe the resistance of CO₂ from the sub-stomatal cavity into the chloroplast up to the carboxylation site. Leaf structural and biochemical components, as well as external environmental factors such as light, temperature, and water, all impact g_m. As an essential factor of plant photosynthesis, light affects plant growth and development and plays a vital role in regulating g_m as well as determining photosynthesis and yield. This review aimed to summarize the mechanisms of g_m response to light. Both structural and biochemical perspectives were combined to reveal the effects of light quality and intensity on the g_m, providing a guide for selecting the optimal conditions for intensifying photosynthesis in plants.