Physiological framework for adaptation of stomata to CO2 from glacial to future concentrations

Mechanistic insights into plant community responses to environmental variables: genome size, cellular nutrient investments, and metabolic tradeoffs

Affecting biodiversity, plants with larger genome sizes (GS) may be restricted in nutrient-poor conditions. This pattern has been attributed to their greater cellular nitrogen (N) and phosphorus (P) investments and hypothesized nutrient-investment tradeoffs between cell synthesis and physiological attributes associated with growth. However, the influence of GS on cell size and functioning may also contribute to GS-dependent growth responses to nutrients. To test whether and how GS is associated with cellular nutrient, stomata, and/or physiological attributes, we examined > 500 forbs and grasses from seven grassland sites conducting a long-term N and P fertilization experiment. Larger GS plants had increased cellular nutrient contents and larger, but fewer stomata than smaller GS plants. Larger GS grasses (but not forbs) also had lower photosynthetic rates and water-use efficiencies. However, nutrients had no direct effect on GS-dependent physiological attributes and GS-dependent physiological changes likely arise from how GS influences cells. At the driest sites, large GS grasses displayed high water-use efficiency mostly because transpiration was reduced relative to photosynthesis in these conditions. We suggest that climatic conditions and GS-associated cell traits that modify physiological responses, rather than resource-investment tradeoffs, largely explain GS-dependent growth responses to nutrients (especially for grasses).

An optimization method for measuring the stomata in cassava (Manihot esculenta Crantz) under multiple abiotic stresses

As a gateway for gas exchange, pores regulate the transport of air and water in carbon assimilation, respiration, and transpiration to quickly adapt to environmental changes. Therefore, the study of stomatal movement characteristics of plants is helpful to strengthen the understanding of the mechanism of plant response to multi-environmental stress, and can improve the function of plant resistance to stresses. The stomatal movement of Arabidopsis leaves was observed by staining the stomata with rhodamine 6G, but this method has not been reported in other plant leaf stomata studies. Taking cassava as an example, the correlation between cassava stomatal movement and cassava response to stress was observed by using and improving the staining method. Rhodamine 6G is a biological stain widely used in cell biology and molecular biology. It was found that 1 μM rhodamine 6G could stain the stomata of cassava without affecting stomatal movement (n = 109, p < 0.05). In addition, we proposed that stomata fixed with 4% concentration of formaldehyde after staining were closest to the stomatal morphology of cassava epidermis, so as to observe stomatal movement under different environmental stresses more accurately. Previous methods of measuring stomatal pore size by autofluorescence of cell wall needs to fix the cells for 6 h, but Rhodamine staining can only be observed in 2 min, which greatly improves the experimental efficiency. Compared with the traditional exfoliation method (e.g., Arabidopsis), this method can reduce the damage of the leaves and observe the stomata of the whole leaves more completely, so that the experimental results are more complete. In addition, the method enables continuous leaf processing and observation. Using this method, we further compared four different cassava varieties (i.e., KU50, SC16, SC8, and SC205) and found that there are differences in stomatal density (SD) among cassava varieties, and the difference in the SD directly affects the stress resistance of cassava (n = 107, p < 0.001). This finding has important implications for studying the mechanism of plant response to environmental stress through stomata.

Stomata Are Driving the Direction of CO2-Induced Water-Use Efficiency Gain in Selected Tropical Trees in Fiji

Understanding plant physiological response to a rising atmospheric CO2 concentration (ca) is key in predicting Earth system plant-climate feedbacks; however, the effects of long-term rising ca on plant gas-exchange characteristics in the tropics are largely unknown. Studying this long-term trend using herbarium records is challenging due to specimen trait variation. We assessed the impact of a ca rise of ~95 ppm (1927-2015) on the intrinsic water-use efficiency (iWUE) and maximum stomatal conductance (gsmax) of five tropical tree species in Fiji using the isotopic composition and stomatal traits of herbarium leaves. Empirical results were compared with simulated values using models that uniquely incorporated the variation in the empirical gsmax responses and species-specific parameterisation. The magnitude of the empirical iWUE and gsmax response was species-specific, ranging from strong to negligible. Stomatal density was more influential than the pore size in determining the gsmax response to ca. While our simulation results indicated that photosynthesis is the main factor contributing to the iWUE gain, stomata were driving the iWUE trend across the tree species. Generally, a stronger increase in the iWUE was accompanied by a stronger decline in stomatal response. This study demonstrates that the incorporation of variation in the gsmax in simulations is necessary for assessing an individual species' iWUE response to changing ca.

Inside-out: Synergising leaf biochemical traits with stomatal-regulated water fluxes to enhance transpiration modelling during abiotic stress

As the global climate continues to change, plants will increasingly experience abiotic stress(es). Stomata on leaf surfaces are the gatekeepers to plant interiors, regulating gaseous exchanges that are crucial for both photosynthesis and outward water release. To optimise future crop productivity, accurate modelling of how stomata govern plant-environment interactions will be crucial. Here, we synergise optical and thermal imaging data to improve modelled transpiration estimates during water and/or nutrient stress (where leaf N is reduced). By utilising hyperspectral data and partial least squares regression analysis of six plant traits and fluxes in wheat (Triticum aestivum), we develop a new spectral vegetation index; the Combined Nitrogen and Drought Index (CNDI), which can be used to detect both water stress and/or nitrogen deficiency. Upon full stomatal closure during drought, CNDI shows a strong relationship with leaf water content (r2 = 0.70), with confounding changes in leaf biochemistry. By incorporating CNDI transformed with a sigmoid function into thermal-based transpiration modelling, we have increased the accuracy of modelling water fluxes during abiotic stress. These findings demonstrate the potential of using combined optical and thermal remote sensing-based modelling approaches to dynamically model water fluxes to improve both agricultural water usage and yields.

2,2',4,4'-Tetrabromodiphenyl Ether (BDE-47) at Environmental Levels Influenced Photosynthesis in the Mangrove Species Kandelia obovata

2,2',4,4'-tetra-bromodiphenytol ether (BDE-47) is one of the ubiquitous organic pollutants in mangrove sediments. To reveal the toxic effects of BDE-47 on mangrove plants, the mangrove species Kandelia obovate was used to investigate the photosynthetic capacity effects and the molecular mechanisms involved after BDE-47 exposure at environment-related levels (50, 500, and 5000 ng g-1 dw). After a 60-day exposure, the photosynthetic capacity was inhibited in K. obovata seedlings, and a decrease in the stomatal density and damage in the chloroplast ultrastructure in the leaves were found. Transcriptome sequencing showed that, following exposure to BDE-47, gene expression in photosynthesis-related pathways was predominantly suppressed in the leaves. The bioinformatics analysis indicated that BDE-47 exerts toxicity by inhibiting photosystem I activity and chlorophyll a/b-binding protein-related genes in the leaves of K. obovata. Thus, this study provides preliminary theoretical evidence for the toxic mechanism effect of BDE-47 on photosynthesis in mangrove species.

An Inverse Scaling Relationship between Stomatal Density and Mean Nearest Neighbor Distance: Evidence from a Photinia Hybrid and One of Its Parents

Stomata are involved in transpiration and CO2 uptake by mediating gas exchange between internal plant tissues and the atmosphere. The capacity for gas exchange depends on stomatal density (SD), stomatal size, and pore dimensions. Most published work on stomatal quantification has assumed that stomatal distribution and stomatal density are spatially homogeneous across the leaf, but this assumption has been seldom tested. We selected 32 leaves from a Photinia hybrid, Photinia × fraseri 'Red Robin', and one of its parents, P. serratifolia. For each leaf, the leaf surface was divided into three or four equidistant layers along the apical-basal axis, and, in each layer, two positions, one closer to the midrib and the other closer to the leaf margin, were further selected. We calculated SD and mean nearest neighbor distance (MNND) for each lamina section and tested the scaling relationship between SD and MNND of the sampled stomatal centers using reduced major axis protocols. In addition, we calculated the stomatal aggregation index (SAI) for each lamina section to examine the spatial arrangement of stomata at the given size of field of view of 1.2 mm × 0.9 mm. We observed that SD decreased from the lamina apex towards the base for central lamina areas but varied little at leaf margins. An inverse scaling relationship between SD and MNND was observed for both species. This relationship could be used for SD estimation using the rapidly estimated trait, MNND. SAI did not vary significantly throughout leaf lamina, and the numerical values of SAI for all fields of view were greater than one, which indicates significant spatial repulsion between stomata. The study suggests that SD varies across leaf lamina to fine-tune plant water use and maximize carbon gain. However, spatial structures of stomata from different lamina sections exhibit similar patterns (i.e., spatial inhibition between stomata at small scales), probably due to hierarchical leaf vein patterns.

Diverse mangroves deviate from other angiosperms in their genome size, leaf cell size and cell packing density relationships

BACKGROUND AND AIMS

While genome size limits the minimum sizes and maximum numbers of cells that can be packed into a given leaf volume, mature cell sizes can be substantially larger than their meristematic precursors and vary in response to abiotic conditions. Mangroves are iconic examples of how abiotic conditions can influence the evolution of plant phenotypes.

METHODS

Here, we examined the coordination between genome size, leaf cell sizes, cell packing densities and leaf size in 13 mangrove species across four sites in China. Four of these species occurred at more than one site, allowing us to test the effect of climate on leaf anatomy.

RESULTS

We found that genome sizes of mangroves were very small compared to other angiosperms, but, like other angiosperms, mangrove cells were always larger than the minimum size defined by genome size. Increasing mean annual temperature of a growth site led to higher packing densities of veins (Dv) and stomata (Ds) and smaller epidermal cells but had no effect on stomatal size. In contrast to other angiosperms, mangroves exhibited (1) a negative relationship between guard cell size and genome size; (2) epidermal cells that were smaller than stomata; and (3) coordination between Dv and Ds that was not mediated by epidermal cell size. Furthermore, mangrove epidermal cell sizes and packing densities covaried with leaf size.

CONCLUSIONS

While mangroves exhibited coordination between veins and stomata and attained a maximum theoretical stomatal conductance similar to that of other angiosperms, the tissue-level tradeoffs underlying these similar relationships across species and environments were markedly different, perhaps indicative of the unique structural and physiological adaptations of mangroves to their stressful environments.

The influences of stomatal size and density on rice abiotic stress resilience

A warming climate coupled with reductions in water availability and rising salinity are increasingly affecting rice (Oryza sativa) yields. Elevated temperatures combined with vapour pressure deficit (VPD) rises are causing stomatal closure, further reducing plant productivity and cooling. It is unclear what stomatal size (SS) and stomatal density (SD) will best suit all these environmental extremes. To understand how stomatal differences contribute to rice abiotic stress resilience, we screened the stomatal characteristics of 72 traditionally bred varieties. We found significant variation in SS, SD and calculated anatomical maximal stomatal conductance (gsmax ) but did not identify any varieties with SD and gsmax as low as transgenic OsEPF1oe plants. Traditionally bred varieties with high SD and small SS (resulting in higher gsmax ) typically had lower biomasses, and these plants were more resilient to drought than low SD and large SS plants, which were physically larger. None of the varieties assessed were as resilient to drought or salinity as low SD OsEPF1oe transgenic plants. High SD and small SS rice displayed faster stomatal closure during increasing temperature and VPD, but photosynthesis and plant cooling were reduced. Compromises will be required when choosing rice SS and SD to tackle multiple future environmental stresses.

Optimal stomatal theory predicts CO2 responses of stomatal conductance in both gymnosperm and angiosperm trees

Optimal stomatal theory predicts that stomata operate to maximise photosynthesis (A_net ) and minimise transpirational water loss to achieve optimal intrinsic water-use efficiency (iWUE). We tested whether this theory can predict stomatal responses to elevated atmospheric CO₂ (eCO₂ ), and whether it can capture differences in responsiveness among woody plant functional types (PFTs). We conducted a meta-analysis of tree studies of the effect of eCO₂ on iWUE and its components A_net and stomatal conductance (g_s ). We compared three PFTs, using the unified stomatal optimisation (USO) model to account for confounding effects of leaf-air vapour pressure difference (D). We expected smaller g_s , but greater A_net , responses to eCO₂ in gymnosperms compared with angiosperm PFTs. We found that iWUE increased in proportion to increasing eCO₂ in all PFTs, and that increases in A_net had stronger effects than reductions in g_s . The USO model correctly captured stomatal behaviour with eCO₂ across most datasets. The chief difference among PFTs was a lower stomatal slope parameter (g₁ ) for the gymnosperm, compared with angiosperm, species. Land surface models can use the USO model to describe stomatal behaviour under changing atmospheric CO₂ conditions.

Trading water for carbon in the future: Effects of elevated CO2 and warming on leaf hydraulic traits in a semiarid grassland

The effects of climate change on plants and ecosystems are mediated by plant hydraulic traits, including interspecific and intraspecific variability of trait phenotypes. Yet, integrative and realistic studies of hydraulic traits and climate change are rare. In a semiarid grassland, we assessed the response of several plant hydraulic traits to elevated CO₂ (+200 ppm) and warming (+1.5 to 3°C; day to night). For leaves of five dominant species (three graminoids and two forbs), and in replicated plots exposed to 7 years of elevated CO₂ , warming, or ambient climate, we measured: stomatal density and size, xylem vessel size, turgor loss point, and water potential (pre-dawn). Interspecific differences in hydraulic traits were larger than intraspecific shifts induced by elevated CO₂ and/or warming. Effects of elevated CO₂ were greater than effects of warming, and interactions between treatments were weak or not detected. The forbs showed little phenotypic plasticity. The graminoids had leaf water potentials and turgor loss points that were 10% to 50% less negative under elevated CO₂ ; thus, climate change might cause these species to adjust their drought resistance strategy away from tolerance and toward avoidance. The C4 grass also reduced allocation of leaf area to stomata under elevated CO₂ , which helps explain observations of higher soil moisture. The shifts in hydraulic traits under elevated CO₂ were not, however, simply due to higher soil moisture. Integration of our results with others' indicates that common species in this grassland are more likely to adjust stomatal aperture in response to near-term climate change, rather than anatomical traits; this contrasts with apparent effects of changing CO₂ on plant anatomy over evolutionary time. Future studies should assess how plant responses to drought may be constrained by the apparent shift from tolerance (via low turgor loss point) to avoidance (via stomatal regulation and/or access to deeper soil moisture).

Stomatal opening ratio mediates trait coordinating network adaptation to environmental gradients

A trait coordination network is constructed through intercorrelations of functional traits, which reflect trait-based adaptive strategies. However, little is known about how these networks change across spatial scales, and what drivers and mechanisms mediate this change. This study bridges that gap by integrating functional traits related to plant carbon gain and water economy into the coordination network of Siberian elm (Ulmus pumila), a eurybiont that survives along a 3800 km environmental gradient from humid forest to arid desert. Our results demonstrated that both stomatal density and stomatal size reached a physiological threshold at which adjustments in these traits were not sufficient to cope with the increased environmental stress. Network analysis further revealed that the mechanism for overcoming this threshold, the stomatal opening ratio, g_ratio , was represented by the highest values for centrality across different spatial scales, and therefore mediated the changes in the trait coordination network along environmental gradients. The mediating roles manifested as creating the highest maximum theoretical stomatal conductance (g_smax ) but lowest possible g_ratio for pathogen defense in humid regions, while maintaining the g_ratio 'sweet spot' (c. 20% in this region) for highest water use efficiency in semihumid regions, and having the lowest g_smax and highest g_ratio for gas exchange and leaf cooling in arid regions. These results suggested that the stomatal traits related to control of stomatal movement play fundamental roles in balancing gas exchange, leaf cooling, embolism resistance and pathogen defense. These insights will allow more accurate model parameterization for different regions, and therefore better predictions of species' responses to global change.

The origin and evolution of stomata

The acquisition of stomata is one of the key innovations that led to the colonisation of the terrestrial environment by the earliest land plants. However, our understanding of the origin, evolution and the ancestral function of stomata is incomplete. Phylogenomic analyses indicate that, firstly, stomata are ancient structures, present in the common ancestor of land plants, prior to the divergence of bryophytes and tracheophytes and, secondly, there has been reductive stomatal evolution, especially in the bryophytes (with complete loss in the liverworts). From a review of the evidence, we conclude that the capacity of stomata to open and close in response to signals such as ABA, CO₂ and light (hydroactive movement) is an ancestral state, is present in all lineages and likely predates the divergence of the bryophytes and tracheophytes. We reject the hypothesis that hydroactive movement was acquired with the emergence of the gymnosperms. We also conclude that the role of stomata in the earliest land plants was to optimise carbon gain per unit water loss. There remain many other unanswered questions concerning the evolution and especially the origin of stomata. To address these questions, it will be necessary to: find more fossils representing the earliest land plants, revisit the existing early land plant fossil record in the light of novel phylogenomic hypotheses and carry out more functional studies that include both tracheophytes and bryophytes.

How Leaf Vein and Stomata Traits Are Related with Photosynthetic Efficiency in Falanghina Grapevine in Different Pedoclimatic Conditions

The increase in severe drought events due to climate change in the areas traditionally suitable for viticulture is enhancing the need to understand how grapevines regulate their photosynthetic metabolism in order to forecast specific cultivar adaptive responses to the changing environment. This study aims at evaluating the association between leaf anatomical traits and eco-physiological adjustments of the ‘Falanghina’ grapevine under different microclimatic conditions at four sites in southern Italy. Sites were characterized by different pedoclimatic conditions but, as much as possible, were similar for plant material and cultivation management. Microscopy analyses on leaves were performed to quantify stomata and vein traits, while eco-physiological analyses were conducted on vines to assess plant physiological adaptation capability. At the two sites with relatively low moisture, photosynthetic rate, stomatal conductance, photosystem electron transfer rate, and quantum yield of PSII, linear electron transport was lower compared to the other two sites. Stomata size was higher at the site characterized by the highest precipitation. However, stomatal density and most vein traits tended to be relatively stable among sites. The number of free vein endings per unit leaf area was lower in the two vineyards with low precipitation. We suggest that site-specific stomata and vein traits modulation in Falanghina grapevine are an acclimation strategy that may influence photosynthetic performance. Overall in-depth knowledge of the structure/function relations in Falanghina vines might be useful to evaluate the plasticity of this cultivar towards site-specific management of vineyards in the direction of precision viticulture.

High intrinsic water use efficiency is underpinned by high stomatal aperture and guard cell potassium flux in C3 and C4 grasses grown at glacial CO2 and low light

We compared how stomatal morphology and physiology control intrinsic leaf water use efficiency (iWUE) in two C3 and six C4 grasses grown at ambient (400 µmol mol-1) or glacial CO2 (180 µmol mol-1) and high (1000 µmol m-2 s-1) or low light intensity (200 µmol m-2 s-1). C4 grasses tended to have higher iWUE and CO2 assimilation rates, and lower stomatal conductance (gs), operational stomatal aperture (aop), and guard cell K+ influx rate relative to C3 grasses, while stomatal size (SS) and stomatal density (SD) did not vary according to the photosynthetic type. Overall, iWUE and gs depended most on aop and density of open stomata. In turn, aop correlated with K+ influx, stomatal opening speed on transition to high light, and SS. Species with higher SD had smaller and faster-opening stomata. Although C4 grasses operated with lower gs and aop at ambient CO2, they showed a greater potential to open stomata relative to maximal stomatal conductance (gmax), indicating heightened stomatal sensitivity and control. We uncovered promising links between aop, gs, iWUE, and K+ influx among C4 grasses, and differential K+ influx responses of C4 guard cells to low light, revealing molecular targets for improving iWUE in C4 crops.

Genome Size and Labellum Epidermal Cell Size Are Evolutionarily Correlated With Floral Longevity in Paphiopedilum Species

Genome size is known to influence phenotypic traits in leaves and seeds. Although genome size is closely related to cellular and developmental traits across biological kingdoms, floral longevity is a floral trait with important fitness consequence, but less is known about the link between floral longevity and sizes of genomes and cells. In this study, we examined evolutionary coordination between genome size, floral longevity, and epidermal cell size in flowers and leaves in 13 Paphiopedilum species. We found that, across all the study species, the genome size was positively correlated with floral longevity but negatively associated with labellum epidermal cell size, and a negative relationship was found between floral longevity and labellum epidermal cell size. This suggested that genome size is potentially correlated with floral longevity, and genome size has an important impact on life-history trait. In addition, genome size was positively correlated with leaf epidermal cell size, which was different from the relationship in flower due to different selective pressures they experienced or different functions they performed. Therefore, genome size constraints floral longevity, and it is a strong predictor of cell size. The impact of genome size on reproduction might have more implications for the evolution of flowering plants and pollination ecology.

Environmental pressures on stomatal size may drive plant genome size evolution: evidence from a natural experiment with Cape geophytes

BACKGROUND AND AIMS

The idea that genome (size) evolution in eukaryotes could be driven by environmental factors is still vigorously debated. In extant plants, genome size correlates positively with stomatal size, leading to the idea that conditions enabling the existence of large stomata in fossil plants also supported growth of their genome size. We test this inductive assumption in drought-adapted, prostrate-leaved Cape (South Africa) geophytes where, compared with their upright-leaved geophytic ancestors, stomata develop in a favourably humid microclimate formed underneath their leaves.

METHODS

Stomatal parameters (leaf cuticle imprints) and genome size (flow cytometry) were measured in 16 closely related geophytic species pairs from seven plant families. In each pair, representing a different genus, we contrasted a prostrate-leaved species with its upright-leaved phylogenetic relative, the latter whose stomata are exposed to the ambient arid climate.

KEY RESULTS

Except for one, all prostrate-leaves species had larger stomata, and in 13 of 16 pairs they also had larger genomes than their upright-leaved relatives. Stomatal density and theoretical maximum conductance were less in prostrate-leaved species with small guard cells (<1 pL) but showed no systematic difference in species pairs with larger guard cells (>1 pL). Giant stomata were observed in the prostrate-leaved Satyrium bicorne (89-137 µm long), despite its relatively small genome (2C = 9 Gbp).

CONCLUSIONS

Our results imply that climate, through selection on stomatal size, might be able to drive genome size evolution in plants. The data support the idea that plants from 'greenhouse' geological periods with large stomata might have generally had larger genome sizes when compared with extant plants, though this might not have been solely due to higher atmospheric CO2 in these periods but could also have been due to humid conditions prevailing at fossil deposit sites.

The influence of stomatal morphology and distribution on photosynthetic gas exchange

The intricate and interconnecting reactions of C₃ photosynthesis are often limited by one of two fundamental processes: the conversion of solar energy into chemical energy, or the diffusion of CO₂ from the atmosphere through the stomata, and ultimately into the chloroplast. In this review, we explore how the contributions of stomatal morphology and distribution can affect photosynthesis, through changes in gaseous exchange. The factors driving this relationship are considered, and recent results from studies investigating the effects of stomatal shape, size, density and patterning on photosynthesis are discussed. We suggest that the interplay between stomatal gaseous exchange and photosynthesis is complex, and that a disconnect often exists between the rates of CO₂ diffusion and photosynthetic carbon fixation. The mechanisms that allow for substantial reductions in maximum stomatal conductance without affecting photosynthesis are highly dependent on environmental factors, such as light intensity, and could be exploited to improve crop performance.

Nitrogen fertilisation influences low CO2 effects on plant performance

Low atmospheric CO2 conditions prevailed for most of the recent evolutionary history of plants. Such concentrations reduce plant growth compared with modern levels, but low-CO2 effects on plant performance may also be affected by nitrogen availability, since low leaf nitrogen decreases photosynthesis, and CO2 concentrations influence nitrogen assimilation. To investigate the influence of N availability on plant performance at low CO2, we grew Elymus canadensis at ambient (~400 μmol mol-1) and subambient (~180 μmol mol-1) CO2 levels, under four N-treatments: nitrate only; ammonium only; a full and a half mix of nitrate and ammonium. Growth at low CO2 decreased biomass in the full and nitrate treatments, but not in ammonium and half plants. Low CO2 effects on photosynthetic and maximum electron transport rates were influenced by fertilisation, with photosynthesis being most strongly impacted by low CO2 in full plants. Low CO2 reduced stomatal index in half plants, suggesting that the use of this indicator in paleo-inferences can be influenced by N availability. Under low CO2 concentrations, nitrate plants discriminated more against 15N whereas half plants discriminated less against 15N compared with the full treatment, suggesting that N availability should be considered when using N isotopes as paleo-indicators.

Links between environment and stomatal size through evolutionary time in Proteaceae

The size of plant stomata (adjustable pores that determine the uptake of CO₂ and loss of water from leaves) is considered to be evolutionarily important. This study uses fossils from the major Southern Hemisphere family Proteaceae to test whether stomatal cell size responded to Cenozoic climate change. We measured the length and abundance of guard cells (the cells forming stomata), the area of epidermal pavement cells, stomatal index and maximum stomatal conductance from a comprehensive sample of fossil cuticles of Proteaceae, and extracted published estimates of past temperature and atmospheric CO₂. We developed a novel test based on stochastic modelling of trait evolution to test correlations among traits. Guard cell length increased, and stomatal density decreased significantly with decreasing palaeotemperature. However, contrary to expectations, stomata tended to be smaller and more densely packed at higher atmospheric CO₂. Thus, associations between stomatal traits and palaeoclimate over the last 70 million years in Proteaceae suggest that stomatal size is significantly affected by environmental factors other than atmospheric CO₂. Guard cell length, pavement cell area, stomatal density and stomatal index covaried in ways consistent with coordinated development of leaf tissues.

Heritable Changes in Physiological Gas Exchange Traits in Response to Long-Term, Moderate Free-Air Carbon Dioxide Enrichment

Atmospheric carbon dioxide ([CO₂]) concentrations significantly alter developmental plant traits with potentially far-reaching consequences for ecosystem function and productivity. However, contemporary evolutionary responses among extant plant species that coincide with modern, anthropogenically driven [CO₂] rise have rarely been demonstrated among field-grown plant populations. Here we present findings from a long-term, free-air carbon dioxide enrichment (FACE) study in a seminatural European grassland ecosystem in which we observe a differential capacity among plant species to acclimate intrinsic water-use efficiencies (WUEs) in response to prolonged multigenerational exposure to elevated [CO₂] concentrations. In a reciprocal swap trial, using controlled environment growth chambers, we germinated seeds from six of the most dominant plant species at the FACE site [Arrhenatherum elatius (L.), Trisetum flavescens (L.), Holcus lanatus (L.), Geranium pratense (L.), Sanguisorba officinalis (L.), and Plantago lanceolata (L.)]. We found that long-term exposure to elevated [CO₂] strongly influenced the dynamic control of WUE_i in the first filial generations (F₁) of all species as well as an unequal ability to adapt to changes in the [CO₂] of the growth environment among those species. Furthermore, despite trait-environment relationships of this nature often being considered evidence for local adaptation in plants, we demonstrate that the ability to increase WUE_i does not necessarily translate to an ecological advantage in diverse species mixtures.

The role of Arabidopsis ABA receptors from the PYR/PYL/RCAR family in stomatal acclimation and closure signal integration

Stomata are microscopic pores found on the surfaces of leaves that act to control CO₂ uptake and water loss. By integrating information derived from endogenous signals with cues from the surrounding environment, the guard cells, which surround the pore, 'set' the stomatal aperture to suit the prevailing conditions. Much research has concentrated on understanding the rapid intracellular changes that result in immediate changes to the stomatal aperture. In this study, we look instead at how stomata acclimate to longer timescale variations in their environment. We show that the closure-inducing signals abscisic acid (ABA), increased CO₂, decreased relative air humidity and darkness each access a unique gene network made up of clusters (or modules) of common cellular processes. However, within these networks some gene clusters are shared amongst all four stimuli. All stimuli modulate the expression of members of the PYR/PYL/RCAR family of ABA receptors. However, they are modulated differentially in a stimulus-specific manner. Of the six members of the PYR/PYL/RCAR family expressed in guard cells, PYL2 is sufficient for guard cell ABA-induced responses, whereas in the responses to CO₂, PYL4 and PYL5 are essential. Overall, our work shows the importance of ABA as a central regulator and integrator of long-term changes in stomatal behaviour, including sensitivity, elicited by external signals. Understanding this architecture may aid in breeding crops with improved water and nutrient efficiency.

The role of Arabidopsis ABA receptors from the PYR/PYL/RCAR family in stomatal acclimation and closure signal integration

Stomata are microscopic pores found on the surfaces of leaves that act to control CO₂ uptake and water loss. By integrating information derived from endogenous signals with cues from the surrounding environment, the guard cells, which surround the pore, 'set' the stomatal aperture to suit the prevailing conditions. Much research has concentrated on understanding the rapid intracellular changes that result in immediate changes to the stomatal aperture. In this study, we look instead at how stomata acclimate to longer timescale variations in their environment. We show that the closure-inducing signals abscisic acid (ABA), increased CO₂, decreased relative air humidity and darkness each access a unique gene network made up of clusters (or modules) of common cellular processes. However, within these networks some gene clusters are shared amongst all four stimuli. All stimuli modulate the expression of members of the PYR/PYL/RCAR family of ABA receptors. However, they are modulated differentially in a stimulus-specific manner. Of the six members of the PYR/PYL/RCAR family expressed in guard cells, PYL2 is sufficient for guard cell ABA-induced responses, whereas in the responses to CO₂, PYL4 and PYL5 are essential. Overall, our work shows the importance of ABA as a central regulator and integrator of long-term changes in stomatal behaviour, including sensitivity, elicited by external signals. Understanding this architecture may aid in breeding crops with improved water and nutrient efficiency.

Response of photosynthesis, growth and water relations of a savannah-adapted tree and grass grown across high to low CO2

BACKGROUND AND AIMS

By the year 2100, atmospheric CO2 concentration ([CO2]a) could reach 800 ppm, having risen from ~200 ppm since the Neogene, beginning ~24 Myr ago. Changing [CO2]a affects plant carbon-water balance, with implications for growth, drought tolerance and vegetation shifts. The evolution of C4 photosynthesis improved plant hydraulic function under low [CO2]a and preluded the establishment of savannahs, characterized by rapid transitions between open C4-dominated grassland with scattered trees and closed forest. Understanding directional vegetation trends in response to environmental change will require modelling. But models are often parameterized with characteristics observed in plants under current climatic conditions, necessitating experimental quantification of the mechanistic underpinnings of plant acclimation to [CO2]a.

METHODS

We measured growth, photosynthesis and plant-water relations, within wetting-drying cycles, of a C3 tree (Vachellia karroo, an acacia) and a C4 grass (Eragrostis curvula) grown at 200, 400 or 800 ppm [CO2]a. We investigated the mechanistic linkages between trait responses to [CO2]a under moderate soil drying, and photosynthetic characteristics.

KEY RESULTS

For V. karroo, higher [CO2]a increased assimilation, foliar carbon:nitrogen, biomass and leaf starch, but decreased stomatal conductance and root starch. For Eragrostis, higher [CO2]a decreased C:N, did not affect assimilation, biomass or starch, and markedly decreased stomatal conductance. Together, this meant that C4 advantages in efficient water-use over the tree were maintained with rising [CO2]a.

CONCLUSIONS

Acacia and Eragrostis acclimated differently to [CO2]a, with implications for their respective responses to water limitation and environmental change. Our findings question the carbon-centric focus on factors limiting assimilation with changing [CO2]a, how they are predicted and their role in determining productivity. We emphasize the continuing importance of water-conserving strategies in the assimilation response of savannah plants to rising [CO2]a.

A leaf-level biochemical model simulating the introduction of C2 and C4 photosynthesis in C3 rice: gains, losses and metabolite fluxes

This work aims at developing an adequate theoretical basis for comparing assimilation of the ancestral C₃ pathway with CO₂ concentrating mechanisms (CCM) that have evolved to reduce photorespiratory yield losses. We present a novel model for C₃ , C₂ , C₂  + C₄ and C₄ photosynthesis simulating assimilatory metabolism, energetics and metabolite traffic at the leaf level. It integrates a mechanistic description of light reactions to simulate ATP and NADPH production, and a variable engagement of cyclic electron flow. The analytical solutions are compact and thus suitable for larger scale simulations. Inputs were derived with a comprehensive gas-exchange experiment. We show trade-offs in the operation of C₄ that are in line with ecophysiological data. C₄ has the potential to increase assimilation over C₃ at high temperatures and light intensities, but this benefit is reversed under low temperatures and light. We apply the model to simulate the introduction of progressively complex levels of CCM into C₃ rice, which feeds > 3.5 billion people. Increasing assimilation will require considerable modifications such as expressing the NAD(P)H Dehydrogenase-like complex and upregulating cyclic electron flow, enlarging the bundle sheath, and expressing suitable transporters to allow adequate metabolite traffic. The simpler C₂ rice may be a desirable alternative.

Stomatal Development and Conductance of a Tropical Forage Legume Are Regulated by Elevated [CO2] Under Moderate Warming

The opening and closing of stomata are controlled by the integration of environmental and endogenous signals. Here, we show the effects of combining elevated atmospheric carbon dioxide concentration (eCO 2; 600 μmol mol-1) and warming (+2°C) on stomatal properties and their consequence to plant function in a Stylosanthes capitata Vogel (C3) tropical pasture. The eCO 2 treatment alone reduced stomatal density, stomatal index, and stomatal conductance (gs ), resulting in reduced transpiration, increased leaf temperature, and leading to maintenance of soil moisture during the growing season. Increased CO2 concentration inside leaves stimulated photosynthesis, starch content levels, water use efficiency, and PSII photochemistry. Under warming, plants developed leaves with smaller stomata on both leaf surfaces; however, we did not see effects of warming on stomatal conductance, transpiration, or leaf water status. Warming alone enhanced PSII photochemistry and photosynthesis, and likely starch exports from chloroplasts. Under the combination of warming and eCO 2, leaf temperature was higher than that of leaves from the warming or eCO 2 treatments. Thus, warming counterbalanced the effects of CO2 on transpiration and soil water content but not on stomatal functioning, which was independent of temperature treatment. Under warming, and in combination with eCO 2, leaves also produced more carotenoids and a more efficient heat and fluorescence dissipation. Our combined results suggest that control on stomatal opening under eCO 2 was not changed by a warmer environment; however, their combination significantly improved whole-plant functioning.

Impact of Stomatal Density and Morphology on Water-Use Efficiency in a Changing World

Global warming and associated precipitation changes will negatively impact on many agricultural ecosystems. Major food production areas are expected to experience reduced water availability and increased frequency of drought over the coming decades. In affected areas, this is expected to reduce the production of important food crops including wheat, rice, and maize. The development of crop varieties able to sustain or improve yields with less water input is, therefore, a priority for crop research. Almost all water used for plant growth is lost to the atmosphere by transpiration through stomatal pores on the leaf epidermis. By altering stomatal pore apertures, plants are able to optimize their CO2 uptake for photosynthesis while minimizing water loss. Over longer periods, stomatal development may also be adjusted, with stomatal size and density being adapted to suit the prevailing conditions. Several approaches to improve drought tolerance and water-use efficiency through the modification of stomatal traits have been tested in the model plant Arabidopsis thaliana. However, there is surprisingly little known about the stomata of crop species. Here, we review the current understanding of how stomatal number and morphology are involved in regulating water-use efficiency. Moreover, we discuss the potential and limitations of manipulating stomatal development to increase drought tolerance and to reduce water loss in crops as the climate changes.

Pores for Thought: Can Genetic Manipulation of Stomatal Density Protect Future Rice Yields?

Rice (Oryza sativa L.) contributes to the diets of around 3.5 billion people every day and is consumed more than any other plant. Alarmingly, climate predictions suggest that the frequency of severe drought and high-temperature events will increase, and this is set to threaten the global rice supply. In this review, we consider whether water or heat stresses in crops - especially rice - could be mitigated through alterations to stomata; minute pores on the plant epidermis that permit carbon acquisition and regulate water loss. In the short-term, water loss is controlled via alterations to the degree of stomatal "openness", or, in the longer-term, by altering the number (or density) of stomata that form. A range of molecular components contribute to the regulation of stomatal density, including transcription factors, plasma membrane-associated proteins and intercellular and extracellular signaling molecules. Much of our existing knowledge relating to stomatal development comes from research conducted on the model plant, Arabidopsis thaliana. However, due to the importance of cereal crops to global food supply, studies on grass stomata have expanded in recent years, with molecular-level discoveries underscoring several divergent developmental and morphological features. Cultivation of rice is particularly water-intensive, and there is interest in developing varieties that require less water yet still maintain grain yields. This could be achieved by manipulating stomatal development; a crop with fewer stomata might be more conservative in its water use and therefore more capable of surviving periods of water stress. However, decreasing stomatal density might restrict the rate of CO₂ uptake and reduce the extent of evaporative cooling, potentially leading to detrimental effects on yields. Thus, the extent to which crop yields in the future climate will be affected by increasing or decreasing stomatal density should be determined. Here, our current understanding of the regulation of stomatal development is summarised, focusing particularly on the genetic mechanisms that have recently been described for rice and other grasses. Application of this knowledge toward the creation of "climate-ready" rice is discussed, with attention drawn to the lesser-studied molecular elements whose contributions to the complexity of grass stomatal development must be understood to advance efforts.

The BIG protein distinguishes the process of CO2 -induced stomatal closure from the inhibition of stomatal opening by CO2

We conducted an infrared thermal imaging-based genetic screen to identify Arabidopsis mutants displaying aberrant stomatal behavior in response to elevated concentrations of CO₂ . This approach resulted in the isolation of a novel allele of the Arabidopsis BIG locus (At3g02260) that we have called CO₂ insensitive 1 (cis1). BIG mutants are compromised in elevated CO₂ -induced stomatal closure and bicarbonate activation of S-type anion channel currents. In contrast with the wild-type, they fail to exhibit reductions in stomatal density and index when grown in elevated CO₂ . However, like the wild-type, BIG mutants display inhibition of stomatal opening when exposed to elevated CO₂ . BIG mutants also display wild-type stomatal aperture responses to the closure-inducing stimulus abscisic acid (ABA). Our results indicate that BIG is a signaling component involved in the elevated CO₂ -mediated control of stomatal development. In the control of stomatal aperture by CO₂ , BIG is only required in elevated CO₂ -induced closure and not in the inhibition of stomatal opening by this environmental signal. These data show that, at the molecular level, the CO₂ -mediated inhibition of opening and promotion of stomatal closure signaling pathways are separable and BIG represents a distinguishing element in these two CO₂ -mediated responses.

Interannual variations in needle and sapwood traits of Pinus edulis branches under an experimental drought

In the southwestern USA, recent large-scale die-offs of conifers raise the question of their resilience and mortality under droughts. To date, little is known about the interannual structural response to droughts. We hypothesized that piñon pines (Pinus edulis) respond to drought by reducing the drop of leaf water potential in branches from year to year through needle morphological adjustments. We tested our hypothesis using a 7-year experiment in central New Mexico with three watering treatments (irrigated, normal, and rain exclusion). We analyzed how variation in "evaporative structure" (needle length, stomatal diameter, stomatal density, stomatal conductance) responded to watering treatment and interannual climate variability. We further analyzed annual functional adjustments by comparing yearly addition of needle area (LA) with yearly addition of sapwood area (SA) and distance to tip (d), defining the yearly ratios SA:LA and SA:LA/d. Needle length (l) increased with increasing winter and monsoon water supply, and showed more interannual variability when the soil was drier. Stomatal density increased with dryness, while stomatal diameter was reduced. As a result, anatomical maximal stomatal conductance was relatively invariant across treatments. SA:LA and SA:LA/d showed significant differences across treatments and contrary to our expectation were lower with reduced water input. Within average precipitation ranges, the response of these ratios to soil moisture was similar across treatments. However, when extreme soil drought was combined with high VPD, needle length, SA:LA and SA:LA/d became highly nonlinear, emphasizing the existence of a response threshold of combined high VPD and dry soil conditions. In new branch tissues, the response of annual functional ratios to water stress was immediate (same year) and does not attempt to reduce the drop of water potential. We suggest that unfavorable evaporative structural response to drought is compensated by dynamic stomatal control to maximize photosynthesis rates.

Genome downsizing, physiological novelty, and the global dominance of flowering plants

The abrupt origin and rapid diversification of the flowering plants during the Cretaceous has long been considered an “abominable mystery.” While the cause of their high diversity has been attributed largely to coevolution with pollinators and herbivores, their ability to outcompete the previously dominant ferns and gymnosperms has been the subject of many hypotheses. Common among these is that the angiosperms alone developed leaves with smaller, more numerous stomata and more highly branching venation networks that enable higher rates of transpiration, photosynthesis, and growth. Yet, how angiosperms pack their leaves with smaller, more abundant stomata and more veins is unknown but linked—we show—to simple biophysical constraints on cell size. Only angiosperm lineages underwent rapid genome downsizing during the early Cretaceous period, which facilitated the reductions in cell size necessary to pack more veins and stomata into their leaves, effectively bringing actual primary productivity closer to its maximum potential. Thus, the angiosperms' heightened competitive abilities are due in no small part to genome downsizing.

The angiosperms, commonly referred to as the flowering plants, are the dominant plants in most terrestrial ecosystems, but how they came to be so successful is considered one of the most profound mysteries in evolutionary biology. Prevailing hypotheses have suggested that the angiosperms rose to dominance through an increase in their maximum potential photosynthesis and whole-plant carbon gain, allowing them to outcompete the ferns and gymnosperms that had previously dominated terrestrial ecosystems. Using a combination of anatomy, cytology, and modelling of liquid water transport and CO₂ exchange between leaves and the atmosphere, we now provide strong evidence that the success and rapid spread of flowering plants around the world was the result of genome downsizing. Smaller genomes permit the construction of smaller cells that allow for greater CO₂ uptake and photosynthetic carbon gain. Genome downsizing occurred only among the angiosperms, and we propose that it was a necessary prerequisite for rapid growth rates among land plants.

A Dynamic Hydro-Mechanical and Biochemical Model of Stomatal Conductance for C4 Photosynthesis

C4 plants are major grain (maize [Zea mays] and sorghum [Sorghum bicolor]), sugar (sugarcane [Saccharum officinarum]), and biofuel (Miscanthus spp.) producers and contribute ∼20% to global productivity. Plants lose water through stomatal pores in order to acquire CO2 (assimilation [A]) and control their carbon-for-water balance by regulating stomatal conductance (gS). The ability to mechanistically predict gS and A in response to atmospheric CO2, water availability, and time is critical for simulating stomatal control of plant-atmospheric carbon and water exchange under current, past, or future environmental conditions. Yet, dynamic mechanistic models for gS are lacking, especially for C4 photosynthesis. We developed and coupled a hydromechanical model of stomatal behavior with a biochemical model of C4 photosynthesis, calibrated using gas-exchange measurements in maize, and extended the coupled model with time-explicit functions to predict dynamic responses. We demonstrated the wider applicability of the model with three additional C4 grass species in which interspecific differences in stomatal behavior could be accounted for by fitting a single parameter. The model accurately predicted steady-state responses of gS to light, atmospheric CO2 and oxygen, soil drying, and evaporative demand as well as dynamic responses to light intensity. Further analyses suggest that the effect of variable leaf hydraulic conductance is negligible. Based on the model, we derived a set of equations suitable for incorporation in land surface models. Our model illuminates the processes underpinning stomatal control in C4 plants and suggests that the hydraulic benefits associated with fast stomatal responses of C4 grasses may have supported the evolution of C4 photosynthesis.

Stomatal Function across Temporal and Spatial Scales: Deep-Time Trends, Land-Atmosphere Coupling and Global Models

Simulating global fluxes of water, carbon, and energy at the land surface requires accurate and versatile models of stomatal conductance, currently represented by structurally similar and interchangeable forms that share weaknesses at environmental extremes.

Hornwort Stomata: Architecture and Fate Shared with 400-Million-Year-Old Fossil Plants without Leaves

As one of the earliest plant groups to evolve stomata, hornworts are key to understanding the origin and function of stomata. Hornwort stomata are large and scattered on sporangia that grow from their bases and release spores at their tips. We present data from development and immunocytochemistry that identify a role for hornwort stomata that is correlated with sporangial and spore maturation. We measured guard cells across the genera with stomata to assess developmental changes in size and to analyze any correlation with genome size. Stomata form at the base of the sporophyte in the green region, where they develop differential wall thickenings, form a pore, and die. Guard cells collapse inwardly, increase in surface area, and remain perched over a substomatal cavity and network of intercellular spaces that is initially fluid filled. Following pore formation, the sporophyte dries from the outside inwardly and continues to do so after guard cells die and collapse. Spore tetrads develop in spore mother cell walls within a mucilaginous matrix, both of which progressively dry before sporophyte dehiscence. A lack of correlation between guard cell size and DNA content, lack of arabinans in cell walls, and perpetually open pores are consistent with the inactivity of hornwort stomata. Stomata are expendable in hornworts, as they have been lost twice in derived taxa. Guard cells and epidermal cells of hornworts show striking similarities with the earliest plant fossils. Our findings identify an architecture and fate of stomata in hornworts that is ancient and common to plants without sporophytic leaves.

Evolution of the Stomatal Regulation of Plant Water Content

Changes in the function of stomata from the earliest bryophytes to derived angiosperms are examined.

Vein density is independent of epidermal cell size in Arabidopsis mutants

Densities of leaf minor veins and stomata are co-ordinated within and across vascular plants. This maximises the benefit-to-cost ratio of leaf construction by ensuring stomata receive the minimum amount of water required to maintain optimal aperture. A 'passive dilution' mechanism in which densities of veins and stomata are co-regulated by epidermal cell size is thought to facilitate this co-ordination. However, unlike stomata, veins are spatially isolated from the epidermis and thus may not be directly regulated by epidermal cell expansion. Here, we use mutant genotypes of Arabidopsis thaliana (L.) Heynh. with altered stomatal and epidermal cell development to test this mechanism. To do this we compared observed relationships between vein density and epidermal cell size with modelled relationships that assume veins and stomata are passively diluted by epidermal cell expansion. Data from wild-type plants were consistent with the 'passive dilution' mechanism, but in mutant genotypes vein density was independent of epidermal cell size. Hence, vein density is not causally linked to epidermal cell expansion. This suggests that adaptation favours synchronised changes to the cell size of different leaf tissues to coordinate veins and stomata, and thus balance water supply with transpirational demand.

Effects of elevated carbon dioxide on stomatal characteristics and carbon isotope ratio of Arabidopsis thaliana ecotypes originating from an altitudinal gradient

Stomatal functioning regulates the fluxes of CO₂ and water vapor between vegetation and atmosphere and thereby influences plant adaptation to their habitats. Stomatal traits are controlled by external environmental and internal cellular signaling. The objective of this study was to quantify the effects of CO₂ enrichment (CE) on stomatal density (SD)-related properties, guard cell length (GCL) and carbon isotope ratio (δ¹³ C) of a range of Arabidopsis thaliana ecotypes originating from a wide altitudinal range [50-1260 m above sea level (asl)], and grown at 400 and 800 ppm [CO₂ ], and thereby elucidate the possible adaptation and acclimation responses controlling stomatal traits and water use efficiency (WUE). There was a highly significant variation among ecotypes in the magnitude and direction of response of stomatal traits namely, SD and stomatal index (SI) and GCL, and δ¹³ C to CE, which represented a short-term acclimation response. A majority of ecotypes showed increased SD and SI with CE with the response not depending on the altitude of origin. Significant ecotypic variation was shown in all stomatal traits and δ¹³ C at each [CO₂ ]. At 400 ppm, means of SD, SI and GCL for broad altitudinal ranges, i.e. low (<100 m), mid (100-400 m) and high (>400 m), increased with increasing altitude, which represented an adaptation response to decreased availability of CO₂ with altitude. δ¹³ C was negatively correlated to SD and SI at 800 ppm but not at 400 ppm. Our results highlight the diversity in the response of key stomatal characters to CE and altitude within the germplasm of A. thaliana and the need to consider this diversity when using A. thaliana as a model plant.

Cell expansion not cell differentiation predominantly co-ordinates veins and stomata within and among herbs and woody angiosperms grown under sun and shade

BACKGROUND AND AIMS

It has been proposed that modification of leaf size, driven by epidermal cell size, balances leaf water supply (determined by veins) with transpirational demand (generated by stomata) during acclimation to local irradiance. We aimed to determine whether this is a general pattern among plant species with contrasting growth habits.

METHODS

We compared observed relationships between leaf minor vein density, stomatal density, epidermal cell size and leaf size in four pairs of herbs and woody species from the same families grown under sun and shade conditions with modelled relationships assuming vein and stomatal densities respond passively to epidermal cell expansion. Leaf lignin content was also quantified to assess whether construction costs of herbaceous leaf veins differ from those of woody plants and the leaf mass fraction invested in veins.

KEY RESULTS

Modelled relationships accurately described observed relationships, indicating that in all species, co-ordinated changes to the density of minor veins and stomata were mediated by a common relationship between epidermal cell size, vein density and stomatal density, with little or no impact from stomatal index. This co-ordination was independent of changes in leaf size and is likely to be an adaptive process driven by the significant proportion of biomass invested in veins (13·1 % of sun leaf dry weight and 21·7 % of shade leaf dry weight). Relative costs of venation increased in the shade, intensifying selective pressure towards economizing investment in vein density.

CONCLUSIONS

Modulation of epidermal cell size appears to be a general mechanism among our experimental species to maintain a constant ratio between leaf anatomical traits that control leaf water fluxes independently of habit. We propose that this process may co-ordinate plasticity in hydraulic supply and demand in the majority of eudicot angiosperms.

Multi-Year Leaf-Level Response to Sub-Ambient and Elevated Experimental CO2 in Betula nana

The strong link between stomatal frequency and CO₂ in woody plants is key for understanding past CO₂ dynamics, predicting future change, and evaluating the significant role of vegetation in the hydrological cycle. Experimental validation is required to evaluate the long-term adaptive leaf response of C3 plants to CO₂ conditions; however, studies to date have only focused on short-term single-season experiments and may not capture (1) the full ontogeny of leaves to experimental CO₂ exposure or (2) the true adjustment of structural stomatal properties to CO₂, which we postulate is likely to occur over several growing seasons. We conducted controlled growth chamber experiments at 150 ppmv, 450 ppmv and 800 ppmv CO₂ with woody C3 shrub Betula nana (dwarf birch) over two successive annual growing seasons and evaluated the structural stomatal response to atmospheric CO₂ conditions. We find that while some adjustment of leaf morphological and stomatal parameters occurred in the first growing season where plants are exposed to experimental CO₂ conditions, amplified adjustment of non-plastic stomatal properties such as stomatal conductance occurred in the second year of experimental CO₂ exposure. We postulate that the species response limit to CO₂ of B. nana may occur around 400–450 ppmv. Our findings strongly support the necessity for multi-annual experiments in C3 perennials in order to evaluate the effects of environmental conditions and provide a likely explanation of the contradictory results between historical and palaeobotanical records and experimental data.

Optimal allocation of leaf epidermal area for gas exchange

A long-standing research focus in phytology has been to understand how plants allocate leaf epidermal space to stomata in order to achieve an economic balance between the plant's carbon needs and water use. Here, we present a quantitative theoretical framework to predict allometric relationships between morphological stomatal traits in relation to leaf gas exchange and the required allocation of epidermal area to stomata. Our theoretical framework was derived from first principles of diffusion and geometry based on the hypothesis that selection for higher anatomical maximum stomatal conductance (gsmax ) involves a trade-off to minimize the fraction of the epidermis that is allocated to stomata. Predicted allometric relationships between stomatal traits were tested with a comprehensive compilation of published and unpublished data on 1057 species from all major clades. In support of our theoretical framework, stomatal traits of this phylogenetically diverse sample reflect spatially optimal allometry that minimizes investment in the allocation of epidermal area when plants evolve towards higher gsmax . Our results specifically highlight that the stomatal morphology of angiosperms evolved along spatially optimal allometric relationships. We propose that the resulting wide range of viable stomatal trait combinations equips angiosperms with developmental and evolutionary flexibility in leaf gas exchange unrivalled by gymnosperms and pteridophytes.

Morphological features of different polyploids for adaptation and molecular characterization of CC-NBS-LRR and LEA gene families in Agave L

Polyploidy has been widely described in many Agave L. species, but its influence on environmental response to stress is still unknown. With the objective of knowing the morphological adaptations and regulation responses of genes related to biotic (LEA) and abiotic (NBS-LRR) stress in species of Agave with different levels of ploidy, and how these factors contribute to major response of Agave against environmental stresses, we analyzed 16 morphological trials on five accessions of three species (Agave tequilana Weber, Agave angustifolia Haw. and Agave fourcroydes Lem.) with different ploidy levels (2n=2x=60 2n=3x=90, 2n=5x=150, 2n=6x=180) and evaluated the expression of NBS-LRR and LEA genes regulated by biotic and abiotic stress. It was possible to associate some morphological traits (spines, nuclei, and stomata) to ploidy level. The genetic characterization of stress-related genes NBS-LRR induced by pathogenic infection and LEA by heat or saline stresses indicated that amino acid sequence analysis in these genes showed more substitutions in higher ploidy level accessions of A. fourcroydes Lem. 'Sac Ki' (2n=5x=150) and A. angustifolia Haw. 'Chelem Ki' (2n=6x=180), and a higher LEA and NBS-LRR representativeness when compared to their diploid and triploid counterparts. In all studied Agave accessions expression of LEA and NBS-LRR genes was induced by saline or heat stresses or by infection with Erwinia carotovora, respectively. The transcriptional activation was also higher in A. angustifolia Haw. 'Chelem Ki' (2n=6x=180) and A. fourcroydes 'Sac Ki' (2n=5x=150) than in their diploid and triploid counterparts, which suggests higher adaptation to stress. Finally, the diploid accession A. tequilana Weber 'Azul' showed a differentiated genetic profile relative to other Agave accessions. The differences include similar or higher genetic representativeness and transcript accumulation of LEA and NBS-LRR genes than in polyploid (2n=5x=150 and 2n=6x=180) Agave accessions, thus suggesting a differentiated selection pressure for overcoming the lower ploidy level of the diploid A. tequilana Weber 'Azul'.

Gas valves, forests and global change: a commentary on Jarvis (1976) 'The interpretation of the variations in leaf water potential and stomatal conductance found in canopies in the field'

Microscopic turgor-operated gas valves on leaf surfaces-stomata-facilitate gas exchange between the plant and the atmosphere, and respond to multiple environmental and endogenous cues. Collectively, stomatal activities affect everything from the productivity of forests, grasslands and crops to biophysical feedbacks between land surface vegetation and climate. In 1976, plant physiologist Paul Jarvis reported an empirical model describing stomatal responses to key environmental and plant conditions that predicted the flux of water vapour from leaves into the surrounding atmosphere. Subsequent theoretical advances, building on this earlier approach, established the current paradigm for capturing the physiological behaviour of stomata that became incorporated into sophisticated models of land carbon cycling. However, these models struggle to accurately predict observed trends in the physiological responses of Northern Hemisphere forests to recent atmospheric CO2 increases, highlighting the need for improved representation of the role of stomata in regulating forest-climate interactions. Bridging this gap between observations and theory as atmospheric CO2 rises and climate change accelerates creates challenging opportunities for the next generation of physiologists to advance planetary ecology and climate science. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.

Increasing water-use efficiency directly through genetic manipulation of stomatal density

Improvement in crop water-use efficiency (WUE) is a critical priority for regions facing increased drought or diminished groundwater resources. Despite new tools for the manipulation of stomatal development, the engineering of plants with high WUE remains a challenge. We used Arabidopsis epidermal patterning factor (EPF) mutants exhibiting altered stomatal density to test whether WUE could be improved directly by manipulation of the genes controlling stomatal density. Specifically, we tested whether constitutive overexpression of EPF2 reduced stomatal density and maximum stomatal conductance (gw(max) ) sufficiently to increase WUE. We found that a reduction in gw(max) via reduced stomatal density in EPF2-overexpressing plants (EPF2OE) increased both instantaneous and long-term WUE without altering significantly the photosynthetic capacity. Conversely, plants lacking both EPF1 and EPF2 expression (epf1epf2) exhibited higher stomatal density, higher gw(max) and lower instantaneous WUE, as well as lower (but not significantly so) long-term WUE. Targeted genetic modification of stomatal conductance, such as in EPF2OE, is a viable approach for the engineering of higher WUE in crops, particularly in future high-carbon-dioxide (CO2 ) atmospheres.

Spatial heterogeneity in stomatal features during leaf elongation: an analysis using Rosa hybrida

Within-leaf heterogeneity in stomatal traits poses a key uncertainty in determining a representative value for the whole leaf. Accounting for this heterogeneity, we studied stomatal initiation on expanding leaves and estimated stomatal conductance (gs) of mature leaves. The entire lamina was evaluated at four percentages of full leaflet elongation (FLE; leaflet length relative to its final length) in Rosa hybrida L. plants grown at 60% relative air humidity (RH), and at 100% FLE following cultivation at elevated (95%) RH. Over 80% of the stomata were initiated between 33 and 67% FLE, whereas stomatal growth mostly occurred afterwards. At 100% FLE, the heterogeneity in stomatal density was the result of uneven stomatal differentiation, while an uneven differentiation of epidermal cells contributed to this variation only at elevated RH. Noticeable within-leaf differences (up to 40%) in gs were calculated at 100% FLE. Avoiding leaflet periphery decreased this heterogeneity. Despite the large promotive effect of elevated RH on stomatal and pore dimensions, the within-leaf variation remained unaffected in all characters, besides pore aperture (and, thus, gs). The noted level of within-leaf variation in stomatal features demands a sampling scheme tailored to the leaf developmental stage, the feature per se and the evaporative demand during growth.

Increasing leaf hydraulic conductance with transpiration rate minimizes the water potential drawdown from stem to leaf

Leaf hydraulic conductance (k leaf) is a central element in the regulation of leaf water balance but the properties of k leaf remain uncertain. Here, the evidence for the following two models for k leaf in well-hydrated plants is evaluated: (i) k leaf is constant or (ii) k leaf increases as transpiration rate (E) increases. The difference between stem and leaf water potential (ΔΨstem-leaf), stomatal conductance (g s), k leaf, and E over a diurnal cycle for three angiosperm and gymnosperm tree species growing in a common garden, and for Helianthus annuus plants grown under sub-ambient, ambient, and elevated atmospheric CO₂ concentration were evaluated. Results show that for well-watered plants k leaf is positively dependent on E. Here, this property is termed the dynamic conductance, k leaf(E), which incorporates the inherent k leaf at zero E, which is distinguished as the static conductance, k leaf(0). Growth under different CO₂ concentrations maintained the same relationship between k leaf and E, resulting in similar k leaf(0), while operating along different regions of the curve owing to the influence of CO₂ on g s. The positive relationship between k leaf and E minimized variation in ΔΨstem-leaf. This enables leaves to minimize variation in Ψleaf and maximize g s and CO₂ assimilation rate over the diurnal course of evaporative demand.

Pore size regulates operating stomatal conductance, while stomatal densities drive the partitioning of conductance between leaf sides

BACKGROUND AND AIMS

Leaf gas exchange is influenced by stomatal size, density, distribution between the leaf adaxial and abaxial sides, as well as by pore dimensions. This study aims to quantify which of these traits mainly underlie genetic differences in operating stomatal conductance (gs) and addresses possible links between anatomical traits and regulation of pore width.

METHODS

Stomatal responsiveness to desiccation, gs-related anatomical traits of each leaf side and estimated gs (based on these traits) were determined for 54 introgression lines (ILs) generated by introgressing segments of Solanum pennelli into the S. lycopersicum 'M82'. A quantitative trait locus (QTL) analysis for stomatal traits was also performed.

KEY RESULTS

A wide genetic variation in stomatal responsiveness to desiccation was observed, a large part of which was explained by stomatal length. Operating gs ranged over a factor of five between ILs. The pore area per stomatal area varied 8-fold among ILs (2-16 %), and was the main determinant of differences in operating gs between ILs. Operating gs was primarily positioned on the abaxial surface (60-83 %), due to higher abaxial stomatal density and, secondarily, to larger abaxial pore area. An analysis revealed 64 QTLs for stomatal traits in the ILs, most of which were in the direction of S. pennellii.

CONCLUSIONS

The data indicate that operating and maximum gs of non-stressed leaves maintained under stable conditions deviate considerably (by 45-91 %), because stomatal size inadequately reflects operating pore area (R(2) = 0·46). Furthermore, it was found that variation between ILs in both stomatal sensitivity to desiccation and operating gs is associated with features of individual stoma. In contrast, genotypic variation in gs partitioning depends on the distribution of stomata between the leaf adaxial and abaxial epidermis.

Environmental adaptation in stomatal size independent of the effects of genome size

Cell sizes are linked across multiple tissues, including stomata, and this variation is closely correlated with genome size. These associations raise the question of whether generic changes in cell size cause suboptimal changes in stomata, requiring subsequent evolution under selection for stomatal size. We tested the relationships among guard cell length, genome size and vegetation type using phylogenetically independent analyses on 67 species of the ecologically and structurally diverse family, Proteaceae. We also compared how genome and stomatal sizes varied at ancient (among genera) and more recent (within genus) levels. The observed 60-fold range in genome size in Proteaceae largely reflected the mean chromosome size. Compared with variation among genera, genome size varied much less within genera (< 6% of total variance) than stomatal size, implying evolution in stomatal size subsequent to changes in genome size. Open vegetation and closed forest had significantly different relationships between stomatal and genome sizes. Ancient changes in genome size clearly influenced stomatal size in Proteaceae, but adaptation to habitat strongly modified the genome-stomatal size relationship. Direct adaptation to the environment in stomatal size argues that new proxies for past concentrations of atmospheric CO2 that incorporate stomatal size are superior to older models based solely on stomatal frequency.

Are fern stomatal responses to different stimuli coordinated? Testing responses to light, vapor pressure deficit, and CO2 for diverse species grown under contrasting irradiances

The stomatal behavior of ferns provides an excellent system for disentangling responses to different environmental signals, which balance carbon gain against water loss. Here, we measured responses of stomatal conductance (gs ) to irradiance, CO2 , and vapor pressure deficit (VPD) for 13 phylogenetically diverse species native to open and shaded habitats, grown under high- and low-irradiance treatments. We tested two main hypotheses: that plants adapted and grown in high-irradiance environments would have greater responsiveness to all stimuli given higher flux rates; and that species' responsiveness to different factors would be correlated because of the relative simplicity of fern stomatal control. We found that species with higher light-saturated gs had larger responses, and that plants grown under high irradiance were more responsive to all stimuli. Open habitat species showed greater responsiveness to irradiance and CO2 , but lower responsiveness to VPD; a case of plasticity and adaptation tending in different directions. Responses of gs to irradiance and VPD were positively correlated across species, but CO2 responses were independent and highly variable. The novel finding of correlations among stomatal responses to different stimuli suggests coordination of hydraulic and photosynthetic signaling networks modulating fern stomatal responses, which show distinct optimization at growth and evolutionary time-scales.

Patterning and processes: how stomatal development defines physiological potential

Stomata present an excellent opportunity for connecting scientific disciplines: they are governed by complex genetic controls and unique cell biology, while also possessing a large influence over plant productivity and relationships with the environment. For this reason, stomata have engaged scientists for many centuries and continue to be a central interest for many fields of research. Recent technological advances have enabled interdisciplinary studies of stomata that were previously out of reach, and as a result, we are beginning to realize new insights about stomatal biology that place them at the intersection of our changing world. This review is intended to describe these interdisciplinary connections, discuss the relevant scales at which they are having an influence, and highlight ways we can capitalize on such novel approaches. While we incorporate knowledge about molecular advances, this is not intended to be an extensive review of that field, but rather, we focus on how those systems inform plant physiology and are connected to global scales.