Leaf Hydraulic Architecture and Stomatal Conductance: A Functional Perspective

Differential impact of commercial rootstocks on the physiological response of a common walnut scion to drought stress

Walnut rootstocks are commonly used in California orchards to provide resistance to soil-borne pests and diseases. However, little information exists about the impact of commercial rootstock on the common scion's physiological response under drought. This is becoming increasingly important since walnuts are commonly cultivated in semi-arid regions where frequent and severe droughts require efficient water use. We previously reported that own-rooted walnut rootstocks (RX1, VX211 and Vlach) differ in their physiological performance under drought. Here, we evaluated whether similar water relations and performance are conferred to a common English walnut scion (Juglans regia cv. Cisco). To do so, we used a mini-lysimeter platform to continuously track soil moisture and transpirational water loss from trees. Along with the canopy's estimated leaf area, changes in canopy shape and texture were evaluated using deep learning as an independent method to analyze canopy response to water stress. In support of our recent findings, the scion grafted onto rootstock RX1 exhibited subtle improvements in physiological performance associated with higher transpiration and canopy conductance under well-watered condition compared to Vlach and VX211 rootstocks. Canopy conductance, texture, and shape were not significantly affected by rootstock under water stress. However, Cisco grafted onto RX1 exhibited higher leaf turgor and water use efficiency, and lower osmotic potentials under water stress. Our results suggest some subtle differences in water relations between the rootstock genotypes, and propose an efficient deep-learning method to screen canopies for water stress-induced response through image processing.

Mapping of drought-induced changes in tissue characteristics across the leaf profile of Populus balsamifera

Leaf architecture impacts gas diffusion, biochemical processes, and photosynthesis. For balsam poplar, a widespread North American species, the influence of water availability on leaf anatomy and subsequent photosynthetic performance remains unknown. To address this shortcoming, we characterized the anatomical changes across the leaf profile in three-dimensional space for saplings subjected to soil drying and rewatering using X-ray microcomputed tomography. Our hypothesis was that higher abundance of bundle sheath extensions (BSE) minimizes drought-induced changes in intercellular airspace volume relative to mesophyll volume (i.e. mesophyll porosity, θIAS) and aids recovery by supporting leaf structural integrity. Leaves of 'Carnduff-9' with less abundant BSEs exhibited greater θIAS, higher spongy mesophyll surface area, reduced palisade mesophyll surface area, and less veins compared with 'Gillam-5'. Under drought conditions, Carnduff-9 showed significant changes in θIAS across leaf profile while that was little for 'Gillam-5'. Under rewatered conditions, drought-induced changes in θIAS were fully reversible in 'Gillam-5' but not in 'Carnduff-9'. Our data suggest that a 'robust' leaf structure with higher abundance of BSEs, reduced θIAS, and relatively large mesophyll surface area provides for improved photosynthetic capacity under drought and supports recovery in leaf architecture after rewatering in balsam poplar.

Using the Montgomery-Koyama-Smith equation to calculate the stomatal area per unit lamina area for 12 Magnoliaceae species

BACKGROUND AND AIMS

The Montgomery-Koyama-Smith (MKS) equation predicts that total leaf area per shoot is proportional to the product of the sum of individual leaf widths and maximum individual leaf length, which has been validated for some herbaceous and woody plants. The equation is also predicted to be valid in describing the relationship between the total stomatal area per micrograph (AT) and the product of the sum of individual stomatal widths (denoted as LKS) and maximum individual stomatal length (denoted by WKS) in any particular micrograph.

METHODS

To test the validity of the MKS equation, 69 931 stomata (from 720 stomatal micrographs from 12 Magnoliaceae species) were examined. The area of each stoma was calculated using empirical measurements of stomatal length and width multiplied by a constant. Six equations describing the relationships among AT, LKS and WKS were compared. The root mean square (RMSE) and the Akaike information criterion (AIC) were used to measure the goodness of fit and the trade-off between the goodness of fit and the structural complexity of each model, respectively.

KEY RESULTS

Analyses supported the validity of the MKS equation and the power-law equation AT ∝ (LKSWKS)α, where α is a scaling exponent. The estimated values of α at the species level and for the pooled data were all statistically smaller than unity, which did not support the hypothesis that AT ∝ LKSWKS. The power-law equation had smaller RMSE and AIC values than the MKS equation for the data from the 12 individual species and the pooled data.

CONCLUSIONS

These results indicate that AT tends to scale allometrically with LKSWKS, and that increases in AT do not keep pace with increases in LKSWKS. In addition, using LKSWKS is better than using only one of the two variables to calculate AT.

Relieving the transfusion tissue traffic jam: a network model of radial transport in conifer needles

Characteristic of all conifer needles, the transfusion tissue mediates the radial transport of water and sugar between the endodermis and axial vasculature. Physical constraints imposed by the needle's linear geometry introduce two potential extravascular bottlenecks where the opposition of sugar and water flows may frustrate sugar export: one at the vascular access point and the other at the endodermis. We developed a network model of the transfusion tissue to explore how its structure and composition affect the delivery of sugars to the axial phloem. To describe extravascular transport with cellular resolution, we construct networks from images of Pinus pinea needles obtained through tomographic microscopy, as well as fluorescence and electron microscopy. The transfusion tissue provides physically distinct pathways for sugar and water, reducing resistance between the vasculature and endodermis and mitigating flow constriction at the vascular flank. Dissipation of flow velocities through the transfusion tissue's branched structure allows for bidirectional transport of an inbound diffusive sugar flux against an outbound advective water flux across the endodermis. Our results clarify the structure-function relationships of the transfusion tissue under conditions free of physiological stress. The presented model framework is also applicable to different transfusion tissue morphologies in other gymnosperms.

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.

Dynamic soil hydraulic resistance regulates stomata

The onset of stomatal closure reduces transpiration during drought. In seed plants, drought causes declines in plant water status which increases leaf endogenous abscisic acid (ABA) levels required for stomatal closure. There are multiple possible points of increased belowground resistance in the soil-plant atmospheric continuum that could decrease leaf water potential enough to trigger ABA production and the subsequent decreases in transpiration. We investigate the dynamic patterns of leaf ABA levels, plant hydraulic conductance and the point of failure in the soil-plant conductance in the highly embolism-resistant species Callitris tuberculata using continuous dendrometer measurements of leaf water potential during drought. We show that decreases in transpiration and ABA biosynthesis begin before any permanent decreases in predawn water potential, collapse in soil-plant hydraulic pathway and xylem embolism spread. We find that a dynamic but recoverable increases in hydraulic resistance in the soil in close proximity to the roots is the most likely driver of declines in midday leaf water potential needed for ABA biosynthesis and the onset of decreases in transpiration.

Mechanical advantage makes stomatal opening speed a function of evaporative demand

Stomatal opening in the light, observed in nearly all vascular land plants, is essential for providing access to atmospheric CO2 for photosynthesis. The speed of stomatal opening in the light is critical for maximizing carbon gain in environments in which light intensity changes, yet we have little understanding of how other environmental signals, particularly evaporative demand driven by vapor pressure deficit (VPD) influences the kinetics of this response. In angiosperms, and some fern species from the family Marsileaceae, a mechanical interaction between the guard cells and the epidermal cells determines the aperture of the pore. Here, we examine whether this mechanical interaction influences the speed of stomatal opening in the light. To test this, we investigated the speed of stomatal opening in response to light across a range of VPDs in seven plant species spanning the evolutionary diversity of guard cell and epidermal cell mechanical interactions. We found that stomatal opening speed is a function of evaporative demand in angiosperm species and Marsilea, which have guard cell and epidermal cell mechanical interactions. Stomatal opening speeds did not change across a range of VPD in species of gymnosperm and fern, which do not have guard cell mechanical interactions with the epidermis. We find that guard cell and epidermal cell mechanical interactions may play a key role in regulating stomatal responsiveness to light. These results provide valuable insight into the adaptive relevance of mechanical advantage.

Functional traits of fossil plants

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

Localized measurements of water potential reveal large loss of conductance in living tissues of maize leaves

The water status of the living tissue in leaves between the xylem and stomata (outside xylem zone (OXZ) plays a critical role in plant function and global mass and energy balance but has remained largely inaccessible. We resolve the local water relations of OXZ tissue using a nanogel reporter of water potential (ψ), AquaDust, that enables an in situ, nondestructive measurement of both ψ of xylem and highly localized ψ at the terminus of transpiration in the OXZ. Working in maize (Zea mays L.), these localized measurements reveal gradients in the OXZ that are several folds larger than those based on conventional methods and values of ψ in the mesophyll apoplast well below the macroscopic turgor loss potential. We find a strong loss of hydraulic conductance in both the bundle sheath and the mesophyll with decreasing xylem potential but not with evaporative demand. Our measurements suggest the OXZ plays an active role in regulating the transpiration path, and our methods provide the means to study this phenomenon.

Characterizing the breakpoint of stomatal response to vapor pressure deficit in an angiosperm

Vapor pressure difference between the leaf and atmosphere (VPD) is the most important regulator of daytime transpiration, yet the mechanism driving stomatal responses to an increase in VPD in angiosperms remains unresolved. Here, we sought to characterize the mechanism driving stomatal closure at high VPD in an angiosperm species, particularly testing whether abscisic acid (ABA) biosynthesis could explain the observation of a trigger point for stomatal sensitivity to an increase in VPD. We tracked leaf gas exchange and modeled leaf water potential (Ψl) in leaves exposed to a range of step-increases in VPD in the herbaceous species Senecio minimus Poir. (Asteraceae). We found that mild increases in VPD in this species did not induce stomatal closure because modeled Ψl did not decline below a threshold close to turgor loss point (Ψtlp), but when leaves were exposed to a large increase in VPD, stomata closed as modeled Ψl declined below Ψtlp. Leaf ABA levels were higher in leaves exposed to a step-increase in VPD that caused Ψl to transiently decline below Ψtlp and in which stomata closed compared with leaves in which stomata did not close. We conclude that the stomata of S. minimus are insensitive to VPD until Ψl declines to a threshold that triggers the biosynthesis of ABA and that this mechanism might be common to angiosperms.

Structural and functional leaf diversity lead to variability in photosynthetic capacity across a range of Juglans regia genotypes

Similar to other cropping systems, few walnut cultivars are used as scion in commercial production. Germplasm collections can be used to diversify cultivar options and hold potential for improving crop productivity, disease resistance and stress tolerance. In this study, we explored the anatomical and biochemical bases of photosynthetic capacity and response to water stress in 11 Juglans regia accessions in the U.S. department of agriculture, agricultural research service (USDA-ARS) National Clonal Germplasm. Net assimilation rate (An ) differed significantly among accessions and was greater in lower latitudes coincident with higher stomatal and mesophyll conductances, leaf thickness, mesophyll porosity, gas-phase diffusion, leaf nitrogen and lower leaf mass and stomatal density. High CO2 -saturated assimilation rates led to increases in An under diffusional and biochemical limitations. Greater An was found in lower-latitude accessions native to climates with more frost-free days, greater precipitation seasonality and lower temperature seasonality. As expected, water stress consistently impaired photosynthesis with the highest % reductions in lower-latitude accessions (A3, A5 and A9), which had the highest An under well-watered conditions. However, An for A3 and A5 remained among the highest under dehydration. J. regia accessions, which have leaf structural traits and biochemistry that enhance photosynthesis, could be used as commercial scions or breeding parents to enhance productivity.

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.

Desiccation of the leaf mesophyll and its implications for CO2 diffusion and light processing

Leaves balance CO2 and radiative absorption while maintaining water transport to maximise photosynthesis. Related species with contrasting leaf anatomy can provide insights into inherent and stress-induced links between structure and function for commonly measured leaf traits for important crops. We used two walnut species with contrasting mesophyll anatomy to evaluate these integrated exchange processes under non-stressed and drought conditions using a combination of light microscopy, X-ray microCT, gas exchange, hydraulic conductance, and chlorophyll distribution profiles through leaves. Juglans regia had thicker palisade mesophyll, higher fluorescence in the palisade, and greater low-mesophyll porosity that were associated with greater gas-phase diffusion (gIAS ), stomatal and mesophyll (gm ) conductances and carboxylation capacity. More and highly-packed mesophyll cells and bundle sheath extensions (BSEs) in Juglans microcarpa led to higher fluorescence in the spongy and in proximity to the BSEs. Both species exhibited drought-induced reductions in mesophyll cell volume, yet the associated increases in porosity and gIAS were obscured by declines in biochemical activity that decreased gm . Inherent differences in leaf anatomy between the species were linked to differences in gas exchange, light absorption and photosynthetic capacity, and drought-induced changes in leaf structure impacted performance via imposing species-specific limitations to light absorption, gas exchange and hydraulics.

Spatial distribution characteristics of stomata at the areole level in Michelia cavaleriei var. platypetala (Magnoliaceae)

BACKGROUND AND AIMS

In hierarchically reticulate venation patterns, smaller orders of veins form areoles in which stomata are located. This study aimed to quantify the spatial relationship among stomata at the areole level.

METHODS

For each of 12 leaves of M. cavaleriei var. platypetala, we assumed that stomatal characteristics were symmetrical on either side of the midrib, and divided the leaf surface on one side of the midrib into six layers equidistantly spaced along the apical-basal axis. We then further divided each layer into three positions equidistantly spaced from midrib to leaf margin, resulting in a total of 18 sampling locations. In addition, for 60 leaves, we sampled three positions from midrib to margin within only the widest layer of the leaf. Stomatal density and mean nearest neighbour distance (MNND) were calculated for each section. A replicated spatial point pattern approach quantified stomatal spatial relationships at different distances (0-300 μm).

KEY RESULTS

A tendency towards regular arrangement (inhibition as opposed to attraction or clustering) was observed between stomatal centres at distances <100 μm. Leaf layer (leaf length dimension) had no significant effect on local stomatal density, MNND or the spatial distribution characteristics of stomatal centres. In addition, we did not find greater inhibition at the centre of areoles, and in positions farther from the midrib.

CONCLUSIONS

Spatial inhibition might be caused by the one-cell-spacing rule, resulting in more regular arrangement of stomata, and it was found to exist at distances up to ~100 μm. This work implies that leaf hydraulic architecture, consisting of both vascular and mesophyll properties, is sufficient to prevent important spatial variability in water supply at the areole level.

Coordinated variation in stem and leaf functional traits of temperate broadleaf tree species in the isohydric-anisohydric spectrum

Stomatal regulation serves as an important strategy for plants to adapt to drought. However, the understanding of how complexes of plant-functional traits vary along the continuum from isohydry to anisohydry remains insufficient. In this study, we investigated a proxy of the degree of iso/anisohydry-the water potential at stomatal closure-and a series of functional traits of leaves and branches in 20 temperate broadleaf species planted in an arid limestone habitat in northern China. The results showed that the water potential at stomatal closure was significantly correlated with many functional traits. At the anisohydric end of the spectrum, species had a higher leaf carbon content and vein density, a greater stomatal length, a thicker lower leaf epidermis, higher embolism resistance, higher wood density, a greater Huber value, a greater ratio of fiber wall thickness to xylem lumen diameter, a larger proportion of total fiber wall area to xylem cross-sectional area, a lower water potential at the turgor loss point (TLP), a smaller relative water content at the TLP, a lower osmotic potential at full turgor and a smaller specific leaf area. It is concluded that a continuum of coordination and trade-offs among co-evolved anatomical and physiological traits gives rise to the spectrum from isohydry to anisohydry spanned by the 20 tree species, and the anisohydric species showed stronger stress resistance, with greater investment in stems and leaves than the isohydric species to maintain stomatal opening under drought conditions.

Understanding airspace in leaves: 3D anatomy and directional tortuosity

The leaf intercellular airspace is a tortuous environment consisting of cells of different shapes, packing densities, and orientation, all of which have an effect on the travelling distance of molecules from the stomata to the mesophyll cell surfaces. Tortuosity, the increase in displacement over the actual distance between two points, is typically defined as encompassing the whole leaf airspace, but heterogeneity in pore dimensions and orientation between the spongy and palisade mesophyll likely result in heterogeneity in tortuosity along different axes and would predict longer traveling distance along the path of least tortuosity, such as vertically within the columnar cell matrix of the palisade layer. Here, we compare a previously established geometric method to a random walk approach, novel for this analysis in plant leaves, in four different Eucalyptus species. The random walk method allowed us to quantify directional tortuosity across the whole leaf profile, and separately for the spongy and palisade mesophyll. For all species tortuosity was higher in the palisade mesophyll than the spongy mesophyll and horizontal (parallel to the epidermis) tortuosity was consistently higher than vertical (from epidermis to epidermis) tortuosity. We demonstrate that a random walk approach improves on previous geometric approaches and is valuable for investigating CO₂ and H₂ O transport within leaves.

Differences in leaf anatomy determines temperature response of leaf hydraulic and mesophyll CO2 conductance in phylogenetically related C4 and C3 grass species

Leaf hydraulic and mesophyll CO₂ conductance are both influenced by leaf anatomical traits, however it is poorly understood how the temperature response of these conductances differs between C₄ and C₃ species with distinct leaf anatomy. This study investigated the temperature response of leaf hydraulic conductance (K_leaf ), stomatal (g_s ) and mesophyll (g_m ) conductance to CO₂ , and leaf anatomical traits in phylogenetically related Panicum antidotale (C₄ ) and P. bisulcatum (C₃ ) grasses. The C₄ species had lower hydraulic conductance outside xylem (K_ox ) and K_leaf compared with the C₃ species. However, the C₄ species had higher g_m compared with the C₃ species. Traits associated with leaf water movement, K_leaf and K_ox , increased with temperature more in the C₃ than in the C₄ species, whereas traits related to carbon uptake, A_net and g_m , increased more with temperature in the C₄ than the C₃ species. Our findings demonstrate that, in addition to a CO₂ concentrating mechanism, outside-xylem leaf anatomy in the C₄ species P. antidotale favours lower water movement through the leaf and stomata that provides an additional advantage for greater leaf carbon uptake relative to water loss with increasing leaf temperature than in the C₃ species P. bisulcatum.

Leaf Water Storage and Robustness to Intermittent Drought: A Spatially Explicit Capacitive Model for Leaf Hydraulics

Leaf hydraulic networks play an important role not only in fluid transport but also in maintaining whole-plant water status through transient environmental changes in soil-based water supply or air humidity. Both water potential and hydraulic resistance vary spatially throughout the leaf transport network, consisting of xylem, stomata and water-storage cells, and portions of the leaf areas far from the leaf base can be disproportionately disadvantaged under water stress. Besides the suppression of transpiration and reduction of water loss caused by stomatal closure, the leaf capacitance of water storage, which can also vary locally, is thought to be crucial for the maintenance of leaf water status. In order to study the fluid dynamics in these networks, we develop a spatially explicit, capacitive model which is able to capture the local spatiotemporal changes of water potential and flow rate in monocotyledonous and dicotyledonous leaves. In electrical-circuit analogs described by Ohm's law, we implement linear capacitors imitating water storage, and we present both analytical calculations of a uniform one-dimensional model and numerical simulation methods for general spatially explicit network models, and their relation to conventional lumped-element models. Calculation and simulation results are shown for the uniform model, which mimics key properties of a monocotyledonous grass leaf. We illustrate water status of a well-watered leaf, and the lowering of water potential and transpiration rate caused by excised water source or reduced air humidity. We show that the time scales of these changes under water stress are hugely affected by leaf capacitance and resistances to capacitors, in addition to stomatal resistance. Through this modeling of a grass leaf, we confirm the presence of uneven water distribution over leaf area, and also discuss the importance of considering the spatial variation of leaf hydraulic traits in plant biology.

Stomatal density and mechanics are critical for high productivity: insights from amphibious ferns

Angiosperm dominance in terrestrial landscapes is partially attributable to high photosynthetic capacities. Angiosperms benefit from diverse anatomical and physiological adaptations, making it difficult to determine which factors may have been prerequisites for the evolution of enhanced photosynthetic rates in this group. We employed a novel approach to this problem: comparisons between angiosperms and Marsileaceae, a family of semi-aquatic ferns that are among the only land plants to match angiosperm photosynthetic rates. We found that Marsileaceae have very high stomatal densities and, like angiosperms but unlike all other ferns previously studied, exhibit wrong-way stomatal responses to excision. These results suggest that stomatal density and a little-studied angiosperm trait, the capacity for lateral displacement of guard cells into neighboring epidermal cells, are crucial for facilitating high rates of gas exchange. Our analysis also associates these adaptations in Marsileaceae with an increased risk of excessive water loss during drought. Our findings indicate that evolution in stomatal physiology was a prerequisite for high photosynthetic capacities in vascular plants and a key driver of the abrupt Cretaceous rise of the angiosperms.

Optimization can provide the fundamental link between leaf photosynthesis, gas exchange and water relations

Tight coordination in the photosynthetic, gas exchange and water supply capacities of leaves is a globally conserved trend across land plants. Strong selective constraints on leaf carbon gain create the opportunity to use quantitative optimization theory to understand the connected evolution of leaf photosynthesis and water relations. We developed an analytical optimization model that maximizes the long-term rate of leaf carbon gain, given the carbon costs in building and maintaining stomata, leaf hydraulics and osmotic pressure. Our model demonstrates that selection for optimal gain should drive coordination between key photosynthetic, gas exchange and water relations traits. It also provides predictions of adaptation to drought and the relative costs of key leaf functional traits. Our results show that optimization in terms of carbon gain, given the carbon costs of physiological traits, successfully unites leaf photosynthesis and water relations and provides a quantitative framework to consider leaf functional evolution and adaptation.

Carboniferous plant physiology breaks the mold

How plants have shaped Earth surface feedbacks over geologic time is a key question in botanical and geological inquiry. Recent work has suggested that biomes during the Carboniferous Period contained plants with extraordinary physiological capacity to shape their environment, contradicting the previously dominant view that plants only began to actively moderate the Earth's surface with the rise of angiosperms during the Mesozoic Era. A recently published Viewpoint disputes this recent work, thus here, we document in detail, the mechanistic underpinnings of our modeling and illustrate the extraordinary ecophysiological nature of Carboniferous plants.

Stomatal behavior following mid- or long-term exposure to high relative air humidity: A review

High relative air humidity (RH ≥ 85%) is frequent in controlled environments, and not uncommon in nature. In this review, we examine the high RH effects on plants with a special focus on stomatal characters. All aspects of stomatal physiology are attenuated by elevated RH during leaf expansion (long-term) in C₃ species. These include impaired opening and closing response, as well as weak diel oscillations. Consequently, the high RH-grown plants are not only vulnerable to biotic and abiotic stress, but also undergo a deregulation between CO₂ uptake and water loss. Stomatal behavior of a single leaf is determined by the local microclimate during expansion, and may be different than the remaining leaves of the same plant. No effect of high RH is apparent in C₄ and CAM species, while the same is expected for species with hydropassive stomatal closure. Formation of bigger stomata with larger pores is a universal response to high RH during leaf expansion, whereas the effect on stomatal density appears to be species- and leaf side-specific. Compelling evidence suggests that ABA mediates the high RH-induced stomatal malfunction, as well as the stomatal size increase. Although high RH stimulates leaf ethylene evolution, it remains elusive whether or not this contributes to stomatal malfunction. Most species lose stomatal function following mid-term (4-7 d) exposure to high RH following leaf expansion. Consequently, the regulatory role of ambient humidity on stomatal functionality is not limited to the period of leaf expansion, but holds throughout the leaf life span.

Linking water relations and hydraulics with photosynthesis

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

Two sides to every leaf: water and CO2 transport in hypostomatous and amphistomatous leaves

Leaves with stomata on both upper and lower surfaces, termed amphistomatous, are relatively rare compared with hypostomatous leaves with stomata only on the lower surface. Amphistomaty occurs predominantly in fast-growing herbaceous annuals and in slow-growing perennial shrubs and trees. In this paper, we present the current understanding and hypotheses on the costs and benefits of amphistomaty related to water and CO₂ transport in contrasting leaf morphologies. First, there is no evidence that amphistomatous species achieve higher stomatal densities on a projected leaf area basis than hypostomatous species, but two-sided gas exchange is less limited by boundary layer effects. Second, amphistomaty may provide a specific advantage in thick leaves by shortening the pathway for CO₂ transport between the atmosphere and the chloroplasts. In thin leaves of fast-growing herbaceous annuals, in which both the adaxial and abaxial pathways are already short, amphistomaty enhances leaf-atmosphere gas-exchange capacity. Third, amphistomaty may help to optimise the leaf-interior water status for CO₂ transport by reducing temperature gradients and so preventing the condensation of water that could limit CO₂ diffusion. Fourth, a potential cost of amphistomaty is the need for additional investments in leaf water transport tissue to balance the water loss through the adaxial surface.

Plant water-use strategy mediates stomatal effects on the light induction of photosynthesis

More efficient gas exchange strategies under dynamic light environments have been hypothesised to contribute to the dominance of angiosperms in the vascular plant flora. However, we still lack a clear understanding of how stomatal dynamics affect photosynthetic dynamics and whether differences exist between lineages. Stomatal and photosynthetic dynamics following changes in irradiance were studied in 15 species, encompassing ferns, gymnosperms and angiosperms. We determined the effect of stomatal speed on dynamic photosynthesis and water loss. Moreover, we assessed whether dynamic behaviour followed evolutionary lineage divisions, or whether ecological adaptation to maximise light fleck use could describe dynamic behaviour. We found that species with fast stomatal opening, such as ferns, forgo less photosynthesis during photosynthetic induction. By contrast, there was no relationship between stomatal closure speed and the water wasted by transiently more-open stomata, because species with higher rates of gas exchange also showed faster stomatal closure. Shade-adapted species possessed fast-opening but slow-closing stomata, consistent with ecological adaptation to maximise light fleck use. Our results suggest dynamic behaviour follows adaptive ecological trends more strongly than evolutionary ones, but angiosperms may benefit from relatively faster photosynthetic induction by adopting a less conservative water-use strategy.

Phloem networks in leaves

The survival of all vascular plants depends on phloem and xylem, which comprise a hydraulically coupled tissue system that transports photosynthates, water, and a variety of other molecules and ions. Although xylem hydraulics has been extensively studied, until recently, comparatively little is known quantitatively about the phloem hydraulic network and how it is functionally coupled to the xylem network, particularly in photosynthetic leaves. Here, we summarize recent advances in quantifying phloem hydraulics in fully expanded mature leaves with different vascular architectures and show that (1) the size of phloem conducting cells across phylogenetically different taxa scales isometrically with respect to xylem conducting cell size, (2) cell transport areas and lengths increase along phloem transport pathways in a manner that can be used to model Münch's pressure-flow hypothesis, and (3) report observations that invalidate da Vinci's and Murray's hydraulic models as plausible constructs for understanding photosynthate transport in the leaf lamina.