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The phytohormone abscisic acid (ABA) plays a major role in closing the stomata of angiosperms. However, recent reports of some angiosperm species having a peaking-type ABA dynamic, in which under extreme drought ABA levels decline to pre-stressed levels, raises the possibility that passive stomatal closure by leaf water status alone can occur in species from this lineage. To test this hypothesis, we conducted instantaneous rehydration experiments in the peaking-type species Umbellularia californica through a long-term drought, in which ABA levels declined to pre-stress levels, yet stomata remain closed. We found that when ABA levels were lowest during extreme drought, stomata reopen rapidly to maximum rates of gas exchange on instantaneous rehydration, suggesting that the stomata of U. californica were passively closed by leaf water status alone. This contrasts with leaves early in drought, in which ABA levels were highest and stomata did not reopen on instantaneous rehydration. The transition from ABA-driven stomatal closure to passively driven stomatal closure as drought progresses in this species occurs at very low water potentials facilitated by highly embolism-resistant xylem. These results have important implications for understanding stomatal control during drought in angiosperms. [ABSTRACT FROM AUTHOR]

ABSTRACT In ecophysiology leaves are frequently stored for hours after sampling before measuring their leaf water potential (Ψleaf). Here, we address a previously unidentified source of error, that metabolic heat generation can cause continuous water loss from leaves stored in impermeable bags, leading to a Ψleaf drop over time. We tested Ψleaf drop rates under various conditions: two bag materials, two species, initial Ψleaf above or below the turgor loss point (Ψtlp), and storage at 25°C versus 4°C. We partitioned leaf water loss due to condensation on the inner bag surface or permeation through the bag. We found that Ψleaf dropped by up to 0.39 MPa per hour, with 41%–89% of the water leaving the leaf condensed on the inner bag surface. Plastic bags showed higher Ψleaf drop rates than aluminium bags, and leaves above Ψtlp experienced greater drops. Storing leaves at 4°C reduced the Ψleaf drop rate by 60% compared to 25°C. Leaves were 0.2–0.3°C warmer than the bags, likely due to metabolic heating. Our energy balance model suggests that water loss is lower when storing leaves at cooler temperatures, using leaves with low stomatal conductance, deflated bags, and leaves with low Ψleaf. [ABSTRACT FROM AUTHOR]

Stomata are the key nodes linking photosynthesis and transpiration. By regulating the opening degree of stomata, plants successively achieve the balance between water loss and carbon dioxide acquisition. The dynamic behaviour of stomata is an important cornerstone of plant adaptability. Though there have been miscellaneous experimental results on stomata and their constituent cells, the guard cells and the subsidiary cells, current theory of stomata regulation is far from clear and unified. In this work, we develop an integrated model to describe the stomatal dynamics of seed plants based on existing experimental results. The model includes three parts: (1) a passive mechanical model of the stomatal aperture as a bivariate function of the guard‐cell turgor and the subsidiary‐cell turgor; (2) an active regulation model with a target ion‐content in guard cells as a function of their water potential; and (3) a dynamical model for the movement of potassium ions and water content. Our model has been used to reproduce abundant experimental phenomena semi‐quantitatively. With accurately measured parameters, our model can also be used to predict stomatal responses to changes of environmental conditions. Summary statement: A stomata model is developed based on existing experiments and reproduces important stomatal behaviour. The model takes into account passive mechanical interaction between guard cells and subsidiary cells, active movement of potassium ions, and water transport driven by water potential. [ABSTRACT FROM AUTHOR]

The regulation of water loss and the spread of xylem embolism have mostly been considered separately. The development of an integrated approach taking into account the temporal dynamics and relative contributions of these mechanisms to plant drought responses is urgently needed. Do conifer species native to mesic and xeric environments display different hydraulic strategies and temporal sequences under drought? A dry-down experiment was performed on seedlings of four conifer species differing in embolism resistance, from drought-sensitive to extremely drought-resistant species. A set of traits related to drought survival was measured, including turgor loss point, stomatal closure, minimum leaf conductance, and xylem embolism resistance. All species reached full stomatal closure before the onset of embolism, with all but the most drought-sensitive species presenting large stomatal safety margins, demonstrating that highly drought-resistant species do not keep their stomata open under drought conditions. Plant dry-down time to death was significantly influenced by the xylem embolism threshold, stomatal safety margin, and minimum leaf conductance, and was best explained by the newly introduced stomatal margin retention index (SMRIΨ50) which reflects the time required to cross the stomatal safety margin. The SMRIΨ50 may become a key tool for the characterization of interspecific drought survival variability in trees. [ABSTRACT FROM AUTHOR]

Summary: Every existing optimal stomatal model uses photosynthetic carbon assimilation as a proxy for plant evolutionary fitness. However, assimilation and growth are often decoupled, making assimilation less ideal for representing fitness when optimizing stomatal conductance to water vapor and carbon dioxide. Instead, growth should be considered a closer proxy for fitness.We hypothesize stomata have evolved to maximize turgor‐driven growth, instead of assimilation, over entire plants' lifetimes, improving their abilities to compete and reproduce. We develop a stomata model that dynamically maximizes whole‐stem growth following principles from turgor‐driven growth models. Stomata open to assimilate carbohydrates that supply growth and osmotically generate turgor, while stomata close to prevent losses of turgor and growth due to negative water potentials.In steady state, the growth optimization model captures realistic stomatal, growth, and carbohydrate responses to environmental cues, reconciles conflicting interpretations within existing stomatal optimization theories, and explains patterns of carbohydrate storage and xylem conductance observed during and after drought.Our growth optimization hypothesis introduces a new paradigm for stomatal optimization models, elevates the role of whole‐plant carbon use and carbon storage in stomatal functioning, and has the potential to simultaneously predict gross productivity, net productivity, and plant mortality through a single, consistent modeling framework. See also the Commentary on this article by Buckley, 238: 457–460. [ABSTRACT FROM AUTHOR]

The sequence of physiological events during drought strongly impacts plants' overall performance. Here, we synthesized the global data of stomatal and hydraulic traits in leaves and stems of 202 woody species to evaluate variations in the water potentials for key physiological events and their sequence along the climatic gradient. We found that the seasonal minimum water potential, turgor loss point, stomatal closure point, and leaf and stem xylem vulnerability to embolism were intercorrelated and decreased with aridity, indicating that water stress drives trait co‐selection. In xeric regions, the seasonal minimum water potential occurred at lower water potential than turgor loss point, and the subsequent stomatal closure delayed embolism formation. In mesic regions, however, the seasonal minimum water potential did not pose a threat to the physiological functions, and stomatal closure occurred even at slightly more negative water potential than embolism. Our study demonstrates that the sequence of water potentials for physiological dysfunctions of woody plants varies with aridity, that is, xeric species adopt a more conservative sequence to prevent severe tissue damage through tighter stomatal regulation (isohydric strategy) and higher embolism resistance, while mesic species adopt a riskier sequence via looser stomatal regulation (anisohydric strategy) to maximize carbon uptake at the cost of hydraulic safety. Integrating both aridity‐dependent sequence of water potentials for physiological dysfunctions and gap between these key traits into the hydraulic framework of process‐based vegetation models would improve the prediction of woody plants' responses to drought under global climate change. [ABSTRACT FROM AUTHOR]

Summary: Evolutionary relationships are likely to play a significant role in shaping plant physiological and structural traits observed in contemporary taxa. We review research on phylogenetic signal and correlated evolution in plant–water relation traits, which play important roles in allowing plants to acquire, use, and conserve water. We found more evidence for a phylogenetic signal in structural traits (e.g. stomatal length and stomatal density) than in physiological traits (e.g. stomatal conductance and water potential at turgor loss). Although water potential at turgor loss is the most‐studied plant–water relation trait in an evolutionary context, it is the only trait consistently found to not have a phylogenetic signal. Correlated evolution was common among traits related to water movement efficiency and hydraulic safety in both leaves and stems. We conclude that evidence for phylogenetic signal varies depending on: the methodology used for its determination, that is, model‐based approaches to determine phylogenetic signal such as Blomberg's K or Pagel's λ vs statistical approaches such as ANOVAs with taxonomic classification as a factor; on the number of taxa studied (size of the phylogeny); and the setting in which plants grow (field vs common garden). More explicitly and consistently considering the role of evolutionary relationships in shaping plant ecophysiology could improve our understanding of how traits compare among species, how traits are coordinated with one another, and how traits vary with the environment. [ABSTRACT FROM AUTHOR]

The isohydric–anisohydric continuum describes the relative stringency of stomatal control of leaf water potential (ψleaf) during drought. Hydroscape area (HA)—the water potential landscape over which stomata regulate ψleaf—has emerged as a useful metric of the iso/anisohydric continuum because it is strongly linked to several hydraulic, photosynthetic and structural traits. Previous research on HA focused on broad ecological patterns involving several plant clades. Here we investigate the relationships between HA and climatic conditions and functional traits across ecologically diverse but closely related species while accounting for phylogeny. Across a macroclimatic moisture gradient, defined by the ratio of mean annual precipitation to mean annual pan evaporation (P/Ep), HA decreased with increased P/Ep across 10 Eucalyptus species. Greater anisohydry reflects lower turgor loss points and greater hydraulic safety, mirroring global patterns. Larger HA coincides with mesophyll photosynthetic capacity that is more sensitive to ψleaf. Hydroscapes exhibit little plasticity in response to variation in water supply, and the extent of plasticity does not vary with P/Ep of native habitats. These findings strengthen the case that HA is a useful metric for characterizing drought tolerance and water‐status regulation. Smaller hydroscape area (increased isohydry) in Eucalyptus species is tied to moister climates and drought‐sensitive functional traits like higher turgor loss points and mesophyll photosynthetic sensitivity to leaf water potential. Hydroscape area shows little plasticity to water availability. [ABSTRACT FROM AUTHOR]

We employed a detailed whole leaf hydraulic model to study the local operation of three stomatal conductance models distributed on the scale of a whole leaf. We quantified the behavior of these models by examining the leaf-area distributions of photosynthesis, transpiration, stomatal conductance, and guard cell turgor pressure. We gauged the models' local responses to changes in environmental conditions of carbon dioxide concentration, relative humidity, and light irradiance. We found that a stomatal conductance model that includes mechanical processes dependent on local variables predicts a spatial variation of physiological activity across the leaf: the leaf functions of photosynthesis and transpiration are not uniformly operative even when external conditions are uniform. The gradient pattern of hydraulic pressure which is needed to produce transpiration from the whole leaf is derived from the gradient patterns of turgor pressures of guard cells and epidermal cells and consequently leads to nonuniform spatial distribution patterns of transpiration and photosynthesis via the mechanical stomatal model. Our simulation experiments, comparing the predictions of two versions of a mechanical stomatal conductance model, suggest that leaves exhibit a more complex spatial distribution pattern of both photosynthesis and transpiration rate and more complex dependencies on environmental conditions when a non-linear relationship between the stomatal aperture and guard cell and epidermal cell turgor pressures is implemented. Our model studies offer a deeper understanding of the mechanism of stomatal conductance and point to possible future experimental measurements seeking to quantify the spatial distributions of several physiological activities taking place over a whole leaf. [ABSTRACT FROM AUTHOR]

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Summary: Experimental evidence that nonstomatal limitations to photosynthesis (NSLs) correlate with leaf sugar and/or leaf water status suggests the possibility that stomata adjust to maximise photosynthesis through a trade‐off between leaf CO2 supply and NSLs, potentially involving source–sink interactions. However, the mechanisms regulating NSLs and sink strength, as well as their implications for stomatal control, remain uncertain.We used an analytically solvable model to explore optimal stomatal control under alternative hypotheses for source and sink regulation. We assumed that either leaf sugar concentration or leaf water potential regulates NSLs, and that either phloem turgor pressure or phloem sugar concentration regulates sink phloem unloading.All hypotheses led to realistic stomatal responses to light, CO2 and air humidity, including conservative behaviour for the intercellular‐to‐atmospheric CO2 concentration ratio. Sugar‐regulated and water‐regulated NSLs are distinguished by the presence/absence of a stomatal closure response to changing sink strength. Turgor‐regulated and sugar‐regulated phloem unloading are distinguished by the presence/absence of stomatal closure under drought and avoidance/occurrence of negative phloem turgor. Results from girdling and drought experiments on Pinus sylvestris, Betula pendula, Populus tremula and Picea abies saplings are consistent with optimal stomatal control under sugar‐regulated NSLs and turgor‐regulated unloading.Our analytical results provide a simple representation of stomatal responses to above‐ground and below‐ground environmental factors and sink activity. [ABSTRACT FROM AUTHOR]

The ability of plants to absorb CO2 for photosynthesis and transport water from root to shoot depends on the reversible swelling of guard cells that open stomatal pores in the epidermis. Despite decades of experimental and theoretical work, the biomechanical drivers of stomatal opening and closure are still not clearly defined. We combined mechanical principles with a growing body of knowledge concerning water flux across the plant cell membrane and the biomechanical properties of plant cell walls to quantitatively test the long-standing hypothesis that increasing turgor pressure resulting from water uptake drives guard cell expansion during stomatal opening. To test the alternative hypothesis that water influx is the main motive force underlying guard cell expansion, we developed a system dynamics model accounting for water influx. This approach connects stomatal kinetics to whole plant physiology by including values for water flux arising fromwater status in the plant. [ABSTRACT FROM AUTHOR]

Chestnut breeding programs have been using a backcross breeding technique to produce trees with a predominantly American chestnut (Castanea dentata (Marsh.) Borkh.) genome and chestnut blight disease resistance from Asian species like the Chinese chestnut (Castanea mollissima Blume). The potential for other physiological changes caused by breeding has yet to be widely studied. We quantify chestnut (American, Chinese, and BC3F3 hybrids) responses to water-stress and measure how co-occurring drought influences disease severity. The experiment was completed using 172 bare-root seedlings organized into a completely randomized factorial design in an outdoor rain-out shelter for one growing season. BC3F3 hybrid gas exchange (Asat, gs) rates were more similar to Chinese than American chestnuts over a 20-day dry-down period, and hybrid turgor loss point showed a more intermediate (between Chinese and American) response. The relationship of stomatal conductance to midday leaf water potential (Ψmd) also exhibited both American and Chinese characteristics in the hybrid trees. There was no effect of drought on the disease severity for any of the chestnut groups. We find evidence that drought physiology has been altered in some BC3F3 hybrids, but do not find changes in disease severity when chestnuts are under co-occurring drought. [ABSTRACT FROM AUTHOR]

Plant cell deformations are driven by cell pressurization and mechanical constraints imposed by the nanoscale architecture of the cell wall, but how these factors are controlled at the genetic and molecular levels to achieve different types of cell deformation is unclear. Here, we used stomatal guard cells to investigate the influences of wall mechanics and turgor pressure on cell deformation and demonstrate that the expression of the pectin-modifying gene PECTATE LYASE LIKE12 (PLL12) is required for normal stomatal dynamics in Arabidopsis thaliana. Using nanoindentation and finite element modeling to simultaneously measure wall modulus and turgor pressure, we found that both values undergo dynamic changes during induced stomatal opening and closure. PLL12 is required for guard cells to maintain normal wall modulus and turgor pressure during stomatal responses to light and to tune the levels of calcium crosslinked pectin in guard cell walls. Guard cell-specific knockdown of PLL12 caused defects in stomatal responses and reduced leaf growth, which were associated with lower cell proliferation but normal cell expansion. Together, these results force us to revise our view of how wall-modifying genes modulate wall mechanics and cell pressurization to accomplish the dynamic cellular deformations that underlie stomatal function and tissue growth in plants. [ABSTRACT FROM AUTHOR]

Phytophthora infestans , the most damaging oomycete pathogen of potato, is specialized to grow sporangiophore through opened stomata for secondary inoculum production. However, it is still unclear which metabolic pathways in potato are manipulated by P. infestans in the guard cell–pathogen interactions to open the stomata. Here microscopic observations and cell biology were used to investigate antagonistic interactions between guard cells and the oomycete pathogen. We observed that the antagonistic interactions started at the very beginning of infection. Stomatal movement is an important part of the immune response of potato to P. infestans infection and this occurs through guard cell death and stomatal closure. We observed that P. infestans appeared to manipulate metabolic processes in guard cells, such as triacylglycerol (TAG) breakdown, starch degradation, H2O2 scavenging, and NO catabolism, which are involved in stomatal movement, to evade these stomatal defense responses. The signal transduction pathway of P. infestans -induced stomatal opening likely starts from H2O2 and NO scavenging, along with TAG breakdown while the subsequent starch degradation reinforces the opening process by strengthening guard cell turgor and opening the stomata to their maximum aperture. These results suggest that stomata are a barrier stopping P. infestans from completing its life cycle, but this host defense system can be bypassed through the manipulation of diverse metabolic pathways that may be induced by P. infestans effector proteins. [ABSTRACT FROM AUTHOR]

Opportunistic diversification has allowed ferns to radiate into epiphytic niches in angiosperm dominated landscapes. However, our understanding of how ecophysiological function allowed establishment in the canopy and the potential transitionary role of the hemi‐epiphytic life form remain unclear. Here, we surveyed 39 fern species in Costa Rican tropical forests to explore epiphytic trait divergence in a phylogenetic context. We examined leaf responses to water deficits in terrestrial, hemi‐epiphytic and epiphytic ferns and related these findings to functional traits that regulate leaf water status. Epiphytic ferns had reduced xylem area (−63%), shorter stipe lengths (−56%), thicker laminae (+41%) and reduced stomatal density (−46%) compared to terrestrial ferns. Epiphytic ferns exhibited similar turgor loss points, higher osmotic potential at saturation and lower tissue capacitance after turgor loss than terrestrial ferns. Overall, hemi‐epiphytic ferns exhibited traits that share characteristics of both terrestrial and epiphytic species. Our findings clearly demonstrate the prevalence of water conservatism in both epiphytic and hemi‐epiphytic ferns, via selection for anatomical and structural traits that avoid leaf water stress. Even with likely evolutionarily constrained physiological function, adaptations for drought avoidance have allowed epiphytic ferns to successfully endure the stresses of the canopy habitat. We detected key trait adaptions with xylem anatomy, frond structural traits and stomatal anatomy for avoidance of dehydration in both epiphytic and hemi‐epiphytic ferns species. Each of the trait adaptations we detected are clearly evident in phylogenetic trait reconstructions, suggesting that our findings provide a foundation on which to explain the evolution of epiphytism in ferns. [ABSTRACT FROM AUTHOR]

Summary: The best predictor of leaf level photosynthetic rate is the porosity of the leaf surface, as determined by the number and aperture of stomata on the leaf. This remarkable correlation between stomatal porosity (or diffusive conductance to water vapour gs) and CO2 assimilation rate (A) applies to all major lineages of vascular plants (Figure 1) and is sufficiently predictable that it provides the basis for the model most widely used to predict water and CO2 fluxes from leaves and canopies. Yet the Ball–Berry formulation is only a phenomenological approximation that captures the emergent character of stomatal behaviour. Progressing to a more mechanistic prediction of plant gas exchange is challenging because of the diversity of biological components regulating stomatal action. These processes are the product of more than 400 million years of co‐evolution between stomatal, vascular and photosynthetic tissues. Both molecular and structural components link the abiotic world of the whole plant with the turgor pressure of the epidermis and guard cells, which ultimately determine stomatal pore size and porosity to water and CO2 exchange (New Phytol., 168, 2005, 275). In this review we seek to simplify stomatal behaviour by using an evolutionary perspective to understand the principal selective pressures involved in stomatal evolution, thus identifying the primary regulators of stomatal aperture. We start by considering the adaptive process that has locked together the regulation of water and carbon fluxes in vascular plants, finally examining specific evidence for evolution in the proteins responsible for regulating guard cell turgor. Significance Statement: Stomata regulate photosynthesis and transpiration in most extant plant species. The evolution of these critical valves provides an important insight into understanding land plant physiology. [ABSTRACT FROM AUTHOR]

This document is a correction notice for an article titled "PECTATE LYASE LIKE12 patterns the guard cell wall to coordinate turgor pressure and wall mechanics for proper stomatal function in Arabidopsis" published in The Plant Cell journal. The correction addresses two typographical errors in the "Materials and methods" section related to the concentrations of fusicoccin and ABA used in the study. The correct concentrations should be listed as 1 μM and 50 μM, respectively, instead of 1 mM and 50 mM. The authors apologize for the error and have made the necessary corrections in this notice. [Extracted from the article]

• Leaf turgor pressure changes to D changes relate to maximum stomatal conductance. • We derived a physiological-based indicator from a leaf turgor pressure-related sensor. • g s,max can be automatically estimated from this indicator previous calibration. • We demonstrated this for three seasons in a commercial olive hedgerow orchard. • Results apply for moderate water stress levels within stomatal functioning ranges. Effective study and management of crops and forests would benefit greatly from useful plant-based indicators of the biological controls on evapotranspiration, and particularly stomatal conductance (g s). Given the strong influence of g s on bulk leaf water potential and turgor pressure (P), in vivo measurement of P may provide useful information about diurnal or seasonal dynamics of g s. Moderate plant water stress affects the diurnal dynamics of P as leaf-to-air vapour pressure deficit (D) varies, and these dynamics correlate to g s. Here, we explored relative changes in P in response to changes in D under mild drought conditions, and how these changes are linked to stomatal behaviour, and specifically to diurnal maximum g s (g s,max), one of the best indicators of plant water stress. We monitored ecophysiological and environmental variables, as well as a relative proxy for P , during three consecutive seasons in a hedgerow olive orchard where trees were supplied with different irrigation treatments to create well-watered and moderately water-stressed conditions. Our results demonstrated that the sensitivity of P to D correlated well with g s,max reached by the trees within a range in which variations in g s are the main diffusional limitation to photosynthesis. We further showed that this correlation held under a wide range of meteorological conditions and soil water availability. This turgor proxy measurement, which is much easier to measure than g s , can facilitate the use of g s,max as an indicator of plant water stress and evapotranspiration in agriculture and plant science research. [ABSTRACT FROM AUTHOR]

ContentsSummary84I.Introduction84II.Stomatal form and biomechanics85III.Stomatal function86IV.Evolution of guard cell ion channels87V.Conclusions88Acknowledgements88Author contributions88References88 Summary: Stomatal pores with apertures that can be adjusted by changes in guard cell turgor have facilitated plant success in dry environments. We explore their evolutionary origins, considering recent findings from bryophytes. Unlike vascular plant stomata, which close to prevent water loss, bryophyte stomata become locked open to promote spore desiccation. We find that the families of ion channels, known to control stomatal movements in angiosperms, are ancient and represented across extant land plants. However, although angiosperm guard cells express specific ion channel genes, none appear specifically expressed in stomata‐bearing moss tissues. Given the evolutionary shift in stomatal function from promotion to prevention of water loss, we postulate that ion channels adopted guard cell‐specific functions after the divergence of bryophytes. [ABSTRACT FROM AUTHOR]

Skin shrivelling of sweet cherry ( Prunus avium L.), referred to here as ‘orange-peel’ disorder, compromises fruit appearance and thus market value. The objectives were to establish a protocol to describe and quantify orange peel disorder and to identify the mechanism and factors determining its incidence. Fruit was stored for 28 to 33 d at 2 °C and 76% RH and orange peel disorder was quantified as a topographical roughness using an interferometer or by rating the fruit for orange peel using a four-step scoring scheme. Fruit with orange peel disorder had a skin topography similar to that of a citrus fruit − just on a finer scale. Under the conditions of the test, orange peel was first visible after ∼7 d and continued to increase in severity thereafter. Orange peel was most severe on the shoulder and in the equatorial and distal regions of the fruit. There was no relationship between the distribution of orange peel and that of stomata or of microcracking. At a microscopic level, the depressions in the fruit surface were markedly larger than the periclinal areas of individual epidermal cells, but similar in size to the mesh formed by the network of minor veins visible just beneath the skin. Susceptibility to orange peel differed among cultivars. Least susceptible were ‘Dönissens Gelbe’ and, ‘Gil Peck’, intermediate were ‘Sam’, ‘Kordia’, ‘Merchant’, and the sour cherry ‘Ungarische Traubige’, and most susceptible were ‘Adriana’, ‘Regina’, and ‘Hedelfinger’. Incidence of orange peel during storage was negatively related to relative humidity (more at lower humidities) but also developed at 100% RH in the absence of transpiration. Submerging fruit in water for 2 d partly reversed orange peel. There was no significant difference in the permeance of the skins or in turgors of cells of the outer mesocarp between fruit without and with orange peel. There was a significant difference between the osmotic potential of the flesh (more negative) and that of the skin (less negative). During storage, the osmotic potentials of flesh and skin both decreased slightly, but the difference between them remained constant. The results show water loss from the skin is causal in orange peel disorder. The water loss from the skin occurs both by two routes: (1) transpiration to the atmosphere and also (2) by osmotic dehydration to the flesh. [ABSTRACT FROM AUTHOR]

The water potential at turgor loss point ( Ψtlp) has been suggested as a key functional trait for determining plant drought tolerance, because of its close relationship with stomatal closure. Ψtlp may indicate drought tolerance as plants, which maintain gas exchange at lower midday water potentials as soil water availability declines also have lower Ψtlp. We evaluated 17 species from seasonally dry habitats, representing a range of life-forms, under well-watered and drought conditions, to determine how Ψtlp relates to stomatal sensitivity (pre-dawn water potential at stomatal closure: Ψg s0) and drought strategy (degree of isohydry or anisohydry; Δ ΨMD between well-watered conditions and stomatal closure). Although Ψg s0 was related to Ψtlp, Ψg s0 was better related to drought strategy (Δ ΨMD). Drought avoiders (isohydric) closed stomata at water potentials higher than their Ψtlp; whereas, drought tolerant (anisohydric) species maintained stomatal conductance at lower water potentials than their Ψtlp and were more dehydration tolerant. There was no significant relationship between Ψtlp and Δ ΨMD. While Ψtlp has been related to biome water availability, we found that Ψtlp did not relate strongly to stomatal closure or drought strategy, for either drought avoiders or tolerators. We therefore suggest caution in using Ψtlp to predict vulnerability to drought. [ABSTRACT FROM AUTHOR]

Species' differences in the stringency of stomatal control of plant water potential represent a continuum of isohydric to anisohydric behaviours. However, little is known about how quasi-steady-state stomatal regulation of water potential may relate to dynamic behaviour of stomata and photosynthetic gas exchange in species operating at different positions along this continuum. Here, we evaluated kinetics of light-induced stomatal opening, activation of photosynthesis and features of quasi-steady-state photosynthetic gas exchange in 10 woody species selected to represent different degrees of anisohydry. Based on a previously developed proxy for the degree of anisohydry, species' leaf water potentials at turgor loss, we found consistent trends in photosynthetic gas exchange traits across a spectrum of isohydry to anisohydry. More anisohydric species had faster kinetics of stomatal opening and activation of photosynthesis, and these kinetics were closely coordinated within species. Quasi-steady-state stomatal conductance and measures of photosynthetic capacity and performance were also greater in more anisohydric species. Intrinsic water-use efficiency estimated from leaf gas exchange and stable carbon isotope ratios was lowest in the most anisohydric species. In comparisons between gas exchange traits, species rankings were highly consistent, leading to species-independent scaling relationships over the range of isohydry to anisohydry observed. [ABSTRACT FROM AUTHOR]

Previous experiments have demonstrated positive effect of Acadian extract of Ascophyllum nodosum on plant stress-resistance, however the mode of action is not fully understood. The aim of this study was to understand the physiological effect of Acadian seaweed extract on the plant response to drought stress. Leaf temperature and leaf angle were measured as early-stage indicators of plant stress with thermal imaging 'in situ' over a 5-day stress-recovery trial. The early stress-response of control became visible on the third day as a rapid wilting of leaves, accompanied with the asymptotic increase of leaf temperature on 4-5 °C to the thermal equilibrium with ambient air temperature. At the same time Acadian treated plants still maintained turgor, accompanied with the linear increase in leaf temperature, which indicated better control of stomatal closure. Re-watering on the fifth day showed better survival of treated plants compared to control. This study demonstrated the ability of Acadian seaweed extract to improve resistance of soybean plants to water stress. [ABSTRACT FROM AUTHOR]

Stomata mediate gas exchange between the inter-cellular spaces of leaves and the atmosphere. CO2 levels in leaves ( Ci) are determined by respiration, photosynthesis, stomatal conductance and atmospheric [ CO2]. [ CO2] in leaves mediates stomatal movements. The role of guard cell photosynthesis in stomatal conductance responses is a matter of debate, and genetic approaches are needed. We have generated transgenic Arabidopsis plants that are chlorophyll-deficient in guard cells only, expressing a constitutively active chlorophyllase in a guard cell specific enhancer trap line. Our data show that more than 90% of guard cells were chlorophyll-deficient. Interestingly, approximately 45% of stomata had an unusual, previously not-described, morphology of thin-shaped chlorophyll-less stomata. Nevertheless, stomatal size, stomatal index, plant morphology, and whole-leaf photosynthetic parameters ( PSII, qP, qN, FV′/ FM′) were comparable with wild-type plants. Time-resolved intact leaf gas-exchange analyses showed a reduction in stomatal conductance and CO2-assimilation rates of the transgenic plants. Normalization of CO2 responses showed that stomata of transgenic plants respond to [ CO2] shifts. Detailed stomatal aperture measurements of normal kidney-shaped stomata, which lack chlorophyll, showed stomatal closing responses to [ CO2] elevation and abscisic acid ( ABA), while thin-shaped stomata were continuously closed. Our present findings show that stomatal movement responses to [ CO2] and ABA are functional in guard cells that lack chlorophyll. These data suggest that guard cell CO2 and ABA signal transduction are not directly modulated by guard cell photosynthesis/electron transport. Moreover, the finding that chlorophyll-less stomata cause a 'deflated' thin-shaped phenotype, suggests that photosynthesis in guard cells is critical for energization and guard cell turgor production. [ABSTRACT FROM AUTHOR]

Because iso- and anisohydric species differ in stomatal regulation of the rate and magnitude of fluctuations in shoot water potential, they may be expected to show differences in the plasticity of their shoot water relations components, but explicit comparisons of this nature have rarely been made. We subjected excised shoots of co-occurring anisohydric J uniperus monosperma and isohydric P inus edulis to pressure-volume analysis with and without prior artificial rehydration. In J. monosperma, the shoot water potential at turgor loss ( ΨTLP) ranged from −3.4 MPa in artificially rehydrated shoots to −6.6 MPa in shoots with an initial Ψ of −5.5 MPa, whereas in P . edulis mean ΨTLP remained at ∼ −3.0 MPa over a range of initial Ψ from −0.1 to −2.3 MPa. The shoot osmotic potential at full turgor and the bulk modulus of elasticity also declined sharply with shoot Ψ in J . monosperma, but not in P . edulis. The contrasting behaviour of J. monosperma and P . edulis reflects differences in their capacity for homeostatic regulation of turgor that may be representative of aniso- and isohydric species in general, and may also be associated with the greater capacity of J . monosperma to withstand severe drought. [ABSTRACT FROM AUTHOR]

Abstract: Stomata respond in a common pattern to various hydraulic perturbations on any part of the ‘soil–plant–air’ system: initial transient ‘wrong-way’ responses and final stationary ‘right-way’ responses. In order to describe this pattern on the basis of statistical physics, we propose a simple model where turgor pressure of a cell is taken to be a power function of its volume, and obtain results in qualitative agreement with experimental data for responses to a variety of hydraulic perturbations: Firstly, stationary stomatal conductance as a function of the vapor pressure deficit divides into three regimes characterized by sensitivities of the stomatal conductance and the transpiration rate with respect to vapor pressure deficit; secondly, for every hydraulic perturbation, the initial transient ‘wrong-way’ responses always appear; thirdly, on condition that water is supplied insufficiently, stomatal oscillations are often observed; finally, stomatal responses following leaf excision exhibit, after the initial transient wrong-way responses, slow relaxation to stomatal closing. In particular, comparison of areoles having different numbers of stomata demonstrates that areoles with small numbers of stomata tend to provoke lack of water in the soil as well as in the plant. In addition, our model also describes well dependence of the stomatal conductance on temperature. It may be extended further to describe stomatal responses to other environmental factors such as carbon dioxide, light, and temperature. [ABSTRACT FROM AUTHOR]

ABSTRACT Carbon uptake and transpiration in plant leaves occurs through stomata that open and close. Stomatal action is usually considered a response to environmental driving factors. Here we show that leaf gas exchange is more strongly related to whole tree level transport of assimilates than previously thought, and that transport of assimilates is a restriction of stomatal opening comparable with hydraulic limitation. Assimilate transport in the phloem requires that osmotic pressure at phloem loading sites in leaves exceeds the drop in hydrostatic pressure that is due to transpiration. Assimilate transport thus competes with transpiration for water. Excess sugar loading, however, may block the assimilate transport because of viscosity build-up in phloem sap. Therefore, for given conditions, there is a stomatal opening that maximizes phloem transport if we assume that sugar loading is proportional to photosynthetic rate. Here we show that such opening produces the observed behaviour of leaf gas exchange. Our approach connects stomatal regulation directly with sink activity, plant structure and soil water availability as they all influence assimilate transport. It produces similar behaviour as the optimal stomatal control approach, but does not require determination of marginal cost of water parameter. [ABSTRACT FROM AUTHOR]

Intracellular NHX proteins are Na+,K+/H+ antiporters involved in K+ homeostasis, endosomal pH regulation, and salt tolerance. Proteins NHX1 and NHX2 are the two major tonoplast-localized NHX isoforms. Here, we show that NHX1 and NHX2 have similar expression patterns and identical biochemical activity, and together they account for a significant amount of the Na+,K+/H+ antiport activity in tonoplast vesicles. Reverse genetics showed functional redundancy of NHX1 and NHX2 genes. Growth of the double mutant nhx1 nhx2 was severely impaired, and plants were extremely sensitive to external K+. By contrast, nhx1 nhx2 mutants showed similar sensitivity to salinity stress and even greater rates of Na+ sequestration than the wild type. Double mutants had reduced ability to create the vacuolar K+ pool, which in turn provoked greater K+ retention in the cytosol, impaired osmoregulation, and compromised turgor generation for cell expansion. Genes NHX1 and NHX2 were highly expressed in guard cells, and stomatal function was defective in mutant plants, further compromising their ability to regulate water relations. Together, these results show that tonoplast-localized NHX proteins are essential for active K+ uptake at the tonoplast, for turgor regulation, and for stomatal function. [ABSTRACT FROM AUTHOR]

Carbon and water flow between plants and the atmosphere is regulated by the opening and closing of minute stomatal pores in surfaces of leaves. By changing the aperture of stomata, plants regulate water loss and photosynthetic carbon gain in response to many environmental stimuli, but stomatal movements cannot yet be reliably predicted. We found that the complexity that characterizes stomatal control in seed plants is absent in early-diverging vascular plant lineages. Lycophyte and fern stomata are shown to lack key responses to abscisic acid and epidermal cell turgor, making their behavior highly predictable. These results indicate that a fundamental transition from passive to active metabolic control of plant water balance occurred after the divergence of ferns about 360 million years ago. [ABSTRACT FROM AUTHOR]

Abstract: Young mahogany (Swietenia macrophylla King) plants were grown under either well-watered (pre-dawn leaf water potential, Ψ pd, ca. −0.40MPa) or drought (Ψ pd, ca. −3.52MPa) conditions to examine some physiological strategies that allow the maintenance of leaf turgor. In well-watered plants, stomatal conductance (g s) was nearly constant (440mmolm−2 s−1) between 7:00 and 13:00h. This was accomplished by significant increases in transpiration (E) and apparent total hydraulic conductance (K T), in which averages were higher at 13:00h. From 13:00 to 17:00h, g s, E, and K T decreased sharply, reaching their lower values at 17:00h. In these plants, significant increases in height (116%), stem diameter (50%) and leaf area (200%) were registered over the experimental period (20 days). Analyses of linear regression between g s or E and leaf-to-air vapor pressure deficit (Δ w) were not significant. In water-stressed plants, g s and E were higher at 7:00h and lower from 9:00 to 17:00h, while K T was higher in early morning (7:00h) and in late afternoon (17:00h) than between 9:00 and 15:00h. Moreover, both g s and E decreased potentially (P <0.001) with the diurnal increases on Δ w. Drought also decreased leaf and leaflet numbers and reduced total leaf area, but had no effect on stem height and diameter. Leaf proline was higher (ca. 400%, between 13:00 and 15:00h) in water-stressed plants, suggesting osmotic adjustment under drought. Twelve hours after resumption of irrigation, Ψ pd was similar (P >0.05) between well-watered and drought-stressed plants, suggesting an ability of plants to recover turgor after stress cessation. Altogether, our data support the hypothesis that young mahogany plants have the ability to satisfactorily tolerate or postpone drought. [Copyright &y& Elsevier]

Stomatal regulation of transpiration constrains leaf water potential (ΨL) within species-specific ranges that presumably avoid excessive tension and embolism in the stem xylem upstream. However, the hydraulic resistance of leaves can be highly variable over short time scales, uncoupling tension in the xylem of leaves from that in the stems to which they are attached. We evaluated a suite of leaf and stem functional traits governing water relations in individuals of 11 lowland tropical forest tree species to determine the manner in which the traits were coordinated with stem xylem vulnerability to embolism. Stomatal regulation of ΨL was associated with minimum values of water potential in branches (Ψbr) whose functional significance was similar across species. Minimum values of Ψbr coincided with the bulk sapwood tissue osmotic potential at zero turgor derived from pressure–volume curves and with the transition from a linear to exponential increase in xylem embolism with increasing sapwood water deficits. Branch xylem pressure corresponding to 50% loss of hydraulic conductivity ( P 50) declined linearly with daily minimum Ψbr in a manner that caused the difference between Ψbr and P 50 to increase from 0.4 MPa in the species with the least negative Ψbr to 1.2 MPa in the species with the most negative Ψbr. Both branch P 50 and minimum Ψbr increased linearly with sapwood capacitance ( C) such that the difference between Ψbr and P 50, an estimate of the safety margin for avoiding runaway embolism, decreased with increasing sapwood C. The results implied a trade-off between maximizing water transport and minimizing the risk of xylem embolism, suggesting a prominent role for the buffering effect of C in preserving the integrity of xylem water transport. At the whole-tree level, discharge and recharge of internal C appeared to generate variations in apparent leaf-specific conductance to which stomata respond dynamically. [ABSTRACT FROM AUTHOR]

Stomata respond to increasing leaf-to-air vapour pressure difference (LAVPD) ( D) by closing. The mechanism by which this occurs is debated. A role for feedback and peristomatal transpiration has been proposed. In this paper, we apply a recent mechanistic model of stomatal behaviour, and compare model and experimental data for the influence of increasing D on stomatal conductance. We manipulated cuticular conductance ( gc) by three independent methods. First, we increased gc by using a solvent mixture applied to both leaf surfaces prior to determining stomatal responses to D; second, we increased gc by increasing leaf temperature at constant D; and third, we coated a small area of leaf with a light oil to decrease gc. In all three experiments, experimental data and model outputs showed very close agreement. We conclude, from the close agreement between model and experimental data and the fact that manipulations of gc, and hence cuticular transpiration, influenced gs in ways consistent with a feedback mechanism, that feedback is central in determining stomatal responses to D. [ABSTRACT FROM AUTHOR]

Given that stomatal movement is ultimately a mechanical process and that stomata are morphologically and mechanically diverse, we explored the influence of stomatal mechanical diversity on leaf gas exchange and considered some of the constraints. Mechanical measurements were conducted on the guard cells of four different species exhibiting different stomatal morphologies, including three variants on the classical ‘kidney’ form and one ‘dumb-bell’ type; this information, together with gas-exchange measurements, was used to model and compare their respective operational characteristics. Based on evidence from scanning electron microscope images of cryo-sectioned leaves that were sampled under full sun and high humidity and from pressure probe measurements of the stomatal aperture versus guard cell turgor relationship at maximum and zero epidermal turgor, it was concluded that maximum stomatal apertures (and maximum leaf diffusive conductance) could not be obtained in at least one of the species (the grass Triticum aestivum) without a substantial reduction in subsidiary cell osmotic (and hence turgor) pressure during stomatal opening to overcome the large mechanical advantage of subsidiary cells. A mechanism for this is proposed, with a corollary being greatly accelerated stomatal opening and closure. Gas-exchange measurements on T. aestivum revealed the capability of very rapid stomatal movements, which may be explained by the unique morphology and mechanics of its dumb-bell-shaped stomata coupled with ‘see-sawing’ of osmotic and turgor pressure between guard and subsidiary cells during stomatal opening or closure. Such properties might underlie the success of grasses. [ABSTRACT FROM AUTHOR]

Water loss from plants is determined by the aperture of stomatal pores in the leaf epidermis, set by the level of vacuolar accumulation of potassium salt, and hence volume and turgor, of a pair of guard cells. Regulation of ion fluxes across the tonoplast, the key to regulation of stomatal aperture, can only be studied by tracer flux measurements. There are two transport systems in the tonoplast. The first is a Ca2+-activated channel, inhibited by phenylarsine oxide (PAO), responsible for the release of vacuolar K+(Rb+) in response to the “drought” hormone, abscisic acid (ABA). This channel is sensitive to pressure, down-regulated at low turgor and up-regulated at high turgor, providing a system for turgor regulation. ABA induces a transient stimulation of vacuolar ion efflux, during which the flux tracks the ion content (volume, turgor), suggesting ABA reduces the set-point of a control system. The second system, which is PAO-insensitive, is responsible for an ion flux from vacuole to cytoplasm associated with inward water flow following a hypo-osmotic transfer. It is suggested that this involves an aquaporin as sensor, and perhaps also as responder; deformation of the aquaporin may render it ion-permeable, or, alternatively, the deformed aquaporin may signal to an associated ion channel, activating it. Treatment with inhibitors of aquaporins, HgCl2 or silver sulfadiazine, produces a large transient increase in ion release from the vacuole, also PAO-insensitive. It is suggested that this involves the same aquaporin, either rendered directly ion-permeable, or signalling to activate an associated ion channel. [ABSTRACT FROM AUTHOR]

ABSTRACT This study examined the linkage between xylem vulnerability, stomatal response to leaf water potential (Ψ L ), and loss of leaf turgor in eight species of seasonally dry tropical forest trees. In order to maximize the potential variation in these traits species that exhibit a range of leaf habits and phenologies were selected. It was found that in all species stomatal conductance was responsive to Ψ L over a narrow range of water potentials, and that Ψ L inducing 50% stomatal closure was correlated with both the Ψ L inducing a 20% loss of xylem hydraulic conductivity and leaf water potential at turgor loss in all species. In contrast, there was no correlation between the water potential causing a 50% loss of conductivity in the stem xylem, and the water potential at stomatal closure (Ψ SC ) amongst species. It was concluded that although both leaf and xylem characters are correlated with the response of stomata to Ψ L, there is considerable flexibility in this linkage. The range of responses is discussed in terms of the differing leaf-loss strategies exhibited by these species. [ABSTRACT FROM AUTHOR]

Background: Stomata respond to vapour pressure deficit (D) - when D increases, stomata begin to close. Closure is the result of a decline in guard cell turgor, but the link between D and turgor is poorly understood. We describe a model for stomatal responses to increasing D based upon cellular water relations. The model also incorporates impacts of increasing levels of water stress upon stomatal responses to increasing D. Results: The model successfully mimics the three phases of stomatal responses to D and also reproduces the impact of increasing plant water deficit upon stomatal responses to increasing D. As water stress developed, stomata regulated transpiration at ever decreasing values of D. Thus, stomatal sensitivity to D increased with increasing water stress. Predictions from the model concerning the impact of changes in cuticular transpiration upon stomatal responses to increasing D are shown to conform to experimental data. Sensitivity analyses of stomatal responses to various parameters of the model show that leaf thickness, the fraction of leaf volume that is air-space, and the fraction of mesophyll cell wall in contact with air have little impact upon behaviour of the model. In contrast, changes in cuticular conductance and membrane hydraulic conductivity have significant impacts upon model behaviour. Conclusion: Cuticular transpiration is an important feature of stomatal responses to D and is the cause of the 3 phase response to D. Feed-forward behaviour of stomata does not explain stomatal responses to D as feedback, involving water loss from guard cells, can explain these responses. [ABSTRACT FROM AUTHOR]

Abstract Humidity in a small area of a Vicia faba L. leaf was perturbed with a flow of dry air from an 80 µm (inside diameter) needle, while the remainder of the leaf was maintained at high and constant humidity. The influence of the needle flow on the humidity at the leaf surface was quantified by using a spatially explicit dewpoint hygrometer to observe condensation patterns. When the dry air from a needle was applied to the leaf, stomata within the influence of the needle opened within the first few minutes of the perturbation, and local epidermal turgor pressure declined within the same time frame. When the needle flow was removed from the leaf, these responses were reversed, but with more variable kinetics. Stomata and epidermal cells outside the influence of the needle flow, which were exposed to a constant and high humidity, showed similar, but smaller, responses when the needle flow was applied to the leaf. Since the opening of these stomata should have had only a small effect on transpiration (because of the high humidity), it is likely that the reduction in epidermal turgor was the cause (rather than the result) of the stomatal opening. The magnitude of the turgor response was only loosely related to the distance from the needle flow up to distances of almost 400 µm. The data support the idea that neighbouring stomata can interact through the influence of transpiration on epidermal turgor. [ABSTRACT FROM AUTHOR]

A method which determines the null point for stomatal aperture has been used to estimate the apoplastic concentrations of potassium and calcium adjacent to the stomatal guard cells of Commelina communis. These two ions are contributors to important aspects of stomatal physiology : the determination of guard cell turgor (K+) and intracellular signalling (Ca2+) when the guard cells respond to the various stimuli that effect changes in stomatal aperture. We obtained estimates of apoplastic K+ concentrations in the range of 50-75 mol m-3, which are in general agreement with those of Bowling (1987). Ca2+ concentrations appear to be in the region of 0.05 mol m-3 adjacent to the guard cells even though much higher concentrations (up to 4 mol m-3) may be delivered to the leaf in the xylem sap. Thus it is suggested that the gradient in apoplastic Ca2+ may be very large over short distances, and may be strictly controlled by cells within the epidermis. [ABSTRACT FROM AUTHOR]

The impairment of the turgor operated modulation of stomatal movement and changes in photosynthetic capacity were studied using two maize (Zea mays L.) varieties of different chilling sensitivity. In the tolerant variety stomatal opening exhibited characteristic transients, while in the more susceptible strain chilling induced a monotonous increase of stomatal apertures. Photosynthetic capacity of the leaves (expressed in terms of 14CO2 incorporation at 25°C) declined as a result of chilling regardless of stomatal opening. Nevertheless, tolerance was reflected in the preservation of a higher level of CO2 fixation capacity and in a more successful recovery. [ABSTRACT FROM AUTHOR]

During a pretreatment period of 1 week, spring wheat plants (Triticum aestivum, L., cv. `Sappo') grown in a sandy soil in pots were adjusted to three levels of soil water osmotic potential induced by watering the plants with a nutrient solution (minus 0.5 bar) and further additions of KNO[sub3] (minus 3 and minus 6 bar). After leaching of the soil, the plants were exposed to two drying cycles separated by rewetting. With severe water stress at the end of drying cycle 1, reduction of wilting was highly correlated with lowering of leaf osmotic potential due to the KNO[sub3] pretreatment. Turgor potential was highest and decreased at lowest soil water matric potentials in KNO[sub3] pretreated plants. Recovery after rewatering was seemingly most accelerated in KNO[sub3] pretreated plants due to a larger potential difference between leaf and soil water. The leaf and soil water matric potentials at which stomata closed were lowered as a result of both KNO[sub3] pretreatment and the effect of previous water stress on leaf osmotic potential. [ABSTRACT FROM AUTHOR]

Seasonal changes in the relationship between stomatal responses and water potential components in detached leaves of sweet cherry (Prunus avium L. cv. Bigarreau Morreau) and plum (Prunus domestica L. cv. Queen Victoria) were examined by the simultaneous measurement of stomatal conductance and water relation parameters derived from pressure-volume curves. The water potential threshold for stomatal closure in sweet cherry leaves throughout the season and in plum leaves until mid-July varied nearly in parallel with changes in the osmotic potential at the turgor loss point. From August onward, closing reactions of plum stomata had no relationship to the turgor loss point. Stomata of sweet cherry leaves opened more widely and showed slower reactions to dehydration early in the season than later. Seasonal changes of stomatal conductance and stomatal responses to dehydration in both species seem thus to be influenced by fruit development and ripening. [ABSTRACT FROM AUTHOR]

Commelina communis L. guard cell protoplasts were induced to swell under low CO2 conditions in the light while incubated in media containing KCl. Precise measurements of changes in the volume of the protoplasts were made including estimates of protoplast non‐osmotic volume by Boyle‐van't Hoff analysis. The amount of K+ which accumulated during the treatment was measured. The observed changes in osmotic volume could be accounted for by the uptake of K+ which appeared to be balanced by an anion or anions with an effective mean charge of – 1.63. The K+ flux rates occurring in guard cell protoplasts were sufficient to explain guard cell turgor regulation in vivo. [ABSTRACT FROM AUTHOR]

The relationship between stomatal aperture (a) and guard cell pressure (Pg) was measured directly in four different species (Vicia faba, Tradescantia virginiana, Ginkgo biloba and Nephrolepis exaltata) using a special cell pressure probe technique. The effect of epidermal turgor (Pep) on this relationship was also measured in T. virginiana. The relationship was sigmoidal for V. faba and T. virginiana, but entirely convex for G. biloba and N. exaltata. Epidermal turgor was found to have a pronounced closing effect on stomata of T. virginiana. Maximum aperture with full epidermal turgor (0·92 MPa) was about half that with zero epidermal turgor. Also, with full epidermal turgor stomata of T. virginiana did not begin to open until Pg was more than 1·25 MPa. These characteristics were used to develop an expression for a as a function of Pg and Pep. Results for the different species are compared and discussed in terms of possible advantages and limitations of water economy. [ABSTRACT FROM AUTHOR]

An empirical model for stomatal conductance (g), proposed by Leuning (1995, this issue) as a modification of Ball, Woodrow & Berry's (1987) model, is interpreted in terms of a simple, steady-state model of guard cell function. In this model, stomatal aperture is a function of the relative turgor between guard cells and epidermal cells. The correlation between g and leaf surface vapour pressure deficit in Leuning's model is interpreted in terms of stomatal sensing of the transpiration rate, via changes in the gradient of total water potential between guard cells and epidermal cells. The correlation between g, CO2 assimilation rate and leaf surface CO2 concentration in Leuning's model is interpreted as a relationship between the corresponding osmotic gradient, irradiance, temperature, intercellular CO2 concentration and stomatal aperture itself. The explicit relationship between osmotic gradient and stomatal aperture (possibly describing the effect of changes in guard cell volume on the membrane permeability for ion transport) results in a decrease in the transpiration rate in sufficiently dry air. Possible extension of the guard cell model to include stomatal responses to soil water status is discussed. [ABSTRACT FROM AUTHOR]

Previous work with clones of Populus trichocarpa demonstrated that the water vapour conductance of leaves from well-watered cuttings of this species does not decline with loss of turgor from the bulk leaf. In the present study, stomatal responses to water potential in Populus were examined with detached epidermal strips. Stomata in epidermal strips from well-watered plants of P. trichocarpa did not close at low water potentials which led to plasmolysis of the guard cells. In contrast, stomata of P. deltoides and a P. trichocarpa × deltoides hybrid closed when the guard cells lost turgor. A period of water stress preconditioning resulted in modified stomatal responses in P. trichocarpa such that stomata of stressed and re-watered plants nearly closed when guard cell turgor was lost. [ABSTRACT FROM AUTHOR]

Moisture release characteristics and field measurements of physiological parameters (conductance and water potential) and environmental parameters (ambient temperature, water vapour saturation deficit and photosynthetic photon flux density) were measured for phyllodes and compound leaves of Acacia koa over a 2 month period at Hawaii Volcanoes National Park, Hawaii, to determine what differences in water relations might occur between leaf types. The phyllodes were found to contain more water at full turgor, use less water in turgor control and have stomatal conductances more closely associated with bulk leaf water status and environmental variables. These results suggest that the phyllodes are more drought adapted, whereas the compound leaves probably promote more rapid early growth during periods of high moisture availability. [ABSTRACT FROM AUTHOR]

Previous mathmatical analyses of stomatal mechanisms have demonstrated, and experimental measurements have confirmed, that the turgor-generated force fo the epidermal cells dominates that of the guard cells in determining aperture. DeMichele & Sharpe (1973) termed the phenomenon the mechanical advantage of the epidermal cells, while Cooke et al. (1976) expressed it as an antagonism ratio. Both of these formulations, however, have theoretical or practical limitations. This study presents a biophysical analysis demonstrating that the efffective forces in the stomatal system may be studied in terms of simple stomatal geometry. From this analysis, the mechanical advantage can be redefined and interpreted based upon simple geometric relationships calculated from measurable anatomical dimensions. [ABSTRACT FROM AUTHOR]

That stomata respond to environmental stimuli by opening and closing is well established. These responses appear to be of advantage to a plant in the way they affect water use and photosynthesis, for example by maximizing photosynthesis while minimizing water loss or by providing increased evaporative cooling at high temperatures. For such responses to occur, the environment must be sensed so that the osmotic potentials of the guard, and possibly the subsidiary cells, will change, resulting in changes in the turgor pressure of these cells and causing the stomatal aperture to change.