Stomatal responses to flooding of the intercellular air spaces suggest a vapor-phase signal between the mesophyll and the guard cells

The poorly-explored stomatal response to temperature at constant evaporative demand

Changes in leaf temperature are known to drive stomatal responses, because the leaf-to-air water vapour gradient (Δw) increases with temperature if ambient vapour pressure is held constant, and stomata respond to changes in Δw. However, the direct response of stomata to temperature (DRST; the response when Δw is held constant by adjusting ambient humidity) has been examined far less extensively. Though the meagre available data suggest the response is usually positive, results differ widely and defy broad generalisation. As a result, little is known about the DRST. This review discusses the current state of knowledge about the DRST, including numerous hypothesised biophysical mechanisms, potential implications of the response for plant adaptation, and possible impacts of the DRST on plant-atmosphere carbon and water exchange in a changing climate.

Calvin cycle and guard cell metabolism impact stomatal function

Stomatal conductance (gs) determines CO2 uptake for photosynthesis (A) and water loss through transpiration, which is essential for evaporative cooling and maintenance of optimal leaf temperature as well as nutrient uptake. Stomata adjust their aperture to maintain an appropriate balance between CO2 uptake and water loss and are therefore critical to overall plant water status and productivity. Although there is considerable knowledge regarding guard cell (GC) osmoregulation (which drives differences in GC volume and therefore stomatal opening and closing), as well as the various signal transduction pathways that enable GCs to sense and respond to different environmental stimuli, little is known about the signals that coordinate mesophyll demands for CO2. Furthermore, chloroplasts are a key feature in GCs of many species, however, their role in stomatal function is unclear and a subject of debate. In this review we explore the current evidence regarding the role of these organelles in stomatal behaviour, including GC electron transport and Calvin-Benson-Bassham (CBB) cycle activity as well as their possible involvement correlating gs and A along with other potential mesophyll signals. We also examine the roles of other GC metabolic processes in stomatal function.

A role for ethylene signaling and biosynthesis in regulating and accelerating CO2 - and abscisic acid-mediated stomatal movements in Arabidopsis

Little is known about long-distance mesophyll-driven signals that regulate stomatal conductance. Soluble and/or vapor-phase molecules have been proposed. In this study, the involvement of the gaseous signal ethylene in the modulation of stomatal conductance in Arabidopsis thaliana by CO₂ /abscisic acid (ABA) was examined. We present a diffusion model which indicates that gaseous signaling molecule/s with a shorter/direct diffusion pathway to guard cells are more probable for rapid mesophyll-dependent stomatal conductance changes. We, therefore, analyzed different Arabidopsis ethylene-signaling and biosynthesis mutants for their ethylene production and kinetics of stomatal responses to ABA/[CO₂ ]-shifts. According to our research, higher [CO₂ ] causes Arabidopsis rosettes to produce more ethylene. An ACC-synthase octuple mutant with reduced ethylene biosynthesis exhibits dysfunctional CO₂ -induced stomatal movements. Ethylene-insensitive receptor (gain-of-function), etr1-1 and etr2-1, and signaling, ein2-5 and ein2-1, mutants showed intact stomatal responses to [CO₂ ]-shifts, whereas loss-of-function ethylene receptor mutants, including etr2-3;ein4-4;ers2-3, etr1-6;etr2-3 and etr1-6, showed markedly accelerated stomatal responses to [CO₂ ]-shifts. Further investigation revealed a significantly impaired stomatal closure to ABA in the ACC-synthase octuple mutant and accelerated stomatal responses in the etr1-6;etr2-3, and etr1-6, but not in the etr2-3;ein4-4;ers2-3 mutants. These findings suggest essential functions of ethylene biosynthesis and signaling components in tuning/accelerating stomatal conductance responses to CO₂ and ABA.

Stomata on the abaxial and adaxial leaf surfaces contribute differently to leaf gas exchange and photosynthesis in wheat

Although stomata are typically found in greater numbers on the abaxial surface, wheat flag leaves have greater densities on the adaxial surface. We determine the impact of this less common stomatal patterning on gaseous fluxes using a novel chamber that simultaneously measures both leaf surfaces. Using a combination of differential illuminations and CO₂ concentrations at each leaf surface, we found that mesophyll cells associated with the adaxial leaf surface have a higher photosynthetic capacity than those associated with the abaxial leaf surface, which is supported by an increased stomatal conductance (driven by differences in stomatal density). When vertical gas flux at the abaxial leaf surface was blocked, no compensation by adaxial stomata was observed, suggesting each surface operates independently. Similar stomatal kinetics suggested some co-ordination between the two surfaces, but factors other than light intensity played a role in these responses. Higher photosynthetic capacity on the adaxial surface facilitates greater carbon assimilation, along with higher adaxial stomatal conductance, which would also support greater evaporative leaf cooling to maintain optimal leaf temperatures for photosynthesis. Furthermore, abaxial gas exchange contributed c. 50% to leaf photosynthesis and therefore represents an important contributor to overall leaf gas exchange.

Stomatal Responses to Light, CO2, and Mesophyll Tissue in Vicia faba and Kalanchoë fedtschenkoi

The responses of stomatal aperture to light intensity and CO₂ concentration were studied in both Vicia faba (C₃) and Kalanchoë fedtschenkoi (Crassulacean acid metabolism; CAM), in material sampled from both light and dark periods. Direct comparison was made between intact leaf segments, epidermises grafted onto exposed mesophyll, and isolated epidermal peels, including transplantations between species and between diel periods. We reported the stomatal opening in response to darkness in isolated CAM peels from the light period, but not from the dark. Furthermore, we showed that C₃ mesophyll has stimulated CAM stomata in transplanted peels to behave as C₃ in response to light and CO₂. By using peels and mesophyll from plants sampled in the dark and the light period, we provided clear evidence that CAM stomata behaved differently from C₃. This might be linked to stored metabolites/ions and signalling pathway components within the guard cells, and/or a mesophyll-derived signal. Overall, our results provided evidence for both the involvement of guard cell metabolism and mesophyll signals in stomatal responses in both C₃ and CAM species.

Guard Cell Metabolism and Stomatal Function

The control of gaseous exchange between the leaf and external atmosphere is governed by stomatal conductance (g_s); therefore, stomata play a critical role in photosynthesis and transpiration and overall plant productivity. Stomatal conductance is determined by both anatomical features and behavioral characteristics. Here we review some of the osmoregulatory pathways in guard cell metabolism, genes and signals that determine stomatal function and patterning, and the recent work that explores coordination between g_s and carbon assimilation (A) and the influence of spatial distribution of functional stomata on underlying mesophyll anatomy. We also evaluate the current literature on mesophyll-driven signals that may coordinate stomatal behavior with mesophyll carbon assimilation and explore stomatal kinetics as a possible target to improve A and water use efficiency. By understanding these processes, we can start to provide insight into manipulation of these regulatory pathways to improve stomatal behavior and identify novel unexploited targets for altering stomatal behavior and improving crop plant productivity.

Role of blue and red light in stomatal dynamic behaviour

Plants experience changes in light intensity and quality due to variations in solar angle and shading from clouds and overlapping leaves. Stomatal opening to increasing irradiance is often an order of magnitude slower than photosynthetic responses, which can result in CO2 diffusional limitations on leaf photosynthesis, as well as unnecessary water loss when stomata continue to open after photosynthesis has reached saturation. Stomatal opening to light is driven by two distinct pathways; the 'red' or photosynthetic response that occurs at high fluence rates and saturates with photosynthesis, and is thought to be the main mechanism that coordinates stomatal behaviour with photosynthesis; and the guard cell-specific 'blue' light response that saturates at low fluence rates, and is often considered independent of photosynthesis, and important for early morning stomatal opening. Here we review the literature on these complicated signal transduction pathways and osmoregulatory processes in guard cells that are influenced by the light environment. We discuss the possibility of tuning the sensitivity and magnitude of stomatal response to blue light which potentially represents a novel target to develop ideotypes with the 'ideal' balance between carbon gain, evaporative cooling, and maintenance of hydraulic status that is crucial for maximizing crop performance and productivity.

Effects of the mesophyll on stomatal responses in amphistomatous leaves

The role of the mesophyll in stomatal functioning in thin amphistomatous leaves was investigated by altering gas exchange for one surface and observing the effects on stomatal conductance for the other surface. Three methods of perturbing gas exchange on the adaxial surface were used. First, gas exchange for the adaxial surface was completely blocked with plastic wrap or vacuum grease. Second, leaves were inverted to induce closure of the adaxial stomata. And third, ambient humidity for the adaxial surface was reduced to induce stomatal closure on that surface. Experiments were performed at low light intensity and three different CO₂ concentrations to test the idea that stomatal responses in thin amphistomatous leaves are partially controlled by a signal from the mesophyll that varies with light and CO₂ . In general, stomata on the abaxial surface opened when gas exchange on the adaxial surface was reduced, with the largest increases in conductance occurring at high CO₂ concentration. The data are discussed with respect to role of a purported signal from the mesophyll and the partitioning of that signal between the two surfaces of the leaf.

Diurnal Variation in Gas Exchange: The Balance between Carbon Fixation and Water Loss

Stomatal control of transpiration is critical for maintaining important processes, such as plant water status, leaf temperature, as well as permitting sufficient CO2 diffusion into the leaf to maintain photosynthetic rates (A). Stomatal conductance often closely correlates with A and is thought to control the balance between water loss and carbon gain. It has been suggested that a mesophyll-driven signal coordinates A and stomatal conductance responses to maintain this relationship; however, the signal has yet to be fully elucidated. Despite this correlation under stable environmental conditions, the responses of both parameters vary spatially and temporally and are dependent on species, environment, and plant water status. Most current models neglect these aspects of gas exchange, although it is clear that they play a vital role in the balance of carbon fixation and water loss. Future efforts should consider the dynamic nature of whole-plant gas exchange and how it represents much more than the sum of its individual leaf-level components, and they should take into consideration the long-term effect on gas exchange over time.

The RopGEF2-ROP7/ROP2 Pathway Activated by phyB Suppresses Red Light-Induced Stomatal Opening

Circadian rhythm of stomatal aperture is mainly regulated by light/darkness. Blue and red light induce stomatal opening through different mechanisms that are mediated by special receptors. ROP2, a member of Rho GTPase family in Arabidopsis (Arabidopsisthaliana), has been found to negatively regulate light-induced stomatal opening. However, the upstream guanine nucleotide exchange factor (GEF) RopGEFs have not been revealed, and it is unclear which photoreceptor is required for the action of RopGEFs-ROPs. Here, we showed that RopGEF2 acted as a negative regulator in phytochrome B (phyB)-mediated red light-induced stomatal opening. Meanwhile, ROP7, another member of ROP family, acting redundantly with ROP2, was regulated by RopGEF2 in this process. RopGEF2 interacted with ROP7 and ROP2 and enhanced their intrinsic nucleotide exchange rates. Furthermore, the direct interactions between phyB and RopGEF2 were detected in vitro and in plants, and phyB enhanced the GEF activity of RopGEF2 toward both ROP7 and ROP2 under light. In addition, RopGEF4 functioned redundantly with RopGEF2 in red light-induced stomatal opening by activating both ROP7 and ROP2, and RopGEF2/RopGEF4 acted genetically downstream of phyB; however, the GEF activity of RopGEF4 was not directly enhanced by phyB. These results revealed that red light-activated phyB enhances the GEF activities of RopGEF2 and RopGEF4 directly or indirectly, and then activate both ROP7 and ROP2 in guard cells. The negative mechanism triggered by phyB prevents the excessive stomatal opening under red light.

Evolution of the Stomatal Regulation of Plant Water Content

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

Exogenous Supplementation of Silicon Improved the Recovery of Hyperhydric Shoots in Dianthus caryophyllus L. by Stabilizing the Physiology and Protein Expression

Hyperhydricity is one of the major problems hindering in vitro propagation of Dianthus caryophyllus L. Silicon (Si) is a well-known beneficial element renowned for its stress amelioration properties in plants. This study has demonstrated the physiological and molecular mechanism behind the Si-mediated recovery from hyperhydricity in D. caryophyllus L. 'Green Beauty'. Four weeks old hyperhydric shoots obtained from temporary immersion system were cultured on the Murashige and Skoog medium supplemented with 0 (control), 1.8 mM, or 3.6 mM of potassium silicate (K2SiO3). After 2 weeks of culture, we observed only 20% of hyperhydric shoots were recovered in control. On the other hand hyperhydricity, shoot recovery percentage in 1.8 mM and 3.6 mM of Si were 44% and 36%, respectively. Shoots in control possessed higher lipid peroxidation rate compared to the Si treatments. Similarly, damaged stomata were detected in the control, while Si treatments restored the normal stomatal development. Expressions of superoxide dismutase, guaiacol peroxidase, and catalase varied between the control and Si treatments. Furthermore, a proteomic analysis showed that as compared with the control Si up-regulated 17 and 10 protein spots in abundance at 1.8 and 3.6 mM of Si, respectively. In comparison to the 3.6 mM, 1.8 mM of Si treatment up-regulated 19 proteins and down-regulated 7 proteins. Identified proteins were categorized into six groups according to their biological roles such as ribosomal binding, oxido-reduction, hormone/cell signaling, metal/ion binding, defense, and photosynthesis. The proteomic results revealed that Si actively involved in the various metabolisms to accelerate the recovery of the shoots from hyperhydricity. Thus, the outcomes of this study can be utilized for addressing the molecular insight of hyperhydricity and its recovery mechanism by the supplementation of Si. Therefore, we conclude that active involvement of Si in the regulation and signaling process of proteins at 1.8 mM concentration could be efficient to trigger the reclamation process of hyperhydric carnation shoots.

Rethinking Guard Cell Metabolism

Stomata control gaseous fluxes between the internal leaf air spaces and the external atmosphere and, therefore, play a pivotal role in regulating CO2 uptake for photosynthesis as well as water loss through transpiration. Guard cells, which flank the stomata, undergo adjustments in volume, resulting in changes in pore aperture. Stomatal opening is mediated by the complex regulation of ion transport and solute biosynthesis. Ion transport is exceptionally well understood, whereas our knowledge of guard cell metabolism remains limited, despite several decades of research. In this review, we evaluate the current literature on metabolism in guard cells, particularly the roles of starch, sucrose, and malate. We explore the possible origins of sucrose, including guard cell photosynthesis, and discuss new evidence that points to multiple processes and plasticity in guard cell metabolism that enable these cells to function effectively to maintain optimal stomatal aperture. We also discuss the new tools, techniques, and approaches available for further exploring and potentially manipulating guard cell metabolism to improve plant water use and productivity.

Elevated-CO2 Response of Stomata and Its Dependence on Environmental Factors

Stomata control the flow of gases between plants and the atmosphere. This review is centered on stomatal responses to elevated CO2 concentration and considers other key environmental factors and underlying mechanisms at multiple levels. First, an outline of general responses in stomatal conductance under elevated CO2 is presented. Second, stomatal density response, its development, and the trade-off with leaf growth under elevated CO2 conditions are depicted. Third, the molecular mechanism regulating guard cell movement at elevated CO2 is suggested. Finally, the interactive effects of elevated CO2 with other factors critical to stomatal behavior are reviewed. It may be useful to better understand how stomata respond to elevated CO2 levels while considering other key environmental factors and mechanisms, including molecular mechanism, biochemical processes, and ecophysiological regulation. This understanding may provide profound new insights into how plants cope with climate change.

Characterization of the Chloride Channel-Like, AtCLCg, Involved in Chloride Tolerance in Arabidopsis thaliana

In plant cells, anion channels and transporters are essential for key functions such as nutrition, ion homeostasis and resistance to biotic or abiotic stresses. We characterized AtCLCg, a member of the chloride channel (CLC) family in Arabidopsis localized in the vacuolar membrane. When grown on NaCl or KCl, atclcg knock-out mutants showed a decrease in biomass. In the presence of NaCl, these mutants overaccumulate chloride in shoots. No difference in growth was detected in response to osmotic stress by mannitol. These results suggest a physiological function of AtCLCg in the chloride homeostasis during NaCl stress. AtCLCg shares a high degree of identity (62%) with AtCLCc, another vacuolar CLC essential for NaCl tolerance. However, the atclcc atclccg double mutant is not more sensitive to NaCl than single mutants. As the effects of both mutations are not additive, gene expression analyses were performed and revealed that: (i)AtCLCg is expressed in mesophyll cells, hydathodes and phloem while AtCLCc is expressed in stomata; and (ii)AtCLCg is repressed in the atclcc mutant background, and vice versa. Altogether these results demonstrate that both AtCLCc and AtCLCg are important for tolerance to excess chloride but not redundant, and form part of a regulatory network controlling chloride sensitivity.

Elevated-CO2 Response of Stomata and Its Dependence on Environmental Factors

Stomata control the flow of gases between plants and the atmosphere. This review is centered on stomatal responses to elevated CO2 concentration and considers other key environmental factors and underlying mechanisms at multiple levels. First, an outline of general responses in stomatal conductance under elevated CO2 is presented. Second, stomatal density response, its development, and the trade-off with leaf growth under elevated CO2 conditions are depicted. Third, the molecular mechanism regulating guard cell movement at elevated CO2 is suggested. Finally, the interactive effects of elevated CO2 with other factors critical to stomatal behavior are reviewed. It may be useful to better understand how stomata respond to elevated CO2 levels while considering other key environmental factors and mechanisms, including molecular mechanism, biochemical processes, and ecophysiological regulation. This understanding may provide profound new insights into how plants cope with climate change.

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

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

Lacking chloroplasts in guard cells of crumpled leaf attenuates stomatal opening: both guard cell chloroplasts and mesophyll contribute to guard cell ATP levels

Controversies regarding the function of guard cell chloroplasts and the contribution of mesophyll in stomatal movements have persisted for several decades. Here, by comparing the stomatal opening of guard cells with (crl-ch) or without chloroplasts (crl-no ch) in one epidermis of crl (crumpled leaf) mutant in Arabidopsis, we showed that stomatal apertures of crl-no ch were approximately 65-70% those of crl-ch and approximately 50-60% those of wild type. The weakened stomatal opening in crl-no ch could be partially restored by imposing lower extracellular pH. Correspondingly, the external pH changes and K(+) accumulations following fusicoccin (FC) treatment were greatly reduced in the guard cells of crl-no ch compared with crl-ch and wild type. Determination of the relative ATP levels in individual cells showed that crl-no ch guard cells contained considerably lower levels of ATP than did crl-ch and wild type after 2 h of white light illumination. In addition, guard cell ATP levels were lower in the epidermis than in leaves, which is consistent with the observed weaker stomatal opening response to white light in the epidermis than in leaves. These results provide evidence that both guard cell chloroplasts and mesophyll contribute to the ATP source for H(+) extrusion by guard cells.

Mesophyll photosynthesis and guard cell metabolism impacts on stomatal behaviour

Stomata control gaseous fluxes between the internal leaf air spaces and the external atmosphere. Guard cells determine stomatal aperture and must operate to ensure an appropriate balance between CO2 uptake for photosynthesis (A) and water loss, and ultimately plant water use efficiency (WUE). A strong correlation between A and stomatal conductance (gs ) is well documented and often observed, but the underlying mechanisms, possible signals and metabolites that promote this relationship are currently unknown. In this review we evaluate the current literature on mesophyll-driven signals that may coordinate stomatal behaviour with mesophyll carbon assimilation. We explore a possible role of various metabolites including sucrose and malate (from several potential sources; including guard cell photosynthesis) and new evidence that improvements in WUE have been made by manipulating sucrose metabolism within the guard cells. Finally we discuss the new tools and techniques available for potentially manipulating cell-specific metabolism, including guard and mesophyll cells, in order to elucidate mesophyll-derived signals that coordinate mesophyll CO2 demands with stomatal behaviour, in order to provide a mechanistic understanding of these processes as this may identify potential targets for manipulations in order to improve plant WUE and crop yield.

Is the signal from the mesophyll to the guard cells a vapour-phase ion?

Previous studies have suggested that the red light and CO2 responses of stomata are caused by a signal from the mesophyll to the guard cells. Experiments were conducted to test the idea that this signal is a vapour-phase ion. Stomata in isolated epidermes of Tradescantia pallida were found to respond to air ions created by an electrode that was positioned under the epidermes. Anthocyanins in the epidermes of this species were observed to change colour in response to these air ions, and this change in colour was attributed to changes in pH. A similar change in lower epidermal colour was observed in intact leaves upon illumination and with changes in CO2 concentration. Based on the change in epidermal colour, the pH of the epidermis was estimated to be approximately 7.0 in darkness and 6.5 in the light. Stomata in isolated epidermes responded to pH when suspended over (but not in contact with) solutions of different pH. We speculate that stomatal responses to CO2 and light are caused by vapour-phase ions, possibly hydronium ions that change the pH of the epidermis.

Opinion: the red-light response of stomatal movement is sensed by the redox state of the photosynthetic electron transport chain

Guard cells regulate CO2 uptake and water loss of a leaf by controlling stomatal movement in response to environmental factors such as CO2, humidity, and light. The mechanisms by which stomata respond to red light are actively debated in the literature, and even after decades of research it is still controversial whether stomatal movement is related to photosynthesis or not. This review summarizes the current knowledge of the red-light response of stomata. A comparison of published evidence suggests that stomatal movement is controlled by the redox state of photosynthetic electron transport chain components, in particular the redox state of plastoquinone. Potential consequences for the modeling of stomatal conductance are discussed.

Unique responsiveness of angiosperm stomata to elevated CO2 explained by calcium signalling

Angiosperm and conifer tree species respond differently when exposed to elevated CO2, with angiosperms found to dynamically reduce water loss while conifers appear insensitive. Such distinct responses are likely to affect competition between these tree groups as atmospheric CO2 concentration rises. Seeking the mechanism behind this globally important phenomenon we targeted the Ca(2+)-dependent signalling pathway, a mediator of stomatal closure in response to elevated CO2, as a possible explanation for the differentiation of stomatal behaviours. Sampling across the diversity of vascular plants including lycophytes, ferns, gymnosperms and angiosperms we show that only angiosperms possess the stomatal behaviour and prerequisite genetic coding, linked to Ca(2+)-dependent stomatal signalling. We conclude that the evolution of Ca(2+)-dependent stomatal signalling gives angiosperms adaptive benefits in terms of highly efficient water use, but that stomatal sensitivity to high CO2 may penalise angiosperm productivity relative to other plant groups in the current era of soaring atmospheric CO2.

Flooding of the apoplast is a key factor in the development of hyperhydricity

The physiological disorder hyperhydricity occurs frequently in tissue culture and causes several morphological abnormalities such as thick, brittle, curled, and translucent leaves. It is well known that hyperhydric shoots are characterized by a high water content, but how this is related to the abnormalities is not clear. It was observed that water accumulated extensively in the apoplast of leaves of hyperhydric Arabidopsis seedlings and flooded apoplastic air spaces almost completely. In hyperhydric Arabidopsis seedlings, the volume of apoplastic air was reduced from 85% of the apoplast to only 15%. Similar results were obtained with hyperhydric shoots of statice. The elevated expression of hypoxia-responsive genes in hyperhydric seedlings showed that the water saturation of the apoplast decreased oxygen supply. This demonstrates a reduced gas exchange between the symplast and its surroundings, which will consequently lead to the accumulation of gases in the symplast, for example ethylene and methyl jasmonate. The impairment of gas exchange probably brings about the symptoms of hyperhydricity. Interestingly, stomatal aperture was reduced in hyperhydric plants, a previously reported response to injection of water into the apoplast. Closure of the stomata and the accumulation of water in the apoplast may be the reasons why seedlings with a low level of hyperhydricity showed improved acclimatization after planting into soil.

Apoplastic mesophyll signals induce rapid stomatal responses to CO2 in Commelina communis

Previous studies have suggested that the mesophyll contributes to stomatal CO(2) responses. The effects of changes in CO(2) concentration (100 or 700 ppm) on stomatal responses in red or white light were examined microscopically in a leaf segment, an epidermal strip and an epidermal strip placed on a mesophyll segment of Commelina communis, all mounted on a buffer-containing gel. In both red and white light, stomata of the leaf segment opened/closed rapidly at low/high CO(2). In red light, epidermal strip stomata barely responded to CO(2). In white light, they opened at low CO(2), but hardly closed at high CO(2). Stomata of the epidermal strip placed on the mesophyll responded in the same manner as those on the leaf segment. Insertion of a doughnut-shaped cellophane spacer (but not polyethylene spacer) between the epidermal strip and the mesophyll hardly altered these responses. Stomata in leaf segments treated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), a photosynthesis inhibitor, did not open in red light, but opened/closed at low/high CO(2) in white light. These results indicate that the apoplast transfer of 'mesophyll signals' and the stomatal opening at low CO(2) are dependent on photosynthesis, whereas the stomatal closure at high CO(2) is independent of photosynthesis.

The dual effect of abscisic acid on stomata

The classical view that the drought-related hormone ABA simply acts locally at the guard cell level to induce stomatal closure is questioned by differences between isolated epidermis and intact leaves in stomatal response to several stimuli. We tested the hypothesis that ABA mediates, in addition to a local effect, a remote effect in planta by changing hydraulic regulation in the leaf upstream of the stomata. By gravimetry, porometry to water vapour and argon, and psychrometry, we investigated the effect of exogenous ABA on transpiration, stomatal conductance and leaf hydraulic conductance of mutants described as ABA-insensitive at the guard cell level. We show that foliar transpiration of several ABA-insensitive mutants decreases in response to ABA. We demonstrate that ABA decreases stomatal conductance and down-regulates leaf hydraulic conductance in both the wildtype Col-0 and the ABA-insensitive mutant ost2-2. We propose that ABA promotes stomatal closure in a dual way via its already known biochemical effect on guard cells and a novel, indirect hydraulic effect through a decrease in water permeability within leaf vascular tissues. Variability in sensitivity of leaf hydraulic conductance to ABA among species could provide a physiological basis to the isohydric or anisohydric behaviour.

Responses of stomatal conductance to simultaneous changes in two environmental factors

To clarify interactions between stomatal responses to two simultaneous environmental changes, the rates of change in stomatal conductance were measured after simultaneously changing two environmental factors from the set of air humidity, leaf water potential (hydraulic environmental factors), air CO(2) concentration and light intensity (photosynthetic environmental factors). The stomatal responses to changes in leaf water potential were not significantly modified by any other simultaneous environmental change. A decrease in air humidity was followed by a decrease in stomatal conductance, and an increase in air humidity was followed by an increase in the conductance, irrespective of the character of the simultaneous change in the photosynthetic environmental factor. If the simultaneous change had an opposite effect on stomatal conductance, the rate of change in stomatal conductance was higher than the theoretical summed rate-the sum of the rate following one environmental change and the rate following another environmental change, measured separately. That is, the stomatal response to air humidity dominated over the responses to photosynthetic environmental factors. Yet, if the simultaneous change in photosynthetic factors had a codirectional effect on stomatal conductance, the rate of stomatal conductance change was lower than the theoretical summed rate. After a simultaneous change of two photosynthetic environmental factors, the rate of stomatal conductance change was very similar to the theoretical rate, if both the environmental changes had a codirectional effect on stomatal conductance. If the changes in the photosynthetic factors had opposite effects on stomatal conductance, the conductance increased, irrespective of the character of the increasing environmental factor. In drought-stressed trees, the rates of change in stomatal conductance tended to differ from the theoretical summed rates more than in well-watered trees. Stomatal closure following an increase in CO(2) concentration was the stomatal response that was most strongly suppressed by the response to another simultaneous environmental change. Six species of temperate deciduous trees were shown to be similar in their relations between the stomatal responses to two simultaneous environmental changes. The mechanism and ecological significance of the interactions between the two signal response pathways of stomata are discussed.

Antisense inhibition of the iron-sulphur subunit of succinate dehydrogenase enhances photosynthesis and growth in tomato via an organic acid-mediated effect on stomatal aperture

Transgenic tomato (Solanum lycopersicum) plants expressing a fragment of the Sl SDH2-2 gene encoding the iron sulfur subunit of the succinate dehydrogenase protein complex in the antisense orientation under the control of the 35S promoter exhibit an enhanced rate of photosynthesis. The rate of the tricarboxylic acid (TCA) cycle was reduced in these transformants, and there were changes in the levels of metabolites associated with the TCA cycle. Furthermore, in comparison to wild-type plants, carbon dioxide assimilation was enhanced by up to 25% in the transgenic plants under ambient conditions, and mature plants were characterized by an increased biomass. Analysis of additional photosynthetic parameters revealed that the rate of transpiration and stomatal conductance were markedly elevated in the transgenic plants. The transformants displayed a strongly enhanced assimilation rate under both ambient and suboptimal environmental conditions, as well as an elevated maximal stomatal aperture. By contrast, when the Sl SDH2-2 gene was repressed by antisense RNA in a guard cell-specific manner, changes in neither stomatal aperture nor photosynthesis were observed. The data obtained are discussed in the context of the role of TCA cycle intermediates both generally with respect to photosynthetic metabolism and specifically with respect to their role in the regulation of stomatal aperture.

Augmentation of abscisic acid (ABA) levels by drought does not induce short-term stomatal sensitivity to CO2 in two divergent conifer species

The stomata of conifers display very little short-term response to changes in atmospheric CO(2) concentration (C(a)), whereas the stomatal responses of angiosperms to C(a) increase in response to water stress. This behaviour of angiosperm stomata appears to be dependent on foliar levels of abscisic acid (ABA(f)). Here two alternative explanations for the stomatal insensitivity of conifers to C(a) are tested: that conifers have either low ABA(f) or a higher or absent threshold for ABA-induced sensitivity. The responsiveness of stomatal conductance (g(s)) to a sequence of transitions in C(a) (386, 100, and 600 μmol mol(-1)) was recorded over a range of ABA(f) in an angiosperm and two divergent conifer species. The different ABA levels were induced by a mild drought cycle. Although the angiosperm and conifer species showed similar proportional increases in ABA(f) following drought, conifer stomata remained insensitive to changes in C(a) whereas angiosperm stomata showed enhanced sensitivity with increasing ABA(f). The conifers, however, had much higher ABA(f) prior to drought than the angiosperm species, suggesting that non-sensitivity to C(a) in these conifers was due to an absent or inactive response/signalling pathway rather than insufficient ABA(f).