Jasmonate‐mediated stomatal closure under elevated CO2 revealed by time‐resolved metabolomics

Foliar stomatal movements are critical for regulating plant water loss and gas exchange. Elevated carbon dioxide (CO₂) levels are known to induce stomatal closure. However, the current knowledge on CO₂ signal transduction in stomatal guard cells is limited. Here we report metabolomic responses of Brassica napus guard cells to elevated CO₂ using three hyphenated metabolomics platforms: gas chromatography‐mass spectrometry (MS); liquid chromatography (LC)‐multiple reaction monitoring‐MS; and ultra‐high‐performance LC‐quadrupole time‐of‐flight‐MS. A total of 358 metabolites from guard cells were quantified in a time‐course response to elevated CO₂ level. Most metabolites increased under elevated CO₂, showing the most significant differences at 10 min. In addition, reactive oxygen species production increased and stomatal aperture decreased with time. Major alterations in flavonoid, organic acid, sugar, fatty acid, phenylpropanoid and amino acid metabolic pathways indicated changes in both primary and specialized metabolic pathways in guard cells. Most interestingly, the jasmonic acid (JA) biosynthesis pathway was significantly altered in the course of elevated CO₂ treatment. Together with results obtained from JA biosynthesis and signaling mutants as well as CO₂ signaling mutants, we discovered that CO₂‐induced stomatal closure is mediated by JA signaling.     

Guard cell photosynthesis is critical for stomatal turgor production,yet does not directly mediate CO2‐ and ABA‐induced stomatal closing

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, F_V′/F_M′) 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.     

A method for measuring whole plant photosynthesis in Arabidopsis thaliana

Measurement of photosynthesis of intact leaves of Arabidopsis thaliana has been prohibitive due to the small leaf size and prostrate growth habit. Because of the widespread use of Arabidopsis for plant science research it is important to have a procedure for accurate, nondestructive measurement of its photosynthesis. We developed and tested a method for analysis of photosynthesis in whole plants of Arabidopsis. Net carbon assimilation and stomatal conductance were measured with an open gas exchange system and photosynthetic oxygen evolution was determined from chlorophyll fluorescence parameters. Individual plants were grown in 50 cubic centimeter tubes that were attached with an air tight seal to an enclosed gas exchange chamber for measurement of carbon dioxide and water exchange by the whole plant. Chlorophyll fluorescence from intact leaves was simultaneously measured with a pulse modulated fluorometer. Photosynthetic CO₂ assimilation and stomatal conductance rates were calculated with established gas exchange procedures and O₂ evolution was determined from chlorophyll fluorescence measurement of Photosystem II yield. Carbon assimilation and oxygen evolution in response to light intensity and ambient CO₂ concentration was measured and is presented here to demonstrate the potential use of this method for investigation of photosynthesis of Arabidopsis plants in controlled environment conditions.     

Effect of Elevated Carbon Dioxide Concentration on the Stomatal Parameters of Rice Cultivars

The response of stomatal parameters of four rice cultivars to atmospheric elevated CO₂ concentration (EC) was studied using open top chambers. EC brought about reduction in stomatal conductance and increase in stomatal index, size of stomatal guard cells, stroma, and epidermal cells. Such acclimation helped the regulation of photosynthesis to EC. These changes in stomatal characters made rice cultivars adjustable to EC environment.     

Malate-sensitive anion channels enable guard cells to sense changes in the ambient CO2 concentration

Malate is a characteristic metabolite in the photosynthesis of C4 and CAM plants. Furthermore, changes in the intracellular concentration of this organic acid provide part of the osmotic motor for guard cells. Since alterations in the malate concentration influence the photosynthetic capacity on one side and stomatal action on the other, it was studied whether the extracellular malate level represents an indicator of changes in the ambient CO₂ concentration and a key regulator of ion transport in guard cells. Here it is demonstrated that alterations in the ambient CO₂ level modify the extracellular malate concentration of Vicia faba leaves. Elevated external malate caused stomatal closure in a concentration-dependent manner (K_m^mal = 0.3 mM). Slight variations in the external malate concentration strongly regulate the voltage-dependent properties of GCAC1, an anion-release channel in the plasma membrane of guard cells. Superfusion of guard cell protoplasts with malate levels in the physiological range (K_m^mal = 0.4 mM) caused the voltage gate to shift towards the resting potential of the cell-activating GCAC1. Single-channel conductance was dependent on the extracellular chloride concentration (K_m^Cl = 3 mM). In the absence of extracellular chloride the plasma membrane lacked anion conductance until the addition of malate induced channel opening. Isophthalate was a powerful agonist in both malate-induced processes, channel regulation and stomatal closure, indicating that modulation of GCAC1 is a key step in stomatal action. It was thus concluded that feedback regulation of volume and turgor with respect to the ambient CO₂ concentration via malate-sensitive anion channels may provide a CO₂ sensor to guard cells.     

From reproduction to production,stomata are the master regulators

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 g_s) 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.     

Interpretation of an empirical model for stomatal conductance in terms of guard cell function

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.     

Growth and onto-morphogenesis of soybean (Glycine max Merril) in an open, naturally CO2-enriched environment

Springs emitting carbon dioxide are frequent in Central Italy and provide a way of testing the response of plants to CO₂ enrichment under natural conditions. Results of a CO₂ enrichment experiment on soybean at a CO₂ spring (Solfatara) are presented. The experimental site is characterized by significant anomalies in atmospheric CO₂ concentration produced by a large number of vents emitting almost pure CO₂ (93%) plus small amounts of hydrogen sulphide, methane, nitrogen and oxygen. Within the gas vent area, plants were grown at three sub-areas whose mean CO₂ concentrations during daytime were 350,652 and 2370 μmol mol^-1, respectively. Weekly harvests were made to measure biomass growth, leaf area and ontogenetic development. Biomass growth rate and seed yield were enhanced by elevated CO₂. In particular, onto-morphogenetic development was affected by elevated CO₂ with high levels of CO₂ increasing the total number of main stem leaf nodes and the area of the main stem trifoliolate leaves. Biochemical analysis of plant tissue suggested that there was no effect of the small amounts of H₂S on the response to CO₂ enrichment. Non-protein sulphydryl compounds did not accumulate in leaf tissues and the overall capacity of leaf extracts to oxidize exogenously added NADH was not decreased. The limitations and advantages of experimenting with crop plants at elevated CO₂ in the open and in the proximity of carbon dioxide springs are discussed.     

Stomatal Responses to CO(2) in Paphiopedilum and Phragmipedium: Role of the Guard Cell Chloroplast

A role of the guard cell chloroplasts in the CO₂ response of stomata was investigated through a comparison of the leaf gas exchange characteristics of two closely related orchids: Paphiopedilum harrisianum, which lacks guard cell chloroplasts and Phragmipedium longifolium, which has chlorophyllous guard cells. Leaves of both species had an apparent quantum yield for assimilation of about 0.05, with photosynthesis saturating at 0.300 to 0.400 millimoles per square meter per second. CO₂ curves were obtained by measuring steady-state assimilation and stomatal conductance under 0.180 or 0.053 millimoles per square meter per second white light, or darkness, at 0 to 400 microliters per liter ambient CO₂. The response of assimilation to changes in CO₂ was similar in the two species, but the response of conductance was consistently weaker in Paphiopedilum than in Phragmipedium. The data suggest involvement of guard cell chloroplasts in the stomatal response to CO₂ and in the coupling of assimilation and conductance in the intact leaf.     

The effects on Arbutus unedo L. of long-term exposure to elevated CO2

Arbutus unedo is a sclerophyllous evergreen, characteristic of Mediterranean coastal scrub vegetation. In Italy, trees of A. unedo have been found close to natural CO₂ vents where the mean atmospheric carbon dioxide concentration is about 2200 μmol mol^?1. Comparisons were made between trees growing in elevated and ambient CO₂ concentrations to test for evidence of adaptation to long-term exposure to elevated CO₂. Leaves formed at elevated CO₂ have a lower stomatal density and stomatal index and higher specific leaf area than those formed at ambient CO₂, but there was no change in carbon to nitrogen ratios of the leaf tissue. Stomatal conductance was lower at elevated CO₂ during rapid growth in the spring. In mid-summer, under drought stress, stomatal closure of all leaves occurred and in the autumn, when stress was relieved, the conductance of leaves at both elevated and ambient CO₂ increased. In the spring, the stomatal conductance of the new flush of leaves at ambient CO₂ was higher than the leaves at elevated CO₂, increasing instantaneous water use efficiency at elevated CO₂. Chlorophyll fluorescence measurements suggested that elevated CO₂ provided some protection against photoinhibition in mid-summer. Analysis of A/C_i curves showed that there was no evidence of either upward or downward regulation of photosynthesis at elevated CO₂. It is therefore anticipated that A. unedo will have higher growth rates as the ambient CO₂ concentrations increase.     

Stomatal density and aperture length in four plant species grown across a subambient CO2 gradient

Stomatal density, stomatal aperture length, area/leaf, and number of stomata/leaf were measured after the annual C₃ agronomic grasses oats (Avena sativa) and wheat (Triticum aestivum), the C, woody legume honey mesquite (Prosopis glandulosa), and the perennial C₄ grass little bluestem (Schizachyrium scoparium) were grown across a subambient carbon dioxide concentration ([CO₂]) gradient from near 200 to 350 μmol/mol in a growth chamber. The purpose was to determine if the size and density of stomata vary in response to atmospheric [CO₂] during growth, across a subambient [CO₂] range representative of the doubling that has occurred since the last ice age. Changes in stomatal density and aperture length with increasing [CO₂] were small when detected. Stomatal density decreased on adaxial flag leaf surfaces of wheat, and aperture length increased slightly with [CO₂], Leaf area and number of stomata/flag leaf increased by similar proportions with [CO₂] in two wheat cultivars. No consistent relationship between [CO₂] and stomatal density or size was detected in mesquite, oats, or little bluestem. We conclude that individual plants of these species lack the plasticity to significantly alter stomatal density and aperture length in response to increasing atmospheric [CO₂] in a single generation (annuals) or growing season (perennials).     

Enhancement of the Stomatal Response to Blue Light by Red Light, Reduced Intercellular Concentrations of CO(2), and Low Vapor Pressure Differences

The effects of environmental parameters on the blue light response of stomata were studied by quantifying transient increases in stomatal conductance in Commelina communis following 15 seconds by 0.100 millimole per square meter per second pulses of blue light. Because conductance increases were not observed following red light pulses of the same or greater (30 seconds by 0.200 millimole per square meter per second) fluences, the responses observed could be reliably attributed to the specific blue light response of the guard cells, rather than to guard cell chlorophyll. In both Paphiopedilum harrisianum, which lacks guard cell chloroplasts, and Commelina, the blue light response was enhanced by 0.263 millimole per square meter per second continuous background red light. Thus, the blue light response and its enhancement do not require energy derived from red-light-driven photophosphorylation by the guard cell chloroplasts. In Commelina, reduction of the intercellular concentration of CO₂ by manipulation of ambient CO₂ concentrations resulted in an enhanced blue light response. In both Commelina and Paphiopedilum, the blue light response was decreased by an increased vapor pressure difference. The magnitude of blue-light-specific stomatal opening thus appears to be sensitive to environmental conditions that affect the carbon and water status of the plant.     

Carbon dioxide diffusion across stomata and mesophyll and photo-biochemical processes as affected by growth CO2 and phosphorus nutrition in cotton

Nutrients such as phosphorus may exert a major control over plant response to rising atmospheric carbon dioxide concentration (CO₂), which is projected to double by the end of the 21st century. Elevated CO₂ may overcome the diffusional limitations to photosynthesis posed by stomata and mesophyll and alter the photo-biochemical limitations resulting from phosphorus deficiency. To evaluate these ideas, cotton (Gossypium hirsutum) was grown in controlled environment growth chambers with three levels of phosphate (Pi) supply (0.2, 0.05 and 0.01 mM) and two levels of CO₂ concentration (ambient 400 and elevated 800 μmol mol⁻¹) under optimum temperature and irrigation. Phosphate deficiency drastically inhibited photosynthetic characteristics and decreased cotton growth for both CO₂ treatments. Under Pi stress, an apparent limitation to the photosynthetic potential was evident by CO₂ diffusion through stomata and mesophyll, impairment of photosystem functioning and inhibition of biochemical process including the carboxylation efficiency of ribulose-1,5-bisphosphate carboxylase/oxyganase and the rate of ribulose-1,5-bisphosphate regeneration. The diffusional limitation posed by mesophyll was up to 58% greater than the limitation due to stomatal conductance (g_s) under Pi stress. As expected, elevated CO₂ reduced these diffusional limitations to photosynthesis across Pi levels; however, it failed to reduce the photo-biochemical limitations to photosynthesis in phosphorus deficient plants. Acclimation/down regulation of photosynthetic capacity was evident under elevated CO₂ across Pi treatments. Despite a decrease in phosphorus, nitrogen and chlorophyll concentrations in leaf tissue and reduced stomatal conductance at elevated CO₂, the rate of photosynthesis per unit leaf area when measured at the growth CO₂ concentration tended to be higher for all except the lowest Pi treatment. Nevertheless, plant biomass increased at elevated CO₂ across Pi nutrition with taller plants, increased leaf number and larger leaf area.     

Roles of intracellular hydrogen peroxide accumulation in abscisic acid signaling in Arabidopsis guard cells

Reactive oxygen species (ROS), including hydrogen peroxide (H₂O₂), are among the important second messengers in abscisic acid (ABA) signaling in guard cells. In this study, to investigate specific roles of H₂O₂ in ABA signaling in guard cells, we examined the effects of mutations in the guard cell-expressed catalase (CAT) genes, CAT1 and CAT3, and of the CAT inhibitor 3-aminotriazole (AT) on stomatal movement. The cat3 and cat1 cat3 mutations significantly reduced CAT activities, leading to higher basal level of H₂O₂ in guard cells, when assessed by 2′,7′-dichlorodihydrofluorescein, whereas they did not affect stomatal aperture size under non-stressed condition. In addition, AT-treatment at concentrations that abolish CAT activities, showed trivial affect on stomatal aperture size, while basal H₂O₂ level increased extensively. In contrast, cat mutations and AT-treatment potentiated ABA-induced stomatal closure. Inducible ROS production triggered by ABA was observed in these mutants and wild type as well as in AT-treated guard cells. These results suggest that ABA-inducible cytosolic H₂O₂ elevation functions in ABA-induced stomatal closure, while constitutive increase of H₂O₂ do not cause stomatal closure.     

Photosynthesis of Cockspur [Echinochloa crus-galli (L.) Beauv.] at Sites of Naturally Elevated CO2 Concentration

High abundance of cockspur (Echinochloa crus-galli) at the geothermal carbon dioxide spring area in Staveinci indicates that this species is able to grow under widely varying CO₂ concentrations. Living cockspur plants can even be found very close to gas-releasing vents where growth is significantly reduced. Plant height correlated well with CO₂ exposure. The ¹³C value of the CO₂ spring air was –3.9 and ¹³C values of high-, medium-, and low-CO₂ plants were –10.14, –10.44, and –11.95 , respectively. Stomatal response directly followed the prevailing CO₂ concentrations, with the highest reduction of stomatal conductance in high CO₂ concentration grown plants. Analysis of the curves relating net photosynthetic rate to intercellular CO₂ concentration (P _N-C_i curves) revealed higher CO₂ compensation concentration in plants growing at higher CO₂ concentration. This indicates adjustment of respiration and photosynthetic carbon assimilation according to the prevailing CO₂ concentrations during germination and growth. There was no difference in other photosynthetic parameters measured.     

CO2 in large-scale and high-density CHO cell perfusion culture

Productivity in a CHO perfusion culture reactor was maximized when pCO₂ was maintained in the range of 30–76 mm Hg. Higher levels of pCO₂ (> 150 mm Hg) resulted in CHO cell growth inhibition and dramatic reduction in productivity. We measured the oxygen utilization and CO₂ production rates for CHO cells in perfusion culture at 5.55×10^-17 mol cell^-1 sec^-1 and 5.36×10^-17 mol cell^-1 sec^-1 respectively. A simple method to directly measure the mass transfer coefficients for oxygen and carbon dioxide was also developed. For a 500 L bioreactor using pure oxygen sparge at 0.002 VVM from a microporous frit sparger, the overall apparent transfer rates (k_La+k_AA) for oxygen and carbon dioxide were 0.07264 min^-1 and 0.002962 min^-1 respectively. Thus, while a very low flow rate of pure oxygen microbubbles would be adequate to meet oxygen supply requirements for up to 2.1×10⁷ cells/mL, the low CO₂ removal efficiency would limit culture density to only 2.4×10⁶ cells/mL. An additional model was developed to predict the effect of bubble size on oxygen and CO₂ transfer rates. If pure oxygen is used in both the headspace and sparge, then the sparging rate can be minimized by the use of bubbles in the size range of 2–3 mm. For bubbles in this size range, the ratio of oxygen supply to carbon dioxide removal rates is matched to the ratio of metabolic oxygen utilization and carbon dioxide generation rates. Using this strategy in the 500 L reactor, we predict that dissolved oxygen and CO₂ levels can be maintained in the range to support maximum productivity (40% DO, 76 mm Hg pCO₂) for a culture at 10⁷ cells/mL, and with a minimum sparge rate of 0.006 vessel volumes per minute.A = volumetric agitated gas-liquid interfacial area at the top of the liquid, 1/mB = cell broth bleeding rate from the vessel, L/minCER = carbon dioxide evolution rate in the bioreactor, mol/min[CO₂] = dissolved CO₂ concentration in liquid, M[CO₂]^* = CO₂ concentration in equilibrium with sparger gas, M[CO₂]^** = CO₂ concentration in equilibrium with headspace gas, MCO₂(1) = dissolved carbon dioxide molecule in water[C_T] = total carbonic species concentration in bioreactor medium, M[C_T]_F = total carbonic species concentration in feed medium, MD = bioreactor diameter, mD_I = impeller diameter, mD_b = the initial delivered bubble diameter, mF = fresh medium feeding rate, L/minH_L = liquid height in the vessel, mk_A = carbon dioxide transfer coefficient at liquid surface, m/mink _infA ^supO = oxygen transfer coefficient at liquid surface, m/minNomenclature     

Stomatal movement and potassium transport in epidermal strips of Zea mays: The effect of CO2

Summary The correlation between stomatal action and potassium movement in the epidermis of Zea mays was examined in isolated epidermal strips floated on distilled water. Stomatal opening in the isolated epidermis is reversible in response to alternate periods of light or darkness, and is always correlated with a shift in the potassium content of the guard cells. K accumulates in guard cells during stomatal opening, and moves from the guard cells into the subsidiary cells during rapid stomatal closure. When epidermal strips are illuminated in normal air, as against CO₂-free air, the stomata do not open and there is a virtually complete depletion of K from the stomatal apparatus. In darkness CO₂-containing air inhibits stomatal opening and K accumulation in guard cells, but does not lead to a depletion of K from the stomata as observed in the light.     

Do Stomata Respond to CO(2) Concentrations Other than Intercellular?

Most studies on stomatal responses to CO₂ assume that guard cells respond only to intercellular CO₂ concentration and are insensitive to the CO₂ concentrations in the pore and outside the leaf. If stomata are sensitive to the CO₂ concentration at the surface of the leaf or in the stomatal pore, the stomatal response to intercellular CO₂ concentration will be incorrect for a `normally' operating leaf (where ambient CO₂ concentration is a constant). In this study asymmetric CO₂ concentrations for the two surfaces of amphistomatous leaves were used to vary intercellular and leaf surface CO₂ concentrations independently in Xanthium strumarium L. and Helianthus annuus L. The response of stomata to intercellular CO₂ concentration when the concentration at the leaf surface was held constant was found to be the same as the response when the surface concentration was varied. In addition, stomata did not respond to changes in leaf surface CO₂ concentration when the intercellular concentration for that surface was held constant. It is concluded that stomata respond to intercellular CO₂ concentration and are insensitive to the CO₂ concentration at the surface of the leaf and in the stomatal pore.     

The Components of the CO2-Indluced Stomatal Movements

Leaf discs of Vicia Faba were allowed to float on water in glass dishes placed in vessels containing KOH. The vessels were kept in darkness at constant temperature. The stomatal width and osmotic values of the guard cells and epidermal cells were measured, generally at one-hour intervals. When the CO₂ content of the air surrounding the leaf specimen falls, it causes a disturbance in the osmotic equilibrium between guard cells and epidermal cells. Sometimes the changes start in the form of falling osmotic values in both kinds of cell. In other cases the values rise, and in still others the changes may be confined chiefly to one of these kinds of cell. Since the changes are not the same in guard cells and epidermal cells, the osmotic difference between them rises or falls. The difference rises during the time immediately after removal of CO₂ from the surrounding air. This causes an osmotic surplus to arise or increase in the guard cells. Later, this change may take place in the opposite direction. The stomatal movements occurring simultaneously follow, on broad lines, the osmotic surplus of the guard cells. Consequently, the CO₂-induced stomatal movement is the result of an interaction between an active component—i.e., the intrinsic osmotic changes in the guard cells—and an osmopassive component, by which is meant here the osmotic changes in the epidermal cells.     

Control of stomatal aperture: A renaissance of the old guard

Stomata, functionally specialized small pores on the surfaces of leaves, regulate the flow of gases in and out of plants. The pore is opened by an increase in osmotic pressure in the guard cells, resulting in the uptake of water. The subsequent increase in cell volume inflates the guard cell and culminates with the opening of the pore. Although guard cells can be regarded as one of the most thoroughly investigated cell types, our knowledge of the signaling pathways which regulate guard cell function remains fragmented. Recent research in guard cells has led to several new hypotheses, however, it is still a matter of debate as to whether guard cells function autonomously or are subject to regulation by their neighboring mesophyll cells. This review synthesizes what is known about the mechanisms and genes critical for modulating stomatal movement. Recent progress on the regulation of guard cell function is reviewed here including the involvement of environmental signals such as light, the concentration of atmospheric CO₂ and endogenous plant hormones. In addition we re-evaluate the important role of organic acids such as malate and fumarate play in guard cell metabolism in this process.Key words: stomata movement, ions, organic acids, malate, fumarate, CO₂, ABA, light