The blue light-specific response of Vicia faba stomata acclimates to growth environment

Stomata in epidermal strips from growth chamber-grown Vicia faba leaves opened less in response to white light than did stomata from greenhouse-grown leaves. Chlorophyll-mediated, red light-stimulated opening was similar in stomata from the two growth conditions, but stomata from the growth chamber environment had a severely reduced response to blue light. Transfer of plants between the two growth conditions resulted in an acclimation of the stomatal blue light response. Stomata lost blue light sensitivity within 1 d of transfer to growth chamber conditions and gained sensitivity to blue light over an 8 d period after transfer to a greenhouse. Short-term transfer experiments confirmed that the rapid loss of blue light sensitivity was an acclimation response, requiring between 12 and 20 h exposure to growth chamber conditions. The acclimation of the stomatal response to blue light was inversely related to a previously reported acclimation response in which stomata change between high CO2 sensitivity under growth chamber conditions and low CO2 sensitivity under greenhouse conditions. The time courses of the blue light and CO2 acclimation responses were virtually identical, suggesting the possibility of a common acclimation mechanism.     

Relative humidity is a key factor in the acclimation of the stomatal response to CO(2)

Previous work has shown that stomata of growth chamber-grown Vicia faba leaves have an enhanced CO2 response when compared with stomata of greenhouse-grown plants. This guard cell response to CO2 acclimatizes to the environmental conditions on the transfer of plants between the two environments. In the present study, air relative humidity is identified as a key environmental factor mediating the changes in stomatal sensitivity to CO2. In the greenhouse environment, elevation of relative humidity to growth chamber levels resulted in an enhanced CO2 response, whereas a reduction in the light level to that comparable to growth chamber conditions had no effect on stomatal CO2 sensitivity. The transfer of plants between humidified and normal greenhouse conditions resulted in an acclimation response with a time-course matching that previously obtained in transfers of plants between greenhouse and growth chamber environments. The high stomatal sensitivity to CO2 of growth chamber-grown plants could be reduced by lowering growth chamber relative humidity and then restored with its characteristic acclimation time-course by an elevation of relative humidity. Leaf temperature was unchanged during this restoration, eliminating it as a primary factor in the acclimation response. Humidity regulation of stomatal CO2 sensitivity could function as a signal for leaves inside dense foliage canopies, promoting stomatal opening under low light, low CO2 conditions.     

Abaxial and adaxial stomata from Pima cotton (Gossypium barbadense L.) differ in their pigment content and sensitivity to light quality

Sensitivity to light quality and pigment composition were analysed and compared in abaxial and adaxial stomata of Gossypium barbadense L. (Pima cotton). In most plants, abaxial (lower) stomatal conductances are higher than adaxial (upper) ones, and stomatal opening is more sensitive to blue light than to red. In greenhouse-grown Pima cotton, abaxial stomatal conductances were two to three times higher than adaxial ones. In contrast, adaxial stomatal conductances were 1·5 to two times higher than abaxial ones in leaves from growth chamber-grown plants. To establish whether light quality was a factor in the regulation of the relationship between abaxial and adaxial stomatal conductances, growth-chamber-grown plants were exposed to solar radiation outdoors and to increased red light in the growth chamber. In both cases, the ratios of adaxial to abaxial stomatal conductance reverted to those typical of greenhouse plants. We investigated the hypothesis that adaxial stomata are more sensitive to blue light and abaxial stomata are more sensitive to red light. Measurements of stomatal apertures in mechanically isolated epidermal peels from growth chamber and greenhouse plants showed that adaxial stomata opened more under blue light than under red light, while abaxial stomata had the opposite response. Using HPLC, we quantified the chlorophylls and carotenoids extracted from isolated adaxial and abaxial guard cells. All pigments analysed were more abundant in the adaxial than in the abaxial guard cells. Antheraxanthin and β-carotene contents were 2·3 times higher in adaxial than in abaxial guard cells, comparing with ad/ab ratios of 1·5–1·9 for the other pigments. We conclude that adaxial and abaxial stomata from Pima cotton have a differential sensitivity to light quality and their distinct responses are correlated with different pigment content.     

The stomata of the fern Adiantum capillus-veneris do not respond to CO2 in the dark and open by photosynthesis in guard cells

The stomata of the fern Adiantum capillus-veneris lack a blue light-specific opening response but open in response to red light. We investigated this light response of Adiantum stomata and found that the light wavelength dependence of stomatal opening matched that of photosynthesis. The simultaneous application of red (2 micromol m(-2) s(-1)) and far-red (50 micromol m(-2) s(-1)) light synergistically induced stomatal opening, but application of only one of these wavelengths was ineffective. Adiantum stomata did not respond to CO2 in the dark; the stomata neither opened under a low intercellular CO2 concentration nor closed under high intercellular CO2 concentration. Stomata in Arabidopsis (Arabidopsis thaliana), which were used as a control, showed clear sensitivity to CO2. In Adiantum, stomatal conductance showed much higher light sensitivity when the light was applied to the lower leaf surface, where stomata exist, than when it was applied to the upper surface. This suggests that guard cells likely sensed the light required for stomatal opening. In the epidermal fragments, red light induced both stomatal opening and K+ accumulation in guard cells, and both of these responses were inhibited by a photosynthetic inhibitor, 3-(3,4-dichlorophenyl)-1,1-dimethylurea. The stomatal opening was completely inhibited by CsCl, a K+ channel blocker. In intact fern leaves, red light-induced stomatal opening was also suppressed by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. These results indicate that Adiantum stomata lack sensitivity to CO2 in the dark and that stomatal opening is driven by photosynthetic electron transport in guard cell chloroplasts, probably via K+ uptake.     

Reversal by green light of blue light-stimulated stomatal opening in intact, attached leaves of Arabidopsis operates only in the potassium-dependent, morning phase of movement

Green light reversal of blue light-stimulated stomatal opening was discovered in isolated stomata. The present study shows that the response also occurs in stomata from intact leaves. Arabidopsis thaliana plants were grown in a growth chamber under blue, red and green light. Removal of the green light opened the stomata and restoration of green light closed them to baseline values under experimental conditions that rule out a mesophyll-mediated effect. Assessment of the response to green light over a daily time course showed that the stomatal sensitivity to green light was observed only in the morning, which coincided with the use of potassium as a guard cell osmoticum. Sensitivity to green light was absent during the afternoon phase of stomatal movement, which was previously shown to be dominated by sucrose osmoregulation in Vicia faba. Hence, the shift away from potassium-based osmoregulation in guard cells is further postulated to entail a shift from blue light to photosynthesis as the primary component of the stomatal response to light. Stomata from intact leaves of the zeaxanthin-less, npq1 mutant of Arabidopsis failed to respond to the removal or restoration of green light in the growth chamber, or to short, high fluence pulses of blue or green light. These data confirm previous studies showing that npq1 stomata are devoid of a specific blue light response. In contrast, stomata from intact leaves of phot1 phot2 double mutant plants had a reduced but readily detectable response to the removal of green light and to blue and green pulses.     

Stomata from growth-chamber-grown Vicia faba have an enhanced sensitivity to CO2

Abaxial stomata from Vicia faba leaves grown in a growth chamber under constant light, temperature and humidity showed an elaborate pattern of aperture changes over the course of a light cycle. These aperture changes were tightly correlated with changes in chamber CO2 concentration (r2=0.83). Changes in chamber [CO2] resulted, in turn, from substantial daily fluctuations in ambient [CO2], typical of the Los Angeles environment, with a constant offset caused by photosynthesis and respiration of the plants within the chamber. The dominant role of the stomatal response to CO2 in the control of aperture was confirmed by manipulation of chamber [CO2]. Fast (15 min) increases and decreases in [CO2] caused rapid decreases and increases in aperture, while constant [CO2] resulted in constant aperture. In contrast, aperture changes in comparable plants grown under greenhouse conditions were tightly correlated with changes in incident solar radiation (r2=0.80), and poorly correlated with changes in [CO2] (r2=0.09). Greenhouse-grown plants transferred to growth chamber conditions showed no apparent response to CO2. These data indicate that growth-chamber-grown V. faba leaves provide an experimental system optimally suited for the study of the stomatal response to CO2, and suggest that acclimation to environmental conditions alters the sensitivity of stomata to CO2.     

Stomatal Responses of Tradescantia albiflora to Changing Air Humidity in Light and in Darkness

Tradescantia albiflora has green variegated and white leaves.Its stomatal apparatus consists of the guard cells and two pairsof subsidiary cells. Investigations were carried out by observingthe stomata microscopically by means of a video system in situin a CO₂ exchange chamber and by simultaneously measuring thegas exchange of the leaves. In response to air humidity changes,stomatal movements in T. albiflora begin, owing to turgor changes,in the polar and lateral subsidiary cells. The stomatal responseof green leaves to changes of air humidity showed typical transientand oscillatory phases prior to steady-state reactions. In darkness,stomata closed when air humidity decreased; however, they didnot reopen when air humidity was raised again. Stomata of illuminatedwhite leaves responded like those of green leaves in darkness.With increasing soil water stress stomata responded to changingair humidity with reductions of the transient phases and a decreasingtendency to reopen when air humidity became high again. CO₂deficiency of the air caused the stomata to open in the dark,and interacted with the air humidity effect in such a way thatstomata of green leaves responded to air humidity changes indarkness in a similar way as they did in light. Key words: Stomata, humidity response, green and white leaf areas, CO₂ deficient air     

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.     

Rapid low temperature-induced stomatal closure occurs in cold-tolerant Commelina communis leaves but not in cold-sensitive tobacco leaves, via a mechanism that involves apoplastic calcium but not abscisic acid

Commelina communis stomata closed within 1 h of transferring intact plants from 27 degrees C to 7 degrees C, whereas tobacco (Nicotiana rustica) stomata did not until the leaves wilted. Abscisic acid (ABA) did not mediate cold-induced C. communis stomatal closure: At low temperatures, bulk leaf ABA did not increase; ABA did not preferentially accumulate in the epidermis; its flux into detached leaves was lower; its release from isolated epidermis was not greater; and stomata in epidermal strips were less sensitive to exogenous ABA. Stomata of both species in epidermal strips on large volumes of cold KCl failed to close unless calcium was supplied. Therefore, the following cannot be triggers for cold-induced stomatal closure in C. communis: direct effects of temperature on guard or epidermal cells, long-distance signals, and effects of temperature on photosynthesis. Low temperature increased stomatal sensitivity to external CaCl(2) by 50% in C. communis but only by 20% in tobacco. C. communis stomata were 300- to 1,000-fold more sensitive to calcium at low temperature than tobacco stomata, but tobacco epidermis only released 13.6-fold more calcium into bathing solutions than C. communis. Stomata in C. communis epidermis incubated on ever-decreasing volumes of cold calcium-free KCl closed on the lowest volume (0.2 cm(3)) because the epidermal apoplast contained enough calcium to mediate closure if this was not over diluted. We propose that the basis of cold-induced stomatal closure exhibited by intact C. communis leaves is increased apoplastic calcium uptake by guard cells. Such responses do not occur in chill-sensitive tobacco leaves.     

Dynamics of adaptation of stomatal behaviour to moderate or high relative air humidity in Tradescantia virginiana

Chlorophyll fluorescence imaging was used to measure stomatalclosure in response to desiccation of Tradescantia virginianaleaves grown under high (90%) and moderate (55%) relative humidities(RHs), or transferred between these humidities. Stomata in leavesgrown at high RH were less responsive to desiccation than thoseof leaves grown at moderate RH. Stomata of plants transferredfrom moderate RH conditions to high RH showed the same diminishedclosure in response to desiccation as did stomata that developedat high RH. This response was found both when the leaves werefully expanded and when still actively expanding during themoderate RH pre-treatment. Four days of exposure to high RHwas the minimal exposure time to induce the diminished closureresponse. When leaves were grown in high RH prior to a 10 dmoderate RH treatment, the reduced stomatal closure responseto desiccation was only reversed in leaves (regions) which wereactively expanding during moderate RH treatment. This indicatesthat with respect to stomatal responses to desiccation, highRH leaf regions have a limited capacity to adapt to moderateRH conditions. The decrease in responsiveness to desiccationof the stomata, induced by long-term exposure to high RH, wasnot due to osmotic adjustment in the leaves. Within 1 d aftertransferring moderate RH-grown plants to a high RH, the abscisicacid (ABA) concentration of their leaves decreased to the lowlevel of ABA found in high RH-grown leaves. The closure responsein leaves exposed to high RH for 5 d, however, could not befully restored by the application of ABA. Transferring plantsfrom high to moderate RH resulted in increased ABA levels within2 d without a recovery of the stomatal closing response. Itis discussed that the diminished stomatal closure in plantsexposed to high RH could be due to changes in the signallingpathway for ABA-related closure of stomata or to an increasedsequestration of ABA by mesophyll tissue or the symplast inthe epidermis, induced by a longer period (several days) ofa low ABA level. Key words: Abscisic acid, desiccation, PSII efficiency, relative water content, stomatal closure, vapour pressure deficit, water potential Received 8 October 2007; Revised 5 November 2007 Accepted 9 November 2007     

Functional Stomata in a Variegated Leaf Chimera of Pelargonium zonale L. without Guard Cell Chloroplasts

Fluorescence microscopy indicated that chlorophyll was absentfrom epidermal and guard cells overlying all white areas andgreen areas (of certain leaves) in variegated leaves of Pelargoniumzonale, cv. Chelsea Gem. Stomata with chlorophyll-free guardcells, in general, responded normally to light and CO₂ as gaugedby direct measurements of stomatal aperture and by transpirationalwater loss studies, although stomata from white regions of variegatedleaves were more reluctant to open than stomata from green regionsof the leaves. Thus, functional stomata without guard cell chloroplastshave been discovered in another genus, namely Pelargonium, besidesthat originally discovered in Paphiopedilum. When stomata withchlorophyll-free guard cells opened, K⁺ accumulated in the guardcells. This indicates that chloroplasts are not essential forthe normal functioning of stomata and that the energy sourcefor driving stomatal movements can come from sources other thanphotophosphorylation. Key words: Guard cell chloroplasts, Leaf chimera, Pelargonium, Stomata     

Changes in apoplastic pH and membrane potential in leaves in relation to stomatal responses to CO2, malate, abscisic acid or interruption of water supply

Low CO2 concentrations open CO2-sensitive stomata whereas elevated CO2 levels close them. This CO2 response is maintained in the dark. To elucidate mechanisms underlying the dark CO2 response we introduced pH- and potential-sensitive dyes into the apoplast of leaves. After mounting excised leaves in a gas-exchange chamber, changes in extracellular proton concentration and transmembrane potential differences as well as transpiration and respiration were simultaneously monitored. Upon an increase in CO2 concentration transient changes in apoplastic pH (occasionally brief acidification, but always followed by alkalinization) and in membrane potential (brief hyperpolarization followed by depolarization) accompanied stomatal closure. Alkalinization and depolarization were also observed when leaves were challenged with abscisic acid or when water flow was interrupted. During stomatal opening in response to CO2-free air the apoplastic pH increased while the membrane potential initially depolarized before it transiently hyperpolarized. To examine whether changes in apoplastic malate concentrations represent a closing signal for stomata, malate was fed into the transpiration stream. Although malate caused apoplastic alkalinization and membrane depolarization reminiscent of the effects observed with CO2 and abscisic acid, this dicarboxylate closed the stomata only partially and less effectively than CO2. Apoplastic alkalinization was also observed and stomata closed partially when KCl was fed to the leaves. Respiration increased on feeding of malate or KCl, or while abscisic acid closed the stomate. From these results we conclude that CO2 signals modulate the activity of plasma-membrane ion channels and of plasmalemma H+-ATPases during changes in stomatal aperture. Responses to potassium malate and KCl are not restricted to guard cells and neighbouring cells.     

Photosynthesis-dependent and -independent responses of stomata to blue, red and green monochromatic light: differences between the normally oriented and inverted leaves of sunflower

The effects of growth light environment on stomatal light responses were analyzed. We inverted leaves of sunflower (Helianthus annuus) for 2 weeks until their full expansion, and measured gas exchange properties of the adaxial and abaxial sides separately. The sensitivity to light assessed as the increase in stomatal conductance was generally higher in the abaxial stomata than in the adaxial stomata, and these differences could not be completely changed by the inversion treatment. We also treated the leaves with DCMU to inhibit photosynthesis and evaluated the photosynthesis-dependent and -independent components of stomatal light responses. The red light response of stomata in both normally oriented and inverted leaves relied only on the photosynthesis-dependent component. The blue light response involved both the photosynthesis-dependent and photosynthesis-independent components, and the relative contributions of the two components differed between the normally oriented and inverted leaves. A green light response was observed only in the abaxial stomata, which also involved the photosynthesis-dependent and photosynthesis-independent components, strongly suggesting the existence of a green light receptor in sunflower leaves. Moreover, acclimation of the abaxial stomata to strong direct light eliminated the photosynthesis-independent component in the green light response. The results showed that stomatal responses to monochromatic light change considerably in response to growth light environment, although some of these responses appear to be determined inherently.     

A scanning electron microscope study of normal and vitrified leaves from Datura insignis plantlets cultured in vitro

The surface anatomy of normal and vitreous leaves of plantlets obtained from Datura insignis Barb Rodr nodal segment cultures was compared using scanning electron microscopy. Normal and vitrified leaves are similar in several ways. They are both amphistomatic, and have similar distributions of glandular and non-glandular trichomes. Stomata have similar length, diameter and distribution in normal and vitreous plants. Immature stomata, which have closed pores, and plugged stomata, which contain an amorphous material between their guard cells, occur in both normal and vitrified leaves. Normal and vitreous leaves differ in the frequency of normal and abnormal stomata. Normal stomata have kidney-shaped guard cells and resemble closely those found in field-grown plants, whereas abnormal stomata have deformed guard cells. Normal stomata represent approximately 80% of the total number of stomata in normal leaves, but only 7% of the total number of stomata in vitreous leaves. Abnormal stomata represent 90% of the total number in vitreous leaves. The deformation of guard cells could possibly be a mechanical impediment to stomatal function.     

The stomatal response to CO2 is linked to changes in guard cell zeaxanthin*

The mechanisms mediating CO₂ sensing and light–CO₂ interactions in guard cells are unknown. In growth chamber-grown Vicia faba leaves kept under constant light (500 μ mol m^–2 s^–1) and temperature, guard cell zeaxanthin content tracked ambient [CO₂] and stomatal apertures. Increases in [CO₂] from 400 to 1200 cm³ m^–3 decreased zeaxanthin content from 180 to 80 mmol mol^–1 Chl and decreased stomatal apertures by 7·0 μ m. Changes in zeaxanthin and aperture were reversed when [CO₂] was lowered. Guard cell zeaxanthin content was linearly correlated with stomatal apertures. In the dark, the CO₂-induced changes in stomatal aperture were much smaller, and guard cell zeaxanthin content did not change with chamber [CO₂]. Guard cell zeaxanthin also tracked [CO₂] and stomatal aperture in illuminated stomata from epidermal peels. Dithiothreitol (DTT), an inhibitor of zeaxanthin formation, eliminated CO₂-induced zeaxanthin changes in guard cells from illuminated epidermal peels and reduced the stomatal CO₂ response to the level observed in the dark. These data suggest that CO₂-dependent changes in the zeaxanthin content of guard cells could modulate CO₂-dependent changes of stomatal apertures in the light while a zeaxanthin-independent CO₂ sensing mechanism would modulate the CO₂ response in the dark.     

Reductions in mesophyll and guard cell photosynthesis impact on the control of stomatal responses to light and CO2

Transgenic antisense tobacco plants with a range of reductions in sedoheptulose-1,7-bisphosphatase (SBPase) activity were used to investigate the role of photosynthesis in stomatal opening responses. High resolution chlorophyll a fluorescence imaging showed that the quantum efficiency of photosystem II electron transport (F(q)(')/F(m)(')) was decreased similarly in both guard and mesophyll cells of the SBPase antisense plants compared to the wild-type plants. This demonstrated for the first time that photosynthetic operating efficiency in the guard cells responds to changes in the regeneration capacity of the Calvin cycle. The rate of stomatal opening in response to a 30 min, 10-fold step increase in red photon flux density in the leaves from the SBPase antisense plants was significantly greater than wild-type plants. Final stomatal conductance under red and mixed blue/red irradiance was greater in the antisense plants than in the wild-type control plants despite lower CO(2) assimilation rates and higher internal CO(2) concentrations. Increasing CO(2) concentration resulted in a similar stomatal closing response in wild-type and antisense plants when measured in red light. However, in the antisense plants with small reductions in SBPase activity greater stomatal conductances were observed at all C(i) levels. Together, these data suggest that the primary light-induced opening or CO(2)-dependent closing response of stomata is not dependent upon guard or mesophyll cell photosynthetic capacity, but that photosynthetic electron transport, or its end-products, regulate the control of stomatal responses to light and CO(2).     

The role of abscisic acid in disturbed stomatal response characteristics of Tradescantia virginiana during growth at high relative air humidity

In this study, the role of abscisic acid (ABA) in altered stomatal responses of Tradescantia virginiana leaves grown at high relative air humidity (RH) was investigated. A lower ABA concentration was found in leaves grown at high RH compared with leaves grown at moderate RH. As a result of a daily application of 20 microM ABA to leaves for 3 weeks during growth at high RH, the stomata of ABA-treated leaves grown at high RH showed the same behaviour as did the stomata of leaves grown at moderate RH. For example, they closed rapidly when exposed to desiccation. Providing a high RH around a single leaf of a plant during growth at moderate RH changed the stomatal responses of this leaf. The stomata in this leaf grown at high RH did not close completely in response to desiccation in contrast to the stomata of the other leaves from the same plant. The ABA concentration on a fresh weight basis, though not on a dry weight basis, of this leaf was significantly lower than that of the others. Moreover, less closure of stomata was found in the older leaves of plants grown at high RH in response to desiccation compared with younger leaves. This was correlated with a lower ABA concentration in these leaves on a fresh weight basis, though not on a dry weight basis. Stomata of leaves grown at moderate RH closed in response to short-term application of ABA or sodium nitroprusside (SNP), while for leaves grown at high RH there was a clear difference in stomatal responses between the leaf margins and main-vein areas. The stomatal aperture in response to short-term application of ABA or SNP at the leaf margins of leaves grown at high RH remained significantly wider than in the main-vein areas. It was concluded that: (i) a long-term low ABA concentration in well-watered plants during growth at high RH could be a reason for less or no stomatal closure under conditions of drought stress; and (ii) the long-term ABA concentration on a fresh weight basis rather than on a dry weight basis is likely to be responsible for structural or physiological changes in stomata during leaf growth.     

Rapid hydropassive opening and subsequent active stomatal closure follow heat-induced electrical signals in Mimosa pudica

In Mimosa pudica L., heat stimulation triggers leaflet folding in local, neighbouring and distant leaves. Stomatal movements were observed microscopically during this folding reaction and electrical potentials, chlorophyll fluorescence, and leaf CO(2)/H(2)O-gas exchange were measured simultaneously. Upon heat stimulation of a neighbouring pinna, epidermal cells depolarized and the stomata began a rapid and pronounced transient opening response, leading to an approximately 2-fold increase of stomatal aperture within 60 s. At the same time, net CO(2) exchange showed a pronounced transient decrease, which was followed by a similar drop in photochemical quantum yield at photosystem (PS) II. Subsequently, CO(2)-gas exchange and photochemical quantum yield recovered and stomata closed partly or completely. The transient and fast stomatal opening response is interpreted as a hydropassive stomatal movement caused by a sudden loss of epidermal turgor. Thus, epidermal cells appear to respond in a similar manner to heat-induced signals as the pulvinar extensor cells. The subsequent closing of the stomata confirms earlier reports that stomatal movements can be induced by electrical signals. The substantial delay (several minutes) of guard cell turgor loss compared with the immediate response of the extensor and epidermal cells suggests a different, less direct mechanism for transmission of the propagating signal to the guard cells.     

Opinion: Stomatal responses to light and CO2 depend on the mesophyll

The mechanisms by which stomata respond to red light and CO₂ are unknown, but much of the current literature assumes that these mechanisms reside wholly within the guard cells. However, responses of guard cells in isolated epidermes are typically much smaller than those in leaves, and there are several lines of evidence in the literature suggesting that the mesophyll is necessary for these responses in leaves. This paper advances the opinion that although guard cells may have small direct responses to red light and CO₂, most of the stomatal response to these factors in leaves is caused by an unknown signal that originates in the mesophyll.