A Model for the Interaction of Low Temperature, ABA, IAA, and CO2 in the Control of Stomatal Behaviour

In the chilling sensitive (C.S.) species Phaseolus vulgarisit was found that at 22 ?C ABA induced stomatal closure butthis effect was dependent on the presence of CO₂. In the absenceof CO₂ the effect of ABA was completely lost. In contrast toABA, the effect of IAA at 22 ?C was to increase stomatal openingas the IAA concentration increased from 10^–2 to 10 molm^–3, and this effect was dependent upon the presence ofCO₂. However, at 5 ?C the action of ABA was reversed and itwas found to induce stomatal opening when fed via the transpirationstream in excised leaves. Similarly, the CO₂ response characteristicswere reversed at low temperatures as removal of CO₂ from theatmosphere caused stomatal closure. However, the effect of IAAat 5 ?C in the presence of CO₂ and with or without ABA was toincrease stomatal aperture with increasing IAA concentration.Significantly, ABA was found to have no effect upon aperturein the presence of CO₂ when IAA was added. The interactive effectsof ABA, IAA, CO₂ and low temperature are discussed in relationto a model proposed by the authors. Key words: IAA, ABA, CO₂, Stomata     

The Influence of Prior Water Stress and Abscisic Acid Foliar Spraying on Stomatal Responses to CO2, IAA, ABA, and Calcium in Leaves of Solanum melongena

The influence of a water stress or foliar ABA spraying pretreatmenton stomatal responses to water loss, exogenous ABA, IAA, Ca²⁺,and CO₂ were studied using excised leaves of Solanum melongena.Both pretreatments increased stomatal sensitivity of water loss,in the presence and absence of CO₂, but decreased stomatal sensitivityto exogenous ABA. CO₂ greatly reduced the effect of exogenouslyapplied ABA. IAA decreased leaf diffusion resistance for controland ABA sprayed leaves, but did not influence the LDR of previouslywater-stressed leaves. CA²⁺ did not influence LDR of any leavesof any treatments. Key words: Water stress, stomatal response, pretreatments     

Control of the Co2 Responses of Stomata by Indol-3ylacetic Acid and Abscisic Acid

Previous studies of stomatal behaviour on detached epidermisof Commelina communis L. have suggested that abscisic acid (ABA)and C02 act independently to cause stomatal closure. Evidenceis presented here that if indol-3ylacetic acid (IAA) is addedto the medium used for incubating the epidermis, an interactionbetween ABA and Co₂ becomes apparent. Increasing concentrationsof IAA reduce the ability of the stomata to respond to CO₂,and ABA appears to antagonize this effect. Recognition of therole of IAA enables us to reconcile earlier conflicting reportsconcerning the interdependence of effects of ABA and Co₂on stomata.     

Delay in the Response of Stomata to Abscisic Acid in CO2-free Air

Abscisic acid (10^–5 M) was fed via their petioles to leavesdetached from well watered plants of Xanthium strumartum, whilethe intercellular spaces were flushed with air of known CO₂content. A closing response to ABA occurred in the presenceor absence of CO₂, and the stomata responded to CO₂ whetheror not ABA was supplied to the leaves. A factorial experimentrevealed no interaction between CO₂ and ABA, and suggested thattheir effect on the rate of closure was purely additive. Theonly evidence of interdependence between the two corn poundswas a delay in the response to ABA in C0 air, which was moremarked in a high light intensity. A hypothesis which is consistentwith the data is that ABA induces stomatal closure by interferingwith the energy supply required for the active transport processeson which guard cell turgor depends. The inhibitory action ofABA takes longer in CO₂-free air because, in the absence ofCO₂ fixation, energy is available from chioroplasts as wellas mitochondria.     

Stomatal Responses to Applied ABA and CO2 in Epidermis Detached from Well-Watered and Water-Stressed Plants of Commelina communis L.

Epidermal strips from either well-watered or water-stressedplants of Commelina communis L. were subjected to a range ofABA concentrations (10^–6–10^–3 mol m^–3)in the presence (330 parts 10^–6 in air) or virtual absence(3 parts 10–6 in air) of CO₂. The stomatal response toCO₂ was greater in epidermis from water-stressed plants, althoughthere was a distinct CO₂ response in epidermis from well-wateredplants. Additions of ABA via the incubation medium had littleeffect on the relative CO₂ response. Stomata responded to ABAboth in the presence and virtual absence of CO₂, but the relativeresponse to ABA was greatest in the high CO₂ treatment. Whenwell-watered plants were sprayed with a 10^–1 mol m^–3ABA solution 1 d prior to use, the stomatal response of detachedepidermis to both CO₂ and ABA was very similar to that of epidermisdetached from water-stressed leaves. It is hypothesized thata prolonged exposure to ABA is necessary before there is anymodification of the CO₂ response of stomata.     

Changing Responses of Stomata to Abscisic Acid and CO2 as Leaves and Plants Age

Willmer, C. M., Wilson, A. B. and Jones, H. G. 1988. Changingresponses of stomata to abscisic acid and CO₂ as leaves andplants age.—J. exp. Bot. 39: 401–410. Stomatal conductances were measured in ageing leaves of Commelinacommunis L. as plants developed; stomatal responses to CO₂ andabscisic acid (ABA) in epidermal strips of C. communis takenfrom ageing leaves of developing plants and in epidermal stripsfrom the same-aged leaves (the first fully-expanded leaf) ofdeveloping plants were also monitored. Stomatal behaviour wascorrelated with parallel measurements of photosynthesis andleaf ABA concentrations. Stomatal conductance in intact leavesdecreased from a maximum of 0-9 cm s^{– 1} at full leaf expansionto zero about 30 d later when leaves were very senescent. Conductancesdeclined more slowly with age in unshaded leaves. Photosynthesisof leaf slices also declined with age from a maximum at fullleaf expansion until about 30 d later when no O₂ exchange wasdetectable. Exogenously applied ABA (0.1 mol m^{– 3}) didnot affect respiration or photosynthesis. In epidermal stripstaken from ageing leaves the widest stomatal apertures occurredabout 10 d after full leaf expansion (just before floweringbegan) and then decreased with age; this decrease was less dramaticin unshaded leaves. The inhibitory effects of ABA on stomatalopening in epidermal strips decreased as leaves aged and wasgreater in the presence of CO₂ than in its absence. When leaveswere almost fully-senescent stomata were still able to open.At this stage, guard cells remained healthy-looking with greenchloroplasts while mesophyll cells were senescing and theirchloroplasts were yellow. Similar data were obtained for stomatain epidermal strips taken from the same-aged leaves of ageingplants. The inhibitory effects of ABA on stomatal opening alsodecreased with plant age. In ageing leaves both free and conjugated ABA concentrationsremained low before increasing dramatically about 30 d afterfull leaf expansion when senescence was well advanced. Concentrationsof free and conjugated ABA remained similar to each other atall times. It is concluded that the restriction of stomatal movements inintact leaves as the leaves and plants age is due mainly toa fall in photosynthetic capacity of the leaves which affectsintracellular CO₂ levels rather than to an inherent inabilityof the stomata to function normally. Since stomatal aperturein epidermal strips declines with plant and leaf age and stomatabecome less responsive to ABA (while endogenous leaf ABA levelsremain fairly constant until leaf senescence) it is suggestedthat some signal, other than ABA, is transmitted from the leafor other parts of the plant to the stomata and influences theirbehaviour. Key words: Abscisic acid, CO₂, Commelina, leaf age, senescence, stomatal sensitivity     

New fava bean guard cell signaling mutant impaired in ABA-induced stomatal closure

We isolated a mutant from Vicia faba L. cv. House Ryousai. Itwilts easily under strong light and high temperature conditions,suggesting that its stomatal movement may be disturbed. We determinedresponses of mutant guard cells to some environmental stimuli.Mutant guard cells demonstrated an impaired ability to respondto ABA in 0.1 mM CaCl₂ and stomata did not close in thepresence of up to 1 mM ABA, whereas wild-type stomata closedwhen exposed to 10 µM ABA. Elevating external Ca²⁺caused a similar degree of stomatal closure in the wild typeand the mutant. A high concentration of CO₂ (700 µlliter^–1) induced stomatal closure in the wild type, butnot in the mutant. On the basis of these results, we proposethe working hypothesis that the mutation occurs in the regiondownstream of CO₂ and ABA sensing and in the region upstreamof Ca²⁺ elevation. The mutant is named fia (fava bean impairedin ABA-induced stomatal closure). ³ Corresponding author: E-mail, smoiwai{at}agri.kagoshima-u.ac.jp;Fax, +81-99-285-8556.     

Stomatal responses to ABA and IAA in isolated epidermal strips ofVicia faba L.

Epidermal strips from well-watered faba-bean plants were subjected to a range of abscisic acid (ABA) and indolyl-3-acetic acid (IAA) concentrations (10^-5 to 1 mM) in the presence or absence of CO₂ in light or dark. ABA had inhibitory effect on abaxial stomatal apertures in all the concentrations studied and retained them closed even after addition of KCl (SO and 100 mM) to the incubation medium. It also influenced stomatal responses to CO₂. In the presence of CO₂ apertures were greater than in its absence in light as well as in darkness. This relationship remained unchanged also after addition of KCl. The action of ABA inhibited accumulation of potassium in the guard cells. IAA stimulated stomatal opening and its effect was quite opposite to ABA; in the presence of CO₂ the apertures were smaller than in its absence. IAA, however, was able to inhibit the closing effect of darkness, CO₂, and ABA, and stimulated potassium accumulation in the guard cells. Simultaneous action of ABA+IAA manifested effects of both substances.     

Effect of Elevated CO2 on Stomatal Density and Distribution in a C4 Grass and a C3 Forb under Field Conditions

Two common tallgrass prairie species, Andropogon gerardii, thedominant C₄ grass in this North American grassland, and Salviapitcheri, a C₃ forb, were exposed to ambient and elevated (twiceambient) CO₂ within open-top chambers throughout the 1993 growingseason. After full canopy development, stomatal density on abaxialand adaxial surfaces, guard cell length and specific leaf mass(SLM; mg cm^-2) were determined for plants in the chambers aswell as in adjacent unchambered plots. Record high rainfallamounts during the 1993 growing season minimized water stressin these plants (leaf xylem pressure potential was usually >-1·5 MPa in A. gerardii) and also minimized differencesin water status among treatments. In A. gerardii, stomatal densitywas significantly higher (190 ± 7 mm^-2; mean ±s.e.) in plants grown outside of the chambers compared to plantsthat developed inside the ambient CO₂ chambers (161 ±5 mm^-2). Thus, there was a significant 'chamber effect' on stomataldensity. At elevated levels of CO₂, stomatal density was evenlower (P < 0·05; 121 ± 5 mm^-2). Most stomatawere on abaxial leaf surfaces in this grass, but the ratio ofadaxial to abaxial stomatal density was greater at elevatedlevels of CO₂. In S. pitcheri, stomatal density was also significantlylower when plants were grown in the open-top chambers (235 ±10 mm^-2 outside vs. 140 ± 6 mm^-2 in the ambient CO₂ chamber).However, stomatal density was greater at elevated CO₂ (218 ±12 mm^-2) compared to plants from the ambient CO₂ chamber. Theratio of stomata on adaxial vs. abaxial surfaces did not varysignificantly in this herb. Guard cell lengths were not significantlyaffected by growth in the chambers or by elevated CO₂ for eitherspecies. Growth within the chambers resulted in lower SLM inS. pitcheri, but CO₂ concentration had no effect. In A. gerardii,SLM was lower at elevated CO₂. These results indicate that stomataland leaf responses to elevated CO₂ are species specific, andreinforce the need to assess chamber effects along with treatmenteffects (CO₂) when using open-top chambers.Copyright 1994, 1999Academic Press Andropogon gerardii, elevated CO₂, Salvia pitcheri, stomatal density, tallgrass prairie     

Comparison of Palaeobotanical Observations with Experimental Data on the Leaf Anatomy of Durmast Oak [Quercus petraea(Fagaceae)] in Response to Environmental Change

To test whether stomatal density measurements on oak leaf remainsare reliable tools for assessing palaeoatmospheric carbon dioxideconcentration [CO₂], under changing Late Miocene palaeoenvironmentalconditions, young seedings of oak (Quercus petraea,Liebl.) weregrown at elevatedvs.ambient atmospheric [CO₂] and at high humiditycombined with an increased air temperature. The leaf anatomyof the young oaks was compared with that of fossil leaves ofthe same species. In the experiments, stomatal density and stomatalindex were significantly decreased at elevated [CO₂] in comparisonto ambient [CO₂]. Elevated [CO₂] induced leaf cell expansionand reduced the intercellular air space by 35%. Leaf cell sizeor length were also stimulated at high air humidity and temperature.Regardless of a temperate or subtropical palaeoclimate, leafcell size in fossil oak was not enhanced, since neither epidermalcell density nor length of the stomatal apparatus changed. Theabsence of these effects may be attributed to the phenologicalresponse of trees to climatic changes that balanced temporalchanges in environmental variables to maintain leaf growth underoptimal and stable conditions.Quercus petraea,which evolvedunder recurring depletions in the palaeoatmospheric [CO₂], maypossess sufficient phenotypic plasticity to alter stomatal frequencyin hypostomatous leaves allowing high maximum stomatal conductanceand high assimilation rates during these phases of low [CO₂].Copyright1998 Annals of Botany Company Atmospheric CO₂, high humidity, elevated temperature,Quercus petraea,durmast oak, Late Miocene, palaeoclimates, leaf anatomy, stomatal density, stomatal index     

Stomatal Density Responses of Egyptian Olea europaea L. Leaves to CO2 Change Since 1327 BC

We have attempted to separate the effects of CO₂ and temperaturechange on stomatal density by examining ancient leaf materialof Olea europaea L. The distribution of this species is confinedto a Mediterranean type climate, so that O. europaea leavesof different ages will have formed under similar temperaturesbut different CO₂ levels over the last 3000 years. Stomataldensity measurements have been made upon leaves of O. europaeaoriginating from King Tutankhamun's tomb dating from 1327 BC,and have been compared with values obtained from Egyptian O.europaea material dating from pre-332 BC, 1818 and 1978 AD.Together, the four dates provide a record of how the plant hasresponded to increases in atmospheric CO₂ concentration duringthat time. The results demonstrate that in accordance with similarstudies examining the stomatal density response of plants overthree time scales (hundreds, thousands and tens of thousandsof years) stomatal density falls as CO₂ levels increase. Sincewe have examined a natural system with leaves developing undersimilar environmental temperatures the results confirm observationsfrom experimental studies in which plants were grown under thesame temperature but different CO₂ regimes.Copyright 1993, 1999Academic Press Olea europaea, stomatal density, atmospheric CO₂, temperature, climate change     

Evidence of a Role for Abscisic Acid in Mediating Stomatal Closure Induced by Obstructing Translocation from Leaves of Pearl Millet (Pennisetum americanum [L.] Leeke)

Application of a heat girdle near the base of the lamina ofthe fifth, fully expanded leaf of young pearl millet (Pennisetumamericanum [L.] Leeke) plants resulted in a decrease in solutepotential, an increase in leaf dry matter content, and a declinein stomatal conductance and in the rate of CO₂ assimilation.Total water potential was largely unaffected by girdling whileturgor potential increased as a consequence of the decreasein solute potential. Abscisic acid (ABA) content of the leaf increased 5 to 6-foldwithin 1 h of girdling, then declined equally rapidly beforeincreasing again at a slower rate. The decline in conductance was correlated with both the decreasein solute potential and the increase in ABA. To determine whichof these factors could be controlling conductance, girdled leaveswere exposed either to 14 h of continuous light or to a similarperiod of darkness followed by a brief light treatment to allowstomata to open. Girdling reduced conductance equally followingdarkness or light but solute accumulation occurred only in thelight. ABA accumulated in girdled leaves in both darkness andlight. Simultaneous measurements of conductance and CO₂ assimilationshowed that intercellular CO₂ concentration did not increasefollowing girdling. It was concluded that the decrease in conductancein millet leaves after girdling was most probably mediated bythe increase in ABA content. Key words: Leaf girdling, Solute accumulation, Stomatal conductance, Abscisic acid; Pennisetum americanum     

Stomatal Characteristics of Four Native Herbs Following Exposure to Elevated CO2

Contrasting effects on the stomatal index (SI), stomatal density,epidermal cell size and number were observed in four chalk grasslandherbs (Sanguisorba minor Scop., Lotus corniculatus L., Anthyllisvulneraria L. and Plantago media L.) following exposure to elevatedcarbon dioxide concentrations (CO₂) in controlled environmentgrowth cabinets. SI of S. minor increased for both leaf surfaces,whilst in A. vulneraria and P. media SI decreased on one surfaceonly. In L. corniculatus , no differences in SI were observedas epidermal cell density changed in parallel with stomataldensity. In L. corniculatus and S. minor stomatal density increasedon both surfaces, whereas in P. media it decreased; in A. vulnerariastomatal density decreased on the abaxial leaf surface alonefollowing exposure to elevated CO₂. In the latter three species,SI changed because stomatal density did not change in parallelwith epidermal cell density. The results suggest elevated CO₂is either directly or indirectly affecting cell differentiationand thus stomatal initiation in the meristem. In S. minor and P. media leaf growth increased in elevated CO₂,because of increased cell expansion of epidermal cells, whereasin L. corniculatus, epidermal cell size decreased and greaterleaf growth was because of an increase in epidermal cell divisions.In A. vulneraria, leaf size did not change, but increased cellexpansion on the adaxial surface suggests CO₂ affects leaf surfacesdifferently, either directly or indirectly at the cell differentiationstage or as the leaf grows. These results suggest component species of a plant communitymay differ in their response to elevated CO₂. Predicting theeffect of environmental change is therefore difficult.Copyright1994, 1999 Academic Press Elevated CO₂, Sanguisorba minor (salad burnet), Lotus corniculatus (birdsfoot trefoil), Anthyllis vulneraria (kidney vetch), Plantago media (hoary plantain), stomatal index, stomatal density, epidermal cell size     

Elevated CO2and Temperature have Different Effects on Leaf Anatomy of Perennial Ryegrass in Spring and Summer

Mature second leaves of Lolium perenne L. cv. Vigor, were sampledin a spring and summer regrowth period. Effects of CO₂enrichmentand increased air temperature on stomatal density, stomatalindex, guard cell length, epidermal cell density, epidermalcell length and mesophyll cell area were examined for differentpositions on the leaf and seasons of growth. Leaf stomatal density was smaller in spring but greater in summerin elevated CO₂and higher in both seasons in elevated temperatureand in elevated CO₂xtemperature relative to the respective controls.In spring, leaf stomatal index was reduced in elevated CO₂butin summer it varied with position on the leaf. In elevated temperature,stomatal index in both seasons was lower at the tip/middle ofthe leaf but slightly higher at the base. In elevated CO₂xtemperature,stomatal index varied with position on the leaf and betweenseasons. Leaf epidermal cell density was higher in all treatmentsrelative to controls except in elevated CO₂(spring) and elevatedCO₂xtemperature (summer), it was reduced at the leaf base. Inall treatments, stomatal density and epidermal cell densitydeclined from leaf tip to base, whilst guard cell length showedan inverse relationship, increasing towards the base. Leaf epidermalcell length and mesophyll cell area increased in elevated CO₂inspring and decreased in summer. In elevated CO₂xtemperatureleaf epidermal cell length remained unaltered in spring comparedto the control but decreased in summer. Stomatal conductancewas lower in all treatments except in summer in elevated CO₂itwas higher than in the ambient CO₂. These contrasting responses in anatomy to elevated CO₂and temperatureprovide information that might account for differences in seasonalleaf area development observed in L. perenne under the sameconditions. Lolium perenne ; perennial ryegrass; elevated CO₂and temperature; stomatal density; stomatal index; cell size     

Stomatal Density and Index of Fossil Plants Track Atmospheric Carbon Dioxide in the Palaeozoic

It has been demonstrated that the leaves of a range of foresttree species have responded to the rising concentration of atmosphericCO₂ over the last 200 years by a decrease in both stomatal densityand stomatal index. This response has also been demonstratedexperimentally by growing plants under elevated CO₂ concentrations.Investigation of Quaternary fossil leaves has shown a correspondingstomatal response to changing CO₂ concentrations through a glacial-interglacialcycle, as revealed by ice core data. Tertiary leaves show asimilar pattern of stomatal density change, using palynologicalevidence of palaeo-temperature as a proxy measure of CO₂ concentration.The present work extends this approach into the Palaeozoic fossilplant record. The stomatal density and index of Early Devonian,Carboniferous and Early Permian plants has been investigated,to test for any relationship that they may show with the changesin atmospheric CO₂ concentration, derived from physical evidence,over that period. Observed changes in the stomatal data givesupport to the suggestion from physical evidence, that atmosphericCO₂ concentrations fell from an Early Devonian high of 10-12times its present value, to one comparable to that of the presentday by the end of the Carboniferous. These results suggest thatstomatal density of fossil leaves has potential value for assessingchanges in atmospheric CO₂ concentration through geologicaltime.Copyright 1995, 1999 Academic Press Aglaophyton major, Sawdonia ornata, Swillingtonia denticulata, Lebachia frondosa, Juncus effusus, Psilotum nudum, Araucaria heterophylla, stomatal density, stomatal index, Palaeozoic CO₂     

Stomatal responses to light and CO2 in greening wheat leaves

The development of two types of stomatal transpiration, oneinduced by light (light-induced stomatal transpiration) andthe other induced by CO₂-free air in the dark (CO₂-sensitivestomatal transpiration), in greening leaves of wheat (Triticumaestivum L.) was studied in respect to the development of CO₂uptake and chlorophyll formation. Light-induced stomatal transpirationwas not observed at all in etiolated leaves and was generatedafter 3 hr of illumination for greening, when the activity ofCO₂ uptake was generated. CO₂-sensitive stomatal transpirationwas low in etiolated leaves and started to increase at the sametime during greening as the start of CO₂ uptake. The activitiesof both light-induced and CO₂-sensitive stomatal transpirationincreased as the activity of CO₂ uptake and the chlorophyllcontent increased. Pre-illumination of etiolated leaves for1 min followed by 4 hr of dark incubation eliminated the lagfor the development of the two types of stomatal transpirationand CO₂ uptake. (Received September 4, 1978; )     

Contrasted Effects of CO2 on the Regulation of Dormancy and Germination in Xanthium pennsylvanicum and Setaria faberi Seeds

The effects of CO₂ on dormancy and germination were examinedusing seeds of cocklebur (Xanthium pennsylvanicum Wallr.) andgiant foxtail (Setaria faberi Herrm.). The rate of germinationof the giant foxtail seeds as well as cocklebur was promotedby exogenously applied CO₂ at a concentration of 30 mmol mol^-1regardless of the sowing conditions. However, seeds which failedto germinate in the presence of CO₂, entered a secondary phaseof dormancy under unfavourable germination conditions. If CO₂was applied to seeds under conditions such as water stress imposedwith a 200 mol m^-3 mannitol solution, a hypoxic atmosphere of100 mmol mol^-1 O₂ or a treatment of 0·1 mol m^-3 ABA,development of secondary dormancy was accelerated. These contrastedeffects of CO₂ were observed in ecological studies. Under naturalfield conditions germination of buried giant foxtail seeds respondedpositively to CO₂ during a period of release from primary dormancyfrom Feb. to May, but CO₂ accelerated secondary dormancy commencingin early Jun. In other words, in the presence of CO₂, both theenvironmental conditions and the germination states of the seedsclearly showed secondary dormancy-inducing effects. Thus, itseems that CO₂ has contrasted effects on regulation of dormancyand germination of seeds depending on the germination conditions.Copyright1995, 1999 Academic Press Xanthium pennsylvanicum, cocklebur, Setaria faberi, giant foxtail, CO₂, water stress, hypoxia, ABA, germination, secondary dormancy     

Effect of Elevated CO2 on Stomatal Size and Distribution in Perennial Ryegrass

Plants of ryegrass (Lolium perenne L. cv. Melle) were grownfrom the early seedling stage in growth cabinets at a day/nighttemperature of 20/15 °C, with a 12-h photoperiod, and aCO₂ concentration of either 340 or 680 ± 15 µl1^–1 CO₂. Young, fully-expanded, acclimated leaves fromprimary branches were sampled for length of stomata, and ofepidermal cells between stomata, numbers of stomata and epidermalcells per unit length of stomatal row, numbers of stomatal rowsacross the leaf and numbers of stomatal rows between adjacentvein ridges. Elevated CO₂ had no significant effect on any ofthe measured parameters. Elevated CO₂, Lolium perenne, ryegrass, stomatal distribution, stomatal size     

The effect of elevated atmospheric CO2 and drought onstomatal frequency in groundnut (Arachis hypogaea (L.))

The effects of elevated atmospheric CO₂, alone or in combinationwith water stress, on stomatal frequency in groundnut (Arachishypogaea (L.) cv. Kadiri-3) were investigated. CO₂ exerted significanteffects on stomatal frequency only in irrigated plants. Theeffects of drought on leaf development out weighed the smallereffects of CO₂ concentration, although reductions in stomatalfrequency induced by elevated atmo-spheric CO₂ were still observed.When stands of groundnut were grown under irrigated conditionswith unrestricted root systems, an increase in atmospheric CO₂from 375 to 700 ppmv decreased stomatal frequency on both leafsurfaces by up to 16% in droughted plants, stomatal frequencywas reduced by 8% on the adaxial leaf surface only. Elevatedatmospheric CO₂ promoted larger reductions in leaf conductancethan the changes in stomatal frequency, indicating partial stomatalclosure. As a result, the groundnut stands grown at elevatedCO₂ utilized the available soil moisture more slowly than thosegrown under ambient CO₂, there by extending the growing period.Despite the large variations in cell frequencies induced bydrought, there was no treatment effect on either stomatal indexor the adaxial/abaxial stomatalfrequency ratio. The data suggestthat the effects of future increases in atmospheric CO₂ concentrationon stomatal frequency in groundnut are likely to be small, especiallyunder conditions of water stress, but that the combination ofassociated reductions in leaf con-ductance and enhanced assimilationat elevated CO₂ will be important in semi-arid regions Key words: Arachis hypogaea L, Leguminosae, groundnu, stomatal frequency, CO₂, drought     

Responses of Stomata in Epidermal Strips of Vicia fabato Carbon Dioxide and Growth Hormones when Incubated on Potassium Chloride and Potassium Iminodiacetate

The effect of various K⁺ levels in combination with Cl^&minus; or iminodiacetate (IDA^{& minus;}) on stomatal responsesin isolated epidermal strips of Vicia faba L. were examinedin order to determine the role of malate during guard cell movements.Responses of guard cells to ABA, kinetin, and varying CO₂ levelswere similar when epidermal strips were floated on KCL or KIDAat 10 mM; such responses were typical in that ABA caused closure,kinetin stimulated opening in ambient air, and apertures weregreater in CO₂-free than ambient air. Maximal stomatal openingwas observed in both ambient and CO₂-free air with KCL at 100mM. The transfer of epidermal strips from 100 mM KCL to solutionsof 100 mM KCL supplemented with ABA or kinetin did not bringabout changes in stomatal aperture. KCL at 100 mM supporteda greater degree of stomatal opening than did 100 mM KIDA irrespectiveof the CO₂ content of the air. In CO₂-free air transfer of epidermalstrips from 100 mM KIDA to solutions containing 100 mM KIDAsupplemented with ABA or kinetin caused little change in stomatalaperture, whereas, in ambient air, the same treatments resultedin stomatal opening. The results are discussed in relation tothe role of malate during guard cell movements.