Stomatal Responses of Variegated Leaves to CO2 Enrichment

The responses of stomatal density and stomatal index of fivespecies of ornamental plants with variegated leaves grown attwo mole fractions of atmospheric CO₂ (350 and 700 µmolmol^-1) were measured. The use of variegated leaves allowed anypotential effects of mesophyll photosynthetic capacity to beuncoupled from the responses of stomatal density to changesin atmospheric CO₂ concentration. There was a decrease in stomataldensity and stomatal index with CO₂ enrichment on both white(unpigmented) and green (pigmented) leaf areas. A similar responseof stomatal density and index was also observed on areas ofleaves with pigmentation other than green indicating that anydifferences in metabolic processes associated with colouredleaves are not influencing the responses of stomatal densityto CO₂ concentrations. Therefore the carboxylation capacityof mesophyll tissue has no direct influence on stomatal densityand index responses as suggested previously (Friend and Woodward1990 Advances in Ecological Research 20: 59-124), instead theresponses were related to leaf structure. The stomatal characteristics(density and index) of homobaric variegated leaves showed agreater sensitivity to CO₂ on green portions, whereas heterobaricleaves showed a greater sensitivity on white areas. These resultsprovide evidence that leaf structure may play an important rolein determining the magnitude of stomatal density and index responsesto CO₂ concentrations.Copyright 1995, 1999 Academic Press Leaf structure, photosynthesis, stomatal conductance, CO₂, stomatal density, stomatal index     

Do fossil plants signal palaeoatmospheric carbon dioxide concentration in the geological past?

Fossil, subfossil, and herbarium leaves have been shown to provide a morphological signal of the atmospheric carbon dioxide environment in which they developed by means of their stomatal density and index. An inverse relationship between stomatal density/index and atmospheric carbon dioxide concentration has been documented for all the studies to date concerning fossil and subfossil material. Furthermore, this relationship has been demonstrated experimentally by growing plants under elevated and reducedcarbon dioxide concentrations. To date, the mechanism that controls the stomatal density response to atmospheric carbon dioxide concentration remains unknown. However, stomatal parameters of fossil plants have been successfully used as a proxy indicator of palaeo-carbon dioxide levels. This paper presents new estimates of palaeo-atmospheric carbon dioxide concentrations for the Middle Eocene (Lutetian), based on the stomatal ratios of fossil Lauraceae species from Bournemouth in England. Estimates of atmospheric carbon dioxide concentrations derived from stomatal data from plants of the Early Devonian, Late Carboniferous, Early Permian and Middle Jurassic ages are reviewed in the light of new data. Semi-quantitative palaeo-carbon dioxide estimates based on the stomatal ratio (a ratio of the stomatal index of a fossil plant to that of a selected nearest living equivalent) have in the past relied on the use of a Carboniferous standard. The application of a new standard based on the present-day carbon dioxide level is reported here for comparison. The resultant ranges of palaeo-carbon dioxide estimates made from standardized fossil stomatal ratio data are in good agreement with both carbon isotopic data from terrestrial and marine sources and long-term carbon cycle modelling estimates for all the time periods studied. These data indicate elevated atmospheric carbon dioxide concentrations during the Early Devonian, Middle Jurassic and Middle Eocene, and reduced concentrations during the Late Carboniferous and Early Permian. Such data are important in demonstrating the long-term responses of plants to changing carbon dioxide concentrations and in contributing to the database needed for general circulation model climatic analogues.     

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     

The Effects of CO2 Enrichment and Nutrient Supply on Growth Morphology and Anatomy of Phaseolus vulgaris L. Seedlings

Plants of Phaseolus vulgaris were grown from seed in open-topgrowth chambers at the present (P, 350 µmol mol^–1)atmospheric CO₂ concentration and at an elevated (E, 700 µmolmol^–1) CO₂ concentration, and at low (L, without additionalnutrient solution) and high (H, with additional nutrient solution)nutrient supply for 28 d The effects of CO₂ and nutrient availabilitywere examined on growth, morphological and biochemical characteristics Leaf area and dry mass were significantly increased by CO₂ enrichmentand by high nutrient supply Stomatal density, stomatal indexand epidermal cell density were not affected by elevated CO₂concentration or by nutrient supply Leaf thickness respondedpositively to CO₂ increasing particularly in mesophyll areaas a result of cell enlargement Intercellular air spaces inthe mesophyll decreased slightly in plants grown in elevatedCO₂ Leaf chlorophyll content per unit area or dry mass was significantlylower in elevated CO₂ grown plants and increased significantlywith increasing nutrient availability The content of reducingcarbohydrates of leaves, stem, and roots was not affected byCO₂ but was significantly increased by nutrient addition inall plant parts Starch content in leaves and stem was significantlyincreased by elevated CO₂ concentration and by high nutrientsupply Phaseolus vulgaris, elevated atmospheric CO₂, CO₂-nutrient interaction, stomatal density, leaf anatomy, chlorophyll, carbohydrates, starch     

Response of Photosynthesis, Stomatal Conductance and Water Use Efficiency to Elevated CO2 and Nutrient Supply in Acclimated Seedlings of Phaseolus vulgaris L.

Plants of Phaseolus vulgaris L were grown from seed in open-topgrowth chambers at present day (350 µmol mol^–1)and double the present day (700 µmol mol^–1) atmosphericCO₂ concentration with either low (L, without additional nutrientsolution) or relatively high (H, with additional nutrient solution)nutrient supply Measurements of assimilation rate, stomatalconductance and water use efficiency were started 17 d aftersowing on each fully expanded, primary leaf of three plantsper treatment Measurements were made in external CO₂ concentrations(C₂) of 200, 350, 450, 550 and 700 µmol mol^–1 andrelated to both C_a and to C₁, the mean intercellular space CO₂concentration Fully adjusted, steady state measurements weremade after approx 2 h equilibration at each CO₂ concentration The rate of CO₂ assimilation by leaves increased and stomatalconductance decreased similarly over the range of C_a or C₁ inall four CO₂ and nutrient supply treatments but both assimilationrate and stomatal conductance were higher in the high nutrientsupply treatment than in the low nutrient treatment The relationbetween assimilation rate or stomatal conductance and C₁ wasnot significantly different amongst plants grown in present-dayor elevated CO₂ concentration in either nutrient supply treatment,i e there was no evidence of down regulation of photosynthesisor stomatal response Increase in CO₂ concentration from 350to 700 µmol mol^–1 doubled water use efficiency ofindividual leaves in the high nutrient supply treatment andtripled water use efficiency in the low nutrient supply treatment The results support the hypothesis that acclimation phenomenaresult from unbalanced growth that occurs after the seed reservesare exhausted, when the supply of resources becomes growth limiting CO₂ enrichment, Phaseolus vulgaris L., net CO₂ assimilation rate, stomatal conductance, water use efficiency     

The Responses of Stomatal Density to CO2 Partial Pressure

Experiments on a range of species of tree, shrub and herb haveshown that stomatal density and stomatal index increase as thepartial pressure of CO₂ decreases over the range from the currentlevel of 34 Pa to 22.5 Pa. Stomatal density responds to thereduced partial pressure of CO₂ in a simulation of high altitude(3000 m), when the CO₂ mole fraction is unchanged. When the partial pressure of CO₂ is increased from 35 to 70Pa stomatal density decreases slightly, with a response to unitchange in CO₂ which is about 10% of that below 34 Pa. Measurements of gas exchange on leaves which had developed indifferent CO₂ partial pressures, but at low saturation vapourpressure deficits in the range of 0.7 to 0.9 kPa, indicatedlower photosynthetic rates but higher stomatal conductancesat reduced CO₂ partial pressures. Experiments on populations of Nardus stricta originating fromaltitudes of 366 m and 810 m in Scotland, indicated geneticdifferences in the responses of stomatal density to CO₂ in pressuressimulating altitudes of sea level and 2 000 m. Plants from thehigher altitude showed greater declines in stomatal densitywhen the CO₂ partial pressure was increased. Key words: Stomata, CO₂, gas exchange, altitude, atmospheric pressure     

Photosynthesis during leaf ontogeny in field-grown Nicotiana tabacum L. lines selected by survival at low CO2 concentrations

Leaf photosynthesis, stomatal conductance, internal CO₂ concentrationand leaf composition (photosynthetic pigments, total solubleprotein and Rubisco content) during leaf ontogeny of field grownNicotiana tabacum L. lines selected for survival at low atmosphericCO₂ concentrations are described. Selection at low CO₂ concentrations resutted in lines with highertotal dry matter production than their parent cultivar, butthis could not be related to improved photosynthesis which theselection method was designed to achieve (shown in previouswork). In the present work we report higher rates of photosynthesisin the selected lines in mature and old leaves, not found inyoung leaves when the capacity for photosynthesis was maximum.Differences in the regulation of the photosynthetic carbon reductioncycle as well as differences in the diffusive characteristicsof the mesophyll, due to changes in the size of the cells, maybe the cause for the higher rates of photosynthesis during leafsenescence in the selected lines. Key words: Photosynthesis, leaf ontogeny, tobacco, lines     

Changes in N and S Leaf Content, Stomatal Density and Specific Leaf Area of 14 Plant Species during the Last Three Centuries of CO2 Increase

Parallel to the increase in atmospheric CO² from 278 µmolmol^–1 in AD 1750 to the current ambient level of 348 µmolmol^–1, there have been overall decreases in leaf nitrogencontent and stomatal density from 144% and 121%, respectively,in AD 1750 to 100% today of herbarium specimens of 14 trees,shrubs, and herbs collected over the last 240 years in Catalonia,a Mediterranean climate area. These decreases were steeper duringthe initial slower increases in CO₂ atmospheric levels as comparedwith the relatively faster CO₂ increases in recent years. Thedeclines in leaf N content and stomatal density have also beenreported in experimental studies on leaves of plants grown underenriched CO₂ environments. Meanwhile, the stomatal index andoverall carbon and sulphur leaf contents have not changed significantly.Leaf S content was higher in the 1940s samples coinciding withthe burning of increased quantities of sulphur-rich coal. Consequently,the epidermal cell density has decreased parallel to the stomataldensity and the C/N ratio of leaves has increased, implyingpossible important consequences on herbivores, decomposers,and ecosystems. An overall decrease in the specific leaf area(SLA) from 184% in the 18th century to 100% today has also beenfound, as would be expected under CO₂ enrichment, but whichmight also be an artifact of prolonged storage. Key words: Carbon dioxide increase, leaf nitrogen content, leaf sulphur content, stomatal density, last centuries     

Effects of CO2 Enrichment on Four Poplar Clones. II. Leaf Surface Properties

The poplar clones Columbia River, Beaupre, Robusta and Raspaljehave been investigated in the present (350 µmol mol^–1)and double the present (700 µmol mol^–1) atmosphericCO₂ concentration. Cuttings were planted in pots and were grownin open-top chambers inside a glasshouse for 92 d. Stomatal density, stomatal index, length of stomatal pore andepidermal cell density were not affected by CO₂ enrichment inany of the clones. Lack of differences in stomatal density orindex indicate that there were no direct effects of CO₂ enrichmenton the initiation of the number of stomata during ontogenesisor on epidermal cell expansion at a later stage. Stomatal conductance decreased because of the effect of CO₂on stomatal opening. The average reduction in both adaxial andabaxial surface has been estimated at 41%. Beaupre showed thelargest response of stomatal conductance and Columbia Riverthe smallest. Poplar clones, CO₂ enrichment, stomatal density, stomatal length, stomatal conductance     

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 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     

新疆准噶尔盆地早侏罗世掌鳞杉科植物化石 及古大气CO2浓度

植物化石气孔参数分析是目前恢复古大气二氧化碳浓度较为精准的方法之一,银杏类和松柏类等是恢复古大气CO_2浓度常用的化石类群。本文利用新疆准噶尔盆地下侏罗统三工河组的松柏类掌鳞杉科Brachyphyllum(Hirmeriella?)sp.化石对早侏罗世大气CO_2浓度进行了重建,获得早侏罗世大气CO_2浓度为～1200ppm,丰富了早侏罗世大气CO_2浓度信息,进一步说明掌鳞杉科植物通过气孔比率法在重建侏罗纪大气CO_2浓度方面的可靠性。掌鳞杉科植物的旱生构造和较高的大气CO_2浓度表明早侏罗世Toarcian期大洋缺氧事件在陆地生态系统内可能产生了一定的响应。     

Effects of Different CO2 Environments on the Photosynthesis-Yield Relationship and the Carbon and Water Balance of a White Clover (Trifolium repens L. cv. Blanca) Sward

Effects of atmospheric CO₂ enrichment to a level above 600 parts10^–6 on leaf and canopy gas exchange characteristics wereinvestigated in Trifolium repens, using an open system for gasexchange measurement. The cuvettes of the system served as growthchambers, allowing continuous measurement in a semi-controlledenvironment of ±350 and ±600 parts 10^–6CO₂, respectively. Carbon balance data were compared with cropyield and effects on the canopy level were compared with measuredleaf responses of photosynthesis and stomatal behaviour. Photosyntheticstimulation by high CO₂ was stronger at the canopy level (103%on average) than for leaves (90% in full light), as a consequenceof accelerated foliage area development. The latter increasedabsolute water consumption by 16%, despite strong stomatal closure.The overall result was a 63% improvement in canopy water useefficiency (WUE), while leaf WVE increased almost 3-fold insaturating light. The stomatal response was such that, whilethe internal CO₂ concentration in the leaf, ch increased withrising atmospherical CO₂ concentration, c_a, ci/c_a was somewhatdecreased. Total canopy resistance, R_c, was generally lowerat high CO₂ levels, despite higher leaf resistance. Higher canopyCO₂ loss at night and faster light extinction in a larger-sizedhigh CO₂ canopy were major drawbacks which prevented a furtherincrease in dry matter production (the harvest index was increasedby a factor 1.83). Key words: CO₂ enrichment, canopy CO₂ exchange, carbon balance, water use efficiency, leaf and canopy resistance     

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     

Variations of stomatal density and carbon isotope values of<Emphasis Type="Italic"> Picea crassifolia</Emphasis> at different altitudes in the Qilian Mountains

The stomatal characteristics, length and dry weight as well as stable carbon isotope composition (i¹³C) of needles and tree rings of Qinghai spruce (Picea crassifolia) at different altitudes in the Qilian mountains were investigated. The results showed that stomatal density, distribution pattern on epidermis, and length and dry weight of needles all increased with altitude below 3,000 m. In contrast, these parameters all decreased with increasing altitude above 3,000 m. Furthermore an assay of tree rings showed that near 3,000 m in altitude was the optimum zone for growth and development of Qinghai spruce. Although atmospheric CO₂ concentration influences stomatal density, the effects of many environmental factors, such as temperature, light and rainfall, could obscure the effects of changes in atmospheric CO₂ concentration on stomatal density at altitudes higher than the optimum for growth. The correlation of stomatal density with atmospheric CO₂ concentration and i¹³C of Qinghai spruce needles was significant below 3,000 m, but was insignificant above 3,000 m altitude. The i¹³C value of needles, however, was negatively correlated with atmospheric CO₂ concentrations. Therefore, the influence of altitude should be considered in research on plant physiological ecology and the relationship of stomatal density with i¹³C value or atmospheric CO₂ concentration.     

Elevated CO2 effects on stomatal density of wheat and sour orange trees

No significant differences were found in stomatal densitiesor stomatal indices of wheat or sour orange trees grown at highCO₂ concentrations in two different CO₂ enrichment systems (Free-AirCO₂ Enrichment for wheat and Open-Top Chambers for orange trees).These results are in accordance with most of the previous resultsobtained in short-term experimental studies which suggest thatplants do not acclimate to increasing CO₂ concentration by changingstomatal density within a single generation. Key words: Triticum aestivum, Citrus aurantium, stomatal density, elevated atmospheric CO₂     

Separation of Direct and Indirect Responses of Stomata to Light: Results from a Leaf Inversion Experiment at Constant Intercellular CO2 Molar Fraction

Previous work has shown that stomata respond directly to light,but it was not clear whether the only additional response isthrough CO₂, or whether some other metabolite is involved inthis response. Gas exchange experiments were done with normallypositioned and inverted leaves of Hedera helix to investigatethis problem. The macroscopic optical properties of the leavesand their anatomical structure were also studied. These experimentssnowed that there is no need to postulate the existence of amessenger other than CO₂ to explain the indirect response ofstomata to light. The experiments also showed that leaf inversionaffects both stomatal conductance and photosynthesis, and highlightthe difficulties involved in the interpretation of the effectof leaf inversion on stomata when stomatal conductance measurementsare not done concurrently with measurements of CO₂ flux densityand intercellular CO₂ molar fraction Key words: Hedera helix, ivy, gas exchange, leaf inversion, stomatal conductance, light, CO₂ flux density, photosynthesis     

Increases in the Diffusion Resistances of Leaves in a Carbon Dioxide-Enriched Atmosphere

The stomatal closing reaction to CO₂, which has been observedin laboratory studies by many workers, was investigated in plant-growthcabinets under conditions similar to those in glass-houses withCO₂-enriched atmospheres. Lettuce and Xanthium plants grownin a normal atmosphere showed the expected stomatal closurein response to increasing CO₂ concentrations, and lettuce plantsgrown in 1000 ppm CO₂ showed the same response, even after 4weeks. Thus there was no evidence of acclimatization of thestomata and it is concluded that they must remain partiallyclosed during CO₂-enrichment. Estimates of diffusion resistancesto CO₂ intake in lettuce leaves showed that in light of 14.4J m^–2 S^–1, 880 ppm CO₂ in the atmosphere resultedin a concentration of 367 ppm near the mesophyll cell walls.If the stomata remained open this same internal concentrationcould be achieved with an external concentration of only 640ppm. There could, therefore, be some economic advantage if stomatalclosure were prevented during CO₂-enrichment.