Increased root growth in elevated CO2: a biophysical analysis of root cell elongation

A biophysical analysis of root expansion was conducted in fourchalk downland herbs (Sanguisorba minor Scop., Lotus corniculatusL., Anthyllis vul-neraria L. and Plantago media L.) exposedto either ambient or elevated CO₂in controlled environment cabinets.Measurements of fine (F) and extra-fine (EF) root extensionrate (RER), water relations, and cell wall tensiometric extensibilityrevealed differences in the diurnal pattern of root growth betweenspecies. After 35 d of exposure to elevated CO₂, RER of bothF and EF roots increased significantly in darkness and on illuminationfor S. minor, whilst for A. vulneraria (EF roots only) and L.corniculatus a significant increase occurred at night whereasfor P. media a significant increase occurred during the day.Cells measured in the zone of elongation were longer in allspecies exposed to elevated CO₂. Water potential (     

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     

Biomass and compositional changes occur in chalk grassland turves exposed to elevated CO2 for two seasons in FACE

Artificial turves composed of 7 chalk grassland species (Festuca ovina L.; Briza media L.; Bromopsis erecta (Hudson) Fourr.; Plantago media L.; Sanguisorba minor Scop.; Anthyllis vulneraria L. and Lotus corniculatus L.) were grown from seed and exposed to two seasons of elevated (600 μmol mol^–1) and ambient (340 μmol mol^–1) CO₂ concentrations in free air CO₂ enrichment (ETH-FACE, Zurich). The turves were clipped regularly to a height of 5 cm and assessed for above ground biomass production and relative abundance based on accumulated clipped dry biomass as well as by point quadrat recording. Below ground biomass production was assessed with root in-growth bags during the second season of growth. Increases in total biomass (> 30%) were noted in elevated CO₂, but the differences did not become significant until the second season of growth. Individual species’ biomass varied in response to elevated CO₂, with significant increases in biomass in elevated CO₂ turves for both legume species, and no significant CO₂ effect on S. minor or P. media. An initial positive CO₂ effect on biomass of combined grass species was reversed by the end of the experiment with less biomass and a significantly smaller proportion of total biomass present in elevated CO₂, which was attributed primarily to changes in proportion of F. ovina. Species relative abundance was significantly affected by elevated CO₂ in the final 4 of the 6 clip events, with the legume species increasing in proportion at the expense of the other species, particularly the grasses. Root length and dry weight were both significantly increased in elevated CO₂ (77% and 89%, respectively), and these increases were greater than increases in shoot biomass (36%) from the same period. Species responses to elevated CO₂, within the model community, were not consistent with predictions made from data on individual species, leading to the conclusion that responses to elevated CO₂, at the community level, and species within the community level, are the result of direct physiological effects and indirect competitive effects. These conclusions are discussed with respect to the ecological responses of natural communities, and the chalk grassland community in particular, to elevated CO₂.     

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     

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     

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     

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     

Mechanism for increased leaf growth in elevated CO2

The effect of exposure to elevated CO₂ on the processes of leafcell production and leaf cell expansion was studied using primaryleaves of Phaseolus vulgaris L. Cell division and expansionwere separated temporally by exposing seedlings to dim red lightfor 10 d (when leaf cell division was completed) followed byexposure to bright white light for 14 d (when leaf growth wasentirely dependent on cell expansion). When plants were exposedto elevated CO₂ during the phase of cell expansion, epidermalcell size and leaf area development were stimulated. Three piecesof evidence suggest that this occurred as a result of increasedcell wall loosening and extensibility, (i) cell wall extensibility(WEx, measured as tensiometric extension using an Instron) wassignificantly increased, (ii) cell wall yield turgor (V, MPa)was reduced and (iii) xyloglucan endotransglycosylase (XET)enzyme activity was significantly increased. When plants wereexposed to elevated CO₂ during the phase of cell division, thenumber of epidermal cells was increased whilst final cell sizewas significantly reduced and this was associated with reducedfinal leaf area, WEx and XET activity. When plants were exposedto elevated CO₂ during both phases of cell division and expansion,leaf area development was not affected. For this treatment,however, the number of epidermal cells was increased, but cellexpansion was inhibited, despite exposure to elevated CO₂ duringthe expansion phase. Assessments were also made of the spatialpatterns of WEx across the expanding leaf lamina and the datasuggest that exposure to elevated CO₂ during the phase of leafexpansion may lead to enhanced extensibility particularly atbasal leaf margins which may result in altered leaf shape. The data show that both cell production and expansion were stimulatedby elevated CO₂, but that leaf growth was only enhanced by exposureto elevated CO₂ in the cell expansion phase of leaf development.Increased leaf cell expansion is, therefore, an important mechanismfor enhanced leaf growth in elevated CO₂, whilst the importanceof increased leaf cell production in elevated CO₂ remains tobe elucidated. Key words: Phaseolus vulgaris L., dwarf beans, elevated CO², biophysics of cell expansion, xyloglucan endotransglycosylase, XET, water relations     

Contrasting CO2 and temperature effects on leaf growth of perennial ryegrass in spring and summer

The effects of increased atmospheric carbon dioxide (CO₂) of700 µmol mol^–1 and increased air temperature of+ 4C were examined in Lolium perenne L. cv. Vigor, growingin semi-controlled greenhouses. Leaf growth, segmental elongationrates (SER), water relations, cell wall (tensiometric) extensibility(%P) and epidermal cell lengths (ECL) were measured in expandingleaves in spring and summer. In elevated CO₂, shoot dry weight (SDW) increased in mid-summer.In both seasons, SDW decreased in elevated air temperatureswith this reduction being greater in summer as compared to spring.Specific leaf area (SLA) decreased in elevated CO₂ and in CO₂ temperature in both seasons. In spring, increased leaf extensionand SER in elevated CO₂ were linked with increased ECL, %P andfinal leaf size whilst in summer all were reduced. In high temperature,leaf extension, SER, %P and final leaf size were reduced inboth seasons. In elevated CO₂ temperature, leaf extension,SER, %P, and ECL increased in spring, but final leaf size remainedunaltered, whilst in summer all decreased. Mid-morning waterpotential did not differ with CO₂ or temperature treatments.Leaf turgor pressure increased in elevated CO₂ in spring andremained similar to the control in summer whilst solute potentialdecreased in spring and increased in summer. Contrasting seasonalgrowth responses of L. perenne in response to elevated CO₂ andtemperature suggests pasture management may change in the future.The grazing season may be prolonged, but whole season productivitymay become more variable than today. Key words: Lolium perenne, ryegrass, CO₂ and temperature, leaf extension, cell wall rheology     

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     

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     

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     

Growth Response of Young Birch Trees (Betula pendulaRoth.) After Four and a Half Years of CO2Exposure

A field experiment consisting of 18 birch trees grown in opentop chambers in ambient and elevated CO₂concentrations was setup with the aim of testing whether the positive growth responseobserved in many short-term studies is maintained after severalgrowing seasons. We present the results of growth and biomassafter 4.5 years of CO₂exposure, one of the longest studies sofar on deciduous tree species. We found that elevated CO₂ledto a 58% increase in biomass at the end of the experiment. However,estimation of stem mass during the growing season showed thatelevated CO₂did not affect relative growth rate during the fourthgrowing season, and therefore, that the large accumulation ofbiomass was the result of an early effect on relative growthrate in previous years. Trees grown in elevated CO₂investedmore carbon into fine roots and had relatively less leaf areathan trees grown in ambient CO₂. In contrast with previous studies,acceleration of growth did not involve a significant declinein nutrient concentrations of any plant tissue. It is likelythat increased fine root density assisted the trees in meetingtheir nutrient demands. Changes in the species composition ofthe ectomycorrhizal fungi associated with the trees grown inelevated CO₂in favour of late successional species supportsthe hypothesis of an acceleration of the ontogeny of the treesin elevated CO₂.Copyright 1997 Annals of Botany Company Betula pendula; silver birch; elevated CO₂; growth; biomass allocation; ectomycorrhizas; tissue composition; nutrients; leaf morphology; specific leaf area; stomatal density; shoot structure     

Elevated carbon dioxide affects the patterning of subsidiary cells in Tradescantia stomatal complexes

The influence of elevated CO₂ concentration (670 ppm) on thestructure, distribution, and patterning of stomata in Tradescantialeaves was studied by making comparisons with plants grown atambient CO₂. Extra subsidiary cells, beyond the normal complementof four per stoma, were associated with nearly half the stomatalcomplexes on leaves grown in elevated CO₂. The extra cells sharedcharacteristics, such as pigmentation and expansion, with thetypical subsidiary cells. The position and shape of the extrasubsidiary cells in face view differed in the green and purplevarieties of Tradescantia. Substomatal cavities of complexeswith extra subsidiary cells appeared larger than those foundin control leaves. Stomatal frequency expressed on the basisof leaf area did not differ from the control. Stomatal frequencybased on cell counts (stomatal index) was greater in leavesgrown in CO₂-enriched air when all subsidiary cells were countedas part of the stomatal complex. This difference was eliminatedwhen subsidiary cells were included in the count of epidermalcells, thereby evaluating the frequency of guard cell pairs.The extra subsidiary cells were, therefore, recruited from theepidermal cell population during development. Stomatal frequencyin plants grown at elevated temperature (29 C) was not significantlydifferent from that of the control (24 C). The linear aggregationsof stomata were similar in plants grown in ambient and elevatedCO₂. Since enriched CO₂ had no effect on the structure or patterningof guard cells, but resulted in the formation of additionalsubsidiary cells, it is likely that separate and independentevents pattern the two cell types. Plants grown at enrichedCO₂ levels had significantly greater internode lengths, butleaf area and the time interval between the appearance of successiveleaves were similar to that of control plants. Porometric measurementsrevealed that stomatal conductance of plants grown under elevatedCO₂ was lower than that of control leaves and those grown atelevated temperature. Tradescantia was capable of regulatingstomatal conductance in response to elevated CO₂ without changingthe relative number of stomata present on the leaf. Key words: Elevated CO₂, stomata, subsidiary cells, patterning     

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     

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     

Elevated atmospheric CO2 concentration ameliorates effects of NaCl salinity on photosynthesis and leaf structure of Aster tripolium L.

This study investigated the interaction of NaCl-salinity andelevated atmospheric CO₂ concentration on gas exchange, leafpigment composition, and leaf ultrastructure of the potentialcash crop halophyte Aster tripolium. The plants were irrigatedwith five different salinity levels (0, 25, 50, 75, 100% seawatersalinity) under ambient and elevated (520 ppm) CO₂. Under salineconditions (ambient CO₂) stomatal and mesophyll resistance increased,leading to a significant decrease in photosynthesis and wateruse efficiency (WUE) and to an increase in oxidative stress.The latter was indicated by dilations of the thylakoid membranesand an increase in superoxide dismutase (SOD) activity. Oxidativestress could be counteracted by thicker epidermal cell wallsof the leaves, a thicker cuticle, a reduced chlorophyll content,an increase in the chlorophyll a/b ratio and a transient declineof the photosynthetic efficiency. Elevated CO₂ led to a significantincrease in photosynthesis and WUE. The improved water and energysupply was used to increase the investment in mechanisms reducingwater loss and oxidative stress (thicker cell walls and cuticles,a higher chlorophyll and carotenoid content, higher SOD activity),resulting in more intact thylakoids. As these mechanisms canimprove survival under salinity, A. tripolium seems to be apromising cash crop halophyte which can help in desalinizingand reclaiming degraded land. Key words: Aster tripolium, cash crop halophyte, elevated CO₂, gas exchange, oxidative stress, photosynthesis, salt tolerance, ultrastructure, water use efficiency Received 29 July 2008; Revised 8 October 2008 Accepted 9 October 2008     

Stem hydraulic conductance of European beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) grown in elevated CO2

Over two seasons in c. 600 ppm CO₂, oak had lower stomatal conductancein CO₂-enriched compared to amblent air. Beech showed no responseto CO₂ concentration on sunny days. Mirroring this pattern,exposure to elevated CO₂ reduced whole-shoot hydraulic conductanceper unit leaf area in oak, but not in beech. Key words: Climate change, Fagaceae, gas exchange, trees, water relations     

Stomatal Responses and the Senescence of Leaves

Guard cell responses were examined in green and senescing leavesof Victa faba using detached epidermal strips to eliminate influencesfrom the mesophyll. Stomatal opening was greater in epidermalstrips from mature leaves than from senescing leaves althoughthe latter still retained the ability to respond to CO₂ andto kinetin. It was concluded that the decline in stomatal activityduring senescence is an independent but parallel process tochanges occurring in the mesophyll. Vicia faba, leaf senescence, stomata, kinetin     

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