Leaf anatomy of alpine plants as related to altitudinal stress

Alopecurus gerardi, Poa alpina, and Carex curvula are spontaneous, perennial forage plants distributed in the high elevation (2300–3200 m) pasture lands of Piedmont and Valle d'Aosta (Italy). Sedum atratum is an annual succulent which grows at elevations up to 3200 m. The three monocotyledons have, in comparison with corresponding plants from the low-land, peculiar organographic and anatomic structures such as curling leaf lamina, vascular bundle sheath layer with chloroplasts arranged in a centrifugal fashion, low stomatal density (stomata number/cm²: upper epidermis x?= 0–11.9; lower epidermis x?= 7.66–11.55), thick cuticles, as well as higher values of S (H₂O; g/dm²; x?= 0.6–0.32), Sm (H₂O g/mg Chi.; x?= 0.11–0.16), SLW (g f. wt/dm²; x?= 0.86–1.36), but lower SLA values (cm²/g f. wt; x?= 75.07–116.77). All these data are correlated to water stress. Even though the leaf lamina anatomy possesses some features typical of C₄ plants, the presence of starch grains in the mesophyll chloroplasts indicates that these plants are probably C₃ ones. In spite of high values of thylacoid grana/thylacoid intergrana ratios, typical of shade plants (mesophyll chloroplasts: x? up to 3.81; bundle sheath chloroplasts: x? up to 5.3), and Chi a/Chl b ratios (x? up to 4.23 in C. curvula), the apparent absence of peroxisomes seem to indicate a very efficient dark phase of photosynthesis. S. atratum, in comparison with the typical CAM succulents, which live in dryer and warmer habitats, has a higher values of stomatal density (upper epidermis, x?= 2.59; lower epidermis, x?= 3.15) and of SLA (x?= 24.98), but lower S (x?= 3.83), Sm (x?= 1.19) and SLW (x?= 4.15).     

The Effect of Light Intensity on the Release of Ethylene from Leaves

When leaf discs of Xanthium strumarium L. a C₃ plant, or Zeamays L. a C₄ plant, are incubated in 1-aminocyclopropane-l-carboxylicacid (ACC) in closed flasks, ethylene is released. The rateof ethylene release appears to be dependent on the levels oflight and CO₂ available for photosynthesis in the tissues. In Xanthium the rate of ethylene release is lower in the lightthan in the dark regardless of the presence or absence of addedbicarbonate as a source of CO₂. The inhibition of ethylene releaseis most apparent in the absence of added bicarbonate (i.e. atthe CO₂ compensation point), and at light intensities sufficientto saturate photosynthesis (had the CO₂ level in the test flaskbeen maintained). In contrast, light dramatically promotes therate of ethylene release from Zea leaf tissue when the CO₂ levelis maintained above the CO2 compensation point. The rate ofethylene release from either Xanthium or Zea, incubated withor without added bicarbonate, does not appear to be alteredby further increasing the light intensity above the minimallevels sufficient to saturate photosynthesis. In the closed system used in these studies and at a light intensitysufficient to saturate photosynthesis, Xanthium and Zea leaftissue both appear to release comparable amounts of ethylenefrom ACC when the data is expressed on a chlorophyll basis.However, in Xanthium the rate of ethylene release is similarin light and dark, while in Zea the rate in the light is muchgreater than in the dark when the data is expressed either ona leaf area or on a chlorophyll basis. It is suggested thatthe different responses of these tissues to light/dark transientsmay reflect differences in their ability to metabolize ACC and/ordifferences in their ability to retain and metabolize ethyleneitself.     

Characterization of the Type of Photosynthetic Carbon Dioxide Fixation in Jute (Corchorus olitorius L.)

Activities of photosynthetic and photorespiratory enzymes viz.,ribulose bisphosphate carboxylase, phosphoenol pyruvate carboxylaseand glycolate oxidase from jute (Corchorus olitorius L.; cv.JRO 632) leaves were compared with those from maize (C₄) andsunflower (C₃) leaves. The photosynthetic CO₂ fixation products,the release of ¹⁴CO₂ in light and dark following photosynthesisin ¹⁴CO₂, chlorophyll a: b ratio, gross leaf photosyntheticrate and dry matter production rate were also studied. The resultsshow that jute is a C₃ plant. Key words: Jute, Corchorus olitorius, C₃ photosynthesis     

The Responses of Net Photosysthesis to Light and Temperature in Spartina townsendii(sensu lato), a C4 Species from a Cool Temperate Climate

Carbon dioxide and water vapour exchanges for single attachedleaves of the temperate C₄ grass Spartina townsendii were measuredunder controlled environment conditions in an open gas-exchangesystem. The responses of net photosynthesis, stomatal resistance,and residual resistance to leaf temperature and photon fluxdensity are described. The light and temperature responses ofnet photosynthesis in S. townsendii are compared to informationon these responses in both temperate C₃ grasses and sub-tropicalC₄ grasses. Adaptation of photosynthesis in this C₄ speciesto a cool temperate climate is indicated both by the light andtemperature responses of net photo-synthesis. Unlike the C₄grasses examined previously, significant rates of net photosynthesiscan be detected at leaf temperatures below 10?C. Rates of netphotosynthesis equal or exceed those reported for temperateC₃ grasses at all of the temperature (5–40?C) and photonflax density (13–2500µmol m^–2 s^–1) conditionsexamined. Maximum rates of net photosynthesis in S. townsendiiare almost double those reported for C₃ herbage grasses. Unliketemperate C₃ grasses, the major limitation to net photosynthesisat low leaf temperatures (10?C and below) is the stomatal resistance,showing that the low residual resistance characteristic of C₄species is maintained in S. townsendii even at low leaf temperatures.     

Growth and Physiology of One-year old Poplar (Populus) Under Elevated Atmospheric CO2 Levels

The effects of elevated atmospheric CO₂ concentrations on theecophysiological responses (gas exchange, chlorophyll a fluorescence,Rubisco activity, leaf area development) as well as on the growthand biomass production of two poplar clones (i.e. Populus trichocarpax P. deltoides clone Beaupré and P. x euramericana cloneRobusta) were examined under open top chamber conditions. Theelevated CO₂ treatment (ambient + 350 µmol mol^-1) stimulatedabove-ground biomass of clones Robusta and Beaupré afterthe first growing season by 55 and 38%, respectively. This increasedbiomass production under elevated CO₂ was associated with asignificant increase in plant height, the latter being the resultof enhanced internode elongation rather than an increased productionof leaves or internodes. Both an increased leaf area index (LAI)and a stimulated net photosynthesis per unit leaf contributedto a significantly higher stem biomass per unit leaf area, andthus to the increased above-ground biomass production underthe elevated CO₂ concentrations in both clones. The larger LAIwas caused by a larger individual leaf size and leaf growthrate; the number of leaves was not altered by the elevated CO₂treatment. The higher net leaf photosynthesis was the resultof an increase in the photochemical (maximal chlorophyll fluorescenceFm and photochemical efficiency Fv/Fm) as well as in the biochemical(increased Rubisco activity) process capacities. No significantdifferences were found in dark respiration rate, neither betweenclones nor between treatments, but specific leaf area significantlydecreased under elevated CO₂ conditions.Copyright 1995, 1999Academic Press Biomass, chlorophyll a fluorescence, elevated CO₂, growth, Populus, poplar, photosynthesis, respiration, Rubisco     

The Seasonal Pattern of Growth and Production of a Temperate C4 Species, Cyperus longus

Growth and production of the temperate C₄ species Cyperus longusL. was measured throughout a growing season in an establishedplot in Eastern Ireland. The maximum standing live biomass reachedwas 2·5 kg m^–2. Estimates of unit leaf rate (ULR)and leaf area index (LAI) were made. The product of these quantitiesgave the crop growth rate (CGR) each week. C. longus was foundto maintain high values of LAI throughout the summer, with amaximum value of about 13 in early August. CGR reached a peakin early July. The optimum LAI was 11·6. Temperaturesat five levels in the plant canopy, and the amount of solarradiation intercepted by the canopy were measured continuouslyduring the summer. The mean daily rate of leaf extension waspositively correlated with the mean daily air temperature abovethe canopy but the temperature coefficient of the process waslow compared with other temperate species. The percentage ofsolar radiation intercepted by the canopy increased rapidlyin early summer, and canopy closure had occurred by mid-June.Rates of net photosynthesis were measured on young and old leafmaterial in situ at the time of peak LAI. In young leaves themaximum rates of net photosynthesis were higher than those publishedfor a range of temperate C₃ species, but similar to those foundin another temperate C₄ species, Spartina townsendii. Key words: C₄ photosynthesis, leaf growth, productivity     

Analysis of chlorophyll a fluorescence in C3 and CAM forms of Mesembryanthemum crystallinum

Comparisons of chlorophyll a fluorescence characteristics ofC₃ and CAM forms of Mesembryanthemum crystallinum were usedto identify features of the photosynthetic mechanism associatedwith CAM. The reduction status, Q, was lower and predicted PSII activityhigher in the C₃ form than in the CAM form throughout the photoperiod.These differences were particularly pronounced during the firsthour of illumination when non-photochemical quenching attributableto the intrathylakoid proton gradient was also at its highestin the CAM form. It is argued that this high proton gradientdiminishes PSII activity and serves a protective role againstphotoinhibition at a time in the CAM cycle when both CO₂ concentrationwithin the leaf, and carbon cycle enzyme activation levels arelikely to be low. Differences in fluorescence characteristics between the C₃ andCAM forms also indicate modification of the energy transductionmechanisms of the CAM form possibly related to the increasedoverall demand for ATP in CAM photosynthesis. Total non-photochemicalquenching was higher in the CAM form than in the C₃ form. Aninverse relationship between fast and slowly-relaxing componentsof non-photochemical quenching can be interpreted in terms ofthe changing demand for ATP in the different phases of CAM. Key words: C₃/CAM photosynthesis, chlorophyll fluorescence, state transitions, cyclic photophosphorylation     

Photosynthetic efficiency of Panicum hians and Panicum milioides in relation to C3 and C4 plants

Panicum hians and Panicum milioides were found to have characteristicsintermediate to those of C₃ and C₄ species with respect to CO₂compensation point, percentage inhibition of photosynthesisby O₂ at various O₂/CO₂ solubility ratios, and water use efficiency.C₄ species have a higher carboxylation efficiency than eitherthe intermediate or C₃ species. During photosynthesis, evenunder 2.5% O₂, C₄ species have a higher affinity for intercellularCO₂ (Km 1.6 µM) apparently due to the initial carboxylationthrough PEP carboxylase. Under low O₂ the intermediate and C₃species had a similar affinity for intercellular CO₂ duringphotosynthesis (Km 5–7 µM) consistent with carboxylationof atmospheric CO₂ through RuDP carboxylase. There were considerablevariation in photosynthesis/unit leaf area at saturating CO₂levels in the species examined which in part is due to differencesin RuDP carboxylase /unit leaf area. The highest rates of photosynthesis/unitleaf area under CO₂-saturating conditions were with the C₃ specieswhich had a correspondingly high level of RuDP carboxylase/unitleaf area. Possibilities for the greater efficiency of P. hiansand P. milioides in comparison to C₃ species in utilizing lowlevels of CO₂ in the presence of atmospheric O₂ are discussed. ¹ This research was supported by the College of Agriculturaland Life Sciences, University of Wisconsin, Madison; and theUniversity of Wisconsin Research Committee with funds from theWisconsin Alumni Research Foundation. (Received June 25, 1977; )     

Comparative Anatomy and Morphology of Leaves between C3 and C4 Species in Panicum

Anatomical and morphological structures of leaf blades werecompared between C₃ and C₄ species in Panicum. Inter-specificvariation of stomatal density, longitudinal vein density andmesophyll thickness was highly correlative either plus or minuswithin respective groups. The two groups could not be distinguishedby a single character, since the variation ranges overlappedeach other. However, the quantitative relations between veindensity and the other two characters differentiated the twogroups well. In C₃, stomatal density seemed to be a primaryfactor for regulating water balance, while in C₄ vein systemwas considered to be important for the regulation. The specieswith intermediate photosynthesis behaved similar to the C₃ species.In the C₃ group, correlative variation was observed betweenleaf width, leaf angle and the three characters mentioned above.Variation of light-receiving area due to the changes of widthand angle of leaf blades was considered to be one of the adaptivestrategies of this group. Increase of light-receiving area wasin connection with the thinning of leaves. On the other hand,in the C₄ correlations between length, width and angle of leaveswere low. Such loose character correlation may be achieved byits efficiency of CO₂ utilization and its well developed veinsystems. Besides, NAD-me type species tended to have relativelylower stomatal and vein densities as compared with the otherdecarboxylation types in this group. Panicum, photosynthesis, C₃, C₄, decarboxylation types, leaf, stomata, vein     

Photosynthetic Characteristics of Cassava (Manihot esculenta Crantz), a C3 Species with Chlorenchymatous Bundle Sheath Cells

Cultivars of cassava, Manihot esculenta Crantz, were studiedto determine the mechanism of photosynthetic carbon assimilationin this species. The results, contrary to recent reports, indicatethat cassava is a C₃ plant based on a number of physiologicaland biochemical photosynthetic characteristics. The CO₂ compensationpoints among 10 cassava cultivars ranged from 55 to 62 µlliter^–1, which was typical for C₃ plants including castorbean, a member of the same family (Euphorbiaceae). The initialproducts of photosynthesis in cassava are C₃-like; the activitiesof several key C₄ enzymes in cassava are low and similar tothose of C₃ plants. Data on the rates of photosynthesis perunit of leaf area and the photosynthetic response of cassavato CO₂ is also consistent with C₃ photosynthesis. Cassava hasa distinctive chlorenchymatous vascular bundle sheath locatedbelow a single layer of palisade cells. Unlike C₃-C₄ intermediatesand C₄ species, the bundle sheaths of cassava are not surroundedby mesophyll cells. The bundle sheath cells which occur at highfrequency in cassava may function in both photosynthesis andtransport of photosynthates in the leaf. (Received July 31, 1990; Accepted September 25, 1990)     

Acclimation in Seedlings of a Tropical Tree, Bischofia javanica, Following a Stepwise Reduction in Light

Acclimation of fully developed leaves of Bischofia javanicaBlume to shadelight was examined. Seedlings were grown undersimulated daylight (1000 µmol m^–2 s^–1), thentransferred to a simulated shadelight (40 µmol m^–2s^–1). When a high-light leaf was transferred to low light, large negativenet photosynthetic rates (P_m) were recorded. This decrease wasrapid, but within 7 d the rate increased and became equal tothe low-light control leaf. These changes in photosynthesisdid not follow the pattern of changes in stomatal conductance(g_s). Transfer to the low light resulted in a dramatic decreasein leaf weight per unit area (L_w), and most of the decreasesin L_w occurred within 3 d of transfer when the P_m of the transferredleaf was well below that of the low-light control leaf. There was a significant decrease in chlorophyll a in the transferredleaf without an appreciable change in chlorophyll b resultingin a large decrease in the chlorophyll a to chlorophyll b ratio.Leaf chlorophylls per unit area were higher in the transferredleaf than the low-light control leaf. Maximum photosyntheticrate in the transferred leaf was decreased by 40% compared tothat for the high-control leaf, but was almost at the same extenthigher than the low-light control leaf The results are discussedin the context of carbon gain capacity of its seedlings underlight-limiting forest understorey habitats. Bischofia, chlorophylls, light, photosynthesis, shade acclimation, tree seedlings, tropical tree     

Prolonged Survival of CAM-Mode Mesembryanthemum crystallinum in Darkness and its Possible Dependence on Malate

A remarkable difference was found in the survival of leavesof Mesembryanthemum crystallinum with plants grown in the C₃versus the CAM mode. With excised leaves (petiole in solution)of C₃-mode plants subjected to 6 days of darkness, there wasa large reduction in the chlorophyll content of the leaf andleaf turgor had decreased. By day 9, the chlorophyll had disappeared,except at the major veins, and the leaf tip had dried and turnedbrown. In contrast, the leaf tissue in the CAM mode showed onlya partial loss of chlorophyll during the same period, and evenafter 17 days of darkness, the tissue at the base was stillalive. Similarly, intact plants grown in the C₃ mode deterioratedmuch faster during 20 days of darkness than did plants grownin the CAM mode. Chlorophyll content, chlorophyll a/b ratio,phosphoenolpyruvate carboxylase, NADP-malic enzyme, malate andstarch content were measured. In both C₃- and CAM-mode plants,the starch content decreased rapidly during the dark periodand was nearly depleted after two days. In the CAM-mode tissue,there was a relatively high level of malate during prolongeddarkness (up to 17 days), with a transitory rise early in thedark period. In contrast, the malate content was low and rapidlydepleted in the C₃-mode leaves kept in darkness. These findingssuggest that malate may be an important source of carbon forsustaining leaves of CAM-mode M. crystallinum during prolongeddarkness. (Received May 20, 1987; Accepted October 23, 1987)     

An Analysis of the Photosynthesis and Productivity of Vegetative Crops in the United Kingdom

The rate of total dry matter production of a vegetative crop,under optimal water and nutrient regimens is related to someleaf and canopy photosynthetic characteristics. Three leaf photosyntheticcharacteristics are examined in detail: the light utilizationefficiency at normal ambient CO₂ and O₂ concentrations, a, therate of light saturated photosynthesis per unit leaf area, F_max,and the ratio of the rates of photorespira tion and gross photosynthesis.The genetic variability in each of these characteristics issought from published data on a wide range of C₃ and C₄ planttypes. Within C₃ and C₄ plant types there are significant genetic differencesonly in F_max,, although differences exist between C₃ and C₄plants in the other two characteristics. The effects of thesedifferences on the rate total dry matter production are estimated,and it is concluded that there is no compelling evidence toindicate that improvements in total dry matter production rates,in the U.K., are likely to result from genetic manipulationof these characteristics in the existing range of plant material.     

Dynamic Model of Leaf Photosynthesis with Acclimation to Light and Nitrogen

A simple model of photosynthesis in a mature C₃leaf is described,based on a non-rectangular hyperbola: the model allows the high-lightasymptote of that equation (P_max) to respond dynamically tolight and nitrogen. This causes the leaf light response equationto acclimate continuously to the current conditions of lightand N nutrition, which can vary greatly within a crop canopy,and through a growing season, with important consequences forgross production. Predictions are presented for the dynamicsof acclimation, acclimated and non-acclimated photosyntheticrates are compared, and the dependence of leaf properties onlight and N availability is explored. There is good correspondenceof predictions with experimental results at the leaf level.The model also provides a mechanism for a down regulation ofphotosynthesis in response to increased carbon dioxide concentrations,whose magnitude will depend on conditions, particularly of nitrogennutrition.Copyright 1998 Annals of Botany Company Leaf, photosynthesis, hyperbola, model, C₃, acclimation, light, nitrogen.     

C3 and C4 Photosynthetic and Anatomical forms of Alloteropsis semialata (R.Br.) Hitchcock: I. Variability in Photosynthetic Characteristics, Water Utilization Efficiency and Leaf Anatomy

The grass Alloteropsis semialata (R.Br.) Hitchcock is uniquein that both Kranz and non-Kranz leaf anatomy has been reportedin this species. The present study investigates Kranz formsof A. semialata collected from a single ecological niche. Theseplants exhibit morphological and anatomical differences withrespect to leaf area, stomatal size and stomatal distribution.Carbon dioxide and water exchange measurements in the two formsshow the expected pattern of higher photosynthetic rate andhigher water utilization efficiency associated with Kranz anatomy.No intermediate physiological response or anatomical form wasobserved in this sample. Alloteropsis semialata (R.Br.) Hitchcock, C₃ photosynthetic, C₄ photosynthesis, water utilization, leaf anatomy, Kranz anatomy     

Photosynthesis in the Chloroembryo of Cyamopsis tetragonaloba Taub

The intraseminal embryo of Cyamopsis tetragonaloba is greenfrom the globular stage onwards. The chloroembryos do not photosynthesizein vivo but are capable of in vitro photosynthesis at a ratewhich is similar to that of leaf tissue and they utilize bicarbonateions in this process. Both ribulose bisphosphate (RuBP) andphospho-enol-pyruvate (PEP) carboxylases are functional in CO₂fixation by the embryos. Application of aminotriazole does notinhibit the development of chlorophyll in chloroembryos Cvamopsis tetragonaloba Taub, cluster bean, chloroembryo, photosynthesis     

A Comparison of the Growth, Photosynthesis, Stomatal Conductance and Water Use Efficiency of Moricandia and Brassica Species

When grown in pots and well-watered, the relative growth ratesof the above ground parts of two species of Moricandia (M. arvensis,an intermediate C₃–C₄ species, and M. moricandioides,a C₃ species) were inferior to those of two cultivated Brassicaspecies (B. campestris and B. napus). The Moricandia specieshad thicker leaves (greater d.wt per unit leaf area) with morechlorophyll than the Brassica species and had slightly greaterrates of photosynthesis per unit leaf area at an irradiance(400–700 nm) of 2000 µmol quanta m^–2 s ^–1.Leaves of M. arvensis, known to have a CO₂ compensation pointbetween that of C₃ and C₄ species, had a lower ratio of theintercellular to atmospheric partial pressure of CO₂ (C₁/C_a)and a greater instantaneous water use efficiency (WUE) thanthose of M. moricandioides and the Brassica species. Carbon isotope discrimination (     

Temperature Response of Early Foliar Expansion of Potato and Wheat

We studied the course of early leaf area expansion and specificleaf area (S_LA) in potato (Solanum tuberosum L.) and wheat (Triticumaestivum L.) genotypes and tested whether air temperature explainsdifferences in these courses within different environments.Such knowledge can be used to improve crop growth modelling.The relative rate of leaf area expansion (R_L) of potato andwheat decreased with thermal time, but was nearly linear upto a leaf area index (L) of 1.0. TheR_L (L < 1; mean: 17.9x 10^-3°C^-1 d^-1) of potato showed an interaction betweengenotype and environment, and varied with year. TheR_L (L <1; mean: 7.1 x 10^-3°C^-1 d^-1) of winter wheat was lower thanthat of spring wheat (mean: 10.9 x 10^-3°C^-1 d^-1), and bothvaried with year. S_LAof potato increased nearly linearly withthermal time from 5 to 15 m² kg^-1at 50% emergence, to 20 to25 m² kg^-1at 155°Cd, and then decreased slightly. The S_LAofboth winter and spring wheat began at 16 to 23 m² kg^-1and inmost cases increased slightly with thermal time. In potato,regression parameters of S_LAwith thermal time were affectedby environment (management conditions and year) and genotype;in wheat they were affected by environment (year and site).Treatment effects on R_Lof potato were not correlated with thoseon S_LA , and were only partly correlated for wheat. Thereforewe conclude that the early foliar expansion of potato is associatedwith a strong increase in S_LA , and not so for wheat. For bothcrops the course of early leaf area expansion and ofS_LA withair temperature is not robust over environments and genotypes.The consequences of these results for modelling are discussed.Copyright 2000 Annals of Botany Company Triticum aestivum, spring wheat, winter wheat, Solanum tuberosum, leaf area expansion, specific leaf area, early growth, genotype, environment, modelling     

Photosynthesis of White Clover Leaves as Influenced by Canopy Position, Leaf Age, and Temperature

Microswards of white clover (Trifolium repens L.) were grownin controlled environments at 10/7, 18/13 and 26/21 °C day/nighttemperatures. The vertical distribution of leaves of differentages and their rates of ¹⁴CO₂-uptake in situ were studied. Extending petioles carried the laminae of young leaves throughthe existing foliage. A final position was reached within 1/4to 1/3 of the time between unfolding and death. Newly unfoldedleaves had higher rates of ¹⁴CO₂-uptake per leaf area than olderones at the same height in the canopy. At higher temperatures,the decrease with age was faster. However, the light-photosynthesisresponse of leaves which were removed from different heightsin the canopy varied much less with leaf age than did the ratesof ¹⁴CO₂-uptake in situ. The comparison of the rates of ¹⁴CO₂-uptake in situ with thelight-photosynthesis response curves suggests that young leavesreceive more light than older ones at the same height in thecanopy. This would imply that young white clover leaves havethe ability to reach canopy positions having a favourable lightenvironment. This ability may improve the chances of survivalof white clover in competition with other species. Trifolium repens L., white clover, photosynthesis, canopy, leaf age, ¹⁴CO₂-uptake, ecotypes, temperature     

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