Effects of the interaction between ozone and carbon dioxide on gas exchange,ascorbic acid content,and visible leaf symptoms in rice leaves

Tropospheric ozone (O₃) decreases photosynthesis, growth, and yield of crop plants, while elevated carbon dioxide (CO₂) has the opposite effect. The net photosynthetic rate (P _N), dark respiration rate (R _D), and ascorbic acid content of rice leaves were examined under combinations of O₃ (0, 0.1, or 0.3 cm³ m⁻³, expressed as O⁰, O^0.1, O^0.3, respectively) and CO₂ (400 or 800 cm³ m⁻³, expressed as C⁴⁰⁰ or C⁸⁰⁰, respectively). The P _N declined immediately after O₃ fumigation, and was larger under O^0.3 than under O^0.1. When C⁸⁰⁰ was combined with the O₃, P _N was unaffected by O^0.1 and there was an approximately 20 % decrease when the rice leaves were exposed to O^0.3 for 3 h. The depression of stomatal conductance (g _s) observed under O^0.1 was accelerated by C⁸⁰⁰, and that under O^0.3 did not change because the decline under O^0.3 was too large. Excluding the stomatal effect, the mesophyll P _N was suppressed only by O^0.3, but was substantially ameliorated when C⁸⁰⁰ was combined. Ozone fumigation boosted the R _D value, whereas C⁸⁰⁰ suppressed it. An appreciable reduction of ascorbic acid occurred when the leaves were fumigated with O^0.3, but the reduction was partially ameliorated by C⁸⁰⁰. The degree of visible leaf symptoms coincided with the effect of the interaction between O₃ and CO₂ on P _N. The amelioration of O₃ injury by elevated CO₂ was largely attributed to the restriction of O₃ intake by the leaves with stomatal closure, and partly to the maintenance of the scavenge system for reactive oxygen species that entered the leaf mesophyll, as well as the promotion of the P _N.     

Effects of elevated atmospheric carbon dioxide on gas exchange and growth of white clover

Effects of rising atmospheric CO₂ concentrations on gas exchange, growth and productivity were investigated on an important grassland species, Trifolium repens L. cv. Blanca. Pure stands of this species were cultivated over an entire growing season in small acrylic greenhouses with an artificial atmosphere of ±367 or ±620 ppm CO₂, respectively. Effects on growth and development were examined in a functional growth analysis, while consequences for gas exchange were determined by photosynthesis and transpiration measurements on canopy level. The stands were regularly clipped for production assessment. Canopies grown at high CO₂ levels showed an average increase in productivity of almost 75%. Growth analysis indicated development of a larger foliage area as the major cause, particularly in the first days of regrowth after cutting. The growth advantage that began in this stage was maintained or bettered during the following weeks. The difference between gas exchange measurements expressed per unit leaf area and per unit ground area suggested that changes in net photosynthesis and respiration did not contribute to the increase in total yield. Transpiration declined under high CO₂ if expressed on a leaf area basis but total canopy transpiration was at least as large as in ambient CO₂ due to the larger leaf area. Water-use efficiency calculations on the summer data indicated a 35% improvement with a doubling of CO₂ concentration.     

Glyphosate effects on carbon assimilation and gas exchange in sugar beet leaves

The mechanism responsible for the inhibition of net carbon exchange (NCE) which was reported previously (DR Geiger et al. 1986 Plant Physiol 82: 468-472) was investigated by applying glyphosate [N-(phosphonomethyl)glycine] to exporting leaves of sugar beet (Beta vulgaris L.). Leaf internal CO₂ concentration (C_i) remained constant despite decreases in stomatal conductance and NCE following glyphosate treatment, indicating that the cause of the inhibition was a slowing of carbon assimilation rather than decreased conductance of CO₂. Throughout a range of CO₂ concentrations, NCE rate at a given C_i declined gradually, with the time-series of response curves remaining parallel. Gas exchange measurements revealed that disruption of chloroplast carbon metabolism was an early and important factor in mediating these glyphosate effects, perhaps by slowing the rate of ribulose bisphosphate regeneration. An increase in the CO₂ compensation point accompanied the decrease in NCE and this increase was hastened by stepwise lowering of the ambient CO₂ concentration. Eventually the CO₂ compensation point approached the CO₂ level of air and the difference between internal and external CO₂ concentrations decreased. In control and in glyphosate-treated plants, both carbon assimilation and photorespiration at atmospheric CO₂ level were inhibited to a similar extent of air level of O₂. Maintaining leaves in low O₂ concentration did not prevent the decline in NCE rate.     

Simultaneous measurements on the effect of oxygen concentration on water vapor and carbon dioxide exchange in leaves

Summary Transpiration rate was unaffected by O₂ concentration in the range 1–80% when illuminated leaves ofAtriplex patula andA. rosea were kept at low CO₂ concentrations. Moreover, the marked increase in the rate of light-saturated photosynthesis that takes place in many plant species when O₂ concentration is reduced from 21% to a few percent was not accompanied by any change in transpiration rate inA. patula andSolanum dulcamara. The results indicate that higher plant photosynthesis in normal air and saturating light is not determined by physical barriers to gas diffusion alone but that it is markedly limited by biochemical processes.C.I.W.-D.P.B.Publ. No. 454.     

Accumulation of phenolic compounds in birch leaves is changed by elevated carbon dioxide and ozone

Atmospheric change may affect plant phenolic compounds, which play an important part in plant survival. Therefore, we studied the impacts of CO₂ and O₃ on the accumulation of 27 phenolic compounds in the short‐shoot leaves of two European silver birch (Betula pendula Roth) clones (clones 4 and 80). Seven‐year‐old soil‐grown trees were exposed in open‐top chambers over three growing seasons to ambient and twice ambient CO₂ and O₃ concentrations singly and in combination in central Finland. Elevated CO₂ increased the concentration of the phenolic acids (+25%), myricetin glycosides (+18%), catechin derivatives (+13%) and soluble condensed tannins (+19%) by increasing their accumulation in the leaves of the silver birch trees, but decreased the flavone aglycons (?7%) by growth dilution. Elevated O₃ increased the concentration of 3,4′‐dihydroxypropiophenone 3‐β‐d ‐glucoside (+22%), chlorogenic acid (+19%) and flavone aglycons (+4%) by inducing their accumulation possibly as a response to increased oxidative stress in the leaf cells. Nevertheless, this induction of antioxidant phenolic compounds did not seem to protect the birch leaves from detrimental O₃ effects on leaf weight and area, but may have even exacerbated them. On the other hand, elevated CO₂ did seem to protect the leaves from elevated O₃ because all the O₃‐derived effects on the leaf phenolics and traits were prevented by elevated CO₂. The effects of the chamber and elevated CO₂ on some compounds changed over time in response to the changes in the leaf traits, which implies that the trees were acclimatizing to the altered environmental conditions. Although the two clones used possessed different composition and concentrations of phenolic compounds, which could be related to their different latitudinal origin and physiological characteristics, they responded similarly to the treatments. However, in some cases the variation in phenolic concentrations caused by genotype or chamber environment was much larger than the changes caused by either elevated CO₂ or O₃.     

Carbonic anhydrase activity in leaves as measured in vivo by18O exchange between carbon dioxide and water

Carbonic anhydrase activity of intactCommelina communis L. leaves was measured using mass spectrometry, by following the¹⁸O-exchange kinetics between¹⁸O-enriched carbon dioxide and water. A gas-diffusion model (Gerster, 1971, Planta97, 155–172) was used to interpret the¹⁸O-exchange kinetics and to determine two constants, one (k) related to the hydration of CO₂ and the other (k_e), related to the diffusion of CO₂. Both constants were determined inCommelina communis L. leaves after stripping the lower epidermis to remove any stomatal influence. The hydration constant (k) was 17200 +2200 ·min^–1 (mean±SD, 12 experiments), i.e., about 8 600 times the uncatalyzed hydration of CO₂ in pure water, and was specifically inhibited by ethoxyzolamide, a powerful inhibitor of carbonic anhydrases, half-inhibition occurring around 10^–5 Methoxyzolamide. The diffusion constant (k_e) was 1.18±0.28·min^–1 (mean±SD, 12 experiments) and was only slightly inhibited (about 20%) by ethoxyzolamide. Carbonic anhydrase activity of stripped leaves was not affected by the leaf water status (up to 50% relative water deficits), was strongly inhibited by monovalent anions such as Cl^– or NO ₃ ^– , and decreased by about 50% when the photon flux density during growth was increased from 100 to 500 mol photons·m^–2·s^–1. By studying the effect of ethoxyzolamide (10^–4 M) on photosynthetic O₂ exchange, measured using¹⁸O₂ and mass spectrometry, we found that inhibition of carbonic anhydrase activity by 92–95% had little effect on the response curves of net O₂ evolution to increased CO₂ concentrations. Ethoxyzolamide had no effect on the photosynthetic electron-transport rate, measured as gross O₂ photosynthesis at high CO₂ concentration (>350 l·^–1), but was found to increase both gross O₂ photosynthesis and O₂ uptake at lower CO₂ levels. The chloroplastic CO₂ concentration calculated from O₂-exchange data was not significantly modified by ethoxyzolamide. We conclude from these results that, under normal conditions of photosynthesis, most of the carbonic anhydrase activity is not involved in CO₂ assimilation. Measurement of carbonic anhydrase activity using¹⁸O-isotope exchange therefore provides a suitable model to study the in-vivo regulation of this chloroplastic enzyme in plants submitted to various environmental conditions.Abbreviations CA carbonic anhydrase - C_cc chloroplastic CO₂ concentration - C_e external CO₂ concentration - EZA ethoxyzolamide - k CO₂ hydration rate constant - k_e CO₂ diffusion rate constan - PPFD photosynthetic photon flux density - Rubisco ribulose-1,5 bisphosphate carboxylase oxygenase - RWD relative water deficit The authors wish to thank P. Carrier for technical assistance with mass-spectrometric experiments and Dr. P. Thibault for helpful suggestions and comments. Dr. A. Vavasseur is gratefully acknowledged for supplyingCommelima communis. cultures. P.C., P.T. and A.V. are all from the CEA, Département de Physiologie Végétale et Ecosystèmes, Cadarache, France.     

Effects of elevated carbon dioxide, ozone and water availability on spring wheat growth and yield

Spring wheat (Triticum aestivum L. cv. Dragon) was exposed to elevated carbon dioxide (CO₂), alone (1995) or in combination with two levels of increased ozone (O₃) (1994) or increased irrigation (1996) during three successive growing seasons as part of the EU ESPACE‐wheat programme and conducted in open‐top chambers (OTCs) and ambient air (AA) plots at Östad, 50 km north‐east of Göteborg, Sweden. Doubling the CO₂ concentration had a positive effect on grain yield in all 3 years (+21, +7 and +11%, respectively), although only statistically significant in 1994. That year was characterised by a warm and dry summer in comparison with 1995 and 1996, in which the summers were more humid and typical for south‐west Sweden. In 1994, the CO₂‐induced increase in grain yield was associated with an increase in the duration of the green leaf area, a positive effect on straw yield and on the number of ears per square metre and a negative effect (?13%) on grain protein concentration. Harvest index was unaffected by the elevated CO₂ concentration. The only statistically significant effect of elevated CO₂ in 1995 was a decrease in the grain protein concentration (?11% in both CO₂ concentrations), and in 1996 an increase (+21%) in the straw yield. In 1996 the soil water potential was less negative in elevated CO₂, which is likely to reflect a lower water consumption of these plants. Addition of extra O₃ significantly affected the grain yield (?6 and ?10%, respectively) and the 1 000‐grain weight negatively (?3 and ?6%). Statistically significant interactions between CO₂ and O₃ were obtained for the number of ears per unit area and for the 1 000‐grain weight. The 1 000‐grain weight was negatively affected by O₃ in low CO₂, but remained unaffected in the high CO₂ treatment. There was a significant decrease (?6%) in the grain protein concentration induced by elevated irrigation. The chambers, compared with AA plots, had a positive effect on plant development and on grain yield in all 3 years.     

Response of photosynthesis and cellular antioxidants to ozone in populus leaves

Atmospheric ozone causes formation of various highly reactive intermediates (e.g. peroxyl and superoxide radicals, H₂O₂, etc.) in plant tissues. A plant's productivity in environments with ozone may be related to its ability to scavenge the free radicals formed. The effects of ozone on photosynthesis and some free radical scavengers were measured in the fifth emergent leaf of poplars. Clonal poplars (Populus deltoides × Populus cv caudina) were fumigated with 180 parts per billion ozone for 3 hours. Photosynthesis was measured before, during, and after fumigation. During the first 90 minutes of ozone exposure, photosynthetic rates were unaffected but glutathione levels and superoxide dismutase activity increased. After 90 minutes of ozone exposure, photosynthetic rates began to decline while glutathione and superoxide dismutase continued to increase. Total glutathione (reduced plus oxidized) increased in fumigated leaves throughout the exposure period. The ratio of GSH/GSSG also decreased from 12.8 to 1.2 in ozone exposed trees. Superoxide dismutase levels increased twofold in fumigated plants. After 4 hours of ozone exposure, the photosynthetic rate was approximately half that of controls while glutathione levels and superoxide dismutase activity remained above that of the controls. The elevated antioxidant levels were maintained 21 hours after ozone exposure while photosynthetic rates recovered to about 75% of that of controls. Electron transport and NADPH levels remained unaffected by the treatment. Hence, elevated antioxidant metabolism may protect the photosynthetic apparatus during exposure to ozone.     

A portable system for measuring carbon dioxide and water vapour exchange of leaves

Abstract. The apparatus described here is a fully portable, steady-state gas exchange system for simultaneous measurements of the CO₂ exchange and transpiration of single, attached leaves. The leaf cuvette provides temperature, humidity, and CO₂-concentration control. The system is suitable for either surveys or detailed studies of photosynthetic and stomatal responses to environmental variables. Representative data demonstrate the response time characteristics of the system and constitute the first field evidence of stomatal behaviour consistent with a recent hypothesis concerning the optimum pattern of stomatal conductance for the maximization of water-use-efficiency.     

Effects of exogenous calcium on mesophyll cell ultrastructure,gas exchange,and photosystem II in tobacco (Nicotiana tabacum Linn.) under drought stress

Photosynthetica - The effects of calcium chloride solution (10 mmol L–1) on mesophyll cell ultrastructure, gas exchange, chlorophyll and carotenoid content, and PSII in tobacco leaf were...     

Carbon dioxide gas exchange and the energy status of leaves of Primula palinuri under water stress

The photosynthetic rate of water stressed leaves of Primula palinuri was reduced drastically by stomatal closure, not by limitations imposed on the capacity of the photosynthetic apparatus, when water loss exceeded 20% of the water content of turgid leaves. The sudden decrease in phtosynthesis was not observed when the lower epidermis of the leaves had been removed. In these ‘stripped’ leaves, inhibition of photosynthesis increased only gradually during the wilting caused by increasing water stress and was complete when the relative water content was as low as 20% compared with the initial value. This corresponded to a water potential of about-40 bar. The light intensity at which half-maximum rates of photosynthesis were observed decreased as stress increased. In intact leaves photosynthesizing in the presence of CO₂, light scattering, which is a measure of thylakoid energization, increased steeply during stomatal closure. The observed increase corresponded to the light scattering level measured in the absence of CO₂. When the lower epidermis was removed, no sudden increase in thylakoid energization could be observed during dehydration. Thylakoid energization remained high even at low water potentials. It decreased drastically only below a relative water content of 20%. Irrespective, of the extent of water stress, CO₂ fixation of stripped leaves increased when the oxygen content of air was reduced from 21% to 2%. Usually the transition from 21 to 2% O₂ was accompanied by increased thylakoid energization. The increase in energization was more pronounced below than above a relative water content of 50%. The data show that energy-dissipating photorespiratory CO₂ turnover in the in tercellular space of water-stressed leaves whose stomata are closed decreases only slowly as water stress increases. Respiratory CO₂ production by leaves in the dark was even more resistant to water stress than photosynthesis. It was still significant at water potentials as low as-80 bar.     

Effects of long-term ozone exposure on citrus: Chlorophyll a fluorescence and gas exchange

Three-years-old trees of Satsuma mandarin (Citrus unshiu [Mak.] Marc.) cv. Okitsu were exposed to O₃ fumigation during long term (one year) in open-top chambers. As a result of the treatment, chlorophyll a fluorescence and gas exchange parameters were modified with respect to trees growing in O₃-free conditions. Net photosynthetic rate and stomatal conductance decreased and intercellular CO₂ concentration increased according to a reduction of the non-cyclic electron flow and a lower capacity to reduce the quinone pool. O₃ also reduced the development of non-photochemical quenching preventing the dissipation of excess excitation energy and, therefore, generated several alterations in photosynthetic apparatus. All these effects were obtained in long-term exposure and higher O₃ concentration. In O₃ ambient conditions, the effects were minor.     

Effects of drought stress on fluorescence characteristics of photosystem II in leaves of Plectranthus scutellarioides

Drought stress has multiple effects on the photosynthetic apparatus. Herein, we aimed to study the effect of drought stress on fluorescence characteristics of PSII in leaves of Plectranthus scutellarioides and explore potentially underlying mechanisms. Plants of P. scutellarioides were grown in a greenhouse and subjected to drought (DS, drought-stressed) or daily irrigation (control group). Leaf chlorophyll (Chl) index and induction kinetics curves of Chl a fluorescence and the JIP-test were used to evaluate effects of drought lasting for 20 d. Our results showed that both the leaf and soil relative water content decreased with increasing treatment duration. The leaf Chl index was reduced to half in the DS plants compared with the control group after 20 d. The minimal fluorescence in the DS plants was higher than that in the control plants after 10 d of the treatment. Maximum photochemical efficiency and lateral reactivity decreased with increasing treatment duration in the DS plants. With the continuing treatment, values of absorption flux per reaction center (RC), trapped energy flux per RC, dissipated energy flux per RC, and electron transport flux per RC increased in the earlier stage in the DS plants, while obviously decreased at the later stage of the treatment. In conclusion, drought stress inhibited the electron transport and reduced PSII photochemical activity in leaves of P. scutellarioides.     

Effects of water stress on photochemical function and protein metabolism of photosystem II in wheat leaves

The effects of osmotic dehydration in wheat leaves ( Triticum aestivum L. cv. Longchun No. 10) on the photochemical function and protein metabolism of PSII were studied with isolated thylakoid and PSII membranes. The results indicated that PSII was rather resistant to water stress as mild water deficit in leaves did nut significantly affect its activity. However, extreme stress conditions such as 40% decrease in relative water content (RWC) or 1.8 MPa in water potential (Ψ) caused ca 50% reduction in O₂ evolution and ca 25% inhibition of DCIP (2.6-dichlorophenol indophenol) photoreduction of PSII. In addition, it was found that the inhibited DCIP photoreduction of PSII could not be reversed by DPC (2.2-diphenylcarbazide), a typical electron donor to PSII, suggesting that water stress did not affect electron donation to PSII. Urea-SDS-PAGE and western blot analysis showed that the steady slate levels of major PSII proteins, including the D1 and D2 proteins in the PSII reaction center, declined on a chlorophyll basis with increasing water stress, possibly as a result of increased degradation. In vitro translation experiments and quantitative analysis of chloroplast RNAs indicated that the potential synthesis of chloroplast proteins from their mRNAs was impaired by water stress. From the results it is concluded that the effects of water stress on PSII protein metabolism, especially on the reaction center proteins, may account for the damage to PSII photochemistry.     

Effects of elevated carbon dioxide and ozone on the phytochemistry of aspen and performance of an herbivore

The purpose of this study was to assess the independent and interactive effects of CO(2), O(3), and plant genotype on the foliar quality of a deciduous tree and the performance of a herbivorous insect. Two trembling aspen (Populus tremuloides Michaux) genotypes differing in response to CO(2) and O(3) were grown at the Aspen FACE (Free Air CO(2) Enrichment) site located in northern Wisconsin, USA. Trees were exposed to one of four atmospheric treatments: ambient air (control), elevated carbon dioxide (+CO(2); 560 microl/l), elevated ozone (+O(3); ambient x1.5), and elevated CO(2)+O(3). We measured the effects of CO(2) and O(3) on aspen phytochemistry and on performance of forest tent caterpillar (Malacosoma disstria Hübner) larvae. CO(2) and O(3) treatments influenced foliar quality for both genotypes, with the most notable effects being that elevated CO(2) reduced nitrogen and increased tremulacin levels, whereas elevated O(3) increased early season nitrogen and reduced tremulacin levels, relative to controls. With respect to insects, the +CO(2) treatment had little or no effect on larval performance. Larval performance improved in the +O(3) treatment, but this response was negated by the addition of elevated CO(2) (i.e., +CO(2)+O(3) treatment). We conclude that tent caterpillars will have the greatest impact on aspen under current CO(2) and high O(3) levels, due to increases in insect performance and decreases in tree growth, whereas tent caterpillars will have the least impact on aspen under high CO(2) and low O(3) levels, due to moderate changes in insect performance and increases in tree growth.     

Effects of solar ultraviolet radiation on the potential efficiency of photosystem II in leaves of tropical plants

The effects of solar ultraviolet (UV)-B and UV-A radiation on the potential efficiency of photosystem II (PSII) in leaves of tropical plants were investigated in Panama (9°N). Shade-grown tree seedlings or detached sun leaves from the outer crown of mature trees were exposed for short periods (up to 75 min) to direct sunlight filtered through plastic or glass filters that absorbed either UV-B or UV-A+B radiation, or transmitted the complete solar spectrum. Persistent changes in potential PSII efficiency were monitored by means of the dark-adapted ratio of variable to maximum chlorophyll a fluorescence. In leaves of shade-grown tree seedlings, exposure to the complete solar spectrum resulted in a strong decrease in potential PSII efficiency, probably involving protein damage. A substantially smaller decline in the dark-adapted ratio of variable to maximum chlorophyll a fluorescence was observed when UV-B irradiation was excluded. The loss in PSII efficiency was further reduced by excluding both UV-B and UV-A light. The photoinactivation of PSII was reversible under shade conditions, but restoration of nearly full activity required at least 10 d. Repeated exposure to direct sunlight induced an increase in the pool size of xanthophyll cycle pigments and in the content of UV-absorbing vacuolar compounds. In sun leaves of mature trees, which contained high levels of UV-absorbing compounds, effects of UV-B on PSII efficiency were observed in several cases and varied with developmental age and acclimation state of the leaves. The results show that natural UV-B and UV-A radiation in the tropics may significantly contribute to photoinhibition of PSII during sun exposure in situ, particularly in shade leaves exposed to full sunlight.     

Soybean leaf growth and gas exchange response to drought under carbon dioxide enrichment

This study was conducted to determine the response in leaf growth and gas exchange of soybean (Glycine max Merr.) to the combined effects of water deficits and carbon dioxide (CO₂) enrichment. Plants grown in pots were allowed to develop initially in a glasshouse under ambient CO₂ and well-watered conditions. Four-week old plants were transferred into two different glasshouses with either ambient (360 μmol mol^-1) or elevated (700 μmol mol^-1) CO₂. Following a 2-day acclimation period, the soil of the drought-stressed pots was allowed to dry slowly over a 2-week period. The stressed pots were watered daily so that the soil dried at an equivalent rate under the two CO₂ levels. Elevated [CO₂] decreased water loss rate and increased leaf area development and photosynthetic rate under both well-watered and drought-stressed conditions. There was, however, no significant effect of [CO₂] in the response relative to soil water content of normalized leaf gas exchange and leaf area. The drought response based on soil water content for transpiration, leaf area, and photosynthesis provide an effective method for describing the responses of soybean physiological processes to the available soil water, independent of [CO₂].     

Effects of ozone and mild drought stress on gas exchange, antioxidants and chloroplast pigments in current-year needles of young Norway spruce [Picea abies (L.) Karst.]

 To investigate the effects of ozone exposure and soil drought, singly and in combination, on gas exchange, antioxidant contents and pigments in current-year needles of Norway spruce [Picea abies (L.) Karst.] 4-year-old seedlings were fumigated in growth chambers with either charcoal-filtered air or with 100 nl l^–1 ozone for 106 days. After 3 weeks a 20% reduction in gas exchange was observed in ozone-treated seedlings. However, no further decrease occurred in spite of continued ozone exposure. Whole needle ascorbate and apoplastic ascorbate increased until the end of the experiment and contents were 62% and 82%, respectively, higher than in ozone-free controls. This increase in ascorbate might have protected net photosynthesis from further decline. Ozone pre-treated plants and ozone-free controls were subjected to soil drought for 38 days which caused stomatal narrowing. Thereby ozone uptake was reduced when compared to well watered seedlings. At the end of the experiment drought alone, and even more in combination with ozone, had also caused an increase in ascorbate. Glutathione increased only in drought-stressed seedlings. The redox states of the ascorbate and the glutathione pools were not affected by any treatment. Superoxide dismutase activity declined under both stresses but was most reduced by ozone alone. While chlorophyll and neoxanthin contents remained unchanged, carotenes were significantly decreased upon drought. The combination of O₃ and drought induced increased lutein contents, an increased pool size of the xanthophyll cycle as well as an increased epoxidation status of the xanthophyll cycle. These results suggest that spruce needles seem to be able to acclimate to ozone stress but also to drought stress by increasing their ascorbate pools and protecting pigments. Received: 15 September 1997 / Accepted: 24 March 1998