Chronic ozone exposure alters the growth of leaves, stems and roots of hybrid Populus

Rooted cuttings of hybrid Populus (DN34, Populus deltoides x nigra) were grown outdoors in pots in open-top chambers at Ithaca, NY (74.5 degrees W, 42.5 degrees N), during 1988 and 1989 (experiment 1) and during 1989 and 1990 (experiment 2). Ambient air was passed through charcoal filters to produce a 0.5 times ambient ozone treatment, and ozone generated from oxygen was added to produce one and two times ambient ozone treatments. In experiment 1, treatments were applied for 8-12 h each day for 112 days of the 1988 growing season, then the plants were grown outdoors with ambient ozone in 1989. In experiment 2, treatments were applied for 9 h each day for 98 days of the 1989 growing season, then the plants were grown outdoors with ambient ozone in 1990. Chronic exposure to ozone caused the following changes (statistically significant in one or both experiments at p<0.05): (1) earlier leaf abscission, (2) decreased stem basal diameter, (3) decreased stem mass, (4) decreased internode length, (5) decreased shoot height p=0.005, and (6) decreased leaf size in the growing season following ozone treatment. There was also strong evidence that ozone increased the number of leaves produced p=0.055. Finally, there was some evidence that ozone increased the ratio of shoot mass to root mass p=0.093.     

Seedling insensitivity to ozone for three conifer species native to Great Smoky Mountains National Park

Field symptoms typical of ozone injury have been observed on several conifer species in Great Smoky Mountains National Park, and tropospheric ozone levels in the Park can be high, suggesting that ozone may be causing growth impairment of these plants. The objective of this research was to test the ozone sensitivity of selected conifer species under controlled exposure conditions. Seedlings of three species of conifers, Table Mountain pine (Pinus pungens), Virginia pine (Pinus virginiana), and eastern hemlock (Tsuga canadensis), were exposed to various levels of ozone in open-top chambers for one to three seasons in Great Smoky Mountains National Park in Tennessee, USA. A combination of episodic profiles (1988) and modified ambient exposure regimes (1989-92) were used. Episodic profiles simulated an average 7-day period from a monitoring station in the Park. Treatments used in 1988 were: charcoal-filtered (CF), 1.0x ambient, 2.0x ambient, and ambient air-no chamber (AA). In 1989 a 1.5x ambient treatment was added, and in 1990, additional chambers were made available, allowing a 0.5x ambient treatment to be added. Height, diameter, and foliar injury were measured most years. Exposures were 3 years for Table Mountain pine (1988-90), 3 years for hemlock (1989-91), and 1 and 2 years for three different sets of Virginia pine (1990, 1990-91, and 1992). There were no significant (p<0.05) effects of ozone on any biomass fraction for any of the species, except for older needles in Table Mountain and Virginia pine, which decreased with ozone exposure. There were also no changes in biomass allocation patterns among species due to ozone exposure, except for Virginia pine in 1990, which showed an increase in the root:shoot ratio. There was foliar injury (chlorotic mottling) in the higher two treatments (1.0x and 2.0x for Table Mountain and 2.0x for Virginia pine), but high plant-to-plant variability obscured formal statistical significance in many cases. We conclude, at least for growth in the short-term, that seedlings of these three conifer species are insensitive to ambient and elevated levels of ozone, and that current levels of ozone in the Park are probably having minimal impacts on these particular species.     

Effects of various ozone exposures on the susceptibility of bean leaves (Phaseolus vulgaris L.) to Botrytis cinerea

The effects of various ozone exposures in predisposing bean leaves (Phaseolus vulgaris L.) to Botrytis cinerea have been investigated under laboratory conditions. Seedlings of two bean cultivars were exposed to incremental ozone concentrations (120, 180 and 270 microg m(-3) for 8-h day(-1)) for five days and primary leaves were subsequently inoculated with conidia suspended in water or in an inorganic phosphate solution (Pi), and with mycelium. Ozone injury increased with increasing ozone concentration and was much higher in the ozone-sensitive cultivar 'Pros' than in the ozone-insensitive 'Groffy'. Ozone only increased the number of lesions on leaves of Pros after inoculation with either of the conidial suspensions. The Pi-stimulated infection in Groffy was reduced by the lower ozone concentrations. Ozone decreased lesion expansion after inoculation with mycelium. In a chronic fumigation experiment, plants of the two cultivars were exposed to 90 microg m(-3) (7-h day(-1)) and the primary and the oldest tree trifoliate leaves were inoculated after five and seven weeks of exposure. Ozone enhanced the senescence-related injury only in Pros. The number of lesions was not influenced by ozone for either cultivar, conidial suspension or inoculation date. Lesion expansion after inoculation with mycelium was generally reduced in exposed plants. Thus, contrasting effects of ozone on the susceptibility of bean leaves to B. cinerea were observed depending on the cultivar, the conidial suspension, the disease parameter and the ozone exposure pattern. In extrapolating the laboratory results to the field, it is suggested that episodic and chronic exposures to ambient ozone are of minor importance in increasing the susceptibility of bean leaves to B. cinerea.     

Evaluation of Vegetation Near Coal-burning Power Plants in Southwestern Pennsylvania. II. Ozone Injury on Foliage of Hybrid Poplar

The objective of this study was to determine if the incidence or severity of foliar injury induced by regional, ambient ozone was influenced by local emissions from a complex of coal-burning power plants in southwestern Pennsylvania. Plantings of an ozonesensitive hybrid poplar clone {Populus maximowizii x trichocarpa, clone NE 388) were established in 1972 at various distances and directions from the power plants. Foliar injury caused by ambient ozone was evaluated annually from 1973 to 1990 in early to mid- August. Data are presented for the 12-year period, 1979 to 1990 inclusive, for which the most complete data sets were available. Injury from ambient ozone varied spatially and temporally, but with little relationship to power plant location. There was an apparent negative relationship between emission trends and ozone-induced symptoms, but only for one power plant. The correlation between annual mean levels of ozone-induced stipple and frequency of days (per year) with a 1-hr ozone maximum exceeding 0.04 ppm was weak, but significant. Ozone-induced bifacial necrosis was not observed on the foliage of the hybrid poplar during the drought year of 1988 in spite of record high levels of ozone; however, ozoneinduced stipple was observed.     

Influence of two growing seasons of experimental ozone fumigation on photosynthetic characteristics of white oak seedlings

White oak (Quercus alba L.) seedlings were exposed to charcoal-filtered air or to above-ambient ozone concentrations for 19-20 weeks during each of two growing seasons in continuously stirred tank reactors in greenhouses. Ozone treatments were 0.15 ppm (300 microg m(-3)) for 8 h day(-1), 3 days week(-1) in 1988, and continuous 15% above ambient in 1989. The seedlings were grown in forest soil watered twice weekly with simulated rain of pH 5.2. Responses of net photosynthesis to photosynthetically active radiation and intercellular CO(2) concentration were measured three times each year. There were no significant differences in light-saturated net photosynthesis or stomatal conductance, dark respiration, quantum or carboxylation efficiencies, and light or CO(2) compensation points on any date between control and ozone-exposed seedlings.     

The effect of winter stress on Ilex aquifolium L.previously fumigated with ozone

European Holly (Ilex aquifolium) received either charcoal-filtered air (CFA) or CFA with 70 nl l(-1) ozone added for 7 h day(-1) over a 28 day period. Plants were then transferred into cooling incubators for hardening (4 degrees C day/2 degrees C night; day length 12 h) for 7 days and then to the frosting stage (2 degrees C day and -5, -10 or -15 degrees C night) for 4 days. The plants were then placed in ambient conditions. Treatment produced significant differences in chlorophyll fluorescence data. Stomatal conductance was significantly higher for the ozone treatments though both showed a general decline over all temperature regimes. Ozone also significantly increased electrolyte leakage and reduced winter survival. These results show that ambient concentrations of ozone can reduce the tolerance of I. aquifolium to freezing stress, which may have serious implications for its establishment and survival.     

Secondary effects of air pollution: ozone decreases brown rust disease potential in wheat

Young wheat plants were fumigated with 170 microg m(-3) ozone for 3 days, or with 210 microg m(-3) ozone for 7 days, for 7 hours a day. At the end of the fumigation period the plants were inoculated with brown rust (Puccinia recondita f. sp. tritici) uredospores. The development of new uredospore pustules on fumigated and control plants was evaluated as a measure of rust disease potential. The number of pustules on the ozone fumigated plants was greatly reduced in comparison with the number of plants treated with charcoal-filtered air.     

Tibouchina pulchra (Cham.) Cogn., a native Atlantic Forest species, as a bio-indicator of ozone: visible injury

Tibouchina pulchra saplings were exposed to carbon filtered air (CF), ambient non-filtered air (NF) and ambient non-filtered air+40 ppb ozone (NF+O3) 8 h per day during two months. The AOT40 values at the end of the experiment were 48, 910 and 12,895 ppb h(-1), respectively, for the three treatments. After 25 days of exposure (AOT40=3871 ppb h(-1)), interveinal red stippling appeared in plants in the NF+O3 chamber. In the NF chamber, symptoms were observed only after 60 days of exposure (AOT40=910 ppb h(-1)). After 60 days, injured leaves per plant corresponded to 19% in NF+O3 and 1% in the NF treatment; and the average leaf area injured was 7% within the NF+O3 and 0.2% within the NF treatment. The extent of leaf area injured (leaf injury index) was mostly explained by the accumulated exposure of ozone (r2=0.89; p<0.05).     

Psidium guajava 'Paluma' (the guava plant) as a new bio-indicator of ozone in the tropics

Psidium guajava 'Paluma' saplings were exposed to carbon filtered air (CF), ambient non-filtered air (NF), and ambient non-filtered air+40ppb ozone (NF+O(3)) 8h per day during two months. The AOT40 values at the end of the experiment were 48, 910 and 12 895ppbh(-1), respectively for the three treatments. After 5 days of exposure (AOT40=1497ppbh(-1)), interveinal red stippling appeared in plants in the NF+O(3) chamber. In the NF chamber, symptoms were observed only after 40 days of exposure (AOT40=880ppbh(-1)). After 60 days, injured leaves per plant corresponded to 86% in NF+O(3) and 25% in the NF treatment, and the average leaf area injured was 45% in NF+O(3) and 5% in the NF treatment. The extent of leaf area injured (leaf injury index) was explained mainly by the accumulated exposure of ozone (r(2)=0.91; p<0.05).     

Ozone,antioxidant spray and meloidogyne hapla effects on tobacco

The relationship between ozone and the northern root-knot nematode on tobacco was investigated. Seedlings of tobacco (Nicotiana tabacum L.) cv. Virginia 115 were inoculated and not inoculated with root-knot (Meloidogyne hapla (Chitwood) prior to transplanting to a field plot. One-half the plants were sprayed at weekly intervals with an antioxidant, EDU at the rate of 1 kg ha⁻¹ to protect against oxidant injury. O₃ concentrations in excess of 80 ppb were recorded 14 times during the summer of 1982. Ambient ozone inhibited growth and yield of tobacco inoculated and not inoculated with M. hapla. Tobacco inoculated with nematode alone developed significantly more ozone injury than other treatments indicating that tobacco infected with M. hapla is more susceptible to ambient O₃. Significantly 20% more galls developed on plants with nematode inoculation compared to plants with nematode inoculation + EDU indicating that EDU indirectly reduced gall development in tobacco. Plants protected with EDU also showed an increase in dry weight of shoot, root and biomass.     

Influence of ozone and ethylenediurea (EDU) on growth and yield of bean (Phaseolus vulgaris L.) in open-top field chambers

Greenhouse and ambient air experiments have shown ethylene diurea (EDU) to be a strong and specific protective suppressant of ozone injury in plants. To examine how EDU affects plant responses to various ozone (O(3)) levels under controlled field conditions, Phaseolus vulgaris L. cv. Lit was treated with 150 ppm EDU every 14 days and exposed in open-top chambers to charcoal-filtered air (CF), nonfiltered air (NF) or two cf treatments with ozone added. The ozone treatments were proportional additions of one (CF1) and two (CF2) times ambient ozone levels. The mean ozone concentrations in the CF, NF, CF1 and CF2 treatments were 0.98, 14.1, 14.98 and 31.56 nl litre(-1). A two-way split plot ANOVA revealed that shoot dry weight was significantly reduced by ozone. EDU treatment was highly significant for leaf dry weight, root dry weight and shoot dry weight, but not for pod dry weight; leading to a higher biomass of EDU-treated plants. Ozone/EDU interactions were significant for root weight only, indicating that EDU reduced growth suppression by ozone. These results show that EDU action on plant biomass could be interpreted as a delay in senescence since EDU-treated plants showed a significant decreased biomass loss even in the CF treatment.     

Interaction of ozone exposure and Fusarium subglutinans inoculation on growth and disease development of loblolly pine seedlings

Seedlings from ten half-sib families of loblolly pine (Pinus taeda) were exposed in open-top chambers to carbon-filtered air (CF), non-filtered air (NF), or air amended with ozone to 1.7 or 2.5 times ambient. After 105 days of exposure, half the seedlings within each family were wounded but not inoculated and half were wounded and inoculated with the pitch canker fungus, Fusarium subglutinans, to which five families were relatively resistant. After an additional 50 days of ozone treatment, seedling growth and canker development were recorded. Cankers were significantly (sigma < or = 0.05) smaller among resistant compared to susceptible families, and were significantly larger among seedlings receiving the highest (2.5) compared to the ambient (NF) ozone treatment. The wound scars of non-inoculated seedlings were also significantly larger among seedlings receiving the 2.5 compared to the NF treatment, but these dimensions did not differ significantly with seedling family or resistance. The weights of needles and large roots were significantly smaller at the 2.5 compared to the 1.7 ozone treatment for inoculated but not for non-inoculated seedlings; this resulted in a significant interaction for ozone and inoculation effects. Among resistant families, root weights were significantly smaller for inoculated seedlings. Diameter growth and dry weights of needles were significantly smaller among inoculated compared to non-inoculated seedlings, but did not differ between NF and 2.5 ozone treatments.     

Ozone episodes in Southern Lower Saxony (FRG) and their impact on the susceptibility of cereals to fungal pathogens

Spring wheat (Triticum aestivum L.) and spring barley (Hordeum vulgare L.) plants were exposed to simulated ozone (O(3)) episodes (7 h day(-1) for 7 days) at maximum concentrations of 120, 180 and 240 microg m(-3) O(3), in comparison to a charcoal-filtered air control. Fumigations were conducted in four closed chambers placed in a climate room. Exposures took place prior to inoculation of the plants with six different facultative leaf pathogens. On wheat, significant enhancement of leaf attack by Septoria nodorum Berk. and S. tritici Rob. ex Desm. appeared, particularly on the older leaves and at the highest level of O(3). The same was true for Gerlachia nivalis W. Gams et E. Müll/Fusarium culmorum (W.F.Sm.) Sacc. on wheat and net blotch (Drechslera teres (Sacc.) Shoem.) or G. nivalis leaf spots on barley. Disease development was promoted both on leaves with and without visible injury following exposure to O(3). Sporulation of the two Septoria species increased at 120 and 180 microg m(-3) O(3); however, it was reduced to the level of the control, if 240 microg m(-3) were applied. No significant effects of predisposition were observed with Bipolaris sorokiniana (Sacc.) Shoem. (syn. Helminthosporium sativum Pamm., King et Bakke), the causal agent of spot blotch, neither on wheat nor on barley. Doses and peak concentrations applied in the experiments were in good agreement with measurements of ambient ozone in Southern Lower Saxony, FRG. Six years' ozone data (1984-1989) revealed the annual occurrence of between 3 and 11 ozone episodes with potentially harmful effects on cereals (three or more consecutive 'ozone days' with 8-h means above 80 microg m(-3)). The frequency of ozone episodes followed by weather periods favourable for infections by facultative pathogens was higher in years with low O(3) pollution than in ozone-rich years, and varied between one and five cases per season. The number of ozone days during the main growing season of cereals (1 April until 31 August) varied from 25 in 1984 to 98 in 1989. However, only 7.9% of ozone days during the 6 years examined were concurrent with weather conditions suitable for fungal infections. It is concluded that the majority of leaf infections in the field happens under low-level concentrations of photooxidants.     

Effects of air filtration on spring wheat grown in open-top field chambers at a rural site. I. Effects on growth, yield and dry matter partitioning

Spring wheat, Triticum aestivum, was grown in open-top field chambers and exposed during the whole life cycle to filtered and non-filtered ambient air. The relatively low ambient pollution level did affect plant growth but had no effect on the overall grain yield of the two spring wheat cultivars Echo (1987) and Pelican (1988). A reduced root growth was found in both years which could be attributed mainly to the deposition of NO2 and SO2. Effects on crop development most likely due to ozone were limited to the 1987 growing season during which the ambient ozone concentrations were enhanced compared to 1988. This resulted in a slightly increased grain harvest index, a reduced 1000-grain weight, straw yield and a greater reduction in root growth. Visible damage resembling ozone injury appeared both years during seedling growth.     

A method for developing ozone exposures that mimic ambient conditions in agricultural areas

Ozone fumigations that mimic ambient ozone distributions facilitate the development of links between

• 1.(1) vegetative effects results that are generated in the laboratory and the field and
• 2.(2) predictive effects models that depend upon ambient air quality data.

Experimental exposure profiles were constructed from a readily available ambient air quality data base (i.e. EPA SAROAD). Air quality data from selected monitoring sites were characterized over the 5-month growing season by identifying

• 1.(a) the number of occurrences of hourly ozone concentrations equal to or above 0.07 ppm,
• 2.(b) the number of days of each episode,
• 3.(c) the number of days between episodes and
• 4.(d) the rate of rise and decline of the daily ozone concentrations.

An episodic profile was constructed incorporating the information into a representative 30-day ozone exposure pattern in which the concentration was changed on an hourly basis. In order to compare treatments having equivalent exposures (sum of hourly ozone concentrations equal to or above a minimum value) but dissimilar temporal distributions of hourly concentrations, a second profile was created. This profile was characterized by a repeated daily incremental rise and decline in ozone concentration that had the same hourly maximum concentration each day. The use of experimental exposure profiles mimicking ambient air quality characteristics and applied under controlled experimental conditions permits the examination of important exposure parameters on plant response.     

Protection of palak (Beta vulgaris L. var Allgreen) plants from ozone injury by ethylenediurea (EDU): Roles of biochemical and physiological variations in alleviating the adverse impacts

Ameliorative effects of ethylenediurea (N-[2-(2-oxo-1-imidazolinidyl) ethyl]-N′ phenylurea, abbreviated as EDU) against ozone stress were studied on selected growth, biochemical, physiological and yield characteristics of palak (Beta vulgaris L. var Allgreen) plants grown in field at a suburban site of Varanasi, India. Mean eight hourly ozone concentration varied from 52 to 73 ppb which was found to produce adverse impacts on plant functioning and growth characteristics. The palak plants were treated with 300 ppm EDU at 10 days after germination at 10 days interval up to the plant maturity. Lipid peroxidation in EDU treated plants declined significantly as compared to non-EDU treated ones. Significant increment in F_v/F_m ratio in EDU treated plants as compared to non-EDU treated ones was recorded. EDU treated plants showed significant increment in ascorbic acid contents and reduction in peroxidase activity as compared to non-EDU treated ones. As a result of the protection provided by EDU against ozone induced stress on biochemical and physiological characteristics of palak, the morphological parameters also responded positively. Significant increments were recorded in shoot length, number of leaves plant⁻¹, leaf area and root and shoot biomass of EDU treated plants as compared to non-EDU treated ones. Contents of Na, K, Ca, Mg and Fe were higher in EDU treated plants as compared to non-EDU treated ones. The present investigation proves the usefulness of EDU in partially ameliorating ozone injury in ambient conditions.     

Modification of the biochemical pathways of plants induced by ozone: what are the varied routes to change?

When plants are observed under a low dose of ozone, some physiological and metabolic shifts occur. Barring extreme injury such as tissue damage or stomata closure, most of these disruptive changes are likely to have been initiated at the level of gene expression. The belief is oxidative products formed in ozone exposed leaves, e.g. hydrogen peroxide, are responsible for much of the biochemical adjustments. The first line of defense is a range of antioxidants, such as ascorbate and glutathione, but if this defense is overwhelmed, subsequent actions occur, similar to systemic acquired resistance or general wounding. Yet there are seemingly unrelated metabolic responses which are also triggered, such as early senescence. We discuss here the current understanding of gene control and signal transduction/control in order to increase our comprehension of how ozone alters the basic metabolism of plants and how plants counteract or cope with ozone.     

Impact of fungicides on active oxygen species and antioxidant enzymes in spring barley (Hordeum vulgare L.) exposed to ozone.

Two modern fungicides, a strobilurin, azoxystrobin (AZO), and a triazole, epoxiconazole (EPO), applied as foliar spray on spring barley (Hordeum vulgare L. cv. Scarlett) 3 days prior to fumigation with injurious doses of ozone (150-250 ppb; 5 days; 7 h/day) induced a 50-60% protection against ozone injury on leaves. Fungicide treatments of barley plants at growth stage (GS) 32 significantly increased the total leaf soluble protein content. Additionally, activities of the antioxidative enzymes superoxide dismutase (SOD), catalase (CAT), ascorbate-peroxidase (APX) and glutathione reductase (GR) were increased by both fungicides at maximal rates of 16, 75, 51 and 144%, respectively. Guiacol-peroxidase (POX) activity was elevated by 50-110% only in AZO treated plants, while this effect was lacking after treatments with EPO. This coincided with elevated levels of hydrogen peroxide (H2O2) only in EPO and not in AZO treated plants. The enhancement of the plant antioxidative system by the two fungicides significantly and considerably reduced the level of superoxide (O2*-) in leaves. Fumigation of barley plants for 4 days with non-injurious ozone doses (120-150 ppb, 7 h/day) markedly and immediately stimulated O2*- accumulation in leaves, while H2O2 was increased only after the third day of fumigation. Therefore, O2*- itself or as precursor of even more toxic oxyradicals appears to be more indicative for ozone-induced leaf damage than H2O2. Ozone also induced significant increases in the activity of antioxidant enzymes (SOD, POX and CAT) after 2 days of fumigation in fungicide untreated plants, while after 4 days of fumigation these enzymes declined to a level lower than in unfumigated plants, due to the oxidative degradation of leaf proteins. This is the first report demonstrating the marked enhancement of plant antioxidative enzymes and the enhanced scavenging of potentially harmful O2*- by fungicides as a mechanism of protecting plants against noxious oxidative stress from the environment. The antioxidant effect of modern fungicides widely used in intense cereal production in many countries represents an important factor when evaluating potential air pollution effects in agriculture.     

Depletion of stratospheric ozone over the Antarctic and Arctic: responses of plants of polar terrestrial ecosystems to enhanced UV-B, an overview

Depletion of stratospheric ozone over the Antarctic has been re-occurring yearly since 1974, leading to enhanced UV-B radiation. Arctic ozone depletion has been observed since 1990. Ozone recovery has been predicted by 2050, but no signs of recovery occur. Here we review responses of polar plants to experimentally varied UV-B through supplementation or exclusion. In supplementation studies comparing ambient and above ambient UV-B, no effect on growth occurred. UV-B-induced DNA damage, as measured in polar bryophytes, is repaired overnight by photoreactivation. With UV exclusion, growth at near ambient may be less than at below ambient UV-B levels, which relates to the UV response curve of polar plants. UV-B screening foils also alter PAR, humidity, and temperature and interactions of UV with environmental factors may occur. Plant phenolics induced by solar UV-B, as in pollen, spores and lignin, may serve as a climate proxy for past UV. Since the Antarctic and Arctic terrestrial ecosystems differ essentially, (e.g. higher species diversity and more trophic interactions in the Arctic), generalization of polar plant responses to UV-B needs caution.     

Interaction of tobacco etch or tobacco vein mottling virus and ozone on biomass changes in burley tobacco

Burley tobacco is susceptible to several different types of virus diseases that suppress plant growth and development. Two viruses, tobacco etch virus (TEV) and tobacco vein mottling virus (TVMV), are particularly damaging to burley. Burley tobacco cultivars resistant to these two viruses are currently being developed. Some of these cultivars also show differential sensitivity to ozone (O3). Recent field observations have suggested that burley tobacco infected with TEV and TVMV was more sensitive to O3 than non-virus-infected tobacco. Experiments were designed to identify interactions between O3 and each of the two virus diseases. Three cultivars, Burley 21, Burley 49, and Greeneville 131, which were differentially sensitive to O3 and both virus diseases, were grown in a charcoal-filtered greenhouse environment. Tobacco plants of each cultivar were inoculated with TEV or TVMV, and virus infected and virus-free plants were exposed to 0.0, 0.05, 0.2, and 0.4 ppm O3 (1 ppm of O3 is equivalent to 1960 microg m(-3)), 3h day(-1), 5 days week(-1) for 3 weeks in continuous-stirred tank reactor exposure chambers in the greenhouse. Exposures were begun after systemic virus symptoms were expressed in inoculated plants. The suppression of lead and stem dry weight by increasing O3 concentrations was less in TEV-infected burley cultivars than in noninfected burley cultivars. Tobacco vein mottling virus infection enhanced biomass suppression by O3 on Burley 21 and on Greeneville 131, but not on Burley 49. Thus, the interactions with O3 were dependent on specific virus-cultivar combinations.