Browsing by Author "Moore, L.D."
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- An analysis of the potential effects of air pollutants emitted during coal combustion on yellow poplar and loblolly pine and influences on mycorrhizal associations of loblolly pineMahoney, Matthew J. (Virginia Polytechnic Institute and State University, 1982)Yellow poplar (Liriodendron tulipifera L.) and loblolly pine (Pinus taeda L.), families 2-8 and 540, seedlings were fumigated with 0.07 ppm ozone, 0.06 ppm sulfur dioxide 0.07 ppm ozone + 0.06 ppm sulfur dioxide, 0.06 ppm sulfur dioxide + 0.10 ppm nitrogen dioxide and 0.07 ppm ozone + 0.06 ppm sulfur dioxide + 0.10 ppm nitrogen dioxide for 35 consecutive days, 6 hr/day. Control seedlings received charcoal-filtered air. Ozone or sulfur dioxide did not significantly affect height growth or dry weight of yellow poplar seedlings. All other treatments significantly reduced height growth and dry weight after 2 weeks of fumigation. Height growth effects of loblolly pine families were not repeatable from one year to the next in replicate experiments and weekly growth trends in the two experiments were reversed. Environmental factors related to time of year were thought to be involved with this growth trend reversal. Root dry weight was found to be a more sensitive indicator of air pollution stress than either shoot dry weight, height growth or visible symptoms. Loblolly families 2-8 and 540 were not found to be differentially sensitive to pollutant treatments. Loblolly pine seedlings, nonmycorrhizal and mycorrhizal with Pisolithus tinctorius, were fumigated with 0.07 ppm ozone and 0.06 ppm sulfur dioxide singly and in combination, 6 hr/day, for 35 consecutive days. Height growth of mycorrhizal and nonmycorrhizal seedlings was not affected by fumigation. Root dry weight of nonmycorrhizal seedlings was significantly reduced by all pollutant treatments in two replicate experiments. A similar reduction in root dry weight of mycorrhizal seedlings did not occur. Shoot dry weight of nonmycorrhizal seedlings was reduced in four of six pollutant treatments, and in one of six treatments of mycorrhizal seedlings. Mycorrhizal formation was extensive regardless of treatment. Apparent photosynthesis, measured every 4 days, was variable and significant differences among treatments did not occur. Total reducing sugar concentrations of roots were an inconclusive indicator of air pollutant stress.
- Assessment of air pollution impacts on vegetation in South AfricaBotha, Amanda Theophila (Virginia Polytechnic Institute and State University, 1989)Field surveys and biomonitoring network experiments were conducted in selected areas in South Africa to assess possible air pollution damage to vegetation. During field surveys, atmospheric fluoride was identified as an important pollutant that damaged vegetation in residential areas north of Cape Town. Gaseous air pollutants, including acid deposition and acidic mist, probably play a major role in the development of characteristic air pollution injury symptoms observed on pine trees in the Eastern Transvaal area. The impact of urban air pollution in the Cape Town area was evaluated by exposing bio-indicator plants in a network of eight biomonitoring network stations from June 1985 to May 1988. Sensitive Freesia and Gladiolus cultivars were used to biomonitor atmospheric fluoride, while a green bean cultivar was used as a biomonitor of atmospheric sulphur dioxide and ozone. At one location, bio-indicator plants were simultaneously exposed in a biomonitoring network station (plant cages), open-top chambers (filtered and unfiltered), as well as in open plots. The responses of plants grown under these different conditions were compared. During both the winter and summer seasons, ambient fluoride concentrations were estimated to be particularly high at the Loumar biomonitoring station (eastern side of Cape Town), as compared to that at the other biomonitoring stations. Elevated levels of atmospheric fluoride, as well as sulphur dioxide, appeared to prevail in the vicinity of industries located in the northern suburbs of Cape Town (Bothasig, Table View and Edgemead). Interveinal bleaching, which is characteristic of sulphur dioxide injury, was regularly observed on bean plants exposed at the Bothasig biomonitoring station and was usually determined to be significantly worse at this location than at the other biomonitoring stations. These results were confirmed by the evaluation of foliar sulphur content Ambient pollutant concentrations appeared to be effectively reduced inside filtered open-top chambers, as exemplified by higher biomass production and lower foliar fluoride and sulphur levels in the relevant bio-indicator species, compared to that of plants grown in the open plots. The methodology used during this research provided baseline data of the impact of air pollution on vegetation in South Africa which, in the absence of networks of sophisticated state of the art equipment, can be applied to aid in air pollution control strategies.
- Biology of virulent and hypovirulent Endothia parasitica on American chestnut (Castanea dentata)Hebard, Frederick Vanuxem (Virginia Polytechnic Institute and State University, 1982)Results of inoculations of grafts, seedlings, excised stems, and intact trees of large, surviving American chestnut indicated that some have heritable blight resistance. Some degree of resistance in American chestnut possibly would assist spread of hypovirulence (H) factors. The pathogenicity of the H isolate but not the V (virulent) isolate appeared to influence the size and sporulating characteristics of biocontrolled H+V cankers. The histopathology of canker development on blight-resistant Chinese and blight-resistant and blight-susceptible American chestnut was examined after inoculation with V and H E-parasitica. Rapid formation and continual growth of mycelial fans distinguished chestnut blight cankers incited by V E-parasitica from those incited by H E-parasitica, and distinguished cankers on susceptible chestnut from cankers on resistant chestnut. Differences among such cankers in wound periderm and other pathological alterations of bark and wood anatomy probably were results of differences in rates of fan formation and growth. Wound periderm formation began at the deepest point of a wound or canker and progressed outward to the bark surface. Superficial cankers arose in resistant trees or with hypovirulent fungus when slow-forming mycelial fans expanded through areas where wound periderm had not formed. Rapid epidemics of chestnut blight occur in recently clearcut areas but not in areas with large trees of species other than American chestnut, most probably because chestnut sprouts released after cutting become larger than they do when overtopped by large forest trees. The size of chestnut sprouts appeared to influence the rate of blight progress by setting a maximum limit on canker size and thus canker sporulation. It may be possible to foster spread of H factors in the Appalachians if blight epidemics in clearcuts can be prolonged. This might be done by restricting inoculum production. Cutting and removing some blighted sprouts, cutting some large unblighted sprouts, or increasing blight resistance in the chestnut population are three potential means of restricting inoculum production.
- The combined effects of ozone, sulfur dioxide and simulated acid rain on the growth of three forest tree speciesChappelka, Arthur H. (Virginia Polytechnic Institute and State University, 1986)Nine-week-old yellow-poplar and green and white ash were exposed to various concentrations of O₃ (0.00 to 0.15 ppm) and/or SO₂ (0.08 ppm), 4 hr/d, 5d/wk in combination with simulated rain (pHs 5.6, 4.3, 3.0), 1 hr/d, 2 d/wk, for 5 or 6 wk under controlled laboratory conditions. Pollutant exposures resulted in alterations in seedling biomass accumulation, growth rates, changes in carbon allocation among plant parts and modification in physiological processes associated with gas exchange. Ozone (0.010 ppm) and SO₂ together caused a significant decrease in height growth and biomass and an increase in leaf area ratio (LAR) in yellow-poplar. Ozone and SO₂ exposures resulted in linear decreases and increases, respectively, in root dry weight, leaf area increase, relative growth rates of all yellow-poplar plant parts and unit leaf rate with decreasing rain pH. Chlorophyll content increased in both O₃ and SO₂ treatments with increasing rain acidity. In green and white ash experiments height growth was inhibited by O₃, SO₂ and O₃ + SO₂ for green ash, whereas only leaf dry weight was decreased by O₃ exposure in white ash. Decreasing rain pH resulted in linear decreases in root/shoot ratio (RSR) and LAR, for white ash. In green ash, a quadratic response to rain pH occurred with these growth variables. Ozone and O₃ + SO₂-treated green ash exhibited a significant quadratic response in leaf weight ratio with increasing rain acidity. Leaf area ratio and RSR exhibited linear increases and decreases, respectively, for O₃ and rain acidity. In SO₂-treated white ash with increasing white ash and yellow-poplar seedlings exposed to various O₃ concentrations and simulated rain for 5 and 6 weeks, respectively, increasing O₃ concentrations caused linear decreases in height and biomass of white ash. Linear decreases in root growth rate and biomass and RSR occurred with decreasing rain pH, across O₃ treatments. Ozone (0.05 or 0.10 ppm) caused linear decreases in these variables in combination with increasing rain acidity. For yellow-poplar, increasing O₃ concentrations caused linear increases in RSR and specific leaf area. At 0.05 and 0.10 ppm O₃, stem and leaf biomass, their relative growth rates and leaf area all decreased with decreasing rain pH. Ozone (0.10 ppm) exposure caused a decrease in stomatal conductance, and decreasing variable. rain pH resulted in a linear decrease in this A linear decrease in net photosynthesis also occurred with increasing rain acidity in O₃-treated (0.10 ppm) plants. These results demonstrate that gaseous pollutants in combination with simulated acid rain can have detrimental effects on growth of three forest tree species, under controlled laboratory conditions.
- Comparative physiology of radish populations with differential sensitivity to O₃ and SO₂Gillespie, Chris Todd (Virginia Polytechnic Institute and State University, 1989)Radish plants (Raphanus sativus L. cv Cherry Belle) were exposed to 0.10 μl l⁻¹ ozone (O₃) or 0.50 μl l⁻¹ sulfur dioxide (SO2) for 4 h d⁻¹, 3 d wk⁻¹ for 3 weeks. From these fumigated plants, individuals were selected that were resistant or sensitive to these pollutants. The selected plants were used as parental material in a breeding program to produce lines differing in resistance to O₃ and SO₂. Non-selected (NS) plants from the original populations served as controls. F₁ populations were raised and exposed to O₃ or SO₂ with the same fumigation regime used for the parents. The plants were harvested 30 days after emergence and dry weights were determined. Plants selected for O₃ resistance (O3R) weighed significantly more than either plants selected for sensitivity to O₃ (O₃S) or NS plants when exposed to either O₃ or SO₂. The hypocotyl was most affected by pollutant exposure, leading to reduced root/shoot ratios. Plants selected for resistance or sensitivity to SO₂ generally had biomass production similar to that of NS plants. Growth analysis at early stages of growth indicated that both O₃R and O₃S plants had less growth under O₃ fumigated conditions; however, by maturity O₃R plants had similar amounts of growth under fumigated or non-fumigated conditions. Ozone fumigations tended to decrease free sugar concentrations in leaves at early stages of growth in both O₃R and O₃S plants, and caused some accumulations of carbohydrates during late stages of growth in O₃S plants. Allocation of Mc was significantly lower to hypocotyls and roots of O₃ fumigated O₃S plants. Allocation to hypocotyls of O₃R plants was not affected, although both O₃R and O₃S plant groups had lower photosynthetic rates due to O₃ fumigation. Ozone did not significantly affect chlorophyll concentrations in leaves of either sensitive or resistant plants, nor was the time of new leaf production affected by fumigation. These experiments demonstrated the potential of O₃ to influence the composition of sensitive plant populations. However, SO₂ was a much less powerful influence on the composition of these populations.
- Growth response and drought susceptibility of forest trees exposed to simulated acidic rain and ozoneLee, Woong S. (Virginia Polytechnic Institute and State University, 1988)One-year-old seedlings of red spruce, loblolly pine, yellow-poplar, and sweetgum were exposed to ozone (0.0 or 0.1 ppm, 4 hr d⁻¹, 3 d wk⁻¹) in combination with simulated acidic rain (pH 5.6 or 3.0, 1 h d⁻¹, 2 d wk⁻¹, 0.75 cm hr⁻¹) for ten weeks. After the ten-week treatment with ozone and simulated acidic rain, the seedlings were submitted to two drought cycles, and water potential, net photosynthesis (Pn), stomatal conductance (Cs), and transpiration (Ts) were measured. Whole-plant fresh weight increment (FWT) and dry weight were significantly reduced in red spruce seedlings after the ten-week treatment with ozone. Ozone also significantly reduced shoot height growth (SHG) and increased the apparent plastochron duration (APD) of sweetgum seedlings. Treatment with simulated rain at pH 3.0 significantly increased FWT and SHG of red spruce compared to pH 5.6 as well as FWT and relative growth rate (RGR) of loblolly pine. The APD was significantly reduced in sweetgum seedlings exposed to simulated acid rain at pH 3.0 compared to 5.6. Significant interactions between ozone and simulated acidic rain occurred in all species except red spruce, and they were additive. The SHG was significantly lower in loblolly seedlings exposed to 0.1 ppm ozone + pH 5.6 than in seedlings exposed to either 0.1 ppm ozone + pH 3.0 or 0.0 ppm ozone + pH 5.6. The APD was significantly increased in yellow-poplar and sweetgum seedlings exposed to 0.1 ppm ozone + pH 5.6 compared to any other treatment. Visible symptoms appeared on the adaxial leaf surface of yellow-poplar seedlings submitted to acid rain at pH 3.0 regardless of ozone treatment. Visible foliar symptoms were also observed on the adaxial surface of sweetgum seedlings exposed to 0.1 ppm ozone. The symptoms were characterized by premature red pigmentation with small brown necrotic lesions. Foliar concentrations of P and S were significantly increased in all seedlings exposed to simulated rain at pH 3.0 compared with pH 5.6 except for red spruce which exhibited the increases of K and S. In general, there were neither significant effects of ozone nor interactions between ozone and rain pH on foliar nutrient concentrations. There were no significant effects of ozone on Pn, Cs, Ts, or water-use efficiency (WUE) prior to the drought cycles for all species. However, after the first drought cycle, Pn and Cs were significantly changed in loblolly pine and sweetgum pre-exposed to 0.1 ppm ozone compared with controls. The ten-week treatment of simulated acidic rain at pH 3.0 significantly increased Pn and Ts of loblolly pine and reduced Cs of yellow-poplar. After the first drought cycle, yellow-poplar treated with pH 3.0 rain showed lower Pn and WUE than seedlings exposed to pH 5.6 solution. A 0.5 MPa shift in the response of net photosynthesis (Pn) to decreasing water potential occurred in red spruce seedlings across all air pollutant treatments after the drought cycles indicating photosynthetic acclimation to drought stress. During the second drought cycle, Pn was more sensitive to water potential in red spruce and loblolly pine seedlings exposed to 0.10 ppm ozone + pH 3.0 solution compared with seedlings exposed to 0.0 ppm ozone + pH 5.6 solution. This increased sensitivity of Pn to water potential might be explained through a change in root hydraulic conductivity (Lp). In general, Lp showed significant effects of ozone and simulated acidic rain after moisture stress. After the first drought cycle, Lp was decreased in yellow-poplar and sweetgum seedlings exposed to 0.10 ppm ozone. After the second drought cycle, Lp was decreased in yellow-poplar and red spruce seedlings exposed to 0.1 ppm ozone. Significant interactions between ozone and simulated acidic rain occurred in the Lp of red spruce and yellow-poplar seedlings after the first drought cycle. The results from this dissertation indicate that ozone and/or simulated acidic rain can alter the growth and drought susceptibility of forest tree species under laboratory conditions. Forest trees which are periodically exposed to ambient ozone and acidic rain in natural forest ecosystems probably have a response similar to the results from this study. Additional research is necessary to determine if such effects are occurring in natural forest ecosystems.
- Inoculum densities of Thielaviopsis basicola in tobacco fields in Virginia, and the relationship of inoculum density to the severity of black root rot and growth of tobaccoSpecht, Lawrence P. (Virginia Polytechnic Institute and State University, 1985)A new selective medium (TB-CEN) was developed for isolating Thielaviopsis basicola, cause of black root rot of tobacco, from soil. TB-CEN medium contains etridiazol and nystatin to inhibit the growth of undesired fungi, and unautoclaved extract from carrot to selectively enhance for the growth of T. basicola. Inoculum and/or population densities of T. basicola in five burley tobacco fields were 74-166 propagules per g of soil, and 0-12 propagules per g of soil in three other burley fields. Inoculum and/or population densities of T. basicola in 12 flue-cured and 2 sun-cured tobacco fields were 0-26 propagules per g of soil, and 101 and 402 propagules per g of soil in two other flue-cured fields. Environmental factors apparently had a strong effect on black root rot development, since root rot and plant stunting were severe in two burley fields that had 148 and 158 propagules per g of soil, but were not severe in the two flue-cured fields that had 101 and 402 propagules per g of soil. All of the cultivars planted in the four fields were susceptible. Black root rot was the major disease associated with the stunting of tobacco plants in the burley region of Virginia, but not in the flue- and sun-cured regions. No evidence was found to indicate that endomycorrhizae were involved in tobacco stunting in Virginia. T basicola inoculum density-disease severity studies were conducted both in soil-temperature tanks and in the field. Tobacco seedlings were grown in temperature tanks (20-23 C) for 30-31 days in naturally infested field soil (pH 6.5). For all cultivars tested (Burley 21, NC95, and Va Gold), the mean percent of roots that were rotted increased significantly (P=0.001) as inoculum density increased (R² range for regressions=0.93-0.97). Severe levels of root rot occurred at inoculum densities of 50-200 propagules per g of soil. Significant (P=0.01) reductions in plant growth occurred at inoculum densities as low as 5-10 propagules per g of soil. In a study conducted on a commercial burley tobacco (cv. B21-Ky10) field, inoculum densities of 150 and 683 propagules per g of soil were associated with moderate and severe levels of black root rot, respectively. Differences between soil-temperature tank and field studies appeared to be due to variations in environmental- and host-related factors. In another burley field study, the fungicide imazalil, which completely inhibited the growth of T. basicola when amended into agar media at a concentration of 1.0 μg a.i./ml, failed to control black root rot when it was added to transplanting water (50 ml/plant) at concentrations as high as 1,500 μg a.i./ml.
- Molecular mechanisms of pathogenesis incited by Erwinia carotovora subsp. carotovoraRoberts, Daniel Paul (Virginia Polytechnic Institute and State University, 1985)Erwinia carotovora subsp. Carotovora (Ecc) incites soft-rot on many plants. It is believed that soft-rot is due to the concerted activity of extracellular enzymes. Recombinant DNA techniques were used to study the molecular basis of pathogenesis incited by Ecc. Specifically, a clone library of Ecc strain EC14 DNA in plasmid pBR322 was constructed and transformed into Escherichia coli strain HB101. Some of the E. coli strains that contain these hybrid plasmids produce pectinases or cellulase(s). Plasmid pDR1 contains a 3.4 kilobase (kb) EC14 DNA fragment and mediates the production of endo-pectate lyases with isoelectric points (pI) of 9.5 and 7.5 in strain HB101. The pI 9.5 enzyme is believed to be the major extracellular pectolytic enzyme in soft-rot while the pI 7.5 enzyme has no documented counterpart in EC14. Subclone and transposon tn5 analyses of pDR1 indicate that 1.5 kb is necessary for the production of the pI 9.5 and pI 7.5 enzymes and that these enzymes are produced independently of other EC14 pectate lyase enzymes. Plasmid pDR30 contains a 2.1 kb EC14 DNA insert that mediates the production of an endo-polygalacturonase and an exo-pectate lyase in HB101. The exo-pectate lyase encoded by pDR30 produces an inducer of endo-pectate lyase synthesis as a reaction product. The endo-polygalacturonase encoded by pDR30 is thought to play a role in plant cell wall pectic polymer degradation. Restriction endonuclease and Southern hybrididizatian analyses indicate that the EC14 genes on plasmids pDR1 and pDR30 are not part of the same operon. Escherichia coli strain HB101 containing plasmid pDR1 or plasmid pDR30 is unable to macerate potato tuber slices. However, HB101 containing plasmids pDR1 and pDR30 can cause limited maceration of potato tuber slices. There appears to be a genetic interaction between plasmids pDR1 and pDR30 in maceration of potato tuber tissue. However, the EC14 gene(s) contained on plasmid pDR1 are transcribed independently of the EC14 genes contained on plasmid pDR30. It is possible that transcription of certain pectolytic enzymes independent of other pectolytic enzymes provides a flexible system for plant cell wall pectic polymer degradation.
- Physiological and ultrastructural effects of sterol-inhibiting fungicides on apple leaves and the apple scab fungusOverton, Santford Vance (Virginia Polytechnic Institute and State University, 1986)The effects of sterol-inhibiting fungicides (SIFs) on the free sterol and free fatty acid composition of apple leaves of Red Delicious and Jonathan cultivars were examiried over a 2 year period. Trees were treated in mornings vs evenings throughout each season and samples collected after 24 and 72 hours after each treatment. Generally, SIFs appeared to have an effect on the free sterol content of apple leaves after 24 hours, but the concentrations of free sterol returned to normal after 72 hours in the leaves of both cultivars. Morning versus evening application had little or no influence on leaf free sterol concentrations. There were increases in unsaturated and total fatty acid concentrations in Red Delicious leaves 24 hours following applications with the SIF, etaconazole, and the non-sterol-inhibiting fungicide (NSIF), metiram, early in the season. There were also increased concentrations of linoleic, linolenic and total free fatty acids in fenarimol and triadimefon-treated Jonathan leaves 72 hours after treatment. Early in the season, the SIF, fenarimol, caused an increase in linolenic acid in both Red Delicious and Jonathan leaves 72 hours after either morning or evening applications. Generally, both the Red Delicious and Jonathan leaves exhibited a decrease in saturated fatty acids following morning application whereas, an increase in saturation following evening application. Although SIFs may have had an effect primarily on the unsaturated fatty acids, particularly linolenic acid, early in the season, particularly linolenic acid, the fatty acid composition of the leaves appeared to return to normal later in the season. Ultrastructural observations were made of Red Delicious leaves 12, 24 and 72 hours after treatment with the SIF, bitertanol. Twelve hours after treatment thylakoids of chloroplasts appeared swollen and irregular resulting in loss of integrity of the organelles. However, after 24 and 72 hours, thylakoids of chloroplasts of treated leaves were similar to the controls. Infection of bitertanol-treated Red Delicious leaves by Spilocaea pomi was also examined at the fine structural level. Nuclear envelopes were not well defined and mitochondrial matrices appeared washed-out after 12 and 72 hours post treatment. There was dissolution of normally plate-like cristae of mitochondria, accompanied by the accumulation of minute electron dense bodies around their periphery. Invaginations and proliferations of the plasmalemma were observed as well as increased vacuolization of the cell. Further electron microscopic observations were made of the in vitro conidial state of Venturia inaequalis following the application of fenarimol. Conidia treated 2 hours with the fungicide for had necrotic areas throughout the cytoplasm. The plasmalemma was not well defined, and appeared to be degrading. Increased vacuolization was observed as were numerous lipid bodies and multivesicular complexes (MVCs) which contained vesicles of varying electron densities. Structural integrity of the organelles was such that they were difficult to discern. After 12 hours, the entire fungal cell was necrotic accompanied by the degradation of the cell wall. Detection of a selected number of SIFs in apple leaf tissue using bioassay procedures were also evaluated. It was found that the leaf disk and leaf extract bioassays examined in this study were ineffective in determining the presence of SIFs in apple leaves.
- Responses of gas exchange and the antioxidant system of soybean cultivars to ozone and/or sulfur dioxideSheng, Wen-Shame (Virginia Tech, 1992)Soybean cultivars (Glycine max (L.) Merr.), "Dare", "Williams" and "Essex", with differential sensitivity to ozone (O₃) based on visible injury were exposed once to 0.20 µl⁻¹ O₃ and/or 0.70 µl 1⁻¹ sulfur dioxide (SO₂) for 4 hr. The cv Dare was considered sensitive, cv Williams intermediate and cv Essex tolerant to O₃. Cultivars exposed to filtered air served as controls. Gas exchange measurements were conducted and antioxidant metabolites (reduced and oxidized glutathione: GSH, GSSG; reduced and oxidized ascorbate: AA, dHAA) concentrations and enzymes (glutathione reductase: GRase; ascorbate peroxidase: APase; superoxide dismutase: SOD) activities were analyzed. Gas exchange rates of all cultivars were significantly inhibited by pollutants exposure. The cv Essex maintained significantly higher net photosynthesis (Pn) at the end of 03 exposure, during the SO₂ exposure and in the first 2 hr of O₃/SO₂ fumigation. During O₃/SO₂ exposure, the estimated pollutant fluxes were 50% and integrated doses were 25-30% of O₃ or SO₂ when fumigated singly. During O₃ fumigation, cv Dare exhibited a higher O₃ flux and integrated O₃ dose. However, cv Essex showed a higher O₃/SO₂ peak flux in O₃/SO₂ fumigation. The cv Williams showed the lowest pollutant flux in all treatments. Ozone fumigation imposed a substantial, but statistically insignificant, effect on some antioxidant components. Relative to the controls, O₃ exposure resulted in increases of GRase and SOD activity in cv Dare. In cv Williams, decreases of GSSG and SOD total activity and increase of dHAA were observed. Decreases of AA and SOD activity and increases of GSSG, dHAA and APase activity in cv Essex were found. The SO₂ exposure resulted in increases of glutathione, particularly GSSG, in all cultivars. The cv Dare responded with increases in AA, APase specific activity and SOD activity. In cv Williams, AA and SOD specific: activity increased and APase activity decreased. Decline of SOD activity in cv Essex was found after SO₂ exposure. The O₃/SO₂ fumigation resulted in increases of glutathione, particularly GSSG, and GRase activity, in all cultivars. Declines of dHAA and SOD activity in cv Williams were found. The cv Essex responded with a decline of AA and increases of dHAA and specific activity of APase and SOD. Ozone and SO₂, singly or in combination, inhibited gas exchange rates in all cultivars, however, cv Essex was the least affected. Stomatal conductance was inhibited greater by O₃ than by SO₂ fumigation. Conversely, Pn was suppressed more by SO₂ than by O₃. The O₃/SO₂ fumigation, however, suppressed Pn and Cs substantially and to a greater extent than individual pollutants. Under the pollutant dose and fumigation profile used in these studies, no consistent responses of different antioxidant components to O₃ and/or SO₂ correlated with differential sensitivity of these soybean cultivars as determined from foliar symptomology.