@article{booker_burkey_pursley_heagle_2007, title={Elevated carbon dioxide and ozone effects on peanut: I. Gas-exchange, biomass, and leaf chemistry}, volume={47}, ISSN={["1435-0653"]}, DOI={10.2135/cropsci2006.08.0537}, abstractNote={The effects of elevated CO2 and ozone (O3) on net photosynthetic rate (A) and growth are generally antagonistic although plant responses are highly dependent on crop sensitivity to the individual gases and their concentrations. In this experiment, we evaluated the effects of various CO2 and O3 mixtures on leaf gas‐exchange, harvest biomass, and leaf chemistry in peanut (Arachis hypogaea L.), an O3–sensitive species, using open‐top field chambers. Treatments included ambient CO2 (about 375 μmol mol−1) and CO2 enrichment of approximately 173 and 355 μmol mol−1 in combination with charcoal‐filtered air (22 nmol O3 mol−1), nonfiltered air (46 nmol O3 mol−1), and nonfiltered air plus O3 (75 nmol O3 mol−1). Twice‐ambient CO2 in charcoal‐filtered air increased A by 23% while decreasing seasonal stomatal conductance (gs) by 42%. Harvest biomass was increased 12 to 15% by elevated CO2 In ambient CO2, nonfiltered air and added O3 lowered A by 21% and 48%, respectively, while added O3 reduced gs by 18%. Biomass was not significantly affected by nonfiltered air, but was 40% lower in the added O3 treatment. Elevated CO2 generally suppressed inhibitory effects of O3 on A and harvest biomass. Leaf starch concentration was increased by elevated CO2 and decreased by O3 Treatment effects on foliar N and total phenolic concentrations were minor. Increasing atmospheric CO2 concentrations should attenuate detrimental effects of ambient O3 and promote growth in peanut but its effectiveness declines with increasing O3 concentrations.}, number={4}, journal={CROP SCIENCE}, author={Booker, Fitzgerald L. and Burkey, Kent O. and Pursley, Walter A. and Heagle, Allen S.}, year={2007}, pages={1475–1487} }
 @article{burkey_booker_pursley_heagle_2007, title={Elevated carbon dioxide and ozone effects on peanut: II. Seed yield and quality}, volume={47}, ISSN={["1435-0653"]}, DOI={10.2135/cropsci2006.08.0538}, abstractNote={Many adverse effects of tropospheric O3 on C3 crop plants are ameliorated by elevated concentrations of atmospheric CO2, but the extent of the interaction can vary, depending on the species, gas concentrations, and other experimental conditions. A 2‐yr open‐top field chamber experiment was conducted to examine this interaction in peanut (Arachis hypogaea L.) by testing the effects of O3 and CO2 mixtures on yield and seed quality. Treatments were ambient CO2 (375 μmol mol−1) and CO2 additions of approximately 173 and 355 μmol mol−1 in combination with charcoal‐filtered (CF) air (22 nmol O3 mol−1), nonfiltered (NF) air (46 nmol O3 mol−1), and NF air plus O3 (75 nmol O3 mol−1). At ambient CO2, pod number was suppressed 16% in NF air and 44% in elevated O3 Pod and seed mass were not significantly affected in NF air but were lowered 33 to 37% in elevated O3 Elevated CO2 increased yield parameters 7 to 17% for plants grown in CF air and restored yield in NF air and elevated O3 treatments to control or higher levels. Gas treatment effects on peanut market grade characteristics were small. No treatment effects were observed on the protein and oil contents of seeds, but there were changes in fatty acid composition. Overall results indicate that increasing concentrations of tropospheric O3 will suppress yield of O3–sensitive peanut cultivars, while elevated CO2 will moderate this response. Elevated O3 and CO2 are not expected to have major effects on peanut seed composition and quality.}, number={4}, journal={CROP SCIENCE}, author={Burkey, Kent O. and Booker, Fitzgerald L. and Pursley, Walter A. and Heagle, Allen S.}, year={2007}, pages={1488–1497} }
 @article{heagle_miller_pursley_2003, title={Growth and yield responses of potato to mixtures of carbon dioxide and ozone}, volume={32}, ISSN={["0047-2425"]}, DOI={10.2134/jeq2003.1603}, abstractNote={ABSTRACTElevated carbon dioxide (CO2) concentrations in the atmosphere can stimulate plant growth and yield, whereas ground‐level ozone (O3) concentrations cause the opposite effect in many areas of the world. Recent experiments show that elevated CO2 can protect some plants from O3 stress, but this has not been tested for most crop species. Our objective was to determine if elevated CO2 protects Irish potato (Solanum tuberosum L.) from foliar injury and suppression of growth and yield caused by O3 An O3–resistant cultivar (Superior) and an O3–sensitive cultivar (Dark Red Norland) were exposed from within 10 d after emergence to maturity to mixtures of three CO2 and three O3 treatments in open‐top field chambers. The three CO2 treatments were ambient (370 μL L−1) and two treatments with CO2 added to ambient CO2 for 24 h d−1 (540 and 715 μL L−1). The O3 treatments were charcoal‐filtered air (15 nL L−1), nonfiltered air (45 nL L−1), and nonfiltered air with O3 added for 12 h d−1 (80 nL L−1). Elevated O3 and CO2 caused extensive foliar injury of Dark Red Norland, but caused only slight injury of Superior. Elevated CO2 increased growth and tuber yield of both cultivars, whereas elevated O3 generally suppressed growth and yield, mainly of Dark Red Norland. Elevated CO2 appeared to protect Dark Red Norland from O3–induced suppression of shoot, root, and tuber weight as measured at midseason but did not protect either cultivar from O3 stress at the final harvest. The results further illustrate the difficulty in predicting effects of O3 + CO2 mixtures based on the effects of the individual gases.}, number={5}, journal={JOURNAL OF ENVIRONMENTAL QUALITY}, author={Heagle, AS and Miller, JE and Pursley, WA}, year={2003}, pages={1603–1610} }
 @article{heagle_2003, title={Influence of elevated carbon dioxide on interactions between Frankliniella occidentalis and Trifolium repens}, volume={32}, ISSN={["0046-225X"]}, DOI={10.1603/0046-225X-32.3.421}, abstractNote={Abstract Elevated CO2 concentrations can increase plant growth and change plant nutritive value for herbivores. Several reports indicate that leaf-chewing insects consume more foliage of plants grown at elevated CO2 concentrations than of plants grown at ambient CO2. Research with additional plant–pest systems is needed to determine if this phenomenon is widespread and if increased insect feeding might affect productivity. Effects of CO2 enrichment on foliar consumption and population size of Western flower thrips [Frankliniella occidentalis (Pergande)]) were measured on white clover (Trifolium repens L.). White clover infested with thrips was exposed for 24 h/d to ≈396 (ambient) or 745 μLL−1 (elevated) CO2 for up to 35 d in 10 greenhouse exposure chambers. At elevated CO2, clover shoot weight and laminae weight were ≈50% greater, and laminar area was ≈20% greater than at ambient CO2. Thrips population size was not significantly affected by CO2, but laminar area scarred by thrips feeding was ≈90% greater at elevated than at ambient CO2. Because of increased growth, however, undamaged leaf area was approximately 15% greater at elevated than at ambient CO2.}, number={3}, journal={ENVIRONMENTAL ENTOMOLOGY}, author={Heagle, AS}, year={2003}, month={Jun}, pages={421–424} }
 @article{heagle_miller_burkey_eason_pursley_2002, title={Growth and yield responses of snap bean to mixtures of carbon dioxide and ozone}, volume={31}, ISSN={["1537-2537"]}, DOI={10.2134/jeq2002.2008}, abstractNote={ABSTRACTElevated CO2 concentrations expected in the 21st century can stimulate plant growth and yield, whereas tropospheric O3 suppresses plant growth and yield in many areas of the world. Recent experiments showed that elevated CO2 often protects plants from O3 stress, but this has not been tested for many important crop species including snap bean (Phaseolus vulgaris L.). The objective of this study was to determine if elevated CO2 protects snap bean from O3 stress. An O3–tolerant cultivar (Tenderette) and an O3–sensitive selection (S156) were exposed from shortly after emergence to maturity to mixtures of CO2 and O3 in open‐top field chambers. The two CO2 treatments were ambient and ambient with CO2 added for 24 h d−1 resulting in seasonal 12 h d−1 (0800–2000 h EST) mean concentrations of 366 and 697 μL L−1, respectively. The two O3 treatments were charcoal‐filtered air and nonfiltered air with O3 added for 12 h d−1 to achieve seasonal 12 h d−1 (0800–2000 h EST) mean concentrations of 23 and 72 nL L−1, respectively. Elevated CO2 significantly stimulated growth and pod weight of Tenderette and S156, whereas elevated O3 significantly suppressed growth and pod weight of S156 but not of Tenderette. The suppressive effect of elevated O3 on pod dry weight of S156 was approximately 75% at ambient CO2 and approximately 60% at elevated CO2 (harvests combined). This amount of protection from O3 stress afforded by elevated CO2 was much less than reported for other crop species. Extreme sensitivity to O3 may be the reason elevated CO2 failed to significantly protect S156 from O3 stress.}, number={6}, journal={JOURNAL OF ENVIRONMENTAL QUALITY}, author={Heagle, AS and Miller, JE and Burkey, KO and Eason, G and Pursley, WA}, year={2002}, pages={2008–2014} }
 @article{costa_kennedy_heagle_2001, title={Effect of host plant ozone stress on Colorado potato beetles}, volume={30}, ISSN={["1938-2936"]}, DOI={10.1603/0046-225X-30.5.824}, abstractNote={Abstract Effects of ozone (O3) stress of potato, Solanum tuberosum L., on fecundity, larval growth and survival of Colorado potato beetles, Leptinotarsa decemlineata (Say), were measured in greenhouse and field experiments. Chronic O3 exposure caused moderate to severe foliar injury on an O3-sensitive cultivar (‘Red Norland’) but caused only minor injury an O3-resistant cultivar (‘Superior’). Foliar injury caused by O3 was greater on old than on young leaves but feeding by adult beetles was greater on young leaves in all experiments. Foliar analyses of the five uppermost leaves (nodes 1–5) showed higher carbon (C) and higher nitrogen (N) concentration in Superior than in Norland. There were no significant O3 effects on C or N and no cultivar × O3 interactions. Egg production by newly emerged adult beetles feeding on plants exposed to high O3 levels was not significantly different from egg production on plants exposed to low O3 levels, regardless of cultivar O3 sensitivity. Feeding and energy conversion efficiency of neonates and survival of larvae to the adult stage were not significantly affected by the O3 treatment. Although present levels of tropospheric O3 are high enough to significantly affect yield of sensitive potato cultivars, our results indicate no significant effect of ambient O3 concentrations on Colorado potato beetle populations.}, number={5}, journal={ENVIRONMENTAL ENTOMOLOGY}, author={Costa, SD and Kennedy, GG and Heagle, AS}, year={2001}, month={Oct}, pages={824–831} }
 @article{mills_ball_hayes_fuhrer_skarby_gimeno_de temmerman_heagle_2000, title={Development of a multi-factor model for predicting the effects of ambient ozone on the biomass of white clover}, volume={109}, ISSN={["0269-7491"]}, DOI={10.1016/S0269-7491(00)00057-9}, abstractNote={Results are presented from the UN/ECE ICP Vegetation (International Cooperative Programme on effects of air pollution on natural vegetation and crops) experiments in which ozone(O(3))-resistant (NC-R) and -sensitive (NC-S) clones of white clover (Trifolium repens cv. Regal) were exposed to ambient O(3) episodes at 14 sites in eight European countries in 1996, 1997 and 1998. The plants were grown according to a standard protocol, and the forage was harvested every 28 days for 4-5 months per year by excision 7 cm above the soil surface. Biomass ratio (NC-S/NC-R) was related to the climatic and pollutant conditions at each site using multiple linear regression (MLR) and artificial neural networks (ANNs). Twenty-one input parameters [e.g. AOT40, 7-h mean O(3) concentration, daylight vapour pressure deficit (VPD), daily maximum temperature] were considered individually and in combination with the aim of developing a model with high r(2) and simple structure that could be used to predict biomass change in white clover. MLR models were generally more complex, and performed less well for unseen data than non-linear ANN models. The ANN model with the best performance had five inputs with an r(2) value of 0.84 for the training data, and 0.71 for previously unseen data. Two inputs to the model described the O(3) conditions (AOT40 and 24-h mean for O(3)), two described temperature (daylight mean and 24-h mean temperature), and the fifth input appeared to be differentiating between semi-urban and rural sites (NO concentration at 17:00). Neither VPD nor harvest interval was an important component of the model. The model predicted that a 5% reduction in biomass ratio was associated with AOT40s in the range 0.9-1.7 ppm x h (microl l(-1) h) accumulated over 28 days, with plants being most sensitive in conditions of low NO(x), medium-range temperature, and high 24-h mean O(3) concentration.}, number={3}, journal={ENVIRONMENTAL POLLUTION}, author={Mills, G and Ball, G and Hayes, F and Fuhrer, J and Skarby, L and Gimeno, B and De Temmerman, L and Heagle, A}, year={2000}, pages={533–542} }
 @article{heagle_miller_pursley_2000, title={Growth and yield responses of winter wheat to mixtures of ozone and carbon dioxide}, volume={40}, ISSN={["0011-183X"]}, DOI={10.2135/cropsci2000.4061656x}, abstractNote={Ozone (O3) in the troposphere can cause plant stress, whereas elevated CO2 generally enhances plant growth. Until recently, few studies have considered whether O3 can affect plant response to CO2 or vice versa. We examined these possibilities for soft red winter wheat (Triticum aestivum L.). Plants were grown in 14‐L pots and exposed in open‐top field chambers to all combinations of three CO2 and three O3 treatments. The CO2 treatments were ambient (approximately 380 μL L−1), or ambient with CO2 added for 24 h d−1 to achieve mean concentrations of approximately 540, or 700 μL L−1 The O3 treatments were charcoal‐filtered air (CF), nonfiltered air (NF), or NF with O3 added for 12 h d−1 (NF+). Mean O3 concentrations in the CF, NF, and NF+ treatments were approximately 27, 45, and 90 nL L−1 In the first experiment, eight cultivars with widely different genetic backgrounds were tested. `Coker 9835' was relatively resistant to O3 and `Coker 9904' was relatively sensitive; these cultivars were tested in Exp. 2. Foliar injury caused by O3 was suppressed by elevated CO2 in both experiments. In Exp. 1, plant size and yield increased with CO2 enrichment in the NF and NF+ treatments, but not in the CF treatment. However, the O3 × CO2 interaction was rarely significant. In Exp. 2, growth and yield of C9904 was suppressed more by O3 than was that of C9835. Because of cultivar differences in sensitivity to O3, CO2 enrichment caused greater amelioration of O3 stress and greater enhancement for C9904 than for C9835. Significant cultivar × O3 × CO2 interactions occurred for all growth and yield measures. These results are similar to results with other crops, and further emphasize the need to consider possible interactions between O3 and CO2 when investigating effects of O3 or CO2 on plant systems.}, number={6}, journal={CROP SCIENCE}, author={Heagle, AS and Miller, JE and Pursley, WA}, year={2000}, pages={1656–1664} }
 @article{heagle_stefanski_2000, title={Relationships between ambient ozone regimes and white clover forage production using different ozone exposure indexes}, volume={34}, ISSN={["1352-2310"]}, DOI={10.1016/s1352-2310(99)00354-4}, abstractNote={Plant responses to seasonal exposure to tropospheric ozone (O3) are mediated by interactions with physical and genetic factors that complicate attempts to develop a measure of O3 exposure (exposure index) that best relates to plant response. Dozens of exposure indexes have been tested for best fit to yield response data from open-top chamber studies. These tests have limited applicability because of possible confounding caused by variability in experimental protocols used in chamber dose–response studies. A 2-yr study in ambient air at eight locations in the USA measured relative effects of ambient O3 on forage weight of a sensitive (NC-S) and a resistant (NC-R) clone of white clover. Protocols included uniform growth medium, irrigation, exposure duration and genetics (clones). Plants were harvested to determine NC-S/NC-R forage biomass ratios after each of four 28-day periods. High ratios indicated low O3 concentrations, and low ratios indicated high concentrations. We used these results in attempts to identify the relative suitability of several exposure indexes in defining O3 exposure–forage biomass relationships. Indexes were calculated using combinations of O3 exposure forms (SUM00, SUM06, W95, W126, and AOT04), diurnal and seasonal accumulating times and harvests. Squared correlations (r2's) between the index and biomass ratio were used as a general indication of relative suitability of the different indexes. Squared correlations were much higher for indexes coupled with harvests 2, 3 and 4, than for harvest 1. Even higher r2's occurred for indexes coupled with the combined mean forage ratio for harvests 2, 3 and 4. Squared correlations were most sensitive to the choice of hourly averaging times. Lowest r2's occurred for the 24 h accumulating period, much higher r2's occurred for the 12 h daylight period, and the highest r2's occurred for periods of 6 h or less during midday, regardless of all other factors. The exposure form was important only for 24 h indexes for which SUM00 gave the lowest r2's. All forms, including SUM00, produced similarly high r2's for 6, 5, and 4 h midday accumulating times.}, number={5}, journal={ATMOSPHERIC ENVIRONMENT}, author={Heagle, AS and Stefanski, LA}, year={2000}, pages={735–744} }
 @article{jackson_rufty_heagle_severson_eckel_2000, title={Survival and development of tobacco hornworm larvae on tobacco plants grown under elevated levels of ozone}, volume={26}, ISSN={["0098-0331"]}, DOI={10.1023/A:1005440025509}, number={1}, journal={JOURNAL OF CHEMICAL ECOLOGY}, author={Jackson, DM and Rufty, TW and Heagle, AS and Severson, RF and Eckel, RVW}, year={2000}, month={Jan}, pages={1–19} }
 @article{heagle_booker_miller_pursley_stefanski_1999, title={Influence of daily carbon dioxide exposure duration and root environment on soybean response to elevated carbon dioxide}, volume={28}, ISSN={["0047-2425"]}, DOI={10.2134/jeq1999.00472425002800020034x}, abstractNote={AbstractLittle is known about effects of daily CO2 enrichment duration and root environment on plant response to elevated CO2. Two experiments were performed with Essex soybean (Glycine max L. Merr.) in open‐top field chambers to address these questions. In one experiment, effects of 12 and 24 h d−1 exposures to double‐ambient CO2 were compared for plants grown in 14 L pots that were either insulated to moderate soil temperature or not insulated. Although never significant statistically, trends at some growth stages suggested that nighttime CO2 enrichment contributed to growth and yield. Plants grew and yielded more in insulated than noninsulated pots, but there were no significant CO2 enrichment × insulation interactions. In the second experiment, response to approximately 1.3, 1.6, and 1.9 times ambient CO2 was compared for plants grown in the ground or 14 L pots. Enhancement of photosynthesis, growth, and yield by CO2 enrichment was similar in pots and in the ground. Linear responses to different CO2 concentrations were significant for all yield components in both root environments, whereas quadratic responses were significant for plants in pots but not for plants in the ground. Tests of proportionality of response for yield components showed no evidence of significant differences between plants in pots and in the ground except weight per 100 seeds. Seed yield enhancement at 1.9 times ambient CO2 was 36% for plants in pots and 33% for plants in the ground. Overall, proportional response of soybean to CO2 enrichment was relatively uniform in spite of large differences in baseline growth and yield.}, number={2}, journal={JOURNAL OF ENVIRONMENTAL QUALITY}, author={Heagle, AS and Booker, FL and Miller, JE and Pursley, WA and Stefanski, LA}, year={1999}, pages={666–675} }
 @article{heagle_miller_booker_pursley_1999, title={Ozone stress, carbon dioxide enrichment, and nitrogen fertility interactions in cotton}, volume={39}, ISSN={["0011-183X"]}, DOI={10.2135/cropsci1999.0011183X003900030021x}, abstractNote={Ozone (O 3 ) in the troposphere can cause plant stress leading to foliar injury and suppressed growth and yield, whereas elevated CO 2 generally enhances growth and yield. Numerous studies have been performed to determine effects of O 3 and CO 2 separately, but relatively few have been performed to determine if 03 can affect plant response to CO 2 or vice versa. Open-top field chambers were used to determine if such interactions occur for cotton (Gossypium hirsutum L.), which is relatively sensitive to O 3 . Nitrogen nutrition is especially important in cotton production so N nutrition was included as an experimental factor. Plants were grown in 14-L pots at low, medium, and high soil N levels and exposed to three CO 2 and two or three 03 treatments in all combinations during two seasons. The CO 2 treatments were ambient (370 μL L -1 ) and two treatments with CO 2 added for 24 h d -1 at approximately 1.5 and 2.0 times ambient. In 1995, the O 3 treatments were charcoal filtered air (CF), and nonfiltered air (NF) with 03 added for 12 h d -1 (NF+). In 1996, a NF treatment was also included to represent ambient 03 conditions. The CF, NF, and NF+ treatments resulted in seasonal 03 concentrations of approximately 23, 51, and 75 nL L -1 . Carbon dioxide enrichment generally stimulated growth and yield whereas 03 exposure suppressed growth and yield. Stimulation induced by CO 2 increased as O 3 stress increased. For example, in 1995 at medium N, the percentage increase in yield caused by doubling CO 2 in CF air was 0%, but was 52% in NF+ air. Comparable values for 1996 were 23% in CF air and 140% in NF+ air. These interactions occurred for a range of soil N levels, and were probably caused by CO 2 -induced prevention of 03 stress. The results emphasize the need to consider O 3 X CO 2 interactions to ensure correct interpretation of cause-effect relationships in CO 2 enrichment studies with crops that are sensitive to O 3 .}, number={3}, journal={CROP SCIENCE}, author={Heagle, AS and Miller, JE and Booker, FL and Pursley, WA}, year={1999}, pages={731–741} }
 @article{hummel_brandenburg_heagle_arellano_1998, title={Effects of ozone on reproduction of twospotted spider mite (Acari : Tetranychidae) on white clover}, volume={27}, ISSN={["1938-2936"]}, DOI={10.1093/ee/27.2.388}, abstractNote={Twospotted spider mite, Tetranychus urticae Koch, is a significant pest of peanut, Arachis hypogeae L., that continues to present problems as an induced pest despite recent widespread implementation of IPM practices. Effects of ozone (O3) on reproduction of twospotted spider mites feeding on an O3-sensitive clone and an O3-resistant clone of white clover, Trifolium repens L., were investigated in a greenhouse in continuous-stirred tank reactor chambers. Mite eggs of narrow age distribution (approximately 6 h) were placed on white clover plants exposed to 5 treatment levels of O3. Constant amounts of O3 were added to charcoal-filtered air for 6 h per day to achieve 5 mean concentrations ranging from 10 to 112 nl per liter. Plants were exposed to O3 approximately 9 d before infestation with mites; daily exposures continued for approximately 20 d after mite infestation. The developmental stage of each mite was recorded at approximately 2-d intervals until females were sexually mature (approximately 10 d) and began ovipositing. Thereafter, the cumulative number of eggs produced per mite was recorded. After approximately 5 d of oviposition, each adult mite was removed and the percentage hatch of eggs remaining on each plant was measured for an additional 5 d. Ozone caused more chlorosis and necrosis on the O3-sensitive clover clone than on the O3-resistant clover clone. Increasing O3 levels caused a significant linear decrease in developmental period of the mites. Estimates of time to 1st oviposition decreased linearly with increasing O3. Estimates of time of 1st hatch of 2nd-generation eggs decreased linearly with increasing O3. Elevated O3 levels appear to decrease the time required for female mites to develop from egg to ovipositing adult, which may have a profound effect on the intrinsic rate of population increase. Different responses by mites feeding on resistant plants versus susceptible plants suggests that this is a plant-mediated response.}, number={2}, journal={ENVIRONMENTAL ENTOMOLOGY}, author={Hummel, RL and Brandenburg, RL and Heagle, AS and Arellano, C}, year={1998}, month={Apr}, pages={388–394} }
 @article{heagle_miller_booker_1998, title={Influence of ozone stress on soybean response to carbon dioxide enrichment: I. Foliar properties}, volume={38}, ISSN={["0011-183X"]}, DOI={10.2135/cropsci1998.0011183X003800010020x}, abstractNote={Tropospheric O3 can cause foliar injury, decreased growth, and decreased yield, whereas CO2 enrichment generally causes opposite effects. Little is known about plant response to mixtures of O3 and CO2. Open‐top field chambers were used to determine if foliar responses of soybean [Glycine max (L.) Merr.] to CO2 enrichment are affected by O3 stress and vice versa. Plants were grown in 14‐L pots and exposed to four CO2 and three O3 concentrations in 12 combinations. The CO2 treatments were ambient (366 μL−) and three treatments with CO2 added for 24 h d 1 at approximately 1.3, 1.6, and 2.0 times ambient. The O3 treatments were charcoal‐filtered air (CF), nonfiltered air (NF), and NF with O3 added for 12 h−1 ( NF+), resulting in seasonal concentrations of approximately 20, 46, and 75 nL L−1. Foliar effects of CO2 enrichment were dependent on the amount of stress caused by O3. In the CF treatment, plants were not stressed by O3, and CO2 enrichment caused chlorosis and decreased chlorophyll. In the NF and NF+ treatments, plants were stressed by 03, and CO2 enrichment suppressed chlorosis and increased chlorophyll. Ozone decreased specific leaf weight, increased foliar N and C, and decreased C/N ratios, whereas CO2 caused opposite responses for these measures. Ozone increased foliar S and B but did not affect P or K concentrations. Conversely, CO2 enrichment suppressed foliar S, B, P, and K concentrations. These interactions between O3 and CO2 emphasize a need to consider the amount of plant stress caused by O3 in studies to measure effects of CO2 enrichment.}, number={1}, journal={CROP SCIENCE}, author={Heagle, AS and Miller, JE and Booker, FL}, year={1998}, pages={113–121} }
 @article{miller_heagle_pursley_1998, title={Influence of ozone stress on soybean response to carbon dioxide enrichment: II. Biomass and development}, volume={38}, ISSN={["0011-183X"]}, DOI={10.2135/cropsci1998.0011183X003800010021x}, abstractNote={Previous research has shown that elevated CO2 concentrations can increase plant growth, whereas the air pollutant O3 is phytotoxic. Because elevated concentrations of these gases will co‐occur, the objective of our experiment was to determine if estimates of plant growth response to future levels of CO2 and O3 require experiments to test the gases in combination. Soybean plants [Glycine max (L.) Merr. cv. Essex) were exposed in open‐top chambers to combinations of O3 and CO2 from plant emergence through physiological maturity. Ozone treatments were charcoal‐filtered air (CF), nonfiltered air (NF), and NF with O3 added for 12 d−1 (NF+) (seasonal mean 12 d−1 O3 concentrations of 20, 50, or 79 nL L−1, respectively). Carbon dioxide exposures were for 24 h d−1 giving seasonal mean 12 d−1 concentrations of 370, 482, 599, or 713 μL L−1. Over the season, elevated CO2 usually stimulated growth and O3 suppressed growth. Elevated CO2 usually increased partitioning of biomass to branches, decreased partitioning to pods, increased specific leaf weight, and decreased leaf area ratio. Ozone suppressed leaf and root weight ratios, increased pod weight ratios, and decreased specific leaf weight. Toward the end of the season, both O3 and CO2 accelerated reproductive development. Elevated CO2 moderated suppression of growth by O3, and the highest CO2 concentration completely ameliorated O3 effects on main stem biomass, root biomass, and leaf area. Ozone, however, limited some positive growth responses to CO2, especially at less than a doubling of CO2 concentrations. These results indicate that in order to understand the future impacts of atmospheric gases such as elevated CO2 and O3 on crop growth, their combined effects should be determined.}, number={1}, journal={CROP SCIENCE}, author={Miller, JE and Heagle, AS and Pursley, WA}, year={1998}, pages={122–128} }
 @article{heagle_miller_pursley_1998, title={Influence of ozone stress on soybean response to carbon dioxide enrichment: III. Yield and seed quality}, volume={38}, ISSN={["0011-183X"]}, DOI={10.2135/cropsci1998.0011183X003800010022x}, abstractNote={Ozone in the troposphere can cause plant stress, whereas elevated CO2 generally causes positive responses. Little is known of how these gases interact to affect plant response. Interactive effects on yield and seed quality of soybean [Glycine max (L.) Merr.] grown in 14‐L pots were measured in open‐top field chambers. Essex was tested in 1993, and Essex, Holladay, and NK 6955 were tested in 1994. Plants were exposed from emergence to maturity to four CO2 levels (ambient and 1.3,1.6, and 2.0 times ambient) and three O3 levels (0.4, 0.9, and 1.5 times ambient) in 12 combinations. Increasing O3 suppressed growth and yield, whereas CO2 enrichment stimulated growth and yield. Carbon dioxide‐induced stimulation was greater for plants stressed by O3 than for non stressed plants. For example, CO2 at 2.0 times ambient increased 2‐yr mean seed yield of Essex by 16, 24, and 81% at O2 levels of 0.4, 0.9, and 1.5 times ambient, respectively. Effects of O3 and CO2 on seed oil content were variable with numerous cultivar differences. Seed protein content was never affected. Elevated O3 suppressed oleic acid content in seeds, whereas CO2 increased it; the nature of the O3 × CO2 interaction for oleic acid was similar to that observed for most yield measures. Carbon dioxide‐induced stimulation of plants stressed by O3 was apparently caused partly by amelioration of O3 stress. Interactions between O3 and CO2 must be considered for proper interpretation of cause‐effect relationships in CO2, enrichment studies.}, number={1}, journal={CROP SCIENCE}, author={Heagle, AS and Miller, JE and Pursley, WA}, year={1998}, pages={128–134} }
 @article{fiscus_reid_miller_heagle_1997, title={Elevated CO2 reduces O-3 flux and O-3-induced yield losses in soybeans: Possible implications for elevated CO2 studies}, volume={48}, ISSN={["0022-0957"]}, DOI={10.1093/jxb/48.2.307}, abstractNote={Soybeans were grown for three seasons in open-top field chambers to determine (1) whether elevated CO2 (360 versus 700 //mol mol"1) alleviates some of the yield loss due to pollutant 0 3, (2) whether the partial stomatal closure resulting from chronic 03 exposure (charcoal-filtered air versus 1.5 x ambient concentrations) is a cause or result of decreased photosynthesis, and (3) possible implications of C0 2 /0 3 interactions to climate change studies using elevated CO2. Leaf conductance was reduced by elevated C02, regardless of O3 level, or by exposure to 03 alone. As. a result of these effects on conductance, high C0 2 reduced estimated midday 03 flux into the leaf by an average of 50% in charcoal-filtered air and 35% in the high 03 treatment. However, while exposure to O 3 reduced seed yields by 41% at ambient CO2 levels, the yield reduction was completely ameliorated by elevated CO2. The threshold midday 03 flux for yield loss appears to be 20-30 nmol m~2 s" 1 in this study. Although elevated CO2 increased total biomass production, it did not increase seed yields. A/C, curves show a large reduction in the stomatal limitation to photosynthesis due to elevated CO2, but no effect of O3. These data demonstrate that (1) reduced conductance due to O3 is the result, and not the cause, of reduced photosynthesis, (2) 700 //mol mol"' CO2 can completely ameliorate yield losses due to 03 within the limits of these experiments, and (3) some reports of increased yields under elevated CO2 treatments may, at least in part, reflect the amelioration of unrecognized suppression of yield by 03 or other stresses.}, number={307}, journal={JOURNAL OF EXPERIMENTAL BOTANY}, author={Fiscus, EL and Reid, CD and Miller, JE and Heagle, AS}, year={1997}, month={Feb}, pages={307–313} }
 @article{heagle_miller_1996, title={Effects of rooting medium and fertilizer rate on response of white clover to tropospheric ozone}, volume={91}, ISSN={["0269-7491"]}, DOI={10.1016/0269-7491(95)00018-M}, abstractNote={Two white clover (Trifolium repens L.) clones with varying sensitivity to O3 are being developed as a system to indicate effects of ambient concentrations of tropospheric O3 on plants. One clone (NC-S) is highly sensitive to O3 and the other (NC-R) is highly resistant. The system relies on periodic measurement of foliar injury, foliar chlorophyll, and forage production of NC-S and NC-R grown in 15-liter pots throughout a summer season. Relative amounts of foliar injury and ratios (NC-S/NC-R) for chlorophyll and forage weight can be used to estimate biologically effective ambient O3 concentrations. The effect of variation in rooting media formulation and fertilizer rate on response of NC-S and NC-R to ambient O3 was determined in the present study. In the rooting medium experiment, clover was grown in three mixtures of sandy loam topsoil:course washed sand:Metro Mix 220 (ratios (by volume) of 2:1:1, 2:1:5, and 6:1:1). In the fertilizer experiment, clover was grown in the 2:1:1 medium at four fertilizer rates (soluble 5-11-26 (N-P-K) at 0.0, 0.5, 1.0, or 2.0 g per pot). Ozone caused more foliar injury, more chlorosis, and a greater decrease in forage production of NC-S than of NC-R in all studies. Rooting media treatments affected both clones similarly and occasional clone x media interactions were judged to be random. Forage production by NC-S, relative to that of NC-R, was generally greater in the 0.0 fertilizer treatment, but the forage ratios were similar at all other fertilizer treatments. The relative response of NC-S and NC-R to O3 is fairly stable under cultural conditions that support normal plant growth.}, number={1}, journal={ENVIRONMENTAL POLLUTION}, author={Heagle, AS and Miller, JE}, year={1996}, pages={113–119} }
 @article{heagle_miller_sherrill_1994, title={A WHITE CLOVER SYSTEM TO ESTIMATE EFFECTS OF TROPOSPHERIC OZONE ON PLANTS}, volume={23}, ISSN={["0047-2425"]}, DOI={10.2134/jeq1994.00472425002300030030x}, abstractNote={AbstractAn ozone‐sensitive (NC‐S) and an ozone‐resistant (NC‐R) clone of white clover (Trifolium repens L.) were tested to determine the feasibility of using them to indicate concentrations of tropospheric ozone (O3) and potential effects of O3 on plants. Plants of each clone were exposed daily in open‐top field chambers to O3 concentrations ranging from 0.5 to 1.5 times the ambient concentrations for three summer seasons near Raleigh, NC. Foliar injury, foliar chlorophyll, and forage production of both clones were related directly to the O3 concentration. Ozone routinely injured leaves, suppressed foliar chlorophyll, and decreased growth of NC‐S more than that of NC‐R. Forage production was highly variable over a wide range of weather conditions, but the relative forage production rate of both clones under such conditions was similar and the seasonal O3 response relationship between NC‐S and NC‐R was relatively stable. The level of response of NC‐S to O3 routinely increased from growth period to growth period suggesting an effect of previous exposure. More work is needed to calibrate the system over a wider range of ambient O3 levels and climatic conditions.}, number={3}, journal={JOURNAL OF ENVIRONMENTAL QUALITY}, author={HEAGLE, AS and MILLER, JE and SHERRILL, DE}, year={1994}, pages={613–621} }
 @article{heagle_miller_sherrill_rawlings_1993, title={EFFECTS OF OZONE AND CARBON-DIOXIDE MIXTURES ON 2 CLONES OF WHITE CLOVER}, volume={123}, ISSN={["0028-646X"]}, DOI={10.1111/j.1469-8137.1993.tb03786.x}, abstractNote={SUMMARYThe effects of mixtures of ozone and carbon dioxide on growth and physiology of an O3‐sensitive (NC‐S) and an O3‐resistant (NC‐R) clone of white clover (Trifolium repens L.) were determined. The experiment was performed in a greenhouse with O3 treatments of 5 and 82 nl l−1 (ppb) for 6 h d−1 and CO2 treatments of 380 (ambient), 490,600, and 710 μl l−1 (ppm) for 24 h d−1. Enrichment with CO2 decreased foliar gas exchange (measured as stomatal resistance) of NC‐R more than that of NC‐S whereas O3 decreased gas exchange of NC‐S more than that of NC‐R. Ozone caused extensive foliar injury of NC‐S but caused only slight injury of NC‐R. CO2 enrichment suppressed O3‐induced foliar injury of NC‐S as measured after 4 wk of exposure, but this effect diminished after 8 wk of exposure. CO2 enrichment decreased the relative chlorophyll content (μg of chlorophyll mg−1 of leaf tissue sampled) but not the total chlorophyll (total chlorophyll in the leaves sampled). There were no O3× CO2 interactions for foliar chlorophyll. High concentrations of CO2 caused reddening of new leaves near the end of the 8 wk exposure period. CO2 enrichment decreased foliar concentrations of N, P, K, S, Cu, B, and Fe, increased foliar concentrations of Mn, but did not affect Zn, Ca, or Mg. Ozone exposure did not modify the CO2 effects on foliar nutrient concentration. Ozone decreased growth of NC‐S but not NC‐R while CO2 enrichment stimulated growth of both clones. The highest CO2 concentration appeared to decrease the effects of O3 on growth of NC‐S. However, except for a transitory effect on foliar injury, there was no evidence that CO2, at concentrations less than the highest used in this study, will protect white clover from the effects of tropospheric O3.}, number={4}, journal={NEW PHYTOLOGIST}, author={HEAGLE, AS and MILLER, JE and SHERRILL, DE and RAWLINGS, JO}, year={1993}, month={Apr}, pages={751–762} }
 @article{heagle_mclaughlin_miller_joyner_1992, title={RESPONSE OF 2 WHITE CLOVER CLONES TO PEANUT STUNT VIRUS AND OZONE}, volume={82}, ISSN={["0031-949X"]}, DOI={10.1094/Phyto-82-254}, abstractNote={Effects of ozone (O 3 ) and peanut stunt virus (PSV) on two clones of white clover (Trifolium repens) were measured in open-top field chambers. An O3-resistant clone (NC-R) and an 03-sensitive clone (NC-S), with and without PSV infection, were exposed to O 3 for 12-h day -1 for 111 days. The exposures were proportional to ambient O 3 and resulted in 12-h day -1 mean concentrations of 26, 45, 64, and 76 nL L -1 for the 111 days of exposure. Plant shoots were harvested five times to measure effects of O 3 and PSV on foliar injury, foliar chlorophyll, and shoot dry weight (...)}, number={3}, journal={PHYTOPATHOLOGY}, author={HEAGLE, AS and MCLAUGHLIN, MR and MILLER, JE and JOYNER, RL}, year={1992}, month={Mar}, pages={254–258} }
 @article{heagle_mclaughlin_miller_joyner_spruill_1991, title={ADAPTATION OF A WHITE CLOVER POPULATION TO OZONE STRESS}, volume={119}, ISSN={["1469-8137"]}, DOI={10.1111/j.1469-8137.1991.tb01008.x}, abstractNote={summaryWhite clover (Trifolium repens L.) ‘Regal’ and tall fescue (Festuca arundinacea Schreb.) ‘Kentucky 31’ were grown together in a field and exposed for two seasons in open‐top chambers to six ozone (O3) regimes ranging from 0.59 to 1.95 times the ambient O3 concentration. Plants that survived were propagated clonally and used in the present study to determine whether selection for resistance or sensitivity to O3 had occurred. Relative foliar sensitivity of surviving clones to various short‐term O3, exposure regimes was determined with and without infection by several viruses. In tests of all surviving clones with viruses present, higher percentages of clones that survived two seasons at the high O3 levels were resistant to short‐term exposure to O3, than were those that survived exposure to the low O3 treatments. Only one of the 33 clones that survived exposure to charcoal‐filtered air (059 treatment) was O3‐resistant while 19 of the 30 clones surviving the 1.95 treatment were O3‐resistant. Conversely, eight clones that survived the 0.59 treatment were sensitive to O3 while none of those that survived the l.95 treatment were sensitive. The results indicate that selection pressure in the presence of O3 stress was for resistance to O3. Various combinations of five common viruses of clover were present among the surviving clones. Shoot‐rip meristem culture was used to free one O3‐resistant and one O3‐sensitive clone from at) viruses. The relative O3 sensitivity of these two clones was not affected by viruses. Further testing is required to determine the relationships between relative foliar sensitivity to short‐term O3 exposure and relative sensitivity to growth effects caused by long‐term exposure.}, number={1}, journal={NEW PHYTOLOGIST}, author={HEAGLE, AS and MCLAUGHLIN, MR and MILLER, JE and JOYNER, RL and SPRUILL, SE}, year={1991}, month={Sep}, pages={61–68} }
 @article{heagle_philbeck_ferrell_heck_1989, title={DESIGN AND PERFORMANCE OF A LARGE, FIELD EXPOSURE CHAMBER TO MEASURE EFFECTS OF AIR-QUALITY ON PLANTS}, volume={18}, ISSN={["0047-2425"]}, DOI={10.2134/jeq1989.00472425001800030021x}, abstractNote={AbstractA 4.66‐m diam., 3.6 m tall, cylindrical open‐top field chamber was designed, constructed, and tested as a tool to measure the effects of air quality on plant function and yield. It is a larger version of the 3‐m diam. chamber that has been used to measure the effects of gaseous pollutants on crop plants. The new chamber has an aluminum‐channel frame covered with clear polyvinyl chloride plastic film. It is equipped with a frustum (truncated cone) that decreases ambient air ingress and can be fitted with a device to exclude rain (rain cap) for studies with simulated rain pH. During the daylight hours, the mean air temperature within the chamber was 0.6 °C greater than ambient on cloudy, cold days, 2.2 °C greater than ambient on partly cloudy, cool days, and 2.8 °C greater than ambient on sunny, warm days. The mean dew point temperature for a wide range of conditions was 0.7 ° greater inside than outside. Mean solar radiation in the chamber, with new plastic panels, was 15% less than ambient with a rain cap and 12% less with no rain cap. Charcoal filtration removed 78% of the O3 in ambient air; long‐term measurements during charcoal filtration showed that the mean O3 concentration in the chamber (all positions and heights) was 77% less than ambient suggesting little or no long‐term ingress through the top. Short‐term gradients in O3 concentrations existed (mostly near the top of the chamber) during infrequent periods of strong winds. During addition of approximately 0.09 µL L−1 of O3 to nonfiltered air, the mean (14‐d) O3 concentrations across all positions and heights varied by less than 0.005 µL L−1 of the overall mean.}, number={3}, journal={JOURNAL OF ENVIRONMENTAL QUALITY}, author={HEAGLE, AS and PHILBECK, RB and FERRELL, RE and HECK, WW}, year={1989}, pages={361–368} }
 @article{shafer_grand_bruck_heagle_1985, title={FORMATION OF ECTOMYCORRHIZAE ON PINUS-TAEDA SEEDLINGS EXPOSED TO SIMULATED ACIDIC RAIN}, volume={15}, ISSN={["0045-5067"]}, DOI={10.1139/x85-012}, abstractNote={ Effects of simulated acidic rain on formation of ectomycorrhizae were studied with Pinustaeda seedlings grown in plastic trays in a greenhouse. Trays of ectomycorrhizal seedlings were exposed 37 times over 16 weeks to simulated rains adjusted to pH 5.6, 4.0, 3.2, or 2.4. After exposures, mean percentages of short roots that were mycorrhizal (%M) were greatest (62.2%) for seedlings exposed to rains of pH 2.4. Values of %M exhibited a quadratic relationship with rain acidity (%M = 146.49 − 48.96 (rain pH) + 5.68 (rain pH)2). Values of R/S (root/shoot ratio) were negatively correlated (P < 0.01) with %M. Responses of ecetomycorrhiza formation and associated shoot growth to acidity of simulated rains suggest that rains of intermediate acidity (pH 4.0 and 3.2) inhibited ectomycorrhiza formation, or that increased soil acidity or other factors induced by rains at pH 2.4 enhanced ectomycorrhiza formation. }, number={1}, journal={CANADIAN JOURNAL OF FOREST RESEARCH-REVUE CANADIENNE DE RECHERCHE FORESTIERE}, author={SHAFER, SR and GRAND, LF and BRUCK, RI and HEAGLE, AS}, year={1985}, pages={66–71} }
 @article{shafer_bruck_heagle_1985, title={INFLUENCE OF SIMULATED ACIDIC RAIN ON PHYTOPHTHORA-CINNAMOMI AND PHYTOPHTHORA ROOT-ROT OF BLUE LUPINE}, volume={75}, ISSN={["0031-949X"]}, DOI={10.1094/Phyto-75-996}, number={9}, journal={PHYTOPATHOLOGY}, author={SHAFER, SR and BRUCK, RI and HEAGLE, AS}, year={1985}, pages={996–1003} }
 @inbook{bruck_heagle_shafer_1982, title={Effects of simulated acid precipitation on foliar and root diseases of forest trees}, booktitle={EPA Acidic Deposition Ecological Effects Research Peer Review}, author={Bruck, R. I. and Heagle, A. S. and Shafer, S. R.}, year={1982}, pages={67–76} }
 @inbook{heagle_1982, title={Interactions between air pollutants and parasitic plant diseases}, booktitle={Effects of gaseous air pollution in agriculture and horticulture}, publisher={Boston: Butterworth Scientific}, author={Heagle, A. S.}, editor={M. H. Unsworth, D. P. OrmrodEditor}, year={1982}, pages={333} }
 @article{heagle_philbeck_letchworth_1979, title={INJURY AND YIELD RESPONSES OF SPINACH CULTIVARS TO CHRONIC DOSES OF OZONE IN OPEN-TOP FIELD CHAMBERS}, volume={8}, ISSN={["0047-2425"]}, DOI={10.2134/jeq1979.00472425000800030021x}, abstractNote={AbstractSpinach (Spinacia oleracea L.) cultivars were exposed continuously during growth to carbon‐filtered air or nonfiltered ambient air in open‐top field chambers. Constant low concentrations of ozone (O3) were added to the varying ambient concentrations in the nonfiltered‐air chambers for 7 hours (0920 to 1620 EDT) per day. There were significant differences in the amount of foliar injury and shoot growth decrease among 11 spinach cultivars exposed for 7 hours/day to 0.13 ppm of O3; America, Winter Bloomsdale, and Seven‐R were less sensitive to foliar injury than Chesapeake, Hybrid‐612, and Dixie Market. Shoot growth of America and Viroflay was affected less and Hybrid‐612 and Dark Green Bloomsdale more than that of most other cultivars. The cultivars America, Winter Bloomsdale, Hybrid 7, and Viroflay were exposed for 38 days to determine threshold doses of O3 for injury and yield effects. The threshold 7‐hour/day mean O3 concentration for foliar injury was between 0.02 and 0.06 ppm. The threshold for a significant decrease in shoot growth of most spinach cultivars was between 0.06 and 0.10 ppm. Shoot fresh weights of plants grown in the ground at 0.06, 0.10, and 0.13 ppm were 18, 37 and 69% less, respectively, than plants grown at 0.02 ppm. Comparative values for potted plants grown in a 1:1:1 mixture of sand/soil/Pro‐Mix BX were 4, 25, and 65%, respectively.}, number={3}, journal={JOURNAL OF ENVIRONMENTAL QUALITY}, author={HEAGLE, AS and PHILBECK, RB and LETCHWORTH, MB}, year={1979}, pages={368–373} }
 @article{heagle_heck_1974, title={PREDISPOSITION OF TOBACCO TO OXIDANT AIR-POLLUTION INJURY BY PREVIOUS EXPOSURE TO OXIDANTS}, volume={7}, ISSN={["0269-7491"]}, DOI={10.1016/0013-9327(74)90033-0}, abstractNote={Pre-exposure of tobacco, cv Bel W3, to ambient oxidant (primarily ozone) air pollution usually predisposed plants to foliar injury from subsequent exposure. Over eight seven-day periods, plants exposed for seven continuous days developed more than twice as much injury as the total injury on plants exposed for only one day each over the same period. The predisposition partly explains the lack of significant quantitative correlations between oxidant dose and plant injury. Thus we need to understand how this predisposition phenomenon and the plant environment interact to affect the amount of injury from a given oxidant dose. Once this is known we will be better able to make quantitative predictions of the effects of various oxidant doses on plants. Until then, the direct assessment of injury to sensitive plant species is the best indication of the incidence and severity of air pollution episodes.}, number={4}, journal={ENVIRONMENTAL POLLUTION}, author={HEAGLE, AS and HECK, WW}, year={1974}, pages={247–252} }