Effects of short-term ozone fumigation on tobacco plants: response of the scavenging system and expression of the glutathione reductase

The role that the constituents of the ascorbate–glutathione cycle play in the mechanism of contrasting ozone sensitivities was examined in mature and old tobacco leaves after acute ozone-fumigation (150 p.p.b., 5 h). Levels of the enzyme activities associated with the detoxifying system were lower in ozone-sensitive Bel W3 control plants than in unfumigated ozone-tolerant Bel B plants. In particular, the endogenous activities of ascorbate peroxidase (APX) and glutathione reductase (GR), and the metabolites ascorbic acid (AA) and reduced glutathione (GSH) were more abundant in Bel B than Bel W3 control plants. These results suggest that the higher tolerance of Bel B to O₃ is associated with a greater initial content of the antioxidant enzymes or metabolites. Only in the mature leaves of the ozone-tolerant Bel B cv. did fumigation trigger activation of APX and, weakly, of dehydroascorbate reductase (DHAR). The activity of these enzymes was significantly lower after ozone treatment in both mature and old leaves of Bel W3 than in control plants. Fumigation had little effect on the ascorbate content. Its main effects on the glutathione pool were that it boosted the oxidized form and lowered the reduced form, particularly in mature Bel W3 leaves. Extractable GR activity remained unchanged in both Bel B and Bel W3 immediately after fumigation, but increased slightly 24 h later, particularly in mature leaves of Bel W3. Exposure to O₃ caused a sharp decline in chloroplastic GR mRNA levels in both cultivars. However, as Western blot analysis failed to detect any major changes in GR protein content at this time, the protein must be highly stable. There is therefore a good correlation between tolerance to O₃ and high endogenous levels of antioxidant metabolites such as AA and GSH in tobacco. In addition, the degree of inducibility of the system discriminates the two cultivars investigated.     

Different responses of two tobacco cultivars and their cell suspension cultures to quercinin,a novel elicitin from Phytophthora quercina

In this study we describe the response of two tobacco cultivars (Nicotiana tabacum L. cv. Bel B and Bel W3) and their cell suspension cultures to quercinin, a novel elicitin produced by the oak pathogen Phytophthora quercina. N-terminal sequencing of the purified protein proved that it belongs to the basic β-elicitins with threonine on position 13. Both tobacco leaves and cells of the cultivar Bel W3 showed hypersensitive cell death after quercinin treatment. Leaves of Bel B also developed quercinin-induced necrosis but higher concentrations of quercinin were necessary as compared to Bel W3. Also Bel B cells showed cell death induction only at the highest quercinin concentration (20 nM). In cell suspension experiments we also measured the quercinin-induced oxidative burst, which occurred in both cultivars. H₂O₂ production in Bel B increased with increasing quercinin concentration and was inhibited only at the highest elicitin concentration (20 nM) whereas the oxidative burst in Bel W3 was completely abolished by 5 nM quercinin. Furthermore we demonstrated that neither H₂O₂ nor superoxide were responsible for cell death induction since neither the inhibitor diphenyleneiodonium (DPI) nor the enzymes catalase (CAT) and superoxide dismutase (SOD) influenced the hypersensitive reaction (HR) in Bel W3 cells. Due to the different response of Bel W3 and Bel B towards the P. quercina elicitin, our system represents an interesting tool to elucidate signaling pathways in tobacco leading to hypersensitive cell death.     

Salicylic acid modulates ozone-induced hypersensitive cell death in tobacco plants

Ozone-tolerant Bel B and ozone-sensitive Bel W3 tobacco cultivars were subjected to acute ozone fumigation (200 p.p.b. for 3 h) and the subcellular localization of H₂O₂ was then studied. H₂O₂ accumulated on the cell walls and plasma membrane of both cultivars but the accumulation pattern differed greatly. H₂O₂ production was high in both cultivars immediately after fumigation, but, in the tolerant Bel B cultivar, after 7 h was only detected in some spongy cells adjacent to epidermal cells. Instead, in the sensitive Bel W3 cultivar, accumulation was still abundant in the cell walls of palisade, spongy and epidermal cells at this time. Significant changes in apoplastic ascorbate pool were noted in both cultivars in the first hours after fumigation. As the reduced ascorbate content remained unchanged, the marked increase in total ascorbate must have originated from the striking increase in dehydroascorbate, particularly in the ozone-sensitive Bel W3. Exposure of plants to ozone resulted in a marked transient increase in both free and conjugated salicylic acid (SA) as well as an increase in the activity of benzoic acid 2-hydroxylase which catalyses SA biosynthesis. SA induction differed greatly in the two cultivars, in that: (1) SA accumulation was far greater in the ozone-sensitive Bel W3 cv. and (2) the maximum SA peak was delayed in Bel W3 and observed only 7 h after fumigation ended. These results suggest that a high SA content, as documented in the ozone-sensitive Bel W3 cultivar, could trigger the production of ROS with subsequent SA-mediated cell-death.     

Illustration of the Genetic Differences in Ozone Sensitivity Between the Varieties Nicotiana tabacum Bel W3 and Bel B Using Various Plant Systems

The ozone-sensitive tobacco variety Bel W₃ was compared with the tolerant cv. Bel B using amphidiploid and amphihaploid genotypes of both. In search of the first genotypical differences, their reaction to acute ozone treatments was investigated with systems of decreasing degree of complexity: whole plants, grown under field, greenhouse and sterile conditions, excised tissues, calli, CCP, MCP and subcellular reactions. It was common to all systems that a fumigation, which clearly exceeds the threshold of the most tolerant material, led to equal reactions in all genotypes in respect of visible injury and membrane leaching. With whole plants and leaf discs growing conditions were found to influence the ozone threshold more than the genotype. Because the most resistant field-grown plants vary widely in their reaction, only sterile or greenhouse grown genotypes were compared. With the exception of whole sterile plants (no genotypical threshold differences), amphihaploids were more susceptible to ozone than their respective amphidiploids as to threshold and sensitivity spectra in all systems investigated. Higher ozone thresholds were detected for Bel B in all systems with one exception: MCP exhibited a lower threshold but also a lower degree of damage in the first buffer range of the sensitivity spectrum than those of Bel W₃. Post-fumigation starch accumulation in mesophyll chloroplasts was the most prominent subcellular ozone reaction.     

Tolerance of a field grown soybean cultivar to elevated ozone level is concurrent with higher leaflet ascorbic acid level, higher ascorbate-dehydroascorbate redox status, and long term photosynthetic productivity

We examined the characteristics of ascorbic acid (ASC) level, dehydroascorbate (DHA) level, and the ASC–DHA redox status in the leaflets of two soybean cultivars grown in a field environment and exposed to elevated ozone (O₃) levels. These two cultivars, one that preliminary evidence indicated to be O₃-tolerant (cv Essex), and one that was indicated to be O₃-sensitive (cv Forrest), were grown in open-top chambers during the summer of 1997. The plants were exposed daily to a controlled, moderately high O₃ level (≈58 nl l⁻¹ air) in the light, beginning at the seedling stage and continuing to bean maturity. Concurrently, control plants were exposed to carbon-filtered, ambient air containing a relatively low O₃ level (≈24 nl l⁻¹ air) during the same period. Elevated O₃ did not affect biomass per plant, mature leaf area accretion, or bean yield per plant of cv Essex. In contrast, elevated O₃ level decreased the biomass and bean yield per plant of cv Forrest by approximately 20%. Daily leaflet photosynthesis rate and stomatal conductance per unit area did not decrease in either cultivar as a result of prolonged O₃ exposure. A 10% lower mature leaflet area in O₃-treated cv Forrest plants contributed to an ultimate limitation in long-term photosynthetic productivity (vegetative and bean yield). Possible factors causing cv Essex to be more O₃ tolerant than cv Forrest were: 1) mature leaflets of control and O₃-treated cv Essex plants consistently maintained a higher daily ASC level than leaflets of cv Forrest plants, and 2) mature leaflets of cv Essex plants maintained a higher daily ASC–DHA redox status than leaflets of cv Forrest plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of high light and ozone fumigation on photosynthesis in Phaseolus vulgaris

In the present work, the aim was to study the combined effects of high irradiance (about 1 000 μmol·m^–2·s^–1) on the effects of O₃ (150 nL·L^–1 for 2 h) on the photosynthetic process of primary pinto (Phaseolus vulgaris L.) leaves. The CO₂ assimilation rate significantly decreased in high light and/or O₃-treated leaves, as did the stomatal conductance. The present work shows that changes occur in photosynthesis-related parameters due to light stress. Photoinhibition was detected as a decrease in the F_v/F_m ratio. This indicates that the light treatment considerably exceeded the photoprotective capacity and resulted in significant photon damage. Moreover O₃ fumigation alone determined an impairment of the photosynthetic process, that in this case is related to the dark reactions of photosynthesis as also showed by the strong increase in (1-q_P). The data presented in this paper illustrate the complexity of photosynthetic response to high light intensities and ozone, which are two stress factors that often occur simultaneously under natural conditions. High light can modify the reaction of leaves to ozone treatment by reducing the chloroplastic electron transport rate and the quantum yield for PSII photochemistry. Thus, high light seems to enhance the detrimental effects of ozone on photosynthesis.     

Responses of Photosynthesis,Carbohydrates and Antioxidants in Needles of Norway Spruce to Slow and Rapid Changes in Ozone

The goal of the present study was to examine the effects of slow and rapid changes of ozone (O₃) concentrations on the physiological behaviour of current-year needles of Norway spruce (Picea abies (L.) Karst.). For this purpose five-year-old spruce seedlings were exposed in growth chambers for 49 days to either charcoal-filtered air, slowly increasing O₃ concentrations from zero up to 100 nl I^?1 in weekly steps of 25 nl I^?1, or immediately to 100 nl I^?1 of O₃. During the investigation period gas exchange, carbohydrate and antioxidant contents of the current flush were measured. In needles which experienced slowly increasing O₃ concentrations, cumulative O₃ uptake was approximately 30 % lower than in needles continuously fumigated with 100 nl I^?1 of O₃. The higher 0₃ uptake in the permanent 100 nl I^?1 O₃ treatment caused a pronounced decline in net photosynthesis, in the efficiency of CO₂ uptake and in the starch content of the seedlings. Initially the ascorbate pool increased, but after 5 weeks of exposure ascorbate concentrations declined and were comparable to values obtained in charcoal-filtered controls, while the thiol contents were enhanced during fumigation with permanent 100 nl I-^?1 O₃. On the contrary, slowly increasing O₃ caused a significant increase in total needle ascorbate throughout the fumigation period, which probably prevented an O₃-induced decline in the photosynthetic machinery as photosynthesis was not affected although the thiol contents were not enhanced. Furthermore, starch content was slightly higher than in O₃-free controls. These results suggest that seedlings of Norway spruce have the possibility to acclimate to O₃ stress, as slowly increasing O₃ concentrations seemed to increase resistance and the seedlings were able to compensate.     

Relationship of antioxidant enzymes to ozone tolerance in branches of mature ponderosa pine (Pinus ponderosa) trees exposed to long-term, low concentration, ozone fumigation and acid precipitation

Seasonal activity of superoxide dismutase (SOD, EC 1.15.1.1). ascorbate peroxidase (APOD, EC 1.11.1.11) and guaiacol-oxidizing enzymes (GPODs, EC 1.11.1.7) was examined in needles of 12- to 15-year-old ponderosa pine (Pinus ponderosa Laws.) trees which received ozone (O₃) and acid precipitation treatment. Individual branches were enclosed in branch exposure chambers delivering either charcoal-filtered (O₃-reduced) air, ambient air, or air with twice ambient (2 x ambient) concentrations of O₃. Acid precipitation treatments were rain of pH 3.0 or 5.1 or no rain. Changes in antioxidant enzyme activity were not a consistent response to O₃ fumigation or acid precipitation, but when observed, they occurred most often in the O₃-sensitive clone and in symptomatic, fumigated branches. In the second year of fumigation. O₃ fleck symptoms appeared on needles of the sensitive clone as early as July and APOD activities were significantly increased by O₃ at all sampling dates. In the tolerant clone, antioxidant enzyme activities were not significantly changed by O₃ in the first season of fumigation (March to December 1990), not even during an episode when ambient O₃ concentrations reached 125 nl 1^?1 (240 nl 1^?1 in 2x ambient chambers). No foliar symptoms were observed on needles of the tolerant clone during this year. However, in the second year of fumigation (1992), O₃ fleck symptoms were observed on the tolerant clone and APOD activities were significantly increased in previous-year needles. The tolerant clone had SOD, APOD, and GPOD activities at least 40% higher than those of the sensitive clone before fumigation and 65, 178, and 119% higher, respectively, during both years of fumigation. The higher constitutive levels of these enzymes may have protected against foliar injury in 1990, however in 1992 we concluded that the stimulations in antioxidant enzyme activities observed in symptomatic branches of both clones were a consequence of O₃ injury. Total (intra- and extracellular) activities of the antioxidant enzymes did not appear to be good indicators of O₃ tolerance. Phenotypically, the O₃-tolerant clone was much more vigorous and in both years of fumigation, gas exchange rates were 30 to 71% higher than in the sensitive clone (P. D. Anderson, unpublished data). The greater vigor of the tolerant clone may allow more carbon allocation to protective and repair processes which include, but are not restricted to, the turnover of antioxidant enzymes and metabolites.     

Ozone Induced Carbon Dioxide Evolution in Tobacco Callus Cultures

Callus derived from Bel–W3 and Bel–B tobacco plants when exposed to ozone turned brown as a consequence of surface cell destruction. Ozone fumigations above a threshold concentration of 0.10 μl/1 for two hoars caused an increase in the rate of tissue carbon dioxide (CO₂) evolution. The maximum increase in CO₂ evolution was about 65 percent for both the ozone sensitive Bel–W3 and resistant Bel–B callus. However, the ozone dosage required to attain maximum increase in CO₂ evolution was approximately two times greater for the resistant variety. Callus cultures that grew roots were observed to be more resistant to ozone. The addition of the antioxidant N,N'dipnenyl–p–phenylenediamine (DPPD) m the nutrient medium retarded ozone induced CO₂ evolution.     

Internal conductance to CO2 transfer of adult Fagus sylvatica: Variation between sun and shade leaves and due to free-air ozone fumigation

Two separate objectives were considered in this study. We examined (1) internal conductance to CO₂ (g_i) and photosynthetic limitations in sun and shade leaves of 60-year-old Fagus sylvatica, and (2) whether free-air ozone fumigation affects g_i and photosynthetic limitations. g_i and photosynthetic limitations were estimated in situ from simultaneous measurements of gas exchange and chlorophyll fluorescence on attached sun and shade leaves of F. sylvatica. Trees were exposed to ambient air (1× O₃) and air with twice the ambient ozone concentration (2× O₃) in a free-air ozone canopy fumigation system in southern Germany (Kranzberg Forest). g_i varied between 0.12 and 0.24 mol m⁻² s⁻¹ and decreased CO₂ concentrations from intercellular spaces (C_i) to chloroplastic (C_c) by approximately 55 μmol mol⁻¹. The maximum rate of carboxylation (V_cmax) was 22–39% lower when calculated on a C_i basis compared with a C_c basis. g_i was approximately twice as large in sun leaves compared to shade leaves. Relationships among net photosynthesis, stomatal conductance and g_i were very similar in sun and shade leaves. This proportional scaling meant that neither C_i nor C_c varied between sun and shade leaves. Rates of net photosynthesis and stomatal conductance were about 25% lower in the 2× O₃ treatment compared with 1× O₃, while V_cmax was unaffected. There was no evidence that g_i was affected by ozone.     

Ozone tolerance and antioxidant enzyme activity in soybean cultivars

The current study confirmed earlier conclusions regarding differential ozone (O₃) tolerances of two soybean cultivars, Essex and Forrest, and evaluated antioxidant enzyme activities of these two varieties based on their performance under environmentally relevant, elevated O₃ conditions. The experiment was conducted in open-top chambers in the field during the 1994 and 1995 growing seasons. Exposure of plants to moderately high O₃ levels (62.9 nl l⁻¹ air, 2-year seasonal average) caused chlorophyll loss and increased membrane permeability when compared to control plants grown in charcoal filtered air (24.2 nl l⁻¹ air). The other effects of O₃ treatment were decrease in seed yield, loss of total sulfhydryl groups, reduction of soluble protein content, and increase in guaiacol peroxidase activity in leaves of both cultivars. The O₃-induced increase in guaiacol peroxidase activity was much smaller in cv. Essex leaflets. Cv. Essex had less leaf oxidative damage and smaller reduction in seed yield than cv. Forrest under elevated O₃ conditions. During ozonation, mature leaflets of the more O₃ tolerant cv. Essex had higher levels of glutathione reductase (30%), ascorbate peroxidase (13%), and superoxide dismutase (45%) activity than did mature leaflets of cv. Forrest. Cu,Zn-superoxide dismutase, which represented 95% of total superoxide dismutase activity in the two cultivars, appeared to be increased by O₃ exposure in the leaflets of O₃ tolerant cv. Essex but not in those of cv. Forrest. Cytosolic ascorbate peroxidase activity was also higher in leaflets of cv. Essex than in cv. Forrest regardless of O₃ level. Stromal ascorbate peroxidase and Mn-superoxide dismutase activity did not appear to be involved in the O₃ tolerance of the two soybean cultivars. This revised version was published online in June 2006 with corrections to the Cover Date.     

Inhibition of photosynthesis ofPopulus euramericana andHelianthus annuus by SO2, NO2 and O3

Populus euramericana cv. I-214 andHelianthus annuus L. cv. Russian Mammoth were exposed to various concentrations of O₃ SO₂ or NO₂ for 2 h in a cylindrical assimilation chamber. The threshold concentrations of air pollutants for inhibition of net photosynthesis differed between the two species and also between the pollutants tested. Furthermore, the lethal concentrations where the net photosynthetic rates were completely inhibited, also differed between species and between pollutants. For SO₂ and NO₂,P. euramericana was more tolerant photosynthetically thanH. annuus when related to the concentration of pollutants used during the experiment. However, when related to the cumulative uptake rate of each pollutant, the photosynthetic tolerance of the two species was similar. In contrast to the effects of SO₂ or NO₂, the influence of O₃ on net photosynthesis was quite different. The relative rates of net photosynthesis in both species showed the same linear relationship with O₃ concentration. However, the relationship between the relative rate of net photosynthesis and the cumulative uptake rate of O₃ differed between the two species, although it was linear in both cases.     

Biochemical Plant Responses to Ozone : III. Activation of the Defense-Related Proteins beta-1,3-Glucanase and Chitinase in Tobacco Leaves

A single pulse of O₃ (0.15 microliter per liter, 5 hours) induced β-1,3-glucanase and chitinase activities in O₃-sensitive and -tolerant tobacco (Nicotiana tabacum L.) cultivars. In the O₃-sensitive cultivar Bel W3, the response was rapid (maximum after 5 to 10 hours) and was far more pronounced for β-1,3-glucanase (40- to 75-fold) than for chitinase (4-fold). In the O₃-tolerant cultivar Bel B, β-1,3-glucanase was induced up to 30-fold and chitinase up to 3-fold under O₃ concentrations that did not lead to visible damage. Northern blot hybridization showed a marked increase in β-1,3-glucanase mRNA in cultivar Bel W3 between 3 and 24 hours following O₃ treatment, a transient induction in cultivar Bel B, and no change in control plants. The induction of β-1,3-glucanase and chitinase activities following O₃ treatment occurred within the leaf cells and was not found in the intercellular wash fluids. In addition, O₃ treatment increased the amount of the β-1,3-glucan callose, which accumulated predominantly around the necrotic spots in cultivar Bel W3. The results demonstrate that near-ambient O₃ levels can induce pathogenesis-related proteins and may thereby alter the disposition of plants toward pathogen attack.     

Growth and functional responses of different cultivars of Lotus corniculatus to aluminum and low pH stress

Aluminum toxicity is an important stress factor in acid soils. Growth, respiration and permeability properties of root cells were studied in five cultivars of Lotus corniculatus subjected to aluminum (Al) or low pH stress. The cultivars showed significant differences in root elongation under stress conditions, which correlated with changes in membrane potential (E_M) of root cortical cells. A pH drop from 5.5 to 4.0 resulted in significant membrane depolarization and root growth inhibition. The strongest inhibition was observed in cv. São Gabriel (33.6%) and least in cv. UFRGS (25.8%). Application of an extremely high Al concentration (2 mM) stopped the root growth in cv. INIA Draco, while inhibition in cv. UFRGS reached only 75%.The E_M values of cortical cells of Lotus roots varied between −115 and −144 mV. Treatment with 250 μM of AlCl₃ (pH 4) resulted in rapid membrane depolarization. The extent of the membrane depolarization ranged between 51 mV (cv. UFGRS) and 16 mV (cv. INIA Draco). The membrane depolarization was followed by a loss of K⁺ from Al-treated roots (2 mM Al) and resulted in a decrease of the diffusion potential (E_D). The total amount of K⁺ in Al-treated roots dropped from 31.4 to 16.8 μmol g⁻¹ FW in sensitive cv. INIA Draco, or from 26.1 to 22.7 μmol g⁻¹ FW in tolerant cv. UFGRS. The rate of root respiration under control conditions as well as under Al treatment was higher in cv. INIA Draco than in cv. UFRGS. Al-induced inhibition of root respiration was 21–34% of the control.     

Physiological responses of Fagus sylvatica L. exposed to low levels of ozone in open-top chambers

Four-year-old beech seedlings were fumigated with three levels of ozone for 2 consecutive years in open-top chambers. During the second growth season different physiological measurements were conducted before and during daily fumigation. A 25–40% decrease in net photosynthesis was seen during fumigation, whereas no differences were detected before fumigation in July. In August lasting effects in net photosynthesis were seen. The apparent quantum yield was decreased after fumigation. Stomatal conductance was generally decreased during fumigation, but transpiration was reduced relatively less than net photosynthesis indicating a lower water use efficiency of the trees exposed to ozone. Chlorophyll fluorescence (F_v/F_m) showed additive reductions in relation to ozone and light.     

The Regulation of Photosynthesis in Leaves of Field-Grown Spring Wheat (Triticum aestivum L., cv Albis) at Different Levels of Ozone in Ambient Air

Wheat (Triticum aestivum L. cv Albis) was grown in open-top chambers in the field and fumigated daily with charcoal-filtered air (0.015 microliters per liter O₃), nonfiltered air (0.03 microliters per liter O₃), and air enriched with either 0.07 or 0.10 microliters per liter ozone (seasonal 8 hour/day [9 am-5 pm] mean ozone concentration from June 1 until July 10, 1987). Photosynthetic ¹⁴CO₂ uptake was measured in situ. Net photosynthesis, dark respiration, and CO₂ compensation concentration at 2 and 21% O₂ were measured in the laboratory. Leaf segments were freeze-clamped in situ for the determination of the steady state levels of ribulose 1,5-bisphosphate, 3-phosphoglycerate, triose-phosphate, ATP, ADP, AMP, and activity of ribulose, 1,5-bisphosphate carboxylase/oxygenase. Photosynthesis of flag leaves was highest in filtered air and decreased in response to increasing mean ozone concentration. CO₂ compensation concentration and the ratio of dark respiration to net photosynthesis increased with ozone concentration. The decrease in photosynthesis was associated with a decrease in chlorophyll, soluble protein, ribulose bisphosphate carboxylase/oxygenase activity, ribulose bisphosphate, and adenylates. No decrease was found for triose-phosphate and 3-phosphoglycerate. The ratio of ATP to ADP and of triosephosphate to 3-phosphoglycerate were increased suggesting that photosynthesis was limited by pentose phosphate reductive cycle activity. No limitation occurred due to decreased access of CO₂ to photosynthetic cells since the decrease in stomatal conductance with increasing ozone concentration did not account for the decrease in photosynthesis. Ozonestressed leaves showed an increased degree of activation of ribulose bisphosphate carboxylase/oxygenase and a decreased ratio of ribulose bisphosphate to initial activity of ribulose bisphosphate carboxylase/oxygenase. Nevertheless, it is suggested that photosynthesis in ozone stressed leaves is limited by ribulose bisphosphate carboxylation possibly due to an effect of ozone on the catalysis by ribulose bisphosphate carboxylase/oxygenase.     

Effect of ozone on spore germination,spore production and biomass production in two <Emphasis Type="Italic">Aspergillus</Emphasis> species

The ability of ozone gas to reduce food spoilage is relatively well documented, but the developmental effects of the gas on food spoilage fungi are not well known. In this study two model aspergilli, Aspergillus nidulans and Aspergillus ochraceus were used to study the effects of ozone on spore germination, sporulation and biomass production. These effects were examined under three levels of ozone; two high level ozone exposures (200 and 300 μmol mol⁻¹) and one low level exposure (0.2 μmol mol⁻¹). The two species behaved noticeably different to each other. Ozone was more effective in reducing growth from spore inocula than mycelia. No spore production could be detected in A. nidulans exposed to continuous low level O₃, whereas the same treatment reduced spores produced in A. ochraceus by 94%. Overall the work suggests that ozone exposure is an effective method to prevent spread of fungal spores in a food storage situation.     

Activation of an oxidative burst is a general feature of sensitive plants exposed to the air pollutant ozone

Ozone exposure stimulates an oxidative burst in leaves of sensitive plants, resulting in the generation and accumulation of hydrogen peroxide (H₂O₂) in tobacco and tomato, and superoxide (O₂^–?) together with H₂O₂ in Arabidopsis accessions. Accumulation of these reactive oxygen species (ROS) preceded the induction of cell death, and both responses co‐occurred spatially in the periveinal regions of the leaves. Re‐current ozone exposure of the sensitive tobacco cv. Bel W3 in closed chambers or in the field led to an enlargement of existing lesions by priming the border cells for H₂O₂ accumulation. Open top chamber experiments with native herbaceous plants in the field showed that Malva sylvestris L. accumulates O₂^–? at those sites that later exhibit plant cell death. Blocking of ROS accumulation markedly reduced ozone‐induced cell death in tomato, Arabidopsis and M. sylvestris. It is concluded that ozone triggers an in planta generation and accumulation of H₂O₂ and/or O₂^–? depending on the species, accession and cultivar, and that both these reactive oxygen species are involved in the induction of cell death in sensitive crop and native plants.