Ozone flux in Glycine max (L.) Merr.: Sites of regulation and relationship to leaf injury

Summary Hood and Dare cultivars of soybean, Glycine max (L.) Merr., vary in their foliar response to ozone. The physiological basis of this variation was investigated as a function of leaf age through an analysis of ozone flux data, leaf developmental morphology, and analogue modelling techniques. At all concentrations (0.25–0.58 l l^-1) and exposure times (1–4 h), resistance to O₃ flux in the gas phase of the diffusive pathway (i.e., boundary layer and stomate) did not account fully for variation in pollutant uptake rates into the leaf interior. Ozone molecules experienced a residual resistance to diffusion that is not shared by effluxing water vapor molecules. Residual resistance to O₃ flux increased with pollutant concentration and exposure time and was associated with age-dependent differences in foliar O₃ response. Leaf morphology data, including stomatal frequency and the ratio of internal to external surface area, did not help explain cultivar or age-dependent differences in O₃ flux. The extent of foliar injury was not consistently related to the magnitude of O₃ flux into the leaf interior. An analysis of the residual resistance to O₃ flux suggests that the gas and liquid phase pathways for O₃, water vapor, and carbon dioxide are not identical.     

Different Effect of Humidity on Growth and Salt Tolerance of Two Soybean Cultivars

Two soybean (Glycine max (L.) Merr.) cultivars, Tachiyutaka and Dare, were grown in pots at 30 and 70 % relative humidity (RH) and treated with 0 (control), 40 (moderate), 80 and 120 (severe) mM NaCl for 3 weeks. Increasing RH enhanced growth of salt sensitive cultivar, Tachiyutaka, but had no effect on salt tolerant cultivar, Dare, under control and moderate saline conditions. Both cultivars benefited from elevated humidity under severe saline conditions. Cultivar Tachiyutaka had poorer ability for controlling translocation of Na⁺ to the leaves, lower Na⁺ exclusion ability in the roots, and lower root activity under NaCl treatment, compared with cv. Dare. The increased growth of cv. Tachiyutaka at high RH was consistent with decreased Na⁺ accumulation in the leaves, increased stomatal conductance and root activity, while the unchanged growth of cv. Dare was consistent with similar Na⁺ accumulation in the leaves, and the decreased root activity.     

Ozone uptake and its effect on photosynthetic parameters of two tobacco cultivars with contrasting ozone sensitivity

Leaf discs of the ozone tolerant tobacco (Nicotiana tabacum L.) cv. Bel B and of the ozone sensitive cv. Bel W3, were exposed to an acute ozone fumigation (300 ppb) for 3 h. We measured ozone uptake by leaves and physiological characteristics before, during and after the treatment, in order to determine if the different O₃ sensitivity was correlated to the leaf uptake. In the tolerant cv. Bel B, O₃ uptake was high during the first 2 h of ozone exposure and then decreased. In the sensitive cv. Bel W3, the rate of O₃ uptake decreased constantly during ozone fumigation. The estimated cumulative uptake over the treatment time was higher (200 ± 30 μmol m^–2) in Bel B than in Bel W3 (130 ± 12 μmol m^–2). Thus, the ozone sensitivity was not correlated with ozone uptake. Stomatal conductance and photosynthesis were significantly inhibited during the fumigation in both cultivars. However, these reductions were strong and irreversible in the cv. Bel W3, while in the cv. Bel B both parameters recovered in the post-fumigation period. Thus, ozone tolerance may be related to a sustained capacity of recovery. There was no linear correlation between ozone uptake and photosynthesis reduction, but a threshold of ozone uptake was found after which photosynthesis was substantially impaired. This threshold may or may not be reached under the same external ozone level, indicating that the AOT40 may not be a sufficiently accurate index for the detection of ozone damage in plants.     

Differential responses in two varieties of winter wheat to elevated ozone concentration under fully open‐air field conditions

Two modern cultivars [Yangmai16 (Y16) and Yangfumai 2 (Y2)] of winter wheat (Triticum aestivum L.) with almost identical phenology were investigated to determine the impacts of elevated ozone concentration (E‐O₃) on physiological characters related to photosynthesis under fully open‐air field conditions in China. The plants were exposed from the initiation of tillering to final harvest, with E‐O₃ of 127% of the ambient ozone concentration (A‐O₃). Measurements of pigments, gas exchange rates, chlorophyll a fluorescence and lipid oxidation were made in three replicated plots throughout flag leaf development. In cultivar Y2, E‐O₃ significantly accelerated leaf senescence, as indicated by increased lipid oxidation as well as faster declines in pigment amounts and photosynthetic rates. The lower photosynthetic rates were mainly due to nonstomatal factors, e.g. lower maximum carboxylation capacity, electron transport rates and light energy distribution. In cultivar Y16, by contrast, the effects of E‐O₃ were observed only at the very last stage of flag leaf ageing. Since the two cultivars had almost identical phenology and very similar leaf stomatal conductance before senescence, the greater impacts of E‐O₃ on cultivars Y2 than Y16 cannot be explained by differential ozone uptake. Our findings will be useful for scientists to select O₃‐tolerant wheat cultivars against the rising surface [O₃] in East and South Asia.     

Responses of transgenic poplar (Populus tremula x P. alba) overexpressing glutathione synthetase or glutathione reductase to acute ozone stress: visible injury and leaf gas exchange

Untransformed hybrid poplar (Populus tremula x P. alba) and transgenic lines overexpressing glutathione synthetase (GshS) in the cytosol (200-300-fold) or glutathione reductase (GR) either in the cytosol 5-fold) or in the chloroplast (150-200-fold) were exposed to 0 (control), 100, 200 or 300 nl l^-1 ozone for 3 d for 7 h d^-1. Following acute ozone stress treatments, wild-type and transgenic poplar suffered from visible foliar injury consisting of dark brown necrotic lesions on the laminae. Necrotic lesions were sharply separated from photosynthetically active cells by a band of red-violet discoloured cell lines showing yellow autofluorescence by blue light, and blue autofluorescence by UV-light excitation. When plants were exposed to 100 nl l^-1 ozone, leaf injury was in general negligible, but when 200 and 300 nl l^-1 ozone was applied, in both untransformed poplar and transgenic lines overexpressing GshS or GR up to 60% and 80%, respectively, visible injury developed on mature leaves. The mean percentage of injured leaf area amounted to 20-30% (200 nl l^-1) and 40-60% (300 nl l^-1). Irrespective of transformation, young leaves of poplar trees were only slightly affected by ozone treatments. In support of these observations, net CO2 assimilation rates of mature leaves were decreased by up to 65% (300 nl l^-1 ozone) in wild-type and transformed poplar, whereas net photosynthesis of young leaves remained unaffected even under severe stress conditions. Leaf conductance was significantly decreased by all ozone treatments, but was in the same range in young and mature leaves, and in wild-type and transformed poplar, pre- and post-exposure to ozone. It can therefore be assumed that the ozone doses effectively taken up into the leaf tissue were not dependent on leaf development and that the strength of the ozone stress exerted was similar in all types of poplar trees investigated in this study.From these data it is concluded that: (i) elevated foliar activities of glutathione synthetase or glutathione reductase alone are not sufficient to improve tolerance of hybrid poplar to acute ozone stress, and (ii) the sensitivity of poplar leaves to acute ozone stress is controlled by unknown factors closely related to leaf development rather than by foliar activities of glutathione synthetase and glutathione reductase, or leaf conductance.     

Interactive effects of ozone and drought stress on pigments and activities of antioxidative enzymes in Pinus halepensis

The aim of this study was to investigate the interactive effects of ozone (O₃) and drought on pigments and antioxidant enzymes of Aleppo pine (Pinus halepensis). Two‐year‐old seedlings were exposed in open‐top chambers to charcoal‐filtered air or non‐filtered air plus an additional 40 nL L^?1 of ozone. After 20 months of O₃ exposure, a subset of plants was subjected to drought stress by withholding water supply for 11 d. Ozone induced higher guaiacol peroxidase, catalase and KCN‐resistant superoxide dismutase (SOD) activities in young needles, while drought stress increased glutathione reductase and CuZnSOD. One‐year‐old needles showed lower capacity to activate these enzymes in response to stress. Both ozone and drought activated the xanthophyll cycle pool and reduced chlorophyll contents in both current and 1‐year‐old needles. The combined effects of ozone and drought decreased antioxidant enzyme activities and the capacity of recovering after re‐watering. Similarly, interactive effects of O₃ and drought reduced xanthophyll‐mediated photoprotection capacity in 1‐year‐old needles but induced a higher conversion of the cycle in current‐year needles. These results showed that ozone modified the Aleppo pine response to drought stress, suggesting that this pollutant might be reducing the ability of this species to withstand other environmental stresses.     

Quantification of ozone influx and apoplastic ascorbate content in needles of Norway spruce trees (Picea abies L., Karst) at high altitude

The objective of the present study was to investigate whether peak concentrations of ozone can deplete the apoplastic ascorbate pool of needles from Norway spruce trees (Picea abies L. Karst.) and, thereby, contribute to damage to forest trees. Twigs of forest trees grown at high altitude (1950m above sea level; Mt Patscherkofel, Austria) were enclosed in situ in chambers and fumigated for 5-5 or 17 h with ozone concentrations ranging from 60 to 798 nmol mol^?1. Adjacent branches were fumigated with filtered air. Ozone influx into the foliage ranging from 1-7 to 17nmolm^?2s^?1 had little effect on whole-needle ascorbate or glutathione contents. However, apoplastic ascorbate decreased by about 30% when the needles were exposed to environmentally relevant ozone concentrations and increased about 3-fold at higher ozone concentrations. This response suggests the induction of ascorbate as a protective system and may also be important under field conditions. Needles of spruce trees from high altitude that were exposed to chronically increased ozone concentrations contained significantly higher apoplastic ascorbate concentrations than needles from spruce trees from lower altitudes with lower mean atmospheric ozone concentrations. The results show that peak concentrations of ozone do not act in spruce via a depletion of the apoplastic ascorbate pool.     

20-HETE Mediates Ozone-Induced,Neutrophil-Independent Airway Hyper-Responsiveness in Mice

Background

Ozone, a pollutant known to induce airway hyper-responsiveness (AHR), increases morbidity and mortality in patients with obstructive airway diseases and asthma. We postulate oxidized lipids mediate in vivo ozone-induced AHR in murine airways.

Methodology/Principal Findings

Male BALB/c mice were exposed to ozone (3 or 6 ppm) or filtered air (controls) for 2 h. Precision cut lung slices (PCLS; 250 µm thickness) containing an intrapulmonary airway (∼0.01 mm² lumen area) were prepared immediately after exposure or 16 h later. After 24 h, airways were contracted to carbachol (CCh). Log EC₅₀ and E_max values were then calculated by measuring the airway lumen area with respect to baseline. In parallel studies, dexamethasone (2.5 mg/kg), or 1-aminobenzotriazol (ABT) (50 mg/kg) were given intraperitoneal injection to naïve mice 18 h prior to ozone exposure. Indomethacin (10 mg/kg) was administered 2 h prior. Cell counts, cytokine levels and liquid chromatography-mass spectrometry (LC-MS) for lipid analysis were assessed in bronchoalveolar lavage (BAL) fluid from ozone exposed and control mice. Ozone acutely induced AHR to CCh. Dexamethasone or indomethacin had little effect on the ozone-induced AHR; while, ABT, a cytochrome P450 inhibitor, markedly attenuated airway sensitivity. BAL fluid from ozone exposed animals, which did not contain an increase in neutrophils or interleukin (IL)-6 levels, increased airway sensitivity following in vitro incubation with a naïve PCLS. In parallel, significant increases in oxidized lipids were also identified using LC-MS with increases of 20-HETE that were decreased following ABT treatment.

Conclusions/Significance

These data show that ozone acutely induces AHR to CCh independent of inflammation and is insensitive to steroid treatment or cyclooxygenase (COX) inhibition. BAL fluid from ozone exposed mice mimicked the effects of in vivo ozone exposure that were associated with marked increases in oxidized lipids. 20-HETE plays a pivotal role in mediating acute ozone-induced AHR.     

Species- and age-dependent sensitivity to ozone in young plants of pea,wheat and spinach: Effects on acyl lipid and pigment content and metabolism

Acyl lipids and pigments were analyzed in young plants of garden pea, spring wheat and spinach exposed to < 5 or 65 nl l^?1 ozone 12 h per day for 6 days. In one set of experiments, the plants were exposed to ¹⁴CO₂ for 2 h 3 days prior to ozone exposure. The plants responded differently to the moderately enhanced level of ozone used Spinach was not at all sensitive while in both pea and wheat, leaves of different ages differed in ozone sensitivity. In pea, ozone sensitivity increased with leaf age. In the second and third oldest leaves, the amounts of galactolipids per leaf area and the proportions of 18:3 of the total lipid extract and of phosphatidylglycerol decreased. In the second oldest leaf, ozone also caused a decreased proportion of 18:3 of monogalactosyldiacylglycerol. In the fourth oldest leaf, lipid composition and galactolipid unsaturation was unaffected, but ozone caused decreased leaf expansion resulting in increased acyl lipid content per leaf area. In both the first and second leaves of wheat, ozone fumigation caused a marked decrease in the content of monogalactosyldiacylglycerol and in the first leaf, the contents of phosphatidylcholine and phosphatidylethanolamine increased. The proportion of 18:3 in phosphatidylcholine was larger in ozone-fumigated than in control plants, while the reverse applied for phosphatidylglycerol. In the oldest sampled leaves of pea and wheat, ozone caused an increase in the radioactivity associated with β-carotene, indicating increased turnover. Thus, while spinach was unaffected, in both pea and wheat ozone caused a decrease in the proportion of chloroplast membrane lipids to non-chloroplast membrane lipids in older leaves while younger leaves were less sensitive.     

Effects of Ozone Exposure on Resistance of Wheat Genotypes to Pyrenophora tritici-repentis

Three spring wheat genotypes, susceptible, moderately resistant or resistant to Pyrenophora tritici-repentis (tan spot fungus) were exposed to charcoal-filtered air and to approx. 80, 160, 240 (g m^?3 ozone for five consecutive days (7 h per day). Visible leaf injury on seedling plants (three-leaf stage) was only observed after fumigation with 160 or 240 (g m^?3 O₃. Amount of injury was four-fold and 10-fold on the susceptible genotype when compared to resistant or moderately resistant genotype at the two highest concentration of ozone, respectively. Genotypic differences to O₃ tolerance were detected at the seedling growth stage (three-leaf stage) and flowering stage but not at the stem elongation stage. A significant increase in tan spot lesion area was observed only on O₃ predisposed second top most leaves of the susceptible genotype at all the three levels of ozone. Predisposition did not enhance tan spot development in resistant and moderately resistant genotypes. In a test with 12 wheat genotypes, a highly significant positive correlation (r = 0· 986, p < 0· 0001) was observed between ozone sensitivity (percent leaf area damaged due to 240 (g m^?3 ozone exposure) and tan spot development (mm² lesion area) following inoculation with P. tritici-repentis. It indicates that wheat genotypes resistant to the tan spot fungus might be tolerant to ozone damage.     

EFFECTS OF ULTRAVIOLET-B RADIATION ON FUNGAL DISEASE DEVELOPMENT IN CUCUMIS SATIVUS

Three cucumber (Cucumis sativus L.) cultivars were exposed to a daily dose of 11.6 kJ m^-2biologically effective ultraviolet-B (UV-B_Be) radiation in an unshaded greenhouse before and/ or after infection by Colletotrichum lagenarium (Pass.) Ell. and Halst. or Cladosporium cucumerinum Ell. and Arth. and analyzed for disease development. Two of these cultivars, Poinsette and Calypso Hybrid, were disease resistant, while the third cultivar, Straight-8, was disease susceptible. Preinfectional treatment of 1 to 7 days with UV-B_BE in Straight-8 led to greater severity of both diseases. Postinfectional UV treatment did not lead to increased disease severity caused by C. lagenarium, while preinfectional UV treatment in both Straight-8 and Poinsette substantially increased disease severity. Although resistant cultivars Poinsette and Calypso Hybrid showed increased anthracnose disease severity when exposed to UV-B, this effect was apparent only on the cotyledons. Both higher spore concentration and exposure to UV-B radiation resulted in greater disease severity. Of the cucumber cultivars tested for UV-B sensitivity, growth in Poinsette was most sensitive and Calypso Hybrid was least sensitive. These preliminary results indicate that the effects of UV-B radiation on disease development in cucumber vary depending on cultivar, timing and duration of UV-B exposure, inoculation level, and plant age.     

Tropospheric ozone-induced structural changes in leaf mesophyll cell walls in grapevine plants

The structural changes in leaves of grapevine plants (Vitis vinifera L.) exposed to different ozone concentrations were investigated. Ozone fumigations were performed in open-top chambers at four different ozone levels (charcoal-filtered air (F), ambient air (N), ambient air + 25 mm³m⁻³ ozone (O-25) and ambient air + 50 mm³m⁻³ ozone (O-50)). The leaves of plants from chambers with increased ozone concentrations (O-25 and O-50) were significantly thicker than the controls (F), owing to increased thickness of the mesophyll layer. Observing O-50 leaves, it was found that the mesophyll cell wall displayed structural changes. In some places cell wall thickness increased up to 1 μm. We found callose deposits on the inner side of the cell walls of mesophyll cells. These data are in accord with the concept that the mesophyll cell wall acts as a barrier against the penetration of tropospheric ozone into the cells.     

Ozone sensitivity of four Pakchoi cultivars with different leaf colors: physiological and biochemical mechanisms

Ozone (O₃) is important air pollutant inducing severe losses of horticultural production. Cultivars of the same species, but with different leaf colors, may differ in their ozone sensitivity. However, it has not been clarified yet if different leaf coloration influences such a sensitivity. In this study, two purple-leafed and two green-leafed cultivars of Pakchoi were selected for ozone fumigation (240 ± 20 nmol mol^–1, 09:00–16:00 h). Elevated O₃ decreased chlorophyll content, increased anthocyanin (Ant) content, damaged cell membrane integrity, enhanced antioxidative enzyme activities, depressed photosynthetic rate (P _N) and stomatal conductance (g _s), inhibited maximal quantum yield (F_v/F_m) and effective quantum yield [Y_II] of PSII photochemistry, and caused visible injury. Purple-leafed cultivars with higher Ant contents were more tolerant than green-leafed cultivars as indicated by lower relative enhancement in malondialdehyde content and lower relative losses in P _N, g _s, F_v/F_m, and Y_II. The higher ability to synthesize Ant in the purple-leafed cultivars contributed to their higher photoprotective ability.     

Contrasting effects of tropospheric ozone on five native herbs which coexist in calcareous grassland

The aim of this study was to examine the effects of increased tropospheric ozone concentrations on the growth and morphology of five native herbs commonly found to coexist in calcareous grassland in areas of Britain and continental Europe: Anthyllis vulneraria L., Cirsium acaule (L.) Scop., Festuca ovina L., Pilosella offtcinarum F. Shultz & Shultz-Bip and Lotus comiculatus L. In a chronic fumigation (mean O₃ concentration of 71 ppb (71 nl 1^?1) for 7 h d^?1 AOT40 4585 ppb-h) which lasted for 21 d, the effects of ozone were assessed using classical growth analysis. Large reductions in mean relative growth rates for shoot and root weight and root length were observed for the two legumes (Fabaceae) Lotus corniculatus and Anthyllis vulneraria, although these were only statistically significant for Lotus corniculatus. Significant reductions in specific root length (length per unit dry weight) were found for Cirsium acaule and Pilosella officinarum (Asteraceae), while for Festuca ovina (Poaceae) the allometric coefficient was reduced significantly following exposure to ozone. An acute fumigation (mean O₃ concentration of 196 ppb, 7 h) resulted in a range of visible injury, from no injury (Festuca ovina and Pilosella officinarum) through moderate levels of injury (Cirsium acaule and Lotus corniculatus) to extensive and widespread injury (Anthyllis vulneraria). Scoring of visible damage showed that this was only statistically significant for the two legumes, Lotus corniculatus and Anthyllis vulneraria. These results suggest that native herbs may differ in their sensitivity to tropospheric ozone. Both chronic and acute exposures revealed that members of the Fabaceae may be most sensitive to ozone pollution, but the study also suggests that subtle changes in root morphology occurred for members of the Asteraceae. These findings are discussed in relation to the critical levels of ozone set recently for plants and the implications of increasing tropospheric ozone for the conservation of native plant communities.     

Expression of arabidopsis cytosolic ascorbate peroxidase gene in response to ozone or sulfur dioxide

The effects of ozone or sulfur dioxide on antioxidant enzymes were investigated in Arabidopsis thaliana. Plants were fumigated with 0.1–0.15 ppm ozone or sulfur dioxide up to about 1 week in an environment-controlled chamber. Both pollutants increased the activities of ascorbate peroxidase and guaiacol per-oxidase in leaves, but had little effect on the activities of superoxide dismutase, catalase, monodehydroascorbate reductase, dehydroascorbate reductase or glutathione reductase. Ozone was more effective than sulfur dioxide in increasing the activities of the peroxidases. Ascorbate peroxidase activity increased 1.8-fold without a lag period during fumigation with 0.1 ppm ozone, while guaiacol peroxidase activity increased 4.4-fold with a 1-day lag. Expression of the APX1 gene encoding cytosolic ascorbate peroxidase was further investigated. Its protein levels in leaves exposed to 0.1 ppm ozone for 4 or 8 days were 1.5-fold higher than in controls. Both ozone and sulfur dioxide elevated APX1 mRNA levels in leaves at 4 and 7 days, whereas at 1 day only ozone was effective. The induction of APX1 mRNA levels by ozone (3.4- to 4.1-fold) was more prominent than that by sulfur dioxide (1.6-to 2.6-fold). The APX1 mRNA level increased by day and decreased by night. Exposure of plants to 0.1 ppm ozone enhanced the APX1 mRNA level within 3 h, which showed a diurnal rhythm similar to that of the control. These results demonstrate that near-ambient concentrations of ozone as well as similar concentrations of sulfur dioxide can induce APX1 gene expression in A. thaliana.Environmental Biology Division     

Variations in ozone-sensitivity in relation to cell cycle and ploidy in Saccharomyces cerevisiae

Summary Cyclic variations in resistance to ozone-inactivation were observed during the cell cycle of a haploid strain of Saccharomyces cerevisiae. The lag and G₁ phases showed the highest sensitivity to ozone whereas there was a progressive increase in resistance from the S to G₂ phases as the proportion of dividing cells increased. A related series of S. cerevisiae varying in ploidy from haploid to tetraploid was exposed to ozone. The results demonstrated clearly an enhancement of ozone resistance directly related to the ploidy of the cells.Taken together these data suggest that resistance to ozone is dependent upon the action of an efficient repair mechanism. Similarities and differences between the biological action of ozone and ionizing radiations in S. cerevisiae are discussed.     

Interaction of ozone pollution and light effects on photosynthesis in a forest canopy experiment

Ozone pollution may reduce net carbon gain in forests, yet data from mature trees are rare and the effects of irradiance on the response of photosynthesis to ozone remain untested. We used an open-air system to expose 10 branches within the upper canopy of an 18-m-tall stand of sugar maple (Acer saccharum Marsh.) to twice-ambient concentrations of ozone (95nmol mol^?1, 0900 to 1700, 1 h mean) relative to 10 paired, untreated controls (45nmol mol^?1) over 3 months. The branch pairs were selected along a gradient from relatively high irradiance (PPFD 14.5 mol m^?2 d^?1) to deep shade (0.7mol m^?2 d^?1). Ozone reduced light-saturated rates of net photosynthesis (A_sat) and increased dark respiration by as much as 56 and 40%, respectively. Compared to sun leaves, shade leaves exhibited greater proportional reductions in A_sat and had lower chlorophyll concentrations, quantum efficiencies, and leaf absorptances when treated with ozone relative to controls. With increasing ozone dose over time, A_sat became uncoupled from stomatal conductance as ratios of internal to external concentrations of carbon dioxide increased, reducing water-use efficiency. Ozone reduced net photosynthesis and impaired stomatal function, with these effects depending on the irradiance environment of the canopy leaves. Increased ozone sensitivity of shade leaves compared to sun leaves has consequences for net carbon gain in canopies.     

The effects of ozone on growth and stomatal response in the F<Subscript>2</Subscript> generation of hybrid poplar (<Emphasis Type="Italic">Populus trichocarpa × Populus deltoides</Emphasis>)

Thirty-six F₂ hybrid poplar (Populus trichocarpa × P. deltoides) clones were fumigated with ozone to record its effects on growth, correlate them with stomatal response and screen for ozone sensitivity. Fumigation was applied for 6 to 9 h each day for approximately 3 months at ozone concentrations of 85 to 128 μg g⁻¹ using open-top chambers. Height, diameter, number of leaves, stomatal conductance, transpiration rate, total biomass, biomass components and root/shoot ratios were reduced by ozone stress. Percent of leaf fall in ozone-treated plants was nearly three times higher than in control plants exposed to charcoal-filtered air. Leaf senescence, because of ozone exposure, did not appear to be associated with reduced biomass production. Some clones had a high percentage of leaf-fall with ozone exposure, but were able to maintain total biomass production near that of the control. Their response may be an example of an ability to adjust or compensate for ozone damage. There was no significant or consistent relationship between stomatal conductance and total biomass or the change in stomatal conductance as a result of ozone exposure and the change in total biomass. Taken together, these results suggest that effects of ozone on poplar growth cannot be solely correlated to changes in stomatal conductance, more physiological and biochemical parameters should be examined.     

Has the sensitivity of soybean cultivars to ozone pollution increased with time? An analysis of published dose–response data

The rising trend in concentrations of ground‐level ozone (O₃) – a common air pollutant and phytotoxin – currently being experienced in some world regions represents a threat to agricultural yield. Soybean (Glycine max (L.) Merr.) is an O₃‐sensitive crop species and is experiencing increasing global demand as a dietary protein source and constituent of livestock feed. In this study, we collate O₃ exposure‐yield data for 49 soybean cultivars, from 28 experimental studies published between 1982 and 2014, to produce an updated dose–response function for soybean. Different cultivars were seen to vary considerably in their sensitivity to O₃, with estimated yield loss due to O₃ ranging from 13.3% for the least sensitive cultivar to 37.9% for the most sensitive, at a 7‐h mean O₃ concentration (M7) of 55 ppb – a level frequently observed in regions of the USA, India and China in recent years. The year of cultivar release, country of data collection and type of O₃ exposure used were all important explanatory variables in a multivariate regression model describing soybean yield response to O₃. The data show that the O₃ sensitivity of soybean cultivars increased by an average of 32.5% between 1960 and 2000, suggesting that selective breeding strategies targeting high yield and high stomatal conductance may have inadvertently selected for greater O₃ sensitivity over time. Higher sensitivity was observed in data from India and China compared to the USA, although it is difficult to determine whether this effect is the result of differential cultivar physiology, or related to local environmental factors such as co‐occurring pollutants. Gaining further understanding of the underlying mechanisms that govern the sensitivity of soybean cultivars to O₃ will be important in shaping future strategies for breeding O₃‐tolerant cultivars.