Phytotoxic Ozone Dose Research Articles

Trends in tropospheric ozone concentrations and forest impact metrics in Europe over the time period 2000\u20132014

In Europe, tropospheric ozone pollution appears as a major air quality issue, and ozone concentrations remain potentially harmful to vegetation. In this study we compared the trends of two ozone metrics widely used for forests protection in Europe, the AOT40 (Accumulated Ozone over Threshold of 40 ppb) which only depends on surface air ozone concentrations, and the Phytotoxic Ozone Dose which is the accumulated ozone uptake through stomata over the growing season, and above a threshold Y of uptake (PODY). By using a chemistry transport model, we found that European-averaged ground-level ozone concentrations (− 2%) and AOT40 metric (− 26.5%) significantly declined from 2000 to 2014, due to successful control strategies to reduce the emission of ozone precursors in Europe since the early 1990s. In contrast, the stomatal ozone uptake by forests increased from 17.5 to 26.6 mmol O3 m−2 despite the reduction in ozone concentrations, leading to an increase of potential ozone damage on plants in Europe. In a climate change context, a biologically-sound stomatal flux-based standard (PODY) as new European legislative standard is needed.

Grapevine and Ozone: Uptake and Effects

The grapevine (Vitis vinifera, L.) has been long since recognized as an ozone-sensitive plant. Ozone molecules can penetrate grapevine leaf tissues when the concentration of ozone in the atmosphere is high due to air pollution. This causes cell damage and interferes with photosynthetic mechanisms, subsequently slowing down plant growth and resulting in premature leaf senescence. Secondary effects include changes in biochemical processes that affect the chemical composition of the must and are likely to alter the quality of the wine. An experiment was conducted during two grapevine-growing seasons in 2010 and 2011 to gain knowledge of the effect of high ozone levels on the yield and on several biochemical characteristics of the plant which could influence the quality of the final product. These factors are economically important for agricultural production; this is especially true for Italy, which is one of the largest wine producers worldwide. The method used was a facility consisting of open top chambers operated at a vineyard in Angera (northern Italy). This facility permitted the study of the effects of different ozone levels. At the end of the experiment, the grapes were weighed and chemical analyses were carried out in order to understand the effects of ozone on the different characteristics of the grapes and on concentrations of several of its chemical substances. In particular, concentrations of yeast assimilable nitrogen, degrees Brix, pH, tartaric and malic acids, and polyphenols, including resveratrol, were considered, as these influence the quality of the wine. Parameters characterizing the different ozone levels were expressed in terms of ozone exposure (AOT40) and phytotoxic ozone dose (POD). The results showed that high ozone levels affect grapevine weight and thus its yield. In addition, the quality of the wine is affected by reductions of polyphenols which diminish the nutritional benefits of the product. In addition, these reductions cause the wine to have a more aggressive taste. These results emphasize the practical importance of the present study.

Large regional differences of soil water limitation effect on ozone induced yield loss for wheat and potato in Switzerland

The accumulated stomatal ozone (O3) uptake over a threshold (Phytotoxic Ozone Dose POD6), calculated by an ozone deposition model, has been shown to be the most appropriate metric to quantify negative effects of O3 on food crops. In this study we used data of 13 sites in different regions of Switzerland with multiple years of O3 measurements to quantify the stomatal O3 uptake and the related yield loss of wheat and potato. Flux patterns for different years were calculated with the DO3SE model to disentangle the influence of contrasting seasonal environmental conditions. Regional and inter-annual differences in meteorological conditions led to considerable variations in soil water conditions and the POD6 values for wheat. Potato stomatal uptake was much less influenced by soil water and showed a more even distribution of POD6 values across sites and years. The estimated nationally and temporally average yield loss was 3.2 ± 1.2% for wheat and 2.4 ± 0.8% for potato, calculated based on an area weighting.It was found that soil water deficit, observed frequently in the western part of Switzerland, had a large attenuation effect on stomatal O3 uptake by wheat and on corresponding yield losses. This highlights the importance of including soil moisture limitation in O3 uptake modelling even in moist climatic regions. The comparison of modelled evapotranspiration with water flux measurements over a wheat field showed a reasonable agreement concerning the temporal pattern and the magnitude. But it is also concluded that the DO3SE soil moisture module will need further testing and adaptation to improve accuracy of the model in dryer conditions.

Impacts of current ozone pollution on wheat yield in China as estimated with observed ozone, meteorology and day of flowering

Wheat yield loss due to ambient ozone (O3) and the associated economic losses in China were evaluated for years 2015 and 2016 using for the first time measured O3 concentration ([O3]) and meteorological data as well as phenological observations. Relative yield losses (RYL) were estimated using response functions based on both O3 exposure and flux metrics: AOT40 (the accumulated hourly [O3] over a threshold of 40 ppb) and POD12 (Phytotoxic Ozone Dose over a threshold of 12 nmol O3 m−2 s−1). Both metrics showed consistent spatial patterns being highest in the North China Plain, the largest wheat-producing area in China, followed by some provinces in Yangtze River region. The higher stomatal conductance in areas with high [O3] in year 2015 contributed to higher POD12 values than in 2016. The RYLs for the whole China estimated with AOT40 were 17.1% and 18.1% for years 2015 and 2016, respectively, whereas those with POD12 were lower, 10.6% and 10.2%, respectively. Wheat production losses as translated to annual economic losses were 10 304 and 10 745 million USD (AOT40) or 5912 and 5554 million USD (POD12), for 2015 and 2016, respectively. The O3 impact estimated with monitoring data suggests its overestimation by some of the previous studies based on model estimates of surface [O3]. The overestimation could be attributed to the uncertainties in estimating meteorological variables and [O3], particularly the omission of the correction for the vertical gradient of [O3] near crop canopy in some studies.

Toward stomatal-flux based forest protection against ozone: The MOTTLES approach.

European standards for the protection of forests from ozone (O3) are based on atmospheric exposure (AOT40) that is not always representative of O3 effects since it is not a proxy of gas uptake through stomata (stomatal flux). MOTTLES "MOnitoring ozone injury for seTTing new critical LEvelS" is a LIFE project aimed at establishing a permanent network of forest sites based on active O3 monitoring at remote areas at high and medium risk of O3 injury, in order to define new standards based on stomatal flux, i.e. PODY (Phytotoxic Ozone Dose above a threshold Y of uptake). Based on the first year of data collected at MOTTLES sites, we describe the MOTTLES monitoring station, together with protocols and metric calculation methods. AOT40 and PODY, computed with different methods, are then compared and correlated with forest-health indicators (radial growth, crown defoliation, visible foliar O3 injury). For the year 2017, the average AOT40 calculated according to the European Directive was even 5 times (on average 1.7 times) the European legislative standard for the protection of forests. When the metrics were calculated according to the European protocols (EU Directive 2008/50/EC or Modelling and Mapping Manual LTRAP Convention), the values were well correlated to those obtained on the basis of the real duration of the growing season (i.e. MOTTLES method) and were thus representative of the actual exposure/flux. AOT40 showed opposite direction relative to PODY. Visible foliar O3 injury appeared as the best forest-health indicator for O3 under field conditions and was more frequently detected at forest edge than inside the forest. The present work may help the set-up of further long-term forest monitoring sites dedicated to O3 assessment in forests, especially because flux-based assessments are recommended as part of monitoring air pollution impacts on ecosystems in the revised EU National Emissions Ceilings Directive.

Ozone exposure- and flux-yield response relationships for maize

A stomatal ozone (O3) flux-response relationship for relative yield of maize was established by parameterizing a Jarvis stomatal conductance model. For the function (fVPD) describing the limitation of stomatal conductance by vapor pressure deficit (VPD, kPa), cumulative VPD during daylight hours was superior to hourly VPD. The latter function is proposed as a methodological improvement of this multiplicative model when stomatal conductance peaks during the morning and it is reduced later as it is the case of maize in this experiment. The model agreed relatively well with the measured stomatal conductance (R2 = 0.63). Based on the comparison of R2 values of the response functions, POD6 (Phytotoxic Ozone Dose over an hourly threshold 6 nmol m−2 s−1) and AOT40 (accumulated hourly O3 concentrations over a threshold of 40 ppb) performed similarly. The critical levels based on POD6 and AOT40 for 5% reduction in maize yield were 1.17 mmol m−2 PLA and 8.70 ppm h, respectively. In comparison with other important crops, the ranking of sensitivity of maize strongly differed depending on the O3 metric used, AOT40 or POD6. The newly proposed response functions are relevant for O3 risk assessment for this crop in Asia.

Ozone dose-response relationships for soil microbial dynamics of winter wheat in North China

Dynamic responses of soil microbial properties of winter wheat (Triticum aestivum L.) to elevated ozone concentration have never been quantified. In this study, the winter wheat was cultivated under two contrasting ozone treatments: non-filtered ambient air (hereinafter called NF) and elevated ozone (effective increase in 9-h mean ozone concentration of 34.91 ± 1.42 nL·L−1 (mean ± SE) above ambient, hereinafter called EO3). Linear regressions between the response ratio of each variable (i.e. EO3/NF) and the increments of exposure- or flux-based ozone indices (i.e. EO3 - NF) were conducted. The results showed that the response ratios of soil microbial biomass C and N, as well as cumulative CO2 and N2O effluxes significantly correlated with the increments of ozone indices. The ΔPOD0 (increment of integrated phytotoxic ozone dose with no threshold) and the ΔAOT40 (increment of accumulated hourly O3 concentrations over a threshold of 40 nL·L−1) performed better than ΔSUM06, ΔW126 and the increments of other flux-based ozone indices. They showed stronger linear relationships with soil microbial biomass than that with gas effluxes. The expanded deviations from the effect-free line indicated distinct cumulative detrimental ozone impacts on soil microbial properties. Ammonia nitrogen content and nitrite reductase activity showed non-significantly adaptive and self-adjustment responses to accumulative ozone stress. The ozone dose-response relationships obtained here will benefit the ecological modeling that evaluates the dynamic responses or predicts the feedback effects of sustainable agro-ecosystems under global climate change.

New Insights into Leaf Physiological Responses to Ozone for Use in Crop Modelling.

Estimating food production under future air pollution and climate conditions in scenario analysis depends on accurately modelling ozone (O3) effects on yield. This study tests several assumptions that form part of published approaches for modelling O3 effects on photosynthesis and leaf duration against experimental data. In 2015 and 2016, two wheat cultivars were exposed in eight hemispherical glasshouses to O3 ranging from 22 to 57 ppb (24 h mean), with profiles ranging from raised background to high peak treatments. The stomatal O3 flux (Phytotoxic Ozone Dose, POD) to leaves was simulated using a multiplicative stomatal conductance model. Leaf senescence occurred earlier as average POD increased according to a linear relationship, and the two cultivars showed very different senescence responses. Negative effects of O3 on photosynthesis were only observed alongside O3-induced leaf senescence, suggesting that O3 does not impair photosynthesis in un-senesced flag leaves at the realistic O3 concentrations applied here. Accelerated senescence is therefore likely to be the dominant O3 effect influencing yield in most agricultural environments. POD was better than 24 h mean concentration and AOT40 (accumulated O3 exceeding 40 ppb, daylight hours) at predicting physiological response to O3, and flux also accounted for the difference in exposure resulting from peak and high background treatments.

Growing season extension affects ozone uptake by European forests

Climate change significantly modifies terrestrial ecosystems and vegetation activity, yet little is known about how climate change and ozone pollution interact to affect forest health. Here we compared the trends of two metrics widely used to protect forests against negative impacts of ozone pollution, the AOT40 (Accumulated Ozone over Threshold of 40 ppb) which only depends on surface air ozone concentrations, and the POD (Phytotoxic Ozone Dose) which relies on the amount of ozone uptaken by plants through stomata. Using a chemistry transport model, driven by anthropogenic emission inventories, we found that European-averaged ground-level ozone concentrations significantly declined (−1.6%) over the time period 2000–2014, following successful control strategies to reduce the ozone precursors emission; as a consequence, the AOT40 metric declined (−22%). In contrast, climate change increased both growing season length (~7 days/decade) and stomatal conductance and thus enhanced the stomatal ozone uptake by forests (5.9%), leading to an overall increase of potential ozone damage on plants, despite the reduction in ozone concentrations. Our results suggest that stomatal-flux based strategies of forest protection against ozone in a changing climate require a proper consideration of the duration of the growing season with a better estimation of start and end of the growing season.

Nitrogen availability does not affect ozone flux-effect relationships for biomass in birch (Betula pendula) saplings

To investigate whether nitrogen (N) load affects the ozone (O3) stomatal flux-effect relationship for birch biomass, three-year old birch saplings were exposed to seven different O3 profiles (24 h mean of 35–66 ppb) and four different N loads (10, 30, 50 and 70 kg ha−1 yr−1) in precision-controlled hemispherical glasshouses (solardomes) in 2012 and 2013. Stomatal conductance (gs) under optimal growth conditions was stimulated by enhanced N supply but was not significantly affected by enhanced O3 exposure. Birch root, woody (stem + branches) and total biomass (root + woody) were not affected by the Phytotoxic Ozone Dose (POD1SPEC) after two seasons of O3 exposure, and enhanced N supply stimulated biomass production independent of POD1SPEC (i.e. there were no POD1SPEC × N interactions). There was a strong linear relationship between the stem cross-sectional area and tree biomass at the end of the experiment, which was not affected by O3 exposure or N load. Enhanced N supply stimulated the stem cross-sectional area at the end of season 2, but not at the end of season 1, which suggests a time lag before tree biomass responded to enhanced N supply. There was no significant effect of POD1SPEC on stem cross-sectional area after either the first or second growing season of the experiment. Contrasting results reported in the literature on the interactive impacts of O3 and N load on tree physiology and growth are likely due to species-specific responses, different duration of the experiments and/or a limitation of the number of O3 and N levels tested.

The role of environmental factors in ozone uptake of Pinus mugo Turra

Montane forests in the High Tatra Mountains are exposed to high ambient ozone (O 3 ) concentrations that may adversely affect the physiological processes and health of plants. This study presents the modelled results of the phytotoxic ozone dose (POD) for dwarf mountain pine ( P. mugo ) in 2016. POD metrics were calculated using the deposition model DO 3 SE, with O 3 concentration and meteorological data measured in three altitudinal zones. In addition, maximal stomatal conductance (G max ) derived from gasometric field measurement in local conditions was included in the model. Field measurements confirmed the robust performance of the DO 3 SE model for stomatal conductance (G sto ). The site-specific stomatal conductance response model was largely congruent with average values estimated by DO 3 SE, though differences in temporal G sto distribution were observed. We determined a moderate limitation of O 3 uptake due to environmental factors (f ENVI ) such as air temperature and relative humidity, solar radiation, and soil water availability. It appears that G max is more relevant for annual POD than f ENVI in the temperate mountain forest. The results indicate a high level of POD and O 3 uptake by P. mugo in the High Tatra Mountains, which corresponds with the O 3 -induced visible injury symptoms observed. We also identified visible O 3 injury in P. mugo needles, which was more prevalent among the two-year old needles than younger individuals.

Dose-response relationships for ozone effect on the growth of deciduous broadleaf oaks in mediterranean environment

This study presents a reanalysis of ozone (O3) exposure experiments performed on deciduous broadleaf oak species in the Mediterranean region and a proposal of critical levels to improve the O3 risk assessment in this area for these widely distributed forest species. Two experiments performed in Spain and Italy were considered, and the following 3 oak species were studied: Quercus pyrenaica, Q. faginea and Q. robur. All the experiments were performed with irrigated potted seedlings growing in Open-Top Chambers exposed to different O3 levels (with charcoal-filtered air as the control treatment) for two consecutive growing seasons. The Phytotoxic Ozone Dose above an instantaneous threshold of 1 nmol O3 m−2 s−1 (POD1) was calculated by applying a Jarvis type model for the estimation of the stomatal conductance (gs), and by adopting a big-leaf resistive scheme to account for the O3 deposition on the vegetation. Two parameterisations were used for the gs multiplicative model: one species-specific based on the “local” gs measurements performed during each experiment, and the other “generic” based on the “Deciduous Mediterranean broadleaf” parameterisation described in the Manual on Methodologies and Criteria for Mapping Critical Loads and Levels and Air Pollution Effects, Risks and Trends of the UN/ECE (CLRTAP, 2015). The two different parameterisations were used to derive dose-response functions and ozone critical levels for the biomass loss of the deciduous oak species. The dose-response functions for roots and total biomass were statistically significant, with both the parameterisations tested (p < 0.05). The O3 critical levels obtained indicate that deciduous broadleaf oaks in Mediterranean environment could be more tolerant to O3 than other European broadleaf species and that O3 is more harmful to the below-ground biomass of the plants rather than the above-ground biomass.

Increased nitrogen wet deposition triggers negative effects of ozone on the biomass production of Carpinus betulus L. young trees

During a two-year Open-Top Chamber experiment performed in 2012 and 2013, young trees of hornbeam (Carpinus betulus L.) were exposed to four levels of ozone (O3) (−40% of ambient O3 in charcoal-filtered OTCs, −5% in non-filtered OTCs, +30% and +75% in O3-enriched OTCs) and two levels of Nitrogen (N) wet deposition (CoN = tap water and +N = tap water + 70 kg N ha−1 yr−1).Stomatal conductance at leaf level and growth parameters (stem diameter and height) were measured during the two years of experimentation, while at the end of each growing season plants dry biomass partition between the roots and the stem was assessed.Hornbeam plants resulted highly tolerant to O3, which did not cause any significant negative effects on the biomass yield of plants grown in CoN conditions. Furthermore, the dose-response relationships based on the Phytotoxic Ozone Dose (POD1) indicated that the stem biomass increased under high levels of O3 (+1.48% per 1 mmol O3 m−2 absorbed by CoN plant), while root biomass was not affected.Nitrogen alone caused a general increase of total biomass in both years of the experiment (+14.8% and +38.1% on average in 2012 and 2013, respectively). However, in 2013 plants subject to the highest level of O3 showed an opposite behavior, with a −11% decrease of total biomass in +N conditions.Ozone and Nitrogen in combination caused a statistically significant negative effect on the stem biomass and stem diameter of plants. Dose-response relationships based on POD1 for plants in +N conditions revealed significant linear regressions for stem, roots and total biomass, which decreased by −1.04%, −0.80% and −0.92% for each mmol m−2 of O3 absorbed, respectively.These results show that on a long-term scale, an increase of N wet deposition could significantly modify C. betulus response to O3 stress, playing a potential role in the risk assessment of O3 impact on the growth of this species.

Ozone-induced stomatal sluggishness changes stomatal parameters of Jarvis-type model in white birch and deciduous oak.

Stomatal ozone flux is closely related to ozone injury to plants. Jarvis-type multiplicative model has been recommended for estimating stomatal ozone flux in forest trees. Ozone can change stomatal conductance by both stomatal closure and less efficient stomatal control (stomatal sluggishness). However, current Jarvis-type models do not account for these ozone effects on stomatal conductance in forest trees. We examined seasonal course of stomatal conductance in two common deciduous tree species native to northern Japan (white birch: Betula platyphylla var. japonica; deciduous oak: Quercus mongolica var. crispula) grown under free-air ozone exposure. We innovatively considered stomatal sluggishness in the Jarvis-type model using a simple parameter, s, relating to cumulative ozone uptake (defined as POD: phytotoxic ozone dose). We found that ozone decreased stomatal conductance of white birch leaves after full expansion (-28%). However, such a reduction of stomatal conductance by ozone fell in late summer (-10%). At the same time, ozone reduced stomatal sensitivity of white birch to VPD and increased stomatal conductance under low light conditions. In contrast, in deciduous oak, ozone did not clearly change the model parameters. The consideration of both ozone-induced stomatal closure and stomatal sluggishness improved the model performance to estimate stomatal conductance and to explain the dose-response relationship on ozone-induced decline of photosynthesis of white birch. Our results indicate that ozone effects on stomatal conductance (i.e. stomatal closure and stomatal sluggishness) are crucial for modelling studies to determine stomatal response in deciduous trees, especially in species sensitive to ozone.

Relationships of CO2 assimilation rates with exposure- and flux-based O3 metrics in three urban tree species

The relationships of CO2 assimilation under saturated-light conditions (Asat) with exposure- (AOTX, Accumulated Ozone exposure over a hourly Threshold of X ppb) and flux-based (PODY, Phytotoxic Ozone Dose over a hourly threshold Y nmol·m−2·s−1) O3 metrics was studied on three common urban trees, Fraxinus chinensis (FC), Platanus orientalis (PO) and Robinia pseudoacacia (RP). Parameterizations for a stomatal multiplicative model were proposed for the three species. RP was the species showing lower species-specific maximum stomatal conductance (gmax) and experiencing lower cumulative O3 uptake along the experiment, but in contrast it was the most sensitive to O3. PODY was slightly better than AOTX metric at estimating relative Asat (R-Asat)for PO and RB but not for FC. The best fittings obtained for the regressions between R-Asat and AOTX for FC, PO and RP were 0.904, 0.868, and 0.876, when the thresholds of X were 60ppb, 55ppb and 30ppb, respectively. However, AOT40 performed also well for all of them, with R2 always >0.83. For PODY, the highest R2 values for FC, PO and RB were 0.863, 0.897 and 0.911 at thresholds Y=7, 5 and 1nmolO3m−2s−1, respectively. Given the potentially higher O3 removal capacity of FC and PO by stomatal uptake and their lower sensitivity to this pollutant than RP, the former two species would be appropriate for urban gardens and areas where O3 levels are high. Parameterization and modeling of stomatal conductance for the main urban tree species may provide reliable estimations of the stomatal uptake of O3 and other gaseous pollutants by vegetation, which may support decision making on the most suitable species for green urban planning in polluted areas.

Modelling long-term impacts of changes in climate, nitrogen deposition and ozone exposure on carbon sequestration of European forest ecosystems

We modelled the effects of past and expected future changes in climate (temperature, precipitation), CO2 concentration, nitrogen deposition (N) and ozone (O3) exposure (phytotoxic ozone dose, POD) on carbon (C) sequestration by European forest ecosystems for the period 1900–2050. Tree C sequestration was assessed by using empirical response functions, while soil C sequestration was simulated with the process-based model VSD, combined with the RothC model. We evaluated two empirical growth responses to N deposition (linear and non-linear) and two O3 exposure relationships (linear function with total biomass or net annual increment). We further investigated an ‘interactive model’ with interactions between drivers and a ‘multiplicative model’, in which the combined effect is the product of individual drivers. A single deposition and climate scenario was used for the period 1900–2050. Contrary to expectations, growth observations at European level for the period 1950–2010 compared better with predictions by the multiplicative model than with the interactive model. This coincides with the fact that carbon responses in kgCha−1yr−1 per unit change in drivers, i.e. per °C, ppm CO2, kgNha−1yr−1 and mmolm−2yr−1 POD, are more in line with literature data when using the multiplicative model. Compared to 1900, the estimated European average total C sequestration in both forests and forest soils between 1950 and 2000 increased by 21% in the interactive model and by 41% in the multiplicative model, but observed changes were even higher. This growth increase is expected to decline between 2000 and 2050. The simulated changes between 1950 and 2000 were mainly due to the increase in both N deposition and CO2, while the predicted increases between 2000 and 2050 were mainly caused by the increase in CO2 and temperature, and to lesser extent a decrease in POD, counteracted by reduced N deposition.

Water stress mitigates the negative effects of ozone on photosynthesis and biomass in poplar plants

Tropospheric ozone (O3) pollution frequently overlaps with drought episodes but the combined effects are not yet understood. We investigated the physiological and biomass responses of an O3 sensitive hybrid poplar clone (‘546’) under three O3 levels (charcoal-filtered ambient air, non-filtered ambient air (NF), and NF plus 40 ppb) and two watering regimes (well-watered (WW) and reduced watering (RW), i.e. 40% irrigation) for one growing season. Water stress increased chlorophyll and carotenoid contents, protecting leaves from pigment degradation by O3. Impairment of photosynthesis by O3 was also reduced by stomatal closure due to water stress, which preserved light-saturated CO2 assimilation rate, and the maximum carboxylation efficiency. Water stress increased water use efficiency of the leaves while O3 decreased it, showing significant interactions. Effects were more evident in older leaves than in younger leaves. Water stress reduced biomass production, but the negative effects of O3 were less in RW than in WW for total biomass per plant. A stomatal O3 flux-based dose-response relationship was parameterized considering water stress effects, which explained biomass losses much better than a concentration-based approach. The O3 critical level of Phytotoxic Ozone Dose over a threshold of 7 nmol O3.m−2.s−1 (POD7) for a 4% biomass loss in this poplar clone under different water regimes was 4.1 mmol m−2. Our results suggest that current O3 levels in most parts of China threaten poplar growth and that interaction with water availability is a key factor for O3 risk assessment.

Testing a ratio of photosynthesis to O3 uptake as an index for assessing O3-induced foliar visible injury in poplar trees.

Visible foliar injury by ozone (ozone visible injury) is known as a biomarker to assess potential phytotoxicity of ozone. We investigated ozone visible injury in an ozone-sensitive poplar (Oxford clone) under a 2-year free-air controlled exposure (FACE) experiment and calculated three ozone indices (i.e., accumulative ozone exposure over 40ppb during daylight hours (AOT40), phytotoxic ozone dose above a flux threshold of 0nmolm-2s-1 (POD0), and the cumulative value of the ratio of hourly ozone uptake to net photosynthesis (ΣU/P n ) to assess the critical level (CL) at the time of the first symptom onset of ozone visible injury. We tested the hypothesis that ozone injury depends both on the amount of ozone entering a leaf and on the capacity for biochemical detoxification or repair with photosynthesis as a proxy. The CLs at the time of the first symptom onset of ozone visible injury were 19ppmh for AOT40, 26mmolm-2 for POD0, and 1.2molmol-1 for ΣU/P n in Oxford clone at the ozone FACE experiment. Our findings were then verified by 4-year observation-based data in central Italy on Oxford clone and white poplar (Populus alba L.). These observation-based data indicated that we found ozone visible injury in Oxford clone even though AOT40 was relatively low (11.7ppmh). On the other hand, when values of POD0 and ΣU/P n exceeded over the CLs, the occurrence of initial symptoms in Oxford clone was shown. White poplar did not show ozone visible injury. ΣU/P n of white poplar at the field sites reached ~1.0molmol-1 (less than the CL=1.2molmol-1, which was obtained from O3 FACE) during May-September, although the values of POD0 were relatively high in white poplar (44-47mmolm-2 during May-September). The result implies that ozone injury may have occurred in poplars when stomatal ozone flux exceeded the critical range of tolerance due to the assimilate shortage for repair and defense against ozone stress.

Ozone exposure- and flux-based response relationships with photosynthesis, leaf morphology and biomass in two poplar clones

Poplar clones 546 (P. deltoides cv. ‘55/56’×P. deltoides cv. ‘Imperial’) and 107 (P. euramericana cv. ‘74/76’) were exposed to five ozone concentrations in 15 open-top chambers (OTCs). Both ozone exposure (AOT40, Accumulation Over a Threshold hourly ozone concentration of 40ppb) and flux-based (POD7, Phytotoxic Ozone Dose above an hourly flux threshold of 7nmol O3 m−2 PLA (projected leaf area) s−1) response relationships were established with photosynthesis, leaf morphology and biomass variables. Increases in both metrics showed significant negative relationships with light-saturated photosynthesis rate, chlorophyll content, leaf mass per area, actual photochemical efficiency of PSII in the light and root biomass but not with stomatal conductance (gs), leaf and stem biomass. Ozone had a greater impact on belowground than on aboveground biomass. The ranking of these indicators from higher to lower sensitivity to ozone was: photosynthetic parameters, morphological index, and biomass. Clone 546 had a higher sensitivity to ozone than clone 107. The coefficients of determination (R2) were similar between exposure- and flux-based dose-response relationships for each variable. The critical levels (CLs) for a 5% reduction in total biomass for the two poplar clones were 14.8ppmh for AOT40 and 9.8mmol O3 m−2 PLA for POD7. In comparison, equivalent reduction occurred at much lower values in photosynthetic parameters (4ppmh for AOT40 and 3mmol O3 m−2 PLA for POD7) and LMA (5.8ppmh for AOT40 and 4mmol O3 m−2 PLA for POD7). While in recent decades different CLs have been proposed for several plant receptors especially in Europe, studies focusing on both flux-based dose-response relationships and CLs are still scarce in Asia. This study is therefore valuable for regional O3 risk assessment in Asia.

Quantification of ozone exposure- and stomatal uptake-yield response relationships for soybean in Northeast China

High ground-level O3 is a new threat to agricultural production in Northeast China with the increasing ambient O3 concentration. Little is known about its impacts on soybean production in this key agricultural region. Accumulated O3 exposure-response and stomatal O3 flux-response relationships were developed during two continuous growing seasons to evaluate O3-induced yield reduction of four typical soybean cultivars in Northeast China. Results showed that critical levels of AOT40 (accumulated hourly O3 concentrations over a threshold of 40nmol·mol−1), SUM06 (sum of all hourly average O3 concentrations over 0.06μmol·mol−1) and W126 (sum of O3 concentrations weighted by a sigmoidal function) in relation to 5% reduction in relative seed yield were 4.2, 7.6 and 6.8μmol·mol−1·h, respectively. The effect of O3 on plants was influenced by leaf position in canopy. An improved Jarvis stomatal conductance model including leaf (node) position fitted well with field measurements. The best linear relationship between stomatal O3 flux and relative soybean yield was obtained when phytotoxic ozone dose was integrated over a threshold of 9.6nmol·m−2·s−1 (POD9.6) to represent the detoxification capacity of soybean. POD9.6 and the commonly used POD6 in relation to 5% reduction in relative seed yield of soybean were 0.9mmol·m−2 and 1.8mmol·m−2, respectively. O3 concentrations above ~38nmol·mol−1 contributed to POD9.6 and caused seed yield loss in soybean. Current annual yield loss of soybean at ambient O3 was estimated to range between 23.4% and 30.2%. The O3 dose-response relationships and corresponding thresholds obtained here will benefit regional O3 risk assessment on soybean production in Northeast China.