The effects of tropospheric ozone on net primary productivity and implications for climate change

Ontogenetic and trans-generational dynamics of a vertically transmitted fungal symbiont in an annual host plant in ozone-polluted settings

Tropospheric ozone is an abiotic stress of increasing importance in the context of global climate change. This greenhouse gas is a potent phytotoxic molecule with demonstrated negative effects on crop yield and natural ecosystems. Recently, oxidative stress has been proposed as a mechanism that could regulate the interaction between cool-season grasses and Epichloë endophytes. We hypothesized that exposure of Lolium multiflorum plants, hosting endophytes to an ozone-polluted environment at different ontogenetic phases, would impact the trans-generational dynamics of the vertically transmitted fungal symbiont. Here, we found that the ozone-induced stress on the mother plants did not affect the endophyte vertical transmission but it impaired the persistence of the fungus in the seed exposed to artificial ageing. Endophyte longevity in seed was reduced by exposure of the mother plant to ozone. Although ozone exposure did not influence either the endophyte mycelial concentration or their compound defences (loline alkaloids), a positive correlation was observed between host fitness and the concentration of endophyte-derived defence compounds. This suggests that fungal defences in grass seeds were not all produced in situ but remobilized from the vegetative tissues. Our study reveals ozone trans-generational effects on the persistence of a beneficial symbiont in a host grass.

Emissions of non-methane volatile organic compounds from a landfill site in a major city of India: impact on local air quality

Emissions from landfills are a significant source of non-methane volatile organic compounds (NMVOCs) in urban environments. NMVOCs play an important role in atmospheric chemistry, and elevated concentrations of some compounds are responsible for air quality deterioration. This study is based on the measurements of a suite of 20 C₂–C₈ NMVOCs at 21 upwind and downwind sites of the largest landfill in western India. Ethane, ethylene and aromatics were the dominant compounds; the concentrations of BTEX in the downwind regions were up to three times higher than their concentrations at upwind sites. The emission ratios of BTEX and other NMVOCs were different from those for residential, commercial, and industrial sources characterizing the emissions from burning and decomposition of organic material. The slope of ΔToluene/ΔBenzene of 0.64 is about three times higher than that determined at the main road junctions of the city. Ranking by Prop-Equiv, the top NMVOCs were isoprene, cis-2-Butene, m + p-xylenes, propylene, ethylene and trans-2-Butene account for 72–75% of the total Prop-Equiv concentrations. Alkenes played the dominant role in ozone formation, followed by aromatic and alkane groups. In addition to landfill emissions, contributions from traffic-related emissions to ambient concentrations of aromatic VOCs were also significant at some sites. Although the experiment was not designed to characterize the emissions from a specific source, the analysis suggests the substantial contributions from both decomposition and burning of landfill materials. The main difficulty in characterizing VOC emissions from landfills is the spatial and temporal variability of emissions from a large area.

Urban Tree Health Classification Across Tree Species by Combining Airborne Laser Scanning and Imaging Spectroscopy

Declining urban tree health can affect critical ecosystem services, such as air quality improvement, temperature moderation, carbon storage, and biodiversity conservation. The application of state-of-the-art remote sensing data to characterize tree health has been widely examined in forest ecosystems. However, such application to urban trees has not yet been fully explored—due to the presence of heterogeneous tree species and backgrounds, severely complicating the classification of tree health using remote sensing information. In this study, tree health was represented by a set of field-assessed tree health indicators (defoliation, discoloration, and a combination thereof), which were classified using airborne laser scanning (ALS) and hyperspectral imagery (HSI) with a Random Forest classifier. Different classification scenarios were established aiming at: (i) Comparing the performance of ALS data, HSI and their combination, and (ii) examining to what extent tree species mixtures affect classification accuracy. Our results show that although the predictive power of ALS and HSI indices varied between tree species and tree health indicators, overall ALS indices performed better. The combined use of both ALS and HSI indices results in the highest accuracy, with weighted kappa coefficients (Kc) ranging from 0.53 to 0.79 and overall accuracy ranging from 0.81 to 0.89. Overall, the most informative remote sensing indices indicating urban tree health are ALS indices related to point density, tree size, and shape, and HSI indices associated with chlorophyll absorption. Our results further indicate that a species-specific modelling approach is advisable (Kc points improved by 0.07 on average compared with a mixed species modelling approach). Our study constitutes a basis for future urban tree health monitoring, which will enable managers to guide early remediation management.

Photosynthesis in a Changing Global Climate: Scaling Up and Scaling Down in Crops

Photosynthesis is the major process leading to primary production in the Biosphere. There is a total of 7000bn tons of CO2 in the atmosphere and photosynthesis fixes more than 100bn tons annually. The CO2 assimilated by the photosynthetic apparatus is the basis of crop production and, therefore, of animal and human food. This has led to a renewed interest in photosynthesis as a target to increase plant production and there is now increasing evidence showing that the strategy of improving photosynthetic traits can increase plant yield. However, photosynthesis and the photosynthetic apparatus are both conditioned by environmental variables such as water availability, temperature, [CO2], salinity, and ozone. The "omics" revolution has allowed a better understanding of the genetic mechanisms regulating stress responses including the identification of genes and proteins involved in the regulation, acclimation, and adaptation of processes that impact photosynthesis. The development of novel non-destructive high-throughput phenotyping techniques has been important to monitor crop photosynthetic responses to changing environmental conditions. This wealth of data is being incorporated into new modeling algorithms to predict plant growth and development under specific environmental constraints. This review gives a multi-perspective description of the impact of changing environmental conditions on photosynthetic performance and consequently plant growth by briefly highlighting how major technological advances including omics, high-throughput photosynthetic measurements, metabolic engineering, and whole plant photosynthetic modeling have helped to improve our understanding of how the photosynthetic machinery can be modified by different abiotic stresses and thus impact crop production.

Yield and economic losses in maize caused by ambient ozone in the North China Plain (2014-2017)

Maize is the second most important crop per harvested area in the world. The North China Plain (NCP) is a highly populated and relevant agricultural region in China, experiencing some of the highest ozone (O₃) concentrations worldwide. It produces ~24% of the total maize production of China in years 2014-2017. For these years, we used observational O₃ data in combination with geostatistic methods to estimate county-level production and economic losses due to O₃ in the NCP. AOT40 (accumulated ozone exposure over an hourly threshold of 40 ppb) values during the maize growing season (90 days before maturity) progressively increased in the four consecutive years: 13.7 ppm h, 15.4 ppm h, 16.9 ppm h and 22.7 ppm h. Mean relative yield losses were 8.2% in 2014, 9.2% in 2015, 10.4% in 2016 and 13.4% in 2017. These yield losses, derived from exposure-response functions, resulted in crop production losses of 530.3 × 10⁴ t, 617.8 × 10⁴ t, 713.8 × 10⁴ t, and 953.4 × 10⁴ t, as well as economic losses of 2343 million USD, 2672 million USD, 1887 million USD, and 2404 million USD from 2014 to 2017. The NCP is a key area in China for monitoring the effectiveness of the clean air action policies aiming at reducing emissions of air pollutants. Despite these measures, O₃ concentrations have increased in NCP, and reduction of this pollutant are challenging. We suggest an increase in the number of rural air quality stations for better characterizing O₃ trends in cropland areas, as well as the application of different mitigation measures. They may involve more stringent air quality regulations and changes in crops, breeding tolerant cultivars and a crop management taking into account O₃ pollution.

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.

Responses of a semi-natural grassland community of tropical region to elevated ozone: An assessment of soil dynamics and biomass accumulation

Despite knowing the phytotoxic effects of tropospheric ozone (O₃), which is of global concern, there is no study so far reported about its impacts on grassland community of tropical regions. Therefore, we assessed the responses of a semi-natural grassland community of Indo-Gangetic plains to elevated O₃ exposure (Ambient + 20 ppb) compared to ambient after three years of exposure using open-top chambers. Percent decreases were found in above (26%; p ≤ 0.002) and belowground (30%; p ≤ 0.003) biomass under elevated compared to ambient O₃ exposure. Percent decrements in total organic carbon (TOC; 24%; p ≤ 0.001), total nitrogen (29%; p ≤ 0.001) and available phosphorus (11%; p ≤ 0.002) in the soil were also observed under elevated O₃ exposure. Exposure at elevated O₃ reduced soil microbial biomass and activities of β-glucosidase, amylase, urease and phosphatase, while polyphenol oxidase and peroxidase showed enhancement in their activities, showing negative effects on belowground soil health. Percent reduction in root shoot ratio (10%; p ≤ 0.05) depicts that less C-allocation towards root system led to a reduction in TOC in the soil, which could affect C-sequestration under elevated O₃ condition in the semi-natural grasslands. Elevated O₃ also affected enzymes participating in N and P-cycles, causing reductions in total nitrogen and phosphorus. The study concludes that projected O₃ concentrations have serious implications for aboveground biomass as well as belowground soil health in tropical areas, identified as hotspots of O₃ in the world.

Toward an impact assessment of ozone on plant carbon fixation using a process-based plant growth model: A case study of Fagus crenata grown under different soil nutrient levels

Ozone (O₃) in the troposphere, an air pollutant with phytotoxicity, is considered as a driver of global warming, because it reduces plant carbon fixation. Recently, a process-based plant growth model has been used in evaluating the O₃ impacts on plants (Schauberger et al., 2019). To make the evaluation more rigorous, we developed a plant growth model and clarified the key factors driving O₃-induced change in the whole-plant carbon fixation amount (C_fix). Fagus crenata seedlings were exposed to three O₃ levels (charcoal-filtered air or 1.0- or 1.5-folds ambient [O₃]) with three soil fertilization levels (non-, low-, or high-fertilized), i.e., a total of nine treatments. The C_fix was reduced in non- and low-fertilized treatments but was unaffected in high-fertilized treatment by O₃ fumigation. Our plant growth model could simulate C_fix accurately (<10% error) by considering the impacts of O₃ on plant leaf area and photosynthetic capacities, including maximum velocities of carboxylation and electron transport (V_cmax and J_max, respectively), and the initial slope and convexity of the curve of the electron transport velocity response to photosynthetic photon flux density (φ and θ, respectively). Furthermore, the model revealed that changes in V_cmax and J_max, φ and θ, or leaf area, caused by 1.5-folds the ambient [O₃] fumigation resulted in the following C_fix changes: -1.6, -5.8, or -16.4% in non-fertilized seedlings, -4.1, -4.4, or -9.3% in low-fertilized seedlings, and -4.6, -7.6, or +5.8% in high-fertilized seedlings. Therefore, photosynthetic capacities (particularly φ and θ) and leaf area are important factors influencing the impact of O₃ on C_fix of F. crenata seedlings grown under various fertilization levels. Further, the impacts of O₃ and soil nutrient on these photosynthetic capacities and plant leaf area should be considered to predict O₃-induced changes in carbon fixation by forest tree species using the process-based plant growth model.

Interactive effects of air pollutants and atmospheric moisture stress on aspen growth and photosynthesis along an urban-rural gradient

Atmospheric pollution could significantly alter tree growth independently and synergistically with meteorological conditions. North China offers a natural experiment for studying how plant growth responds to air pollution under different meteorological conditions, where rapid economic growth has led to severe air pollution and climate changes increase drought stress. Using a single aspen clone (Populus euramericana Neva.) as a 'phytometer', we conducted three experiments to monitor aspen leaf photosynthesis and stem growth during in situ exposure to atmospheric pollutants along the urban-rural gradient around Beijing. We used stepwise model selection to select the best multiple linear model, and we used binned regression to estimate the effects of air pollutants, atmospheric moisture stress and their interactions on aspen leaf photosynthesis and growth. Our results indicated that ozone (O₃) and vapor pressure deficit (VPD) inhibited leaf photosynthesis and stem growth. The interactive effect of O₃ and VPD resulted in a synergistic response: as the concentration of O₃ increased, the negative impact of VPD on leaf photosynthesis and stem growth became more severe. We also found that nitrogen (N) deposition had a positive effect on stem growth, which may have been caused by an increase in canopy N uptake, although this hypothesis needs to be confirmed by further studies. The positive impact of aerosol loading may be due to diffuse radiation fertilization effects. Given the decline in aerosols and N deposition amidst increases in O₃ concentration and drought risk, the negative effects of atmospheric pollution on tree growth may be aggravated in North China. In addition, the interaction between O₃ and VPD may lead to a further reduction in ecosystem productivity.

Assessment of growth, physiological, and yield attributes of wheat cultivar HD 2967 under elevated ozone exposure adopting timely and delayed sowing conditions

The present study was conducted to assess the impact of elevated levels of O₃ and shifting of crop calendar practice, singly, and in combination on Triticum aestivum cv. HD 2967 on its growth, gas exchange parameters, and yield attributes in open-top chambers (OTCs). Two sowing dates were considered: timely sown and late sown. Late sowing was delayed by 20 days from the timely sowing date. The result revealed that wheat plants under elevated O₃ and timely sown conditions (ET) showed reductions in growth parameters, while such effects were synergistic when plants were exposed to elevated O₃ under late sown conditions (EL). Photosynthetic rate, stomatal conductance, and water use efficiency reduced significantly under EL followed by ET and AL as compared with AT (ambient O₃ + timely sown) whereas transpiration rate showed maximum increment under EL. Grain yield reduced by 45.3% in EL as compared with AT and 16.2% in ET as compared with AT. The growth parameters and yield attributes obtained from the present experiment revealed that (i) O₃ is affecting the growth and productivity of the wheat and (ii) late sowing practice has not proved to be a feasible adaptation strategy for the wheat cultivation against O₃-induced production losses under the prevailing conditions of Indo-Gangetic Plain. This is the first report documenting the shifting of crop calendar practice at the present and future scenario of O₃ concentration under agro-ecological conditions in the tropical region of India.

Effect of elevated ozone, nitrogen availability and mesophyll conductance on the temperature responses of leaf photosynthetic parameters in poplar

Although ozone (O3) concentration and nitrogen (N) availability are well known to affect plant physiology, their impacts on the photosynthetic temperature response are poorly understood. We addressed this knowledge gap by exposing seedlings of hybrid poplar clone '107' (Populous euramericana cv. '74/76') to elevated O3 (E-O3) and N availability variation in a factorial experiment. E-O3 decreased light-saturated net photosynthesis (Asat), mesophyll conductance (gm) and apparent maximum rate of carboxylation (Vcmax, based on intercellular CO2 concentration) but not actual Vcmax (based on chloroplast CO2 concentration) and increased respiration in light (Rd) at 25 °C. Nitrogen fertilization increased Asat, gm, Vcmax and the maximum rate of electron transport (Jmax) and reduced Rd at 25 °C and the activation energy of actual Vcmax. No E-O3 or E-O3 x N interaction effects on the temperature response parameters were detected, simplifying the inclusion of O3 impacts on photosynthesis in vegetation models. gm peaked at 30 °C, apparent Vcmax and Jmax at 32-33 °C, while the optimum temperatures of actual Vcmax and Jmax exceeded the measured temperature range (15-35 °C). Ignoring gm would, thus, have resulted in mistakenly attributing the decrease in Asat at high temperatures to reduced biochemical capacity rather than to greater diffusion limitation.

Towards a multiscale crop modelling framework for climate change adaptation assessment

Predicting the consequences of manipulating genotype (G) and agronomic management (M) on agricultural ecosystem performances under future environmental (E) conditions remains a challenge. Crop modelling has the potential to enable society to assess the efficacy of G × M technologies to mitigate and adapt crop production systems to climate change. Despite recent achievements, dedicated research to develop and improve modelling capabilities from gene to global scales is needed to provide guidance on designing G × M adaptation strategies with full consideration of their impacts on both crop productivity and ecosystem sustainability under varying climatic conditions. Opportunities to advance the multiscale crop modelling framework include representing crop genetic traits, interfacing crop models with large-scale models, improving the representation of physiological responses to climate change and management practices, closing data gaps and harnessing multisource data to improve model predictability and enable identification of emergent relationships. A fundamental challenge in multiscale prediction is the balance between process details required to assess the intervention and predictability of the system at the scales feasible to measure the impact. An advanced multiscale crop modelling framework will enable a gene-to-farm design of resilient and sustainable crop production systems under a changing climate at regional-to-global scales.

Interactive effects of ozone exposure and nitrogen addition on tree root traits and biomass allocation pattern: An experimental case study and a literature meta-analysis

Ground-level ozone (O₃) pollution often co-occurs with anthropogenic nitrogen (N) deposition. Many studies have explored how O₃ and soil N affect aboveground structure and function of trees, but it remains unclear how belowground processes change over a spectrum of N addition and O₃ concentrations levels. Here, we explored the interactive impact of O₃ (five levels) and soil N (four levels) on fine and coarse root biomass and biomass allocation pattern in poplar clone 107 (Populus euramericana cv. '74/76'). We then evaluated the modifying effects of N on the responses of tree root biomass to O₃ via a synthesis of published literature. Elevated O₃ inhibited while N addition stimulated root biomass, with more pronounced effects on fine roots than on coarse root. The root:shoot (R:S) ratio was markedly decreased by N addition but remained unaffected by O₃. No interactive effects between O₃ and N were observed on root biomass and R:S ratio. The slope of log-log linear relationship between shoot and root biomass (i.e. scaling exponent) was increased by N, but not significantly affected by O₃. The analysis of published literature further revealed that the O₃-induced reduction in tree root biomass was not modified by soil N. The results suggest that higher N addition levels enhance faster allocation of shoot biomass while shoot biomass scales isometrically with root biomass across multiple O₃ levels. N addition does not markedly alter the sensitivity of root biomass of trees to O₃. These findings highlight that the biomass allocation exhibits a differential response to environmentally realistic levels of O₃ and N, and provide an important perspective for understanding and predicting net primary productivity and carbon dynamics in O₃-polluted and N-enriched environments.

Evidence of Ozone-Induced Visible Foliar Injury in Hong Kong Using Phaseolus Vulgaris as a Bioindicator

(1) Background: Hong Kong is one of the most densely populated cities in the world, with millions of people exposed to severe air pollution. Surface ozone, mostly produced photochemically from anthropogenic precursor gases, is harmful to both humans and vegetation. The phytotoxicity of ozone has been shown to damage plant photosynthesis, induce early leaf death, and retard growth. (2) Methods: We use genotypes of bush bean Phaseolus vulgaris with various degrees of sensitivity to ozone to investigate the impacts of ambient ozone on the morphology and development of the beans. We use ozone-induced foliar injury index and measure the flowering and fruit production to quantify the ozone stress on the plants. (3) Results: We expected that the ozone-sensitive genotype would suffer from a reduction of yield. Results, however, show that the ozone-sensitive genotype suffers higher ozone-induced foliar damage as expected but produces more pods and beans and heavier beans than the ozone-resistant genotype. (4) Conclusions: It is postulated that the high ozone sensitivity of the sensitive genotype causes stress-induced flowering, and therefore results in higher bean yield. A higher than ambient concentration of ozone is needed to negatively impact the yield production of the ozone-sensitive genotype. Meanwhile, ozone-induced foliar damage shows a graduated scale of damage pattern that can be useful for indicating ozone levels. This study demonstrates the usefulness of bioindicators to monitor the phytotoxic effects of ozone pollution in a subtropical city such as Hong Kong.

Dry Deposition of Ozone over Land: Processes, Measurement, and Modeling

Dry deposition of ozone is an important sink of ozone in near surface air. When dry deposition occurs through plant stomata, ozone can injure the plant, altering water and carbon cycling and reducing crop yields. Quantifying both stomatal and nonstomatal uptake accurately is relevant for understanding ozone's impact on human health as an air pollutant and on climate as a potent short-lived greenhouse gas and primary control on the removal of several reactive greenhouse gases and air pollutants. Robust ozone dry deposition estimates require knowledge of the relative importance of individual deposition pathways, but spatiotemporal variability in nonstomatal deposition is poorly understood. Here we integrate understanding of ozone deposition processes by synthesizing research from fields such as atmospheric chemistry, ecology, and meteorology. We critically review methods for measurements and modeling, highlighting the empiricism that underpins modeling and thus the interpretation of observations. Our unprecedented synthesis of knowledge on deposition pathways, particularly soil and leaf cuticles, reveals process understanding not yet included in widely-used models. If coordinated with short-term field intensives, laboratory studies, and mechanistic modeling, measurements from a few long-term sites would bridge the molecular to ecosystem scales necessary to establish the relative importance of individual deposition pathways and the extent to which they vary in space and time. Our recommended approaches seek to close knowledge gaps that currently limit quantifying the impact of ozone dry deposition on air quality, ecosystems, and climate.

Assessment of O3-induced yield and economic losses for wheat in the North China Plain from 2014 to 2017, China

Tropospheric ozone (O₃) is a pollutant of widespread concern in the world and especially in China for its negative effects on agricultural crops. For the first time, yield and economic losses of wheat between 2014 and 2017 were estimated for the North China Plain (NCP) using observational hourly O₃ data from 312 monitoring stations and exposure-response functions based on AOT40 index (accumulated hourly O₃ concentration above 40 ppb) from a Chinese study. AOT40 values from 2014 to 2017 during the wheat growing seasons (75-days, 44 before and 30 after mid-anthesis) ranged from 3.1 to 14.9 ppm h, 4.9-17.5 ppm h, 7.3-17.6 ppm h, and 0.5-18.6 ppm h, respectively. The highest AOT40 values were observed in the Beijing-Tianjin-Hebei region. The values of relative yield losses from 2014 to 2017 were in the ranges of 6.4-30.5%, 10.0-35.8%, 14.9-34.1%, and 21.6-38.2%, respectively. The total wheat production losses in NCP for 2014-2017 accounted for 18.5%, 22.7%, 26.2% and 30.8% in the whole production, while the economic losses amounted to 6,292 million USD, 8,524 million USD, 10,068 million USD, and 12,404 million USD, respectively. The important impact of O₃ in this area, which is of global importance, should be considered when assessing wheat yield production. Our results also show an increasing trend in AOT40, relative yield loss, total crop production loss and economic loss in the four consecutive years.

O3-Induced Priming Defense Associated With the Abscisic Acid Signaling Pathway Enhances Plant Resistance to Bemisia tabaci

Elevated ozone (O₃) modulates phytohormone signals, which subsequently alters the interaction between plants and herbivorous insects. It has been reported that elevated O₃ activates the plant abscisic acid (ABA) signaling pathway, but its cascading effect on the performance of herbivorous insects remains unclear. Here, we used the ABA-deficient tomato mutant notabilis (not) and its wild type, Ailsa Craig (AC), to determine the role of ABA signaling in mediating the effects of elevated O₃ on Bemisia tabaci in field open-top chambers (OTCs). Our results showed that the population abundance and the total phloem-feeding duration of B. tabaci were decreased by O₃ exposure in AC plants compared with not plants. Moreover, elevated O₃ and B. tabaci infestation activated the ABA signaling pathway and enhanced callose deposition in AC plants but had little effect on those in not plants. The exogenous application of a callose synthesis inhibitor (2-DDG) neutralized O₃-induced resistance to B. tabaci, and the application of ABA enhanced callose deposition and exacerbated the negative effects of elevated O₃ on B. tabaci. However, the application of 2-DDG counteracted the negative effects of O₃ exposure on B. tabaci in ABA-treated AC plants. Collectively, this study revealed that callose deposition, which relied on the ABA signaling pathway, was an effective O₃-induced priming defense of tomato plants against B. tabaci infestation.

Tropospheric ozone and cadmium do not have interactive effects on growth, photosynthesis and mineral nutrients of Catalpa ovata seedlings in the urban areas of Northeast China

Heavy metal contamination and tropospheric ozone (O₃) pollution often co-occur in heavy industrial urban areas, adversely affecting urban plant health. Little is known about the characteristics of growth, physiological metabolism, bioaccumulation of cadmium (Cd) and mineral nutrients in urban trees under the combination of soil Cd contamination and elevated O₃ exposure. In this study, one-year-old street tree Catalpa ovata G. Don seedlings were exposed to Cd contaminated soil (0, 100, 500 mg/kg soil) with 40 µg/m³ O₃ (ambient air) and 120 µg/m³ O₃ (elevated O₃ exposure) for 4 weeks. The results revealed that 500 mg/kg soil Cd addition alone decreased net photosynthetic rate, stomatal conductance, peroxidase activity and increased abscisic acid content and oxidative injury in the leaves of C. ovata. Furthermore, Cd soil contamination decreased leaf, stem, root and total biomass and affected Cd, Mg, Fe, and Zn contents in leaves (P < 0.01), but it did not affect Mg, Fe and Zn contents in roots. O₃ exposure did not affect growth, net photosynthetic rate, Cd accumulation and mineral nutrient contents of C. ovata. No interactive effect between Cd and O₃ was found on growth, oxidative injury, photosynthetic rate, and the contents of Cd, Mg, Fe and Zn in plant tissues (P > 0.05). Our findings suggest that C. ovata is an appropriate tree species for urban greening and afforestation in heavy industrial urban areas with high O₃ pollution in Northeast China. To ensure successful afforestation in heavy industrial areas, the long-term and large scale studies are needed to advance our understanding of the combined effects from extreme climate conditions and multi-pollutant exposure on the metabolism of mature urban trees.

Effects of Elevated Temperature and Ozone in Brassica juncea L.: Growth, Physiology, and ROS Accumulation

Global warming and ozone (O3) pose serious threats to crop yield and ecosystem health. Although neither of these factors will act individually in reality, most studies have focused on the responses of plants to air pollution or climate change. Interactive effects of these remain poorly studied. Therefore, this study was conducted to assess the effects of optimal (22/20 °C day/night) and elevated temperature (27/25 °C) and/or ambient (10 ± 10 nL L−1) and elevated O3 concentrations (100 ± 10 nL L−1) on the growth, physiology, and reactive oxygen species (ROS) accumulation of leaf mustard (Brassica juncea L.). The aim was to examine whether elevated temperature increase the O3 damage due to increasing stomatal conductance, and thus, O3 flux into the leaf. Significant reductions in photosynthetic rates occurred under O (elevated O3 with optimal temperatures) and OT (elevated O3 and temperature) conditions compared to C (controls). Stomatal conductance was significantly higher under T than in the C at 7 DAE. Under OT conditions, O3 flux significantly increased compared to that in O conditions at 7 days after exposure (DAE). Significant reductions in total fresh and dry weight were observed under OT conditions compared to those under O. Furthermore, significant reductions in levels of carotenoids and ascorbic acid were observed under OT conditions compared to O. Lipid peroxidation and accumulation of ROS such as hydroxyl radical, hydrogen peroxide, and superoxide radical were higher under O and OT conditions than in C conditions at 7 and 14 DAE. As a result of O3 stress, the results of the present study indicated that the plant injury index significantly increased under OT compared to O conditions. This result suggested that elevated temperature (+5 °C) may enhance O3 damage to B. juncea by increasing stomatal conductance and O3 flux into leaves.

Global change biology: A primer

Because of human action, the Earth has entered an era where profound changes in the global environment are creating novel conditions that will be discernable far into the future. One consequence may be a large reduction of the Earth's biodiversity, potentially representing a sixth mass extinction. With effective stewardship, the global change drivers that threaten the Earth's biota could be alleviated, but this requires clear understanding of the drivers, their interactions, and how they impact ecological communities. This review identifies 10 anthropogenic global change drivers and discusses how six of the drivers (atmospheric CO₂ enrichment, climate change, land transformation, species exploitation, exotic species invasions, eutrophication) impact Earth's biodiversity. Driver impacts on a particular species could be positive or negative. In either case, they initiate secondary responses that cascade along ecological lines of connection and in doing so magnify the initial impact. The unique nature of the threat to the Earth's biodiversity is not simply due to the magnitude of each driver, but due to the speed of change, the novelty of the drivers, and their interactions. Emphasizing one driver, notably climate change, is problematic because the other global change drivers also degrade biodiversity and together threaten the stability of the biosphere. As the main academic journal addressing global change effects on living systems, GCB is well positioned to provide leadership in solving the global change challenge. If humanity cannot meet the challenge, then GCB is positioned to serve as a leading chronicle of the sixth mass extinction to occur on planet Earth.

Dynamics of plant responses to combinations of air pollutants

The focus of this review is on how plants respond to combinations of multiple air pollutants. Global pollution trends, plant physiological responses and ecological perspectives in natural and agricultural systems are all discussed. In particular, we highlight the importance of studying sequential or simultaneous exposure of plants to pollutants, rather than exposure to individual pollutants in isolation, and explore how these responses may interfere with the way plants interact with their biotic community. Air pollutants can alter the normal physiology and metabolic functioning of plants. Here we describe how the phenotypic and molecular changes in response to multiple pollutants can differ compared to those elicited by single pollutants, and how different responses have been observed between plants in the field and in controlled laboratory conditions and between trees and crop plants. From an ecological perspective, we discuss how air pollution can result in greater susceptibility to biotic stressors and in direct or indirect effects on interactions with organisms that occupy higher trophic levels. Finally, we provide an overview of the potential uses of plants to mitigate air pollution, exploring the feasibility for pollution removal via the processes of bio-accumulation and phytoremediation. We conclude by proposing some new directions for future research in the field.

Advancing global change biology through experimental manipulations: Where have we been and where might we go?

This commentary summarizes the publication history of Global Change Biology for works on experimental manipulations over the past 25 years and highlights a number of key publications. The retrospective summary is then followed by some thoughts on the future of experimental work as it relates to mechanistic understanding and methodological needs. Experiments for elevated CO2 atmospheres and anticipated warming scenarios which take us beyond historical analogs are suggested as future priorities. Disturbance is also highlighted as a key agent of global change. Because experiments are demanding of both personnel effort and limited fiscal resources, the allocation of experimental investments across Earth's biomes should be done in ecosystems of key importance. Uncertainty analysis and broad community consultation should be used to identify research questions and target biomes that will yield substantial gains in predictive confidence and societal relevance. A full range of methodological approaches covering small to large spatial scales will continue to be justified as a source of mechanistic understanding. Nevertheless, experiments operating at larger spatial scales encompassing organismal, edaphic, and environmental diversity of target ecosystems are favored, as they allow for the assessment of long-term biogeochemical feedbacks enabling a full range of questions to be addressed. Such studies must also include adequate investment in measurements of key interacting variables (e.g., water and nutrient availability and budgets) to enable mechanistic understanding of responses and to interpret context dependency. Integration of ecosystem-scale manipulations with focused process-based manipulations, networks, and large-scale observations will aid more complete understanding of ecosystem responses, context dependence, and the extrapolation of results. From the outset, these studies must be informed by and integrated with ecosystem models that provide quantitative predictions from their embedded mechanistic hypotheses. A true two-way interaction between experiments and models will simultaneously increase the rate and robustness of Global Change research.

The Combined Effect of Elevated O3 Levels and TYLCV Infection Increases the Fitness of Bemisia tabaci Mediterranean on Tomato Plants

Global change and biotic stress, such as tropospheric contamination and virus infection, can individually modify the quality of host plants, thereby altering the palatability of the plant for herbivorous insects. The bottom-up effects of elevated O3 and tomato yellow leaf curl virus (TYLCV) infection on tomato plants and the associated performance of Bemisia tabaci Mediterranean (MED) were determined in open-top chambers. Elevated O3 decreased eight amino acid levels and increased the salicylic acid (SA) and jasmonic acid (JA) content and the gene expression of pathogenesis-related protein (PR1) and proteinase inhibitor (PI1) in both wild-type (CM) and JA defense-deficient tomato genotype (spr2). TYLCV infection and the combination of elevated O3 and TYLCV infection increased eight amino acids levels, SA content and PR1 expression, and decreased JA content and PI1 expression in both tomato genotypes. In uninfected tomato, elevated O3 increased developmental time and decreased fecundity by 6.1 and 18.8% in the CM, respectively, and by 6.8 and 18.9% in the spr2, respectively. In TYLCV-infected tomato, elevated O3 decreased developmental time and increased fecundity by 4.6 and 14.2%, respectively, in the CM and by 4.3 and 16.8%, respectively, in the spr2. These results showed that the interactive effects of elevated O3 and TYLCV infection partially increased the amino acid content and weakened the JA-dependent defense, resulting in increased population fitness of MED on tomato plants. This study suggests that whiteflies would be more successful at TYLCV-infected plants than at uninfected plants in elevated O3 levels.

Uncovering hidden genetic variation in photosynthesis of field-grown maize under ozone pollution

Ozone is the most damaging air pollutant to crops, currently reducing Midwest US maize production by up to 10%, yet there has been very little effort to adapt germplasm for ozone tolerance. Ozone enters plants through stomata, reacts to form reactive oxygen species in the apoplast and ultimately decreases photosynthetic C gain. In this study, 10 diverse inbred parents were crossed in a half-diallel design to create 45 F1 hybrids, which were tested for ozone response in the field using free air concentration enrichment (FACE). Ozone stress increased the heritability of photosynthetic traits and altered genetic correlations among traits. Hybrids from parents Hp301 and NC338 showed greater sensitivity to ozone stress, and disrupted relationships among photosynthetic traits. The physiological responses underlying sensitivity to ozone differed in hybrids from the two parents, suggesting multiple mechanisms of response to oxidative stress. FACE technology was essential to this evaluation because genetic variation in photosynthesis under elevated ozone was not predictable based on performance at ambient ozone. These findings suggest that selection under elevated ozone is needed to identify deleterious alleles in the world's largest commodity crop.

A quantitative assessment of hormetic responses of plants to ozone

Evaluations of ozone effects on vegetation across the globe over the last seven decades have mostly incorporated exposure levels that were multi-fold the preindustrial concentrations. As such, global risk assessments and derivation of critical levels for protecting plants and food supplies were based on extrapolation from high to low exposure levels. These were developed in an era when it was thought that stress biology is framed around a linear dose-response. However, it has recently emerged that stress biology commonly displays non-linear, hormetic processes. The current biological understanding highlights that the strategy of extrapolating from high to low exposure levels may lead to biased estimates. Here, we analyzed a diverse sample of published empirical data of approximately 500 stimulatory, hormetic-like dose-responses induced by ozone in plants. The median value of the maximum stimulatory responses induced by elevated ozone was 124%, and commonly <150%, of the background response (control), independently of species and response variable. The maximum stimulatory response to ozone was similar among types of response variables and major plant species. It was also similar among clades, between herbaceous and woody plants, between deciduous and evergreen trees, and between annual and perennial herbaceous plants. There were modest differences in the stimulatory response between genera and between families which may reflect different experimental designs and conditions among studies. The responses varied significantly upon type of exposure system, with open-top chambers (OTCs) underestimating the maximum stimulatory response compared to free-air ozone-concentration enrichment (FACE) systems. These findings suggest that plants show a generalized hormetic stimulation by ozone which is constrained within certain limits of biological plasticity, being highly generalizable, evolutionarily based, and maintained over ecological scales. They further highlight that non-linear responses should be taken into account when assessing the ozone effects on plants.

Leaf Traits That Contribute to Differential Ozone Response in Ozone-Tolerant and Sensitive Soybean Genotypes

Ozone (O₃) is a phytotoxic air pollutant that limits crop productivity. Breeding efforts to improve yield under elevated O₃ conditions will benefit from understanding the mechanisms that contribute to O₃ tolerance. In this study, leaf gas exchange and antioxidant metabolites were compared in soybean genotypes (Glycine max (L.) Merr) differing in ozone sensitivity. Mandarin (Ottawa) (O₃-sensitive) and Fiskeby III (O₃-tolerant) plants grown under charcoal-filtered (CF) air conditions for three weeks were exposed for five days to either CF conditions or 70 ppb O₃ in continuously stirred tank reactors (CSTRs) in a greenhouse. In the CF controls, stomatal conductance was approximately 36% lower for Fiskeby III relative to Mandarin (Ottawa) while the two genotypes exhibited similar levels of photosynthesis. Ozone exposure induced significant foliar injury on leaves of Mandarin (Ottawa) associated with declines in both stomatal conductance (by 77%) and photosynthesis (by 38%). In contrast, O₃ exposure resulted in minimal foliar injury on leaves of Fiskeby III with only a small decline in photosynthesis (by 5%), and a further decline in stomatal conductance (by 30%). There was a general trend towards higher ascorbic acid content in leaves of Fiskeby III than in Mandarin (Ottawa) regardless of treatment. The results confirm Fiskeby III to be an O₃-tolerant genotype and suggest that reduced stomatal conductance contributes to the observed O₃ tolerance through limiting O₃ uptake by the plant. Reduced stomatal conductance was associated with enhanced water-use efficiency, providing a potential link between O₃ tolerance and drought tolerance.

Evaluation of O3 Effects on Cumulative Photosynthetic CO2 Uptake in Seedlings of Four Japanese Deciduous Broad-Leaved Forest Tree Species Based on Stomatal O3 Uptake

The current level of tropospheric ozone (O3) is expected to reduce the net primary production of forest trees. Here, we evaluated the negative effects of O3 on the photosynthetic CO2 uptake of Japanese forest trees species based on their cumulative stomatal O3 uptake, defined as the phytotoxic O3 dose (POD). Seedlings of four representative Japanese deciduous broad-leaved forest tree species (Fagus crenata, Quercus serrata, Quercus mongolica var. crispula and Betula platyphylla var. japonica) were exposed to different O3 concentrations in open-top chambers for two growing seasons. The photosynthesis–light response curves (A-light curves) and stomatal conductance were measured to estimate the leaf-level cumulative photosynthetic CO2 uptake (ΣPn_est) and POD, respectively. The whole-plant-level ΣPn_est were highly correlated with the whole-plant dry mass increments over the two growing seasons. Because whole-plant growth is largely determined by the amount of leaf area per plant and net photosynthetic rate per leaf area, this result suggests that leaf-level ΣPn_est, which was estimated from the monthly A-light curves and hourly PPFD, could reflect the cumulative photosynthetic CO2 uptake of the seedlings per unit leaf area. Although the O3-induced reductions in the leaf-level ΣPn_est were well explained by POD in all four tree species, species-specific responses of leaf-level ΣPn_est to POD were observed. In addition, the flux threshold appropriate for the linear regression of the responses of relative leaf-level ΣPn_est to POD was also species-specific. Therefore, species-specific responses of cumulative photosynthetic CO2 uptake to POD could be used to accurately evaluate O3 impact on the net primary production of deciduous broad-leaved trees.

Ozone flux and ozone deposition in a mountain spruce forest are modulated by sky conditions

In order to understand the main driving factors of ozone (O₃) deposition we tested the hypothesis that sky conditions (cloudy, partly cloudy, and clear sky) modulate O₃ flux in forest ecosystems via stomatal regulation. The hypothesis is based on the fact that complex microclimate conditions under cloudy sky usually stimulate stomatal conductance. O₃ fluxes were inferred from a concentration gradient in a mountainous Norway spruce forest in the Czech Republic (Central Europe) for years 2012-2016 and measured directly by eddy-covariance during the summer of 2017. Daily and seasonal O₃ depositions were calculated separately for days with cloudy, partly cloudy, and clear sky conditions. The data show unequivocally that more O₃ is taken up under cloudy and partially cloudy skies. Moreover, we found significant interactive effects of sky conditions and season on O₃ flux. Though there are other mechanisms and pathways involved in the transport of O₃ to the plant-soil system, the highest O₃ deposition was associated to the highest stomatal conductance during partly cloudy and cloudy sky conditions in all seasons, while lower O₃ ecosystem fluxes were observed under clear sky conditions despite the highest O₃ concentrations at this time. These findings suggest that forests growing at sites where conditions are predominantly cloudy are expected to deposit higher extent of O₃ than less-cloudy forests being thus more threatened by phytotoxic O₃.

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.

Photosynthetic Responses of Canola and Wheat to Elevated Levels of CO2, O3 and Water Deficit in Open-Top Chambers

The effects of elevated CO2 (700 ppm) and O3 (80 ppb) alone and in combination on the photosynthetic efficiency of canola and wheat plants were investigated in open-top chambers (OTCs). The plants were fumigated for four weeks under well-watered and water-stressed (water deficit) conditions. The fast chlorophyll a fluorescence transients were measured after 2 and 4 weeks of fumigation, as well as in control plants, and analyzed by the JIP-test, which is a non-destructive, non-invasive, informative, very fast and inexpensive technique used to evaluate the changes in photosynthetic efficiency. Biomass measurements were taken only after 4 weeks of fumigation. The performance index (PItotal), an overall parameter calculated from the JIP-test formulae, was reduced by elevated CO2 and O3 under well-watered conditions. In the absence of any other treatment, water stress caused a decrease of the PItotal, and it was partly eliminated by fumigation with elevated CO2 and CO2 + O3. This finding was also supported by the biomass results, which revealed a higher biomass under elevated CO2 and CO2 + O3. The decrease in biomass induced by elevated O3 was likely caused by the decline of photosynthetic efficiency. Our findings suggest that elevated CO2 reduces the drought effect both in the absence and presence of O3 in canola and wheat plants. The study also indicates that elevated O3 would pose a threat in future to agricultural crops.

Light Energy Partitioning under Various Environmental Stresses Combined with Elevated CO2 in Three Deciduous Broadleaf Tree Species in Japan

Understanding plant response to excessive light energy not consumed by photosynthesis under various environmental stresses, would be important for maintaining biosphere sustainability. Based on previous studies regarding nitrogen (N) limitation, drought in Japanese white birch (Betula platyphylla var. japonica), and elevated O3 in Japanese oak (Quercus mongolica var. crispula) and Konara oak (Q. serrata) under future-coming elevated CO2 concentrations, we newly analyze the fate of absorbed light energy by a leaf, partitioning into photochemical processes, including photosynthesis, photorespiration and regulated and non-regulated, non-photochemical quenchings. No significant increases in the rate of non-regulated non-photochemical quenching (JNO) were observed in plants grown under N limitation, drought and elevated O3 in ambient or elevated CO2. This suggests that the risk of photodamage caused by excessive light energy was not increased by environmental stresses reducing photosynthesis, irrespective of CO2 concentrations. The rate of regulated non-photochemical quenching (JNPQ), which contributes to regulating photoprotective thermal dissipation, could well compensate decreases in the photosynthetic electron transport rate through photosystem II (JPSII) under various environmental stresses, since JNPQ+JPSII was constant across the treatment combinations. It is noteworthy that even decreases in JNO were observed under N limitation and elevated O3, irrespective of CO2 conditions, which may denote a preconditioning-mode adaptive response for protection against further stress. Such an adaptive response may not fully compensate for the negative effects of lethal stress, but may be critical for coping with non-lethal stress and regulating homeostasis. Regarding the three deciduous broadleaf tree species, elevated CO2 appears not to influence the plant responses to environmental stresses from the viewpoint of susceptibility to photodamage.

The effects of elevated ozone on the accumulation and allocation of poplar biomass depend strongly on water and nitrogen availability

Ozone (O₃) pollution can alter carbon allocation and reduce tree growth - both above and below ground, but the extent of these effects depends on the variation in soil water and nutrient availability. Here we present the accumulation and allocation of biomass in poplar clone 546 (Populus deltoides cv. '55/56' × P. deltoides cv. 'Imperial') for one growing season at two O₃ concentrations (charcoal-filtered air [CF] and non-filtered air + 40 ppb of O₃ [E-O₃]), two watering regimes (well-watered [WW] and reduced watering at 40% of WW irrigation [RW]) and two soil nitrogen addition treatments (no addition [N0] and the addition of 50 kg N ha^-1 year^-1 [N50]). We found that the deleterious effects of E-O₃ depended on the supply of water and nitrogen. Specifically, when the supplies of water and/or N (WW and/or N50) were abundant, E-O₃ significantly reduced whole plant biomass by >15% but had no significant effect on biomass when these supplies were limited (RW and N0). A significant reduction of biomass by E-O₃ occurred earlier in fine roots than in other plant organs, indicating greater sensitivity of fine root to E-O₃. These results suggest that rising O₃ concentrations may not ubiquitously lead to a large reduction in plant biomass since plant growth is often jointly constrained by water and nutrients.

Nitrogen loading increases the ozone sensitivity of larch seedlings with higher sensitivity to nitrogen loading

Larch (Larix sp.) tree is a critical species for the future afforestation in Northeast Asia. The impacts of elevated concentrations of ground-level ozone (O₃) and nitrogen (N) deposition are raising concerns. However, knowledge of the combined effects of elevated O₃ and N loading are still limited. We investigated whether nitrogen loading mitigates the negative impacts of ozone on two larch species: the Japanese larch (L. kaempferi) and its hybrid larch F₁ (L. gmelinii var. japonica × L. kaempferi) or not. We used open-top cambers and compared responses of the larch seedlings. Results showed the N loading mitigated the negative effects of O₃ on Japanese larch. However, in hybrid larch F₁, N loading did not mitigate O₃-induced inhibition of growth and photosynthetic capacity. Mitigation effect of N loading on negative O₃ impacts may vary between the two Larix spp., Hybrid larch F₁ could be more affected by the combined effects of O₃ and N loading due to its higher growth response to N loading. Elevated O₃ also reduced leaf nitrogen/phosphorus (N/P) ratio by elevated O₃, with significant effects in hybrid larch F₁, particularly under N loading. In the present study, leaf N/P ratio was utilized to validate the hypothesis that a positive effect of N loading may be observed if O₃ does not induce P limitation in Larix spp. We demonstrated a potential leaf N/P ratio function, which could reflect responses to O₃ and N loading in hybrid larch F₁.

Modeling the Effects of Climate Change on Surface Ozone during Summer in the Yangtze River Delta Region, China

Future climate change can impact ozone concentrations by changing regional meteorological factors related to ozone (O3) pollution. To better understand the variations of meteorological factors and their effects on O3 formation processes under future climate conditions, we model the present and the future meteorology and air quality in summer over the Yangtze River Delta (YRD) region by using the Weather Research and Forecasting Model with Chemistry module (WRF/Chem), which is driven by the outputs of Community Climate System Model version 4 (CCSM4). The simulations predict that solar radiation, 2-m air temperature, and wind speed increase in the daytime over most of the YRD region. Absolute humidity and precipitation increase in the north and decrease in the south, while the planetary boundary layer height (PBLH) has an opposite change pattern displaying a decrease in the north and an increase in the south. The southerly wind will be strengthened in the daytime. At night, the change patterns of the meteorological factors are similar to the daytime but with small variations. Meanwhile, O3 and its precursors all increase in the north and decrease in the south. The increases of NOx, volatile organic compounds (VOC), and CO are related with the decreases of PBLH and the input effect of stronger southerly wind, while the decreases are attributed to the output effect of the stronger southerly wind. During the daytime, the increase of surface O3 in the north is dominated by the chemical processes related with the increases of solar radiation, air temperature, and O3 precursors. The decrease of surface O3 in the south is mainly caused by the transport process changing with the strengthened southerly wind. At night, the surface O3 changing the amplitude is less than the daytime. The less O3 variations at night can be attributed to an O3 titration reaction with NO, the changes in NOx concentrations, and the increases of nocturnal PBLH. With the aid of H2O2/HNO3, O3 formation in the YRD region is found to be easily affected by NOx in the future. The findings can help to understand the changing trend of O3 in the YRD region and can propose reasonable pollution control policies.

Ascorbic acid and thiols as potential biomarkers of ozone tolerance in tropical wheat cultivars

Tropospheric ozone (O₃) has been identified as the most damaging air pollutant to crop plants in terms of growth and yield reductions. Considering the negative effect of O₃ in tropical regions, fourteen commonly grown Indian wheat cultivars with known sensitivity to O₃ were tested for their sensitivity/tolerance with respect to two major antioxidants (ascorbic acid and thiols) and grain yield responses against elevated O₃ (ambient + 30 ppb) exposure. The objectives of the study were to assess the usefulness of the biochemical markers in the screening of wheat cultivars having differential level of sensitivity to O₃ and different release time (modern and old cultivars). Ozone exposure led to an upsurge of ascorbic acid, thiols as well as their ratio greatly in the tolerant group followed by the intermediately sensitive group while least in sensitive one. Both ascorbic acid and thiol contents offered more resistance to early released cultivars compared to modern ones. Ascorbic acid served to be the most influential parameter for determining varietal response under elevated O₃ stress and directly linked with O₃ tolerance. Overall, the sensitive group suffered maximum yield losses while the minimum was observed in the tolerant group due to the differential enhancement of tolerance offered by antioxidants. Higher concentrations of antioxidants at early growth stages were highly correlated with final yield responses suggesting the role of antioxidants as a determinant of final yield. Findings of this study will help in the identification of O₃ tolerant and sensitive wheat cultivars for future screening programs using ascorbic acid and thiols as important markers of O₃ tolerance.

Asian Culturally Specific Predictors in a Large-Scale Land Use Regression Model to Predict Spatial-Temporal Variability of Ozone Concentration

This paper developed a land use regression (LUR) model to study the spatial-temporal variability of O₃ concentrations in Taiwan, which has typical Asian cultural characteristics with diverse local emission sources. The Environmental Protection Agency's (EPA) data of O₃ concentrations from 2000 and 2013 were used to develop this model, while observations from 2014 were used as the external data verification to assess model reliability. The distribution of temples, cemeteries, and crematoriums was included for a potential predictor as an Asian culturally specific source for incense and joss money burning. We used stepwise regression for the LUR model development, and applied 10-fold cross-validation and external data for the verification of model reliability. With the overall model R² of 0.74 and a 10-fold cross-validated R² of 0.70, this model presented a mid-high prediction performance level. Moreover, during the stepwise selection procedures, the number of temples, cemeteries, and crematoriums was selected as an important predictor. By using the long-term monitoring data to establish an LUR model with culture specific predictors, this model can better depict O₃ concentration variation in Asian areas.

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 (g_s) 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.

Elevated Ozone Concentration Reduces Photosynthetic Carbon Gain but Does Not Alter Leaf Structural Traits, Nutrient Composition or Biomass in Switchgrass

Elevated tropospheric ozone concentration (O₃) increases oxidative stress in vegetation and threatens the stability of crop production. Current O₃ pollution in the United States is estimated to decrease the yields of maize (Zea mays) up to 10%, however, many bioenergy feedstocks including switchgrass (Panicum virgatum) have not been studied for response to O₃ stress. Using Free Air Concentration Enrichment (FACE) technology, we investigated the impacts of elevated O₃ (~100 nmol mol^-1) on leaf photosynthetic traits and capacity, chlorophyll fluorescence, the Ball⁻Woodrow⁻Berry (BWB) relationship, respiration, leaf structure, biomass and nutrient composition of switchgrass. Elevated O₃ concentration reduced net CO₂ assimilation rate (A), stomatal conductance (g_s), and maximum CO₂ saturated photosynthetic capacity (V_max), but did not affect other functional and structural traits in switchgrass or the macro- (except potassium) and micronutrient content of leaves. These results suggest that switchgrass exhibits a greater O₃ tolerance than maize, and provide important fundamental data for evaluating the yield stability of a bioenergy feedstock crop and for exploring O₃ sensitivity among bioenergy feedstocks.

Impacts of air pollution on human and ecosystem health, and implications for the National Emission Ceilings Directive: Insights from Italy

Across the 28 EU member states there were nearly half a million premature deaths in 2015 as a result of exposure to PM_2.5, O₃ and NO₂. To set the target for air quality levels and avoid negative impacts for human and ecosystems health, the National Emission Ceilings Directive (NECD, 2016/2284/EU) sets objectives for emission reduction for SO₂, NOx, NMVOCs, NH₃ and PM_2.5 for each Member State as percentages of reduction to be reached in 2020 and 2030 compared to the emission levels into 2005. One of the innovations of NECD is Article 9, that mentions the issue of "monitoring air pollution impacts" on ecosystems. We provide a clear picture of what is available in term of monitoring network for air pollution impacts on Italian ecosystems, summarizing what has been done to control air pollution and its effects on different ecosystems in Italy. We provide an overview of the impacts of air pollution on health of the Italian population and evaluate opportunities and implementation of Article 9 in the Italian context, as a case study beneficial for all Member States. The results showed that SO₄^2- deposition strongly decreased in all monitoring sites in Italy over the period 1999-2017, while NO₃^- and NH₄⁺ decreased more slightly. As a consequence, most of the acid-sensitive sites which underwent acidification in the 1980s partially recovered. The O₃ concentration at forest sites showed a decreasing trend. Consequently, AOT40 (the metric identified to protect vegetation from ozone pollution) showed a decrease, even if values were still above the limit for forest protection (5000 ppb h^-1), while PODy (flux-based metric under discussion as new European legislative standard for forest protection) showed an increase. National scale studies pointed out that PM10 and NO2 induced about 58,000 premature deaths (year 2005), due to cardiovascular and respiratory diseases. The network identified for Italy contains a good number of monitoring sites (6 for terrestrial ecosystem monitoring, 4 for water bodies monitoring and 11 for ozone impact monitoring) distributed over the territory and will produce a high number of monitored parameters for the implementation of the NECD.

Investigation of ozone deposition to vegetation under warm and dry conditions near the Eastern Mediterranean coast

Dry deposition of ozone (O₃) to vegetation is an important removal pathway for tropospheric O₃, while O₃ uptake through plant stomata negatively affects vegetation and leads to climate change. Both processes are controlled by vegetation characteristics and ambient conditions via complex mechanisms. Recent studies have revealed that these processes can be fundamentally impacted by coastal effects, and by dry and warm conditions in ways that have not been fully characterized, largely due to lack of measurements under such conditions. Hence, we hypothesized that measuring dry deposition of O₃ to vegetation along a sharp spatial climate gradient, and at different distances from the coast, can offer new insights into the characterization of these effects on O₃ deposition to vegetation and stomatal uptake, providing important information for afforestation management and for climate and air-quality model improvement. To address these hypotheses, several measurement campaigns were performed at different sites, including pine, oak, and mixed Mediterranean forests, at distances of 20-59 km from the Eastern Mediterranean coast, under semiarid, Mediterranean and humid Mediterranean climate conditions. The eddy covariance technique was used to quantify vertical O₃ flux (F_tot) and its partitioning to stomatal flux (F_st) and non-stomatal flux (F_ns). Whereas F_st tended to peak around noon under humid Mediterranean and Mediterranean conditions in summer, it was strongly limited by drought under semiarid conditions from spring to early winter, with minimum average F_st/F_tot of 8-11% during the summer. F_ns in the area was predominantly controlled by relative humidity (RH), whereas increasing F_ns with RH for RH < 70% indicated enhancement of F_ns by aerosols, via surface wetness stimulation. At night, efficient turbulence due to sea and land breezes, together with increased RH, resulted in strong enhancement of F_tot. Extreme dry surface events, some induced by dry intrusion from the upper troposphere, resulted in positive F_ns events.

Mesophyll conductance limitation of photosynthesis in poplar under elevated ozone

Finite mesophyll conductance (g_m) reduces the rate of CO₂ diffusion from the leaf intercellular space to the chloroplast and constitutes a major limitation of photosynthesis in trees. While it is well established that g_m is decreased by stressors such as drought and high temperature, few studies have investigated if the phytotoxic air pollutant ozone (O₃) affects g_m. We quantified the relative importance of three different types of limitations of photosynthesis in poplar trees exposed to elevated O₃: decreases in stomatal conductance, g_m and biochemical photosynthetic capacity. The O₃-induced reductions in light-saturated net photosynthesis were linked to significant declines in g_m and biochemical photosynthetic capacity (in particular carboxylation). There was no significant effect of O₃ on stomatal conductance. Of the O₃-induced limitations on photosynthesis, g_m limitation was by far the most important (-16%) while biochemical limitation (-8%) was rather small. Both limitations grew in magnitude over the study period and varied in response to leaf-specific O₃ exposure. Our findings suggest that declines in g_m may play a key role in limiting photosynthesis of plants exposed to elevated O₃, an effect hitherto overlooked.

Spatial-temporal variations of summertime ozone concentrations across a metropolitan area using a network of low-cost monitors to develop 24 hourly land-use regression models

Ten relatively-low-cost ozone monitors (Aeroqual Series 500 with OZL ozone sensor) were deployed to assess the spatial and temporal variability of ambient ozone concentrations across residential areas in the Monroe County, New York from June to October 2017. The monitors were calibrated in the laboratory and then deployed to a local air quality monitoring site where they were compared to the federal equivalent method values. These correlations were used to correct the measured ozone concentrations. The values were also used to develop hourly land use regression models (LUR) based on the deletion/substitution/addition (D/S/A) algorithm that can be used to predict the spatial and temporal concentrations of ozone at any hour of a summertime day and given location in Monroe County. Adjusted R2 values were high (average 0.83) with the highest adjusted R2 for the model between 8 and 9 AM (i.e. 1-2 h after the peak of primary emissions during the morning rush hours). Spatial predictors with the highest positive effects on ozone estimates were high intensity developed areas, low and medium intensity developed areas, forests + shrubs, average elevation, Interstate + highways, and the annual average vehicular daily traffic counts. These predictors are associated with potential emissions of anthropogenic and biogenic precursors. Maps developed from the models exhibited reasonable spatial and temporal patterns, with low ozone concentrations overnight and the highest concentrations between 11 AM and 5 PM. The adjusted R2 between the model predictions and the measured values varied between 0.79 and 0.87 (mean = 0.83). The combined use of the network of low-cost monitors and LUR modeling provide useful estimates of intraurban ozone variability and exposure estimates that will be used in future epidemiological studies.

Current ambient and elevated ozone effects on poplar: A global meta-analysis and response relationships

The effects of current and future elevated O₃ concentrations (e[O₃]) were investigated by a meta-analysis for poplar, a widely distributed genus in the Northern Hemisphere with global economic importance. Current [O₃] has significantly reduced CO₂ assimilation rate (P_n) by 33% and total biomass by 4% in comparison with low O₃ level (charcoal-filtered air, CF). Relative to CF, an increase in future [O₃] would further enhance the reduction in total biomass by 24%, plant height by 17% and plant leaf area by 19%. Isoprene emissions could decline by 34% under e[O₃], with feedback implications in reducing the formation of secondary air pollutants including O₃. Reduced stomatal conductance and lower foliar area might increase runoff and freshwater availability in O₃ polluted areas. Higher cumulated O₃ exposure over a threshold of 40 ppb (AOT40) induced larger reductions in P_n, total biomass and isoprene emission. Relationships of light-saturated photosynthesis rates (A_sat), total biomass and chlorophyll content with AOT40 using a global dataset are provided. These relationships are expected to improve O₃ risk assessment and also to support the inclusion of the effect of O₃ in models addressing plantation productivity and carbon sink capacity.

Predicting the effect of ozone on vegetation via linear non-threshold (LNT), threshold and hormetic dose-response models

The nature of the dose-response relationship in the low dose zone and how this concept may be used by regulatory agencies for science-based policy guidance and risk assessment practices are addressed here by using the effects of surface ozone (O₃) on plants as a key example for dynamic ecosystems sustainability. This paper evaluates the current use of the linear non-threshold (LNT) dose-response model for O₃. The LNT model has been typically applied in limited field studies which measured damage from high exposures, and used to estimate responses to lower concentrations. This risk assessment strategy ignores the possibility of biological acclimation to low doses of stressor agents. The upregulation of adaptive responses by low O₃ concentrations typically yields pleiotropic responses, with some induced endpoints displaying hormetic-like biphasic dose-response relationships. Such observations recognize the need for risk assessment flexibility depending upon the endpoints measured, background responses, as well as possible dose-time compensatory responses. Regulatory modeling strategies would be significantly improved by the adoption of the hormetic dose response as a formal/routine risk assessment option based on its substantial support within the literature, capacity to describe the entire dose-response continuum, documented explanatory dose-dependent mechanisms, and flexibility to default to a threshold feature when background responses preclude application of biphasic dose responses.

CAPSULE

The processes of ozone hazard and risk assessment can be enhanced by incorporating hormesis into their principles and practices.