A chamberless field exposure system for ozone enrichment of short vegetation

Only few studies have been conducted as yet which focus on the effects of rising tropospheric ozone levels on semi-natural vegetation under free-air conditions. A new technical approach was used to examine the response of calcareous grassland to ozone employing a chamberless fumigation system. Four different ozone regimes were applied (1-, 1.33-, 1.66- and 2-fold ambient air levels) with five replicates each. Ozone enrichment was carried out on circular plots of 2 m in diameter by a computer controlled exposure system. Transparent windscreens encircling each plot accelerated the mixing of ambient air and ozone released. Thus, the use of blowers could be avoided. The exposure system presented here is regarded as an appropriate technique for free-air trace gas enrichment on short vegetation avoiding microclimatic alterations known to affect plant growth and pollutant uptake. Furthermore, the chosen technical set-up was rather cost-effective. Hence, it enabled the establishment of a larger number of replications providing the basis for results of higher statistical power.     

Assessing vegetation exposure to ozone: properties of the AOT40 index and modifications by deposition modelling

A discussion is presented on the application of micrometeorological deposition modelling principles to improve the characterisation of vegetation exposure to ozone and thus the use of critical levels as the basis of targeted emission control. The AOT40 (accumulated exposure over a threshold of 40 ppb or nl l(-1)) ozone exposure index is shown to impose a differential weighting that results in a high sensitivity, by a factor of two to 10 depending on the pollution climate, with respect to concentration. This makes it necessary to correct for systematic effects, such as the concentration profile below the measurement height, in order to justify a comparison with the biological data obtained from well-mixed exposure chambers. Available studies indicate a 50-70% lower AOT40 at the vegetation height. The resistance method for estimating the profile is extended to allow for stomatal effects that potentially bias the plant response predicted with an exposure index. This integrated profile-uptake correction refines the current approach and serves as a transitional step towards a real flux-based approach. For the latter, a new deposition parameterisation is tested against field observations.     

Compensation processes of Aleppo pine (Pinus halepensis Mill.) to ozone exposure and drought stress

A long-term experiment was performed to study the effects of O3 and drought-stress (DS) on Aleppo pine seedlings (Pinus halepensis Mill.) exposed in open-top chambers. Ozone reduced gas exchange rates, ribulose-1,5-biphosphate carboxylase/oxygenase activity (Rubisco), aboveground C and needle N concentrations and C/N ratio and Ca concentrations of the twigs under 3 mm (twigs<3) and the aerial biomass. Also it increased phosphoenolpyruvate carboxylase (PEPc) and N and K concentrations of the twigs<3. Water stress decreased gas exchange rates, predawn needle water potential (PsiPd), C/N ratio, twigs<3 Ca, plant growth, aerial biomass and increased N, twigs with a diameter above 3 mm P and Mg concentrations. The combined exposure to both stresses increased N concentrations of twigs<3 and roots and aboveground biomass K content and decreased root C, maximum daily assimilation rate and instantaneous water use efficiency. The sensitivity of Aleppo pine to both stresses is determined by plant internal resource allocation and compensation mechanisms to cope with stress.     

Crops' responses to ozone in Mediterranean environments

The Mediterranean environment, and most of the Italian peninsula, presents some peculiarities in terms of crop response to O₃ since most physiological mechanisms activated upon O₃ exposure, such as stomatal closure, often overlap and interact with those that underlie plant adaptation to drought and hyperosmotic stress, which are typical of these environments. OTC and EDU experiments have demonstrated that O₃ causes strong yield losses when crops are grown without water limitations. However, exposure to water or saline stress significantly reduced O₃ effects on crop yield. In this review, we present the methodological approaches that have been used to study plant-ozone interactions in Italy as well as biochemical, physiological and agronomic responses for representative cropping systems of the Mediterranean climate.     

Influence of the atmospheric conductivity on the ozone exposure of plants under ambient conditions: Considerations for establishing ozone standards to protect vegetation

This paper provides results of ozone flux density measurements above a permanent grassland ecosystem as they relate to an establishment of air quality guidelines or standards. Using a resistance analogue, the product of zone concentration measured at a standard measurement height and the conductivity of the atmosphere reflect the maximum possible ozone flux density towards the envelope of the plants. In other words, this product can be regarded as the ozone exposure potential of the atmosphere for plants. It could be shown that ozone concentrations between 100 and 180 microg m(-3) are likely to have a great phytotoxic potential and are more important than concentrations greater than 180 microg m(-3). From the results presented one can deduce that the application of dose-response relationships based on chamber experiments to ambient conditions results in an overestimation of, for example, yield loses. Any guideline or standard has to take into account the influence of the atmospheric conductivity on the absorbed dose of ozone.     

Visible injury, crown condition, and growth responses of selected Italian forests in relation to ozone exposure

The impact of ozone on forest ecosystems in Italy is monitored within the CONECOFOR programme. Ozone levels are measured in 30 plots using passive samplers. Response parameters used are: crown condition (transparency), BAI (basal area increment), and visible symptoms on spontaneous vegetation. Levels of AOT40 are above the concentration-based critical level of 5 ppmh in all sites, but the evidence of impact on forest vegetation remains limited. Ozone is a predictor of crown transparency residuals in beech sites over two consecutive years, but the variance explained amounts to less than 10%. The relation between BAI reduction and ozone is even less certain. Transparency and BAI are more readily explainable in terms of ecological conditions of the site and climate fluctuations. The interpretation of visible symptoms is doubtful, and is conditioned by the prevailing ecological factors in the areas.     

A California air standard to protect vegetation from ozone

Evidence shows that the current national primary ambient air quality standard, if attained, would still permit substantial injury to vegetation. Thus, in March 1987, the California Air Resources Board (CARB) began consideration of the evidence for the effects of ozone (O3) on vegetation, and of several possible state ambient air quality standards designed to protect vegetation, especially crops, from O3 injury. In its review, the CARB addressed a number of issues relevant to such a standard. One issue considered by the CARB is the relationship of an ambient air quality standard to natural background levels of O3, which would greatly influence the practicality of attainment. Attainment of a standard close to natural background could entail excessive costs. Another issue considered is the occurrence of oxidants other than O3 that can damage vegetation. Throughout much of California, O3 accounts for over 90% of the oxidant air pollutants, and the CARB considered whether, in keeping with current practice, O3 should be used as a surrogate for total oxidant air pollutants. A major new piece of information presented to the CARB was an assessment of the economic effects of several potential standards. This assessment, produced by University of California scientists at Riverside and Davis, calculated the benefits of the potential standards in comparison to current O3 levels and estimated natural O3 background. This assessment was developed using field chamber response data, local crop data, and local O3 concentration data as inputs to the California Agricultural Resources Model, which accounts for both supply and demand effects. Because of California's varied climate, agricultural production occurs on a year-round basis, with overlapping growing seasons for many crops. Over long periods of time, O3 levels may vary markedly because of the influence of various factors, and a 1-h standard may not be an accurate indicator of growing season O3 exposure. A moving three-month averaging time has been proposed as a way to approximate the growing seasons of California's 200 crops. However, a sufficiently stringent 1-h standard would serve as a surrogate for a growing season standard. The CARB reviewed evidence supporting both long-term and short-term standards. Agriculture dominates the economies of some regions within California but is a minor components of other regional economies. Because the San Joaquin Valley is California's most important agricultural area, the CARB reviewed evidence for a regional standard for this area that would be more stringent than standards for other parts of the state.     

A comparison of indices that describe the relationship between exposure to ozone and reduction in the yield of agricultural crops

The objective of this study is to compare the use of several indices of exposure in describing the relationship between O₃ and reduction in agricultural crop yield. No attempt has been made to determine which exposure-response models best fit the data sets examined. Hourly mean O₃ concentration data, based on two-three measurements per hour, were used to develop indices of exposure from soybean and winter wheat experiments conducted in open-top chambers at the Boyce Thompson Institute, Ithaca, New York NCLAN field site. The comparative efficacy of cumulative indices (i.e. number of occurrences equal to or above specific hourly mean concentrations, sum of all hourly mean concentrations equal to or above a selected level, and the weighted sum of all hourly mean concentrations) and means calculated over an experimental period to describe the relationship between exposure to O₃ and reductions in the yield of agricultural crops was evaluated. None of the exposure indices consistently provided a best fit with the Weibull and linear models tested. The selection of the model appears to be important in determining the indices that best describe the relationship between exposure and response. The focus of selecting a model should be on fitting the data points as well as on adequately describing biological responses. The investigator should be careful to couple the model with data points derived from indices relevant to the length of exposure. While we have used a small number of data sets, our analysis indicates that exposure indices that weight peak concentrations differently than lower concentrations of an exposure regime can be used in the development of exposure-response functions. Because such indices may have merit from a regulatory perspective, we recommend that additional data sets be used in further analyses to explore the biological rationale for various indices of exposure and their use in exposure-response functions.     

Growth-stage dependent crop yield response to ozone exposure

Data from four crop yield-loss field trials were examined to determine if analysis using an imposed phenological weighting function based on seasonal growth stage would provide a more accurate indication of impact of ozone exposure. Alfalfa (Medicago sativa L. cv. Moapa 69), dry bean (Phaseolus vulgaris L. cv. California Dark Red kidney), fresh market and processing tomato (Lycopersicon esculentum Mill. cv. 6718 VF and VF-145-B7879, respectively) were grown at 9-11 ambient field plots within southern California comprising an ambient gradient of ozone. The growing season for each crop was artificially divided into 'quarters' composed of equal numbers of whole days and roughly corresponding to specific growth stages. Ozone exposure was calculated for each of these 'quarters' and regressed against final crop yield using 163 different exposure statistics. Weighting functions were developed using reciprocal residual mean square (1/RMS) or percentage of the best 100 exposure statistics of the 163 tested (TOP100) for each of the quarters. The third quarter of the alfalfa season was clearly most responsive to ozone as measured by both of the weighting functions. Third quarter ozone was also weighted highest by both weighting functions for dry bean. Fresh market and processing tomato were each influenced the greatest by second quartero zone as demonstrated by both weighting functions. The occurrence of ozone during physiologically important events (flowering and initial fruit set in second quarter for tomato; pod development in third quarter for dry bean) appeared to influence the yield of these crops the greatest. Growth-stage-dependent phenological weighting of pollutant exposure may result in more effective predictions of levels of ozone exposure resulting in yield reductions.     

Atmospheric ozone: formation and effects on vegetation

Ozone (O(3)) is present both in the troposphere and the stratosphere. Troposphere O(3) is predominantly produced by photochemical reactions involving precursors generated by natural processes and to a much larger extent by man's activities. There is evidence for a trend towards increasing tropospheric O(3) concentrations. However, tropospheric O(3) is known to account for only 10% of the vertical O(3) column above the earth's surface. The stratosphere accounts for an additional 90% of the O(3) column. There is evidence to suggest that there are losses in the stratospheric O(3) due to the updraft of O(3) destroying pollutants generated by both natural processes and by human activity. Such a loss in stratospheric O(3) can result in alterations of incidence in the ultraviolet (UV) radiation to the earth's surface. Tropospheric O(3) is known to be highly phytotoxic. Appropriate exposures to O(3) can result in both acute (symptomatic) and chronic (changes in growth, yield or productivity and quality) effects. Chronic effects are of great concern in terms of both crops and forests. A number of experimental techniques are available to evaluate the chronic effects of O(3) on plants. There are limitations attached to the use of these techniques. However, results obtained, with such techniques are valuable if interpreted in the appropriate context. Among all field evaluation techniques, open-top chambers are the most frequently used method for evaluating the chronic effects of O(3) on crops. The National Crop Loss Assessment Program (NCLAN) of the United States is the largest such effort. However, given the limitations of the open-top chambers and the experimental aspects of NCLAN, its results must be interpreted with caution. On the other hand, acute effects can be evaluated with less complexity through the use of biological indicator plants. The numerical modelling of such effects are also far less complicated than establishing numerical cause and effects relationships for chronic effects. Confounding the acute or chronic responses of plants to O(3), is the presence of other kinds and forms of pollutants in the ambient atmosphere and the incidence of pathogens and pests. The resulting complex interactions and joint effects on plants are poorly understood. Future research must address these issues. In the final analysis we have re-emphasized the fact that plant health is the product of its interaction with the physical and chemical climatology and pathogens and pests. What we have described in this context is the importance of tropospheric O(3) within the chemical climatology of our environment and its effects on vegetation.     

Response of Brazilian native trees to acute ozone dose

Ozone (O₃) is a toxic secondary pollutant able to cause an intense oxidative stress that induces visual symptoms on sensitive plant species. Controlled fumigation experiment was conducted with the aim to verify the O₃ sensibility of three tropical species: Piptadenia gonoachanta (Mart.) Macbr. (Fabaceae), Astronium graveolens Jacq. (Anacardiaceae), and Croton floribundus Spreng. (Euphorbiaceae). The microscopical features involved in the oxidative stress were recognized based on specific histochemical analysis. The three species showed visual symptoms, characterized as necrosis and stippling between the veins, mostly visible on the adaxial leaf surface. All the studied species presented hypersensitive-like response (HR-like), and peroxide hydrogen accumulation (H₂O₂) followed by cell death and proanthocyanidin oxidation in P. gonoachanta and A. graveolens. In P. gonoachanta, a decrease in chlorophyll autofluorescence occurred on symptomatic tissues, and in A. graveolens and C. floribundus, a polyphenol compound accumulation occurred. The responses of Brazilian native species were similar to those described for sensitive species from temperate climate, and microscopical markers may be useful for the detection of ozone symptoms in future studies in the field.     

Meta-analysis of the relative sensitivity of semi-natural vegetation species to ozone

This study identified 83 species from existing publications suitable for inclusion in a database of sensitivity of species to ozone (OZOVEG database). An index, the relative sensitivity to ozone, was calculated for each species based on changes in biomass in order to test for species traits associated with ozone sensitivity. Meta-analysis of the ozone sensitivity data showed a wide inter-specific range in response to ozone. Some relationships in comparison to plant physiological and ecological characteristics were identified. Plants of the therophyte lifeform were particularly sensitive to ozone. Species with higher mature leaf N concentration were more sensitive to ozone than those with lower leaf N concentration. Some relationships between relative sensitivity to ozone and Ellenberg habitat requirements were also identified. In contrast, no relationships between relative sensitivity to ozone and mature leaf P concentration, Grime's CSR strategy, leaf longevity, flowering season, stomatal density and maximum altitude were found. The relative sensitivity of species and relationships with plant characteristics identified in this study could be used to predict sensitivity to ozone of untested species and communities.     

Physiological and growth responses of differentially irrigated cotton to ozone

This study was conducted to determine the physiological and growth responses of cotton (Gossypium hirsutum L.) to the interaction of ozonee (O3) and drought stress. Cotton (cv SJ-2) was grown in open-top chambers in the field at three levels of soil water and exposed to charcoal-filtered air (CF), nonfiltered air (NF), and NF x 1.25, and NF x 1.5 ambient O3 concentrations in Riverside, CA, from June to October 1986. Ozone reduced carbon fixation an average of 74.6% in optimally watered (OW) plots, 63.4% in suboptimal (SO) plots, but only 19.3% in severely water-stressed (SS) plots. Leaf and stem biomass in OW and SO plots showed similar linear reductions in mass response to increased O3 concentrations, but SS plots showed no response to O3 except at the highest O3 treatment (seasonal 12-h O3 mean of 0.111 ppm 218 microm(-3)). These results showed that moderately water-stressed cotton had similar physiological and growth responses to O3 as well-watered plants, but severely water-stressed cotton showed little response to O3 at ambient O3 concentrations.     

Physiological and biochemical stress responses in grassland species are influenced by both early-season ozone exposure and interspecific competition

The effects of two-year early season ozone exposure on physiological and biochemical stress response were investigated in model plant communities. Achillea millefolium and Veronica chamaedrys target plants were grown in monocultures and in mixed cultures with Poa pratensis (phytometer) and exposed in open-top chambers over two years for five weeks to charcoal-filtered (CF) air plus 25 nl l(-1) O3 (control) and non-filtered (NF) air plus 50 nl l(-1) O3. Significant O3 effects were detected in different physiological and biochemical parameters, evidencing interspecific differences in metabolic stress responses and a strong influence of the competition factor. O3 induced strong oxidative effects in Achillea irrespective to the different growth modality. Veronica showed less O3-induced effects in monoculture than when grown in competition with the phytometer. Poa exhibited a different behaviour against O3 depending on the species in competition, showing an overall higher sensitivity to O3 when in mixture with Achillea.     

Wheat-kernel growth characteristics during exposure to chronic ozone pollution

Chronic exposure to ozone (O(3)) air pollution can reduce yield in wheat; however, little is known concerning the effects of O(3) stress on kernel development. A field study was conducted to investigate the effects of chronic O(3) exposure on kernel-growth components of two soft red winter-wheat genotypes (Seven and MD5518308). Five air-quality treatments, including charcoal-filtered air (CF), non-filtered air (NF), NF + 20, and NF + 40 and NF + 80 nl O(3) liter(-1) air were applied 4 h d(-1), 5 d wk(-1) through maturity. In the case of the NF + treatments, O(3) was added to existing ambient O(3) levels. Spike samples were collected 16, 20, 24, 28, and 32 days after anthesis (DAA). Linear and quadratic equations were fitted to kernel-weight data to estimate kernel-growth rate (KGR) and kernel-fill duration (KFD). Effective filling period (EFP) and assimilate utilization (AU) were also determined. Rates of growth for individual kernels were 0.74 mg d(-1) and 1.07 mg d(-1) for the NF + 80 and CF treatments, respectively. The NF + 80 nL litter(-1) O(3) treatment significantly reduced KGR and AU compared with the CF treatment. Severn had a significantly loger KFD than MD5518308, but O(3) had no significant effect on KFD of either genotype. Each genotype had similar EFP values, and O(3) had no significant effect on EFP. Linear relationships between O(3) exposure and kernel weight suggests that O(3) effects on kernel weight begin soon after anthesis in MD5518308, but, in Severn, O(3) has a greater effect on kernel weight during the later stages of kernel development. These data suggest that decreased economic yield associated with chronic O(3) exposure is primarily the result of decreased KGR.     

A new flux-orientated concept to derive critical levels for ozone to protect vegetation

The current European critical levels for ozone (O3) to protect crops, natural and semi-natural vegetation and forest trees are based on a relative small number of open-top chamber experiments with a very limited number of plant species. Therefore, the working group "Effects of Ozone on Plants" of the Commission on Air Pollution Prevention of the Association of German Engineers and the German Institute of Standardization reanalysed the literature on O3 effects on European plant species published between 1989 and 1999. An exposure-response relationship for wild plant species and agricultural crops could be derived from 30 experiments with more than 30 species and 90 data points; the relationship for conifer and deciduous trees is based on 20 experiments with nine species and 50 data points. From these relationships maximum O3 concentrations for different risk stages are deduced, below which the vegetation type is protected on the basis of the respective criteria. Because it is assumed that the fumigation concentrations reflect the O3 concentrations at the top of the canopy, i.e. the upper surface boundary of the quasi-laminar layer if the micrometeorological big-leaf approach is applied, the application of these maximum O3 concentrations requires the transformation of O3 concentrations measured at a reference height above the canopy to the effective phytotoxic concentrations at the top of the canopy. Thus, the approach described in this paper is a synthesis of the classical concept of toxicology of air pollutants (critical concentrations) and the more toxicological relevant dose concept.     

Critical levels for ozone effects on vegetation in Europe

The evidence of detrimental effects of ozone on vegetation in Europe, and the need to develop international control policies to reduce ozone exposures which are based on the effects of the pollutant, has led to attempts to define so-called critical levels of ozone above which adverse effects on trees, crops and natural vegetation may occur. This review is a critical assessment of the scientific basis of the concepts used to define critical levels for ozone and identifies the key limitations and uncertainties involved. The review focuses on the Level I critical level approach, which provides an environmental standard or threshold to minimise the effects of ozone on sensitive receptors, but does not seek to quantify the impacts of exceeding the critical level under field conditions. The concept of using the AOT (accumulated exposure over a threshold) to define long-term ozone exposure is demonstrated to be appropriate for several economically important species. The use of 40 ppb (giving the AOT40 index) as a threshold concentration gives a good linear fit to experimental data from open-top chambers for arable crops, but it is less certain that it provides the best fit to data for trees or semi-natural communities. Major uncertainties in defining critical level values relate to the choice of response parameter and species; the absence of data for many receptors, especially those of Mediterranean areas; and extrapolation to field conditions from relatively short-term open-top chamber experiments. The derivation of critical levels for long-lived organisms, such as forest trees, may require the use of modelling techniques based on physiological data from experimental studies. The exposure-response data which have been applied to derive critical levels should not be used to estimate the impacts of ozone over large areas, because of the uncertainties associated with extrapolation from the open-top chamber method, especially for forest trees, and because of spatial variation in atmospheric and environmental conditions, which may alter ozone uptake.