Stratospheric ozone depletion, UV-B radiation and crop disease

Ultraviolet-B radiation (UV-B: 290-315 nm) is expected to increase as the result of stratospheric ozone depletion. Within the environmental range, UV-B effects on host plants appear to be largely a function of photomorphogenic responses, while effects on fungal pathogens may include both photomorphogenesis and damage. The effects of increased UV-B on plant-pathogen interactions has been studied in only a few pathosystems, and have used a wide range of techniques, making generalisations difficult. Increased UV-B after inoculation tends to reduce disease, perhaps due to direct damage to the pathogen, although responses vary markedly between and within pathogen species. Using Septoria tritici infection of wheat as a model system, it is suggested that even in a species that is inherently sensitive to UV-B, the effects of ozone depletion in the field are likely to be small compared with the effects of variation in UV-B due to season and varying cloud. Increased UV-B before inoculation causes a range of effects in different systems, but an increase in subsequent disease is a common response, perhaps due to changes in host surface properties or chemical composition. Although it seems unlikely that most crop diseases will be greatly affected by stratospheric ozone depletion within the limits currently expected, the lack of a detailed understanding of the mechanisms by which UV-B influences plant-pathogen interactions in most pathosystems is a significant limit to such predictions.     

The Greenhouse effect: impacts of ultraviolet-B (UV-B) radiation, carbon dioxide (CO2), and ozone (O3) on vegetation

There is a fast growing and an extremely serious international scientific, public and political concern regarding man's influence on the global climate. The decrease in stratospheric ozone (O3) and the consequent possible increase in ultraviolet-B (UV-B) is a critical issue. In addition, tropospheric concentrations of 'greenhouse gases' such as carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) are increasing. These phenomena, coupled with man's use of chlorofluorocarbons (CFCs), chlorocarbons (CCs), and organo-bromines (OBs) are considered to result in the modification of the earth's O3 column and altered interactions between the stratosphere and the troposphere. A result of such interactions could be the global warming. As opposed to these processes, tropospheric O3 concentrations appear to be increasing in some parts of the world (e.g. North America). Such tropospheric increases in O3 and particulate matter may offset any predicted increases in UV-B at those locations. Presently most general circulation models (GCMs) used to predict climate change are one- or two-dimensional models. Application of satisfactory three-dimensional models is limited by the available computer power. Recent studies on radiative cloud forcing show that clouds may have an excess cooling effect to compensate for a doubling of global CO2 concentrations. There is a great deal of geographic patchiness or variability in climate. Use of global level average values fails to account for this variability. For example, in North America: 1. there may be a decrease in the stratospheric O3 column (1-3%); however, there appears to be an increase in tropospheric O3 concentrations (1-2%/year) to compensate up to 20-30% loss in the total O3 column; 2. there appears to be an increase in tropospheric CO2, N2O and CH4 at the rate of roughly 0.8%, 0.3% and 1-2%, respectively, per year; 3. there is a decrease in erythemal UV-B; and 4. there is a cooling of tropospheric air temperature due to radiative cloud forcing. The effects of UV-B, CO2 and O3 on plants have been studied under growth chamber, greenhouse and field conditions. Few studies, if any, have examined the joint effects of more than one variable on plant response. There are methodological problems associated with many of these experiments. Thus, while results obtained from these studies can assist in our understanding, they must be viewed with caution in the context of the real world and predictions into the future. Biomass responses of plants to enhanced UV-B can be negative (adverse effect); positive (stimulatory effect) or no effect (tolerant). Sensitivity rankings have been developed for both crop and tree species. However, such rankings for UV-B do not consider dose-response curves. There are inconsistencies between the results obtained under controlled conditions versus field observations. Some of these inconsistencies appear due to the differences in responses between cultivars and varieties of a given plant species; and differences in the experimental methodology and protocol used. Nevertheless, based on the available literature, listings of sensitive crop and native plant species to UV-B are provided. Historically, plant biologists have studied the effects of CO2 on plants for many decades. Experiments have been performed under growth chamber, greenhouse and field conditions. Evidence is presented for various plant species in the form of relative yield increases due to CO2 enrichment. Sensitivity rankings (biomass response) are agein provided for crops and native plant species. However, most publications on the numerical analysis of cause-effect relationships do not consider sensitivity analysis of the mode used. Ozone is considered to be the most phytotoxic regional scale air pollutant. In the pre-occupation of loss in the O3 column, any increases in tropospheric O3 concentrations may be undermined relative to vegetation effects. As with the other stress factors, the effects of O3 have been studied both under controlled and field conditions. Thboth under controlled and field conditions. The numerical explanation of cause-effect relationships of O3 is a much debated subject at the present time. Much of the controversy is directed toward the definition of the highly stochastic, O3 exposure dynamics in time and space. Nevertheless, sensitivity rankings (biomass response) are provided for crops and native vegetation. The joint effects of UV-B, CO2 and O3 are poorly understood. Based on the literature of plant response to individual stress factors and chemical and physical climatology of North America, we conclude that nine different crops may be sensitive to the joint effects: three grain and six vegetable crops (sorghum, oat, rice, pea, bean, potato, lettuce, cucumber and tomato). In North America, we consider Ponderosa and loblolly pines as vulnerable among tree species. This conclusion should be moderated by the fact that there are few, if any, data on hardwood species. In conclusion there is much concern for global climate change and its possible effects on vegetation. While this is necessary, such a concern and any predictions must be tempered by the lack of sufficient knowledge. Experiments must be designed on an integrated and realistic basis to answer the question more definitively. This would require very close co-operation and communication among scientists from multiple disciplines. Decision makers must realize this need.     

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.     

Depletion of stratospheric ozone over the Antarctic and Arctic: responses of plants of polar terrestrial ecosystems to enhanced UV-B, an overview

Depletion of stratospheric ozone over the Antarctic has been re-occurring yearly since 1974, leading to enhanced UV-B radiation. Arctic ozone depletion has been observed since 1990. Ozone recovery has been predicted by 2050, but no signs of recovery occur. Here we review responses of polar plants to experimentally varied UV-B through supplementation or exclusion. In supplementation studies comparing ambient and above ambient UV-B, no effect on growth occurred. UV-B-induced DNA damage, as measured in polar bryophytes, is repaired overnight by photoreactivation. With UV exclusion, growth at near ambient may be less than at below ambient UV-B levels, which relates to the UV response curve of polar plants. UV-B screening foils also alter PAR, humidity, and temperature and interactions of UV with environmental factors may occur. Plant phenolics induced by solar UV-B, as in pollen, spores and lignin, may serve as a climate proxy for past UV. Since the Antarctic and Arctic terrestrial ecosystems differ essentially, (e.g. higher species diversity and more trophic interactions in the Arctic), generalization of polar plant responses to UV-B needs caution.     

上海城市化对气象要素和臭氧浓度的影响

为探讨城市化引起的土地利用变化对上海近地面气象要素和臭氧(O3)浓度的影响,运用美国国家大气研究中心等机构共同开发的WRF-Chem模式,在考虑扩大城市用地、运用城市冠层模式以及城市人为热影响的基础上,针对上海地区2个不同发展时期的下垫面土地利用类型,就2007年3次高浓度O3天气过程,设置4组灵敏性试验进行模拟。结果表明,以虹桥机场站为代表的市区受城市化影响温度升高、相对湿度降低、风速减小,日平均温度最高上升3.5℃,日平均相对湿度最大降低20%,日平均风速最大减小1.5m/s;但以青浦站和川沙站为代表的郊区受城市化影响不明显。此外,以卢湾站为代表的市区,O3浓度普遍增加,日均值最高可增加8.3μg/m3;但以川沙站和淀山湖站为代表的郊区,O3浓度的变化随着个例的不同有增加也有减少。     

UV-B radiation and acclimation in timberline plants

Research has shown that some plants respond to enhanced UV-B radiation by producing smaller and thicker leaves, by increasing the thickness of epidermis and concentration of UV-B absorbing compounds of their surface layers and activation of the antioxidant defence system. The response of high-altitude plants to UV-B radiation in controlled conditions is often less pronounced compared to low-altitude plants, which shows that the alpine timberline plants are adapted to UV-B. These plants may have a simultaneous co-tolerance for several stress factors: acclimation or adaptation to the harsh climate can also increase tolerance to UV-B radiation, and vice versa. On the other hand, alpine timberline plants of northern latitudes may be less protected against increasing UV-B radiation than plants from more southern latitudes and higher elevations due to harsh conditions and weaker preadaptation resulting from lower UV-B radiation exposure. It is evident that more long-term experimental field research is needed in order to study the interaction of climate, soil and UV-B irradiance on the timberline plants.     

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.     

The impact of vegetation on sedimentary organic matter composition and PAH desorption

Relationships between sedimentary organic matter (SOM) composition and PAH desorption behavior were determined for vegetated and non-vegetated refinery distillate waste sediments. Sediments were fractionated into size, density, and humin fractions and analyzed for their organic matter content. Bulk sediment and humin fractions differed more in organic matter composition than size/density fractions. Vegetated humin and bulk sediments contained more polar organic carbon, black carbon, and modern (plant) carbon than non-vegetated sediment fractions. Desorption kinetics of phenanthrene, pyrene, chrysene, and C₃-phenanthrene/anthracenes from humin and bulk sediments were investigated using Tenax^® beads and a two-compartment, first-order kinetic model. PAH desorption from distillate waste sediments appeared to be controlled by the slow desorbing fractions of sediment; rate constants were similar to literature values for k_slow and k_{very slow}. After several decades of plant colonization and growth (Phragmites australis), vegetated sediment fractions more extensively desorbed PAHs and had faster desorption kinetics than non-vegetated sediment fractions.     

The exchange of ozone between vegetation and atmosphere: micrometeorological measurement techniques and models

The European critical levels (CLs) to protect vegetation are expressed as an accumulative exposure over a threshold of 40 ppb (nl l(-1)). In view of the fact that these chamber-derived CLs are based on ozone (O(3)) concentrations at the top of the canopy the correct application to ambient conditions presupposes the application of Soil-Vegetation-Atmosphere-Transfer (SVAT) models for quantifying trace gas exchange between phytosphere and atmosphere. Especially in the context of establishing control strategies based on flux-oriented dose-response relationships, O(3) flux measurements and O(3) exchange simulations are needed for representative ecosystems. During the last decades several micrometeorological methods for quantifying energy and trace gas exchange were developed, as well as models for the simulation of the exchange of trace gases between phytosphere and atmosphere near the ground. This paper is a synthesis of observational and modeling techniques which discusses measurement methods, assumptions, and limitations and current modeling approaches. Because stomatal resistance for trace gas exchange is parameterized as a function of water vapor or carbon dioxide (CO(2)) exchange, the most important micrometeorological techniques especially for quantifying O(3), water vapor and CO(2) flux densities are discussed. A comparison of simulated and measured O(3) flux densities shows good agreement in the mean.     

Sources of errors in assessing ozone visible symptoms on native vegetation

This paper aims to identify the problems regarding the evaluation of ozone (and ozone-like) symptoms, by examining the results of the 4th UN/ECE ICP-Forests Intercalibration Course for the assessment of ozone-induced visible symptoms. Trees, shrubs and herbaceous species were evaluated in a tree nursery, at Lattecaldo (Switzerland) and under open field conditions at Moggio (Italy). The main findings were: (i) the most expert surveyors tended to be grouped in the same cluster and, during the field exercises, they tended to assess in a more conservative manner compared to the less trained participants; (ii) the agreement was greater in assessing the absence rather than the presence of symptoms; (iii) typical interveinal stippling on the upper leaf surface was more accurately evaluated than discoloration; (iv) uncertainties resulted mainly for species which showed greater variability in their symptom manifestation, and for certain herbaceous species.     

The impact of ozone on assimilate partitioning in plants: a review

Numerous studies have shown that ozone (O(3)) reduces plant growth and changes assimilate partitioning. The pattern of such changes varies with species, but trends suggest a comprehensive model. O(3) generally reduces the amount of dry matter in the whole plant. In plants which have not flowered or set fruit, and at low O(3) levels, the remaining available assimilate is generally diverted to leaves and stems at the expense of roots and crowns. As the plant matures, flowers and develops seeds, these sinks receive a relatively high proportion of the available assimilate. O(3) may reduce the number of flowers or seeds, but the remaining seeds often have a total dry matter accumulation comparable to that in non-stressed plants. At higher O(3) levels, assimilate accumulation is greatly depressed, and partitioning changes are not as obvious. However, it is significant that the storage organs of plants-those organs which supply energy for new growth in perennial plants such as trees-are the organs most affected by O(3)-induced partitioning changes when O(3) concentrations are in the range commonly observed in polluted ambient air.     

Possible impacts of changes in UV-B radiation on North American trees and forests

Approximately 35 species representing 14 tree genera have been evaluated for responses to UV-B radiation in North America. The best representation has been in the conifers where some 20 species representing three genera have been studied. Overall, about 1/3 of these have demonstrated some deleterious response to UV-B. However, most negative impacts have been observed under controlled environment conditions where sensitivity may be enhanced. Therefore, it seems unlikely that expected levels of ozone depletion will result in direct losses in productivity. However, the role that ambient or enhanced levels of UV-B may play in forest ecosystem processes is more difficult to access. One possible indirect response of forests to changes in UV-B radiation levels could be via alterations in plant secondary metabolites. Increases in phenolics and flavonoids that enhance epidermal UV-screening effectiveness may also influence leaf development, water relations or ecosystem processes such as plant-herbivore interactions or decomposition.     

Increasing risk for negative ozone impacts on vegetation in northern Sweden

Trends were found for increasing surface ozone concentrations during April-September in northern Sweden over the period 1990-2006 as well as for an earlier onset of vegetation growing season. The highest ozone concentrations in northern Sweden occurred in April and the ozone concentrations in April showed a strong increasing trend. A model simulation of ozone flux for Norway spruce indicated that the provisional ozone flux based critical level for forests in Europe is exceeded in northern Sweden. Future climate change would have counteracting effects on the stomatal conductance and needle ozone uptake, mediated on the one hand by direct effect of increasing air temperatures and on the other through increasing water vapour pressure difference between the needles and air. Thus, there is a substantial and increasing risk for negative impacts of ozone on vegetation in northern Sweden, related mainly to increasing ozone concentrations and an earlier onset of the growing season.     

The impact of ozone on a salt marsh cordgrass (Spartina alterniflora)

Spartina alterniflora plants were collected from salt marshes within New Jersey, South Carolina, and Georgia USA and shipped to The Pennsylvania State University. New plants were grown from rhizomes in six open-top field chambers. Three chambers received charcoal-filtered air, and three received charcoal-filtered air plus 80 ppb ozone, 8 h/day for 65 days. Flower, leaf, and shoot number per plant were recorded weekly. Photosynthetic rates were measured in week 5, and foliar injury was assessed during week 9. Final dry weight of roots, shoots, and rhizomes were determined. While ozone-treated plants from all states expressed symptoms of ozone injury, plants from South Carolina exhibited no effect of ozone on any other measured variable. Plants from the Georgia site showed ozone-induced reductions in all measured variables except leaf dry weight. Ozone-treated plants from New Jersey showed reductions in photosynthetic rate, leaf and shoot number, and root dry weights. Only plants from New Jersey produced flowers, with ozone treatment causing delay in flowering and reduction in the number of flower spikes produced.     

Climate change: potential effects of increased atmospheric carbon dioxide (CO2), ozone (O3), and ultraviolet-B (UV-B) radiation on plant diseases

Continued world population growth results in increased emission of gases from agriculture, combustion of fossil fuels, and industrial processes. This causes changes in the chemical composition of the atmosphere. Evidence is emerging that increased solar ultraviolet-B (UV-B) radiation is reaching the earth's atmosphere, due to stratospheric ozone depletion. Carbon dioxide (CO(2)), ozone (O(3)) and UV-B are individual climate change factors that have direct biological effects on plants. Such effects may directly or indirectly affect the incidence and severity of plant diseases, caused by biotic agents. Carbon dioxide may increase plant canopy size and density, resulting in a greater biomass of high nutritional quality, combined with a much higher microclimate relative humidity. This would be likely to promote plant diseases such as rusts, powdery mildews, leaf spots and blights. Inoculum potential from greater overwintering crop debris would also be increased. Ozone is likely to have adverse effects on plant growth. Necrotrophic pathogens may colonize plants weakened by O(3) at an accelerated rate, while obligate biotroph infections may be lessened. Ozone is unlikely to have direct adverse effects on fungal pathogens. Ozone effects on plant diseases are host plant mediated. The principal effects of increased UV-B on plant diseases would be via alterations in host plants. Increased flavonoids could lead to increased diseased resistance. Reduced net photosynthesis and premature ripening and senescence could result in a decrease in diseases caused by biotrophs and an increase in those caused by necrotrophs. Microbial plant pathogens are less likely to be adversely affected by CO(2), O(3) and UV-B than are their corresponding host plants. Changes in host plants may result in expectable alterations of disease incidence, depending on host plant growth stages and type of pathogen. Given the importance of plant diseases in world food and fiber production, it is essential to begin studying the effects of increased CO(2), O(3) and UV-B (and other climate change factors) on plant diseases. We know very little about the actual impacts of climate change factors on disease epidemiology. Epidemiologists should be encouraged to consider CO(2), O(3) and UV-B as factors in their field studies.     

The contribution of reactive carbon emissions from vegetation to the carbon balance of terrestrial ecosystems

From November 1998 to October 2000, measurements of soil respiration were performed on the Spanish plateau for two patches of non-irrigated barley, one managed with conventional tillage (CT) and the other with reduced tillage (RT). Soil CO2 flux showed seasonal variation on both patches, with an increase from March to October, peaking in May, and a decrease during the winter period by a factor of around 2. The mean value for both combined years was 2.03 and 1.70 micromol m(-2) S(-1), in the CT and RT patches, respectively. In order to analyse the influence of RT on soil CO2 flux, two tests were performed. The first one was the Kruskal-Wallis test to compare whether the differences between the medians in both patches were statistically significant. The results obtained revealed statistically significant differences during the second year, at a 85% and 95% significance level, use being made of annual data and that recorded during the period of maximum interest, March-October, respectively. The decrease in soil respiration in the RT patch was around 24%. The second test was aimed at describing and comparing the influence of soil temperature on soil CO2 flux. By using the data of both patches recorded during the first year, an empirical equation on 10-cm soil temperature was fitted and tested on the data corresponding to the second year in each of the patches. Then, a comparison between the medians of the differences between the estimated and observed values was again performed by means of the Kruskal-Wallis test. The over-prediction of the model in the RT patch, statistically significant at a 90% significance level, was roughly 23%, confirming again the decrease in soil respiration one year after this agricultural management practice had been implemented.     

Volatile organic compounds from Italian vegetation and their interaction with ozone

Volatile Organic Compounds (VOCs) emitted from vegetation (particularly isoprenoids) represent an important source of atmospheric hydrocarbons almost double the anthropogenic source. When biogenic VOC mix with NO_x in the presence of UV radiation, ozone (O₃) is formed. In Italy, optimal conditions for O₃ formation in terms of VOC/NO_x ratios and abundance of UV radiation occur for long periods of the year. Moreover, Italian vegetation includes several species that are strong and evergreen isoprenoid emitters, and high temperatures for part of the year further stimulate these temperature-dependent emissions. We review emission of isoprenoids from Italian vegetation, current knowledge on the impact of rising O₃ levels on isoprenoid emission, and evidence showing that isoprenoids can increase both the O₃ flux to the plant and protection against oxidative stress because of their antioxidant functions. This trait not only influences plant tolerance to O₃ but also may substantially alter the flux of O₃ between atmosphere and biosphere.     

Effects of the ultraviolet-B radiation (UV-B) on conifers: a review

The current knowledge on conifer responses to enhanced ultraviolet-B (UV-B) radiation is mainly based on greenhouse or growth chamber experiments of one growing season in duration. However, the biomass losses observed in greenhouses do not occur in field-grown trees in their natural habitats. Moreover, the majority of the 20 conifer species studied have been 1-year-old seedlings, and no studies have been undertaken on mature trees. Fully grown needles, with their glaucous waxy surfaces and thick epidermal cells with both soluble and wall-bound UV-B screening metabolites, are well protected against UV-B radiation. However, it is not known whether these are sufficient protectants in young emerging needles or during the early spring period of high UV-B levels reflected from snow. In order to understand all the mechanisms that result in the protection of conifer needles against UV-B radiation, future research should focus on the epidermal layer, separating the waxes, cuticle and epidermal and hypodermal cells. Parallel studies should consist of wall-bound and soluble secondary metabolite analysis, antioxidant measurements and microscopic observations.     

Effects of atmospheric ammonia (NH3) on terrestrial vegetation: a review

At the global scale, among all N (nitrogen) species in the atmosphere and their deposition on to terrestrial vegetation and other receptors, NH3 (ammonia) is considered to be the foremost. The major sources for atmospheric NH3 are agricultural activities and animal feedlot operations, followed by biomass burning (including forest fires) and to a lesser extent fossil fuel combustion. Close to its sources, acute exposures to NH3 can result in visible foliar injury on vegetation. NH3 is deposited rapidly within the first 4-5 km from its source. However, NH3 is also converted in the atmosphere to fine particle NH4+ (ammonium) aerosols that are a regional scale problem. Much of our current knowledge of the effects of NH3 on higher plants is predominantly derived from studies conducted in Europe. Adverse effects on vegetation occur when the rate of foliar uptake of NH3 is greater than the rate and capacity for in vivo detoxification by the plants. Most to least sensitive plant species to NH3 are native vegetation > forests > agricultural crops. There are also a number of studies on N deposition and lichens, mosses and green algae. Direct cause and effect relationships in most of those cases (exceptions being those locations very close to point sources) are confounded by other environmental factors, particularly changes in the ambient SO2 (sulfur dioxide) concentrations. In addition to direct foliar injury, adverse effects of NH3 on higher plants include alterations in: growth and productivity, tissue content of nutrients and toxic elements, drought and frost tolerance, responses to insect pests and disease causing microorganisms (pathogens), development of beneficial root symbiotic or mycorrhizal associations and inter species competition or biodiversity. In all these cases, the joint effects of NH3 with other air pollutants such as all-pervasive O3 or increasing CO2 concentrations are poorly understood. While NH3 uptake in higher plants occurs through the shoots, NH4+ uptake occurs through the shoots, roots and through both pathways. However, NH4+ is immobile in the soil and is converted to NO3- (nitrate). In agricultural systems, additions of NO3- to the soil (initially as NH3 or NH4+) and the consequent increases in the emissions of N2O (nitrous oxide, a greenhouse gas) and leaching of NO3- into the ground and surface waters are of major environmental concern. At the ecosystem level NH3 deposition cannot be viewed alone, but in the context of total N deposition. There are a number of forest ecosystems in North America that have been subjected to N saturation and the consequent negative effects. There are also heathlands and other plant communities in Europe that have been subjected to N-induced alterations. Regulatory mitigative approaches to these problems include the use of N saturation data or the concept of critical loads. Current information suggests that a critical load of 5-10 kg ha(-1) year(-1) of total N deposition (both dry and wet deposition combined of all atmospheric N species) would protect the most vulnerable terrestrial ecosystems (heaths, bogs, cryptogams) and values of 10-20 kg ha(-1) year(-1) would protect forests, depending on soil conditions. However, to derive the best analysis, the critical load concept should be coupled to the results and consequences of N saturation.