Does ascorbate in the mesophyll cell walls form the first line of defence against ozone? Testing the concept using broad bean (Vicia faba L.)

Chitooligosaccharide Maintained Cell Membrane Integrity by Regulating Reactive Oxygen Species Homeostasis at Wounds of Potato Tubers during Healing

Chitooligosaccharide (COS) is a degradation product of chitosan. Although COS increased fruit resistance by regulating the metabolism of reactive oxygen species (ROS), few reports are available on whether COS regulates ROS homeostasis at wounds of potato tubers during healing. In this study, COS increased gene expression and activities of NADPH oxidase and superoxide dismutase, and promoted the generation of O₂^●- and H₂O₂. Moreover, COS increased gene expression and activities of catalase, peroxidase, and AsA-GSH cycle-related enzymes, as well as the levels of ascorbic acid and glutathione levels. In addition, COS elevated the scavenging ability of DPPH, ABTS⁺, and FRAP, and reduced cell membrane permeability and malondialdehyde content. Taken together, COS could maintain cell membrane integrity by eliminating excessive H₂O₂ and improving the antioxidant capacity in vitro, which contributes to the maintainance of cell membrane integrity at wounds of potato tubers during healing.

Cellular Responses Required for Oxidative Stress Tolerance of the Necrotrophic Fungus Alternaria alternata, Causal Agent of Pear Black Spot

To establish successful infections in host plants, pathogenic fungi must sense and respond to an array of stresses, such as oxidative stress. In this study, we systematically analyzed the effects of 30 mM H₂O₂ treatment on reactive oxygen species (ROS) metabolism in Alternaria alternata. Results showed that 30 mM H₂O₂ treatment lead to increased O²⁻ generation rate and H₂O₂ content, and simultaneously, increased the activities and transcript levels of NADPH oxidase (NOX). The activities and gene expression levels of enzymes related with ascorbic acid-glutathione cycle (AsA-GSH cycle) and thioredoxin systems, including superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), ascorbate peroxidase (AXP) and thioredoxin (TrxR), were remarkably enhanced by 30 mM H₂O₂ stress treatment. Additionally, 30 mM H₂O₂ treatment decreased the glutathione (GSH) content, whereas it increased the amount of oxidized glutathione (GSSG), dehydroascorbate (DHA) and ascorbic acid (AsA). These results revealed that cellular responses are required for oxidative stress tolerance of the necrotrophic fungus A. alternata.

Biochemical responses of hairgrass (Deschampsia caespitosa) to hydrological change

Plant growth and development are closely related to water availability. Water deficit and water excess are detrimental to plants, causing a series of damage to plant morphology, physiological and biochemical processes. In the long evolutionary process, plants have evolved an array of complex mechanisms to combat against stressful conditions. In the present study, the duration-dependent changes in ascorbate (AsA) and glutathione (GSH) contents and activities of enzymes involved in the AsA-GSH cycle in hairgrass (Deschampsia caespitosa) in response to water stress was investigated in a pot trial using a complete random block design. The treatments were as follows: (1) heavily waterlogging, (2) moderate waterlogging, (3) light waterlogging, (4) light drought, (5) moderate drought, (6) heavily drought, and (7) a control (CK) with plant be maintained at optimum water availability. The hairgrass plants were subjected to waterlogging or drought for 7, 14, 21 and 28 days and data were measured following treatment. Results revealed that hairgrass subjected to water stress can stimulate enzymatic activities of ascorbate peroxidase (APX), glutathione peroxidase (GPX), glutathione reductase (GR), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR) and L-galactono-1, 4-lactone dehydrogenase (GalLDH), switched on the ascorbate-glutathione (AsA-GSH) cycle and the L-galactose synthesis, up-regulated the contents of AsA and GSH, and maintained higher ratios of ascorbate to dehydroascorbate (AsA/DHA) and reduced glutathione to oxidized glutathione (GSH/GSSG) to alleviate potential oxidative damage. However, the light waterlogging did not induce hairgrass under stress to switch on the AsA-GSH pathway. In general, the critic substances and enzyme activities in AsA-GSH metabolic pathway increased as the increase of water stress intensity. As the increase of exposure duration, the critic antioxidant substances content and enzyme activities increased first and then maintained a relatively stable higher level. Our findings provide comprehensive information on biochemical responses of hairgrass to hydrological change, which would be a major step for accelerating ecological restoration of degradation alpine marshes in the Qinghai-Tibetan Plateau.

Ozone uptake at night is more damaging to plants than equivalent day-time flux

MAIN CONCLUSION

Plants exposed to equivalent ozone fluxes administered during day-time versus night-time exhibited greater losses in biomass at night and this finding is attributed to night-time depletion of cell wall-localised ascorbate. The present study employed Lactuca sativa and its closest wild relative, L. serriola, to explore the relative sensitivity of plants to ozone-induced oxidative stress during day-time versus night-time. By controlling atmospheric ozone concentration and measuring stomatal conductance, equivalent ozone uptake into leaves was engineered during day and night, and consequences on productivity and net CO2 assimilation rate were determined. Biomass losses attributable to ozone were significantly greater when an equivalent dose of ozone was taken-up by foliage at night compared to the day. Linkages between ozone impacts and ascorbic acid (AA) content, redox status and cellular compartmentation were probed in both species. Leaf AA pools were depleted by exposure of plants to darkness, and then AA levels in the apoplast and symplast were monitored on subsequent transfer of plants to the light. Apoplast AA appeared to be more affected by light-dark transition than the symplast pool. Moreover, equivalent ozone fluxes administered to leaves with contrasting AA levels resulted in contrasting effects on the light-saturated rate of CO2 assimilation (Asat) in both species. Once apoplast AA content recovered to pre-treatment levels, the same ozone flux resulted in no impacts on Asat. The results of the present investigation reveal that plants are significantly more sensitive to equivalent ozone fluxes taken-up at night compared with those during the day and were consistent with diel shifts in apoplast AA content and/or redox status. Furthermore, findings suggest that some thought should be given to weighing regional models of ozone impacts for extraordinary night-time ozone impacts.

Quantifying determinants of ozone detoxification by apoplastic ascorbate in peach (Prunus persica) leaves using a model of ozone transport and reaction

Ascorbate in leaf apoplast (ASC_apo ) reacts with ozone (O₃ ) and thereby reduces O₃ flux reaching plasmalemma (F_pl ). Some studies have shown significant protection of cells from O₃ by ASC_apo , while others have questioned its efficacy. Hypothesizing that the protection by ASC_apo depends on other variables, we quantified determinants of O₃ detoxification with a model of O₃ transport and reaction in apoplast. The model determines ascorbic acid concentration in apoplast (AA_apo ) using measured values of O₃ concentration (c_o ), leaf tissue ascorbic acid concentration (AA_leaf ), cell wall thickness (L₃ ), apoplastic pH (pH_apo ), and stomatal conductance (G_sw ). We compared the measured and model-estimated AA_apo in leaves of peach (Prunus persica) grown in open-top chambers under non-filtered air (NF) and elevated (EO₃ : NF + 80 ppb) O₃ concentrations. The estimated AA_apo in individual leaves agreed well with the measured values (R²  = .91). Analyses of the simulation results yielded the following findings: (a) The efficacy of O₃ reduction with ASC_apo as quantified by fractional reduction (ϕ₃ ) of O₃ flux at the surface of plasmalemma (F_pl ) was lowered from 70% in NF to 40% in EO₃ due to the reduction of L₃ . The EO₃ reduced AA_apo , but the lower G_sw and L₃ in EO₃ increased AA_apo resulting in no significant change in AA_apo due to EO₃ . ϕ₃ can be calculated with measured values of AA_apo and L₃ , and F_pl can be estimated with the measurement-based ϕ₃ . (b) When c₀ is increased, F_pl increased curvilinearly with the increase of F_st : nominal O₃ flux via stomatal diffusion, exhibiting apparent threshold on F_st . The deviation of F_pl from F_st became greater when L₃ , pH_apo , and AA_leaf were increased. The quantification of ϕ₃ and F_pl using leaf traits shall facilitate the understanding of the mechanisms of differential plant sensitivity to O₃ and improve quantification of the O₃ impacts on plants.

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.

Ozone effects on plants in natural ecosystems

Tropospheric ozone (O₃ ) is an important stressor in natural ecosystems, with well-documented impacts on soils, biota and ecological processes. The effects of O₃ on individual plants and processes scale up through the ecosystem through effects on carbon, nutrient and hydrologic dynamics. Ozone effects on individual species and their associated microflora and fauna cascade through the ecosystem to the landscape level. Systematic injury surveys demonstrate that foliar injury occurs on sensitive species throughout the globe. However, deleterious impacts on plant carbon, water and nutrient balance can also occur without visible injury. Because sensitivity to O₃ may follow coarse physiognomic plant classes (in general, herbaceous crops are more sensitive than deciduous woody plants, grasses and conifers), the task still remains to use stomatal O₃ uptake to assess class and species' sensitivity. Investigations of the radial growth of mature trees, in combination with data from many controlled studies with seedlings, suggest that ambient O₃ reduces growth of mature trees in some locations. Models based on tree physiology and forest stand dynamics suggest that modest effects of O₃ on growth may accumulate over time, other stresses (prolonged drought, excess nitrogen deposition) may exacerbate the direct effects of O₃ on tree growth, and competitive interactions among species may be altered. Ozone exposure over decades may be altering the species composition of forests currently, and as fossil fuel combustion products generate more O₃ than deteriorates in the atmosphere, into the future as well.

Ascorbic Acid and Ozone: Novel Perspectives to Explain an Elusive Relationship

A huge amount of studies highlighted the importance of high ascorbic acid (AA) content in ozone tolerance, yet the relationship between them appears more complex than a simple direct correlation. Sometimes the connection is clear, for example, two Arabidopsis mutants defective in the main AA biosynthetic pathway (vtc mutants) were identified by means of their ozone sensitivity. However, some low-AA containing mutants are relatively tolerant, suggesting that AA location/availability could be more relevant than total content. A clear distinction should also be made between ozone tolerance obtained when AA content is increased by experimental supplementation (exogenous AA), and the physiological role of plant-synthesized AA (endogenous AA), whose amount is apparently subjected to tight regulation. Recent findings about the role of AA in signal transduction and epigenetic regulation of gene expression open new routes to further research.

Exogenous Melatonin Counteracts NaCl-Induced Damage by Regulating the Antioxidant System, Proline and Carbohydrates Metabolism in Tomato Seedlings

Melatonin, a natural agent, has multiple functions in animals as well as in plants. However, its possible roles in plants under abiotic stress are not clear. Nowadays, soil salinity is a major threat to global agriculture because a high soil salt content causes multiple stresses (hyperosmotic, ionic, and oxidative). Therefore, the aim of the present study was to explore: (1) the involvement of melatonin in biosynthesis of photosynthetic pigments and in regulation of photosynthetic enzymes, such as carbonic anhydrase (CA) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco); (2) the role of melatonin in osmoregulation by proline and carbohydrate metabolism; and (3) the function of melatonin in the antioxidant defense system under salinity. Outcomes of the study reveal that under non-saline conditions, application of melatonin (20 and 50 µM) improved plant growth, viz. shoot length, root length, shoot fresh weight (FW), root FW, shoot dry weight (DW), root DW and leaf area and physio-biochemical parameters [chlorophyll (Chl) a and b, proline (Pro) and total soluble carbohydrates (TSC) content, and increased the activity of CA and Rubisco]. However, tomato seedlings treated with NaCl exhibited enhanced Chl degradation, electrolyte leakage (EL), malondialdehyde (MDA) and reactive oxygen species (ROS; superoxide and hydrogen peroxide). ROS were detected in leaf and root. Interestingly, application of melatonin improved plant growth and reduced EL, MDA and ROS levels through upregulation of photosynthesis enzymes (CA, Rubisco), antioxidant enzymes (superoxide dismutase, catalase, glutathione reductase and ascorbate reductase) and levels of non-enzymatic antioxidants [ascorbate (ASC) and reduced glutathione (GSH)], as well as by affecting the ASC-GSH cycle. Additionally, exogenous melatonin also improved osmoregulation by increasing the content of TSC, Pro and Δ¹-pyrroline-5-carboxylate synthetase activity. These results suggest that melatonin has beneficial effects on tomato seedlings growth under both stress and non-stress conditions. Melatonin's role in tolerance to salt stress may be associated with the regulation of enzymes involved in photosynthesis, the antioxidant system, metabolism of proline and carbohydrate, and the ASC-GSH cycle. Also, melatonin could be responsible for maintaining the high ratios of GSH/GSSG and ASC/DHA.

Understanding and improving global crop response to ozone pollution

Concentrations of ground-level ozone ([O3 ]) over much of the Earth's land surface have more than doubled since pre-industrial times. The air pollutant is highly variable over time and space, which makes it difficult to assess the average agronomic and economic impacts of the pollutant as well as to breed crops for O3 tolerance. Recent modeling efforts have improved quantitative understanding of the effects of current and future [O3 ] on global crop productivity, and experimental advances have improved understanding of the cellular O3 sensing, signaling and response mechanisms. This work provides the fundamental background and justification for breeding and biotechnological approaches for improving O3 tolerance in crops. There is considerable within-species variation in O3 tolerance in crops, which has been used to create mapping populations for screening. Quantitative trait loci (QTL) for O3 tolerance have been identified in model and crop species, and although none has been cloned to date, transcript profiling experiments have identified candidate genes associated with QTL. Biotechnological strategies for improving O3 tolerance are also being tested, although there is considerable research to be done before O3 -tolerant germplasm is available to growers for most crops. Strategies to improve O3 tolerance in crops have been hampered by the lack of translation of laboratory experiments to the field, and the lack of correlation between visual leaf-level O3 damage and yield loss to O3 stress. Future efforts to screen mapping populations in the field and to identify more promising phenotypes for O3 tolerance are needed.

Diurnal variation of apoplastic ascorbate in winter wheat leaves in relation to ozone detoxification

Besides stomatal closure, biological detoxification is an important protection mechanism for plants against ozone (O3). This study investigated the diurnal changes of ascorbate (a major detoxification agent) in the apoplast and leaf tissues of winter wheat grown under ambient air field conditions. Results showed the reduced ascorbate in the apoplast (ASCapo) exhibited a peak in late morning or midday, mismatching with either the maximum external O3 concentrations in mid-afternoon or the maximum stomatal O3 uptake between late morning and mid-afternoon. In contrast, the ASC in leaf tissues remained stable throughout the day. The investigations conducted in a Free-Air Concentration Elevation of O3 system confirmed that the diurnal variations of the ASCapo were induced more by the daily variations of O3 concentrations rather than the cumulative O3 effects. In conclusion, the O3-stress detoxification should be a dynamic variable rather than a fixed threshold as assumed in the stomatal flux-based O3 dose metrics.

Role of L-ascorbate in alleviating abiotic stresses in crop plants

L-ascorbic acid (vitamin C) is a major antioxidant in plants and plays a significant role in mitigation of excessive cellular reactive oxygen species activities caused by number of abiotic stresses. Plant ascorbate levels change differentially in response to varying environmental stress conditions, depending on the degree of stress and species sensitivity. Successful modulation of ascorbate biosynthesis through genetic manipulation of genes involved in biosynthesis, catabolism and recycling of ascorbate has been achieved. Recently, role of ascorbate in alleviating number of abiotic stresses has been highlighted in crop plants. In this article, we discuss the current understanding of ascorbate biosynthesis and its antioxidant role in order to increase our comprehension of how ascorbate helps plants to counteract or cope with various abiotic stresses.

Glutathione and transpiration as key factors conditioning oxidative stress in Arabidopsis thaliana exposed to uranium

Although oxidative stress has been previously described in plants exposed to uranium (U), some uncertainty remains about the role of glutathione and tocopherol availability in the different responsiveness of plants to photo-oxidative damage. Moreover, in most cases, little consideration is given to the role of water transport in shoot heavy metal accumulation. Here, we investigated the effect of uranyl nitrate exposure (50 μM) on PSII and parameters involved in water transport (leaf transpiration and aquaporin gene expression) of Arabidopsis wild type (WT) and mutant plants that are deficient in tocopherol (vte1: null α/γ-tocopherol and vte4: null α-tocopherol) and glutathione biosynthesis (high content: cad1.3 and low content: cad2.1). We show how U exposure induced photosynthetic inhibition that entailed an electron sink/source imbalance that caused PSII photoinhibition in the mutants. The WT was the only line where U did not damage PSII. The increase in energy thermal dissipation observed in all the plants exposed to U did not avoid photo-oxidative damage of mutants. The maintenance of control of glutathione and malondialdehyde contents probed to be target points for the overcoming of photoinhibition in the WT. The relationship between leaf U content and leaf transpiration confirmed the relevance of water transport in heavy metals partitioning and accumulation in leaves, with the consequent implication of susceptibility to oxidative stress.

A diminution in ascorbate oxidase activity affects carbon allocation and improves yield in tomato under water deficit

The regulation of carbon allocation between photosynthetic source leaves and sink tissues in response to stress is an important factor controlling plant yield. Ascorbate oxidase is an apoplastic enzyme, which controls the redox state of the apoplastic ascorbate pool. RNA interference was used to decrease ascorbate oxidase activity in tomato (Solanum lycopersicum L.). Fruit yield was increased in these lines under three conditions where assimilate became limiting for wild-type plants: when fruit trusses were left unpruned, when leaves were removed or when water supply was limited. Several alterations in the transgenic lines could contribute to the improved yield and favour transport of assimilate from leaves to fruits in the ascorbate oxidase lines. Ascorbate oxidase plants showed increases in stomatal conductance and leaf and fruit sugar content, as well as an altered apoplastic hexose:sucrose ratio. Modifications in gene expression, enzyme activity and the fruit metabolome were coherent with the notion of the ascorbate oxidase RNAi lines showing altered sink strength. Ascorbate oxidase may therefore be a target for strategies aimed at improving water productivity in crop species.

Expression of a heterologous SnRK1 in tomato increases carbon assimilation, nitrogen uptake and modifies fruit development

SnRK1 (sucrose non-fermenting-1-related protein kinase 1) plays an important role in plant carbon metabolism and development. To understand the mechanism of carbon and nitrogen metabolism regulated by MhSnRK1 from pingyitiancha (Malus hupehensis Rehd. var. pinyiensis Jiang), two transgenic lines (T2-7 and T2-9) over expressing this gene in tomato were studied. SnRK1 activity in the leaves of 2 transgenic lines was increased by 15-16% compared with that in the wild-type. The leaf photosynthetic rate in transgenic tomatoes was higher than the wild-type. The activity of sucrose synthase breakdown and ADP-glucose pyrophosphorylase was also increased, by approximately 25-36% and 44-48%, respectively, whereas sucrose synthase synthesis and sucrose phosphate synthase activities were unchanged. The content of starch in the leaves and red-ripening fruits was higher than that of the wild-type. The transgenic fruit ripened ∼10 days earlier than the wild-type. The nitrate reductase activity (mgplant⁻¹ h⁻¹) shows no significant difference between the transgenic plant and the wild-type, but the N-uptake efficiency and root/shoot ratio in the T2-9 line were 15% and 35% higher than that in the wild-type, respectively. These results suggest that over expressing MhSnRK1 can increase both the carbon and nitrogen assimilation rate of the plant as well as regulate the development of fruit.

Enhanced oxidative stress in the ethylene-insensitive (ein3-1) mutant of Arabidopsis thaliana exposed to salt stress

To better understand the role of ethylene signaling in plant stress tolerance, salt-induced changes in gene expression levels of ethylene biosynthesis, perception and signaling genes were measured in Arabidopsis thaliana plants exposed to 15 days of salinity. Among the genes analyzed, EIN3 showed the highest expression level increase under salt stress, suggesting a key role for this ethylene-signaling component in response to salt stress. Therefore, we analyzed the salt stress response over 15 days (by adding 100 mM NaCl to the nutrient solution) in the ein3-1 mutant compared to the wild-type (Col-0) in terms of growth, oxidative stress markers (lipid peroxidation, foliar pigments and low-molecular-weight antioxidants) and levels of growth- and stress-related phytohormones (including cytokinins, auxins, gibberellins, abscisic acid, jasmonic acid and salicylic acid). The ein3-1 mutant grew similarly to wild-type plants both under control and salt stress conditions, which was associated with a differential time course evolution in the levels of the cytokinins zeatin and zeatin riboside, and the auxin indole-3-acetic acid between the ein3-1 mutant and the wild-type. Despite showing no signs of physiological deterioration under salt stress (in terms of rosette biomass, leaf water and pigment contents, and PSII efficiency) the ein3-1 mutant showed enhanced lipid peroxidation under salt stress, as indicated by 2.4-fold increase in both malondialdehyde and jasmonic acid contents compared to the wild-type. We conclude that, at moderate doses of salinity, partial insensitivity to ethylene might be compensated by changes in endogenous levels of other phytohormones and lipid peroxidation-derived signals in the ein3-1 mutant exposed to salt stress, but at the same time, this mutant shows higher oxidative stress under salinity than the wild-type.

NADPH oxidase in plasma membrane is involved in stomatal closure induced by dehydroascorbate

Stoma is surrounded by two guard cells, and regulates the contents of water and CO(2) in plant, its opening and closing was affected by various factors. Recently, dehydroascorbate was found to induce stomata closure and H(2)O(2) generation. However, the mechanism of H(2)O(2) production is not clear. DPI and imidazole inhibit the flavoprotein and the b(-type) cytochrome components of the NADPH oxidase complex. Application of DPI or imidazole with DHA together impaired stomatal closure and elevation of H(2)DCF-DA fluorescent intensity induced by DHA in guard cells. CoCl(2) and PD98059, as the blocker of calcium channel and the inhibitor of MAPKKK, both impaired stomatal closure induced by DHA. The results suggested that DHA-induced H(2)O(2) generation via activation of NADPH oxidase, and thus resulting in stomatal closure. Moreover, Ca(2+) channel and MAPK cascades were involved in stomatal closure induced by DHA.

Apoplastic ascorbate contributes to the differential ozone sensitivity in two varieties of winter wheat under fully open-air field conditions

We studied leaf apoplastic ascorbates in relation to ozone (O(3)) sensitivity in two winter wheat (Triticum aestivum L.) varieties: Yangfumai 2 (Y2) and Yangmai 16 (Y16). The plants were exposed to elevated O(3) concentration 27% higher than the ambient O(3) concentration in a fully open-air field from tillering stage until final maturity. The less sensitive variety Y16 had higher concentration of reduced ascorbate in the apoplast and leaf tissue by 33.5% and 12.0%, respectively, than those in the more sensitive variety Y2, whereas no varietal difference was detected in the decline of reduced ascorbate concentration in response to elevated O(3). No effects of O(3) or variety were detected in either oxidized ascorbate or the redox state of ascorbate in the apoplast and leaf tissue. The lower ascorbate concentrations in both apoplast and leaf tissue should have contributed to the higher O(3) sensitivity in variety Y2.

The cellular redox state in plant stress biology--a charging concept

Different redox-active compounds, such as ascorbate, glutathione, NAD(P)H and proteins from the thioredoxin superfamily, contribute to the general redox homeostasis in the plant cell. The myriad of interactions between redox-active compounds, and the effect of environmental parameters on them, has been encapsulated in the concept of a cellular redox state. This concept has facilitated progress in understanding stress signalling and defence in plants. However, despite the proven usefulness of the concept of a redox state, there is no single, operational definition that allows for quantitative analysis and hypothesis testing.

Nitrogen mediates above-ground effects of ozone but not below-ground effects in a rhizomatous sedge

Ozone and atmospheric nitrogen are co-occurring pollutants with adverse effects on natural grassland vegetation. Plants of the rhizomatous sedge Carex arenaria were exposed to four ozone regimes representing increasing background concentrations (background-peak): 10-30, 35-55, 60-80 and 85-105 ppb ozone at two nitrogen levels: 12 and 100 kg N ha(-1) yr(-1). Ozone increased the number and proportion of senesced leaves, but not overall leaf number. There was a clear nitrogen x ozone interaction with high nitrogen reducing proportional senescence in each treatment and increasing the ozone dose (AOT40) at which enhanced senescence occurred. Ozone reduced total biomass due to significant effects on root biomass. There were no interactive effects on shoot:root ratio. Rhizome tissue N content was increased by both nitrogen and ozone. Results suggest that nitrogen mediates above-ground impacts of ozone but not impacts on below-ground resource translocation. This may lead to complex interactive effects between the two pollutants on natural vegetation.

The impact of atmospheric composition on plants: a case study of ozone and poplar

Tropospheric ozone is the main atmospheric pollutant that causes damages to trees. The estimation of the threshold for ozone risk assessment depends on the evaluation of the means that this pollutant impacts the plant and, especially, the foliar organs. The available results show that, before any visible symptom appears, carbon assimilation and the underlying metabolic processes are decreased under chronic ozone exposure. By contrast, the catabolic pathways are enhanced, and contribute to the supply of sufficient reducing power necessary to feed the detoxification processes. Reactive oxygen species delivered during ozone exposure serve as toxic compounds and messengers for the signaling system. In this review, we show that the contribution of genomic tools (transcriptomics, proteomics, and metabolomics) for a better understanding of the mechanistic cellular responses to ozone largely relies on spectrometric measurements.

A new stable isotope approach identifies the fate of ozone in plant-soil systems

* We show that the stable isotope (18)O can be used to trace ozone into different components of the plant-soil system at environmentally relevant concentrations. * We exposed plants and soils to (18)O-labelled ozone and used isotopic enrichment in plant dry matter, leaf water and leaf apoplast, as well as in soil dry matter and soil water, to identify sites of ozone-derived (18)O accumulation. * It was shown that isotopic accumulation rates in plants can be used to infer the location of primary ozone-reaction sites, and that those in bare soils are dependent on water content. However, the isotopic accumulation rates measured in leaf tissue were much lower than the modelled stomatal flux of ozone. * Our new approach has considerable potential to elucidate the fate and reactions of ozone within both plants and soils, at scales ranging from plant communities to cellular defence mechanisms.

Ozone risk assessment for plants: central role of metabolism-dependent changes in reducing power

The combination of stomatal-dependent ozone flux and total ascorbate level is currently presented as a correct indicator for determining the degree of sensitivity of plants to ozone. However, the large changes in carbon metabolism could play a central role in the strategy of the foliar cells in response to chronic ozone exposure, participating in the supply of reducing power and carbon skeletons for repair and detoxification, and modifying the stomatal mode of functioning. To reinforce the accuracy of the definition of the threshold for ozone risk assessment, it is proposed to also consider the redox pool (NAD(P)H), the ratio between carboxylases and the water use efficiency as indicators of the differential ozone tolerance of plants.

Modification of the biochemical pathways of plants induced by ozone: what are the varied routes to change?

When plants are observed under a low dose of ozone, some physiological and metabolic shifts occur. Barring extreme injury such as tissue damage or stomata closure, most of these disruptive changes are likely to have been initiated at the level of gene expression. The belief is oxidative products formed in ozone exposed leaves, e.g. hydrogen peroxide, are responsible for much of the biochemical adjustments. The first line of defense is a range of antioxidants, such as ascorbate and glutathione, but if this defense is overwhelmed, subsequent actions occur, similar to systemic acquired resistance or general wounding. Yet there are seemingly unrelated metabolic responses which are also triggered, such as early senescence. We discuss here the current understanding of gene control and signal transduction/control in order to increase our comprehension of how ozone alters the basic metabolism of plants and how plants counteract or cope with ozone.

Pre-exposure of calli to ozone promotes tolerance of regenerated Lycopersicon esculentum cv. PKM1 plantlets against acute ozone stress

Studies were performed to evaluate the effects of pre-exposure of calli to ozone in promoting tolerance of the regenerated Lycopersicon esculentum cv. PKM1 (tomato) plantlets against acute ozone stress (AOS). Calli induced from tomato leaf explants were subjected to pre-treatment with ozone: T(1)=100 ppb, T(2)=200 ppb and T(3)=300 ppb. For the control (C) calli, charcoal-filtered air was supplied to test differential sensitivity of regenerated plantlets to acute ozone stress. All treated calli were subsequently transferred to shooting, rooting medium and acclimatized. The plantlets regenerated from the respective ozone (T(1), T(2), T(3))-treated calli are referred to here as T(1), T(2), T(3) plantlets and the plantlets regenerated from control calli are referred to as control plantlets. The frequencies of regeneration of tomato plantlets from the calli were T(1)=86%, T(2)=82% and T(3)=67%, and 92% regeneration was obtained from control calli. In order to evaluate the ozone tolerance, all the regenerated plantlets were exposed to the acute ozone exposure (AOE). After AOE, the T(2) plantlets endured remarkably well by experiencing reduced ozone stress, which was evident from the lower level of hydrogen peroxide and oxidative stress-related enzymes such as ascorbate peroxidase (EC 1.11.1.11) and superoxide dismutase (EC 1.15.1.1) activities relative to T(3), T(1) and C plantlets. All T(2) plantlets showed enhanced tolerance against AOE by upholding enhanced soluble phenol content, a higher level of foliar and apoplastic ascorbic acid, elevated dehydroascorbate reductase (EC 1.8.5.1) and glutathione content. The present study reveals that the calli pre-exposed to T(2) ozone treatment resulted in an increase in the level of antioxidants and provided the plants greater protection against acute ozone stress.

Seasonal differences and within-canopy variations of antioxidants in mature spruce (Picea abies) trees under elevated ozone in a free-air exposure system

The effect of free-air ozone fumigation and crown position on antioxidants were determined in old-growth spruce (Picea abies) trees in the seasonal course of two consecutive years (2003 and 2004). Levels of total ascorbate and its redox state in the apoplastic washing fluid (AWF) were increased under double ambient ozone concentrations (2xO3), whilst ascorbate concentrations in needle extracts were unchanged. Concentrations of apoplastic and symplastic ascorbate were significantly higher in 2003 compared to 2004 indicating a combined effect of the drought conditions in 2003 with enhanced ozone exposure. Elevated ozone had only weak effects on total glutathione levels in needle extracts, phloem exudates and xylem saps. Total and oxidised glutathione concentrations were higher in 2004 compared to 2003 and seemed to be more affected by enhanced ozone influx in the more humid year 2004 compared to the combined effect of elevated ozone and drought in 2003 as observed for ascorbate.

Apoplastic antioxidative system responses to ozone stress in strawberry leaves

Cell wall polysaccharides, pectin composition, as well as apoplastic superoxide dismutase and peroxidase activities were investigated in strawberry (Fragaria x ananassa) cultivars (cvs) Korona and Elsanta differing in their ozone sensitivity. Plants were exposed to 140-170 microg m(-3) ozone either short-term for 7 days or long-term for 2 months in order to investigate whether differences in ozone sensitivity were due to differences in the apoplastic antioxidative systems. Cell wall polysaccharides were increased after 7 days and 2 months of ozone stress. While water-soluble pectins, low methoxy pectinates and NaOH-soluble pectinates were elevated after 7 days, their contents were unaffected (water-soluble pectins) or lower (low methoxy pectinates, NaOH-soluble pectinates) after 2 months. In cv. Elsanta, ozone treatment resulted in a significant reduction of superoxide dismutase activity after 7 days and 2 months, while it remained similar in cv. Korona. After 7 days, peroxidase activity was significantly higher in ozone-exposed leaves of cv. Korona, whereas after 2 months it was similar to or higher than in controls. Superoxide dismutase in cv. Korona detoxified ozone and its products in the apoplast, and the resulting elevated levels of H(2)O(2) were balanced within 7 days by an increase in peroxidase activity. Long-term peroxidase activity may not play a comparably significant role in ozone defence, but the increase in cell wall polysaccharides and cell wall thickness measured after 2 months, resulting in a decrease in specific leaf area, reflected structural modifications that limited activities of reactive oxygen species efficiently. In contrast, the reduction of superoxide dismutase activity in cv. Elsanta indicated a less efficient apoplastic radical scavenging system, at least during the first 7 days of ozone stress, which was accompanied by membrane leakage and contributed to accelerated leaf senescence. Long-term, the reduction of intercellular air space volume in leaves contributed to ozone tolerance of cv. Elsanta as in cv. Korona.

Leaf extracellular ascorbate in relation to O(3) tolerance of two soybean cultivars

Soybean [Glycine max (L.) Merr.] cultivars Essex and Forrest that exhibit differences in ozone (O(3)) sensitivity were used in greenhouse experiments to investigate the role of leaf extracellular antioxidants in O(3) injury responses. Charcoal-filtered air and elevated O(3) conditions were used to assess genetic, leaf age, and O(3) effects. In both cultivars, the extracellular ascorbate pool consisted of 80-98% dehydroascorbic acid, the oxidized form of ascorbic acid (AA) that is not an antioxidant. For all combinations of genotype and O(3) treatments, extracellular AA levels were low (1-30nmolg(-1) FW) and represented 3-30% of the total antioxidant capacity. Total extracellular antioxidant capacity was twofold greater in Essex compared with Forrest, consistent with greater O(3) tolerance of Essex. The results suggest that extracellular antioxidant metabolites in addition to ascorbate contribute to detoxification of O(3) in soybean leaves and possibly affect plant sensitivity to O(3) injury.

Factors affecting the ozone sensitivity of temperate European grasslands: an overview

This overview of experimentally induced effects of ozone aims to identify physiological and ecological principles, which can be used to classify the sensitivity to ozone of temperate grassland communities in Europe. The analysis of data from experiments with single plants, binary mixtures and multi-species communities illustrates the difficulties to relate individual responses to communities, and thus to identify grassland communities most at risk. Although there is increasing evidence that communities can be separated into broad classes of ozone sensitivity, the database from experiments under realistic conditions with representative systems is too small to draw firm conclusions. But it appears that risk assessments, based on results from individuals or immature mixtures exposed in chambers, are only applicable to intensively managed, productive grasslands, and that the risk of ozone damage for most of perennial grasslands with lower productivity tends to be less than previously expected.

Ozone stress and antioxidant substances in Trifolium repens and Centaurea jacea leaves

Ozone-sensitive (NC-S clone) and resistant plants (NC-R clone) of Trifolium repens and Centaurea jacea were exposed to moderate ozone concentrations in ambient air. The aim of this study was the investigation of the relation between ozone-sensitivity and leaf concentrations of antioxidants (ascorbic acid, total phenolics and total antioxidant capacity). NC-R clone showed the highest concentrations of antioxidants with 50-70% more ascorbic acid than NC-S. NC-R had about 5 times more ascorbic acid in the young leaves and 9 times more in the old leaves than Centaurea. In a fumigation experiment with acute ozone stress (100 nl L(-1)) the antioxidant levels changed profoundly. The ozone-injured leaves of NC-S had 6-8 times more total phenolics than uninjured leaves. Generally older leaves had lower antioxidant concentrations and were more prone to ozone injury than younger leaves. Ascorbic acid concentrations were closer related to the appearance of visible ozone injury than the other antioxidative parameters.

Antioxidative defence of old growth beech (Fagus sylvatica) under double ambient O3 concentrations in a free-air exposure system

In this study the influence of chronic free-air ozone exposure and of different meteorological conditions in the very dry year 2003 and the more humid year 2004 on the antioxidative system in sun and shade leaves of adult FAGUS SYLVATICA trees were investigated. Contents of ascorbate, glutathione, and alpha-tocopherol, as well as chloroplast pigments were determined under ambient (1 x O(3)) and double ambient (2 x O(3)) ozone concentrations. Ozone affected the antioxidative system in June and July, causing lower ascorbate contents in the apoplastic space, a more oxidized redox state of ascorbate and glutathione and an increase in pigment contents predominantly in the shade crown. For all measured parameters significant differences between the years were observed. In 2004 the redox state of ascorbate and glutathione was in a more reduced state and leaf contents of alpha-tocopherol, pigments of the xanthophyll cycle, beta-carotene, lutein, neoxanthin, and alpha-carotene were lower compared to 2003. Contents of total glutathione and chlorophyll a + b were increased in the second year. These results indicate a strong influence of the drought conditions in 2003 on the antioxidative system of beech overruling the ozone effects. Shade leaves showed lower contents of ascorbate in both years and the redox states of ascorbate and glutathione were more oxidized compared to sun leaves. Contents of photoprotective and accessory pigments generally were enhanced and the de-epoxidation state of the xanthophyll cycle was lower in the shade compared to the sun crown. Exhibiting less antioxidants shade leaves seem to be more sensitive against ozone than sun leaves.

Alteration of oxidative and carbohydrate metabolism under abiotic stress in two rice (Oryza sativa L.) genotypes contrasting in chilling tolerance

Abiotic stress is a major limiting factor in crop production. Physiological comparisons between contrasting abiotic stress-tolerant genotypes will improve understanding of stress-tolerant mechanisms. Rice seedlings (S3 stage) of a chilling-tolerant (CT) genotype (CT6748-8-CA-17) and a chilling-sensitive (CS) genotype (INIAP12) were subjected to abiotic stresses including chilling (13/12 degrees C), salt (100mM NaCl), and osmotic (200mM mannitol). Measures of physiological response to the stresses included changes in stress-related sugars, oxidative products and protective enzymes, parameters that could be used as possible markers for selection of improved tolerant varieties. Metabolite analyses showed that the two genotypes responded differently to different stresses. Genotype survival under chilling-stress was as expected, however, CT was more sensitive to salt stress than the CS genotype. The CT genotype was able to maintain membrane integrity better than CS, perhaps by reduction of lipid peroxidation via increased levels of antioxidant enzymes during chilling stress. This genotype accumulated sugars in response to stress, but the accumulation was usually less than in the CS genotype. Chill-stressed CT accumulated galactose and raffinose whereas these saccharides declined in CS. On the other hand, the tolerance mechanism in the more salt- and water-deficit-tolerant CS may be associated with accumulation of osmoprotectants such as glucose, trehalose and mannitol.

Seasonal profiles of leaf ascorbic acid content and redox state in ozone-sensitive wildflowers

Cutleaf coneflower (Rudbeckia laciniata L.), crown-beard (Verbesina occidentalis Walt.), and tall milkweed (Asclepias exaltata L.) are wildflower species native to Great Smoky Mountains National Park (U.S.A.). Natural populations of each species were analyzed for leaf ascorbic acid (AA) and dehydroascorbic acid (DHA) to assess the role of ascorbate in protecting the plants from ozone stress. Tall milkweed contained greater quantities of AA (7-10 micromol g(-1) fresh weight) than crown-beard (2-4 micromol g(-1) fresh weight) or cutleaf coneflower (0.5-2 micromol g(-1) fresh weight). DHA was elevated in crown-beard and cutleaf coneflower relative to tall milkweed suggesting a diminished capacity for converting DHA into AA. Tall milkweed accumulated AA in the leaf apoplast (30-100 nmol g(-1) fresh weight) with individuals expressing ozone foliar injury symptoms late in the season having less apoplast AA. In contrast, AA was not present in the leaf apoplast of either crown-beard or cutleaf coneflower. Unidentified antioxidant compounds were present in the leaf apoplast of all three species. Overall, distinct differences in antioxidant metabolism were found in the wildflower species that corresponded with differences in ozone sensitivity.

Predicting leaf-level fluxes of O3 and NO2: the relative roles of diffusion and biochemical processes

Pollutants like O(3) and NO(2) enter leaves through the stomata and cause damage during reactions with components of biological cell membranes. The steady-state flux rates of these gases into the leaf are determined by a series of physical and biochemical resistances including stomatal aperture, reactions occurring within the cell wall and the ability of the leaf to remove the products of apoplastic reactions. In the present study, multiple regression models incorporating stomatal conductance, apoplastic and symplastic ascorbate concentrations, and nitrate reductase (NR) activities were generated to explain the observed variations in leaf-level flux rates of O(3) and NO(2). These measurements were made on the plant Catharanthus roseus (Madagascar periwinkle). The best-fit model explaining NO(2) flux included stomatal conductance, apoplastic ascorbate and NR activity. This model explained 89% of the variation in observed leaf fluxes and suggested physical resistances, reaction between NO(2) and apoplastic ascorbate, and the removal rate of nitrate (generated by reactions of NO(2) and water) from the apoplast all play controlling roles in NO(2) flux to leaves. O(3) flux was best explained by stomatal conductance and symplastic ascorbate explaining 66% of the total variation in leaf flux. Both models demonstrate the importance of measuring processes other than stomatal conductance to explain steady-state leaf-level fluxes of pollutant gases.

Seasonal patterns of ascorbate in the needles of Scots Pine (Pinus sylvestris L.) trees: correlation analyses with atmospheric O3 and NO2 gas mixing ratios and meteorological parameters

In the present field study the role of ascorbate in scavenging the harmful atmospheric trace gases O3 and NO2 was examined. For this purpose ascorbate contents were determined in needles of adult Scots pine trees (Pinus sylvestris L.) during three consecutive years. Ascorbate contents were correlated with ambient tropospheric O3 and NO2 concentrations and with meteorological parameters. The results showed a strong correlation of atmospheric O3 but not of atmospheric NO2 concentrations with the apoplastic content of ascorbate during the seasonal course. Ascorbate contents in needle extracts did not correlate with ambient trace gas concentrations. In the apoplastic space, but not in needle extracts ascorbate contents correlate highly significantly with global radiation. From these results it is assumed that apoplastic ascorbate in Scots pine needles is adapted to the actual atmospheric O3 concentration to mediate immediate detoxification of O3, while the atmospheric O3 concentration itself is largely dependent on light intensity.

Increasing tolerance to ozone by elevating foliar ascorbic acid confers greater protection against ozone than increasing avoidance

Ascorbic acid (Asc) is the most abundant antioxidant in plants and serves as a major contributor to the cell redox state. Exposure to environmental ozone can cause significant damage to plants by imposing conditions of oxidative stress. We examined whether increasing the level of Asc through enhanced Asc recycling would limit the deleterious effects of environmental oxidative stress. Plants overexpressing dehydroascorbate reductase (DHAR), which results in an increase in the endogenous level of Asc, were exposed to acute or chronic levels of ozone. DHAR-overexpressing plants had a lower oxidative load, a lower level of oxidative-related enzyme activities, a higher level of chlorophyll, and a higher level of photosynthetic activity 24 h following an acute exposure (2 h) to 200 ppb ozone than control plants, despite exhibiting a larger stomatal area. Reducing the size of the Asc pool size through suppression of DHAR expression had the opposite effect. Following a chronic exposure (30 d) to 100 ppb ozone, plants with a larger Asc pool size maintained a larger stomatal area and a higher oxidative load, but retained a higher level of photosynthetic activity than control plants, whereas plants suppressed for DHAR had a substantially reduced stomatal area, but also a substantially lower level of photosynthetic activity. Together, these data indicate that, despite a reduced ability to respond to ozone through stomatal closure, increasing the level of Asc through enhanced Asc recycling provided greater protection against oxidative damage than reducing stomatal area.

Suppressed expression of the apoplastic ascorbate oxidase gene increases salt tolerance in tobacco and Arabidopsis plants

Transgenic tobacco plants expressing the ascorbate oxidase (AAO) gene in sense and antisense orientations, and an Arabidopsis mutant in which the T-DNA was inserted into a putative AAO gene, were used to examine the potential roles of AAO for salt-stress tolerance in plants. AAO activities in the transgenic tobacco plants expressing the gene in sense and antisense orientations were, respectively, about 16-fold and 0.2-fold of those in the wild type. Under normal growth conditions, no significant differences in phenotypes were observed, except for a delay in flowering time in the antisense plants. However, at high salinity, the percentage germination, photosynthetic activity, and seed yields were higher in antisense plants, with progressively lower levels in the wild type and the sense plants. The redox state of apoplastic ascorbate in sense plants was very low even under normal growth conditions. Upon salt stress, the redox state of symplastic and apoplastic ascorbate decreased among the three types of plants, but was lowest in the sense plants. The hydrogen peroxide contents in the symplastic and apoplastic spaces were higher in sense plants, progressively lower in the wild type, followed by the antisense plants. The Arabidopsis T-DNA inserted mutant exhibited very low ascorbate oxidase activity, and its phenotype was similar to that of antisense tobacco plants. These results suggest that the suppressed expression of apoplastic AAO under salt-stress conditions leads to a relatively low level of hydrogen peroxide accumulation and a high redox state of symplastic and apoplastic ascorbate which, in turn, permits a higher seed yield.

Components of apoplastic ascorbate use in Betula pendula leaves exposed to CO2 and O3 enrichment

Here, the aim was to estimate loads imposed on the apoplastic ascorbate (ASC) pool by enzymatic and nonenzymatic reactions in Betula pendula exposed to doubled CO2 and O3 concentrations in open-top chambers. Leaf apoplastic extracts were analysed for peroxidase and oxidase activities in vitro, using different substrates. Partial loads in vivo were deduced using measured kinetic constants and substituted-enzyme catalysis approaches. Ascorbate use in O3 scavenging was calculated using measured stomatal conductances and ASC concentrations. Under elevated O3, stomatal conductance and O3 uptake were higher. O3 fluxes to the plasmalemma were levelled off by higher apoplastic ASC concentrations. The effect of CO2 enrichment on ASC concentrations under elevated O3 was minor. Under ambient O3, the ascending hierarchy of ASC users was: peroxidases, O3 scavenging, oxidases, coniferyl alcohol re-reduction. Under elevated O3, ASC use in O3 scavenging was higher than by oxidases. The redox state of ASC was not depressed by O3; there was no leaf injury. The cell wall/plasmalemma/cytosol system in birch had sufficient capacity to maintain ASC redox status in the apoplast, without necessity to restrict O3 uptake by stomatal closure.

Apoplastic ascorbate metabolism and its role in the regulation of cell signalling

The apoplast has crucial functions in plant biology. It comprises all the compartments beyond the plasmalemma, including the cell wall. As the reservoir of information on the biotic and abiotic environment surrounding the cell and a major conduit of information between cells, the apoplast has functions in stress perception and the subsequent appropriate control of growth and defence. The oxidative burst phenomenon, caused by environmental challenges and pathogen attack in particular, oxidises the apoplast. Ascorbic acid (AA), the major and probably the only antioxidant buffer in the apoplast, becomes oxidised in these conditions. The apoplastic enzyme ascorbate oxidase (AO) also regulates the reduction/oxidation (redox) state of the apoplastic ascorbate pool. We propose that a key function of the oxidative burst and of AO is to modify the apoplastic redox state in such a way as to modify receptor activity and signal transduction to regulate defence and growth.

Ecological issues related to ozone: agricultural issues

Research on the effects of ozone on agricultural crops and agro-ecosystems is needed for the development of regional emission reduction strategies, to underpin practical recommendations aiming to increase the sustainability of agricultural land management in a changing environment, and to secure food supply in regions with rapidly growing populations. Major limitations in current knowledge exist in several areas: (1) Modelling of ozone transfer and specifically stomatal ozone uptake under variable environmental conditions, using robust and well-validated dynamic models that can be linked to large-scale photochemical models lack coverage. (2) Processes involved in the initial reactions of ozone with extracellular and cellular components after entry through the stomata, and identification of key chemical species and their role in detoxification require additional study. (3) Scaling the effects from the level of individual cells to the whole-plant requires, for instance, a better understanding of the effects of ozone on carbon transport within the plant. (4) Implications of long-term ozone effects on community and whole-ecosystem level processes, with an emphasis on crop quality, element cycling and carbon sequestration, and biodiversity of pastures and rangelands require renewed efforts. The UNECE Convention on Long Range Trans-boundary Air Pollution shows, for example, that policy decisions may require the use of integrated assessment models. These models depend on quantitative exposure-response information to link quantitative effects at each level of organization to an effective ozone dose (i.e., the balance between the rate of ozone uptake by the foliage and the rate of ozone detoxification). In order to be effective in a policy, or technological context, results from future research must be funnelled into an appropriate knowledge transfer scheme. This requires data synthesis, up-scaling, and spatial aggregation. At the research level, interactions must be considered between the effects of ozone and factors that are either directly manipulated by man through crop management, or indirectly changed. The latter include elevated atmospheric CO(2), particulate matter, other pollutants such as nitrogen oxides, UV-B radiation, climate and associated soil moisture conditions.

Environmental oxidant pollutant effects on biologic systems: a focus on micronutrient antioxidant-oxidant interactions

Oxidative atmospheric pollutants represent a significant source of stress to both terrestrial plants and animals. The biosurfaces of plants and surface-living organisms are directly exposed to these pollutant stresses. These surfaces, including respiratory tract surfaces, contain integrated antioxidant systems that would be expected to provide a primary defense against environmental threats caused by atmospheric reactive oxygen species. When the biosurface antioxidant defenses are overwhelmed, oxidative stress to the cellular components of the exposed biosurfaces can be expected, inducing inflammatory, adaptive, injurious, and reparative processes. Studies of mutants and/or transformed plants and insects, with specific alterations in key components of antioxidant defense systems, offer opportunities to dissect the complex systems that maintain surface defenses against environmental oxidants. In this article, we use a comparative approach to consider interactions of atmospheric oxidant pollutants with selected biosystems, with focus on O3 as the pollutant; plants, flies, skin, and lungs as the exposed biosystems; and nonenzymatic micronutrient antioxidants as significant contributors to overall antioxidant defense strategies of these varied biosystems. Parallelisms among several living organisms, with regard to their protective strategies against environmental atmospheric oxidants, are presented.