Ozone pollution threatens the production of major staple crops in East Asia.

East Asia is a hotspot of surface ozone (O3) pollution, which hinders crop growth and reduces yields. Here, we assess the relative yield loss in rice, wheat and maize due to O3 by combining O3 elevation experiments across Asia and air monitoring at about 3,000 locations in China, Japan and Korea. China shows the highest relative yield loss at 33%, 23% and 9% for wheat, rice and maize, respectively. The relative yield loss is much greater in hybrid than inbred rice, being close to that for wheat. Total O3-induced annual loss of crop production is estimated at US$63 billion. The large impact of O3 on crop production urges us to take mitigation action for O3 emission control and adaptive agronomic measures against the rising surface O3 levels across East Asia.

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

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

Responses of an old and a modern Indian wheat cultivar to future O3 level: Physiological, yield and grain quality parameters.

A field study was conducted to understand the physiological responses, yield and grain quality of an old (HUW234) and a modern (HD3118) wheat cultivar exposed to elevated ozone (O3). The cultivars were grown under ambient O3 (NF) and ambient +20 ppb O3 (NF+) conditions using open-top chambers (OTCs). The comparative study of an old and a modern cultivar showed variable physiological responses under elevated O3 exposure. Elevated O3 in old cultivar caused high reductions in Rubisco activity (Vcmax) and electron transport rate (J) compared to modern cultivar with simultaneous reductions in the rate of photosynthesis and chlorophyll fluorescence. In modern cultivar, high stomatal density and conductance caused higher O3 uptake thereby triggering more damage to the adjacent stomatal cells and photosynthetic pigments coupled with reductions in photosynthetic rate and photosynthetic nitrogen use efficiency (PNUE). Modern cultivar also showed relatively high reduction in grain yield compared to old one under NF + treatment. Furthermore, grain quality traits (such as starch, protein and amino acids) of modern cultivar were better than old cultivar under ambient O3, but showed more deterioration under NF + treatment. Results thus indicated that modern cultivar is relatively more susceptible to O3 and showed more negative impacts on plant performance, yield and quality of grains compared to old cultivar.

Responses of tropical tree species to urban air pollutants: ROS/RNS formation and scavenging.

Air pollution in an urban environment is the major stress factor for vegetation due to the direct generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS). To quantify urban air pollution-induced ROS/RNS formation, damage and detoxification, nine different biochemical parameters related to free radical formation, scavenging and membrane damage were estimated in twelve tropical tree species. The experiment was performed in three different seasons at four distinct urban environments in Varanasi city located in the Indo-Gangetic plain of India. Redundancy analysis was performed to statistically assess the relationship between air pollutants (PM2.5, NO2, SO2 and O3) and temperature with ROS/RNS generation and their detoxification. Significant effects of air pollution exposure and temperature on ROS/RNS formation, scavenging and membrane damage were recorded with increasing pollution load in the city for all the tree species. The extent of variability (47-87%) in responses of different tree species was due to their intrinsic ability to scavenge free radicals which minimized the membrane damage. PM2.5, NO2 and O3 were identified as major pollutants that influenced trees to different extents in regulating ROS/RNS. However, the response was maximum against NO2 (34-72%) followed by PM2.5 (16-64%) and O3 (3-31%), indicating that under urban environment, trees are considerably sensitive to the combined effects of both particulate and gaseous pollutants. Reactive oxygen intermediate release, total free radical scavenging activity, NO scavenging activity and membrane stability index were identified as major parameters which showed distinct responses with increasing pollution load. Caesalpinia sappan, Ficus religiosa and Albizia lebbeck were identified as most tolerant tree species having higher ROS/RNS scavenging potential resulted in lower membrane damage. Thus responses of urban trees to air pollution are governed by their intrinsic defence mechanisms to scavenge ROS/RNS by maintaining the membrane integrity through integrated cross-talk between different antioxidative pathways.

Ozone phytotoxicity to Panicum maximum and Cenchrus ciliaris at Indo-Gangetic plains: an assessment of antioxidative defense and growth responses.

Two common tropical grassland species, Panicum maximum Jacq. (Guinea grass) and Cenchrus ciliaris (Buffel grass) of Indo-Gangetic plains were assessed for their responses under future level of O3 (ambient +30 ppb) using open top chambers. Plants were assessed for foliar injuries, pigments, growth, biomass accumulation, histochemical localization of reactive oxygen species (ROS), antioxidant defense system and ROS scavenging activities at two stages. Foliar injuries were noticed at an early stage in P. maximum compared to C. ciliaris. Significant reductions were observed in total chlorophyll, growth and total biomass in both species. Significant increases in contents of melondialdehyde and ascorbic acid in P. maximum while total phenolics and thiols in C. ciliaris were found. Histochemical analysis showed more production of superoxide radicals and hydrogen peroxide in leaf tissues of P. maximum compared to C. ciliaris. It can be concluded that higher level of primary antioxidants (total phenolics and thiols) along with superoxide dismutase and ascorbate peroxidase scavenged O3 effectively in C. ciliaris causing less reduction of biomass which is used as a feed for cattles. In P. maximum, more photosynthates were allocated for defense, leading to higher reduction in total biomass compared to C. ciliaris. The leaf area ratio was higher in P. maximum compared to C. ciliaris under elevated O3. The study further suggests higher susceptibility of P. maximum compared to C. ciliaris under future level of O3 exposure.

Assessment of Ozone Sensitivity in Three Wheat Cultivars Using Ethylenediurea.

Three wheat (Triticum aestivum L.) cultivars [HD 2987 (ozone (O3) sensitive), PBW 502 (intermediately sensitive) and Kharchiya 65 (O3 tolerant)] with known sensitivity to O3 were re-evaluated using ethylenediurea (EDU; 400 ppm) to ascertain the use of EDU in determiningO3 sensitivity under highly O3-polluted tropical environments. EDU treatment helped in improving the growth, biomass, photosynthetic pigments and the antioxidative defense system of all the wheat cultivars. Under EDU treatment, PBW 502 retained more biomass, while HD 2987 showed better performance and ultimately the greatest increment in yield. Cultivar Kharchiya 65 also showed a positive response to EDU as manifested with an increase in pigment contents, total biomass and enzymatic antioxidants; however, this increment was comparatively lower compared to the other two cultivars. The results indicated that EDU did not have many physiological effects on cultivars but helped in counteracting O3 primarily by scavenging reactive oxygen species and enhancing the antioxidative defense system where superoxide dismutase emerged as the major responsive biochemical parameter against ambient O3. The observed results clearly indicated that differential O3 sensitivity in three wheat cultivars established by the previous study is in accordance with the present study using EDU as a sensitivity tool, which is an easy and efficient technology in comparison to chamber and Free-Air Carbon dioxide Enrichment (FACE) experiments although its mechanistic understanding needs to be further validated.

ROS production and its detoxification in early and late sown cultivars of wheat under future O3 concentration.

The present field study was planned with an objective to unravel the mechanisms behind the differential responses of early and late sown wheat cultivars with respect to their defense capacity to scavenge ROS induced under elevated O3 (EO3). Experiments were performed under ambient and elevated levels of O3 (ambient + 20 ppb) to plants inside open-top chambers (OTCs). Ozone concentrations, stomatal flux of O3 and meteorological parameters were measured throughout the experiment. Contents of superoxide radicals (O2-) and hydrogen peroxide (H2O2) and their localization, lipid peroxidation, antioxidative enzyme activities, ascorbic acid and total phenolic contents were measured at vegetative and reproductive developmental stages. EO3 exposure induced higher stomatal flux of O3 in early sown cultivars. Higher contents of O2-, H2O2 and lipid peroxidation were noticed under EO3 in all the cultivars but the magnitude of increases was higher in late sown cultivars at the reproductive stage. Activities of glutathione reductase (GR) and ascorbate peroxidase (APX) were higher in late sown cultivars under EO3. Ascorbic acid and total phenolic contents were significantly higher in early sown than late sown cultivars under EO3 treatment. The present study concludes that early sown cultivars are more efficient in their defense response due to higher induction of enzymatic and non-enzymatic antioxidants, while the induction of enzymatic antioxidants was more distinct in late sown cultivars. Non-enzymatic linked defense mechanism requires additional metabolic cost than enzymatic defense, making early sown cultivars more susceptible to EO3. Differential response of early and late sown cultivars with respect to antioxidative defense against O3 stress suggests that yield responses are governed by the time of sowing and intrinsic defense responses of the cultivars. In future with rising trend of O3, early sown cultivars are expected to be more vulnerable to oxidative stress compared to late sown cultivars.

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

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

The influence of urban stress factors on responses of ground cover vegetation.

A comprehensive study was conducted to evaluate the effects of ambient air pollution, land use, and soil properties on ground cover vegetation in the urban area of Varanasi city, situated in the Indo Gangetic Plain of India. Twelve leaf functional traits were assessed on eight most dominant herbaceous species belonging to four angiospermic families in three different land uses with varying air pollution loads and soil properties. Particulate matter (PM10 and TSP), gaseous pollutants (SO2, NO2, and O3), land uses (built-up area, shrub, and grass cover), and soil properties showed significant variability among the land uses. Air pollution was identified as the major stress factor which influenced leaf functional traits of ground cover vegetation followed by soil properties and land uses. Among the plants, Croton sparsiflorus was found to be the most responsive plants to all the factors. Plants responded differently under varying environmental factors as Euphorbia hirta was maximally influenced by air pollution, whereas the effect of land use was maximum in C. sparsiflorus. Influence of soil properties was highest in Digitaria ciliaris and Scoparia dulcis. All the environmental factors in combination maximally influenced non-enzymatic antioxidants (ascorbic acid and polyphenolics) followed by photosynthetic pigments among the different leaf functional traits. Among the environmental factors, NO2 and PM were identified as the most influencing factors regulating leaf functional traits followed by K level in soil and shrub cover. It can be concluded that responses of different leaf functional traits of ground cover vegetation varied with different environmental factors and responses were mostly species specific.

Assessment of ozone toxicity among 14 Indian wheat cultivars under field conditions: growth and productivity.

Tropospheric ozone (O3) is a well-known threat to global agricultural production. Wheat (Triticum aestivum L.) is the second most important staple crop in India, although little is known about intra-specific variability of Indian wheat cultivars in terms of their sensitivity against O3. In this study, 14 wheat cultivars widely grown in India were exposed to 30 ppb elevated O3 above ambient level using open top chambers to evaluate their response against O3 stress. Different growth and physiological parameters, foliar injury and grain yield were evaluated to assess the sensitivity of cultivars and classified them on the basis of their cumulative stress response index (CSRI). Due to elevated O3, growth parameters, plant biomass, and photosynthetic rates were negatively affected, whereas variable reductions in yield were observed among the test cultivars. Based on CSRI values, HD 2987, DBW 50, DBW 77, and PBW 550 were classified as O3 sensitive; HD 2967, NIAW 34, HD 3059, PBW 502, HUW 213, and HUW 251 as intermediately sensitive, while HUW12, KUNDAN, HUW 55, and KHARCHIYA 65 were found to be O3-tolerant cultivars. Cultivars released after year 2000 were found to be more sensitive compared to earlier released cultivars. Path analysis approach showed that leaf area, plant biomass, stomatal conductance, net assimilation rate, and absolute growth rate were the most important variables influencing yield under O3 stress. Findings of the current study highlight the importance of assessing differential sensitivity and tolerance of wheat cultivars and response of different traits in developing resistance against elevated O3.

Use of GLM approach to assess the responses of tropical trees to urban air pollution in relation to leaf functional traits and tree characteristics.

Responses of urban vegetation to air pollution stress in relation to their tolerance and sensitivity have been extensively studied, however, studies related to air pollution responses based on different leaf functional traits and tree characteristics are limited. In this paper, we have tried to assess combined and individual effects of major air pollutants PM10 (particulate matter ≤ 10 µm), TSP (total suspended particulate matter), SO2 (sulphur dioxide), NO2 (nitrogen dioxide) and O3 (ozone) on thirteen tropical tree species in relation to fifteen leaf functional traits and different tree characteristics. Stepwise linear regression a general linear modelling approach was used to quantify the pollution response of trees against air pollutants. The study was performed for six successive seasons for two years in three distinct urban areas (traffic, industrial and residential) of Varanasi city in India. At all the study sites, concentrations of air pollutants, specifically PM (particulate matter) and NO2 were above the specified standards. Distinct variations were recorded in all the fifteen leaf functional traits with pollution load. Caesalpinia sappan was identified as most tolerant species followed by Psidium guajava, Dalbergia sissoo and Albizia lebbeck. Stepwise regression analysis identified maximum response of Eucalyptus citriodora and P. guajava to air pollutants explaining overall 59% and 58% variability's in leaf functional traits, respectively. Among leaf functional traits, maximum effect of air pollutants was observed on non-enzymatic antioxidants followed by photosynthetic pigments and leaf water status. Among the pollutants, PM was identified as the major stress factor followed by O3 explaining 47% and 33% variability's in leaf functional traits. Tolerance and pollution response were regulated by different tree characteristics such as height, canopy size, leaf from, texture and nature of tree. Outcomes of this study will help in urban forest development by selection of specific pollutant tolerant tree species and leaf traits, which is suitable as air pollution mitigation measure.

A Global Perspective of Fine Particulate Matter Pollution and Its Health Effects.

Fine particulate matter (PM) in the ambient air is implicated in a variety of human health issues throughout the globe. Regulation of fine PM in the atmosphere requires information on the dimension of the problem with respect to variations in concentrations and sources. To understand the current status of fine particles in the atmosphere and their potential harmful health effects in different regions of the world this review article was prepared based on peer-reviewed scientific papers, scientific reports, and database from government organizations published after the year 2000 to evaluate the global scenario of the PM2.5 (particles <2.5 µm in aerodynamic diameter), its exceedance of national and international standards, sources, mechanism of toxicity, and harmful health effects of PM2.5 and its components. PM2.5 levels and exceedances of national and international standards were several times higher in Asian countries, while levels in Europe and USA were mostly well below the respective standards. Vehicular traffic has a significant influence on PM2.5 levels in urban areas; followed by combustion activities (biomass, industrial, and waste burning) and road dust. In urban atmosphere, fine particles are mostly associated with different health effects with old aged people, pregnant women, and more so children being the most susceptible ones. Fine PM chemical constituents severely effect health due to their carcinogenic or mutagenic nature. Most of the research indicated an exceedance of fine PM level of the standards with a diverse array of health effects based on PM2.5 chemical constituents. Emission reduction policies with epidemiological studies are needed to understand the benefits of sustainable control measures for fine PM mitigation.

Pollution Response Score of Tree Species in Relation to Ambient Air Quality in an Urban Area.

Multivariate statistical techniques were employed on twelve leaf traits in four selected common tree species (Mangifera indica L., Polyalthia longifolia Sonn., Ficus benghalensis L. and Psidium guajava L.) to evaluate their responses with respect to major air pollutants in an urban area. Discriminant analysis (DA) identified chlorophyll/carotenoid ratio, leaf dry matter content, carotenoids, net water content and ascorbic acid as the major discriminating leaf traits, which varied maximally with respect to the pollution status. Pollution response score (PRS), calculated on the basis of discriminate functional coefficient values, increased with an increase in air pollution variables for all the tested species, with the highest increase in P. longifolia and the lowest in F. benghalensis. The study highlights the usefulness of DA for evaluation of plant specific traits and PRS for selection of tolerant species.