Supriya Tiwari

Annual and seasonal variations in tropospheric ozone concentrations around Varanasi

This study examines the annual, seasonal and diurnal variations in the ambient concentrations of ozone at a suburban site of Varanasi, India, during 2002–2006. Prominent seasonal variations in ozone concentrations were recorded. Ozone concentrations were higher during the warmer months. Daytime 12‐hourly mean monthly ozone concentrations varied from 45.18 to 62.35 ppb during summer, from 28.55 to 44.25 ppb during winter and from 24 to 43.85 ppb during the rainy season from 2002 to 2006. Distinct diurnal variations in ozone concentrations were also observed. Daytime maxima in ozone concentration were recorded between 1200 and 1400 h, whereas morning and evening hours showed lower concentrations of ozone. Ozone concentrations in the atmosphere depended on several meteorological factors. Monthly average ozone concentration was significantly correlated with maximum temperature (p<0.001) and mean monthly temperature (p<0.05), maximum relative humidity (p<0.001), minimum relative humidity (p<0.001) and mean monthly relative humidity (p<0.001), and sunshine hours (p<0.001). Ozone concentrations in the ambient air have shown an increase in the past decade that was more in the winter and rainy seasons than in the summer. This study suggests that ozone concentrations around Varanasi were sufficiently high to cause significant damage to agricultural production. The present work can be extended to a regional level by incorporating modelling studies using recent remote sensing tools.

Effectiveness of different EDU concentrations in ameliorating ozone stress in carrot plants.

Ethylenediurea (EDU) is suggested for use to evaluate plant response under ambient ozone (O(3)) concentrations. Four EDU treatments, viz. 0 (non-EDU), 150, 300 and 450 mg L(-1), applied as soil drench at 10 days interval to carrot (Daucus carota L. var. Pusa Kesar), grown at a tropical suburban site of Varanasi experiencing mean O(3) concentration of 36.1 ppb during the experimental period. EDU treated plants showed significantly higher antioxidative defense, assimilation capability and reduced membrane lipid peroxidation, which led to better growth and significant yield increments compared to non-EDU treated ones. The magnitude of positive responses was highest at 150 mg L(-1) EDU treatment at 60 DAG, representing the metabolically most active phase of root filling in carrot. This study suggests that the lowest EDU concentration was sufficient to provide protection against negative effects of O(3).

Effects of elevated ozone on photosynthesis and stomatal conductance of two soybean varieties: a case study to assess impacts of one component of predicted global climate change

Global climatic change scenarios predict a significant increase in future tropospheric ozone (O(3)) concentrations. The present investigation was done to assess the effects of elevated O(3) (70 and 100 ppb) on electron transport, carbon fixation, stomatal conductance and pigment concentrations in two tropical soybean (Glycine max L.) varieties, PK 472 and Bragg. Plants were exposed to O(3) for 4 h.day(-1) from 10:00 to 14:00 from germination to maturity. Photosynthesis of both varieties were adversely affected, but the reduction was higher in PK 472 than Bragg. A comparison of chlorophyll a fluorescence kinetics with carbon fixation suggested greater sensitivity of dark reactions than light reactions of photosynthesis to O(3) stress. The O(3)-induced uncoupling between photosynthesis and stomatal conductance in PK 472 suggests the reduction in photosynthesis may be attributed to a factor other than reduced stomatal conductance. An increase in internal CO(2) concentration in both O(3)-treated soybean varieties compared suggests that the reduction in photosynthesis was due to damage to the photosynthetic apparatus, leading to accumulation of internal CO(2) and stomatal closure. The adverse impact of O(3) stress increased at higher O(3) concentrations in both soybean varieties leading to large reductions in photosynthesis. This study suggests that O(3)-induced reductions in photosynthesis in tropical and temperate varieties are similar.

Ozone damage, detoxification and the role of isoprenoids – new impetus for integrated models

High concentrations of ozone (O3) can have significant impacts on the health and productivity of agricultural and forest ecosystems, leading to significant economic losses. In order to estimate this impact under a wide range of environmental conditions, the mechanisms of O3 impacts on physiological and biochemical processes have been intensively investigated. This includes the impact on stomatal conductance, the formation of reactive oxygen species and their effects on enzymes and membranes, as well as several induced and constitutive defence responses. This review summarises these processes, discusses their importance for O3 damage scenarios and assesses to which degree this knowledge is currently used in ecosystem models which are applied for impact analyses. We found that even in highly sophisticated models, feedbacks affecting regulation, detoxification capacity and vulnerability are generally not considered. This implies that O3 inflicted alterations in carbon and water balances cannot be sufficiently well described to cover immediate plant responses under changing environmental conditions. Therefore, we suggest conceptual models that link the depicted feedbacks to available process-based descriptions of stomatal conductance, photosynthesis and isoprenoid formation, particularly the linkage to isoprenoid models opens up new options for describing biosphere-atmosphere interactions.

Ethylenediurea as a potential tool in evaluating ozone phytotoxicity: a review study on physiological, biochemical and morphological responses of plants

Present-day climate change scenario has intensified the problem of continuously increasing ground-level ozone (O3), which is responsible for causing deleterious effects on growth and development of plants. Studies involving use of ethylenediurea (EDU), a chemical with antiozonant properties, have given some promising results in evaluating O3 injury in plants. The use of EDU is especially advantageous in developing countries which face a more severe problem of ground-level O3, and technical O3-induced yield loss assessment techniques like open-top chambers cannot be used. Recent studies have detected a hormetic response of EDU on plants; i.e. treatment with higher EDU concentrations may or may not show any adverse effect on plants depending upon the experimental conditions. Although the mode of action of EDU is still debated, it is confirmed that EDU remains confined in the apoplastic regions. Certain studies indicate that EDU significantly affects the electron transport chain and has positive impact on the antioxidant defence machinery of the plants. However, the mechanism of protecting the yield of plants without significantly affecting photosynthesis is still questionable. This review discusses in details the probable mode of action of EDU on the basis of available data along with the impact of EDU on physiological, biochemical, growth and yield response of plants under O3 stress. Data regarding the effect of EDU on plant ‘omics’ is highly insufficient and can form an important aspect of future EDU research.

Mitigation of Ozone Stress

The photochemical reactions leading to O3 formation and the variables on which these reactions depend are undergoing rapid alterations owing to the present climate change scenario. The multifarious set-up related to O3 formation in the troposphere makes it difficult to check the continuously increasing concentration of O3 around the globe. O3 concentration has already crossed the standard limit for vegetation set by European Union (EU) in most of the parts of the World, which is evident by a number of O3 induced crop yield reduction studies. Therefore the demand of the time is to develop certain strategies that will help in alleviating the deleterious effects of O3 on plant performance. This target can be achieved by adopting different approaches such as improved agronomic practices; selection of O3 resistant cultivars, improving photosynthetic efficiencies of O3 exposed plants etc. Several strategies have been followed to achieve these targets, important ones being CO2 fertilization and soil nutrient amendments. In addition to this, air quality management practices using CH4 emission control is also considered to be an important strategy in minimizing O3 induced stress in plants. It has been shown that CO2 fertilization increased the carbon input which can be incorporated in plant biomass and subsequently helps in maintaining yield of plants exposed to O3 stress. Treatment of additional nutrient helps in the repair of the O3 injured plants thus sustaining their yield. As apparent through a number of studies, these strategies have proved quite efficient in partially mitigating O3 stress in plants. However, more experimentation is required before confirming the use of these approaches in mitigating O3 injury and implementing them in daily agricultural practices.

Effect of Ozone on Physiological and Biochemical Processes of Plants

The oxidizing nature of O3 is responsible for its phytotoxic effects on plants. O3 enters the plants through stomata and dissolves in the aqueous phase of the substomatal cavity to generate reactive oxygen species (ROS) such as superoxide anions (O2°-), hydrogen peroxide (H2O2), hydroxyl radicals (OH°) and singlet oxygen (1O2). Although ROS are an inevitable part of normal cellular metabolism and are continuously produced in the subcellular compartments like mitochondria, peroxisomes, chloroplasts, etc., O3 exposure stimulates the overproduction of ROS which exceeds the scavenging capacity of the cells intrinsic defense machinery. Plants have incorporated a constitutive antioxidative system which operates to scavenge the ROS generated under normal as well as stress conditions. The defense mechanism of plants has both enzymatic as well as non enzymatic components and works towards annihilating the ROS generated in apoplast as well as symplast. Excess of ROS that are not scavenged by apoplastic antioxidants, target the membrane permeability via the lipid peroxidation of the bilipid layer of the membranes. O3 also brings about alterations in the physiological process by affecting the biochemistry of photosynthetic machinery, disrupting the chlorophyll fluorescence kinetics and light as well as dark reactions of photosynthesis. O3 also alters the biophysical parameters like stomatal conductance and internal CO2 concentration which directly affects the rate of photosynthesis. In addition to this, excess of ROS stimulates the enhanced biosynthesis of cellular antioxidants. O3 stress also brings about changes in allocation of photosynthates, as more biomass is utilized in O3 injury repair rather than being converted to storage sugar, starch. O3 also affects the enzymes of nitrogen metabolism, thus influencing the biosynthesis of amino acids. This chapter investigates the role O3 in ROS generation and stimulation of antioxidant production. Effect of O3 on physiological processes, metabolite contents and nitrogen metabolism is also discussed.

Ozone Biomonitoring, Biomass and Yield Response

The negative consequences of surface ozone on agricultural crops will be an important threat to global food security in coming years. Several approaches have been adopted from time to time to evaluate the harmful effects of O3 on crop plants. The technique of O3 biomonitoring is used to estimate the level of O3 induced injury in plants. The concept of biomonitoring basically applies two methods, either the estimation of foliar injury symptoms or by analyzing the response of plants using specific chemical protectants which act as antiozonants. Biomonitoring programmes provide us with valuable comparative information regarding the O3 concentrations at different sites, such that the impact of ambient O3 is directly measured. The technique is specifically useful in developing countries, where extensive research facilities are not present. Effect of O3 on plant biomass is an important factor contributing towards the yield of the plants. The biomass accumulation and allocation pattern adopted by the plants in response to O3 stress not only determines the yield loss but also specifies the cultivar sensitivity/resistivity towards O3. Different crop loss assessment programmes carried out in different parts of the world have depicted the necessity for further exploring the O3 induced yield losses in coming times. In addition to programmes like NCLAN and EOTC, several individual experiments have also been conducted to evaluate yield losses in different crop plants. In the past few decades, several models like MOZRAT, MOZRAT 2, have been proposed which have predicted the yield losses of different agricultural crops in coming few decades. As per the modeling results, the regions of south and south-east Asia are specifically prone to greater yield reductions and therefore, there is an urgent need to develop extensive crop loss evaluation programmes in these areas.

Ozone Concentrations in Troposphere: Historical and Current Perspectives

The concentration of ground level ozone (O3) has registered an unpredictably high increase in the last few decades. It has been observed that the background O3 concentration has doubled since the nineteenth century with more prominent effects in Northern hemisphere. Although the formation of O3 largely depends upon the regional emission of O3 precursors, the increasing O3 concentrations have acquired a global significance. With the implementation of air quality legislations, the anthropogenic emission of O3 precursors has declined in North America and Europe but the problem still persists in Asia. The long lifetime of O3 superimposed by its intercontinental transport from Asia play an important role increasing global background O3 concentration in North America and Europe. Modeling studies have recognized South and East Asia as the major hotspots where O3 concentrations are expected to show maximum increase in near future. The present chapter throws light on the historical background of O3 monitoring and discusses the present scenario of ground level O3 with emphasis on the recent trends in different continental zones along with the seasonal and diurnal variations.

Tropospheric Ozone and its Impacts on Crop Plants

The concentration of ground level ozone (O3) has registered an unpredictably high increase in the last few decades. It has been observed that the background O3 concentration has doubled since the nineteenth century with more prominent effects in Northern hemisphere. Although the formation of O3 largely depends upon the regional emission of O3 precursors, the increasing O3 concentrations have acquired a global significance. With the implementation of air quality legislations, the anthropogenic emission of O3 precursors has declined in North America and Europe but the problem still persists in Asia. The long lifetime of O3 superimposed by its intercontinental transport from Asia play an important role increasing global background O3 concentration in North America and Europe. Modeling studies have recognized South and East Asia as the major hotspots where O3 concentrations are expected to show maximum increase in near future. The present chapter throws light on the historical background of O3 monitoring and discusses the present scenario of ground level O3 with emphasis on the recent trends in different continental zones along with the seasonal and diurnal variations.