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Impact of tropospheric ozone on crop growth and productivity – a review Article in Journal of scientific and industrial research · February 2012

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BHATIA at al: IMPACT OF TROPOSPHERIC OZONE ON CROP GROWTH AND PRODUCTIVITY – A REVIEW

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Journal of Scientific & Industrial Research Vol. 71, February 2012, pp. 97-112

Impact of tropospheric ozone on crop growth and productivity – a review A Bhatia*, R Tomer, V Kumar, S D Singh and H Pathak Division of Environmental Sciences, Indian Agricultural Research Institute, New Delhi 110 012, India Received 26 July 2011; revised 10 December 2011; accepted 19 December 2011 This review presents tropospheric ozone (O3 ), an air pollutant affecting agriculture by reducing crop yield and deteriorating quality of produce. O 3 enters leaves through stomata and diffuses within the apoplast, producing many oxidizing compounds and affecting various physiological and biochemical processes, crop growth and yield. O3 affects above and below ground carbon allocation and its dynamics, N cycling, microbial content and emission of greenhouse gases (GHG) from soil. This review focuses on the impact of O3 on crop growth with special emphasis on productivity in Asian region, methods for evaluating O3 responses in field, amelioration of O3 induced injury and providing dose-response functions for economic assessments. Keywords: Carbon allocation, Carbon dioxide, Crop productivity, Ozone (O3 )

Introduction Tropospheric ozone (O 3 ) suppresses crop productivity on a large scale 1,2 . It is one of the most damaging air pollutants, whose concentration has increased rapidly as a result of overuse of fossil fuels and nitrogen fertilizers3 . Also, increased recognition of transboundary transport of O 3 precursors [nitrogen oxides (NOx) & volatile organic compounds (VOCs)] in troposphere4 , result in higher O3 concentration than critical level to plants in agricultural, forest and remote rural areas5 . Under favorable meteorological conditions, O3 may accumulate in troposphere and reach a level that causes significant decrease in growth and yield of O3 sensitive plant species. Elevated O3 concentrations are now recognized as extending far beyond city limits, and in rural regions significantly affect crop yields, forest productivity, and natural ecosystems 6 . Adverse effects of O3 on plants were first identified in 1950s, and now it is the most important rural air pollutant, affecting human health, materials and vegetation. Being toxic to plants at concentrations as low as 30 ppb, high O 3 concentrations affect physiological and biochemical processes as well as biomass growth and yield of agricultural crops 7,8 . Although O3 at the ground level is also a greenhouse gas (GHG), as it absorbs outgoing long wave radiation and *Author for correspondence Tel: +91 11 25841490; Fax: +91 11 25841866 E-mail: [email protected]

plays a role in regulating air temperature contributing to 7% to the total warming effect9 . It also absorbs solar radiation, in particular UV-B radiation. This review presents impact of O3 on crop growth with special emphasis on productivity in Asian region, methods for evaluating O3 responses in field, amelioration of O3 induced injury and providing dose-response functions for economic assessments. Tropospheric Ozone (O3 ) Formation Chemistry

Atmospheric O3 (90%) layer is located in stratosphere. Tropospheric O3 contributes only 10% to the total O3 column, but its concentrations have been steadily rising during last 100 years. Tropospheric O3 production10 is the result of photochemical reactions of carbon monoxide (CO), methane (CH4 ), and other hydrocarbons in the presence of NOx (NO + NO2 ). O 3 destruction is also the result of photochemical reactions, involving NO, HO2 , or OH. NOx is primarily a product of fossil fuel combustion (63%), but is secondarily a result of biomass burning (14%), lightning (10%), soils (11%), and other small sources11 . Hydrocarbons are also the result of fossil fuel emissions, as well as direct evaporation of fuel, solvent use, chemical manufacturing, and natural vegetation1 . In urban regions with high concentrations of NOx, O3 production is generally VOC-limited, whereas in suburban or rural regions with low NOx levels, O3 production is NOx-limited. O3 is also

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J SCI IND RES VOL 71 FEBRUARY 2012

Fig. 1— Surface ozone concentrations (ppb) in different cities of India

transported into a region by local winds 12 . Different spatial distributions of NOx and VOC production, as well as NO destruction of O3 , often result in the largest O3 concentrations downwind of urban centers, rather than in urban areas13 . O3 production occurs during times of high temperature and solar radiation. Current and Background Ozone Concentrations in Troposphere

A wide range of O3 concentration (10-100 ppb) exists simultaneously in troposphere at any given time; highest concentrations are usually associated with emission of precursors (NOx, VOC) from urban areas and are often found downwind of these locations. At a certain location, O3 displays daily and annual concentration pattern that reflects the presence and changes in the precursors and scavengers 14 . Tropospheric O3 levels are dependent upon the hour of day, season, geographical location and meteorological conditions15 . Daily pattern shows an early morning minimum and early afternoon maximum. O3 is smaller during winter when production capacity is lower. O3 production occurs during times of high temperature and

solar radiation, such as during stagnant high pressure systems in summer1 . Whereas natural O3 production is expected to reach a maximum in early spring16 ; current maxima often occur during summer due to increased NOx and VOC emissions. Industrial regions tend to have maximum O3 in late afternoon and minimum in early morning hours. In contrast, marine and high latitude sites have maximum O3 before sunrise and lowest in the afternoon due to low NOx concentrations and therefore low O3 production and strong O3 destruction12 . At low elevation sites, a clear 24-h periodicity is usually observed, with low concentration at night and maximum levels before midday. At high elevation sites, diurnal variation is not observed. Background O3 levels in unpolluted air can be between 20-50 ppb17,18, though19 occasional background levels over 60 ppb resulting from stratospheric input. Polluted regions can have O3 levels peaking as high as 400 ppb17 . Studies of background ozone concentrations in the mid-latitude northern hemisphere suggest an increase of 0.5-2% per year20 , which modeling studies suggest O3 is primarily increasing due to rising NOx


emissions, augmented by intercontinental transport. Volz & kley21 reported that background O 3 concentration have been increasing from 10-20 ppb at beginning of 20t h century to 20-40 ppb in recent years. Stagnant air masses, varying in duration from one to several days, will result in high surface O3 concentrations (>80 nl l-1), with other periods of relatively low concentrations (150 nmol mol-1)4 . In contrast, chronic responses include lesions that develop over days to weeks under lower O3 concentrations, and accelerate senescence where lesions might not form. Under field conditions, accelerated senescence may be difficult to identify in the absence of a clean air control or other reference point (O3 tolerant cultivar) because cumulative effects on foliar senescence at the end of season are separated in time from O3 events causing the effects. O 3 is known to adversely affect carbon flow to roots and consequently their biology and biomass. Visible injury resulting from chronic exposure to low O3 concentrations includes changes in pigmentation or bronzing, chlorosis, and premature senescence after chronic exposure to low O 3 concentrations. Flecking and stippling may occur after acute exposure to high O 3 levels. Deleterious effects of O3 have often been attributed to premature leaf senescence, decreases in light interception, chlorophyll content and photosynthesis, consequent reductions in assimilate availability and alterations in assimilate partitioning88 . O 3 causes negative effects on water use efficiency, rate of senescence, dry matter production, and yield, increased turnover of antioxidant systems, damage to reproductive processes89 especially flowering and pollen tube extension, increased dark respiration90 , lowered carbon transport to roots91 , reduced decomposition of early successional communities92 , and reduced forage quality of C 4 plants93 . Sensitivity of seed crops to O3 was greatest between flowering and seed maturity. Effect on Germination

Bosac94 reported that germination is slightly delayed relative to the control plants in seeds obtained from racemes of oilseed rape plants, which were exposed to 100 nl l–1 O3 for 6 h during flowering. Stewart95 found that germination was promoted in seed harvested from Brassica compestris plants, in which terminal raceme was exposed to 100 nl l-1 O3 (6 h d-1) on single or multiple occasions during flowering. This response was apparent for all positions on raceme, suggesting a general effect on seed development. By contrast, in seed from plants,


in which vegetative and reproductive structures were both exposed to 70 nl l–1 O3 for 10 days before first flowering, although the final germination percentage was unaffected, it delayed germination. Stewart95 also noted that some seeds germinated precociously within pods of B. compestris following exposure to O3 , particularly in older, more mature pods. Precocious germination has also been recorded for B. compestris cv. Macro and B. napus cv. Lair 96 following O3 treatment. Agrawal et al.97 exposed rice seedlings to 200 ppb O3 and found a clear damage in rice proteome with specific leaf injury. Bhatia et al30 studied germination of seven upland rice varieties (PS-2, PS-3, PS-4, PS-5, Pusa-44, Pusa Basmati-1, Pusa-1460) and seven lowland rice varieties (PNR-381, PNR-519, CR-1009, PR-1018, Utkal Parbha, Sarla and Tapasvini) under 100 ppb O3 and found Pusa44 (100% germination) to be the most tolerant of all rice cultivars. Decrease in germination under elevated O 3 as compared to ambient control ranged from 55 to 100%. O3 exposure led to an increased fungal attack in seedlings. Effect on Plant Growth

High concentration O3 in the atmosphere is capable of inhibiting photosynthesis, carbon (sugar) production and altering carbon allocation to roots and stems and reducing carbohydrate formation of mycorrhizae (a symbiotic fungus/root relationship), uptake of important minerals (N, P, K and S), and root and stem growth. Progressive increase of O3 has a significant impact on agricultural production by inhibiting photosynthesis, accelerating leaf senescence, reducing plant growth and impairing yield attributes98,99. In crop leaves, exposed long term to O3 , the onset of senescence is advanced and accelerated catalysis leads to rapid loss of protein and chlorophyll71,100 . O3 impairs phloem loading and assimilates partitioning to roots and grain is often reduced while carbohydrates are retained in leaves2 . In turn, this contributes to: i) higher shoot/root biomass ratio; ii) lower harvest index (together with the effect of a reduced length of grain filling period); and iii) altered leaf chemistry. McKee & Long101 suggested that O 3 effects on allocation and development are more important for reductions in final yield than effects on photosynthesis and biomass accumulation. O3 reduces root growth more than shoot growth in a wide range of plant species. Effect on Pollen Germination, Flower Initiation and Reproduction

Studies of oilseed rape B. napus ,1 0 2 and B. compestris95 indicated that exposure to realistic O3

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episodes at critical developmental stages can influence reproductive structures directly, promoting significant effect on floral development, seed yield and quality and seedling vigour. Exposure of pollen either in vivo on anthers103 or in vitro 104 reduces germination and / or tube growth in various species. Pollen from tomato (Lycopersicon esculentum) and oilseed rape (B. napus cv Libravo) is insensitive105 even to relatively high concentrations of O3 under some exposure conditions. However, Stewart 95 reported that exposure of B. compestris pollen to 120 nl 1-1 O3 for 6 h had significant effects, suggesting the existence of a threshold concentration for damage. Exposure to O 3 has been reported to delay flowering in various species, including soybean (Glycine max) and cotton (Gossypium hirsutum). Fernandez-Bayon et al106 reported that flower production was significantly reduced in two cultivars of watermelon (Citrullus lanatus cvs Suger Baby and De La Reina) and muskmelon (Cucumis melo cvs Verde Tendral Tardio and Amarillo Temprano) following 21 days exposure to 70 nl1 -1 O3 for 6 h d–1 . Bosac et al107 reported that a single 6 h exposure of inflorescences of B. napus cvs Tapidor and Libravo to 100 nl l-1 O3 was sufficient to increase flower bud abortion 5 days after exposure. Effect on Plant Biochemical Processes

Via the production of ROS, O 3 impairs photosynthetic CO2 fixation by impairing Rubisco activity or stomatal functioning, and/or indirectly via acceleration of leaf senescence and thus protein (Rubisco) and chlorophyll degradation, particularly in leaves formed during flowering 1 0 8. Reduction in CO 2 fixation by ribulosebisphosphate carboxylase is a typical symptom found in leaves exposed to O3 over longer periods of time. Further inhibition of CO2 assimilation results from direct or indirect inhibition of stomatal opening that reduces uptake. Reduction in chlorophyll content109 and carboxylation efficiency contributes to decrease in net photosynthesis under elevated O 3 8 . The latter occurs due to oxidation of –SH groups of Rubisco58 . Stimulated dark respiration often occurs together with reduced photosynthesis, probably due to increased respiration associated with maintenance and repair 110 . Combined effects of reduced assimilation and increased respiratory loss of CO2 consist of an overall reduction of assimilate production and export from the source leaves. Moreover, exposure to O 3 typically reduces effective leaf area, thus decreasing light interception and quantity of assimilate available to support the growth of economic yield component.

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O3 after passing through stomatal pore reacts with organic molecules (ethylene, isoprene) in intercellular air space or with components of extra cellular fluid. In both cases, secondary oxidants (primary ozonides, hydroxyhydroperoxides) may be formed, and could react with protein component of cell membrane. This reaction is prevented to some extent by the presence of radical scavengers (ascorbic acid and polyamines) 53 . Formaldehyde, formate and acetate accumulate in damaged tissue, possibly as a result of the reaction between O3 and ethylene or between O3 and phenylpropanoid residues of lignin111 . Ethylene formation determines sensitivity of plants to O 3 112 . Interactions of O3 with ethylene or other hydrocarbons are thought to be part of mechanism leading to injury. Ethylene is considered to be the major cause of accelerated senescence and leaf abscission113 . Response of crop quality to increasing O3 is not straightforward. An analysis by Pleijel et al114 for wheat revealed that loss in grain yield is accompanied by increased grain quality. Fuhrer et al115 showed that higher than ambient O3 concentrations reduced starch concentration in wheat grains. A more comprehensive analysis for wheat116 confirmed that O 3 increases grain protein concentration while decreasing protein yield. Nitrogen metabolic cycle of plants is mainly affected by O3 through changes in amino acid and protein contents1 1 7. Seed protein content increases with O3 concentration in soybean1 1 8 and wheat 1 1 9, but is decreased in oilseed rape 102 . Exposure to O3 increased concentrations of Ca, Mg, K and P contents of grain. In potato, sugar and starch content of tubers decreased significantly with increased O3 , while ascorbic acid concentrations increased120 . No effect of increasing O 3 on seed composition and quality was observed in peanut (Arachis hypogaea)121 . Effect on Yield Parameters and Crop Yields

Research in Europe and North America, using opentop chamber (OTC) technique with charcoal-filtered air, non-filtered ambient air and O3 enriched air, have produced a wealth of data and exposure yield relationships 122 , which were used as input to economic crop loss assessments1021 . Mills et al123 classified wheat, watermelon, pulses, cotton, turnip, tomato, onion, soybean and lettuce as the most O3 -sensitive crops; sugarbeet, potato, oilseed rape, tobacco, rice, maize, grape and broccoli as moderately O3 -sensitive and barley, plum and strawberry as O3 -tolerant. Wheat,

Table 1—Yield loss due to surface ozone in different Asian countries Country

Crop

Pakistan

Wheat 130, 132 Rice131, 132 Soybean133 Barley 134 Wheat 135,137 Rice30 Maize30 Tomato27 Mustard 128 Mungbean129 Rice63 Rice136

India

Malaysia China

Average O3 conc.,h ppb 22-52 10-55 33-63 71 40-48 56 56 37 62 10-59 32.5 23-39

Yield reduction % 33-47 37-51 64 13-44 11-21 10-15 13-39 24 20 18-79 6.3 14-20

soybean and corn were identified as especially sensitive to O3 due to likely co-occurrence of peak levels of O3 and growing season of these crops 1 2 4. Wang & Mauzerall125 calculated that in China, Japan and South Korea, wheat, rice and corn yields (1-9%) and soybean yield (23-27%) were lost due to 1990 levels of O3 and that losses may exceed 30% by 2020. Amundson et al126 reported that exposure of wheat commencing at anthesis reduced yield by decreasing only grain weight, suggesting that timing of exposure might be important in determining the response of individual yield components. Season-long exposure of spring wheat in open-top chambers to a seasonal mean of 60 nl l-1 O3 (7 h d-1) had no significant effect on grain yield. However, on exposure to 84 nl l–1 , yield was reduced by 30% due to decrease in the number of ears, grains per ear and per spikelet, and individual grain weight127 . Agrawal et al128,129 observed negative effects of ambient air pollution containing O 3 in a mixture with other pollutants on yield and seed quality of mung bean (Vigna radiata) during summer and of wheat (Triticum aestivum) and mustard (B. campestris) during winter. Yield of tropical wheat in eastern Gangetic Plain was strongly reduced with a mean O3 concentration of 40 ppb as compared to charcoal-filtered air 134,135. O 3 affects photosynthesis, biomass, leaf area index (LAI), grain number and grain mass for rice138 . Wahid et al139 reported reductions of 42% and 37% in grain yield of two cultivars of rice (Oryza sativa L.), primarily due to reduction in mean panicle number per plant. Yield reduction due to surface O3 levels has been observed in cereals, pulses, vegetable and oilseeds grown in different Asian countries


(Table 1). Thus impact of O 3 on specific yield components appears to vary depending on factors including O3 concentration, exposure conditions, timing of exposure, crop species and cultivars. There is a considerable effect of polluted air on a wide range of agricultural yield reduction23 . Yield losses ranged from near 0 up to 39% and complete reduction of yield were observed during O3 episodes. Excess O3 uptake not only reduces crop growth and yield and alters crop quality but in longer term may also lead to changes in species and genetic composition of semi-natural plant communities140 . Impact on Belowground Carbon (C) Allocation

Higher O3 concentration adversely affect plant growth, but to a greater extent roots than shoots80 . Cooley & Manning91 and Spence et al141 reported that elevated O3 reduces belowground C allocation. Many O 3 induced reactions (repair processes and production of secondary compounds) in leaves cause an increase in C demand, and thus a reduction in C allocation belowground79,142. Decreased allocation belowground alters C flux to soils, leading to effects on soil processes79 and long term system C balance143 . Because of the relatively increased shoot sink strength for repair of O3 -injured leaves 144,145, allocation of C to roots decreases. This C limitation decreases root biomass and growth as well as root carbohydrate concentrations 146,147 and decreased root and soil respiration rates148 . In addition to reducing root growth, exposure to elevated O3 can also decrease the amount of root exudates149,150 and pore water dissolved organic carbon (DOC)151 . Andersen et al152 reported that O3 reduce leaf N content and thus affects quality of leaf litter. However, all knowledge of O 3 related changes in litter quality is related to either forest trees or grasslands. Pregitzer et al153 found that long term exposure to elevated O3 increased aspen fine-root production and mortality. Impact on N Cycle

Elevated O3 may lead to significant alterations in plant residue decomposition (both litter quality and quantity), nutrient cycling and microbial activities, which directly affect N cycle. Leaf litter is a major nutrient supply, since plant residues are degraded in the soil by microorganisms and nutrients once bound to plant material are hence mineralized1 5 4. Elevated O3 may affect decomposition processes by reducing residue mass input, as reported in soybean and blackberry155 . Such changes in litter quantity may cause modifications in mineralization

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of organic C and other nutrients such as N are reduced, thus whole N cycling process is affected155 . Soil microbial activities (nitrification and denitrification) are of vital importance to N cycling. O 3 -induced changes in N cycling and available C would be expected to slow potential nitrification and denitrification. An earlier study156 with forest trees only reports no changes in potential microbial activities. As soil nitrification and denitrification are related to N2 O emissions 1 5 7, decreases in substrate availability (changes in quantity and quality of leaf litter, and root exudates) for nitrifying and denitrifying bacteria may also decrease N2 O emissions 1 5 8. O 3 causes reduction in C allocation and root exudates, and it also has an impact on soil microbial content, and C and N cycling and emission of GHGs from soils 84 . Impact on Microbial Content

Soil microbial biomass is linked to plant roots and decomposition of plant residue, as microbes feed on roots, root exudates or litter154 . Microbial growth and activity are constrained by availability of organic C; therefore, a decline in C inputs combined with reduction in soil N through altered decomposition could lead to shifts in the size and composition of soil microbial biomass and affects metabolic activities of microbes 79,159,160. Assumed decrease in soil microbial population size may also cause changes in structure of microbial communities, and this, in turn, may have significant effects on the functioning (including emissions of GHGs and nutrient cycling) of microbial community and its interaction with plant community79,161. Alteration of abundance of certain soil organisms (mycorrhizal fungi) may have a greater impact on ecosystem productivity than others162 . Chronic O3 exposure163 increases microbial biomass and organic acid concentrations in peatland. O3 has led to changes164 or no changes165 in soil microbial community structure. Thus, impact of elevated O3 on complicated interactions found in N cycle and its microbial activities are poorly understood79,166. O3 seems to have very little effect on soil nematodes, but have more effect on soil bacteria than soil fungi. Treatment of strawberry fields with high rates of O3 improved colonization of Trichoderma, so there must have been either an initial knock back of competing microbial or releases of nutrients favorable for Trichoderma sp. growth167 . Interaction of O 3 and CO2 on Belowground Processes

Concurrent with rise in O 3 , global CO2 concentrations have progressively increased to today’s level of 380 ppm

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and are predicted to increase to 550 ppm by mid-21st century. Biological effects of O3 on plants have been studied for over 50 years168-170. Increasing atmospheric CO2 affects most plant processes positively171 ; it is possible that adverse effect of increasing O3 on crop yields may have been at least partially offset by positive influence of concomitant increases in CO2 . Long et al172 have shown that magnitude of response to elevated CO2 is highly variable, not only across but within species and cultivars. Amelioration of O 3 injury by elevated CO2 was largely attributed to the restriction of O 3 intake by leaves with stomatal closure, and partly to maintenance of scavenge system for reactive oxygen species that entered leaf mesophyll, as well as the promotion of photosynthetic rate 173 . Most relevant interaction is the reduction in stomatal conductance at elevated levels of CO2 that has the effect of preventing reductions of photosynthetic rate, growth, and yield in many crops grown in the presence of toxic levels of O3 155,174-177. Although CO2 enrichment reduced the damaging impact of O3 on radiation interception, chlorophyll content, photosynthesis and vegetative growth, it did not prevent significant O3 induced yield losses178 . In C3 species, photosynthetic process is not CO 2 -saturated under present-day conditions because atmospheric concentrations of 385 ppb is well below the saturation point of 670 ppb. In short term experiments, increased atmospheric CO2 may increase photosynthetic rates in C 3 species, provided that sufficient water and nutrients are available. Although it is expected that assimilation ratios in elevated CO2 are substantially higher than control4 . Analysis of compiled shoot biomass and yield data 1 7 6 show that increased photosynthetic rate does get translated into an average 30% increase in biomass but often not into yield in clean (charcoal-filtered) air 179 . Balaguer et al180 showed that dry matter accumulation in wheat increased under elevated CO2 , whereas elevated O3 reduced growth by 10%, when both gases were applied. In soybean (Glycine max), seed yield decreased in elevated O 3 and elevated CO2 by only half on the average, as compared to the decrease observed in ambient CO 2 and elevated O3 . SoyFACE experiment at University of Illinois at Urbana-Champaign studied effects of CO2 and O3 on soybean, and observed that slower senescence from elevated CO2 levels of 550 ppm was offset by accelerated senescence from elevated O3 levels at 23% above ambient levels66 . Morgan et al181 found substantial

decrease (11-23%) in aboveground net primary production (NPP) as a result of increased O 3 levels. Long et al182 found maize and rice as less susceptible to O3 than wheat and soybean. Studies on belowground processes have shown amelioration155,183 and no amelioration156,184 with elevated CO 2 . Phillips et al 1 6 4 reported alterations in the composition of microbial communities following CO2 enrichment. Studies have mainly concentrated on the effects of these gases on N transformations 156 , microbial biomass C and soil organic C quantity 185,159 , C allocation1 8 6, soil C formation 183 , decomposition 1 5 5, microbial community composition164 , mycorrhiza 187 and soil fauna 188 of forests. Abiotic factors (water status, nutrient availability, temperature and relative humidity) may also alter magnitude of O3 and CO2 effects on plants189,190. O3 and CO2 fertilization effects are thus clearly the same order of magnitude, but their relative magnitudes and inte ractive effects are still highly uncertain and dependent on both species and environmental conditions. Amelioration of O 3 -induced Injury

Application of some protective chemicals may prevent injury in crops occurring due to O3 exposure. Some of the chemicals were found to protect sensitive crops from O3 -induced injury with varied effectiveness, while others were ineffective or produced unacceptable side effects 1 9 1. Antioxidant ascorbate, antiozonant ethylenediurea (EDU), and systemic fungicide benomyl may be used as protective chemicals in mitigating negative impacts of O 3 on plant growth and yield. Ethylene diurea, a protective chemical, prevents onset of foliar injury and estimated production loss by O 3 , and is applied as a foliar spray, a soil drench or by direct injection192,193. Ascorbic acid is an antioxidant in plants57 . Application of ascorbic acid to leaves increases concentration of ascorbic acid in cell walls and results in considerable protection from O3 injury1 9 4. Systemic fungicide, benomyl, shows antiozonant and antisenescence in plants195 . Rodriguez et al196 reported that diphenylamine (DPA) markedly decreased yield reduction due to O 3 . In Egypt, Hassan197 found a significant increase tuber weight of potato after EDU treatment. Oleg & Nataliya 198 tested some natural extracts having antioxidants activity as O3 protectant. Blum et al193 reported protective effect of growth regulators Emistym C and Agrostimulin against O3 damage of plants. Application of these natural and synthetic antioxidants may be helpful in overcoming yield losses in major crops due to high O3 levels.


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In order to estimate the impact of ambient O3 on agricultural crops, most used method is the use of opentop chambers (OTCs)199 , which are suitable for studying the effects of O3 because plants can be grown in close to natural conditions while O3 concentrations can be maintained below phytotoxic levels with filtration or increased by additions of O3 . OTCs have provided consistent indications of yield losses for a wide variety of plants due to O 3 exposure200,201. In OTCs, experimental unit is replicable, a range of treatments are available, control of exogenous factors is possible, and simulation of field losses is possible and relatively accurate23 . Cylindrical OTCs consist of a metallic frame, plastic film panel, and a set-up to sample the air inside the chamber uniformly. One major concern about OTC has been the modification of microclimatic conditions due to constant air flow and turbulence within the chamber202 . In OTC, daytime temperatures can increase, while photosynthetic active radiation and wind speed can decrease 2 0 3. However, modification of microclimatic conditions does not appear to affect relative plant responses to O3 204 . OTCs have been used to study the impact of elevated O3 on growth and productivity of different crops in India at Varanasi205 and at New Delhi30 . No significant change was observed in microclimatic conditions at both the locations between OTC control plots and chamber less plots. On the other hand, direct application of OTC studies in yield loss and economic studies has caused considerable debate because of possible confounding effects of chamber environment, which could lead to an over estimation of impacts206 . To overcome this, experiments have also been conducted in free air O3 concentration enrichment (O3 -FACE) experiments to study the effects of surface O 3 on plants207-209 and also interactive effects along with CO2 210,211. Free-air concentration (FACE) enrichment provides a realistic method for evaluating effects of elevated gaseous concentrations on growth, development, yield, and water use of agricultural crops under natural and fully open-air conditions. FACE technology uses no confinement structures, rather an array of vertical or horizontal vent pipes to release jets of CO2 -enriched air or pure CO2 gas at the periphery of vegetation plots. FACE relies on natural wind and diffusion to disperse CO2 across the experimental area. FACE design allows good temporal and spatial control of gas concentrations throughout crop canopies2 1 2.

Weighting factor

Methodology for Studying Impact of Ambient O 3 on Agricultural Crops

AOT40 SUM06 W126

Hourly mean concentration of ozone, ppb Fig. 2—Dose-response indices for ozone concentrations

Economic cost of experimentation though is manifold as compared to OTCs201 . Dose-response Relationship

Dose-response functions have been developed to quantify the effects of O3 on crop yields. Dose is the amount of O3 available during the response period and is defined as O3 concentration multiplied by duration of exposure. While doses measure concentration over a period of time, because of antioxidant defenses, O3 is often observed to affect yields only after surpassing certain threshold levels. O3 exposure is typically summarized by suitable statistical indices, which can be characteristics of the frequency distribution of hourly concentrations measured near the surface in ppmV (partsper-million by volume) or ppbV (parts per billion by volume) such as mean, median, maximum or percentiles. To be more receptor-specific, calculations are limited to growing season, and daylight hours. Results from OTC studies suggested that indices that give greater weight to peak concentrations and those accumulating exposure were best related to yield changes140 . Since 1980s, cumulative indices (SUM06, AOT40, or W126; that preferentially weight higher conc.) have been used in conjunction with mean indices for developing exposure-response relationships. AOT40 index is sum of the amounts, by which hourly O 3 concentrations exceed a threshold of 40 ppb over growing season and during daylight hours. Critical levels for O 3 are calculated as a cumulative exposure over a threshold of 40 ppbv; resulting index was AOT40 (accumulated exposure over a threshold of 40 ppbv) 22 . AOT40 value of 5.3 ppm had been proposed for crop plants, calculated for 3 months

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at day-light hour22 . In a study conducted at Indian agricultural research institute (IARI), New Delhi, an AOT 40 of 12.3 ppmvh under elevated O3 conditions led to a 15% decline in rice yield as compared to control30 . SUM06 index is the sum of hourly O3 concentrations equal to or greater than 60 ppb over the daylight period. Daily sums are then added over a specified time period; a 3-day SUM06 is used for assessment of acute effects, whereas a seasonal (3-month) SUM06 is used for assessment of chronic effects. Although SUM06 clearly encapsulates some aspects of plant exposure that are important in plant response (cumulative exposure over a time period and relative importance of peak concentrations), there are other factors demonstrably important in determining plant response (phenology, time of day etc.) that are not accounted for in SUM06 index. Furthermore, there is no biological basis for assuming that concentrations below 60 ppb O3 are not significant in plant response. In the future it may be possible to develop a more biologically relevant index. Examples of cumulative indices (Fig. 2) in units of ppb h (or ppm h, using hourly means) are as follows: AOT40 = Σ ([O3 ]40) for [O3 ] ≥ 40 ppb during daylight hours; and SUM06 = Σ ([O3 ] for [O3 ] ≥ 60 ppb. An index involving a continuous weighting function is W126 = Σ ([O3 ] W, where W (sigmoidal weighting function) is W=1/(1+4403(exp (-0.126 [O3 ]))). Meta-analyses of O 3 effects studies on rice, soybean and wheat found that seasonal O3 concentrations averaging 62, 45 and 42 nl-1 lowered yields by 14%, 10% and 18%, respectively, compared with CF air controls 138,177,213. An extensive survey of season-long field studies conducted in OTCs found that bean, cotton, lettuce, onion, soybean, tomato, turnip, watermelon and wheat suffered 5% yield losses at seasonal AOT40 values of 6 ppm h or less (O3 -sensitive crops)123 . Yields of broccoli (B. oleracae), grape, maize, potato, rape, rice, sugarbeet and tobacco were suppressed by 5% at seasonal AOT40s of 8.6 to 20 ppm h (moderately O3 sensitive crops)123 . Though economic assessment of crop losses due to O 3 have not been carried out in developing countries but studies conducted in USA showed that agronomic crop yield losses due to ambient O3 is estimated to be worth $US 3-5 billion annually 4,201.

and quality aspects of affected vegetation can be lowered by O3 as well. In the absence of an effective emission control of O3 precursors, it is expected that ambient O 3 concentration will increase in future. Rising levels of atmospheric CO2 will likely ameliorate deleterious O3 effects on vegetation, although the converse is also true that O 3 may suppress the potential CO2 aerial fertilization effect in some plants as well. Damaging effects of ambient O 3 on yield and quality of many crops plants will continue in many areas of the world and require further scientific evaluation of magnitude, distribution and mechanism. Understanding is needed to quantify the impact of ambient O 3 under open field conditions on new crop cultivars, which are being grown without specific consideration of their sensitivity to O3 . Crop breeding programs need to incorporate selection of traits for improved plant tolerance to ambient O3 in order to maintain and increase crop yields and nutritive quality. Acknowledgements Authors are grateful to DST, Govt of India, for funding during the course of this study. References 1

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Conclusions Tropospheric O3 has a negative impact on growth, development, and productivity of crops. Measurable yield losses due to O 3 toxicity are likely occurring in food crops

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