Impact of Elevated Ozone on Growth , Yield and Nutritional Quality of Two Wheat Species in Northern India

Sensitivity to tropospheric ozone is highly variable in cultivars of different plant species. Wheat, an important cereal crop, has been found to be sensitive to elevated ozone levels leading to differences in grain yields. The objective of this study was to compare the effects of elevated tropospheric ozone on growth, yield and nutritional quality of two species of wheat, Triticum aestivum (PBW 343) and Triticum durum (HD 2936), which are tropical wheat cultivars commonly grown in northern India. Experiments were conducted growing winter wheat (rabi season) under elevated tropospheric ozone in northern India for two years in open-top chambers (OTCs) under charcoal-filtered air (CF), non-filtered air (NF), open air (OA) and elevated ozone (EO) concentration (NF + 25–35 ppb O3). There were different species responses to EO, with the modern aestivum wheat cultivar being more sensitive than durum wheat. The declines in all growth and yield parameters were greater in T. aestivum than T. durum in both the years. On average there was a 7% greater reduction in the photosynthetic rate and stomatal conductance in T. aestivum as compared to T. durum under EO at the flowering stage, and a 6% more reduction in leaf chlorophyll was observed on T. aestivum as compared to T. durum. Exposure to elevated O3 caused a decrease in the number of leaves and leaf area index, rubisco enzyme activity and chlorophyll in both the species. More reductions in grain yield were observed in T. aestivum (15 and 19%) as compared to T. durum (9 and 13%) under EO in the two years, respectively. Filtration of air significantly increased all growth and yield parameters in both species of wheat.


INTRODUCTION
O 3 is one of the most damaging tropospheric air pollutants affecting plant growth and productivity (Leisner and Ainsworth, 2012).Concentrations of tropospheric O 3 have increased considerably since the Industrial Revolution, especially during the past 60 years, and are likely to continue to increase by 10-30 parts per billion (ppb) by 2100 if current high emission rates continue in the Northern Hemisphere (The Royal Society, 2008).The IPCC Fourth Assessment Report projects an increase in tropospheric O 3 across the globe of 20-25% by 2050 (Jaggard et al., 2010).Due to its strong oxidative property, ozone damages crops by reducing photosynthesis and other important physiological functions, resulting in weaker plants, inferior crop quality, and decreased yields (Fuhrer, 2009;Singh et al., 2013).Ozone enters into the underside of the leaf of plants through their stomata and most likely reacts with molecules in the cell wall that end up triggering production of ROS molecules, which damage the cells, inhibiting photosynthesis, accelerating leaf senescence, reducing plant growth and impairing yield attributes (Bhatia et al., 2012).
Impacts of tropospheric O 3 on crop plants are reported from many developing countries, which are rapidly industrializing like China (Wang et al., 2007(Wang et al., , 2008)), India (Bhatia et al., 2011) and Pakistan (Wahid, 2006;Rai et al., 2007;Wahid et al., 2012).O 3 impairs plant metabolism leading to yield reduction in agricultural crops and its influence is dependent on dose, genetic background and the developmental phase of plants and varies between species and cultivars (Fiscus, 2005;Rai et al., 2010).Cereals are highly sensitive and have shown decreased yields with increasing O 3 levels (Singh et al., 2010).Wheat, one of the crucially important crops in the global food supply, has been found to be particularly sensitive to O 3 , yields decreasing with increasing O 3 (Mills et al., 2007;Feng et al., 2008Feng et al., , 2010)).Wheat (Triticum sps.) is the second most important food crop in India after rice both in terms of area and production, contributing substantially to the national food security (Mughal et al., 2011).Globally, India is the second largest wheat producing country, averaging an annual production of 85.93 million tonnes (MOA, 2013) and accounts for nearly 12 per cent of world wheat production.There have been some reports that ground level ozone is having tremendous impact on yield of cereals especially wheat due to tropospheric ozone (Ambasht and Agrawal, 2003;Wilkinson et al., 2012).Wheat has been identified as especially sensitive to O 3 due to likely co-occurrence of peak levels of O 3 during growing season of wheat (Avnery et al., 2011).Wheat is grown during the months of November to April in Northern and central India.95% of the cultivated wheat is of the aestivum (hexaploid) type used for the preparation of bread and other baked products and the remaining 5% is durum (tetraploid) wheat, which is used essentially for making pasta and macaroni (Patnaik and Khurana, 2003).With urbanization the demand for durum wheat is on the rise due to higher fast food consumption in India.Picchi et al. (2010) studied the potential risk of O 3 damage for modern wheat cultivars; however, there are not many studies on the impact of elevated ozone levels on growth and productivity of T. durum species of wheat especially in the tropical region.Thus the present study was carried out to assess the impact of elevated ozone on growth, yield and nutritional quality of two wheat species grown in Northern India.

Experimental Site and Design
A field experiment was conducted growing wheat cultivars (T.aestivum and T. durum variety PBW 343 and HD 2936 during rabi season (winter) from December to April in 2008-2009 and 2009-2010 at the experimental farm of the Indian Agricultural Research Institute, New Delhi, India.The site is located in the Indo-Gangetic alluvial tract at 28°40′N and 77°12′E, at an altitude of 228 m above mean sea level.The climate of the region is subtropical, semi-arid and January is the coldest month of the year with a minimum temperature ranging from 5 to 7°C.The mean maximum and minimum temperatures from December to April are 36.6 and 22.6°C, respectively.Average rainfall of this area is 72 cm annually, approximately 80% of which occurs during the kharif (monsoon) season.The alluvial soil of experimental site was sandy loam texture (46% sand, 33% silt and 21% clay) (Typic Ustochrept) with bulk density of 1.42 g/cm 3 , pH (1:2 soil:water) of 8.61, electrical conductivity of 0.158 dS/m, cation exchange capacity of 7.3 C mol (p+)/kg; and organic carbon, total N, Olsen P, and ammonium acetate extractable K contents of 0.32%, 156.8 kg/ha, 13.83 kg/ha, and 171.56 kg/ha, respectively.The experimental soil was under rice-wheat rotation for the past two years.

Treatments and Crop Management
The experiment was carried out growing wheat in open top chambers (OTCs) of 3 meter diameter and 2.5 meter height consisting of a circular aluminum frame covered with transparent film.The experiment was carried out with four treatments arranged in randomized block design with three replications.The treatments were: charcoal filtered air (CF), elevated ozone (EO), non-filtered control (NF) and open air (OA) plots (Table 1).The OTCs were fitted with an inert PVC pipe of 10 cm diameter (adjustable height) with many small holes which released either charcoal filtered air (CF), non-filtered air (NF) or elevated ozone along with non-filtered air (EO) at the crop canopy level.Air was blown into the OTCs through a fan that provided uniform air speeds.The ventilation rates were kept at 3 air changes per minute to keep the leaf boundary layer resistances down and the chamber temperature close to ambient.Charcoal filters were used to adsorb ozone from ambient air reduced by around 80-85% blown inside the CF treatment.In NF treatment only air was blown in the open top chambers.The NF treatment was the open top chamber control.In the EO treatment 25-35 ppb of additional ozone was maintained over the non-filtered air levels.O 3 was applied for 7 h/d from 09.30-16.30h in the elevated O 3 chambers.Additional O 3 was generated from oxygen with the help of reaction with UV radiation < 200 nm using ozone generators (Systocom, Varanasi, India).Air was sampled from the middle of each OTC at the crop canopy level and fed to an O 3 analyzer (Model APOA-370, Horiba, Germany) for measuring the ozone concentrations daily using a cross flow modulated ultraviolet absorption method.The light intensity inside and outside the OTCs was measured using a portable light meter (Metravi 1332), temperature with a constantan-copper thermocouple and relative humidity was measured using a digital humidity sensor at 10.00 and 16.00 h daily.Cumulative ozone exposure above 40 ppb during daylight hours at different growth stages was characterized by the AOT40 index (Fuhrer et al., 1997) and is listed in Table 1.

Experimental Practices
Two most popular wheat cultivars of northern India T. aestivum (variety PBW 343) and T. durum (variety, HD 2936) were grown in open top chambers for the study.The seeds were sown at a recommended spacing of 20 × 15 (row × row).Nitrogen (120 kg/N/ha) was applied in 3 splits, 50% as basal and 25% each at top dressing while all P (60 kg/P/ha) and K (40 kg/K/ha) and zinc sulphate (20 kg/ha) was applied basally.A week after sowing of crops, gap filling was done to maintain uniform plant population.Irrigation was given at crown root initiation (CRI), tillering, flowering, milking and dough stages respectively.Weed management was carried out as per standard procedures.At every harvesting, three plants were taken for growth and biochemical measurements.At the final harvest, the yield of one meter square area was recorded.
Three destructive plant harvestings were performed during the crop growth period at 34, 69 and 105 DAS (Days after Sowing) in I st year and 35, 71 and 108 DAS in II nd year respectively at tillering, flowering and maturing stages for measurement of plant height, root dry weight, shoot dry weight and number of leaves and tillers per plant.

Growth and Physiology
Single-leaf net photosynthetic rates (P N ), and stomatal conductance (g s ), were measured with portable photosynthesis systems (LI-6400-40 Portable Photosynthesis System).Leaf area index (LAI) was calculated using leaf area meter (Model LI-3100, LI-COR, Inc., USA).Total chlorophyll content of the flag leaf was determined as per method described by Barnes et al. (1992).Total chlorophyll, photosynthetic rates and stomatal conductance were measured at tillering and flowering and Leaf Area Index (LAI) was measured at tillering, flowering and maturing stages in both the years.The rubisco enzyme activity was also measured at tillering stage (43 DAS) in both the years as per method described by Kobza and Seemann (1988).

Final Harvest
Final harvesting of the crop was carried out at123 DAS and 135 DAS in the I st year and II nd year respectively, when the crop had a golden yellow colour, by measuring various yield parameters of the straw and ears, viz., average number of tillers, average number of spikes, average number of grains/spike, grain yield, total biomass and 1000 grain weight.The grains were separated from the straw and straw weights for wheat in both cultivars viz.T. aestivum and T. durum and dry weight was got by oven drying at 65°C reaching a constant weight.

Grain Quality Parameters
Total Soluble sugar, starch and protein content were analyzed in the wheat grain.The total soluble sugar of grain was analyzed as per the procedure described by Mc. Cready et al. (1950).The starch content of grain was analyzed by using Bradford Dye Binding method as per the procedure described by Bradford, (1976).Protein content was calculated from percentages of total nitrogen by multiplication by conversion factor of 5.7 for wheat grains by macro Kjeldahl method (Jackson, 1973).

Statistical Analysis
Data of growth, yield and quality parameters were subjected to two ways ANOVA to examine the individual and combined effects of species, treatments and their interaction.Duncan's multiple range tests were performed for various measurements after subjecting to one way ANOVA test.All the statistical tests were performed using Statistical package for agricultural research (SPAR, version 2.0).

RESULTS AND DISCUSSION
Variations were observed in O 3 concentration during the crop growth period.Mean hourly ozone concentrations in the CF chambers did not exceed 10 ppb as the charcoal filters removed 85-90% of ambient ozone from the OTCs.The average daytime O 3 concentration in non-filtered air during the growing season was 30.9 ± 1.4 and 34.9 ± 1.9 ppb in I st year and II nd year respectively.The mean 7-h ozone concentration in the EO treatment was 59.2 ± 2.5 and 65.35 ± 3.7 ppb in the two years, respectively (Table 1).Levels of sun light on the crop canopy were, on an average, reduced by 12% inside the OTC, whereas mean air temperature inside the chamber was 1.06 ± 0.2°C higher than that on outside the chambers (Table 2).Relative humidity was higher inside the chambers and average mean difference of relative humidity was 4.6 and 2.9% in I st year and II nd year, respectively (Table 2).Higher concentrations of ozone were observed during the second year as compared to the I st year.During the experiment variations were observed in O 3 concentration during the crop growth period.Higher daily ambient O 3 concentration (7-hr mean) was observed during the flowering and post-flowering period i.e., February to March (40-67 ppb) as compared to the vegetative phase i.e., December and January (18-35 ppb).The average O 3 concentrations during grain filling period were higher during the second year (49 ppb) as compared to first year (42 ppb) of the study.Highest daily O 3 concentration during the crop growth period was observed during the month of April in both the years of the study (65-73 ppb) (Fig. 1).The AOT 40 under EO was higher in the second year of the study (Table 1).The AOT 40 under EO at the different growth stages of sowing to tillering, tillering to flowering and flowering to maturity varied from 4.35, 4.52 and 7.76 ppmv h in 2009 to 4.28, 5.04 and 8.65 ppmv h in 2010 respectively.

Effect on Growth Stomatal Conductance (g s ) and Photosynthetic Rate (P N )
Previous research has shown that higher O 3 sensitivity of modern day cultivars of wheat is due to greater stomatal conductance (g s ) leading to higher O 3 flux (Pleijel et al., 2006).In our study, at tillering the difference observed in g s among the species were not significant, while at flowering stage a clear difference was observed in g s between the species (Fig. 2(a)).At flowering, reduction in g s was more in T. durum than T. aestivum.An average decrease of 12% was observed in T. durum whereas it was 19% in T. aestivum under EO.This decrease in g s was probably due to stomatal closure which is caused by an increase in internal CO 2 concentration resulting from O 3 -induced inhibition of carbon assimilation by chloroplasts (Heath and Taylor, 1997).
Partial closure of stomata in response to high O 3 concentration was responsible for decreased photosynthetic rate (P N ) of the plants.A significant lowering in P N was obtained in the two wheat species under EO at the tillering stage (results not shown).As the plant growth progressed  (Noormets et al., 2010;Rai et al., 2010) and ultimately reduction in photosynthetic rate.In our study the P N of T. durum was significantly higher than T. aestivum in CF at all growth stages.

Plant Height and above Ground Biomass
Wheat is an O 3 sensitive crop (Mishra et al., 2013) and in our study we observed a differential response in the two species under different levels of ozone.At tillering stage there were significant differences observed in plant height between the two species of wheat, however no significant differences were observed among the different treatments at this stage.At flowering stage under EO, more decrease was observed in T. aestivum (16-18%) as compared to T. durum (11-13%) in the two years of the study (Table 3(a)).The plant growth was visibly very healthy in CF treatment in both the wheat species.There significant differences in plant height at flowering stage under the two species of wheat (Table 3(b)).Rai et al. (2007) observed reduced shoot length in wheat under non filtered air as compared to filtered air which had higher ozone levels.Feng et al., (2008) while carrying out a meta-analysis of wheat responses to elevated ozone observed similar reduction (5%) in stem height of wheat under elevated ozone levels.No significant reductions in plant dry weight were observed between the two species however significant differences were observed under the O 3 treatments.In T. aestivum, average plant dry weight reduced by 19% while it reduced by 13% in T. durum under EO (Table 3(a)).At flowering, the plant dry weight was significantly reduced in the EO treatment possibly due in part to reductions in plant height which were significant.The presence of elevated levels of O 3 impacted the plant growth by reducing the g s which inhibited the photosynthesis, leading to a reduction in dry matter accumulation.As photosynthesis and C-fixation decreased more in T. aestivum, this led to lower biomass production in T. aestivum as compared to T. durum.Accelerating leaf senescence, reducing plant growth and impaired yield attributes were also obtained by Sarkar et al. (2010) and Wahid (2006) under higher ozone levels in wheat.However, no significant differences in plant dry weight under CF at flowering was observed among the species.

Leaf Area Index
Leaf area index (LAI) is expressed as total leaf area of the crop per unit ground area occupied by the crop.Significant differences were observed in LAI under the two species in the different O 3 treatments at flowering (Table 3(b)).The decrease in LAI was more in T. aestivum species than in T. durum.During I st year, the LAI decreased from 3.50 in T. aestivum and 3.40 in T. durum in CF to 2.60 in T. aestivum and 3.0 in T. durum under EO at flowering stage.In II nd year it decreased by 10% in T. aestivum and 6.3% in T. durum under EO as compared to NF at flowering.The decrease in LAI was due to a decrease in number of leaves per plant under EO (Table 3(a)).The reduction in numbers of leaves was more in T. aestivum as compared to T. durum at flowering and maturity in EO.In our study there was significant reduction in number of leaves per plant between the two species and the treatments, however, no significant differences was observed in the species × treatment interaction (Table 3(b)).Under EO while senescence is accelerated, formation of new leaves may be slowed by decreased supply of assimilates and translocation (Ewert and Pleijel, 1999;Grantz and Farrar, 1999;Morgan et al., 2003).Leaf senescence in our study was characterised by a general yellowing, which was more pronounced at the leaf tip in the beginning and then it spread downwards along the leaf.In EO treatment, the leaves colour changed from green to yellow.Symptoms of ozone-induced premature senescence of basal leaves became evident during the latter part of February in the EO treatment.Senescence rate was higher in T. aestivum as compared to T. durum.Plants under EO treatment were accompanied by black or brown darkening of the affected area after 65 DAS and it was more visible in T. aestivum than T. durum (Fig. 3).Visibly the numbers of dark spots were more in T. aestivum as compared to T. durum species of wheat.Feng et al. (2010) also observed accelerated senescence of flag leaves in wheat under higher O 3 concentrations.Kharel and Amgain, (2010) observed that elevated O 3 reduced the number of leaves per plant, leaf area and LAI in crop plants.Dermody et al. (2006) reported that lowering in LAI was primarily due to accelerated senescence under elevated ozone in soybean.

Total Chlorophyll and Rubisco Enzyme Activity
Total chlorophyll content of leave was also more affected in T. aestivum as compared to T. durum in both the years.At tillering stage decrease in chlorophyll was not significant between the two species.At flowering total chlorophyll significantly decreased under EO in both the species (Fig. 2(c)).In T. aestivum, the decrease was 6% more than in T. durum.Under CF also the two species showed different chlorophyll levels.A 17% increase in chlorophyll was observed in T. aestivum and 10% in T. durum in CF over NF in both the years.
Ozone induced ROS can damage chloroplasts by changing membrane permeability, thereby causing chlorophyll destruction.Thus, in our study the reduction in chlorophyll was due to higher ozone induced ROS in EO.More reduction in chlorophyll in T. aestivum as compared to T. durum under exposure to O 3 was probably due to higher O 3 uptake (less decrease in g s in aestivum as compared to durum) and a greater loss of photosynthetic activity (more reduction in photosynthetic rate in aestivum as compared to durum).Zhu et al. (2011) growing winter wheat under elevated O 3 in FACE-ozone system observed corpus adiposum inside chloroplast broken down, resulting in a significant decrease of the chlorophyll content.Pleijel et al. (2006) reported significant reduction of chlorophyll in the flag leaf in different cultivars of wheat after exposure to O 3 above 50 ppb for 1 month, which shortened the grain filling period and resulted in lower yields under elevated O 3 .It has been reported that the decline in photosynthetic capacity induced  by O 3 is caused primarily by a decrease in the maximum in vivo rate of Rubisco carboxylation due to a reduction in the activity and/or quantity of Rubisco (Long and Naidu, 2002;Biswas and Jiang, 2011).The decrease in ribulose bisphosphate carboxylase/oxygenase (Rubisco) activity was significantly different between the two species of wheat (Fig. 4) and might have been a reason for differential response in our study between the two species of wheat.It reduced by 19% in T. aestivum and by 13% in T. durum under EO as compared to NF. Rubisco is the key protein involved in net CO 2 fixation during photosynthesis (Miziorka and Lorimer, 1983).

Effect on Yield Parameters
The results of final harvest of crop showed reduction in yield parameters for both the varieties under EO, though changes were more significant in case of T. aestivum species.In contrast, the filtration of air (CF) significantly increased the major yield components, in both the species.There was significant decline in average no. of spikelet/spike, average no. of grains/spike and 1000 grain weight (Table 4) under different treatments (Table 4(a)) however, only significant difference in average no. of grains/spike was observed among the species and in species × treatment interaction (Table 4(b)).As EO affected more the T. aestivum species (growth and physiological parameters described earlier) so that plants of T. aestivum were more affected under EO as compared to the T. durum species.In T. aestivum, the average number of spikelet/spike reduced by 8% and in T. durum, it decreased by 5% under EO (Table 4(a)).The effect on average grains/spike was largely due to an increase in percentage sterility in EO treatment.Percentage sterility increased by 8% T. aestivum and by 5% in T. durum under EO.Accelerated senescence observed under EO (result discussed earlier) shortened the grain filling duration and there by reduced average no. of grains/spike and ultimately to lower grain yield under elevated O 3 (Gelang et al., 2000).
From our results it was observed that T. aestivum was more affected by EO as compared to T. durum and this ultimately resulted in a greater decline in grain yield of wheat in T. aestivum.Significant differences were obtained between T. aestivum and T. durum in grain yield under EO in both the years (Table 4(b)).The grain yield decreased by 17% in T. aestivum and by 11% in T. durum.Grain yield ranged from 345 g/m 2 to 538 g/m 2 in the different treatments (Table 4(a)).Under low O 3 levels of CF treatment the grain yield increased by 19% T. aestivum and by 16% in T. durum over NF.Significant impact of EO was also observed on 1000 grain weight, however no significant differences were obtained among the two wheat species (Table 4(b)).Feng et al. (2009) in a meta analysis of impacts of elevated O 3 on wheat obtained results indicating decreased grain yield due to decreased 1000 grain weight, grains/spike, spikes/plant, relative to ambient air.This was due to higher translocation of carbohydrates to sink, leading to higher grain filling and grain weight (Wahid et al., 2006) in filtered air.Picchi et al. (2010) studied the potential risk of O 3 damage for modern wheat cultivars and found durum wheat to be more sensitive showing more severe symptoms than common aestivum wheat.However, in our study we observed that T. aestivum was more sensitive than T. durum with respect many yield parameters under EO.
Straw yield was significantly affected among the two wheat species which was largely due to reduced level of tillering under EO in T. aestivum.It decreased by 10.5% in T. aestivum and, and by 4.5% in T. durum under EO (Table 3(a)).Significant reduction was also observed in the average number of productive tillers among the two species (Table 4(a)).Harvest index ranged from 0.40 to 0.42 in I st year and 0.38 to 0.43 in II nd year among the different treatments in both the years, however no significant species × treatment interaction was observed in HI (Table 4(b)).Decline in HI under EO showed that there was less biomass portioning towards grain as compared to non-filtered air.All the yield related parameters showed more decline in the II nd year of the study which might be due to higher ambient O 3 levels between flowering to maturity stage (Fig. 1).

Effect of Ozone Treatments on Grain Quality
In addition to reductions in grain yield, there was also an impact of O 3 levels on nutritional quality of the two wheat species (Fig. 5).The sugar content decreased by 10 and 15% in T. aestivum and by 8 and 9% in T. durum in the two years of the study and the differences were significant (Fig. 5(a)).The decline in starch content however, was significant among the species only in the first year of the study.Ozone reduces photosynthesis, leading to lower translocation of carbon to the grain resulting in the reduced sugar and starch in the grain (Bhatia et al., 2012).Mishra et al. (2013) observed similar decrease in soluble sugar and protein content in grain in tall and dwarf wheat cultivars under EO.In our study there was a decline in grain protein in both the species however no significant difference was observed.The decrease in grain protein content was due to growth dilution effect which occurs due to dilution of accumulation of more nonnitrogenous compounds in the grain.Evans (1993) and Mishra et al. (2013) observed a negative correlation between grain yield (GY) and grain protein concentration (GPC) due to growth dilution effect.The decrease observed in carbohydrate (sugar and starch) were more in T. aestivum than T. durum showing that the more primitive wheat (T.durum) showed less decline than the modern species (T.aestivum) under EO.No significant differences were observed in nutritional quality among the two species in the CF (Fig. 5(b)).

CONCLUSIONS
Comparison of responses of wheat species (T.aestivum and T. durum) with plants grown in different environments with filtered, non-filtered air, and elevated O 3 showed that the current ambient levels of O 3 may be significantly affecting the growth, yield and nutritional quality of wheat and it may vary from species to species.Elevated ozone caused more decline in physiological and growth parameters, of T. aestivum as compared to T. durum which ultimately resulted in significant reduction in yield and nutritional quality, however more studies are required to be carried out evaluating different wheat cultivars of the two wheat species.Effect of tropospheric O 3 on both growth and yield of an important cereal crop like wheat needs attention to maintain the productivity of agricultural systems in future climate change scenarios.

Fig. 2 .
Fig. 2. Impact of elevated ozone on (a) stomatal conductance, (b) photosynthetic rate and (c) total chlorophyll in wheat varieties at flowering.

*
Impact of ozone treatments on growth parameters at flowering stage in wheat.Different letters within a row indicate significant differences among treatments at p < 0.05 by Duncan's test.

Fig. 4 .
Fig. 4. Impact of different treatments on rubisco enzyme activity at tillering.

*
Different letters within a row indicate significant differences among treatments at p < 0.05 by Duncan's test.

Fig. 5 .
Fig. 5. Impact of different treatments on (a) decrease in grain quality parameters under elevated ozone (b) increase in grain quality parameters in charcoal filtered air.

Table 2 .
Mean monthly microclimatic conditions inside and outside the open top chambers (OTCs) during 2008-2009 and 2009-2010 wheat season.

Daily mean ambient O 3 concentration (ppb) Date (Day/month) 2009 2010 Fig. 1.
Cao et al. (2009)t ozone concentrations during crop growth period in 2009 and 2010.morereduction in P N was observed at flowering stage.The P N was less in T. aestivum (15.70 and 17.02 µmol/m 2 /s) as compared to T. durum (26.29 and 20.95 µmol/m 2 /s) under EO in both the years (Fig.2(b)).On an average there was 7% more reduction in P N in T. aestivum as compared to T. durum under EO.Cao et al. (2009)also observed a significant decrease of photosynthetic rate in wheat cultivars grown under O 3 treatment.After entering through stomata, O 3 is converted to reactive oxygen species (ROS) causing membrane damage, resulting in injuries and destroying photosynthetic pigments

Table 4 (a). Impact
of ozone treatments on yield parameters at final harvest.