Ozone impact on wheat in Europe, Asia and North America - A comparison

Chronic ozone exposure affects nitrogen remobilization in wheat at key growth stages

The interaction between nitrogen storage and translocation, senescence, and late phase photosynthesis is critical to the post-anthesis grain fill period in wheat, but ozone's effect on nitrogen dynamics within the wheat plant is not well understood. This study used solardomes to expose a widely grown elite spring wheat cultivar, cv. Skyfall, to four levels of ozone (30 ppb, 45 ppb, 70 ppb, 85 ppb) for 11 weeks, with two levels of nitrogen fertilization, 140 kg ha-1 and 160 kg ha-1, the higher rate including an additional 20 kg N ha-1 at anthesis. Chronic ozone exposure triggered earlier senescence in the 4th, 3rd and 2nd leaves but not the flag leaf, with a similar pattern of reduced chlorophyll content in the lower, older leaf cohorts, which started before senescence became visible. At anthesis there was no evidence of any effect of ozone on nitrogen storage in upper plant parts. However, high ozone increased levels of residual nitrogen found within plant parts at harvest, with concomitant reductions in C:N ratios and Nitrogen Remobilization Efficiency. Extra nitrogen fertilization applied at anthesis appeared to ameliorate the effect of ozone on nitrogen content and nitrogen translocation. The application of 15N ammonium nitrate at anthesis confirmed that the majority of post-anthesis nitrogen uptake had been translocated to the ear/grain by harvest, with no effect of ozone on the translocation of nitrogen around the plant. These data can inform future modelling of ozone's effect on nitrogen dynamics and global wheat yields.

Wheat yield response to elevated O3 concentrations differs between the world's major producing regions

Ground-level ozone (O3) concentration is rising in Asia, which accommodates the world's top-two wheat producers (China and India). Because wheat is among the species of high O3 sensitivity, yield loss due to rising O3 in Asia is a major threat to global wheat supply. We estimated the relationships between O3 dose on AOT40 (accumulated daytime O3 concentrations above 40 ppb for 90 days) and relative wheat yield for four wheat producing regions: China, India, Europe and North America using results of O3 elevation experiments conducted therein. When compared on the same AOT40, the estimated yield loss was greatest in China followed by India, Europe, and North America in this order. In China, Europe and North America, the yield loss was primarily due to the reduction of single grain weight, whereas in India reduction of the number of grains contributed more to the yield loss than single grain weight. The greater response of the number of grains to O3 in India can be explained by the earlier start of O3 elevation, but the seasonal change in O3 concentrations cannot explain the lower yield loss in North America than China and India. Referring to the past reports of lower yield sensitivity to O3 in older cultivars, we compared the year of release of cultivars between the regions. In North America, they used cultivars released in 1980s or earlier, whereas in China they used cultivars released in 2000s. In Europe and India, most cultivars were released between those in North America and China. The difference in cultivars could therefore be a cause the differential yield response among the regions. We argue that the O3-induced yield loss should be quantified using the dose-response relationships for each region accounting for the effects of seasonal change in O3 concentrations, cultivars and climate on the yield response.

Similar photosynthetic but different yield responses of C3 and C4 crops to elevated O3

The deleterious effects of ozone (O3) pollution on crop physiology, yield, and productivity are widely acknowledged. It has also been assumed that C4 crops with a carbon concentrating mechanism and greater water use efficiency are less sensitive to O3 pollution than C3 crops. This assumption has not been widely tested. Therefore, we compiled 46 journal articles and unpublished datasets that reported leaf photosynthetic and biochemical traits, plant biomass, and yield in five C3 crops (chickpea, rice, snap bean, soybean, and wheat) and four C4 crops (sorghum, maize, Miscanthus × giganteus, and switchgrass) grown under ambient and elevated O3 concentration ([O3]) in the field at free-air O3 concentration enrichment (O3-FACE) facilities over the past 20 y. When normalized by O3 exposure, C3 and C4 crops showed a similar response of leaf photosynthesis, but the reduction in chlorophyll content, fluorescence, and yield was greater in C3 crops compared with C4 crops. Additionally, inbred and hybrid lines of rice and maize showed different sensitivities to O3 exposure. This study quantitatively demonstrates that C4 crops respond less to elevated [O3] than C3 crops. This understanding could help maintain cropland productivity in an increasingly polluted atmosphere.

The interaction of O3 and CO2 concentration, exposure timing and duration on stem rust severity on winter wheat variety 'Coker 9553'

Wheat rusts, elevated ozone (O3), and carbon dioxide (CO2) are simultaneously impacting wheat production worldwide, but their interactions are not well understood. This study investigated whether near-ambient O3 is suppressive or conducive to stem rust (Sr) of wheat, considering the interactions with ambient and elevated CO2. Winter wheat variety 'Coker 9553' (Sr-susceptible; O3 sensitive) was inoculated with Sr (race QFCSC) following pre-treatment with four different concentrations of O3 (CF, 50, 70, and 90 ppbv) at ambient CO2 levels. Gas treatments were continued during the development of disease symptoms. Disease severity, measured as percent sporulation area (PSA), significantly increased relative to the CF control only under near-ambient O3 conditions (50 ppbv) in the absence of O3-induced foliar injury. Disease symptoms at higher O3 exposures (70 and 90 ppbv) were similar to or less than the CF control. When Coker 9553 was inoculated with Sr while exposed to CO2 (400; 570 ppmv) and O3 (CF; 50 ppbv) in four different combinations, and seven combinations of exposure timing and duration, PSA significantly increased only under continuous treatment with O3 for six weeks or pre-inoculation treatment for three weeks, suggesting that O3-predisposes wheat to the disease rather than enhancing disease post-inoculation. O3 singly and in combination with CO2 increased PSA on flag leaves of adult Coker 9553 plants while elevated CO2 alone had little effect on PSA. These findings show that sub-symptomatic O3 conditions are conducive to stem rust, contradicting the current consensus that biotrophic pathogens are suppressed by elevated O3. This suggests that sub-symptomatic O3 stress may enhance rust diseases in wheat-growing regions.

The Effects of Elevated Tropospheric Ozone on Carbon Fixation and Stable Isotopic Signatures of Durum Wheat Cultivars with Different Biomass and Yield Stability

Tropospheric ozone (O₃) enrichment caused by human activities can reduce important crop yields with huge economic loss and affect the global carbon cycle and climate change in the coming decades. In this study, two Italian cultivars of durum wheat (Claudio and Mongibello) were exposed to O₃ (80 ppb, 5 h day^-1 for 70 consecutive days), with the aim to investigate the changes in yield and biomass, ecophysiological traits, and stable carbon and nitrogen isotope values in plants, and to compare the stable isotope responses under environmental stressors. Both cultivars showed a relative O₃ tolerance in terms of photosynthetic performance, but in cultivar Mongibello, O₃ was detrimental to the grain yield and plant biomass. The δ¹³C values in the leaves of plants identified that the impact of O₃ on CO₂ fixation by RuBisCO was dominant. The δ¹⁵N value showed significant differences between treatments in both cultivars at seven days from the beginning of the exposure, which could be considered an early indicator of ozone pollution. Under increasingly frequent extreme climates globally, the relationships among stable isotope data, ecophysiological traits, and agronomic parameters could help breed future cultivars.

Bioclimatic modeling and FACE study forecast a bleak future for wheat production in India

Since the impact of future climate change on wheat productivity is inconsistent, we studied geographic distribution and yield of wheat using two global General Circulation Models (GCMs) and Free Air CO_2/O₃ Enrichment (FACE) experiments. The GCMs (IPSL-CM5A-LR and NIMR-HADGEM2-AO) with four Representative Concentration Pathways (RCPs) and 19 bioclimatic variables were used for distribution/ecological niche modeling (ENM). Currently cultivated eight wheat cultivars were exposed to individual treatment of (i) ambient CO₂, temperature, and ozone (ACO + AO + AT) representing the present climate scenario, and (ii) elevated CO₂ (550 ppm) (ECO), (iii) elevated temperature (+ 2 °C) (ET), (iv) elevated O₃ (ambient + 20 ppb) (EO), (v) elevated CO₂ + elevated O₃ (ECO + EO), and (vi) elevated CO₂ + elevated temperature + elevated O₃ (ECO + EO + ET) under FACE facility simulating the future climate change scenarios in 2050. The niche models predicted a reduction in climatically suitable areas for wheat, and identified "maximum temperature" as the most influencing factor for area reduction. The elevated CO₂, O₃, and temperature individually and in combinations had differential impacts on the yield of wheat cultivars. Only two cultivars, viz., DBW 184 and DBW 187 did not exhibit yield decline suggesting their suitability in the future climate change scenario. Since the performance of six out of eight cultivars significantly declined under simulated FACE experiment, and ENM predicted reduction in wheat cultivation area under RCP 8.5 in 2050, it was concluded that future of wheat cultivation in India is bleak. The study further indicates that coupling of bioclimatic modeling and FACE experiment can effectively predict the impact of climate change on different crops.

Ozone modelling and mapping for risk assessment: An overview of different approaches for human and ecosystems health

Tropospheric ozone (O₃) is one of the most concernedair pollutants dueto its widespread impacts on land vegetated ecosystems and human health. Ozone is also the third greenhouse gas for radiative forcing. Consequently, it should be carefully and continuously monitored to estimate its potential adverse impacts especially inthose regions where concentrations are high. Continuous large-scale O₃ concentrations measurement is crucial but may be unfeasible because of economic and practical limitations; therefore, quantifying the real impact of O₃over large areas is currently an open challenge. Thus, one of the final objectives of O₃ modelling is to reproduce maps of continuous concentrations (both spatially and temporally) and risk assessment for human and ecosystem health. We here reviewedthe most relevant approaches used for O₃ modelling and mapping starting from the simplest geo-statistical approaches andincreasing in complexity up to simulations embedded into the global/regional circulation models and pro and cons of each mode are highlighted. The analysis showed that a simpler approach (mostly statistical models) is suitable for mappingO₃concentrationsat the local scale, where enough O₃concentration data are available. The associated error in mapping can be reduced by using more complex methodologies, based on co-variables. The models available at the regional or global level are used depending on the needed resolution and the domain where they are applied to. Increasing the resolution corresponds to an increase in the prediction but only up to a certain limit. However, with any approach, the ensemble models should be preferred.

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.

Reductions in crop yields across China from elevated ozone

Exposure of crops to high concentrations of ozone can cause substantial reductions in yield that pose a serious threat to global food security. Here we provide comprehensive estimates of yield losses for key crops across China between 2014 and 2017 attributed to ozone using a number of new approaches. We use an air quality model at 5 km resolution and crop-specific dose-response functions developed for both concentration- and flux-based metrics. We bias correct modelled ozone concentrations and metrics using observations from more than 1000 locations. We find that on a 4-year average basis, production losses of key crops are 34-91 million metric tonnes (Mt/yr), dependent on the approach used, with highest losses in Henan province. At a national level, loss of winter wheat production derived using a China-specific dose-response function increased by 82% from 2014 to 2017, with large interannual variations in the North China Plain and in eastern China. Winter wheat losses estimated using flux-based functions, which require robust simulation of stomatal conductance and underlying vegetation physiology, are significantly lower, at 30 Mt/yr. We show that the definition of the growing season may have a greater impact on estimated losses than small biases in ozone surface concentrations. Although uncertainties remain, our findings demonstrate that increasing ozone concentrations have substantial adverse impacts on crop yields and threaten food security in China. It is important to control ozone concentrations to mitigate these negative impacts.

Estimating the impact of ground ozone concentrations on crop yields across China from 2014 to 2018: A multi-model comparison

Ground level ozone exerts a strong impact on crop yields, yet how to properly quantify ozone induced yield losses in China remains challenging. To this end, we employed a series of O₃-crop models to estimate ozone induced yield losses in China from 2014 to 2018. The outputs from all models suggested that the total Relative Yield Losses (RYL) of wheat in China from 2014 to 2018 was 18.4%-49.3% and the total RYL of rice was 6.2%-52.9%. Consequently, the total Crop Production Losses (CPL) of wheat and rice could reach 63.9-130.4 and 28.3-35.4 million tons, and the corresponding Total Economic Losses (TEL) could reach 20.5-44.7 and 11.0-15.3 billion dollars, stressing the great importance and urgency of national ozone management. Meanwhile, the estimation outputs highlighted the large variations between different regional O₃-crop models when applying to large scales. Instead of applying one unified O₃-crop models to all regions across China, we also explored the strategy of employing specific O₃-crop models in corresponding (and neighboring) regions to estimate ozone induced yield loss in China. The comparison of two strategies suggested that the mean value from multiple models may still present an inconsistent over/underestimation trend for different crops. Therefore, it is a preferable strategy to employ corresponding O₃-crop models in different regions for estimating the national crop losses caused by ozone pollution. However, the severe lack of regional O₃-crop models in most regions across China makes a robust estimation of national yield losses highly challenging. Given the large variations between O₃-crop interactions across regions, a systematic framework with massive regional O₃-crop models should be properly designed and implemented.

Genetic dissection of bread wheat diversity and identification of adaptive loci in response to elevated tropospheric ozone

Rising tropospheric ozone affects the performance of important cereal crops thus threatening global food security. In this study, genetic variation of wheat regarding its physiological and yield responses to ozone was explored by exposing a diversity panel of 150 wheat genotypes to elevated ozone and control conditions throughout the growing season. Differential responses to ozone were observed for foliar symptom formation quantified as leaf bronzing score (LBS), vegetation indices and yield components. Vegetation indices representing the carotenoid to chlorophyll pigment ratio (such as Lic2) were particularly ozone-responsive and were thus considered suitable for the non-invasive diagnosing of ozone stress. Genetic variation in ozone-responsive traits was dissected by a genome-wide association study (GWAS). Significant marker-trait associations were identified for LBS on chromosome 5A and for vegetation indices (NDVI and Lic2) on chromosomes 6B and 6D. Analysis of linkage disequilibrium (LD) in these chromosomal regions revealed distinct LD blocks containing genes with a putative function in plant redox biology such as cytochrome P450 proteins and peroxidases. This study gives novel insight into the natural genetic variation in wheat ozone response, and lays the foundation for the molecular breeding of tolerant wheat varieties.

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

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

The ozone sensitivity of five poplar clones is not related to stomatal conductance, constitutive antioxidant levels and morphology of leaves

Ground-level ozone (O₃) is an important phytotoxic air pollutant in China. In order to compare the sensitivity of common poplar clones to O₃ in China and explore the possible mechanism, five poplar clones, clone DQ (Populus cathayana), clone 84 K (P. alba × P. glandulosa), clone WQ156 (P. deltoids × P. cathayana), clone 546 (P. deltoides cv. '55/56' × P. deltoides cv. 'Imperial') and clone 107 (P. euramericana cv. '74/76') were exposed to four O₃ treatments. According to the date of the initial visible O₃ symptom and the slopes of O₃ exposure-response relationships with the relative light-saturated rate of CO₂ assimilation, we found that clone DQ and clone 546 were the most sensitive to O₃, clone 84 K and clone WQ156 were the less sensitive, and clone 107 was the most tolerant, which could provide a basis to select O₃ tolerant clones for poplar planting at areas with serious O₃ pollution. Elevated O₃ significantly reduced photosynthetic parameters, total phenols content, potential antioxidant capacity, leaf mass per area and biomass of five poplar clones, and there were significant interactions between O₃ and clones for most photosynthetic parameters. Elevated O₃ also significantly increased malondialdehyde content and total ascorbate content. The responses of total antioxidant capacity for poplar clones to elevated O₃ were different, as indicated by the increase for clone 107 and reduction for other clones under elevated O₃ treatment. Our results on the sensitivity of different poplar clones to O₃ are not related to leaf stomatal conductance, leaf constitutive antioxidant levels or leaf morphology of plant grown in clean air. The possible reason is little difference in leaf traits among clones within close species, suggesting that more properties of plants should be considered for exploring the sensitivity mechanism of close species, such as mesophyll conductance, antioxidant enzyme activity and apoplastic antioxidants.