Effects of ozone on maize (Zea mays L.) photosynthetic physiology, biomass and yield components based on exposure- and flux-response relationships

Exogenous protectants alleviate ozone stress in Trifolium repens: Impacts on plant growth and endophytic fungi

Industrialization-driven surface ozone (O3) pollution significantly impairs plant growth. This study evaluates the effectiveness of exogenous protectants [3 mg L⁻1 abscisic acid (ABA), 400 mg L⁻1 ethylenediurea (EDU), and 80 mg L⁻1 spermidine (Spd)] on Trifolium repens subjected to O3 stress in open-top chambers, focusing on plant growth and dynamics of culturable endophytic fungal communities. Results indicate that O3 exposure adversely affects photosynthesis, reducing root biomass and altering root structure, which further impacts the ability of plant to absorb essential nutrients such as potassium (K), magnesium (Mg), and zinc (Zn). Conversely, the application of ABA, EDU, and Spd significantly enhanced total biomass and chlorophyll content in T. repens. Specifically, ABA and Spd significantly improved root length, root surface area, and root volume, while EDU effectively reduced leaves' malondialdehyde levels, indicating decreased oxidative stress. Moreover, ABA and Spd treatments significantly increased leaf endophytic fungal diversity, while root fungal abundance declined. The relative abundance of Alternaria in leaves was substantially reduced by these treatments, which correlated with enhanced chlorophyll content and photosynthesis. Concurrently, EDU and Spd treatments increased the abundance of Plectosphaerella, enhance the absorption of K, Ca, and Mg. In roots, ABA treatment increased the abundance of Paecilomyces, while Spd treatment enhanced the presence of Stemphylium, linked to improved nitrogen (N), phosphorus (P), and K uptake. These findings suggest that specific symbiotic fungi mitigate O3-induced stress by enhancing nutrient absorption, promoting growth. This study highlights the potential of exogenous protectants to enhance plant resilience against O3 pollution through modulating interactions with endophytic fungal communities.

Opposing effects of warming on the stability of above- and belowground productivity in facing an extreme drought event

Climate warming, often accompanied by extreme drought events, could have profound effects on both plant community structure and ecosystem functioning. However, how warming interacts with extreme drought to affect community- and ecosystem-level stability remains a largely open question. Using data from a manipulative experiment with three warming treatments in an alpine meadow that experienced one extreme drought event, we investigated how warming modulates resistance and recovery of community structural and ecosystem functional stability in facing with extreme drought. We found warming decreased resistance and recovery of aboveground net primary productivity (ANPP) and structural resistance but increased resistance and recovery of belowground net primary productivity (BNPP), overall net primary productivity (NPP), and structural recovery. The findings highlight the importance of jointly considering above- and belowground processes when evaluating ecosystem stability under global warming and extreme climate events. The stability of dominant species, rather than species richness and species asynchrony, was identified as a key predictor of ecosystem functional resistance and recovery, except for BNPP recovery. In addition, structural resistance of common species contributed strongly to the resistance changes in BNPP and NPP. Importantly, community structural resistance and recovery dominated the resistance and recovery of BNPP and NPP, but not for ANPP, suggesting the different mechanisms underlie the maintenance of stability of above- versus belowground productivity. This study is among the first to explain that warming modulates ecosystem stability in the face of extreme drought and lay stress on the need to investigate ecological stability at the community level for a more mechanistic understanding of ecosystem stability in response to climate extremes.

Using soil nitrogen amendments in mitigating ozone stress in agricultural crops: a case study of cluster beans

The climate change scenario in the coming years is liable to have serious negative consequences on agricultural productivity. Increasing tropospheric ozone concentration is an important aspect of climate change, which, due to its oxidative nature, is injurious to the plants. Due to the multifarious nature and continuously increasing concentration of tropospheric ozone, it is prerequisite to develop strategies to manage ozone stress in plants. Present study not only evaluates the potential of soil nitrogen amendments in ameliorating ozone stress in plants, but also focuses upon the mechanistic approaches adopted by the different plant cultivars to combat ozone stress. Three doses of nitrogen amendments, recommended (N1), 1.5× recommended (N2) and 2× recommended (N3), were given to two cultivars (S-151 and PUSA-N) of Cymopsis tetragonoloba exposed to ambient ozone stress. Control plants were also maintained in which no nitrogen treatment was given. Nitrogen supplementation reduced the root nodulation frequency and leghaemoglobin content, which subsequently increased the cellular nitrogen metabolism as evident through increase in the activities of nitrate reductase and nitrite reductase in both the test cultivars. The positive effects of nitrogen amendments are clearly evident in the 1D protein profile studies which showed a greater accumulation of larger sub-units of RuBisCO in nitrogen amended plants. The results clearly indicate that N2 treatment effectively enhanced the yield of both the cultivars (84.8% and 76.37%, in S-151 and PUSA-N, respectively); however, the mechanistic approach adopted by the two cultivars was different. Whereas the yield quantity showed higher increments in S-151, the yield quality parameters (carbohydrates and nitrogen contents) responded more positively in PUSA-N.

Effect of ozone stress on crop productivity: A threat to food security

Tropospheric ozone (O3), the most important phytotoxic air pollutant, can deteriorate crop quality and productivity. Notably, satellite and ground-level observations-based multimodel simulations demonstrate that the present and future predicted O3 exposures could threaten food security. Hence, the present study aims at reviewing the phytotoxicity caused by O3 pollution, which threatens the food security. The present review encompasses three major aspects; wherein the past and prevailing O3 concentrations in various regions were compiled at first, followed by discussing the physiological, biochemical and yield responses of economically important crop species, and considering the potential of O3 protectants to alleviate O3-induced phytotoxicity. Finally, the empirical data reported in the literature were quantitatively analysed to show that O3 causes detrimental effect on physiological traits, photosynthetic pigments, growth and yield attributes. The review on prevailing O3 concentrations over various regions, where economically important crop are grown, and their negative impact would support policy makers to implement air pollution regulations and the scientific community to develop countermeasures against O3 phytotoxicity for maintaining food security.

Cultivar assortment index (CAI): a tool to evaluate the ozone tolerance of Indian Amaranth (Amaranthus hypochondriacus L.) cultivars

The adverse impact of climate change on crop yield has accelerated the need for identification of crop cultivars resistant to abiotic stress. In the present study, a cultivar assortment index (CAI) was generated for the evaluation of forty Amaranthus hypochondriacus cultivars response to elevated ozone (EO) concentrations (AO + 30 ppb) in Free Air Ozone Enrichment (FAOE) facility using the parameters viz. foliar injury, gaseous exchange attributes, namely, net photosynthetic rate, stomatal conductance, transpiration rate, intercellular carbon dioxide, and water use efficiency along with above ground biomass and grain yield attributes. The dataset was used to identify key indicator parameters responsive to EO through principal component analysis (PCA) and further transformed to obtain linear score and weighted score. The CAI varied from 70.49 to 193.43. Cultivars having CAI value less than 151 were ozone tolerant (OT) whereas cultivars with CAI values between 150 and 170 were moderately tolerant (MOT). The cultivars exhibiting CAI values above 170 were ozone sensitive (OS). The cultivars exhibited differential sensitivity to EO with IC-5994 (CAI = 187.26) being the most affected cultivar whereas IC-5576 (CAI = 83.38) and IC-5916 (CAI = 70.49) being the least affected ones. The CAI, based on linear score and weighted score, offers easy identification of ozone sensitive (OS) and ozone tolerant (OT) cultivars. This index could help researchers to define a clear and strong basis for identification of OT cultivars which will reduce the time required for preliminary screening and further evaluation of crop cultivars for the development of climate smart crops.

Elevated ozone decreases the multifunctionality of belowground ecosystems

Elevated tropospheric ozone (O₃ ) affects the allocation of biomass aboveground and belowground and influences terrestrial ecosystem functions. However, how belowground functions respond to elevated O₃ concentrations ([O₃ ]) remains unclear at the global scale. Here, we conducted a detailed synthesis of belowground functioning responses to elevated [O₃ ] by performing a meta-analysis of 2395 paired observations from 222 publications. We found that elevated [O₃ ] significantly reduced the primary productivity of roots by 19.8%, 16.3%, and 26.9% for crops, trees and grasses, respectively. Elevated [O₃ ] strongly decreased the root/shoot ratio by 11.3% for crops and by 4.9% for trees, which indicated that roots were highly sensitive to O₃ . Elevated [O₃ ] impacted carbon and nitrogen cycling in croplands, as evidenced by decreased dissolved organic carbon, microbial biomass carbon, total soil nitrogen, ammonium nitrogen, microbial biomass nitrogen, and nitrification rates in association with increased nitrate nitrogen and denitrification rates. Elevated [O₃ ] significantly decreased fungal phospholipid fatty acids in croplands, which suggested that O₃ altered the microbial community and composition. The responses of belowground functions to elevated [O₃ ] were modified by experimental methods, root environments, and additional global change factors. Therefore, these factors should be considered to avoid the underestimation or overestimation of the impacts of elevated [O₃ ] on belowground functioning. The significant negative relationships between O₃ -treated intensity and the multifunctionality index for croplands, forests, and grasslands implied that elevated [O₃ ] decreases belowground ecosystem multifunctionality.

Maize yield reduction and economic losses caused by ground-level ozone pollution with exposure- and flux-response relationships in the North China Plain

Ground-level ozone (O₃) has negative effects on agricultural crops. Maize is an important grain crop in China. The North China Plain (NCP) serves as the major crops' production area of China and experiences severe ozone pollution. Using the ground-level ozone simulated by an atmospheric chemistry transport model (WRF-Chem), we quantified the yield reduction and economic losses of maize during 2015-2018 over NCP based on exposure-response AOT40 (accumulation of hourly O₃ concentration exceed 40 ppb) and flux-response POD₆ (phytotoxic dose of ozone over 6 nmol m^-2 s^-1). Results showed that the ozone concentration, AOT40, and POD₆ clearly increased from 2015 to 2018 in growing season of maize over NCP. The four-year annual mean ozone concentration, AOT40, and POD₆ were 0.055 ppm, 18.02 ppm h, and 5.02 mmol m^-2, respectively. At county level, the relative loss of maize yield (MRYL) based on AOT40 and POD₆ had clearly spatio-temporal differences in NCP. The average MRYLs of AOT40 and of POD₆ from 2015 to 2018 were 10.4% and 21.4%, respectively, and these reductions were associated with 2399 million and 5637 million US dollars, respectively. This study suggests that surface ozone increased the yield losses of maize, and indicates that further reductions in ozone concentrations can enhance the food security in China.

Ambient volatile organic compounds in tropical environments: Potential sources, composition and impacts - A review

Volatile organic compounds (VOCs) are widely recognized to affect the environment and human health. This review provides a comprehensive presentation of the types and levels of VOCs, their sources and potential effects on human health and the environment based on past and current observations made at tropical sites. Isoprene was found to be the dominant biogenic VOC in the tropics. Tropical broad leaf evergreen trees are the main emitters of isoprene, making up more than 70% of the total emissions. The VOCs found in the tropical remote marine atmosphere included isoprene (>100 ppt), dimethyl sulfide (≤100 ppt) and halocarbons, i.e. bromoform (≤8.4 ppt), dibromomethane (≤2.7 ppt) and dibromochloromethane (≤1.6 ppt). VOCs such as benzene, toluene, ethylbenzene and xylene (BTEX) are the most monitored anthropogenic VOCs and are present mainly due to motor vehicles emissions. Additionally, biomass burning contributes to anthropogenic VOCs, especially high molecular weight VOCs, e.g. methanol and acetonitrile. The relative contributions of VOC species to ozone are determined through the level of the Ozone Formation Potential (OFP) of different species. Emissions of VOCs (e.g. very short-lived halogenated gases) in the tropics are capable of contributing to stratospheric ozone depletion. BTEX has been identified as the main types of VOCs that are associated with the cancer risk in urban areas in tropical regions. Finally, future studies related to VOCs in the tropics and their associated health risks are needed to address these concerns.

Examining the effectiveness of biomass-derived biochar for the amelioration of tropospheric ozone-induced phytotoxicity in the Indian wheat cultivar HD 2967

A pot study was performed to evaluate the influence of O₃ stress with different biochar treatments on a wheat cultivar (HD 2967). Plants were subjected to ambient and elevated (ambient+20 ppb) O₃ along with three doses of biochar (0%, 2.5%, and 5%). Elevated ozone alone reduced most of the growth parameters, negatively affecting the test cultivar's physiology. Although enzymatic antioxidants were up-regulated by elevated O₃, damage to the membrane integrity was evident by higher MDA content in the wheat leaves. Besides, the uptake of nutrients was observed to be reduced under elevated O₃ due to the reduced phyto-availability of the soil's nutrients and cation exchange capacity. Such limitation of assimilates and nutrients marked a trade-off between growth and defence, translating to grain yield loss. However, applying biochar as a soil conditioner ameliorated the detrimental effects of O₃ with respect to the economic yield of wheat. Biochar alone improved soil properties and nutrient phyto-availability, which translated to better plant growth, stronger physiological capacity, and higher crop productivity. Thus, the study inferred that altered nutrient phyto-availablity and its uptake, likely associated with biochar-induced improved soil properties, relayed stronger plant physiology and antioxidative defence system to combat O₃ induced oxidative stress.

Effects of elevated ozone on maize under varying soil nitrogen levels: Biomass, nitrogen and carbon, and their allocation to kernel

Effects of ozone (O₃) on maize have been increasingly studied, but only few studies have focused on the combined impacts of O₃ and nitrogen (N) on this important crop with C₄ carbon (C) fixation. In this study, a maize cultivar with the largest acreage in China was exposed to two O₃ treatments (NF: ambient air O₃ concentration; NF60: NF plus 60 ppb O₃) and four N levels (farmers' N practice: 240 kg N ha^-1 yr^-1; 150%, 50% and 25% of farmers' N practice). Generally, O₃ and N significantly influenced biomass, N and C, but did not change their allocation to kernel. There were significant interactions between O₃ and N in stem biomass, C concentration and uptake, and leaf biomass and C uptake, with significant O₃ effects mainly occurring at N120 and N240. Based on the coefficient of determination (R²), root C:N ratio rather than the most commonly used leaf C:N ratio was the best trait to indicate maize productivity. Furthermore, O₃ significantly increased the regression slopes between root C:N ratio and kernel N uptake, kernel C uptake and plant N uptake, strengthened the correlation of C:N ratio and kernel C uptake, and weakened the correlation of C:N ratio and hundred-kernels weight. These suggest that O₃ pollution can change the relationship of C:N ratio and productivity in maize. The weak correlation between kernel harvest index (HI) and N harvest index (NHI) indicated that future breeding researches should consider how to improve the coupling between biomass and N-related nutrition allocations in crop edible parts. Our results not only are helpful to accurately estimate O₃ impacts on maize with consideration of N but also provide a new insight into the relationship between plant traits and its productivity under O₃ pollution.

Elevated Ozone Levels Affect Metabolites and Related Biosynthetic Genes in Tartary Buckwheat

Global climate change and the industrial revolution have increased the concentration of tropospheric ozone, a photochemical air pollutant that can negatively affect plant growth and crop production. In the present study, we investigated the effects of O₃ on the metabolites and transcripts of tartary buckwheat. A total of 36 metabolites were identified by gas chromatography coupled with time-of-flight mass spectrometry, and principal component analysis was performed to verify the metabolic differences between nontreated and O₃-treated tartary buckwheat. The content of threonic acid increased after 2 days of the O₃ treatment, whereas it decreased after 4 days of exposure, after which it gradually increased until the eighth day of exposure. In addition, the levels of most metabolites decreased significantly after the O₃ treatment. On the contrary, the levels of two anthocyanins, cyanidin-3-O-glucoside and cyanidin-3-O-rutinoside, increased more than 11.36- and 11.43-fold, respectively, after the O₃ treatment. To assess the effect of O₃ on the genomic level, we analyzed the expression of anthocyanin biosynthesis pathway genes in O₃-treated and nontreated buckwheat using quantitative real-time reverse transcription polymerase chain reaction (PCR). We found that the expression of all anthocyanin pathway genes increased significantly in the O₃-treated buckwheat compared to that in the nontreated buckwheat. Altogether, our results suggested that O₃ affected the transcripts and metabolites of tartary buckwheat, which would eventually cause phenotypic changes in plants.

Impact of ozone pollution on nitrogen fertilization management during maize (Zea mays L.) production

The impacts of ozone (O₃) on crops have been extensively studied and are well understood. However, little information is available on the response of crops (especially maize) to the interactive effects of O₃ and nitrogen (N) fertilizer. To this end, a maize cultivar (Zheng dan 958, ZD958) that is common in China was exposed to two O₃ treatments and four N levels. We found that (1) the interactions between O₃ and N were non-significant for grain yield, plant biomass, C and N, although N addition significantly increased all parameters except C concentrations in grain and plant; (2) compared to NF (non-filtered ambient air O₃ concentration), NF60 (NF plus an extra 60 ppb O₃) increased the optimum N application rates (N_opt) in grain yield and plant biomass, but not grain yield and plant biomass potentials, thus resulting in lower N use efficiencies (NUE) and a larger risk of N-related environmental pollution (e.g., increased N₂O emission) under N_opt in NF60; (3) because of higher optimum plant N uptake (PN_opt) in NF60, relative to NF, plant N-saturated conditions for grain yield potential can be gradually turned into N-limited conditions as O₃ pollution increases. These findings manifest that O₃ is a vital global change factor impacting the management of N fertilization. If current O₃ pollution is substantially reduced, maize yield and biomass potentials can be increased under reductions in N input and N-related environmental pollution. In addition, these results can also contribute in developing and verifying N_opt model considering O₃ pollution in the future.

Effects of ethylenediurea (EDU) on regulatory proteins in two maize (Zea mays L.) varieties under high tropospheric ozone phytotoxicity

Rising tropospheric ozone is a major threat to the crops in the present climate change scenario. To investigate the EDU induced changes in proteins, two varieties of maize, the SHM3031 and the PEHM5, (hereafter S and P respectively) were treated with three EDU applications (0= control, 50 and 200 ppm) (hereafter 0= A, 1 and 2 respectively) (SA, S1, S2, PA, P1, P2 cultivar X treatments). Data on the morpho-physiology, enzymatic activity, and protein expression (for the first time) were collected at the vegetative (V, 45 DAG) and flowering (F, 75 DAG) developmental stages. The tropospheric ozone was around 53 ppb enough to cause phytotoxic effects. Protective effects of EDU were recorded in morpho-physiologically and biochemically. SOD, CAT and APX together with GR performed better under EDU protection in SHM3031 variety than PEHM5. The protein expression patterns in SHM3031 at the vegetative stage (28% proteins were increased, 7% were decreased), and at the flowering stage (17% increased, 8% decreased) were found. In PEHM5, a 14% increase and an 18% decrease (vegetative stage) whereas a 16% increase and a 20% decrease (flowering stage) were recorded in protein expression. Some protein functional categories, for instance, photosynthesis, carbon metabolism, energy metabolism, and defense were influenced by EDU. Rubisco expression was increased in SHM3031 whereas differentially expressed in PEHM5. Germin like protein, APX, SOD, and harpin binding proteins have enhanced defense regulatory mechanisms under EDU treatment during prevailing high tropospheric O₃. The present study showed EDU protective roles in C4 plants as proven in C3.

Yield and economic losses in maize caused by ambient ozone in the North China Plain (2014-2017)

Maize is the second most important crop per harvested area in the world. The North China Plain (NCP) is a highly populated and relevant agricultural region in China, experiencing some of the highest ozone (O₃) concentrations worldwide. It produces ~24% of the total maize production of China in years 2014-2017. For these years, we used observational O₃ data in combination with geostatistic methods to estimate county-level production and economic losses due to O₃ in the NCP. AOT40 (accumulated ozone exposure over an hourly threshold of 40 ppb) values during the maize growing season (90 days before maturity) progressively increased in the four consecutive years: 13.7 ppm h, 15.4 ppm h, 16.9 ppm h and 22.7 ppm h. Mean relative yield losses were 8.2% in 2014, 9.2% in 2015, 10.4% in 2016 and 13.4% in 2017. These yield losses, derived from exposure-response functions, resulted in crop production losses of 530.3 × 10⁴ t, 617.8 × 10⁴ t, 713.8 × 10⁴ t, and 953.4 × 10⁴ t, as well as economic losses of 2343 million USD, 2672 million USD, 1887 million USD, and 2404 million USD from 2014 to 2017. The NCP is a key area in China for monitoring the effectiveness of the clean air action policies aiming at reducing emissions of air pollutants. Despite these measures, O₃ concentrations have increased in NCP, and reduction of this pollutant are challenging. We suggest an increase in the number of rural air quality stations for better characterizing O₃ trends in cropland areas, as well as the application of different mitigation measures. They may involve more stringent air quality regulations and changes in crops, breeding tolerant cultivars and a crop management taking into account O₃ pollution.

Facile synthesis of N, P-doped carbon dots from maize starch via a solvothermal approach for the highly sensitive detection of Fe3+

Nitrogen/phosphorus-doped carbon dots (N, P-CDs) with a quantum yield as high as 76.5% were synthesized by carbonizing maize starch via a facile ethanol solvothermal approach and utilized for the detection of Fe³⁺.