The response of plants to O₃ varies depending on species, cultivar, and developmental stage ( Table 2 ). A few varieties of wheat (Triticum aestivum L.) and rice show visible signs of foliage injury during their early growth stages (Ramya et al., 2021b). Ozone causes early leaf senescence and abscission, which can affect biomass and growth by allocating carbon to edible plant portions. Additionally, the O₃-induced reduction in root growth is greater than the reduction in shoot growth because of impaired carbohydrate partitioning in the roots (Witting et al., 2009). However, it adversely affects shoot growth. Yadav et al. (2021) observed significant effects on plant growth when experimenting with four Indian wheat cultivars (early-sown cultivars HUW468 and HD3086; late-sown cultivars HUW234 and HD3118). They reported a greater reduction (26%) in the aboveground biomass of early-sown cultivars than that of late-sown cultivars (21%) under ambient and elevated O₃ (ambient+20 ppb).

Ozone-induced phytotoxicity negatively affects plant physiology and metabolism, including photosynthesis, respiration, transpiration, relative water content, and secondary metabolite accumulation in various crop plants (Cho et al., 2011; Ainsworth et al., 2012; Hassan et al., 2017). In particular, evaluating physiological processes is a more accurate method for assessing intrinsic O₃-induced injuries in plants because physiological damage can start at lower O₃ concentrations and before the onset of visible impairment (Pandey et al., 2019). Several studies have evaluated the detrimental effects of ambient and elevated O₃ on the physiological processes in plants ( Table 3 ). O₃-induced injuries are also related to reduced dry mass accumulation in leaves, lower leaf area-based antioxidant levels, and altered g_s . However, the O₃ uptake is linked to g_s, which varies according to the absorption capacity of the cuticle and stem in different plant species, such as the cuticle and stem. For example, increasing the amount of O₃ leads to a reduction in g_s and stomatal pore area in tomatoes (Solanum lycopersicum L.) (Thwe et al., 2014). Moreover, Yendrek et al. (2015) reported that legume crops, such as peas (Pisum sativum L.), soybeans, and beans, display reduced net photosynthetic rates and leaf longevity. Cabbage (Brassica oleracea var. capitata) (cv. Tekila and Primero) plants showed decreased photosynthetic rates (71.2%), stomatal conductance (81.03%), and chlorophyll content (32.98%) (Ramakrishnan et al., 2023) when exposed to O₃ (200 ppb). Ozone-induced stomatal closure occurs because of the inhibition of carbon assimilation in chloroplasts, which leads to an accelerated internal carbon dioxide (CO₂) concentration (Ainsworth et al., 2012). Moreover, O₃ exposure directly impacted the net CO₂ assimilation rate and CO₂ fixation ability negatively. Total carbon sequestration and transpiration rates were also related to reduced g_s and photosynthetic rates. It has also been observed that O₃-induced alterations in the CO₂ assimilation rate further influence plant respiration, leading to reduced crop growth and productivity (Ainsworth, 2017). In addition to ROS production, the concentration of secondary metabolites in plant cells is altered by O₃-induced stress. Higher concentrations of O₃ activate the first enzyme of the phenylpropanoid pathway, leading to a higher accumulation of flavonoids, phenolic acids, monolignols, GSH, gamma-aminobutyric acid (GABA), terpenoids, and volatile organic compounds such as isopropanoids (Mikkelsen et al., 2015).

Ozone exposure also causes a nutrient imbalance in plant cells by altering the allocation of nutritional elements and their ratios in the belowground (root) and aboveground parts (stem and leaves) and disrupting other physiological activities. For example, the potassium (K), calcium (Ca), sodium (Na), iron (Fe), and zinc (Zn) contents in potato tubers are lower under elevated O₃ than under ambient conditions, which lowers tuber quality (Kumari and Agrawal, 2014). Similarly, Ghosh et al. (2020a) revealed that the concentrations of nitrogen (N), phosphorus (P), K, magnesium (Mg), and Ca were reduced in the leaves and shoots of wheat under ambient and elevated (ambient+20 ppb) O₃ stress from 2 weeks after germination to maturity for 4 h. However, carbon was enhanced under the same stressful conditions, which led to an increase in the C:N and C:K ratios in the leaves. Similarly, as O₃ alters the absorption and distribution of macronutrients owing to changes in organic matter mineralization, the uptake of other nutrients is also influenced. Under O₃ stress, copper (Cu) concentration was significantly reduced in the leaves, shoots, and roots, although the reduction was higher in the shoots than in the roots. Ozone-induced reduction in leaf N has also been observed in previous studies (Chen et al., 2015; Pandey et al., 2018).

Reproductive development is the key determinant of plant productivity and species distribution. Increased O₃ has a detrimental effect on the reproductive system, primarily because it alters the allocation of carbon among tissues and directly affects plant reproductive processes (Gillespie et al., 2015). Several studies have demonstrated the effects of O₃ on flower initiation, floral development, pod formation, seed quality, seedling germination, and seedling vigor have been demonstrated in several studies. For example, Gillespie et al. (2015) demonstrated that O₃ adversely affected pollen viability, pollen germination, germ tube growth, pollen-stigma interactions, and fertilization in tomatoes, leading to decreased seed size, weight, and quality. Consequently, O₃-induced lower pollen viability or ovule development leads to reduced fertilization. Moreover, elevated O₃ levels promoted flower and pod abscission. For example, decreased pod production without an effect on flower production was observed in soybeans when plants were exposed to 150 ppb of O₃ inside O₃ chambers. Fruit number, fruit size, seed number, and seed size also decrease at this level of O₃ stress in soybeans (Leisner et al., 2014). Additionally, research suggests that the effects of O₃ exposure vary depending on the stage of plant growth, affecting flowering patterns in various ways and affecting the pollination and reproduction of annual crops and wild species. According to a recent investigation by Duque et al. (2021a), wild mustard (Sinapis arvensis L.) plants exposed to 120 ppb O₃ for 6 h day^-1 at earlier stages had more open flowers than the corresponding controls, whereas plants exposed to the same concentrations of O₃ at later stages tended to have fewer open flowers. Similarly, when S. arvensis plants were exposed to 120 ppb O₃ for 6 h day⁻¹ during the flower initiation stage, flowering accelerated, increasing the proportion of open flowers in O₃-exposed plants at the start of the flowering phase (Duque et al., 2021b). Furthermore, the O₃-induced reduction in the photosynthesis rate inhibits the accumulation of carbohydrates in pollen in some plant species, conferring adverse effects on pollen germination (Zhang et al., 2017). O₃-induced reductions in the number, size, weight, and quality of grains in cereal crops such as rice and wheat have also been observed in some studies (Banerjee and Roychoudhury, 2019; Schauberger et al., 2019). Although O₃ has been found to have detectable effects on reproductive growth, the precise sites of action and mechanisms underlying these effects remain unknown.

A significant proportion of crop yield losses is caused by tropospheric O₃, which is a transient, volatile, secondary air pollutant and a powerful phytotoxic compound (Ainsworth, 2017). According to the results of several controlled environmental and field studies, current O₃ concentrations in the environment have been found to negatively influence the yield and quality of several crop species worldwide, according to the results of several controlled environment and field studies ( Table 4 ). For example, according to McGrath et al. (2015), yield losses of 10% and 20% were observed in soybean and maize (Zea mays L.), respectively, in combination with dry conditions and high seasonal temperatures, and 5% and 10%, respectively, under rainfed conditions at field levels from 1980 to 2011 due to O₃ exposure. Additionally, when two cultivars of tropical maize were exposed to two different doses of O₃ (ambient+15 ppb and ambient+30 ppb), a reduction in the test weight of kernel plant^-1 of 6% and 10%, respectively, in DHM117, and 4% and 6%, respectively, in HQPM1 was observed (Singh et al., 2014a). Another long-term investigation conducted by Sinha et al. (2015) revealed that yield losses in rice, wheat, and maize were 21–26%, 27–41%, and 3–5%, respectively, under elevated O₃. Additionally, according to the Intergovernmental Panel on Climate Change’s Special Report on Emission Scenarios (IPCC SRES) A2 Scenario, O₃-induced worldwide yield losses in 2030 will range from 5 to 26% for wheat and 4 to 9% for maize (Avnery et al., 2011). Moreover, a recent study by Ghosh et al. (2020a) revealed that the grain yield was reduced by 45% in wheat cv. HD 2967 under ambient and elevated (ambient+20 ppb) O₃ stress for 4 hours day⁻¹ from 2 weeks after germination to maturity. Baqasi et al. (2018) reported a 49% decline in grain dry mass in wheat after 50 ppb O₃ exposure. Ozone also influences the quality of crops in terms of starch, protein, nutrients, and oil content. For example, it affects grain quality by decreasing starch content and increasing the protein and nutritional contents of crops such as wheat and rice (Broberg et al., 2015; Frei, 2015).

Leaf health, g_s , photosynthesis, and photosynthetic machinery are hampered by elevated O₃ levels. Improving photosynthesis may be an important approach for attaining higher plant tolerance to O₃ exposure. Ozone-mediated chlorophyll decline causes early senescence, but ethylene diurea (EDU) supplementation delays senescence in maize by increasing the chlorophyll content (Gupta et al., 2020; Poornima et al., 2022; Dhevagi et al., 2023). Increased expression of carbon metabolism-related proteins, which are part of the Calvin cycle and glycolysis, contributed to higher O₃ tolerance by accumulating more starch, which was reflected in better biomass production in EDU-treated maize. In addition, EDU-induced elevation of RuBisCO activity also supports higher photosynthesis in combating O₃ stress. Exogenous EDU increases the C₃ and C₄ photosynthesis rates by increasing the decarboxylation rate and expression of pyruvate phosphate dikinase protein, respectively, leading to higher O₃ tolerance (Alfonso and Brüggemann, 2012). Sensitive wheat cultivars showed higher photosynthetic rates with exogenous EDU application under elevated O₃ stress, which was correlated with EDU-induced higher chlorophyll content (Fatima et al., 2019). Therefore, improved photosynthesis and chlorophyll content may explain the higher biomass and yield, which later resulted in increased O₃ tolerance due to EDU supplementation.

Exogenous catechin (5 mM) supplementation in rice under elevated O₃ conditions reversed O₃-induced damage by enhancing chlorophyll content and its precursor (Mg²⁺ content), g_s , which resulted in higher grain production (Kittipornkul et al., 2020). Catechins can improve photosynthetic processes, thereby improving O₃ tolerance. Recently, calcium acetate application was shown to increase photosystem (PS)-II efficiency and improve the yield performance of rice under O₃ stress (Lakaew et al., 2022). There is a lack of knowledge regarding the mechanisms involved in improving the photosynthetic pathways to regulate plant growth and yield under O₃ stress conditions. Further in-depth research is required to elucidate these mechanisms and the associated pathways.

Plant antioxidant defenses must be upregulated to scavenge O₃-induced excess ROS, thereby protecting cellular functions (Gupta et al., 2020). The inhibition of cellular component peroxidation and the maintenance of the subsequent stability of the cell membrane under O₃-induced ROS overgeneration of ROS are distinct features of O₃-tolerant plants, which are made possible by the activation of defense mechanisms, including both enzymatic and non-enzymatic antioxidant components (Gill and Tuteja, 2010; Noctor et al., 2014; Pellegrini et al., 2019; Hasanuzzaman et al., 2020). Similarly, Czarnocka and Karpińsky (2018) stated that insufficient responses to plant antioxidants cause oxidative damage and strengthen the defense responses of plants, which are required to develop tolerance to O₃.

Both AsA and GSH are major antioxidants in the AsA-GSH cycle and are involved in regulating oxidative damage by scavenging ROS and maintaining cellular redox balance (Hasanuzzaman et al., 2020). Biogenic AgNPs and EDU-mediated higher antioxidant content (AsA and GSH) and enzymatic activities (SOD, CAT, APX, and GR) in wheat resulted in improved O₃ tolerance, which was correlated with lower H₂O₂ and MDA accumulation (Pellegrini et al., 2019).

Fatima et al. (2018) evaluated the ROS-scavenging capability of O₃-sensitive and tolerant wheat genotypes through their antioxidant responses. Kharchiya 65 (tolerant) displayed a maximum level of AsA, GSH, and flavonoids along with high free radical scavenging activities as well as lower ROS content than genotypes such as HD 2987 (sensitive) and PBW 502 (intermediately sensitive). At high O₃ (ambient+30 ppb), both enzymatic and non-enzymatic antioxidant responses varied among the three cultivars. The highest SOD, peroxidase (POD), GR, and GPX activities were observed in HD 2987, whereas Kharchiya 65 and PBW 502 showed the lowest increases. The maximum APX and the lowest CAT activity was observed in HD 2987 and Kharchiya 65. Sensitive cultivars showed higher enzymatic antioxidant responses when they suffered from O₃-induced elevated ROS levels. However, lowered SOD and POD activities were required in the tolerant cultivars, where ROS levels were lower than those in the sensitive ones. Higher free radical scavenging activities were observed in the tolerant (Kharchiya 65) cultivar than in the sensitive cultivar. Non-enzymatic antioxidant levels, such as those of AsA, GSH, and flavonoids, were also higher in the tolerant cultivar than in the sensitive cultivar, which is probably the most efficient mechanism for combating the elevated O₃. This may be because higher O₃ tolerance is highly associated with the genetic competence to preserve high AsA/DHA (Burkey et al., 2003). In cabbage (cv. Tekila and Primero), proline content was increased by 32.24%, ascorbic acid by 64.75%, CAT activity by 3.58%, and POD activity by 56%, which helped to reduce oxidative stress under O₃ stress (200 ppb) (Ramakrishnan et al., 2023). Therefore, variations in antioxidant responses to counteract O₃-induced oxidative stress are highly dependent on the crop species and the cultivars of the same crop species (Singh et al., 2014b).

Plant researchers are becoming increasingly interested in selecting appropriate techniques to increase plant tolerance by stimulating antioxidant activity. The supplementation of exogenous chemical substances to O₃-exposed plants is one of the most efficient approaches for reducing oxidative stress and cellular damage through the enrichment of antioxidant defense systems (Qiu et al., 2019).

Ethylene diurea is widely used as an anti-ozonant to increase plant tolerance to O₃ phytotoxicity and protect plants from damage (Manning et al., 2011). EDU-induced plant protection under O₃ stress depends on the activation of antioxidant activity (Gupta et al., 2021). Therefore, EDU supplementation in wheat revealed an EDU-mediated active role of apoplastic SOD, CAT, and amino methyltransferase, which facilitated the maintenance of ROS at optimum levels and decreased O₃-induced damage (Gupta et al., 2021).

Foliar spraying of catechin and SA can significantly affect the overexpression of APX and CAT genes, followed by their higher enzymatic activities, leading to lower lipid peroxidation (MDA), and thus increased tolerance of rice to O₃ (Kittipornkul et al., 2020).

Similar to ROS and Ca, phytohormones, such as abscisic acid (ABA), SA, jasmonic acid (JA), and ethylene (ET), are involved in the regulation of stomatal aperture movement upon O₃ exposure, leading to increased plant tolerance, which is mainly related to the cell signaling cascade (Pellegrini et al., 2016). Moreover, O₃-induced stomatal movement is controlled by anion channels, such as slow anion channel 1 and open stomata 1 (Vahisalu et al., 2010). These channels are regulated by ABA (Negi et al., 2008). However, variations in O₃ tolerance have been attributed to g_s and other protective mechanisms involved (Castagna and Ranieri, 2009). How phytohormones are associated with plant tolerance to O₃ exposure needs to be explored to better understand plant responses to stressful conditions. It was reported that the O₃-mediated increases in JA, asmonoyl-l-isoleucine, and ABA reduce leaf damage in Habataki rice (Tsukahara et al., 2015). Stress-induced apoplastic ROS exacerbates SA synthesis, which contributes to OsORAP1 expression and causes O₃ sensitivity and tolerance (Ueda et al., 2015).

Salicylic acid is essential for maintaining antioxidant defense mechanisms and cellular redox responses in plants upon O₃ exposure (Hasan et al., 2021). Ozone-induced leaf damage accompanying ET biosynthesis has been confirmed by the inhibition of ET biosynthesis in tobacco following O₃ treatment (Bandurska et al., 2009). This suggests that the suppression of ET biosynthesis can increase O₃ tolerance in plants. Abscisic acid controls ET and ABA biosynthesis by limiting ABI1 phosphatase activity and ROS homeostasis to induce O₃ tolerance (Pellegrini et al., 2016). Jasmonic acid is responsible for suppressing ROS-dependent leaf damage under O₃ stress (Hasan et al., 2021). Ozone exposure induces ET-dependent damage, which can be inhibited by JA when AT2G24850 and AT5G24770 are induced by JA (Wang et al., 2017). The significant role of SA in increasing the tolerance of rice to O₃ was studied by Kittipornkul et al. (2020), where exogenous 100 µM SA supplementation decreased MDA due to higher activity of CAT, APX upon 100–150 ppb (8 h day⁻¹) O₃. Additionally, plants inhibit O₃ uptake by increasing SA under O₃ exposure as a mechanism of O₃ tolerance (Pheomphun et al., 2019). Therefore, a comprehensive research on exogenous phytohormones is required to understand the mechanisms underlying their protective roles in the development of O₃ tolerance in cultivated crops.

Because abiotic stress is inevitable, it is crucial to develop strategies to combat stress-induced losses in crop production. Agronomic practices, such as changing the cropping season, air quality management, proper irrigation, and adequate plant protection measures, can be used to manage O₃-induced damage in crops. As O₃ is strongly linked to seasonal and regional changes, shifting the crop growing season by manipulating the sowing time has been suggested by Teixeira et al. (2011). Seasonal variation in crops can influence the physiological responses of plants by altering their gas exchange capacity, PS I function, and stomatal density upon exposure to O₃ stress. The generation of adaptive measures has been recorded through decreased g_s , increased stomatal density, and increased PS I activity when plants are exposed to stress at a later stage in their life cycle (Fusaro et al., 2016). Moreover, early sown crops exhibit higher sensitivity to O₃ owing to their longer life cycles, lengthy post-anthesis stages, higher g_s, and lower threshold levels. In contrast, the comparatively higher enzymatic antioxidant activity of late-grown crops, with increased energy allocation toward growth, facilitates reduced O₃-induced damage in crops (Yadav et al., 2019; Yadav et al., 2021). Air quality management to check for O₃ precursors is beneficial during O₃ exposure. For instance, decreasing methane, an important precursor of O₃ and greenhouse gases, has proven to be beneficial in combating O₃ (Shindell et al., 2012). Furthermore, controlling nitrogen oxide emissions in air is beneficial for reducing O₃ stress in northern China (Lu et al., 2021). By calculating the O₃ depletion potential of the substances (responsible for O₃ depletion), Ravishankara et al. (2009) reported that nitrous oxide (N₂O) is one of the most important greenhouse gases responsible for O₃ layer depletion. Nitrous oxide production can be mitigated using mulches (e.g., rice straw) and by minimizing fertilizer requirements in the soil. It is evident that soil water-filled pore spaces play an important role in N₂O emissions in the field; therefore, altering nitrogenous fertilizer with mulch can contribute to N₂O mitigation (Wu et al., 2018). Intercultural operations, such as mulching, can also reduce O₃-induced losses.

Although irrigation is desirable for improving crop production, it can enhance the susceptibility of crops to O₃ toxicity. In irrigated crops, the widely open leaf pores are subjected to stimulated g_s with elevated uptake of O₃ (Mills et al., 2018). This phenomenon is in agreement with another study (Harmens et al., 2019) that concluded that reduced irrigation could be an effective strategy to mitigate O₃-induced negative impacts partially or completely by delaying adversities on flag leaves at the time of flower initiation or during the gain-filling period of wheat. Therefore, it is imperative to manage irrigation properly to protect crops from the adverse effects of O₃ without causing water stress. For example, alternate wetting and drying irrigation was found to be beneficial for increasing rice productivity with decreasing g_s (Carrijo et al., 2017) and was effective in mitigating the adverse impacts of O₃. In addition, protective measures should be implemented to control plant competition and prevent plant injury in response to O₃ stress. It has been reported that when weed infestation is coupled with O₃, it enhances the susceptibility of crops to increased losses compared with O₃ exposure alone (Li et al., 2016). Ghosh et al. (2020b) reported a higher yield loss in wheat under O₃ stress owing to weed competition and concluded that strong weed management should be introduced to combat O₃ stress-affected production loss.

Although nutrient supplementation is important for enriching soil fertility, improper nutrient maintenance can increase crop vulnerability to various stressors, including O₃ (Tiwari and Agrawal, 2018). Zhang et al. (2018) conducted an experiment to assess the O₃ risk management capacity of plant nutrients, where N fertilizer was recorded to elevate the sensitivity of plants to stress; conversely, P improved tolerance by increasing the critical level of O₃ exposure in crops. Biomass loss caused by O₃ also differed between the two nutrients. Nitrogen fertilization caused the maximum g_s , which increased the sensitivity to O₃ and, in turn, resulted in a loss of biomass production, whereas P fertilization decreased g_s . In another study (Tatsumi et al., 2019), it was claimed that a lack of N supplementation in rice caused N deficiency while giving rise to photosynthetic assimilate translocation to the roots in an attempt to increase nutrient uptake, thereby protecting against O₃-induced damage to plant growth. Nutrient supplementation contributes to detoxification of ROS-induced O₃. Both N and P were recorded to encourage the mobilization of integrated participation of antioxidant compounds (carotenoid and AsA) and osmoprotectant (proline) and consequently reduce oxidative stress by keeping minimum O₂ ^•− and H₂O₂ while maintaining membrane integrity (Podda et al., 2019). Moreover, N addition can participate in diversifying stored carbohydrates and photosynthates to synthesize amino acids that help repair damage caused by O₃ (Podda et al., 2019). Although some studies have shown the negative impacts of O₃ and nutrient interactions, improved plant nutrient management can help mitigate O₃ stress to some extent (Gautam and Tiwari, 2020).

Plant growth can be stimulated by the subsidiary carbon supply provided by CO₂, which is known as CO₂ fertilization. This phenomenon can help relieve stress by reducing ROS production during the oxidative damage caused by various stressors, including O₃ (AbdElgawad et al., 2016). When CO₂ is used under O₃ concentrations in chickpeas (Cicer arietinum L.), a source-sink imbalance changes with the accumulation of photosynthates in leaves and, subsequently, an alteration of phenological characteristics that ultimately accelerate the crop life cycle to early maturity to escape the damage caused by O₃ exposure (Singh et al., 2021). Elevated CO₂ can counteract the damaging effects of O₃ by increasing shoot biomass and pod weight compared to O₃-exposed plants alone. Moreover, CO₂ fertilization can increase protein, starch, and certain mineral nutrients, even under O₃ exposure, thus combatting the negative impacts of imposed stress (Bhatia et al., 2021). A similar compensation tendency was observed in maize (Yadav et al., 2020). Interactive treatment with CO₂ and O₃ resulted in a positive result, as this combination resulted in a higher photosynthesis rate with improved growth attributes and, consequently, an increased yield component compared to the O₃ treatment alone. Carbon dioxide fertilization also results in improved g_s and carbon assimilation (Yadav et al., 2020). When elevated CO₂ levels were coupled with elevated O₃, reduced lipid peroxidation and solute leakage decreased, indicating improved cell membrane integrity. Enhanced antioxidant enzyme activity, which indicates reduced oxidative stress, has also been observed (Kumari et al., 2015). Therefore, CO₂ fertilization can help mitigate O₃-induced damage to some extent by reducing O₃ uptake, increasing carbon assimilation, and reducing oxidative damage, which helps to overcome the detrimental effects on plant growth, physiology (especially photosynthesis), and yield.

The use of tolerant cultivars can be an effective strategy that needs to be expanded by including agricultural practices, particularly in O₃ risk areas (Tiwari and Agrawal, 2018). A noticeable variation in rice genotypes in response to stress was observed by Arshad (2021), in which plant height, dry mass, leaf area, plant damage, and leaf damage were visible between susceptible and tolerant genotypes. Biochemical attributes such as total amino acids, sugar, protein profile, and phenolic content were not affected in varieties tolerant to O₃ stress, indicating tolerance capacity, whereas a trend of reduction was observed in susceptible genotypes. The susceptible varieties showed early visual symptoms through yellow to brown spots, which later turned into necrosis and early leaf senescence; however, this phenomenon was slower in the tolerant varieties (Arshad, 2021). In wheat, the tolerant variety (HD2967) sensed O₃ stressors at an early vegetative stage through increased MDA and initiated ROS scavenging activity, and consequently uplifted better antioxidant defense prior to the reproductive stage to protect against yield losses compared to the sensitive genotype (Sonalika), which was later in sensing (Pandey et al., 2018). Moreover, they added that the tolerant variety has tended to translocate more photosynthetic assimilates to increase biomass and husk weight with better resource partitioning, thus ensuring better protection of the reproductive parts under higher levels of O₃ exposure. These findings agree with those of Dhevagi et al. (2021), who concluded that the tolerant variety of mung beans (Vigna mungo L.) had higher AsA under exaggerated O₃ concentrations, which resulted in better morphological, physiological, biochemical performance, and antioxidant defense than the susceptible variety. Bailey et al. (2019) demonstrated the distinctiveness of a soybean-tolerant variety in the exclusion of O₃ uptake through stomata by lowering g_s and the transpiration rate, followed by higher water-use efficiency than the sensitive variety. Furthermore, under O₃ exposure, variations in phosphoenolpyruvate, carboxylase activity, and RuBisCO content and activity controlled the tolerance mechanism of maize hybrids more than the antioxidant defense mechanism of g_s . Stable leaf N content and RuBisCO activity under reduced O₃ exposure indicated better tolerance to late senescence and better yield than sensitive hybrids (Choquette et al., 2020). Therefore, the selection of tolerant varieties under O₃ exposure could be an effective approach for adaptation to O₃.

Elicitors with different chemical structures can be used against different stressors through exogenous application to crops or incorporation of transcription factors through breeding (Chakraborty et al., 2019). The use of various chemical elicitors to protect against O₃-induced phytotoxicity has also been demonstrated. For example, chitosan positively influenced wheat growth under O₃ exposure (Picchi et al., 2021). In addition to improving crop yield and quality, chitosan serves as a protectant that enhances the defense metabolism of plants by increasing the concentration of AsA within a short period by activating the APX enzyme to control H₂O₂ and oxidative stress. It also showed limited symptoms on the leaf surface area of chitosan-treated plants compared to O₃-stressed plants. The same trend of increasing antioxidant enzyme levels under stress conditions was reported by Kittipornkul et al. (2020), using catechins in rice. Application of catechin to rice under O₃ exposure activated antioxidant enzymes and helped to maintain chlorophyll, g_s , and Mg contents at the vegetative stage, which consecutively resulted in increased panicle number, filled grain weight, and starch, conferring protection against stress. These chemical elicitors protect plants by forming chemical barriers that detoxify O₃ (Li et al., 2018). It has been shown that trichomes have the capacity to deplete O₃ near the leaf surface, resulting in reduced O₃ uptake through stomata (Li et al., 2018). In addition, stress-induced lipoxygenase (LOX) activity (a consequence of O₃ uptake) were maintained through trichome density, providing plant tolerance to O₃ exposure. Ethylene diurea is widely used to enhance tolerance to O₃ exposure, and its effectiveness has been demonstrated in rice (Ashrafuzzaman et al., 2018), maize (Gupta et al., 2020), groundnuts (Arachis hypogaea L.; Chaudhary and Rathore, 2020) and castors (Ricinus communis L.; Rathore and Chaudhary, 2018). As EDU actively participates in the upregulation of crop growth, photosynthesis, and maintenance of improved membrane properties by activating defense metabolism by increasing AsA and flavonoid content, it can protect against O₃ stress (Rathore and Chaudhary, 2018; Chaudhary and Rathore, 2020). Furthermore, in addition to improving plant mechanisms (increased SOD, CAT, and APX activities) to regulate the defense system, EDU enhances protein accumulation in plants, which in turn enhances metabolic functions to mitigate damage under O₃ stress (Gupta et al., 2020). Foliar spraying with calcium acetate and calcium chloride improved tolerance through protectant-induced mechanisms of the antioxidant defense system in O₃-stressed rice plants (Lakaew et al., 2022). Calcium acetate-treated plants tolerated longer periods of O₃ exposure by augmenting NAD kinase and NADPH activities. This calcium acetate-mediated increase in NADPH content was associated with higher AsA and GSH levels, and higher APX and GR activities, resulting in an approximately 29% reduction in MDA generation. This calcium acetate-mediated oxidative stress mitigation contributes to improved plant growth and yield (Lakaew et al., 2022).