Air pollution is only one of several environmental issues that threaten food production. Tropospheric ozone (O₃) is one of the major air pollutants harmful to plants (Feng et al., 2015). The O₃ pollution is expected to worsen in the future due to a warmer climate, depending on the location, and increased anthropogenic emissions of O₃ precursors.

Following the Industrial Revolution, the percentage of ground-level O₃ has been steadily rising, reaching 35–40 ppb globally. In the 21st century, it may continue to rise more rapidly in developing countries with fast-growing economies (Proietti et al., 2016). The tropospheric O₃ level might exceed 70 ppb by 2050, according to the Environmental Protection Agency, if worldwide emissions of O₃ precursors continue at their current rate (Pfister et al., 2014). Tropospheric O₃ concentrations are increasing at a higher rate in tropical regions due to favorable conditions for O₃ formation (Agrawal et al., 2003; Tiwari et al., 2008). Following the mid-1990s, the decadal increase in tropospheric O₃ levels has been 2–14% in the tropics, 2–7% in northern mid-latitude areas, and 8–14% in the South Asian region (IPCC, 2021). Tropospheric O₃ is created as a secondary pollutant in the atmosphere through photochemical interactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of sunlight (Lefohn et al., 2017). Due to increased economic growth accompanied by higher NOx and VOC emissions, tropospheric O₃ levels are rising and are expected to continue to do so across the Asian region, leading to more crop losses (Ashmore, 2005). The O₃ concentration ranged from 45 to 65 ppb across the Indian region (David and Nair, 2013; Deb Roy et al., 2009). In most regions of the world, ozone is now the most significant air pollutant that has a detrimental effect on the growth and productivity of crops. Tropospheric O₃ is also a greenhouse gas (GHG) and contributes to climate change (Feng et al., 2019). Exposure to O₃ leads to stomatal closure, which limits carbon dioxide (CO₂) and water uptake by plants, further compromising photosynthetic efficiency and causing reduced growth and decreased grain filling in crop plants (Feng and Kobayashi, 2009). Several studies have quantified the impact of O₃ on crop development and yield (Ainsworth, 2008; Mills et al., 2018; Tai et al., 2014; Yadav et al., 2021). There are reports of reduced photosynthesis rates, leaf senescence, and changes in assimilate partitioning in plants due to exposure to elevated levels of O₃ (Mina et al., 2016; Tomer et al., 2015). Increased concentrations of surface O₃ diminish carbon storage in vegetation, leading to reduced growth and yield of crops (Tai et al., 2014). On the other hand, elevated atmospheric CO₂ concentrations lead to the accumulation of carbon, thereby increasing crop growth and also reducing plant nitrogen (N) and protein content (Abebe et al., 2016; Chakrabarti et al., 2020; Raj et al., 2019). The global average concentration of CO₂ has increased from 280 ppm to 409.7 ppm (NOAA, ESRI, 2019). An increase in atmospheric CO₂ concentrations improves the growth and productivity of crop plants (Bhatia et al., 2012; Singh et al., 2013; Deryng et al., 2016).

The world’s most important food crop, rice, is vulnerable to a variety of contaminants, especially air pollutants such as tropospheric O₃. Global food security is likely to suffer if the productivity of rice declines. Elevated tropospheric O₃ levels cause stress to rice plants during both the vegetative and reproductive stages, affecting their physiology, yield, and grain quality. According to Xia et al. (2021), major food crops, such as rice, wheat, and maize, are less tolerant to elevated O₃ than trees, such as spruce, silver fir, and pine. Elevated O₃ levels can alter physiological processes in crop plants, leading to changes in crop morphology and reduced crop growth (Bhatia et al., 2013).

Some reports elevated O₃ levels reduced yield by 11.4–12.3% compared to ambient levels in rice crops (Bhatia et al., 2011). Pandey et al. (2018) observed that grain yield and grain N content in wheat decreased under elevated O₃ concentrations. Along with reduced plant growth and productivity, higher O₃ levels also affect grain quality in crops as it is a strong oxidant and reduces important physiological processes in plants (Avnery et al., 2011; Broberg et al., 2015). Some researchers have reported that protein content in crops gets negatively affected by elevated O₃, but to a lesser extent than grain yield (Broberg et al., 2015; Grünhage et al., 2012). The interactive effect of elevated O₃ and CO₂ on crop growth, yield, and plant nutrient content will differ from the individual effect of O₃. Phothi et al. (2016) reported that elevated CO₂ levels would mitigate the negative impact of elevated tropospheric O₃ in rice crops. A simultaneous increase in both O₃ and CO₂ concentrations could nullify the negative effects of elevated O₃ on the ecosystem (Bhatia et al., 2012). Therefore, the negative effects of elevated tropospheric O₃ would be overestimated if the impact of elevated CO₂ concentrations is not considered (Xia et al., 2021).

Crop productivity is affected by the availability of N supplied to plants through various sources, including inorganic fertilizers. The interaction between O₃ and CO₂ will also affect the nutrient levels in crops (Phothi et al., 2016). As crop growth is reduced under elevated O₃ conditions (Pleijel et al., 2014), the demand and uptake of nutrients will also get altered under such situations, thereby reducing total nutrient content in plants. A study comprising data analysis of peer-reviewed literature depicted that the translocation of nitrogen from straw and leaves to grain in wheat crops was negatively affected by O₃. As N fertilizer use efficiency is reduced under elevated O₃ conditions, the risk of N losses from agroecosystems increases (Broberg et al., 2017). Elevated O₃ also affects soil microbial function and N transformation in the soil (Chen et al., 2015; Wu et al., 2016), which could, in turn, affect nutrient availability and uptake in plants. As crop productivity is often limited by nitrogen, N management becomes very crucial, especially under changing climatic conditions (Bhatia et al., 2010). A study by Singh et al. (2009) showed that the application of 1.5 times of recommended NPK can help ameliorate the negative effects of elevated tropospheric O₃ in mustard crops.

Estimating the effects of crop loss caused by tropospheric O₃ is crucial for nations, such as India, that are experiencing rapid urbanization and population growth. A few studies have investigated the interactive effects of elevated O₃ and CO₂ on crop plants. However, studies on elevated O₃ and CO₂ interactions with rice productivity and plant N are very limited. In addition, most studies were conducted earlier using open-top chambers (OTCs). It is imperative to conduct studies on the interaction between elevated levels of O₃ and CO₂ in open-field conditions, as such research is rarely reported. There is a need for a better understanding of cultivar-specific responses to elevated O₃ and CO₂ interactions to identify suitable adaptation options in rice under ozone stress conditions. In addition, N management strategies need to be formulated to sustain crop productivity and quality under elevated O₃ and CO₂ conditions. The hypothesis is that elevated O₃ and CO₂ can negatively affect rice productivity and quality. An additional N dose may counteract the harmful effects by improving growth, yield, and plant N uptake under elevated O₃ and CO₂ conditions. Therefore, the following study was conducted using a free air ozone and carbon dioxide enrichment (FAOCE) facility under open-field conditions (1) to quantify the yield and N uptake in rice varieties under the interaction of elevated O₃ and CO₂ and (2) to investigate the effect of an increased N dose on the yield and plant N in rice under elevated O₃ and CO₂ conditions.

Tropospheric O₃ is a secondary air pollutant generated through photochemical reactions among precursors such as nitrogen oxides (NOx), volatile organic compounds (VOCs), and carbon monoxide (CO), primarily released from anthropogenic activities (Broberg et al., 2015). Several researchers have reported the substantial impact of elevated O₃ levels on crop productivity and quality (Singh et al., 2018; Yadav et al., 2019). The results from the current study showed that elevated O₃ reduced the photosynthesis rates of the Pusa Basmati 1121 and IR64 Drt1 varieties of the rice crop, while the elevated O₃ plus CO₂ treatment was able to maintain the photosynthesis rates of the crops. The elevated O₃ and elevated O₃ plus CO₂ treatments significantly reduced the stomatal conductance and transpiration rates of the Pusa Basmati 1121 and IR64 Drt1 varieties. Previous studies have also indicated that elevated O₃ could hinder photosynthetic carbon acquisition in crops (Guidi et al., 2001; Morgan et al., 2003; Singh et al., 2009; Bhatia et al., 2012). There are reports that elevated CO₂ concentrations reduce stomatal conductance and transpiration rates in rice crops (Maity et al., 2023; Zhang et al., 2022). The elevated O₃ level substantially decreased the yield of the Pusa Basmati 1121 and IR64 Drt1 varieties. In contrast, the Nagina 22 variety demonstrated comparatively less or no yield reduction. Nagina 22 is known for its stress tolerance, and the results indicate that the increase in the O₃ concentration had minimal to no effect on its yield. Such yield losses under the O₃ exposure occurred due to the reduction in photosynthesis, which reduced the supply of assimilates required for reproductive development and seed growth (Feng et al., 2010). Tatsumi et al. (2019) also reported that brown rice yield was significantly reduced under elevated O₃ concentrations. A similar decrease in grain yield with exposure to elevated O₃ concentrations was reported for wheat cultivars in north India (Daripa et al., 2016; Yadav et al., 2019). In our study, an elevated CO₂ concentration of 550 ppm was able to partially mitigate the reduction in the grain yield due to increased O₃ exposure in the Pusa Basmati 1121 and IR64 Drt1 varieties, while it fully compensated for the yield reduction in the Nagina 22 variety. Elevated CO₂ has a fertilization effect on crops, which has helped compensate for yield losses in different rice varieties. The yield loss in the Nagina 22 variety, which is stress-tolerant, was already less than the other varieties. Therefore, the reduction in the yield in Nagina 22 was fully compensated under the elevated CO₂ plus O₃ treatment. Various researchers have earlier reported the beneficial effects of increased CO₂ levels in terms of increasing yields in different crops (Dey et al., 2017; Kobayashi et al., 1999; Pramanik et al., 2018; Sanyal et al., 2023). Bhatia et al. (2021) observed that a high CO₂ concentration of 554 ppm was able to counter the harmful impacts of O₃ exposure on yield and nutrient content in chickpeas. A study conducted with rice showed that elevated CO₂ could counter yield reduction due to O₃ exposure by more than 40% (Kumar et al., 2021).

The application of a 25% higher dose of N increased the photosynthesis rates and different yield parameters, such as the number of panicles and the number of grains per panicle, in the rice crops, thereby increasing the yield to a certain extent under the elevated O₃ concentration. According to Chen et al. (2018), decreased photosynthesis in nitrogen-deficient leaves increases the yield sensitivity of crops to high O₃ concentrations. Therefore, the increased dose of N had a positive effect on the yield parameters of the rice crops even under the elevated O₃ condition. The beneficial impact of higher N doses was also reported by Raj et al. (2019), who observed that increased atmospheric CO₂ concentrations and higher N doses synergistically contributed to an increase in the panicle number, thereby improving the yield of rice crops.

The elevated O₃ plus CO₂ interaction decreased the grain N concentration and N uptake in the rice varieties compared to the ambient condition. As O₃ is a strong oxidant, it harmfully affects important physiological functions in plants, which leads to reduced quality in crops (Avnery et al., 2011). Similar findings have been reported by earlier researchers, who found that protein content in plants is negatively affected by increased O₃ (Broberg et al., 2015; Grünhage et al., 2012). Broberg et al. (2017) reported that O₃ impairs the N translocation from straw to grain, thereby reducing the N use efficiency of crops. In addition, the applied N fertilizer is used less efficiently under elevated O₃, leading to adverse effects on grain protein content in crops. Under elevated CO₂ conditions, higher photosynthesis rates lead to greater carbon assimilation in rice, thereby reducing plant N concentrations due to the dilution effect (Kim et al., 2001). Reports indicate that protein content and N content in plants, especially cereal crops, decrease under elevated CO₂ conditions (Abebe et al., 2016; Chakrabarti et al., 2020; Raj et al., 2019).

Soil available N is also negatively affected by elevated O₃ and CO₂ treatment, which may be attributed to increased N losses from crop fields under elevated O₃ (Broberg et al., 2017). Under O₃ exposure, plant photosynthesis and grain filling duration are shortened (Gelang et al., 2000), which reduces the nutrient uptake period and increases the likelihood of greater N loss from the soil. Earlier studies (Chakrabarti et al., 2020; Maity et al., 2020) have also reported a decrease in soil available N under elevated CO₂ conditions due to increased crop growth resulting in higher N demand by the crop. As plant photosynthesis is hampered under increased O₃ concentrations, reduced photosynthates in the roots affect the root system and various soil processes (Chen et al., 2008). This may further affect soil N transformation rates (Wu et al., 2016), thereby lowering the available N status in the soil. The application of a higher N dose increased both the grain N uptake and soil available N in rice. As the crop growth was greater under the higher N doses, this generated more aboveground and belowground biomass of the crops. This might have led to increased microbial activity in the rhizosphere, leading to enhanced availability of N to the plants.

The study showed that although the negative effects of elevated O₃ on the rice yield were negated by the elevated CO₂ concentration, the adverse effect of elevated O₃ on the grain N content could not be compensated for. The response of rice to elevated CO₂ may be limited when nitrogen levels are sub-optimal. However, the decrease in plant N could be alleviated to a certain extent by applying higher doses of N (Maity et al., 2020). Our study showed that the application of the 25% RDN improved both the grain N concentration and grain N uptake, as well as soil available N, in the rice crops under the elevated O₃ and CO₂ interaction.

The grain yield of the rice varieties decreased under the elevated O₃ condition. An elevated CO₂ concentration of 550 ppm was able to compensate for the yield loss by 3.9–4.6% in the Pusa Basmati 1121 rice variety and by 4.6–8.0% in the IR64 Drt1 variety. Although elevated CO₂ was able to compensate for the yield loss due to elevated O₃, the N content in the rice grains was further reduced in the elevated O₃ plus CO₂ treatment. The application of an additional 25% of the recommended dose of N improved the grain N uptake by 15.3–40.8% in the different rice varieties compared to the 100% RDN in the elevated O₃ plus CO₂ interaction treatment. The study shows that nitrogen in rice grains and soil available N will decrease under elevated O₃ plus CO₂ conditions. An elevated CO₂ concentration of 550 ppm will be able to compensate for yield loss to a certain extent, but grain quality will further deteriorate in the elevated O₃ plus CO₂ treatment. The application of an additional 25% of the recommended dose of N could help in sustaining rice yield and also maintaining plant and soil N under elevated O₃ and CO₂ conditions in the future. However, the response of rice varieties to elevated O₃ and CO₂ might vary with different climate types. Therefore, the findings of the study could be further improved by testing different N fertilizer formulations to suggest the best N management options for the sustaining productivity and quality of rice under elevated O₃ and CO₂ conditions.