Air pollution is a severe concern to humans, crops, and animals and primary significant emitter is human beings. The primary air pollutants such as sulfur dioxide(SO2), carbon monoxide(CO), methane(CH4), non-methane volatile organic compounds(NMVOC) and nitrogen oxides(NOX) formed the tropospheric ozone, a secondary air pollutant by the oxidation of CO, CH4 or NMVOCs in presence of NOX(nitrogen oxides) (Wedow et al., 2021) and the concentration of ozone increased due to industrialization (Grulke and Heath, 2020). Surface level ozone concentration is increasing in Asia and decreasing or stagnating in Europe and North America, which possess great threats to food production and food quality in Asia (Feng et al., 2021).The endemic richness area such as Atlantic islands, Mediterranean Basin, equatorial Africa, Ethiopia, the Indian coastline, the Himalayan region, southern Asia, and Japan are under high risk of biodiversity loss due to increasing concentration of surface level ozone by 2100 under RCP 2.6, RCP 4.5 and RCP 8.5 climate change scenarios (Agathokleous et al., 2020). There was a significant correlation of AOT 40 ozone exposure and human health, food production in China and in the year 2015, China lost 7% of the GDP due to ozone exposure (Feng et al., 2019).
In India, tropospheric ozone level is higher during winter, in this case, transport and residential sectors are most leading source for increasing tropospheric levels whereas, the northern parts of India contribute more compare to southern parts of India (Gao et al., 2019). Ozone is a short-lived pollutant, which significantly declines the growth, yield and productivity of crops globally (Ainsworth, 2016), when its concentration increased in the atmosphere's troposphere. Ozone enters into the plants through stomata and forms reactive oxygen species (ROS) directly by reacting with organic molecules present in apoplast, as well as induction of endogenous formation of ROS that can collectively cause cell damage and programmed cell death (Ainsworth, 2016; Li et al., 2017) when O3 concentration exceeds the critical levels (Kanagendran et al., 2017). Even, present surface level ozone concentration is causing damage to sensitive crops in Asia, North America and Europe and the expected yield loss due to projected ozone concentration under optimistic emission scenario would be 0.1 to 11% wheat, soya bean and maize by 2030 (Emberson, 2020).
Increase in concentration of ozone causes leaf injury, senescence, abscission, stomatal closure leading to reduction of photosynthesis, reduction of root growth, reduction of leaf growth, reduction of plant biomass and reduction of phloem translocation efficiency, which resulted in a significant decrease in crop yield (Wilkinson et al., 2012). According to the effect of ozone, few crops has been classified so far as sensitive (Wheat, watermelon, pulses, cotton, turnip, tomato, onion, soybean and lettuce), moderately sensitive (Potato, sugar beet, potato, oilseed rape, tobacco, rice, grape, broccoli and maize) and tolerant (Barley, plum and strawberry) (Emberson et al., 2009). The crops exposed to higher concentrations of ozone show some common symptoms on foliar i.e., ozone injury. Acute injuries are flecking and stippling, whereas the chronic injuries are bronzing, chlorosis and premature senescence. The estimated wheat loss due to ozone is 39.7%, 31.8% and 26.8% in Developed Countries, Upper Middle-Income Countries and Territories and Lower Middle-Income Countries and Territories respectively (Mills et al., 2018). Furthermore, the increase in ozone concentration would alter the foliar chemical composition and thereby changing plant - insect interaction and finally affects the ecosystem services (Agathokleous et al., 2020).
In increasing radiative forcing, O3 as a greenhouse gas modifies water and carbon exchange between flora and atmosphere regionally and globally by affecting transpiration and photosynthesis of plants (Lombardozzi et al., 2012). The O3 has potential to destruct photosynthesis in several ways when it enters the leaf via stomata, it oxidizes the cellular membrane by altering mesophyll cells, thus lowering Rubisco activity and carbon fixation. This results in reducing chlorophyll content (Kulshrestha and Saxena, 2016). The regulation of photosynthesis and transpiration is modified by stomatal conductance. The direct way of affecting the plants is by changing guard cell turgor pressure and signaling pathways and indirectly by stomatal closure in case of a rise in intercellular CO2 concentration (Lombardozzi et al., 2013). In few studies with respect to chronic O3 exposure, there was increase in stomatal conductance (Lombardozzi et al., 2013). Photosynthesis is directly proportional to chlorophyll content and net assimilation rate (Kobayakawa and Imai, 2017), hence, declining of photosynthetic rates resulted in lowering of biomass in crops (Chen et al., 2018) due to increased level of tropospheric ozone concentration. The negative effects of elevated tropospheric O3 concentration include decreased plant growth and altering the plant metabolism that significantly reduced crop yield (Saxena et al., 2019).
Ozone sensitivity varies across species due to species-specific biochemical, physiological and morphological traits (Li et al., 2016), stomatal conductance, and leaf antioxidant capacity (Brosche et al., 2010; Fares et al., 2013). An increase in the concentration of ozone suppressed photosynthetic activity and stomatal conductance, resulting in premature leaf fall and reduced biomass content that showed changes in physiology and growth of crops (Tetteh et al., 2015). The net photosynthesis reduction in wood plants also occurred due to elevated levels (Li et al., 2016). Reductions in stomatal conductance (gs), net photosynthetic CO2 assimilation and carboxylation efficiency had all been associated with O3 exposure to sorghum (Li et al., 2021). A decline in photosynthetic rate in O3 exposed plants was associated with damage to the photosynthetic machinery that leads to reduced fixation and increased CO2 concentration (Ci), resulting in reduced stomatal conductance (gs) (Yadav et al., 2020).
Ozone protectant studies are also being carried out globally to reduce the ozone impact on crops and improve crop yield. A unique preparation of Panchagavya is by both water (milk, urine, curd, dung, tender coconut water) and fat (ghee, milk with fat) products (Boomiraj and Christopher 2004) where polar and non-polar natural antioxidants are possibly produced by panchagavya that are essential for intracellular and intercellular parts of leaves in scavenging oxidative stress of elevated ozone. Furthermore, it has antioxidant potential by the experiment conducted by Athavale et al., 2012 with total phenols, FRAP and DPPH assays. Panchagavya were studied as ozone protectants in cauliflower (Sethupathi et al., 2018) and different plants and tree species of shola forest (Murugaragavan et al., 2018). Similarly, neem oil, which used in the study have highest percentage of antioxidant activity imputed to total phenol content under ethanol extract and it generates more free radicals, which have ability to inhibit highest percentage of DPPH radical. (Nahak and Sahu, 2011). Neem oil as ozone protectants studies in different crops like cauliflower (Sethupathi et al., 2018) and in different plants and tree species of shola forest (Murugaragavan et al., 2018). Ascorbic acid as a source of vitamin c plays a vital role to reduce the ozone impact in plants such as groundnut (Chaudhary et al., 2020), Soyabean (Jiang et al., 2018), Wheat (Fatima et al., 2019), Arabidopsis (Bellini and Tullio, 2019), Cauliflower (Sethupathi et al., 2018). The application of ascorbic acid, neem oil, and panchagavya was done to assess their capability to reduce the ozone impact on garlic crops in this study. Thus, the current study intended to study physiological responses and quality of garlic to elevated different ozone levels.