Elevated tropospheric ozone and crop production: potential negative effects and plant defense mechanisms

Ozone (O₃) levels on Earth are increasing because of anthropogenic activities and natural processes. Ozone enters plants through the leaves, leading to the overgeneration of reactive oxygen species (ROS) in the mesophyll and guard cell walls. ROS can damage chloroplast ultrastructure and block photosynthetic electron transport. Ozone can lead to stomatal closure and alter stomatal conductance, thereby hindering carbon dioxide (CO₂) fixation. Ozone-induced leaf chlorosis is common. All of these factors lead to a reduction in photosynthesis under O₃ stress. Long-term exposure to high concentrations of O₃ disrupts plant physiological processes, including water and nutrient uptake, respiration, and translocation of assimilates and metabolites. As a result, plant growth and reproductive performance are negatively affected. Thus, reduction in crop yield and deterioration of crop quality are the greatest effects of O₃ stress on plants. Increased rates of hydrogen peroxide accumulation, lipid peroxidation, and ion leakage are the common indicators of oxidative damage in plants exposed to O₃ stress. Ozone disrupts the antioxidant defense system of plants by disturbing enzymatic activity and non-enzymatic antioxidant content. Improving photosynthetic pathways, various physiological processes, antioxidant defense, and phytohormone regulation, which can be achieved through various approaches, have been reported as vital strategies for improving O₃ stress tolerance in plants. In plants, O₃ stress can be mitigated in several ways. However, improvements in crop management practices, CO₂ fertilization, using chemical elicitors, nutrient management, and the selection of tolerant crop varieties have been documented to mitigate O₃ stress in different plant species. In this review, the responses of O₃-exposed plants are summarized, and different mitigation strategies to decrease O₃ stress-induced damage and crop losses are discussed. Further research should be conducted to determine methods to mitigate crop loss, enhance plant antioxidant defenses, modify physiological characteristics, and apply protectants.