Ozone phytotoxicity to Panicum maximum and Cenchrus ciliaris at Indo-Gangetic plains: an assessment of antioxidative defense and growth responses

Spatiotemporal variations and risk assessment of ambient air O3, PM10 and PM2.5 in a coastal city of China

Rapid industrialization and urbanization has created significant air pollution problems that have recently begin to impact the lives and health of human beings in China. This study systematically investigated the spatiotemporal variations and the associated health risks of ambient O₃, PM₁₀ and PM_2.5 between 2016 and 2019. The relationships between the target air pollutants and meteorological conditions were further analyzed using the Spearman rank correlation coefficient method. The results demonstrated that the annual mean concentrations of PM₁₀ and PM_2.5 experienced a decreasing trend overall, and PM_2.5 significantly decreased from 1.54 μg/m³ in 2016 to 1.48 μg/m³ in 2019. In contrast, the annual mean concentrations of O₃ were nearly constant during the study period with a slight increasing trend. The pollutants exhibited different seasonal variations and cyclical diurnal variations. The most highest O₃ pollution was seen in spring and summer, while spring and winter were the seasons with the most PM₁₀ and PM_2.5 pollution. The highest concentrations of O₃ appeared in periods of strong solar radiation intensity and photochemical reactions. The highest concentrations of PM₁₀ and PM_2.5 appeared at commuting time. The pollutant concentrations were significantly affected by meteorological conditions. Finally, the non-carcinogenic risks from exposure to O₃, PM₁₀ and PM_2.5 were at an acceptable level (HI < 0.96) and O₃ accounted for ~50% of the total non-carcinogenic risks. However, PM_2.5 posed highly carcinogenic risks (2.5 × 10^-4 < CR < 1.6 × 10^-1) and O₃ exposure showed high potential ecological impacts on vegetation (AOT40: 23.3 ppm-h; W126: 29.0 ppm-h).

Salinity alleviates the toxicity level of ozone in a halophyte Mesembryanthemum crystallinum L

Mesembryanthemum crystallinum (Ice plant) is an annual halophytic plant species spread in the coastal areas of the Mediterranean Sea, Egypt. Information about the behaviour of halophytes under the future concentration of ozone (O₃) is scanty. Therefore, we have assessed the effects of elevated O₃ (ambient + 20 ppb), moderate salinity (200 mM NaCl), and their combined treatment (salinity + elevated O₃) on various morphological, growth, physiological, biochemical and anatomical parameters of Egyptian ice plant. Under salinity stress, plant growth, percentage of pigmented leaf and its thickness, ROS levels, antioxidative enzymes, and ROS scavenging activities were increased, while photosynthetic pigments and efficiency were decreased compared to the control. Elevated O₃ exposure led to reductions in most of the growth parameters and pigments, while ROS levels, histochemical localization of H₂O₂ and ·O₂^-, antioxidative enzymes and non-enzymatic antioxidants (betacyanin, phenolics, thiols and ascorbic acid) showed increases. Surprisingly, salinity alleviated the oxidative stress of elevated O₃ due to the rise of SOD activity, antioxidant compounds, and a decrease of ·O₂^- production rate with concomitant increases of most of the growth parameters. Thick lower collenchyma and enhancement of xylem parenchyma under O₃ and combined treatment suggested that anatomical acclimation also operated under O₃ stress and salinity played a vital role in the growth of this plant under combined stress. Results showed that salt is essential for the optimum development of this species and its role is extended to alleviate the oxidative damage caused by elevated O₃. The results further recommend the use of Egyptian M. crystallinum as a O₃ tolerant crop for saline areas along the Mediterranean Sea coast.