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.