Identification of key responsive leaf traits for ozone tolerance in six modern indica and japonica rice cultivars (Oryza sativa L.) over two years

Increasing ozone concentrations cause rice yield loss in many polluted regions, it renders the use of ozonetolerant rice cultivars to mitigate its negative effects. To evaluate the sensitivity of rice cultivars to ozone, however, simple diagnostic procedures, such as alterations in leaf properties, have not been systematically established. As a result, a comparative investigation on the upper leaf properties of three indica and three japonica modern rice cultivars was carried out in Yangzhou, China, utilizing an ozone fumigation platform. From the start of tillering until the end of harvest in the growing seasons of 2016 and 2017, the plants were subjected to 100 ppb of ozone for 8 h each day, as well as control conditions (approximately 10 ppb). At 20 days after heading, eighteen leaf characteristics, including leaf SPAD, chlorophyll (CHL), morphological parameters, malondialdehyde (MDA), antioxidants content, and antioxidative enzyme activity, were evaluated. The results demonstrated that ozone stress accelerated leaf senescence, as shown by an increase in MDA content and faster reductions in pigment quantities (SPAD value, CHL), light-saturated photosynthesis (Asat) and stomatal conductance (gs). Ozone considerably reduced leaf area, length, and dry weight (DW), but did not affect stomatal density (SD) and dry weight per unit area (WPA). Ozone stress significantly enhanced total ascorbate (ASA), but enzyme activity of superoxide dismutase (SOD), peroxidase (POD), and hydrogen peroxidase (CAT) showed a trend of decrease. Analysis of variance (ANOVA) demonstrated a substantial interaction impact between ozone and year on the majority of leaf attributes, which may be ascribed to the high temperatures during the heading stage in 2016. The ozone-induced changes in leaf characteristics of the indica cultivar, such as leaf SPAD value, CHL, area, Asat, gs, MDA, and CAT, were greater than those of the japonica cultivar, demonstrating a substantial interaction between rice type and ozone. In combination with grain yield, this study demonstrated that the yield response differences across rice cultivars under ozone stress were closely related to the major leaf characteristics, including leaf pigments, Asat, gs, and MDA. These findings provide a significant theoretical basis for selecting ozone-tolerant rice cultivars and implementing cultivation strategies to reduce ozone's negative impact on rice yield.