Yield and photosynthetic characteristics of flag leaves in Chinese rice (Oryza sativa L.) varieties subjected to free-air release of ozone

The effects of elevated O3 concentration ([O3]) on grain yield and photosynthetic characteristics of flag leaves in rice (Oryza sativa L.) were studied using a hybrid indica cultivar SY63 and a conventional Japonica cultivar WYJ3. Both cultivars were grown in the field under ambient (A-O3) and elevated [O3] (E-O3, ambient × 1.5) using free-air O3 enrichment in Jiangdu city, Jiangsu province, China. The E-O3 significantly reduced above-ground biomass (p = 0.047) and grain yield (p = 0.0046), but had no significant effect on harvest index (p = 0.18) across cultivars. The hybrid Indica cultivar SY63 was more sensitive to E-O3 than the conventional Japonica cultivar WYJ3. The above-ground biomass was reduced by 17.6% for SY63 and 6.6% for WYJ3, whereas the grain yield was reduced by 20.7% for SY63 and 6.3% for WYJ3. A barely non-significant (p = 0.079) interaction between O3 and cultivar on grain yield suggested a higher sensitivity of the hybrid variety: SY63. In SY63, flag leaves exhibited an earlier and stronger response in photosynthetic characteristics to E-O3. Depression in light-saturated photosynthetic rate (Asat) was first observed at 237 day of year (DOY) in SY63, and the seasonal mean Asat was reduced by 23.1%. In contrast, for WYJ3, the conventional cultivar, the impact of E-O3 on Asat was negligible until 266 DOY, and seasonal mean decrease of Asat was only 9.4%. The same trend was found in chlorophyll a fluorescence parameters. In SY63, actual quantum yield of PSII (p = 0.007) and qp (photochemical quenching) (p = 0.002) were significantly decreased at 248 DOY and the downward trends persisted throughout the rest of the life span of flag leaves. No such changes were observed in WYJ3. In the ozonated flag leaves of SY63, necrotic damages occurred and chlorophyll contents declined significantly by 12.6–43.6% through the entire functional duration of the leaf. As for WYJ3, chlorophyll remained unaffected until a 25.7% decrease appeared at 278 DOY under E-O3.