Ozone-induced reduction in rice yield is closely related to the response of spikelet density under ozone stress.

Six modern rice cultivars, including three indica and three japonica cultivars were exposed to 100 ppb ozone (8 h per day) and control conditions throughout cropping season in 2016 to 2017 at Yangzhou, China. Ozone decreased plant height and inhibited tillering development as well as panicle number per plant of all cultivars, but had no effect on the productive tiller ratio. Ozone significantly decreased spikelet number per panicle, spikelet number per plant, fully-filled grain percentage and fully-filled grain weight, resulting in yield decrease by 39.3% on average for all cultivars and by 42.6 and 34.4% in the indica and the japonica groups, respectively. The response of aboveground biomass to ozone was similar to that of yield, albeit with a lower decrease, which led to a 7.6% decrease in harvest index. In terms of grain positions (grains attached to the upper primary rachis: superior spikelet (SS); grains attached to the lower secondary rachis: inferior spikelet (IS), and the remaining grains: medium spikelet (MS)), the ozone-induced change in yield traits (yield, spikelet number per panicle, spikelet number per plant, fully-filled grain percentage, and empty grain percentage) followed the order IS > MS > SS, as indicated by the significant interaction between ozone and grain position. Although ozone had negative effects on all yield traits, only ozone-induced reduction in spikelet density (spikelet number per panicle and or spikelet number per plant) was significantly correlated to yield loss. Grain yield showed significant ozone by cultivar and ozone by year interactions, indicating ozone impacts on rice yield varied with meteorological conditions and cultivars.