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.

[Effects of ozone stress on amylose content and starch RVA profile in grains located at diffe-rent positions on a panicle]. / 臭氧胁迫对稻穗不同部位糙米直链淀粉含量和RVA谱特征值的影响.

The increase of ground-level ozone concentration significantly reduces rice yield, but its effect on grain quality in association with the positions on a panicle was largely unknown. The effects of ozone stress on amylose content and RVA profile of rice grains located at different positions of panicles were studied by using a sunlit gas fumigation platform. Eight varieties representing different types of rice were fumigated under ambient (9 nL·L-1) or elevated ozone (100 nL·L-1) concentrations from transplanting until maturity. The results showed that elevated ozone treatment significantly reduced amylose content, maximum viscosity, hot viscosity, breakdown and cold viscosity by 5.9%, 7.6%, 5.9%, 11.6%, 2.9%, respectively, but increased the setback and gelatinization temperature by 24.9% and 1.0%. There were significant differences among varieties for amylose content and all parameters in RVA profile. The grains located at different positions on a panicle differed in amylose content, maximum viscosity, hot viscosity, breakdown and cold viscosity. The superior grains located at the upper part of a panicle had the highest value and the inferior grains located at the lower part of a panicle had the lowest value. However, the setback in RVA profile showed a different trend, with the superior grains having the lowest setback but inferior grains having the highest setback. In most cases, there were significant interactive effects of ozone by year or ozone by variety on amylose content and RVA profile. No significant ozone by grain position interaction on RVA profile was found, although the responses of superior grains to ozone stress was slightly smaller than those of inferior grains or grains located at the middle part of a panicle. The results demonstrated that ozone fumigation of 100 nL·L-1 during rice growing season deteriorated rice quality, with the magnitude of deterioration varying with growth seasons and varieties and little impacts of grain positions on a panicle.