Impact of ozone stress on thermodynamic characteristic values of rice starch and the differences between superior and inferior grains

As a strong oxidant, ozone pollution not only threatens human health but also negatively affects plant life. Elevated concentrations of ground-level ozone reduces rice yield and tends to deteriorate grain quality traits, including appearance, nutritional value and taste. However, the effect of ozone stress on the thermodynamic characteristics (DSC) of rice starch in respect of growth season, variety, and grain position on a panicle is not well-known. In this study, a glasshouse-type gas fumigation platform was used to examine how ozone stress affected the DSC of rice grains located at different positions on a panicle. Plants of eight rice varieties were exposed to ozone fumigation from the plant tilling stage until plant maturity. Two levels of ozone concentration, 100 nL·L^-1 and 9 nL·L^-1 were applied to rice plants as ozone stress treatment and control, respectively. At harvest, according to their positions on a panicle, rice grains were separated into three groups:superior grains, inferior grains, and medium grains, according to their position in a panicle-upper part, lower part and middle part of panicle. The DSC values of rice starch from the different groups were measured. The study showed that ozone stress significantly reduced the enthalpy value of brown rice by 4.15% compared with the control, but it had no significant effect on the gelatinization starting temperature, the peak gelatinization temperature, the gelatinization termination temperature, and the peak width and peak height of the DSC curve. In 2017, the enthalpy value, gelatinization peak temperature, gelatinization starting temperature, and peak height of brown rice were significantly higher than the values in 2016. However, the opposite trend was observed for the peak width of the DSC curve. There were significant differences among rice varieties in respect of the DSC characteristic values of rice starch. All the DSC values of grains at different positions on a panicle were in the order of upper part > middle part > lower part, and the differences were statistically significant, apart from the case of the gelatinization starting temperature. Results of ANOVA revealed significant ozone by year interactions for enthalpy value, gelatinization peak temperature, and gelatinization termination temperature, and significant ozone by variety interactions for gelatinization peak temperature, gelatinization termination temperature, and peak width and peak height of the DSC curve. Meanwhile, ANOVA revealed significant ozone by grain position interactions only for the peak width of DSC curve. The findings demonstrated that the DSC thermodynamic parameters of rice grains varied with the growing season, the varieties tested, and the grain position on a panicle. Ozone fumigation during the rice growing season reduced the enthalpy value of grains at different positions on a panicle, which indicated ozone-stressed rice grains are prone to gelatinization.