Effect of elevated surface layer ozone concentration on grain quality of two rice cultivars — A FACE study

Tropospheric ozone has been assumed to be the most phytotoxic air pollutant, which has created a severe concern for environmental pollution due to its negative impact on crop production. However, high ozone concentration also affects crop quality, which has so far not been treated in sufficient detail. Rice (Oryza sativa L.) is one of the most important food crops in the world, providing a significant proportion of human daily dietary needs. The accurate assessment of the impact of elevated concentration of surface layer ozone on rice quality is critical for reducing uncertainties in predicting future global food security. Using a Free-Air gas Concentration Enrichment (FACE) facility in Xiaoji Town (which is in Jiangdu County, Jiangsu Province, China), we conducted a field experiment to investigate the impacts of ozone stress on rice grain quality, including processing quality, appearance quality, cooking/eating quality and nutritional quality. Two rice cultivars (a conventional japonica cultivar ‘Wujing 15’ and a hybrid japonica cultivar ‘Lingfengyou 18’) were exposed to either ambient or elevated ozone concentration (ca. 21% above ambient values) from tillering to harvest. The results showed that elevated ozone resulted in a small decrease in brown rice percentage, milled rice percentage and head rice percentage, with the effect on milled rice percentage significant at the 0.05 level. Averaged across the two cultivars, elevated ozone concentration increased chalky grain percentage, area of chalkiness and degree of chalkiness by 15.0% (P = 0.10), 42.0% (P < 0.05) and 60.5% (P < 0.05), respectively. On average, elevated ozone concentration decreased gel consistency by 7.1% (P < 0.05), while no significant effect was observed on amylose concentration and gelatinization temperature for both cultivars. Measurements of RVA profile (Rapid Visco Analyser) indicated no significant changes in maximum viscosity, breakdown, cold viscosity, setback, and return due to elevated ozone. Elevated ozone concentration caused a non-significant increase in grain protein concentration for both cultivars. Analysis of variance indicated that in most cases, the effect of cultivar differences was significant. However, the interaction of ozone with cultivar was not detected for the tested traits of grain quality, suggesting that the response to ozone stress was similar for the two rice cultivars. The open-air field experiment suggested that moderate high ozone environment greatly increased grain chalkiness and significantly decreased gel consistency, but had little effect on other quality parameters of both cultivars.