How do drought and heat affect the response of soybean seed yield to elevated O? An analysis of 15 seasons of free-air O concentration enrichment studies.

The coincidence of rising ozone concentrations ([O]), increasing global temperatures, and drought episodes is expected to become more intense and frequent in the future. A better understanding of the responses of crop yield to elevated [O] under different levels of drought and high temperature stress is, therefore, critical for projecting future food production potential. Using a 15-year open-air field experiment in central Illinois, we assessed the impacts of elevated [O] coupled with variation in growing season temperature and water availability on soybean seed yield. Thirteen soybean cultivars were exposed to a wide range of season-long elevated [O] in the field using free-air O concentration enrichment. Elevated [O] treatments reduced soybean seed yield from as little as 5.3% in 2005 to 35.2% in 2010. Although cultivars differed in yield response to elevated [O] (R), ranging from 17.5% to -76.4%, there was a significant negative correlation between R and O dosage. Soybean cultivars showed greater seed yield losses to elevated [O] when grown at drier or hotter conditions compared to wetter or cooler years, because the hotter and drier conditions were associated with greater O treatment. However, year-to-year variation in weather conditions did not influence the sensitivity of soybean seed yield to a given increase in [O]. Collectively, this study quantitatively demonstrates that, although drought conditions or warmer temperatures led to greater O treatment concentrations and O-induced seed yield reduction, drought and temperature stress did not alter soybean's sensitivity to O. Our results have important implications for modeling the effects of rising O pollution on crops and suggest that altering irrigation practices to mitigate O stress may not be effective in reducing crop sensitivity to O.