Drought does not mitigate reductions in soybean photosynthesis and yield caused by elevated ozone

The co-occurrence of elevated tropospheric ozone concentrations and drought in agricultural regions is anticipated to increase with climate change. Both stressors negatively impact leaf photosynthetic capacity and stomatal conductance, contributing to reductions in biomass and yield. The interaction of ozone and drought stress is complex and under-researched, particularly in field settings. Stomatal closure in response to soil drying may provide protection from high ozone influx to leaves. Conversely, elevated ozone may prevent drought-induced stomatal closure, leading to depletion of soil water resources and exacerbation of drought stress. Here, we used Free Air Concentration Enrichment of ozone (100 ppb) and rainfall exclusion canopies (intercepting ∼40% of seasonal rainfall) to test potential interaction effects of elevated ozone and drought stress on soybean (Glycine max) leaf-level physiology and yield. Elevated ozone consistently reduced soybean Rubisco carboxylation capacity (-17%) and maximum electron transport capacity (-9%) across 3 yrs of study. Elevated ozone did not alter the relationships between soil moisture, abscisic acid, and stomatal conductance. Thus, there was no evidence indicating that ozone exposure prevented stomata from responding during drought. Yield was significantly reduced in soybeans exposed to elevated ozone, resulting from fewer seeds per plot and reduced seed size. The reduced precipitation treatment only affected yields in the driest growing season. These findings suggest that the effects of elevated ozone and drought are additive, rather than interactive, and dose dependent. The persistence of ozone damage under soil moisture depletion is likely to be exacerbated by global climate change.