Reduced soil water availability did not protect two competing grassland species from the negative effects of increasing background ozone

Two common (semi-) natural temperate grassland species, Dactylis glomerata and Ranunculus acris, were grown in competition and exposed to two watering regimes: well-watered (WW, 20–40% v/v) and reduced-watered (RW, 7.5–20% v/v) in combination with eight ozone treatments ranging from pre-industrial to predicted 2100 background levels. For both species there was a significant increase in leaf damage with increasing background ozone concentration. RW had no protective effect against increasing levels of ozone-induced senescence/injury. In high ozone, based on measurements of stomatal conductance, we propose that ozone influx into the leaves was not prevented in the RW treatment, in D. glomerata because stomata were a) more widely open than those in less polluted plants and b) were less responsive to drought. Total seasonal above ground biomass was not significantly altered by increased ozone; however, ozone significantly reduced root biomass in both species to differing amounts depending on watering regime. - Highlights: ► Enhanced ozone increased senescence in Ranunculus acris and Dactylis glomereata. ► Reduced soil water availability had no protective mechanism on senescence. ► Interactions between ozone and water status on root biomass were species-specific. ► Reduced soil water availability reduced below-ground allocation in R. acris. ► Reduced-watering failed to close stomata in ozonated D. glomerata plants. - Ozone reduces stomatal response to soil drying leading to increased injury/senescence and alterations in carbon allocation.