Combined Acute Ozone and Water Stress Alters the Quantitative Relationships between O(3) Uptake, Photosynthetic Characteristics and Volatile Emissions in Brassica nigra

Ozone (O(3)) entry into plant leaves depends on atmospheric O(3) concentration, exposure time and openness of stomata. O(3) negatively impacts photosynthesis rate (A) and might induce the release of reactive volatile organic compounds (VOCs) that can quench O(3), and thereby partly ameliorate O(3) stress. Water stress reduces stomatal conductance (g(s)) and O(3) uptake and can affect VOC release and O(3) quenching by VOC, but the interactive effects of O(3) exposure and water stress, as possibly mediated by VOC, are poorly understood. Well-watered (WW) and water-stressed (WS) Brassica nigra plants were exposed to 250 and 550 ppb O(3) for 1 h, and O(3) uptake rates, photosynthetic characteristics and VOC emissions were measured through 22 h recovery. The highest O(3) uptake was observed in WW plants exposed to 550 ppb O(3) with the greatest reduction and poorest recovery of g(s) and A, and elicitation of lipoxygenase (LOX) pathway volatiles 10 min–1.5 h after exposure indicating cellular damage. Ozone uptake was similar in 250 ppb WW and 550 ppb WS plants and, in both treatments, O(3)-dependent reduction in photosynthetic characteristics was moderate and fully reversible, and VOC emissions were little affected. Water stress alone did not affect the total amount and composition of VOC emissions. The results indicate that drought ameliorated O(3) stress by reducing O(3) uptake through stomatal closure and the two stresses operated in an antagonistic manner in B. nigra.