Concentration‐ and flux‐based dose–responses of isoprene emission from poplar leaves and plants exposed to an ozone concentration gradient

Concentration‐ and flux‐based O₃ dose–responses of isoprene emission from single leaves and whole plants were developed. Two poplar clones differing in O₃ sensitivity were exposed to five O₃ levels in open‐top chambers for 97 d: charcoal‐filtered ambient air (CF), non‐filtered ambient air (NF) and NF plus 20 ppb (NF + 20), 40 ppb (NF + 40) and 60 ppb (NF + 60). At both leaf and plant level, isoprene emission was significantly decreased by NF + 40 and NF + 60 for both clones. Although intra‐specific variability was found when the emissions were up‐scaled to the whole plant, both leaf‐ and plant‐level emissions decreased linearly with increasing concentration‐based (AOT40, cumulative exposure to hourly O₃ concentrations >40 ppb) and flux‐based indices (POD_Y, cumulative stomatal uptake of O₃ > Y nmol O₃ m^?2 PLA s^?1). AOT40‐ and POD₇‐based dose–responses performed equally well. The two clones responded differently to AOT40 and similarly to POD_Y (with a slightly higher R² for POD₇) when the emission was expressed as change relative to clean air. We thus recommend POD₇ as a large‐scale risk assessment metric to estimate isoprene emission responses to O₃ in poplar.