Response of isoprene emission from poplar saplings to ozone pollution and nitrogen deposition depends on leaf position along the vertical canopy profile

We investigated isoprene (ISO) emission and gas exchange in leaves from different positions along the vertical canopy profile of poplar saplings (Populus euramericana cv. ‘74/76’). For a growing season, plants were subjected to four N treatments, control (NC, no N addition), low N (LN, 50 kg N ha1 year1 ), middle N (MN, 100 kg N ha1 year1 ), high N (HN, 200 kg N ha1 year1 ) and three O3 treatments (CF, charcoalfiltered ambient air; NF, non-filtered ambient air; NF þ O3, NF þ 40 ppb O3). Our results showed the effects of O3 and/or N on standardized ISO rate (ISOrate) and photosynthetic parameters differed along with the leaf position, with larger negative effects of O3 and positive effects of N on ISOrate and photosynthetic parameters in the older leaves. Expanded young leaves were insensitive to both treatments even at very high O3 concentration (67 ppb as 10-h average) and HN treatment. Significant O3 N interactions were only found in middle and lower leaves, where ISOrate declined by O3 just when N was limited (NC and LN). With increasing light-saturated photosynthesis and chlorophyll content, ISOrate was reduced in the upper leaves but on the contrary increased in middle and lower leaves. The responses of ISOrate to AOT40 (accumulated exposure to hourly O3 concentrations > 40 ppb) and PODY (accumulative stomatal uptake of O3 > Y nmol O3 m2 PLA s1 ) were not significant in upper leaves, but ISOrate significantly decreased with increasing AOT40 or PODY under limited N supply in middle leaves but at all N levels in lower leaves. Overall, ISOrate changed along the vertical canopy profile in response to combined O3 and N exposure, a behavior that should be incorporated into multi-layer canopy models. Our results are relevant for modelling regional isoprene emissions under current and future O3 pollution and N deposition scenarios.