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 hayear), middle N (MN, 100 kg N hayear), high N (HN, 200 kg N hayear) and three O treatments (CF, charcoal-filtered ambient air; NF, non-filtered ambient air; NF + O, NF + 40 ppb O). Our results showed the effects of O and/or N on standardized ISO rate (ISO) and photosynthetic parameters differed along with the leaf position, with larger negative effects of O and positive effects of N on ISO and photosynthetic parameters in the older leaves. Expanded young leaves were insensitive to both treatments even at very high O concentration (67 ppb as 10-h average) and HN treatment. Significant O × N interactions were only found in middle and lower leaves, where ISO declined by O just when N was limited (NC and LN). With increasing light-saturated photosynthesis and chlorophyll content, ISO was reduced in the upper leaves but on the contrary increased in middle and lower leaves. The responses of ISO to AOT40 (accumulated exposure to hourly O concentrations > 40 ppb) and POD (accumulative stomatal uptake of O > Y nmol O m PLA s) were not significant in upper leaves, but ISO significantly decreased with increasing AOT40 or POD under limited N supply in middle leaves but at all N levels in lower leaves. Overall, ISO changed along the vertical canopy profile in response to combined O 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 O pollution and N deposition scenarios.