Whole-plant compensatory responses of isoprene emission from hybrid poplar seedlings exposed to elevated ozone

It is still unclear whether the responses of isoprene (ISO) emission to elevated O₃ vary with biological organization level (i.e. leaf and whole-plant). To study such responses and the possible reasons explaining their variation, we investigated the effect of O₃ (CF: charcoal-filtered ambient air; E-O₃: non-filtered ambient air enriched with O₃) on ISO emission rate (ISO_rate), net photosynthetic rate (Pn), leaf nitrogen and carbon contents, and leaf growth traits in poplar seedlings (Populus deltoides cv. 55/56 × P. deltoides cv. Imperial) during one growing season. Opposite effects of E-O₃ on Pn were found between upper leaves (positive effect) and lower leaves (negative effect). Compared to CF, E-O₃ significantly decreased leaf mass per area, number of leaves, and leaf biomass, but increased leaf nitrogen content and individual leaf size. In the framework of such compensatory responses, poplar seedlings further increased ISO_rate in upper leaves and decreased ISO_rate in lower leaves, thus preventing significant decrease in the overall whole-plant ISO_rate by E-O₃. The measured whole-plant ISO_rate also showed that the simplistic estimation approaches based on the linear regression between chlorophyll content indicated by soil plant analysis development meter (SPAD value) and leaf-level ISO_rate could not accurately reflect the true response of whole plant to elevated O₃. For more accurate predictions, the potential ISO compensatory response to increasing O₃ concentration should be incorporated into the climate biogeochemical models related to ISO emission.