Biomass response of young Holmoak trees after one season of ozone treatment in well watered condition

An Open-Top Chambers (OTC) experiment with ozone enrichment has been performed on young trees of Quercus ilex from April to September 2013 in Northern Italy.\r\nForty-eight 3 years old plantlets were potted and placed in 12 Open-Top Chambers arranged in 3 randomised blocks where the ozone factor assumed 4 different levels (CF-40%, NF, NF+30%, NF+74%). The plants were irrigated every night with about 0.4 L of water.\r\nAt the end of the season all the plants were harvested and biomass production was separately assessed for stem, roots and leaves after oven drying.\r\nThe stomatal ozone dose was calculated by applying a jarvisian model for stomatal conductance following the parameterisation published in the UN-ECE Mapping Manual, but with the gmax value calculated from gas exchange measurements performed in June and September. The latter measurements have been also used to check the model fit.\r\nThe 99th percentile of the measured stomatal conductance to water with CO2 concentration between 350 and 450 ppm was 358 mmol m-2 s-1, and it was taken as gmax in the stomatal model. This value is only 50 unit greater than that found by Fares et al. (2013), and about 90 units greater than the value proposed by the UNE-CE Mapping Manual.\r\nThe biomass reduction was quite generalised in the ozonized plantlets, but greater for the roots which showed a 27% decrease in the OZ++ treatment. The stem reduction and the foliar biomass in the same treatment were respectively -16% and -17%. \r\nThe relationship between ozone exposure (daylight AOT40) and the biomass reduction was significant (p<0.05) for roots and showed a 5% of biomass decrease every 10'000 ppb.h of AOT40 (R2=0.39). A similar relationship for shoots biomass was slightly less significant (p<0.09), while for leaf biomass was non-significant (p<0.16)\r\nAt the conference a flux-effect relationship for roots and shoots biomass decrease, based on stomatal flux, will be presented too. Elaboration are still in progress.