Photosynthetical response to ozone exposure and nitrogen enrichment of C. betulus and Q. robur saplings : results of the first year of experimentation under the ECLAIRE project

A manipulation experiment on ozone enrichment and increased nitrogen deposition has been performed in 2012 in Northern Italy on young trees of Quercus robur and Carpinus betulus, two of the most representative species of forest vegetation of the Po valley and the pre-alpine region. Two hundreds and twentyfour saplings of each species have been potted and placed in 12 Open-Top Chambers following a split-plot design with 3 randomized blocks and two factors: ozone concentration, the main factor, at 4 different levels (CF, AA, AA+35%, AA+70%), and nitrogen irrigation, the nested factor, at 2 different levels (tap water, tap water + 70Kg of N*ha*y-1). Saplings growth and physiological have been monitored during the whole growing season from April to the end of September. Gas exchange measurements were performed once a month as well as photosynthetic performance of PSII and stomatal conductance measurements (3 daily cycles). Moreover A/Ci and A/Light response curves have been made in June and September to assess the maximum carboxylation rate of RuBisCO, the maximum assimilation at non-limiting Ci concentration, photorespiration and dark respiration rates. At the end of the growing season half of the saplings have been harvested in order to estimate the total biomass production and its root/shoot partition. Preliminary results of the first year of experimentation (ECLAIRE experiment will last two years) reveal significant positive effects of nitrogen and negative effects of ozone at photosynthetic level in both species. Effects on growth and biomass partition were detected only for nitrogen, particularly for Q. robur, while the ozone effects were weak and scattered. The only detectable effect of ozone was a clear reduction of stomatal conductance starting from the mid-season. This fact highlights the need to include a f(O3) modifying function in the stomatal conductance models, one of which is proposed for these two species. The interaction between the two factors shows an antagonistic effects of nitrogen and ozone on the photosynthetic parameters. The observed incoherence among the ozone effects on photosynthetic parameters and the real plant growth rises a question about the possibility to predict ozone impacts on plant growth with a purely modellistic approach based upon photosynthesis and, once again, highlights the need to set up and support pluriannual experiments on field