Stomatal conductance, photosynthesis and growth response of Hornbeam and Oak young trees after a two-years treatment with ozone and nitrogen addition

An Open-Top Chambers (OTC) experiment with ozone enrichment and increased nitrogen deposition has been performed during two consecutive years (2012 and 2013) in Northern Italy on young trees of Quercus robur and Carpinus betulus. Two hundreds and sixteen saplings of each species were 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-45%, NF, NF+35%, NF+70%), and nitrogen irrigation (NDep), the nested factor, at 2 different levels (tap water for control, tap water +NDep of 70Kg of N*ha*y-1). These treatments were applied for two consecutive growing seasons. In both years stomatal conductance (gs) measurements and CO2 assimilation response curves have been made throughout the season to assess the impacts on physiological and photosynthetical parameters. Half of the saplings was harvested at the end of 2012 season, while the remaining half was harvested at the end of 2013, in order to estimate the effects of both stresses, alone and in combination, on the total biomass production and on the root/shoot partition of the plants. After two years of treatments, a general positive effect of nitrogen deposition on biomass production was found in both species, as it could be easily expected. This biomass increase was particularly intense in C. betulus (+76% of the total biomass, + 65% of roots biomass). Q. robur showed a greater response to ozone than C. betulus in the control Ndep conditions. Oak plants showed a 10% and 18% of reduction of the total and root biomass in NF+70% treatment. This response was also found in the plants subjected increased to nitrogen deposition (-15% for total biomass, -16% for roots biomass). Hornbeam seemed stimulated by O3 when no nitrogen was added (+5% of total biomass). However, NDep treatment made hornbeam more susceptible to ozone, which caused a 30% biomass decrease in both shoots and roots. Looking at gs as a possible driver of the plants\u2019 response to ozone, we found that ozone lead to a 21% decrease of gs in oak plants when the had a total biomass reduction of 10%. This response is confirmed in NDep treatment (-18% in gs, -15% in total biomass). Thus, oak seemed to be more vulnerable to gs limitation because of the consequent reduction of CO2 assimilation. In hornbeam, without nitrogen addition, ozone caused a slight reduction on gs leading to a decrease of stomatal dose and a small increase in total biomass (+5%). On the contrary, nitrogen deposition caused a significant increase of gs (+23%) that lead to an increased ozone uptake, thus suggesting an overwhelming of the detoxifying defence (-30% in total biomass). Stomatal conductance, thus, reveals to be a key driver of the plant\u2019s response to ozone, but the final manifestation of the effect seems to be modulated by the detoxifying capacity of the plants. The general decrease of gs in both species caused by ozone (also found in 2012 measurements), suggests the need to include an f(O3) modifying function in the stomatal conductance models which will be defined for these two species. Some of biomass responses are partially in disagreement with the results of the first year, likely for the presence of a carry over effect. This fact highlights the importance of performing long-term experiments (more than 1 year) for the investigation on ozone and nitrogen effects on biomass