Validation of the stomatal flux approach for the assessment of ozone visible injury in young forest trees. Results from the TOP (transboundary ozone pollution) experiment at Curno, Italy G. Gerosa a, * , R. Marzuoli a , R. Desotgiu b , F. Bussotti b , A. Ballarin-Denti a a Department of Mathematics & Physics, Catholic University, via dei Musei 41, 25125 Brescia, Italy b Department of Plant Biology, University of Florence, Piazzale delle Cascine 28, 50144 Florence, Italy The stomatal uptake of ozone is an important factor to evaluate visible injury appearance and evolution in plants. article info Article history: Received 10 September 2008 Accepted 12 September 2008 Keywords: Ozone stomatal fluxes AOT40 Forest trees Visible foliar injury abstract This paper summarises some of the main results of a two-year experiment carried out in an Open-Top Chambers facility in Northern Italy. Seedlings of Populus nigra, Fagus sylvatica, Quercus robur and Fraxinus excelsior have been subjected to different ozone treatments (charcoal-filtered and non-filtered air) and soil moisture regimes (irrigated and non-irrigated plots). Stomatal conductance models were applied and parameterised under South Alpine environmental conditions and stomatal ozone fluxes have been calculated. The flux-based approach provided a better performance than AOT40 in predicting the onset of foliar visible injuries. Critical flux levels, related to visible leaf injury, are proposed for P. nigra and F. sylvatica (ranging between 30 and 33 mmol O 3 m 2 ). Soil water stress delayed visible injury appearance and development by limiting ozone uptake. Data from charcoal-filtered treatments suggest the existence of an hourly flux threshold, below which may occur a complete ozone detoxification. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The Po Valley in Northern Italy is one of the most polluted regions in Europe according to the current air quality standards (Gerosa et al., 1999; EEA, 2007b). High concentrations of tropo- spheric ozone (O 3 ) have been reported in the last 15 years with increasing background levels, and the limit values set by the 2002/ 3/EC European Directive have been frequently exceeded (EEA, 2007a). Ozone is a phytotoxic pollutant which is responsible of a heavy impact on crops, semi-natural vegetation and forests as extensively documented in the last decades (Matyssek and Innes, 1999; Krupa et al., 2001; Paoletti, 2007). Its effects are present at biochemical, physiological and morphological level, and eventually affect plant productivity. In some species, ozone produces only visible leaf injuries (Novak et al., 2003; Bussotti et al., 2005); while in others it causes a significant decrease in grain yield and biomass produc- tion.. (Fumagalli et al., 2001; Pleijel et al., 2004). The intensity of the phytotoxic responses depends on the amount of O 3 directly absorbed by vegetation (i.e. the dose), but this parameter is often difficult to measure and quantify. In order to solve this problem, different exposure indices have been developed (Lee et al., 1988), such as the AOT40 (accumulated ozone over a threshold of 40 ppb), widely employed in Europe, which is based on the time- integrated atmospheric concentration of the pollutant (C) AOT40 ¼ X cCðtÞ>40 ppb ½C air ðt Þ 40Dt The AOT40 index assumes that the harmful O 3 concentrations for vegetation are only those exceeding a threshold of 40 ppb. However, plants absorb O 3 almost exclusively through the stomata, while other uptake pathways, such as cuticular absorption, are negligible (Kerstiens et al., 1992). For this reason, the AOT40 index is calculated only for daylight hours, when stomata are open, and precisely when the solar radiation is above 50 W m 2 (Karenlampi and Ska ¨rbi, 1996). AOT40 has been largely used in investigations focused on the identification of O 3 exposure–effect relationships (Mills et al., 2007). In the UN/ECE framework, under the Convention on Long Range Transboundary Air Pollution (http://www.unece.org/env/ lrtap), these experimentations allowed the definition of AOT40 critical levels for crops, semi-natural and forest vegetation (UN/ECE, 2004) and their consequent adoption in the EU regulations. Nevertheless, it has become evident that AOT40 is not fully reliable for impact predictions, since it does not take into account * Corresponding author. E-mail address: giacomo.gerosa@unicatt.it (G. Gerosa). Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol 0269-7491/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2008.09.042 Environmental Pollution 157 (2009) 1497–1505