“…The threshold ozone levels, beyond which necrosis appears, is very low i.e., 40 ppb for plant exposed for a period of 8 h (Larsen and Heck 1976). Since the beginning of the 1960s, this Bel-W3 variety has often been used as an indicator of the presence of ozone, especially in the United States (Craker et al 1974;Walter and Barlow 1974), The Netherlands (Posthumus 1976), the United Kingdom (Ashmore et al 1978(Ashmore et al , 1980, Israel (Goren and Donagi 1979), France (Bonte 1985;Van Haluwyn and Garrec 1995;Garrec and Livertoux 1997;Vergé 2000;Laffray et al 2007), Italy (Nali et al 2006, 2007), and Brazil (Sant'Anna et al 2008. However, the sensitive tobacco indicator system has never been used in ozone biomonitoring programs in North African countries because of the relatively little knowledge about plant response to air pollutants in harsh climatic and physiologic factors susceptible to alter the type and the severity of foliar symptoms (Madkour and Laurence 2002).…”