Eects of pre-exposure to ultraviolet-B radiation on responses of tomato (Lycopersicon esculentum cv. New Yorker) to ozone in ambient and elevated carbon dioxide X. Hao a , B.A. Hale b , D.P. Ormrod c, *, A.P. Papadopoulos a a Greenhouse and Processing Crops Research Centre, Agriculture and Agri-Food Canada, Harrow, Ontario, Canada N0R 1G0 b Department of Land Resource Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1 c Centre for Forest Biology, University of Victoria, Victoria, BC, Canada V8W 2Y2 Received 7 June 1999; accepted 3 November 1999 ``Capsule'': Pre-exposure to enhanced UV-B mitigated ozone damage to leaf photosynthesis at elevated carbon dioxide concentrations. Abstract Patterns of environmental change in the biosphere include concurrent and sequential combinations of increasing ultraviolet (UV-B) and ozone (O 3 ) at increasing carbon dioxide (CO 2 ) levels; long-term changes are resulting mainly from stratospheric O 3 depletion, greater tropospheric O 3 photochemical synthesis, and increasing CO 2 emissions. Eects of selected combinations were evaluated in tomato (Lycopersicon esculentum cv. New Yorker) seedlings using sequential exposures to enhanced UV-B radiation and O 3 in dierential CO 2 concentrations. Ambient (7.2 kJ m 2 day 1 ) or enhanced (13.1 kJ m 2 day 1 ) UV-B ¯uences and ambient (380 mll 1 ) or elevated (600 mll 1 ) CO 2 were imposed for 19 days before exposure to 3-day simulated O 3 episodes with peak concentrations of 0.00, 0.08, 0.16 or 0.24 mll 1 O 3 in ambient or elevated CO 2 . CO 2 enrichment increased dry mass, leaf area, speci®c leaf weight, chlorophyll concentration and UV-absorbing compounds per unit leaf area. Exposure to enhanced UV-B increased leaf chlorophyll and UV-absorbing compounds but decreased leaf area and root/shoot ratio. O 3 exposure generally inhibited growth and leaf photosynthesis and did not aect UV-absorbing compounds. The highest dose of O 3 eliminated the sti- mulating eect of CO 2 enrichment after ambient UV-B pre-exposure on leaf photosynthesis. Pre-exposure to enhanced UV-B mitigated O 3 damage to leaf photosynthesis at elevated CO 2 . # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Tomato, Lycopersicon esculentum; Ozone; Carbon dioxide; Ultraviolet-B; Growth; UV-absorbing compounds 1. Introduction Stratospheric ozone (O 3 ) depletion, increasing tropo- spheric O 3 pollution and rising concentrations of atmos- pheric carbon dioxide (CO 2 ) are on the forefront of concerns about global climate change. Stratospheric O 3 plays a very important role in shielding the earth's surface from ultraviolet (UV) radiation. Anthropogenic increases in the global concentration of chloro¯uoro- carbons, methane and nitrous oxide are eroding strato- spheric O 3 (Cicerone, 1987). Measurements in Toronto, Canada, from 1989 to 1993 have shown increases in solar UV-B radiation (near 300 nm) of 35% per year in winter and 7% per year in summer (Kerr and McElroy, 1993). Increases in the solar UV-B radiation reaching the biosphere may have many eects on terrestrial plants (Tevini, 1993) and are ``likely to cause signi®cant biological damage'' although protective and repair mechanisms exist to counteract the eects of increased UV-B (Jansen et al., 1998). Tropospheric O 3 , a product of photochemical reactions, is a highly phytotoxic air pollutant (Krupa and Manning, 1988) that is increasing by 0.5±2% a year (Penkett, 1988). The photochemical formation of tropospheric O 3 is dependent on emissions of NO x mainly from urban and industrial areas. The photolysis of NO 2 at wavelengths less than 420 nm, peaking in the UV-A, provides the singlet oxygen required for O 3 formation (Finlayson-Pitts and Pitts, 0269-7491/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0269-7491(99)00305-X Environmental Pollution 110 (2000) 217±224 www.elsevier.com/locate/envpol * Corresponding author. Tel.: +1-250-721-7118; fax: +1-250-721- 7120. E-mail address: dpo@uvic.ca (D.P. Ormrod).