A time-concentration study on the effects of ozone on spring wheat (Triticum aestivum L.). 3: Effects on leaf area and ¯ag leaf senescence J.M. Finnan b,* , M.B. Jones a , J.I. Burke b a Department of Botany, Trinity College, Dublin 2, Ireland b Teagasc, Oak Park Research Centre, Carlow, Ireland Received 20 May 1997; accepted 29 December 1997 Abstract Spring sown wheat (Triticum aestivum L. cv. `Promessa') was exposed to different concentrations of ozone in open-top chambers over a three year period at a site in the south-east of Ireland. Filtered and non-®ltered treatments were used in addition to fumigated treatments in which additional ozone was added to the non-®ltered air. Ozone was added either as short term high concentration exposures or as long term low concentration exposures. In 1991, exposure to the A50 (Ambient50 ppb ozone) treatment resulted in a substantial reduction in the green leaf area index and enhanced both the onset and the rate of leaf yellowing. The A50 treatment in 1993 also resulted in a reduction in green leaf area in addition to accelerating chlorophyll and soluble protein loss in ¯ag leaves. This treatment caused a more gradual loss of chlorophyll and green leaf area in comparison to the same treatment in 1991 which received a greater amount of potentially damaging ozone. In contrast the A25 treatment in 1992 delayed leaf yellowing and enhanced green leaf retention in comparison to the ®ltered control treatment. This treatment received the same total cumulative ozone exposure as the A50 treatment in 1991 but was supplied as a lower concentration over a longer time interval. High concentrations applied over short time intervals proved more phytotoxic than identical exposures in which lower concentrations were applied over longer time intervals. Changes in leaf senescence and in leaf area index induced by ozone were closely paralleled by changes in the grain yield. The results suggest that changes in grain yield are the result of ozone induced alterations in leaf senescence which lead to changes in the leaf area available for photosynthate production. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Ozone; Spring Wheat; Senescence; Chlorosis; Soluble Protein; Leaf area 1. Introduction Levels of ozone, a photochemical oxidant, in the troposphere have more than doubled in the past one hundred years and are predicted to continue rising at an even faster rate in the future (Hough and Derwent, 1990). It has been demonstrated that ambient levels of ozone over large areas of the United States are high enough to reduce the yield of ®eld grown crops (Heck et al., 1983). Reductions in the yield of winter wheat crops from ambient ozone have been observed in Agriculture, Ecosystems and Environment 69 (1998) 27±35 *Corresponding author. Tel.: +353 503 31425; fax: +353 503 42423. 0167-8809/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0167-8809(98)00088-7