A time-concentration study on the effects of ozone on spring wheat (Triticum aestivum L.). 3: Effects on leaf area and flag leaf senescence

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-filtered treatments were used in addition to fumigated treatments in which additional ozone was added to the non-filtered 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 flag 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 filtered 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. (C) 1998 Elsevier Science B.V. All rights reserved.