Foliar Injuries Caused by Ozone in Betula pubescens Ehrh. and Phleum pratense L. as Influenced by Climatic Conditions Before and During O3 Exposure
Seedlings of Betula pubescens Ehrh. (mountain birch) and Phleum pratense L. (timothy) were grown for 42 days under full light or 50% shade in the field at 12°C, and at comparable photosynthetic active radiation (PAR) levels in a greenhouse at 18°C. Plants from the four pretreatments were exposed to 78 nmol mol-1 (ppb) O3 (8 h day-1) under two temperatures (15 and 25°C), two relative air humidities (50 and 80% RH) or two CO2 concentrations (400 and 750 mol mol-1) during 7 days. The accumulated O3 dose over 40 nmol mol-1 O3 (AOT40) was 2.6 mol mol-1-hours (ppm-h). Decreasing the temperature during exposure significantly increased the amount of injury induced by O3 in leaves of birch (yellow mottling/bronzing) as well as timothy (chlorosis/necrosis). Increasing the air humidity or decreasing the CO2 concentration strongly enhanced the injuries caused by O3 in timothy, but not in birch. In general, both birch and timothy plants grown in the greenhouse and in the field had the same O3 sensitivity. However, decreasing the PAR level during the pretreatment enhanced leaf injury in birch but not in timothy. At the most sensitive exposure climate, 15°C/80% RH, leaf injury developed at an AOT40 of 0.7-0.9 ppm-h in both species.