Effect of tropospheric ozone on photosynthetic activity of ozone-resistant and ozone-sensitive test plants (Nicotiana tabacum L., Phaseolus vulgaris L.)

Ozone-sensitive and -resis- tant tobacco plants and an ozone-sensitive bean were employed in this experiment. Plants were exposed at two different sites varying in ozone level, within Poznan city and in a remote forestry area. Net photosynthetic rate (P N ), stomatal con- ductance (g s ) and intercellular CO 2 concentration (C i ) were measured every 7 th day until the 28 th day of the experiment. The ozone concentrations were higher at the forest site (called here the high ozone site) than in the city (low ozone site). Mean hour ozone concentrations at forest site varied between 34.2-45.5 ppb and 30.2-39.2 ppb, while cumulative ozone concentrations (AOT 40) were 2032 ppb h -1 and 611, respectively. The aim of the study was to compare the results of exposed ozone-sensitive and resistant test plants accord- ing to the potential effect of tropospheric ozone on natural vegetation. The experiment revealed the variability between exposure sites and plant species, as well as changes of photosynthetic pa- rameters during the whole experimental season. Common bean assimilation parameters revealed the best correlation with the tropospheric ozone level. Tobacco test plants - ozone-sensitive and -resistant - varied in response to stress factors. There were no statistical differences between ex- posure sites and plant response. However, both ozone-sensitive and -resistant tobacco plants re- vealed lower levels of PN at the low ozone site, while ozone-sensitive bean revealed the opposite relation. Moreover, plants revealed gas exchange relations which were not previously observed in fumigation experiments (such as small differenc- es in Ci levels of tobacco plant in the first three weeks). This might be a result of relatively low levels of tropospheric ozone during the experi- mental period. Hence, it is suggested to continue ambient air investigations to confirm or reject the results of this experiment.