Responsiveness of Viburnum lantana L. to tropospheric ozone: field evidence under contrasting site conditions in Trentino, northern Italy.

Specific visible foliar injuries were demonstrated to occur on plants of Viburnum lantana L. (wayfaring tree) when exposed to ozone in open-top chamber experiments. However, although evidence of visible injury was reported even for native plants, no comprehensive testing has been carried out under real field conditions. Thus, the extent to which V. lantana may match the requirements to be used as an in situ bioindicator is not fully known. To investigate the actual responsiveness of native V. lantana plants to ozone under field condition, two 1 × 1 km quadrates (named "Margone" and "Lasino"), for which the occurrence of different ozone levels was known, were considered. There, a fully randomized design was adopted to ensure within-quadrate replications and to select V. lantana plants. Measurements confirmed different exposure levels (Margone, Accumulated ozone concentrations Over a Threshold of 40 ppb h (AOT40): 31 952 ppb h; Lasino, AOT40: 23 259 ppb h). Ozone visible foliar symptoms (i) matched the known symptomatology, (ii) were easy to be identified, (iii) confirmed by microscopical validation, and (iv) observed at both quadrates. However, higher frequency of symptoms, earlier date of onset and faster development occurred at the quadrate with the highest ozone exposure (Margone), although not always proportionally with the difference in ozone exposure. This may be partly due to inherent covariation of environmental variables (higher ozone exposure occurred at the sites with higher relative humidity and cooler air temperature, a set of conditions that may promote ozone uptake), and partly due to a set of (unmeasured) other factors that may cause additional oxidative stress to plants. Implications for biomonitoring are discussed.