Risposte morfologiche, fisiologiche e geniche all’ozono della specie arbustiva Viburnum lantana L.

Because its oxidative power, tropospheric ozone is considered, at the large scale, the most harmful pollutant to vegetation. The exceedances of critical levels set to protect vegetation are quite high and widespread, so that large parts of crops and forests in Europe are exposed to potentially harmful levels of ozone. The impact of ozone on vegetation is the result of multiple factors such as the concentration in the atmosphere, the stomatal uptake - which depends on environmental and physiological factors -, and the detoxification potential of plants. The complexity of these factors and their interactions can make it difficult to establish a clear relationship between ozone and plant response under field conditions. The use of plants as bioindicators may be a solution because they reflect and summarize all processes that occur between ozone exposure and the response of the plant. The aim of this research is to explore the potential of the shrub species Viburnum lantana L. as a bioindicator in situ to assess the potential effects of ozone on native vegetation. This species is known to be sensitive to ozone, has a specific response (visible foliar injuries, consisting in red stipples on the upper leaf surface ), and a wide spatial distribution. However it is not fully known if V. lantana meets all the requirements to be used as a bioindicator. In particular, the actual responsiveness to ozone of native plants and the relationship between the intensity of responses and the levels of exposure to the pollutant under field conditions remain to be evaluated. For these purposes, two field studies were carried out in the Province of Trento (North Italy) at local (1) and large scale (2). Moreover, a study under controlled conditions (3) was carried out in order to deepen the knowledge about the effects of ozone on V. lantana at biochemical, physiological and genetic level. (1) The first study was carried out in 2009 and aimed at assessing the time course of plant responses to ozone during the growing season. To this, two 1x1 km quadrates were considered. The two quadrates, located 3 km apart, were characterized by different levels of ozone. The adoption of a completely randomized experimental design ensured replication in each area and the selection of plants. Plants were monitored for the development of ozone-specific foliar symptoms, the chlorophyll content (SPAD) and the fluorescence of chlorophyll a during the entire growing season. (2) The second study was carried out in 2010 and aimed at verifying the response of V. lantana to different levels of ozone exposure. To this end, a stratified random sampling design (elevation x ozone) was adopted to select 30 1x1 km quadrates. On all quadrates, the assessment of symptomatic plants was carried out. For this second field study, the spatial domain was the entire surface of the province of Trento (6.200 km2). (3) For the study in controlled environment, 9 potted plants of V. lantana were subjected to fumigation with known concentrations of ozone (60 ppb for 45 days, 5 hours per day) (treated), while other 9 plants were maintained under the same environmental conditions with the exception of ozone (control). Plants were analyzed for the presence and development of foliar symptoms, chlorophyll content (SPAD), fluorescence of chlorophyll a, leaf content of photosynthetic pigments and carbohydrate (HPLC) and gene expression. Overall, the results of the two field studies allowed to verify (1) a temporal development of the responses of V. lantana consistent with the trend of ozone exposure; and (2) an higher frequency of symptomatic plants where ozone levels were also higher. However, the frequency of symptoms was not always proportionate to the level of ozone exposure. At the same time of the onset and spread of foliar symptoms, a decrease in the foliar chlorophyll content and in photosynthetic performance occurred. The analysis of the fluorescence transient of chlorophyll a showed an early response to ozone for the parameter ΔVI-P, that indicates the ability of the final electron acceptor to be reduced. Interestingly, when comparing similar ranges of ozone exposures, symptomatic plants were always more frequent at higher altitudes (above 700 m a.s.l.): this may suggests that they are subjected to an additional oxidative stress (e.g. due to solar radiation), and/or that environmental conditions are more favourable to ozone uptake (high relative humidity and relative lower temperature). The physiological and metabolic analysis carried out on plants treated with ozone, confirmed their reduced photosynthetic capacity and their lower content of chlorophyll, as well as a not completely effective system to protect plants against photo-inhibition. This behavior is probably the cause of the sensitivity of this species in relation to the ozone. V. lantana - whose sensitivity and specificity of response to ozone were verified also in real field conditions - seems suitable as a bioindicator in situ to qualitatively assess the potential impact of ozone, for large-scale surveys and in remote areas. Foliar symptoms on this species are also confirmed as valid response indicators of ozone, although their interpretation in terms of potential damage to vegetation always requires great caution.