Morpho-anatomical and physiological responses of Robinia pseudoacacia L. plants to anthropogenic dust deposition in the Vesuvius National Park.

Anthropogenic impacts affect plants also in several protected areas of the Mediterranean basin. These impacts can also produce changes in vegetation dynamics. In Southern Italy, some areas of the Vesuvius National Park are particularly exposed to the impact of seasonal tourism. The frequent and intense presence of tourists, both hiking and moving by means of vehicles, is responsible for the formation of dust clouds which are mainly deposited on the vegetation close to the trails (Fig.1). Dust deposition on leaf lamina may negatively affect gas exchanges, compromising the plant health status. In this study, we analysed the effects of dust deposition on the leaf surface in Robinia pseudoacacia L., an invasive tree, native to north America, widespread in many areas of the Vesuvius National Park. Samplings were conducted in different sub-areas either located a) along the side of the roads, where a high deposition of dust (HD) was reported, or b) at least five meters far from the roads/trails, where plants receive a lower amount of dust (LD). For each site, the amount of dust on leaves was quantified in different periods of the year (springsummer-autumn), which have a different touristic pressure. On both HD and LD leaves, the effects of the dust deposition on anatomical and physiological traits were evaluated by performing eco-physiological measurements (e.g. chlorophyll fluorescence emission, photosynthetic pigment extraction and leaf functional traits) and light microscopy analysis (e.g. thickness of spongy and palisade tissues, incidence of intercellular spaces, stomatal size and density) (Fig. 2). Our results showed dust-induced changes in some of the anatomical and physiological traits, suggesting that dust deposition acted as a natural screen to protect the leaf surface especially in HD plantsIn HD plants, photochemistry was not affected by the presence of dust, being comparable to LD plants thus showing no sign of photoinhibition. HD plants also presented a higher amount of photosynthetic pigments, which could be an adaptation to the shielding phenomenon by which the plant increment chlorophylls and carotenoids in order to capture more light for photosynthesis especially during the summer, when the photosynthetic tissues are exposed to the excess of sunlight. These responses can be considered additional adaptive traits when the leaf heliotropic movements are not sufficient to avoid photo-inhibition. The leaf morphological and physiological plasticity of R. pseudoacacia in response to dust deposition is another critical trait which allows this species to be particularly competitive, and to spread in natural areas of the Vesuvius National Park subjected to anthropogenic impact.