The cuticle micromorphology of extant and fossil plants as indicator of environmental conditions. A pioneer study on the influence of volcanic gases on the cuticle structure in extant plants

The leaves of many tracheophytes are covered with a cuticle, an extracellular membrane covering aerial organs of plants. The gas exchanges between the plant and the surrounding atmosphere are mediated by the cuticle; its acts as the main barrier to air pollutants. The study of the plant cuticle, in particular the stomatal apparatuses of conifer, is largely used as a tool analysis revealing ecological and paleoecological features. It is worth noting that little is known about the long-term response of micromorphology of natural vegetation to volcanic toxic gases. Fortunately, Campania Region with its numerous volcanic localities (Pisciarelli, Solfatara, complexe du Somma-Và©suve) represents a natural laboratory allowing experiments involving plant-volcano interactions. The object of this research is to study the conifer and angiosperms potentialities (extant and fossil) as ecological indicators useful in the identification of the environmental parameters variations. That is why, macroscopical and microscopical observations in vascular plants in relation to various environmental factors (volcanic gases, light intensity, water availability and salinity), have been analysed. A number of localities have been sampled and SEM, TEM and EDS equipments have been used together with statistic. Observations made on extant plants allowed for the first time, the study of the effects of volcanic gases on the cuticle ultrastructure of Pinus halepensis [Aleppo pine; Pisciarelli (fumigated) and Cigliano (not fumigated) localities] and Erica arborea [tree heather; Solfatara, Pisciarelli (fumigated) and Cigliano (not fumigated) localities]. TEM observations on P. halepensis cuticles fumigated or not by volcanic gases revealed insignificant thickness variations of the cell wall plus cuticle among current- and first-yearold needles of both fumigated and not fumigated trees. In particular, the needle cuticles experiencing chronic fumigation display (TEM) a calcium oxalate accumulation. Moreover, in respect to the cell surface, fibrils are parallel disposed. SEM and TEM observations allowed an identification key enabling distinction between not fumigated and fumigated material with 9 characters, providing a good tool detecting the influence of volcanism for extant and fossil plants. In specimens of E. arborea fumigated or not by volcanic gases, the total thickness of cuticles varies significantly. In plant experiencing chronic fumigation the A2 layer records an increase of its thickness. Within three localities, a good correlation between the atmospheric CO2 concentration and the thickness variation of A2 layer has been found. The cuticle micromorphology of extant and fossil plants as indicator of environmental conditions. A pioneer study on the influence of volcanic gases on the cuticle structure in extant plants. This fact confirms that the cuticle is the main mediator between the plant and the atmosphere. As for fossil plants, the cuticles of Cretaceous Fossil-Lagertà¤tten of Cusano Mutri (Late Aptian) and Pietraroja (Lower Albian) have been studied. In the former: 1) numerous taxa belonging to conifers have been identified; 2) the new species Frenelopsis cusanensis Bartiromo et al. bearing xeromorphic features has been described; 3) the occurrence of Montsechia vidalii is recorded for the first time outside of Spain. Taxonomical studies carried out on Cretaceous cuticles from Cusano Mutri and Pietraroja allowed the description of typical Euro-Sinian fossil plants. Sedimentological and taxonomical studies suggest semi-arid or arid conditions in a subtropical or tropical climate. It is worth noting as for Cusano Mutri locality, evidence of wildfire (fusain) suggests a periodic combination of arid periods, high temperatures and lightning strikes. This study (at least for extant plants) can be considered pioneering, because, for the first time, the relationships between cuticle ultrastructure variations and volcanic gases have been studied.