Morpho-anatomical responses of woody species to urban air pollution

Woody species respond to urban air pollution in different ways, according to the complexity of the considered level, spanning from whole plant responses to organ, tissue, and cell level responses. The whole response of woody species mainly includes growth inhibition, alteration in plant height and shoot growth, biomass reduction, and alteration in the root: shoot ratio, altered plant shape, altered reproduction capacity, as well as suspended flowering and fruiting. These alterations are attributable to those observed at the organ level, with a generally reported reduction in leaf surface, stem diameter, root length, and number and functionality of reproductive structures. Similarly, the observed variations observed in organ (mainly leaves) shapes and colors could be related to the alterations observed at the tissue and cell levels, mainly palisade and spongy tissues, as well as stomata-related properties (number and traits). Overall, more investigations, above all at the cellular level (cell wall and membrane alterations, such as thickening cell wall and lipid peroxidation, plasmolysis and cellcollapse) and intracellular level (disruption of vacuoles, chloroplasts, mitochondria, peroxisomes, abnormal nucleus shapes) are desirable, in order to know and prevent damages at the three levels and loss of ecosystem services associated with urban vegetation. It is also advisable more investigation about multiple influential parameters affecting several tree species to different levels of air pollution.