|I would like to know what is it, specifically, that makes some trees salt tolerant and air pollution tolerant.
|I can't say specifically what makes some plants more tolerant than others to different stresses. My hunch would be that it depends, in part, on the environement in which the plant evolved. Plants that evolved in a seaside environment developed mechanisms to allow them to tolerate salt--for example, they may have evolved certain characteristics in their cell membranes, so higher salt concentrations wouldn't affect the cell interior. (I'm just speculating on this.)
Other plants may have a waxy cuticle (thin covering on leaf surfaces) that they evolved to protect them from hot, dry conditions; this may also protect them from some environmental pollutants.
Also, plant breeders can select and breed plants that show resistant to certain stresses.