Distribution of isoprenoid emitters in the Quercus genus around the world:chemo-taxonomical implications and evolutionary considerations based on the ecological function of the trait

Isoprenoid (isoprene and monoterpenes) emission is widespread among oak species. A number of studies investigating the emission by oak species around the world are here compiled. These studies indicate that isoprenoids are reliable taxonomical markers at subgenera level. Isoprene-emitters belong to the subgenera Lobateae, Quercus sensu strictu, and Protobalanus and are therefore found in every continent of the northern hemisphere. Non-emitters and monoterpene-emitters belong to Cerris and Ilex (= Schlerophyllodris) subgenus, respectively, and are particularly, if not solely, diffused in the Mediterranean area. The results suggest that the taxonomical assignment of some species should be reviewed. Experimental data indicate that isoprenoids have a protective role against environmental stresses and a strong antioxidant action. On the basis of the present knowledge I speculate that in oaks the ancestral trait is the emission of isoprene, and that the other traits evolved as adaptive response to the environment.