Ultrastructure, chemical composition, and recrystallization of epicuticular waxes: transversely ridged rodlets

Transversely ridged rodlets (Aristolochia-type) are of high systematic sign ificance characterizing the ancestral Aristolochiales, Magnoliales, and Lau rales. Sporadically, they also occur in various unrelated derived taxa. The ultrastructure, chemistry, and recrystallization of epicuticular waxes of nine species were investigated by high resolution scanning electron microsc opy, gas chromatography, and mass spectrometry. Chemical analyses show that transversely ridged rodlets clearly differ in their composition. Waxes of one group are characterized by ketones, whereas a second group completely l acks ketones and is dominated by alkanes. Hentriacontan-16-one (palmitone) was found to be characteristic for transversely ridged rodlets of Aristoloc hia, Laurus, and Paeonia. Standard solutions were taken for recrystallizati on experiments under different conditions of solvent, crystallization veloc ity, and temperature. It was shown that transversely ridged rodlets or rela ted crystals grow from total waxes of all species but never crystallize fro m individual compounds such as alkanes orpalmitone. We concluded that trans versely ridged crystals are formed by self-assembly based on a slow crystal lization process and the presence of additives. This paper shows that trans versely ridged rodlets occur convergently within angiosperms based on a sim ilar mode of crystallization but a different chemical composition. The role of palmitone as a chemotaxonomic character of ancestral angiosperms is dis cussed.