Ultrastructure, chemical composition, and recrystallization of epicuticular waxes: transversely ridged rodlets
Transversely ridged rodlets (Aristolochia-type) are of high systematic significance characterizing the ancestral Aristolochiales, Magnoliales, and Laurales. 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 microscopy, 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 lacks ketones and is dominated by alkanes. Hentriacontan-16-one (palmitone) was found to be characteristic for transversely ridged rodlets of Aristolochia, Laurus, and Paeonia. Standard solutions were taken for recrystallization experiments under different conditions of solvent, crystallization velocity, and temperature. It was shown that transversely ridged rodlets or related crystals grow from total waxes of all species but never crystallize from individual compounds such as alkanes or palmitone. We concluded that transversely ridged crystals are formed by self-assembly based on a slow crystallization process and the presence of additives. This paper shows that transversely ridged rodlets occur convergently within angiosperms based on a similar mode of crystallization but a different chemical composition. The role of palmitone as a chemotaxonomic character of ancestral angiosperms is discussed.Key words: plant cuticle, epicuticular waxes, chemistry, ultrastructure, recrystallization, systematics.