Surface microstructures of daisy florets (Asteraceae) and characterization of their anisotropic wetting

Abstract

The surface microstructures on ray florets of 62 species were characterized and compared with modern phylogenetic data of species affiliation in Asteraceae to determine sculptural patterns and their occurrence in the tribes of Asteraceae. Their wettability was studied to identify structural-induced droplet adhesion, which can be used for the development of artificial surfaces for water harvesting and passive surface water transport. The wettability was characterized by contact angle (CA) and tilt angle measurements, performed on fresh ray florets and their epoxy resin replica. The CAs on ray florets varied between 104° and 156°, but water droplets did not roll off when surface was tilted at 90°. Elongated cell structures and cuticle folding orientated in the same direction as the cell elongation caused capillary forces, leading to anisotropic wetting, with extension of water droplets along the length axis of epidermis cells. The strongest elongation of the droplets was also supported by a parallel, cell-overlapping cuticle striation. In artificial surfaces made of epoxy replica of ray florets, this effect was enhanced. The distribution of the identified four structural types exhibits a strong phylogenetic signal and allows the inference of an evolutionary trend in the modification of floret epidermal cells.

Cuticular Patterns on Stylar Hairs in Asteraceae: a New Micromorphological Feature

“…The number of stylar cuticular patterns is higher than that of petal cuticular patterns since the stylar hairs in contrast to petal cells can exhibit a combination of longitudinal and transverse cuticular striae. A crested pattern, which on the stylar hairs is confined to the Onoserideae and Mutisieae, is found on the petals in Mutisieae as well as in Cichorieae, Arctotideae, Calenduleae, and Inuleae (Baagøe 1977a(Baagøe , 1978Koch et al 2013). It is noteworthy that in Gazania (Arctotideae, fig.…”

“…The cuticular pattern of only longitudinal striae occurs in Senecioneae, Calenduleae, Astereae, Helenieae, Coreopsideae, Heliantheae, and Madieae on the stylar hairs ( fig. 5) as well as on the petal surfaces (Baagøe 1977a(Baagøe , 1977b(Baagøe , 1978Koch et al 2013). However, in the tribes Senecioneae, Calenduleae, and Astereae, the pattern of transverse striae on the petal surfaces (Baagøe 1978;Koch et al 2013) is not found on the stylar hairs.…”

“…5) as well as on the petal surfaces (Baagøe 1977a(Baagøe , 1977b(Baagøe , 1978Koch et al 2013). However, in the tribes Senecioneae, Calenduleae, and Astereae, the pattern of transverse striae on the petal surfaces (Baagøe 1978;Koch et al 2013) is not found on the stylar hairs. In contrast, Cichorieae and Vernonieae are characterized by the cuticular pattern of transverse striae on the stylar hairs ( fig.…”

“…Since the petal cuticular patterns have been described differently with genus names (Baagøe 1977a(Baagøe , 1977b(Baagøe , 1978Koch et al 2013), we prefer structural descriptions of the stylar hair cuticular pattern investigated by us to avoid any confusion. As characteristic in the definitions and line drawings ( fig.…”

“…Baagøe (1977aBaagøe ( , 1977bBaagøe ( , 1978; see also Hansen 1991) found three main types of cuticular structures on the adaxial petal surfaces in Asteraceae (helianthoid, mutisioid, senecionoid type), whereas Koch et al (2013) described four types of surface structures of petals in Asteraceae (Bellis, Calendula, Chrysanthemum, Gazania type). Apart from the Mutisieae (s.l.…”