Interspecific variations in the surface wettability and morphological traits of petals across 125 plant species

Abstract

Petal wettability is an important characteristic of flowering plants, as excess surface water on a petal can cause color loss and collapse of shape. However, few studies have examined natural variations in the wettability and the related morphological traits of petals. The present study investigated these characteristics across 125 species from a warm temperate habitat (Tokyo, Japan) by evaluating the contact angles (CAs) of water droplets on the petal epidermis as an index of surface wettability and cell curvature (= cell height/width) and the presence of epicellular structures, such as cuticular folds and trichomes as an index of the petal surface roughness. We found that more than half of the species possessed convex epidermal cells with epicellular structures. In addition, the majority of species had petals with low wettability (CAs varying between 90 and 140°), and only 2.4% and 12% of species possessed super-hydrophobic and hydrophilic petals, respectively. This natural distribution of petal wettability was less variable than that of leaves. Among the less wettable petals (CA > 90°), there was also a significant correlation between cell curvature and CA, and this relationship differed between the adaxial and abaxial sides. To assess the ecological importance of petal wettability, its relationships with life form and flower direction were examined. This showed that herbaceous plants have more convex epidermal cells than woody plants, but no significant difference was detected in their CAs. Besides, among upward-facing flowers, the adaxial side had a more convex and less wettable epidermis compared to the abaxial side, which may protect sky-facing adaxial epidermis from wetting by raindrops. However, there was no significant difference in these features between the adaxial and abaxial sides of sideward- and downward-facing flowers. The less clear tendency of petal wettability was attributable to multiple functions of the petal surface.