title: A theoretical approach to the relationship between wettability and surface microstructures of epidermal cells and structured cuticles of flower petals.
abstract: OBJECTIVE
  The epidermal surface of a flower petal is composed of convex cells covered with a structured cuticle, and the roughness of the surface is related to the wettability of the petal. If the surface remains wet for an excessive amount of time the attractiveness of the petal to floral visitors may be impaired, and adhesion of pathogens may be promoted. However, it remains unclear how the epidermal cells and structured cuticle contribute to surface wettability of a petal.
  
  
  METHODS
  By considering the additive effects of the epidermal cells and structured cuticle on petal wettability, a thermodynamic model was developed to predict the wetting mode and contact angle of a water droplet at a minimum free energy. Quantitative relationships between petal wettability and the geometries of the epidermal cells and the structured cuticle were then estimated. Measurements of contact angles and anatomical traits of petals were made on seven herbaceous species commonly found in alpine habitats in eastern Nepal, and the measured wettability values were compared with those predicted by the model using the measured geometries of the epidermal cells and structured cuticles.
  
  
  RESULTS
  The model indicated that surface wettability depends on the height and interval between cuticular steps, and on a height-to-width ratio for epidermal cells if a thick hydrophobic cuticle layer covers the surface. For a petal epidermis consisting of lenticular cells, a repellent surface results when the cuticular step height is greater than 0·85 µm and the height-to-width ratio of the epidermal cells is greater than 0·3. For an epidermis consisting of papillate cells, a height-to-width ratio of greater than 1·1 produces a repellent surface. In contrast, if the surface is covered with a thin cuticle layer, the petal is highly wettable (hydrophilic) irrespective of the roughness of the surface. These predictions were supported by the measurements of petal wettability made on flowers of alpine species.
  
  
  CONCLUSIONS
  The results indicate that surface roughness caused by epidermal cells and a structured cuticle produces a wide range of petal wettability, and that this can be successfully modelled using a thermodynamic approach.
authors: Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo, 113-0033, Japan, National Herbarium and Plant Laboratories, Department of Plant Resources, Kathomandu, Nepal and The University Museum, The University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo, 113-0033, Japan taneda@bs.s.u-tokyo.ac.jp.
authors: Haruhiko
authors: H
authors: Taneda
authors: Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo, 113-0033, Japan, National Herbarium and Plant Laboratories, Department of Plant Resources, Kathomandu, Nepal and The University Museum, The University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo, 113-0033, Japan.
authors: Ayako
authors: A
authors: Watanabe-Taneda
authors: Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo, 113-0033, Japan, National Herbarium and Plant Laboratories, Department of Plant Resources, Kathomandu, Nepal and The University Museum, The University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo, 113-0033, Japan.
authors: Rita
authors: R
authors: Chhetry
authors: Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo, 113-0033, Japan, National Herbarium and Plant Laboratories, Department of Plant Resources, Kathomandu, Nepal and The University Museum, The University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo, 113-0033, Japan.
authors: Hiroshi
authors: H
authors: Ikeda
mesh_terms: D017753:Ecosystem; D035264:Flowers / Q000166:cytology* / Q000648:ultrastructure; D008962:Models, Theoretical; D019441:Plant Epidermis / Q000166:cytology* / Q000648:ultrastructure; D013816:Thermodynamics; D014867:Water; D017155:Wettability
chemical_list: D014867:Water
keywords: Cassie–Baxter wetting; Epidermis; Wenzel wetting; alpine plant; cuticle structure; lotus effect; petal effect; petal wettability; thermodynamic model; water run-off