Wax Layers on Cosmos bipinnatus Petals Contribute Unequally to Total Petal Water Resistance

Cuticular waxes coat all primary aboveground plant organs as a crucial adaptation to life on land. Accordingly, the properties of waxes have been studied in much detail, albeit with a strong focus on leaf and fruit waxes. Flowers have life histories and functions largely different from those of other organs, and it remains to be seen whether flower waxes have compositions and physiological properties differing from those on other organs. This work provides a detailed characterization of the petal waxes, using Cosmos bipinnatus as a model, and compares them with leaf and stem waxes. The abaxial petal surface is relatively flat, whereas the adaxial side consists of conical epidermis cells, rendering it approximately 3.8 times larger than the projected petal area. The petal wax was found to contain unusually high concentrations of C₂₂ and C₂₄ fatty acids and primary alcohols, much shorter than those in leaf and stem waxes. Detailed analyses revealed distinct differences between waxes on the adaxial and abaxial petal sides and between epicuticular and intracuticular waxes. Transpiration resistances equaled 3 × 10⁴ and 1.5 × 10⁴ s m⁻¹ for the adaxial and abaxial surfaces, respectively. Petal surfaces of C. bipinnatus thus impose relatively weak water transport barriers compared with typical leaf cuticles. Approximately two-thirds of the abaxial surface water barrier was found to reside in the epicuticular wax layer of the petal and only one-third in the intracuticular wax. Altogether, the flower waxes of this species had properties greatly differing from those on vegetative organs.The flowers of many plants are especially adapted to ensure reproductive success by attracting, orienting, and assisting pollinators. Petals must also resist unfavorable environmental conditions such as a desiccating atmosphere. Some characteristics that increase reproductive success, including their high surface areas and surface permeability to small scent molecules, may also make petals more vulnerable to drying out (Goodwin et al., 2003; Bergougnoux et al., 2007). Thus, despite their ephemeral nature, petals may need to compromise between competing physiological and ecological functions. This raises questions: How effective are petal skins at blocking water? Do petal skin compositions differ from those on other plant parts in order to balance multiple functions?To answer these questions, both the chemical composition and the transpiration barrier properties of petal skins must be determined. It is well established that petals are covered by cuticles comparable to those on vegetative organs (Whitney et al., 2011). The waxes coating all primary parts of shoots consist of very-long-chain compounds, including alkanes, aldehydes, primary and secondary alcohols, fatty acids, esters, and ketones ranging in chain length from 20 to 70 carbons (Jetter et al., 2007). The ratio between these derivatives varies temporally and spatially between organs and layers within the cuticle (Jenks et al., 1995, 1996; Jetter and Schäffer, 2001). As well, wax may contain cyclic compounds such as pentacyclic triterpenoids (Buschhaus and Jetter, 2011). Even though it has long been known that the waxes, rather than the accompanying cutin polymer, are essential for the cuticular transpiration barrier (Schönherr, 1976), it is currently not clear how individual wax components contribute to this physiological function.In contrast to other organs, relatively few studies so far have addressed the chemical composition of petal waxes. Noteworthy exceptions are detailed analyses of petal waxes for Crataegus monogyna and three cultivars of Rubus idaeus (Griffiths et al., 2000), Antirrhinum majus (Goodwin et al., 2003), Vicia faba (Griffiths et al., 1999), Cistus albidus (Hennig et al., 1988), Petunia hybrida (King et al., 2007), Arabidopsis (Arabidopsis thaliana; Shi et al., 2011), and Rosa damascena (Stoianova-Ivanova et al., 1971). Selected compound classes have been investigated for some more species, including selected Ericaceae (Salasoo, 1989), Rosaceae (Wollrab, 1969a, 1969b), and Asteraceae (Akihisa et al., 1998) species. Some major plant families, such as the Asteraceae, have not been investigated in much detail.Along with chemical analyses, the physiological properties of waxes on fruits and leaves of diverse plant species also have been investigated in the past. The effectiveness of a water barrier may be characterized by quantifying the permeance for water (P; m s⁻¹) or, inversely, the transpiration resistance (s m⁻¹; Riederer and Schreiber, 1995). These characteristics may, in turn, be determined by measuring the water flux (J; kg m⁻² s⁻¹) across the cuticle under controlled conditions according to the equation P = J/Δc (where Δc is the water concentration gradient driving the diffusion across the barrier). Because both permeance and resistance are physiological characteristics independent of water concentration, their values enable comparisons between water barriers of different plant species and organs. Water permeance values and the corresponding barrier effectiveness vary widely between plant species and organs, with a range of 0.36 to 200 × 10⁻⁶ m s⁻¹ (Kerstiens, 1996; Schreiber and Riederer, 1996). The mean and median leaf permeances (1.42 × 10⁻⁵ and 0.58 × 10⁻⁵ m s⁻¹, respectively) were lower than those of fruit (9.93 × 10⁻⁵ and 9.46 × 10⁻⁵ m s⁻¹), leading to the conclusion that leaves typically produce a better barrier against water movement than does fruit (Kerstiens, 1996). This difference in the physiological performance of waxes on different organs raises the question of how effective the transpiration barrier of cuticular waxes on petals may be. However, to date, water permeance values for petals have not been published and thus cannot be compared with those for other organs.To fill important gaps in our understanding of cuticle function and composition, we initiated a detailed analysis of petal waxes using Cosmos bipinnatus as a first model. We recently reported the identification of novel compounds from the C. bipinnatus petal waxes (Buschhaus et al., 2013) but not the overall wax composition of the petal waxes. Therefore, the ray flowers of this species were examined here to determine (1) the wax composition on the adaxial and abaxial petal surfaces in comparison with the stem and leaf wax and (2) the corresponding petal water permeances.