Structural features of reconstituted wheat wax films

Abstract

Cuticular waxes are essential for the well-being of all plants, from controlling the transport of water and nutrients across the plant surface to protecting them against external environmental attacks. Despite their significance, our current understanding regarding the structure and function of the wax film is limited. In this work, we have formed representative reconstituted wax film models of controlled thicknesses that facilitated an ex vivo study of plant cuticular wax film properties by neutron reflection (NR). Triticum aestivum L. (wheat) waxes were extracted from two different wheat straw samples, using two distinct extraction methods. Waxes extracted from harvested field-grown wheat straw using supercritical CO2 are compared with waxes extracted from laboratory-grown wheat straw via wax dissolution by chloroform rinsing. Wax films were produced by spin-coating the two extracts onto silicon substrates. Atomic force microscopy and cryo-scanning electron microscopy imaging revealed that the two reconstituted wax film models are ultrathin and porous with characteristic nanoscale extrusions on the outer surface, mimicking the structure of epicuticular waxes found upon adaxial wheat leaf surfaces. On the basis of solid–liquid and solid–air NR and ellipsometric measurements, these wax films could be modelled into two representative layers, with the diffuse underlying layer fitted with thicknesses ranging from approximately 65 to 70 Å, whereas the surface extrusion region reached heights exceeding 200 Å. Moisture-controlled NR measurements indicated that water penetrated extensively into the wax films measured under saturated humidity and under water, causing them to hydrate and swell significantly. These studies have thus provided a useful structural basis that underlies the function of the epicuticular waxes in controlling the water transport of crops.

Structural Features of Reconstituted Cuticular Wax Films upon Interaction with Nonionic Surfactant C12E6

“…The process was programmed to run for 20 s at 3000 rpm, with an initial acceleration of 1000 rpm for 3 s. This coating procedure for forming reconstituted wax films was discussed in the previous study and was found to generate reproducible uniform films within the thickness range of cuticular wax films found in cereal crops. 22 For SE measurements, Si wafers (2 cm × 2 cm) were spin coated in the fashion similar to the Si blocks used for NR, with 200 μl of solution deposited onto the surface prior to coating.…”

“…[35][36][37][38] The reflectivity profile which represents the structural distribution of an interfacial material is described by the Abeles layer model through a discrete series of homogeneous slabs each characterized by their SLD and thickness. 28,30 In this study, the general solution to the optical matrix used to describe any standard stratified layer 22,23 was modified to more accurately describe the observed interfacial roughness and density variations of the reconstituted cuticular wax films. It has been shown that the material's volume fraction and SLD distribution profiles could be adequately described using a 2-layer model based on the error function model developed by Hughes.…”

“…Further details of this model and definitions of the parameters used are described in the previous study of the reconstituted wax films 22 , and in the Electronic Supplementary Material.…”

“…Using the aforementioned Hughes error function model 22,38 , the wax film coverage, distribution and the extent of water penetration were obtained from the best-fitted density profiles. Using…”

“…We speculate that structural features such as porosity and defects as often observed in wax films must play a crucial role in regulating transport through the plant surface barriers. 15,20 Following the previous success in the development of reconstituted wax films 22 , this work aims to continue the theme of neutron reflection-led studies by examining how surfactants interact with the wax films, thereby providing useful indications of the role played by surfactants. Whilst NR provides reliable information about the structure and composition of the interface before and after surfactant binding, spectroscopic ellipsometry (SE) offers a real time, in-situ observation of the dynamic process.…”