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.
Highlights
Plant surfaces are covered by a waxy cuticle that is primarily composed of an outer cuticular wax film and an inner polymeric cutin, with the cuticular wax often well embedded within the extracellular cutin matrix [1]
atomic force microscopy (AFM) imaging was carried out upon model films formed from waxes of field-grown wheat straw extracted by supercritical CO2 (SCW) and of wheat straw grown in controlled laboratory conditions extracted via dissolution in chloroform (LGW), with representative images shown in figure 1a,b over a 20 mm2 region
Films formed from laboratory-grown wheat’ (LGW) and SCW waxes were characterized in air, under various moisture-controlled conditions and at the solid –liquid interface under three solvent contrasts; D2O, water contrast matched to silicon (CMSi) and water contrast matched to air (CMA)
Summary
Plant surfaces are covered by a waxy cuticle that is primarily composed of an outer cuticular wax film and an inner polymeric cutin, with the cuticular wax often well embedded within the extracellular cutin matrix [1]. A key target in adjuvant and plant science is to characterize the wax structure and transport properties within the cuticle This will provide us with an understanding of the influence of the environment and wax composition upon the plant’s interaction with adjuvants as they are deposited on a leaf surface. Neutron reflectivity has become a widely used technique over the last two decades in the field of surface chemistry and physics It has allowed the soft matter community to investigate the structural properties of adsorbed surfactants, lipids or proteins and it is being used to achieve a better structural determination of cuticular wax films formed upon the surface of plants [15,16,17,18]. 20.29Â1026 pave the way for the investigation of more complex nonhomogeneous structures found in nature
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