No abstract

As part of the plant cuticle, epicuticular waxes build the boundary layer of a plant to its environment, fulfilling many vital functions. Epicuticular waxes are small crystalline structures which originate by self-assembly. The morphology of β-diketone tubules on Eucalyptus gunnii Hook.f. leaves was studied by field emission scanning electron microscopy (FE-SEM) and regeneration of removed waxes was investigated in real time by atomic force microscopy (AFM) on leaf surfaces. Smooth tubules as well as helically wound ribbons and transitional forms of tubules were found on adaxial leaf surfaces. Leaves of different developmental stages revealed no differences in their wax morphologies, but in the amount of wax allocation. After removal of the waxes regeneration was observed on leaves of all investigated ages. The regeneration of wax crystals started directly after wax removal and tubule growth could be observed in real time.

The cuticle with its superimposed epicuticular waxes represents the barrier of all aboveground parts of higher plant primary tissues. Epicuticular waxes have multiple effects on the interaction of plants with their living and non-living environment, whereby their shape, dimension, arrangement, and chemical composition play significant roles. Here, the ability of self-assembly of wax after isolation from the leaves was used to develop a small-scale wax-coated artificial leaf surface with the chemical composition and wettability of wheat (Triticum aestivum) leaves. By thermal evaporation of extracted plant waxes and adjustment of the evaporated wax amounts, the wettability and chemical character of the microstructure of the surface of wheat leaves were transferred onto a technical surface. For the use of these artificial leaves as a test system for biotic (e.g., germination of fungal pathogens) and non-biotic (e.g., applied surfactants) interactions on natural leaf surfaces, the chemical composition and the wetting behavior should be the same in both. Therefore, the morphology, chemistry, and wetting properties of natural and artificial surfaces with recrystallized wax structures were analyzed by scanning electron microscopy, gas chromatography–mass spectrometry, and by the determination of water contact angles, contact angle hysteresis, and tilting angles. Wheat leaves of different ages were covered exclusively with wax platelets. The extracted wheat wax was composed of alcohols, aldehydes, esters, and acids. The main component was 1-octacosanol. The waxes recrystallized as three-dimensional structures on the artificial surfaces. The three tested wetting parameters resembled the ones of the natural surface, providing an artificial surface with the chemical information of epicuticular waxes and the wetting properties of a natural leaf surface.

Eucalyptus trees and many plants from the grass family (Poaceae) and the heather family (Ericaceae) have a protective multifunctional wax coating on their surfaces made of branched ß-diketone tubules. ß-diketone tubules have a different size, shape, and chemical composition than the well-described nonacosanol tubules of the superhydrophobic leaves of lotus (Nelumbo nucifera). Until now the formation process of ß-diketone tubules is unknown. In this study, extracted wax of E. gunnii leaves and pure ß-diketone were recrystallized on two different artificial materials and analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) to study their formation process. Both the wax mixture and pure ß-diketone formed tubules similar to those on E. gunnii leaves. Deviating platelet-shaped and layered structures not found on leaves were also formed, especially on areas with high mass accumulation. High-resolution AFM images of recrystallized ß-diketone tubules are presented for the first time. The data showed that ß-diketone tubules are formed by self-assembly and confirmed that ß-diketone is the shape-determining component for this type of tubules.

Wax chemistry, especially long‐chain aldehydes, and surface wettability are discussed to have a stimulating effect on the development of Blumeria graminis, the pathogen of powdery mildew on wheat. Here, these specific surface properties are investigated on leaves of different wheat varieties and on developed test systems coated with plant wax and wax components. So far, the wettability of leaves has not been achieved by artificial substrates used in in vitro studies. With the test systems developed here, a wettability comparable to that on leaf surfaces and the signal character of wax chemistry could be investigated individually and in combination. The results show that wax morphology and chemistry (analyzed by scanning electron microscopy and gas chromatography) as well as wettability of leaves have no differences that would suggest a relationship with variety susceptibility. Furthermore, the influence of wax chemistry and wettability on the process of prepenetration of B. graminis on leaves and test systems is investigated. Germination and differentiation are not stimulated on any of the test systems compared to surfaces without the signals offered. Wettability, wax chemistry, and components such as long‐chain aldehydes could not be identified as decisive signals for the germination development of B. graminis.