A Double-Edged Sword of Surfactant Effect on Hydrophobic Surface Broccoli Leaf as a Model Plant: Promotion of Pathogenic Microbial Contamination and Improvement to Disinfection Efficiency of Ozonated Water

Pathogenic microbial contamination is significantly influenced by the crop surface properties and surfactant use, which are crucial factors for the postharvest washing process. However, there is little information on the interaction between surfactant and food pathogens on food crops. Thus, this study (1) investigated whether the attachment of Salmonella increases as pesticides denature epicuticular wax crystals and (2) tested if the antibacterial effect of ozonated water can be improved on waxy produce surfaces by adding surfactant to ozonated water. As a result, significantly lower levels of Salmonella Typhimurium attached to waxy leaf surfaces than they did to glossy and pesticide-treated waxy leaf surfaces (3.28 as opposed to 4.10 and 4.32 Log colony forming units (CFU)/cm2, respectively), suggesting that the pesticide containing a surfactant application increased the attachment of S. Typhiumurium on waxy leaf surfaces. There was no significant washing effect on waxy leaf surfaces washed with ozonated water. On the other hand, S. Typhimurium were not detected on waxy leaf surfaces after washing with surfactant-added ozonated water.

Developmental pattern of grapevine (Vitis vinifera L.) berry cuticular wax: Differentiation between epicuticular crystals and underlying wax

The grapevine berry surface is covered by a cuticle consisting of cutin and various lipophilic wax compounds. The latter build the main barrier for transpirational water loss and protect the fruit against environmental factors e.g. pests, mechanical impacts or radiation. The integrety of the fruit surface is one important key factor for post-harvest quality and storage of fruits. Nonetheless, the developmental pattern of cuticular wax was so far only investigated for a very limited number of fruits. Therefore, we performed comparative investigations on the compositional and morphological nature of epicuticular wax crystals and underlying wax during fruit development in Vitis vinifera. The main compound oleanolic acid belongs to the pentacyclic triterpenoids, which occur very early in the development in high amounts inside the cuticle. The amount increases until veraison and decreases further during ripening. In general, very-long chain aliphatic (VLCA) compounds are present in much smaller amounts and alcohols and aldehydes follow the same trend during development. In contrast, the amount of fatty acids constantly increases from fruit set to ripening while wax esters only occur in significant amount at veraison and increase further. Wax crystals at the fruit surface are solely composed of VLCAs and the morphology changes during development according to the compositional changes of the VLCA wax compounds. The remarkable compositional differences between epicuticular wax crystals and the underlying wax are important to understand in terms of studying grape-pest interactions or the influence of environmental factors, since only wax crystals directly face the environment.

Slippery flowers as a mechanism of defence against nectar-thieving ants

Background and AimsThe great diversity of floral characters among animal-pollinated plants is commonly understood as the result of coevolutionary interactions between plants and pollinators. Floral antagonists, such as nectar thieves, also have the potential to exert selection on floral characters, but adaptation against floral antagonists has attracted comparatively little attention. We found that the corollas of hornet-pollinated Codonopsis lanceolata (Campanulaceae) and the tepals of bee-pollinated Fritillaria koidzumiana (Liliaceae) are slippery to nectar-thieving ants living in the plant’s habitat; because the flowers of both species have exposed nectaries, slippery perianths may function as a defence against nectar-thieving ants.MethodsWe conducted a behavioural experiment and observed perianth surface microstructure by scanning electron microscopy to investigate the mechanism of slipperiness. Field experiments were conducted to test whether slippery perianths prevent floral entry by ants, and whether ant presence inside flowers affects pollination.Key ResultsScanning electron microscopy observations indicated that the slippery surfaces were coated with epicuticular wax crystals. The perianths lost their slipperiness when wiped with hexane. Artificial bridging of the slippery surfaces using non-slippery materials allowed ants to enter flowers more frequently. Experimental introduction of live ants to the Codonopsis flowers evicted hornet pollinators and shortened the duration of pollinator visits. However, no differences were found in the fruit or seed sets of flowers with and without ants.ConclusionsSlippery perianths, most likely based on epicuticular wax crystals, prevent floral entry by ants that negatively affect pollinator behaviour. Experimental evidence of floral defence based on slippery surfaces is rare, but such a mode of defence may be widespread amongst flowering plants.

Sequences of epicuticular wax structures along stems in four selected tree species

Wax layer formation accompanies the processes of epidermis and cuticle formation. To examine these changes, observationsalong current-year long shoots of four woody species (Acer negundo, A. rufinerve, Gymnocladus dioica, and Gingko biloba) were made. Long shoots are suitable objects for such observations, because from the same stem, several samples can be obtained that represent a well-defined sequence of fragments of different ages. The data that were obtained from different and sometimes distant stem regions suggest that epicuticular wax crystals appear on the stems very early and quickly, since they are found near the stem apex during intensive plant growth. This was confirmed for the four examined species. Some differences, however, were noted. Very rapid production of prominent wax crystals was observed in stems. In an experiment involving mechanical wax removal from the A. negundo stem surface and subsequent analysis of the surface after several days via scanning electron microscopy (SEM), it was shown that wax regenerates very quickly. After only several hours, new emerging crystals were observed and their morphologies were essentially the same. Nearly full regeneration was observed several days after removal.

Impact of cell shape in hierarchically structured plant surfaces on the attachment of male Colorado potato beetles (Leptinotarsa decemlineata)

Plant surfaces showing hierarchical structuring are frequently found in plant organs such as leaves, petals, fruits and stems. In our study we focus on the level of cell shape and on the level of superimposed microstructuring, leading to hierarchical surfaces if both levels are present. While it has been shown that epicuticular wax crystals and cuticular folds strongly reduce insect attachment, and that smooth papillate epidermal cells in petals improve the grip of pollinators, the impact of hierarchical surface structuring of plant surfaces possessing convex or papillate cells on insect attachment remains unclear. We performed traction experiments with male Colorado potato beetles on nine different plant surfaces with different structures. The selected plant surfaces showed epidermal cells with either tabular, convex or papillate cell shape, covered either with flat films of wax, epicuticular wax crystals or with cuticular folds. On surfaces possessing either superimposed wax crystals or cuticular folds we found traction forces to be almost one order of magnitude lower than on surfaces covered only with flat films of wax. Independent of superimposed microstructures we found that convex and papillate epidermal cell shapes slightly enhance the attachment ability of the beetles. Thus, in plant surfaces, cell shape and superimposed microstructuring yield contrary effects on the attachment of the Colorado potato beetle, with convex or papillate cells enhancing attachment and both wax crystals or cuticular folds reducing attachment. However, the overall magnitude of traction force mainly depends on the presence or absence of superimposed microstructuring.

Comparative and functional morphology of hierarchically structured anti-adhesive surfaces in carnivorous plants and kettle trap flowers

Plant surfaces that are slippery for insects have evolved independently several times in the plant kingdom, mainly in the groups of carnivorous plants and kettle trap flowers. The surface morphologies of 53 species from both groups were investigated by scanning electron microscopy. It was found that the surfaces possess highly diverse topographical structures. We present a classification of 12 types of anti-adhesive surfaces, in regard to the assembly and hierarchy of their structural elements. The observed structural elements are different combinations of epidermal cell curvatures with cuticular folds or 3D epicuticular wax crystals and idioblastic elements.