Plant Skin: Of Interest to Humans and Insects

By Sophia Rhizopoulou, University of Athens

Millions of years before people began making and manipulating synthetic structures, plant biological systems were developing three-dimensional complex microstructures and special features to produce striking visual and functional effects. Such features are typically associated with the plants’ environmental conditions and their physiological circumstances. Currently, sophisticated imaging techniques are available for viewing and investigating living plant material at high resolution. Investigation of plant tissue microscupltures and assessing their functionality provides us with knowledge of many of the adaptations present in nature. Of note here is the catch phrase, “Art imitates nature”, which implies that much of humankind’s art may be based on a natural object. Some may ask, “Does it make sense to study the outermost layer of a plant?” Recent studies indicate that the appropriate answer is “Yes”.

The study of biological surfaces has become an active area of research related to the development of functional manufactured materials. Some of those materials are based on mimicry of characteristics and properties of living organisms. Some of the materials being developed have functions associated with water regulation, self-cleaning, energy conversion, energy conservation, adhesion, coloration and thermal insulation; each of those functions have examples in nature—many in the plant world.

The surfaces of plants (plant skin or plant cuticle) form multifunctional interfaces between the internal plant tissues and the plant’s environment. As such, plant skin may act like a complex sensor of various environmental stimuli. In considering the multitude of functions of this boundary layer, it is worth noting that plant skins may combine many aspects of so-called “smart” materials, which are designed to respond to environmental stimuli such as light, temperature, moisture and dust, with particular changes in some variables. Moreover, plant surfaces appear to have evolved in a way that allows them to have many different roles simultaneously.

Detailed visualization of plant samples reveals interesting features, particularly when plant tissues are imaged at the nanometer (1/1,000,000,000 of a meter) level. Such viewing reveals that microsculptural details increase the surface area of the examined tissues, and this surface enhancement may be particularly important for the tissue’s performance. For example, the increase in surface relief might affect the effectiveness of the tissue’s adhesive and physical properties, or it may affect its water regulation, dehydration, rehydration functions and its energy and gas exchanges. Irregular surfaces increase the tissue’s roughness, which can enhance its wetting characteristics and influence the water status of a plant as water droplets can penetrate into the depressions of a rough surface and wet the tissue. It seems likely that rough plant surfaces allow a plant to retain water during a prolonged dry period. Research reveals that many nanometer-scale features of plant surfaces are related to the plant’s adaptations to microclimatic conditions, mainly associated with water and light availability.

Surface relief on a plant’s cuticle can reduce the loss of water through the epidermis and can protect the plant’s tissues against physical, chemical and biological attack. Also, it can reduce the absorbance of ultraviolet radiation that reaches plant cells, and quite importantly, relief-related structures form favourable microsculptural features that allow insect pollinators to walk on plant parts, particularly flowers.

In addition, many plant surface cells are associated with the capture of incident light and serve as interfaces between the plant and its environment. A prominent microsculptural feature of plant cuticle tissues is the conical shape of the epidermal cells, which reduce light reflection and increase the proportion of incident light that enters the cells. This enhances light absorption by plant pigments and produces maximum pigment brilliance. Below the epidermal cell layer, the mesophyll layer contains numerous light-reflecting interfaces that can increase light scattering within the tissue and affect colouration and pigment brilliance.

Cuticular boundary layers and epidermal cells of plants are exposed to environmental stimulations and are able to exhibit many reactions to such stimuli, often reacting simultaneously to many different stimulation types. It is likely that the natural systems within plant skin can still offer examples for further transfer to artificial materials. As some branches of modern industry have shown increasing demands for colourful and water-repellent materials, many of these may be related to the development of special light and colour reacting materials.

Having discovered visible and hydrophobic effects of plant skin, by looking at the surfaces of leaves and flowers we have also had to learn to look at the outer micrometer of plants in an entirely different way.

Sophia Rhizopoulou
University of Athens
www.atomiumculture.eu