Most of the aerial organs of vascular plants are covered by a protective layer known as the cuticle, the main purpose of which is to limit transpirational water loss. Cuticles consist of an amphiphilic polyester matrix, polar polysaccharides that extend from the underlying epidermal cell wall and become less prominent towards the exterior, and hydrophobic waxes that dominate the surface. Here we report that the polarity gradient caused by this architecture renders the transport of water through astomatous olive and ivy leaf cuticles directional and that the permeation is regulated by the hydration level of the cutin-rich outer cuticular layer. We further report artificial nanocomposite membranes that are inspired by the cuticles’ compositionally graded architecture and consist of hydrophilic cellulose nanocrystals and a hydrophobic polymer. The structure and composition of these cuticle-inspired membranes can easily be varied and this enables a systematic investigation of the water transport mechanism.
Main conclusion Time-dependent contact angle measurements of pure water on barley leaf surfaces allow quantifying the kinetics of surfactant diffusion into the leaf. Abstract Barley leaf surfaces were sprayed with three different aqueous concentrations (0.1, 1.0 and 10%) of a monodisperse (tetraethylene glycol monododecyl ether) and a polydisperse alcohol ethoxylate (BrijL4). After 10 min, the surfactant solutions on the leaf surfaces were dry leading to surfactant coverages of 1, 10 and 63 µg cm−2, respectively. The highest surfactant coverage (63 µg cm−2) affected leaf physiology (photosynthesis and water loss) rapidly and irreversibly and leaves were dying within 2–6 h. These effects on leaf physiology did not occur with the lower surfactant coverages (1 and 10 µg cm−2). Directly after spraying of 0.1 and 1.0% surfactant solution and complete drying (10 min), leaf surfaces were fully wettable for pure water and contact angles were 0°. Within 60 min (0.1% surfactant) and 6 h (1.0% surfactant), leaf surfaces were non-wettable again and contact angles of pure water were identical to control leaves. Scanning electron microscopy investigations directly performed after surfactant spraying and drying indicated that leaf surface wax crystallites were partially or fully covered by surfactants. Wax platelets with unaltered microstructure were fully visible again within 2 to 6 h after treatment with 0.1% surfactant solutions. Gas chromatographic analysis showed that surfactant amounts on leaf surfaces continuously disappeared over time. Our results indicate that surfactants, applied at realistic coverages between 1 and 10 µg cm−2 to barley leaf surfaces, leading to total wetting (contact angles of 0°) of leaf surfaces, are rapidly taken up by the leaves. As a consequence, leaf surface non-wettability is fully reappearing. An irreversible damage of the leaf surface fine structure leading to enhanced wetting and increased foliar transpiration seems highly unlikely at low surfactant coverages of 1 µg cm−2.
The plant cuticle, which covers all aerial parts of plants in their primary developmental stage, is the major barrier against water loss from leaves. Accumulation of cutin and waxes has often been linked to drought tolerance. Here we investigated whether cutin and waxes play a role in the drought adaption of barley mimicked by osmotic stress acting on roots. We compared the cuticle properties of cultivated barley (Hordeum vulgare spp. vulgare) with wild barley (Hordeum vulgare spp. spontaneum), and tested whether wax and cutin composition or amount and cuticular transpiration could be future breeding targets for more drought‐tolerant barley lines. In response to osmotic stress, accumulation of wax crystals was observed. This coincides with an increased wax and cutin gene expression and a total increase of wax and cutin amounts in leaves, which seems to be a general response triggered through root shoot signalling. Stomatal conductance decreased fast and significantly, whereas cuticular conductance remained unaffected in both wild and cultivated barley. The often‐made conclusion that higher amounts of wax and cutin necessarily reduce cuticular transpiration and thus enhance drought tolerance is not always straightforward. To prevent water loss, stomatal regulation under water stress is much more important than regulation or adaptation of cuticular transpiration in response to drought.
Surfactants are known to enhance the foliar uptake of agrochemicals by plasticizing the transport-limiting barrier of plant cuticles. The effects of two different polydisperse alcohol ethoxylates with a low degree [mean ethoxylation of 5 ethylene oxide units (EOs)] and a high degree (mean ethoxylation of 10 EOs) of ethoxylation on cuticular barrier properties were investigated. The diffusion of the lipophilic organic molecule 14C-epoxiconazole and of polar 3H-water across cuticles isolated from six different plant species was investigated. At low surfactant coverages (10 μg cm–2), the diffusion of water across the cuticles was not affected by the two surfactants. Only at very high surfactant coverages (100–1000 μg cm–2) was the diffusion of water enhanced by the two surfactants between 5- and 50-fold. Unlike that of water, the diffusion of epoxiconazole was significantly enhanced 12-fold at surfactant coverages of 10 and 100 μg cm2 by the surfactant with low ethoxylation (5 EOs), and it decreased to 6-fold at a surfactant coverage of 1000 μg cm–2. The alcohol ethoxylate with a high degree of ethoxylation (10 EOs) only weakly increased the epoxiconazole diffusion. Our results clearly indicate that those surfactants that significantly enhance the uptake of the lipophilic agrochemicals (e.g., epoxiconazole) at a realistic leaf surface coverage of 10 μg cm–2, as is applied in the field, do not interfere with cuticular transpiration as an unwanted negative side effect.
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