Interaction of surfactants with barley leaf surfaces: time-dependent recovery of contact angles is due to foliar uptake of surfactants

Abstract

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.

Transcriptomic changes in barley leaves induced by alcohol ethoxylates indicate potential pathways of surfactant detoxification

“…At this stage, the 2nd leaf was fully developed and used for the experiments. In order to be consistent with our preceding study, investigating the uptake of AEs into barley leaves 24 , we have been working with exactly this plant and leaf age. Surfactants were sprayed on the leaf surfaces at aqueous concentrations of 0.1% (v/v) using an airbrush system (Start Single Action Airbrush-Pistole, Conrad, Germany).…”

“…However, since it’s mean chemical structure is C 12 E 4 it can be assumed to have a similar lipophility as the monodisperse surfactant. This high lipophility of these chosen alcohol ethoxylates ensures an efficient penetration across the lipophilic cuticle inside the living leaf 24 .…”

“…In a previous study, we showed that AEs sprayed to leaf surfaces of barley plants are rapidly taken up by the leaves within a few hours 24 . At higher concentrations (1% and 10%), leaf photosynthesis was severely affected and leaves were rapidly dying within a couple of hours indicating the toxicity of AEs.…”

Adhesive properties of Aphrophoridae spittlebug foam

“…due to gravity or desiccation), the liquid content reduces, and the bubble shape turns pentagonal. Dried foam leaves lamellate and filamentous, obviously protein and mucopolysaccharide residues on plant and insect surfaces or between frames, suggesting similar substrate surface alteration as known by synthetic surfactants [71,73].…”

“…Considering the diversity of host plants, the foam must be universal, wetting a broad range of plant surfaces having water contact angles from 0°up to 163° [68][69][70]. One wonders if spittlebug foam behaves similarly to synthetic surfactant solutions on leaf surfaces, which lead to higher wettability, surfactant coverages, plant surface alterations and surfactant-induced biochemical plant responses [71][72][73]. In this context, surface/solid-foam fluid interactions and foam properties matter.…”