Hydraulic activation: Establishment and effects of liquid water bridges across the stomata
The ecological function of dew and fog is strongly linked to the question of foliar water uptake (‘imbibition’). Several recent studies have clearly shown that an efficient way of imbibition into the leaves must exist. While the cuticle is largely water impermeable, the penetration of water droplets into stomata is prevented by the water surface tension, the stomatal geometry, and the cuticular hydrophobicity. However, these physical constraints preventing the stomatal uptake of droplets do not impede stomatal uptake of thin films. Thin water films along stomatal walls could theoretically establish by various processes, including fungal hyphae, mucilage type substances within the stomata, and hygroscopic aerosols. We have been studying the contribution of deposited atmospheric aerosols to the chemistry and physicochemistry of dew. While atmospheric aerosols act as cloud condensation nuclei in the atmosphere, they become similarly activated on leaf surfaces, fostered by the humid boundary layer of transpiring leaves. Condensation is thus initiated considerably below 100% relative humidity, leading to the formation of highly concentrated, almost saturated salt solutions with specific physicochemical properties. We studied the chemical composition of dew and the electrical conductance of leaf surfaces in a coniferous forest. We used environmental scanning electron microscopy (ESEM) to study the relevant processes for the development of the stomatal pathway. We measured gas exchange and used aerosol exclusion experiments to determine the ecological role of aerosols for plant functioning.
Ion concentrations of dew samples reflected the dry deposition of aerosols. Thin films of liquid water on the surface of spruce needles were capable of dissolving NH3, indicating the presence of liquid water during apparently dry conditions. ESEM videos clearly show that specific deliquescent salts containing ‘chaotropic’ ions of the Hofmeister series can readily enter the stomata, due to low surface tension. There are indications that NaCl and other salts might also (although less efficiently) be capable of creating a stomatal pathway over time.
Once established, this ‘hydraulic activation of stomata’ (HAS) enables the bidirectional transport of water, solutes, and hydraulic signals. Aerosols have always been part of the atmospheric environment of plants, and aerosol concentrations have been stable over evolutionary timescale, so plants may have adapted to them. Deposited aerosols may contribute to the stomatal system as a cumulative environmental factor. This system may be delicate and may become disturbed by air pollution.