Hygroscopic particles on leaves: nutrients or desiccants?

Abstract

Aerosols have always been part of the atmosphere, and plant surfaces are a major aerosol sink. Given the nutrient content of aerosols and the natural stability of aerosol concentrations over evolutionary time, plants may have developed adaptations to aerosol input. Although little is known about such adaptations, leaf surface micro‐roughness appears to play a key role. This review focuses on the deposition and fate of fine aerosols that are less than 2.5 μm in diameter. Most of these aerosols are hygroscopic, and they are often deliquescent (liquid) on transpiring leaves. Such concentrated solutions may be taken up by both the cuticle and stomata, contradicting previous concepts. The establishment of a continuous liquid water connection along stomatal walls affects individual stomata and is a new concept called “hydraulic activation of stomata” (HAS). HAS enables the efficient bidirectional transport of water and solutes between the leaf interior and leaf surface and makes stomatal transpiration partly independent of stomatal aperture. The response of plants to changes in humidity can be explained by the split transpiration in an HAS pore and its interaction with neighboring stomata, i.e., as an emergent property of a stomatal patch. Normally, HAS affects only a few stomata, but if too many are activated by excessive particle accumulation or additional surfactants, hygroscopic particles may work as “desiccants,” reducing the drought tolerance of plants. This is made use of when hygroscopic salts and acids are sprayed to kill potato vine, but may cause problems in foliar fertilization. Excessive particle accumulation may also be caused by air pollution. It is hypothesized that deliquescent hygroscopic particles, due to their amorphous appearance, may have been misinterpreted as “degraded waxes.” Degraded waxes have been highly correlated to leaf loss, decreased drought tolerance, and decreased frost tolerance of trees. No sound explanation for degraded waxes has been found, and they have been interpreted as symptoms of forest decline. Because hygroscopic particles may affect the drought tolerance of trees, they could be drivers of regional tree die‐off and especially affect those trees that have adapted to capture aerosols. Several research questions are identified.

Leaf surface structures enable the endemic Namib desert grass Stipagrostis sabulicola to irrigate itself with fog water

“…It is also possible that some of these structures represent salt particles that were deposited on the plant surface in the form of aerosols. Such airborne particles are almost indistinguishable from irregular surface waxes [22]. By using AFM, the roughness of the surface was measured as the ratio of actual surface area to the projected surface area.…”

Quantitative Retention of Atmospherically Deposited Elements by Native Vegetation Is Traced by the Fallout Radionuclides 7Be and 210Pb

“…1−3 Insofar as 70% of the Earth's landmass is vegetated, the deposition and subsequent fate of aerosols is typically mediated by plant surfaces. 4,5 However, processes that regulate the efficiency and time scale of aerosol retention by foliage are not well understood. The long-standing paradigm of aerosol fate was developed from observations of fission radionuclides which were dispersed into the atmosphere by nuclear bombtesting in the 1960s and 1970s, 6,7 it predicts that various "washoff" processes remove atmospheric fallout from vegetation, and that the rate of removal can be described using a first-order rate constant (λ e ) and a characteristic "environmental half-life" (ln2/λ e ) on the order of weeks.…”

Cuticular wax load and surface wettability of leaves and fruits collected from sweet cherry (Prunus avium) trees grown under field conditions or inside a polytunnel

“…Furthermore, in both cultivation systems, leaves were exposed to natural dust contamination, which is known to affect surface wettability (Burkhardt 2010). In the case of leaves grown outside, deposited particles were at least partially removed by occasional rainwater.…”

“…The importance of cuticular wax as a limiting factor against movement of substances between the plant and the atmosphere, as well as its importance as target site for interaction with insects, pathogens, dust, and externally applied compounds, has been well exploited (e.g. Bargel et al 2006;Carver and Gurr 2006;Müller 2006;Krämer et al 2009;Burkhardt 2010). When considering the intrinsic variations between surfaces from different species and organs, a clear relationship between the chemical composition of the cuticular waxes and their microstructure on the leaf surface has been established (Jeffree et al 1976;Jetter and Riederer 1994;Hunsche et al 2006; Bargel et al 2006).…”