Hygroscopic particles on leaves: nutrients or desiccants?

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. ne aerosols that are less than 2.5 mu m 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.