Some plants have adapted to competing for nutrient uptake by creating stomata that open in the presence of humidity. When these stomata open, the nutrients that have collected on the surface of the leaf are absorbed and transported into the leaf apoplast. The mechanism through which these plants do this is known as hydraulic activation of stomata, or "HAS." The surfaces of plants are filled with grooves that collect aerosols over time. When these plants are in the presence of high humidity, moisture also begins to collect on the surface of the plant. As it does, the aerosols containing hygroscopic salts bind with the water. The presence of excess moisture from the humidity also causes the activation of certain stomata. As these stomata open, the fine aerosol particles that have bonded to the water are transported to the interior of the leaf where they can then be absorbed and utilized for their nutritional value.
This review focuses on the deposition and fate of ?ne 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 ef?cient 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 identi?ed." (Burkhardt 2010: 369)