Foliar absorption from liquid solutions may take place via the cuticle, cuticular cracks and imperfections, through stomata, trichomes or specialised epidermal cells. Two mechanisms of foliar absorption have been discussed and characterized to a certain extent, namely, the cuticular pathway and the stomatal pathway. Regarding the mechanisms of cuticular absorption, apolar, lipophilic compounds have been proposed to follow a diffusion-dissolution process. In contrast, the mechanisms of absorption of hydrophilic solutes through the cuticle are currently not fully understood and it has been hypothesized, it may occur via “aqueous pores”. There is only indirect evidence for the existence of polar pores, since it has not been possible so far to detect such structures in plant materials with the microscopical techniques currently available.
While the significance of the stomatal pathway on the uptake of foliar sprays has been a matter of controversy for many years, recent evidence shows that it can largely contribute to the absorption process. Investigations carried out with fluorescent tracers provided evidence for stomatal absorption along the surface via a diffusion process. Foliar absorption requires dissolution of the applied salt. Hence, once the drop of fertilizer solution hits the leaf surface, the time remaining in a liquid phase seems to be crucial. An expanded liquid phase on the leaf surface is influenced by two factors, relative humidity (RH) and deliquescence humidity (DRH) of the salt, which describes the minimal water vapor pressure enabling sugars and salts to absorb water from the surrounding air and to go into solution. The relative humidity at which deliquescence first occurs is a characteristic of the individual substance.
Boric acid for example, has a DRH of 98% indicating that foliar-applied solutions containing pure boric acid will only stay in solution in almost water saturated air. Mixtures of two substances lower the DRH below the DRH of the individual substance.
Since nutrient sprays are applied in the field, there is a limited chance to control the mechanisms associated with the rate of absorption and bioactivity of the nutrients applied to the foliage, but spraying plants when stomata are open and under conditions limiting the rate of solution drying might be a possible control tool. However, the importance of improving the properties of foliar nutrient formulations has long been recognized, and is currently a matter of scientific interest as key strategy to optimize the performance of foliar sprays. Additives, such as surfactants lowering the surface tension of water and facilitating the process of stomatal infiltration or humectants prolonging the process of droplet drying, have been shown to improve the effectiveness of nutrient sprays when applied to horticultural crops.