Guttation is the exudation of drops of xylem sap on the tips or edges of leaves of some vascular plants, such as grasses, and a number of fungi. Guttation is not to be confused with dew, which condenses from the atmosphere onto the plant surface.
At night, transpiration usually does not occur, because most plants have their stomata closed. When there is a high soil moisture level, water will enter plant roots, because the water potential of the roots is lower than in the soil solution. The water will accumulate in the plant, creating a slight root pressure. The root pressure forces some water to exude through special leaf tip or edge structures, hydathodes or water glands, forming drops. Root pressure provides the impetus for this flow, rather than transpirational pull. Guttation is most noticeable when transpiration is suppressed and the relative humidity is high, such as during the night. The process of guttation formation in fungi is unknown.
Guttation fluid may contain a variety of organic and inorganic compounds, mainly sugars, and potassium. On drying, a white crust remains on the leaf surface.
Girolami et al. (2005) found that guttation drops from corn plants germinated from neonicotinoid-coated seeds could contain amounts of insecticide consistently higher than 10 mg/l, and up to 200 mg/l for the neonicotinoid imidacloprid. Concentrations this high are near those of active ingredients applied in field sprays for pest control and sometimes even higher. It was found that when bees consume guttation drops collected from plants grown from neonicotinoid-coated seeds, they die within a few minutes. This phenomenon may be a factor in bee deaths and, consequently, colony collapse disorder.
If high levels of nitrogen appear in the fluid, it is a sign of fertilizer burn. Excess nitrogen must be leached from the soil by addition of large quantities of water. This is the best way to restore soil fertility, but it may result in water pollution.