What are water droplets on leaves called?

The water droplets that form on plant leaves are commonly referred to as guttation drops or guttation water. Guttation is the process by which drops of xylem sap are exuded from the tips or edges of leaves of some vascular plants. Guttation generally occurs during the night or in early morning hours when transpiration is suppressed, and root pressure is high.

What causes guttation?

Guttation occurs when the roots absorb more water than the plant can transpire through its leaves. This excess water builds up hydraulic pressure in the xylem vessels, forcing water out through special leaf tip or edge structures called hydathodes. Hydathodes are natural openings in a plant’s epidermis that allow for secretion of water droplets. They are found along the margins or tips of leaves and are surrounded by a few specialized epidermal cells known as water pores or water stomata.

During the day, when transpiration rates are high, guttation generally does not occur because the tension created by transpiration pull prevents exudation of pressurized root water. At night however, when stomata are closed and transpiration is minimal, root pressure can overcome the decreased tension and push xylem sap out through the hydathodes.

Cool weather conditions, high soil moisture, and young seedlings with high root pressure are also associated with increased guttation. The lower nighttime temperatures reduce transpiration, while the damp soil and shallow roots of seedlings allow for increased root water absorption and hydraulic root pressure.

Composition of guttation fluid

The drops that form on leaf tips and edges through guttation are largely composed of water and contain various dissolved mineral nutrients absorbed by the roots. The exuded sap can have high concentrations of potassium, calcium, magnesium, silica, chloride, sulfate, bicarbonate, amino acids, and inorganic acids like malic acid.

In some plant species, guttation fluid may also contain significant amounts of sugars, particularly sucrose. This is because the root pressure can force phloem sap contents through the xylem back into the leaves. In certain toxic plants like lily of the valley, guttation drops may contain poisonous compounds that discourage herbivores from feeding on the leaves.

Table of components commonly found in guttation fluid

Component	Description
Water	The main component, making up over 90% of the guttation fluid
Minerals	Potassium, calcium, magnesium, silica, chloride, sulfate, bicarbonate
Organic acids	Malic acid, citric acid
Sugars	Sucrose, glucose, fructose
Amino acids	Primarily glutamine and asparagine
Alkaloids and toxins	In some plants like lily of the valley

Benefits and functions of guttation

Guttation serves several important functions for plants:

Relieves root pressure

Guttation allows plants to relieve excess root pressure that could potentially damage water conducting xylem vessels.

Cooles leaf surfaces

The evaporation of guttation drops from leaf tips and margins can help cool the leaf surfaces.

Moisturizes air

The water released by guttation can increase local air humidity around the leaves.

Removes excess salts

Guttation allows plants to excrete excess mineral salts from their systems that could potentially accumulate to toxic levels.

Deters herbivores

In some plants, distasteful or toxic compounds in the guttation fluid can deter insects and other herbivores from feeding on leaves.

Differences between guttation and dew

Guttation fluid is sometimes confused with dew droplets that can also accumulate on the tips of leaves overnight. However, there are a few key differences between these:

Origin

Guttation originates from within the plant, while dew forms from water vapor condensing from the atmosphere onto the leaf surface.

Distribution

Guttation occurs specifically through hydathodes on leaf margins and tips, whereas dew can form across the entire leaf surface.

Chemical composition

Guttation contains minerals and metabolites from xylem sap, while dew is simply condensed atmospheric moisture.

Temperature

Guttation is associated with lower nighttime temperatures, while dew forms when air temperature drops below the dew point.

Differences between guttation and bleeding sap

In some cases, guttation fluid can be confused with bleeding sap, another type of plant exudate. However, they have distinct causes and compositions:

Origin

Guttation originates from xylem tissue, while bleeding sap comes from phloem tissue.

Composition

Guttation contains more water and minerals, whereas bleeding sap is rich in sugars.

Causation

Guttation is caused by root pressure forcing xylem sap out, while bleeding sap results from phloem damage.

Color

Guttation fluid is usually clear to white, while bleeding sap is often darker from plant pigments.

Environmental conditions that favor guttation

Guttation is most prevalent when the following environmental conditions occur:

• Cool nighttime temperatures – Reduces transpiration and allows root pressure to build up.
• High relative humidity – Prevents evapotranspiration.
• Moist soil – Promotes water absorption by roots.
• Young seedlings – Have lower transpiration rates and high root pressure.
• Just after rains – Increases soil moisture.

Plants that exhibit guttation

Guttation can occur in many types of vascular plants, but is especially common in:

• Herbaceous plants – Such as tomatoes, cabbages, strawberries.
• Ornamentals – Including roses, chrysanthemums, geraniums.
• Turfgrasses – Ryegrass, bluegrass, fescue.
• Vegetable seedlings – Lettuce, peppers, broccoli.
• Succulents – Such as aloe vera, jade plants.
• Aquatic plants – Like water lilies.

Is guttation good or bad for plants?

In most cases, guttation is a natural and beneficial process for plants. It allows them to:

• Relieve root pressure.
• Cool leaf surfaces.
• Get rid of excess salts and compounds.

However, guttation can potentially facilitate the entry of bacterial and fungal pathogens through the hydathodes. Disease organisms like those causing blights and leaf spots can infect leaves through water droplets. Preventing excessive guttation by avoiding overwatering and increasing air circulation can help reduce this disease risk.

Can guttation cause plant wilting?

Guttation does not directly cause wilting in plants. Wilting is most commonly caused by excessive transpiration and inadequate root water absorption. However, if guttation occurs frequently due to overwatering, it can sometimes contribute to root rot diseases that damage roots and impair their ability to absorb water. This root dysfunction can then lead to wilting of leaves and stems.

Is guttation water safe for plants and humans?

The water droplets from guttation are generally safe for most plants. However, consumption of guttation fluid is not recommended for humans or animals. Some plants exude sap that contains toxic compounds through hydathodes. Consuming guttation water from ornamental plants may introduce unnecessary minerals, organic acids, pathogens, pesticides, or other hazardous substances into the body. It is best to avoid ingesting guttation droplets, especially from plants where the sap chemistry is unknown.

That being said, brief topical exposure to guttation is not dangerous. Accidentally getting some sap on your skin from handling wet plants is unlikely to cause harm. Just be sure to wash it off afterwards.

Conclusion

Guttation is a fascinating botanical process that serves necessary functions for many plants. The water droplets that appear on leaf tips overnight result from root pressure forcing xylem sap out through specialized pores called hydathodes. Guttation allows excess water and salts to be excreted, provides evaporative cooling, and deters herbivores in some species. While generally beneficial, guttation can also facilitate foliar pathogen entry when excessive. To prevent disease issues, avoid overwatering while providing adequate air circulation around plants prone to guttation. Overall, this intriguing plant phenomenon highlights the dynamic vascular processes occurring just underneath the surface, essential for growth and adaptation to the environment.