Guttation is the term given to the exudation of the xylem sap in the form of liquid droplets. These droplets are present on the tips and edges of the leaves of tropical plants. Guttation often occurs in moist soils during damp, humid nights, when water absorption is high, but transpiration is minimal. This phenomenon is easily observable in the early morning hours, and often can be seen during the later hours of the day as well. Guttation is observed among several angiospermous plants, especially the ones that grow in tropical regions. Examples include grasses, grapevines, strawberries, cereal crops, ornamental plants like hibiscus, hydrangea, etc.
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Guttation is a result of positive hydrostatic root pressure. At night time, transpiration occurs at a negligible rate, but the root cells still actively absorb moisture and nutrients from the soil, due to the lower water potential inside the roots. Low transpiration and active absorption of water and minerals by the roots results in the accumulation of minerals within the xylem of the vascular bundles. Water also moves up from the root cortex to the vascular bundles. This “pushes” the xylem sap (water and solutes) upwards and into the leaves, via the uphill active transport using ATP (“ascent of sap”). In the absence of transpiration, this positive root pressure results in the phenomenon of guttation, wherein excess xylem sap is exuded from the pores on the marginal surface of the leaves.
Hydathodes is secretory structures through which guttation occurs. They are pores found in the margins and tips of leaves and are present on the adaxial and abaxial surfaces. Unlike the stomatal pores, these pores are always open. Each hydathode is made up of small, loosely-arranged parenchymatous cells, called epithem, which lack chloroplasts.
The xylem vessels and tracheids open into the epithem, which is situated below a “sub-epidermal chamber”. The subepidermal chamber opens into the pore, on the leaf margin/ leaf apex.
The pores of the hydathode tissues are sometimes also referred to as the water stomata since they are pores, similar to stomata but instead of exchanging gases, they exude water out from the leaves.
| Hydathode | Stomata |
|---|---|
These are the structures that facilitate guttation | These are the structures through which transpiration occurs |
Pores are always open as there is no regulatory mechanism for the opening and closing | Pores open only during the day, the opening of the pores governed by guard cells |
Not associated with any sort of gaseous exchanges | Stomata are instrumental in the exchange of gases |
Occur at the leaf margins on the sides and the apex and the tips of leaf veins | present on the epidermis of leaves, and are present in larger number at the abaxial surface |
No such apparatus is present | Surrounded by a pair of guard cells, one cell on either side of the pore |
Cells surrounding the pore does not contain chloroplasts | cells that surround the stomatal opening contain chloroplasts |
A sub-epidermal chamber and epithem surround the pore | A sub-stomatal cavity present below the stomatal pore |
No subsidiary cells found | Subsidiary cells present around the stomatal opening |
Although both processes are examples of the ascent of sap theory and help in the release of water to the external environment, they are two very distinct processes. The most distinguishing point in terms of the mechanism of guttation and transpiration is that guttation is a result of xylem sap being “pushed up”, contrary to transpiration, which is more of a “pulling up” situation.
| Guttation | Transpiration |
|---|---|
The exudation of excess xylem sap which may be pure water or water mixed with organic and inorganic constituents | The release of water from above-ground parts of a plant |
Occurs through hydathodes | Occurs through stomata |
Can occur only at the margins of the leaves, where the veins end. | Occurs on the phylloplane |
Occurs mainly during the night time | Occurs during the day time |
It is a result of positive hydrostatic root pressure | It is a result of negative root pressure |
It is the exudation of the xylem sap (liquid) | Water vapour is released |
Guttation is an exudation process | Process of simple diffusion |
Guttation doesn't cause excessive loss of water | Almost 90% of the water from the plant system is lost to transpiration |
Hydathodes always remain open, hence guttation is not regulated | Opening and closing of stomata help in regulating transpiration |
Mostly found to occur in small, herbaceous plants | Universal process, occurs in all plants with vascular bundles |
Although mostly seen as a purposeless process, many researchers believe that guttation does in fact help in removing the wasteful materials from the plant body. It also helps in maintaining a balance of slats and other nutrients and water in the plant system, by exuding off the excess xylem sap. Moreover, it helps in balancing off the excess root pressure that develops in the plant system.
Guttation is, in most cases, a harmless process. However, in some instances, it can translate into a problem for the plant. An often cited instance is that of leaf injuries due to the exuded sap. The salts that are exuded along with the xylem sap often tend to accumulate at the leaf surface, and the margins and lead to necrosis of the leaves.
Guttation is the release of xylem sap from excretory structures called hydathodes that resemble stomata
Guttation occurs during the night hours, when transpiration is negligible and humidity is high
Hydathodes are structures found on the leaf margins and apices, at the tips of veins
Although mostly seen as a wasteful process, guttation does help in releasing excess pressure and excess water and minerals, thus maintaining a much need balance in the plant system
Q1. Is dew the same thing as guttation?
Ans. Dew drops are a result of condensation of the atmospheric water vapour on the surface of the leaves. Like guttation, dew is also observed in the morning hours, but the droplets occur all over the phylloplane, not just the leaf margins, unlike guttation.
Q2. What is the composition of the guttation fluid?
Ans. The fluid exuded from the hydathodes (i.e., guttant) may be pure water or sap-i.e., water and some dissolved sugars and inorganic compounds released from the xylem tissues.
Q3. What is the ascent of sap theory?
Ans. The ascent of water and minerals from the soil to the aerial parts such as leaves and stems is known as the ascent of sap. This theory has been explained by the cohesion-tension theory, aka transpirational pull
Q4. What are two types of hydathodes?
Ans. In most angiosperms, hydathodes are of two kinds- the epithemal hydathodes and the epidermal hydathodes. The epithemal hydathodes are passively involved in the exudation of sap, while the epidermal hydathodes exude the fluid actively.
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