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Guttation, Practice Problems and FAQs

Did you know that plants utilise roughly 2% of the water that they absorb from the soil? So what happens to the rest of the water? It is lost through transpiration. You are also aware that transpiration helps to create a suction force in the plants which not only helps in the ascent of sap through xylem but also reaches till the root hair cells and helps them to absorb more and more water. But, transpiration mostly occurs during the day, right? This is because a major percentage of transpiration in plants occurs through the stomata which remains closed at night. But does the absorption of water by the roots stop during night time? No it does not. But if the plant utilises only a small percentage of the absorbed water, then what happens to the excess water that is absorbed? You must have seen tiny drops of water forming on the edges of leaves during the very dark and early hours of the morning. The answer to the previous question lies in these droplets. Come, let’s try and understand what these droplets are and why they are produced.

Table of contents

  • Guttation
  • Demonstration of Guttation
  • Mechanism of Guttation
  • Composition of Water Released due to Guttation
  • Difference between Guttation and Transpiration
  • Practice Problems
  • FAQs

Guttation

Guttation is the exudation of watery drops from the edges of leaves of intact plants. This phenomenon is seen in some plants only during humid seasons when the rate of transpiration is very low, for example, spring and rainy season. Some common plants that show guttation are balsam, tomato, Luffa, Cucurbita, oat, garden, Nasturtium, Colocasia, etc. Guttation occurs through special permanent pores present on the margins of the leaves known as hydathodes.


                                                 Fig: Guttation

Guttation occurs in small vascular plants, such as grass, wheat, strawberry, etc. However, it generally does not occur in tall trees like conifers because a lot of root pressure would be required to push the water out of the margins of leaves which lie at the top of the trees, and such a huge amount of root pressure is not generated in tall trees.

Structure of hydathodes

Each hydathode consists of two immobile hydathode guard cells which enclose a pore known as the water pore or water stoma. This pore overlies the area of the vein ending or terminal tracheids which is surrounded by a mass of loosely arranged colourless parenchymatous cells called epithem.


                                      Fig: Hydathode structure

Difference between hydathodes and stomata

While the hydathodes help in guttation, the stomata are the structures that lie in the epidermis of leaves and help in evaporative loss of water through the process of transpiration. Let us try to understand how these structures are different to each other:

Stomata

Guard cells

1. These are found all over the leaf surface and in young stems.

1. These are found on the edges of leaves

2. Guard cells surrounding the stoma contain chloroplasts.

2. Chloroplasts are absent in hydathode guard cells.

3. Guard cells are either kidney shaped or dum-bell shaped.

3. Hydathode guard cells are oval, barrel shaped in outline.

4. Guard cells show turgor movements and can swell and shrink to open or close the stomatal pore.

4. Do not show turgor movements.

5. Subsidiary cells are often found surrounding the guard cells.

5. Lack the subsidiary cells around guard cells.

6. Stomata open internally into an air cavity and are not connected to vein endings.

6. Air cavity is absent and the pores are present over terminal tracheids or vein endings.

7. Epithem is absent.

7. Epithem surrounds the water pore or water stoma.

8. Release water in the form of vapour.

8. Release water droplets.

9. Stomata do not pass out solutes along with transpired water.

9. Release some solutes dissolved in the water droplets.

Fig: Stomatal apparatus

Fig: Hydathode structure

Demonstration of Guttation

Procedure

Take a well-watered small potted plant of garden Nasturtium or balsam and cover the soil and outer surfaces of the pot with an oil cloth to prevent evaporation. Place the potted plant over a glass plate and invert a dry, clean bell jar over the potted plant. Seal the edges of the bell jar with the help of vaseline and place the apparatus in a dark, warm and humid place.

Observation

Drops of liquid water appear on the margin of the leaves but no drops are seen on the inner surface of the bell jar.

Conclusion

Presence of liquid drops on the leaf margins and their absence from the inner surface of the bell jar indicate that water drops are not formed due to condensation of transpired water vapour. They are actually due to exudation of liquid drops from inside the leaf due to guttation.

Precautions

Use only a healthy potted plant and be sure to cover the soil and the surface of the pot to avoid evaporation. Sealing the edges of the bell jar is important to prevent the entry of water vapour from the outside air.

Mechanism of Guttation

The stomata remain closed during the night and hence transpiration does not occur during the night. When the moisture level in the soil is high, especially in well-watered soil, the roots continue to absorb water due to high water potential in the soil solution and less water potential in the root hair cells. As the excess water is not lost by transpiration, the root pressure within the plants keeps building up. This root pressure causes water to ooze out of the leaf margins through the hydathodes as small water droplets.


                                 Fig: Water oozing from hydathode

The amount of guttation varies from plant to plant. A single leaf of Colocasia may pass out 100-200 ml of liquid water in a single day. The rate of guttation is high under conditions of high humidity because lack of transpiration under such conditions leads to build up of root pressure. There is generally a daily rhythm of guttation, being present during early morning and absent for the rest of the day when transpiration occurs. The process of guttation is prevented by mineral deficiency, excess minerals, water logged and water deficient soils.

Composition of Water Released due to Guttation

The water droplets released by guttation do not contain pure water. It contains both inorganic and organic solutes. The concentration of solutes is around 0.25-2.5 gm per litre. The inorganic solutes include minerals, mainly potassium and the organic solutes include all types of soluble organic compounds, especially sugars. Mixing of organic solutes may occur through sieve tubes located in the vein endings. Evaporation of water released by guttation leaves the solutes over the leaf which might form a white crust.

Difference between Guttation and Transpiration

Transpiration

Guttation

1. It is loss of water from aerial parts of the plant, in the form of vapour.

1. It is the loss of water in the form of liquid droplets, through the margin of the leaves, due to an increase in root pressure.

2. Water is lost in its pure form.

3. Water lost due to guttation has dissolved organic and inorganic solutes.

3. Transpiration does not leave any incrustation.

3. Evaporation of water released due to guttation can leave behind solutes which might eventually form a white crust.

4. Transpiration is a common phenomenon in all vascular plants.

4. Guttation does not occur in very tall trees due to lack of adequate root pressure.

5. Transpiration through stomata occurs only during the day.

5. Guttation mostly occurs at night or early morning when transpiration is absent or occurs at a very slow rate.

6. It occurs through stomata, lenticels and cuticles.

6. It occurs through pores on the leaf margins known as hydathodes.

7. It generates a negative suction pressure on xylem sap.

7. Guttation occurs due to positive root pressure on the xylem sap.

8. It occurs in all seasons.

8. It occurs in seasons with high humidity, such as spring and rainy season.

9. Excessive transpiration results in wilting of plants.

9. Guttation does not result in wilting of plants.

10. Its rate is high under conditions of low humidity.

10. The amount of guttation increases with increase in humidity.

Practice Problems

  1. Find out the incorrect sentence.
  1. The water stoma is surrounded by a mass of loosely arranged colourless sclerenchymatous cells called epithem
  2. The water stoma is surrounded by oval barrel shaped guard cells
  3. The water stoma lies just over terminal tracheids
  4. The hydathodes are located on the margin of the leaves.

Solution: Guttation is the exudation of watery drops from the edges of leaves of intact plants. Guttation occurs through special permanent pores present on the margins of the leaves known as hydathodes. Each hydathode consists of two oval barrel shaped immobile hydathode guard cells which enclose a pore known as the water pore or water stoma. This pore overlies the area of the vein ending or terminal tracheids which is surrounded by a mass of loosely arranged colourless parenchymatous cells called epithem. Thus, the correct option is a.

2. Which of the following conditions is not conducive to the occurrence of guttation?

  1. High root pressure
  2. Humid environment
  3. Dry soil
  4. Poor or no transpiration

Solution: Guttation is the loss of water in the form of droplets from the margins of leaves of intact plants through special pores known as hydathodes. The stomata remain closed during the night and hence transpiration does not occur during the night. When the moisture level in the soil is high, especially in well-watered soil, the roots continue to absorb water due to high water potential in the soil solution and less water potential in the root hair cells. As the excess water is not lost by transpiration, the root pressure within the plants keeps building up. This root pressure causes water to ooze out of the leaf margins through the hydathodes as small water droplets.

Thus, guttation is favoured by highly humid conditions and a well-watered soil because it leads to an excessive build up of root pressure due to poor or no transpiration in the plants. Hence, the correct option is c.

3. Which of the following is true for stomata but not hydathodes?

  1. Surrounding guard cells do not show turgor movements
  2. The guard cells are not surrounded by subsidiary cells
  3. These open internally to an air cavity
  4. Water is released in its liquid form, along with dissolved solutes

Solution: The stomata help in release of water in the form of vapour during transpiration. They are found in the epidermis of leaves and young green shoots and are surrounded by bean shaped or dumb bell shaped guard cells which regulate their opening and closing. The guard cells can show turgor movements and swell up to open the stomata and shrink down to close them. The guard cells around the stomata are surrounded by subsidiary cells. The stomata open internally to a substomatal cavity which is filled with air.

The hydathodes are permanently open pores located at the margin of leaves, over the terminal tracheids or vein endings. They do not open internally to air cavities. Due to build up in root pressure in the plants, hydathodes allow excess xylem sap to ooze out in the form of water droplets having dissolved solutes in it. The pore of a hydathode is known as a water stoma and is surrounded by oval barrel shaped guard cells which do not show any turgor movements. These guard cells are not surrounded by subsidiary cells.

Thus, the correct option is c.

4. Water released due to guttation

  1. is in its pure form
  2. is in its gaseous state
  3. has dissolved organic and inorganic solutes in it
  4. leaves a white crust on leaves when it evaporates
  1. I and IV
  2. II and IV
  3. Only III
  4. III and IV

Solution: During guttation, water is released in its liquid form.The water droplets released by guttation do not contain pure water. It contains both inorganic and organic solutes. The concentration of solutes is around 0.25-2.5 gm per litre. The inorganic solutes include minerals, mainly potassium and the organic solutes include all types of soluble organic compounds, especially sugars. Mixing of organic solutes may occur through sieve tubes located in the vein endings. Evaporation of water released by guttation leaves the solutes over the leaf which might form a white crust.

Thus, the correct option is d.

FAQs

  1. Does guttation occur in any organism other than plants?

Answer: Guttation is also seen in fungi although the mechanism behind the process is still not understood. It is mostly observed in the stages of fast growth in the fungi and hence it is assumed that due to high metabolic rate during such a stage, the fungi must produce a lot of extra water due to respiration which might be exuded through the process of guttation.

  1. Are the water droplets released due to guttation the same as dew drops?

Answer: Water released due to guttation is not the same as dew drops. Dew drops are formed on the surface of leaves due to condensation of atmospheric water vapour during late nights or early morning, when the ambient temperature drops considerably. The water droplets released during guttation are released due to high root pressure build up within the plant in the absence of transpiration. These droplets are basically excess xylem sap that ooze out through pores called hydathodes from the margin of leaves.

  1. Is guttation useful or harmful?

Answer: Guttation has negligible significance to plants. It is a natural process that occurs in plants due to excessive root pressure under humid conditions and lack of adequate transpiration. It is not always harmful to plants but it might occasionally damage the leaf margins due to excessive deposition of salts that are released dissolved in the water droplets that ooze out through the hydathodes during guttation. This is particularly seen in plants growing in highly fertilised soils.

The salt deposited due to evaporation of the water released due to guttation leaves behind white crusts which marr the beauty of ornamental plants. The guttated liquid can also act as a point of entry for plant pathogens.

  1. What does high levels of nitrogen in the guttated liquid signify?

Answer: High levels of nitrogen in the guttated liquid indicate a fertiliser burn due to excessive fertilisers being added to the soil. Thus the xylem sap has an excess of nitrates and nitrites which upon oozing through the hydathodes, get deposited at the tips and burn or damage the leaf tissues.

  1. Why is guttation more common in tropical rainforests but not so common in the woody plants of the temperate zone?

Answer: Guttation requires a combination of warm temperature, moist and well-watered soil and humid atmosphere for its occurrence. These conditions are more prevalent in the tropics but rarely occur together in the temperate zone. Thus, guttation is more common in tropical rainforests.

YOUTUBE LINK:

Related Topics

Transpiration

Root Pressure

Uptake and transport of mineral nutrients: Uptake of mineral ions and Translocation of mineral ions

Long Distance Transport of Water

Stomata: Opening and Closing

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