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Introduction

Plants lack a well-sophisticated circulatory system like human beings. However, they obtain maximum water from the ground by transpiration. Moreover, the speed of ascent of water can reach as high as 15 m per hour! So, how is the water getting pulled up without any heart to do the job? The driving force of transpiration pull lies in the evaporation of water through leaves through transpiration. Well, we shall know here the physiology and mechanism involved and how the water properties help the plant obtain water and minerals.

Transpiration

Who knew that plants use less than 1% of the water they obtain while the rest 99% gets lost through transpiration?

Transpiration is the process of water loss through evaporation from aerial plant parts. Depending on the site, it is of three types: stomatal, cuticular and lenticular transpiration.

Stomatal Transpiration

  • It is the water loss as vapours through stomata, present on the surface of leaves
  • It is the major type of transpiration that accounts for 50-97% of transpiration
  • It is responsible for the ascent of sap/ transpiration pull

Stomatal Apparatus

It comprises guard cells, stoma, and subsidiary cells.

  • Stoma: A small pore surrounded by guard cells and forms stomata
  • Guard Cells: Specialised epidermal cells contain chloroplasts and have radially-oriented cellulose microfibrils

Transpiration Pull: Water Translocation Model by Dixon and Jolly

The transpiration pull phenomenon is also known as the cohesion-tension transpiration pull model.

Water properties contribute to the success of the ascent of sap:

  • Cohesion

The cohesive property of water is due to the attraction of one molecule with another similar one due to cohesive force.

  • Adhesion

 It is the property by which the molecules of different substances experience the force of attraction. It occurs between the xylem and water molecules.

  • Surface Tension

It is the attraction of the water molecules on the surface by the molecules in the bulk of water. Water molecules in a liquid state are highly one another than in a gaseous state. 

  • Tensile Strength

The property of the water column is to take on maximum load and resist breakage due to pulling force. A high tensile strength results due to high cohesive and adhesive forces.

  • Capillary Action

Water can rise in thin porous tubes like xylem vessels and tracheids. High capillary actions occur due to stronger adhesive force between the xylem wall and water molecules than cohesive forces between water molecules.

Let’s discuss xylem and tracheary elements that support water and mineral translocation.

Tracheary Elements of Xylem

Xylem conducts water and minerals from the roots to the leaves and stem. It comprises xylem vessels, tracheids, xylem fibres, and xylem parenchyma. Xylem vessels and tracheids come under tracheary elements and are highly specialised cells.

Xylem Vessels

  • These are long, cylindrical tube-like structures with lignified walls and lack protoplasm
  • Xylem vessels are formed from perforated inter-connected vessel members and are the characteristic features of angiosperms
  • Gymnosperms lack vessels

Tracheids

  • They are long tube-like elongated structures with tapering ends
  • They are dead with lignified walls and no protoplasm
  • They are broad of five types, depending upon their different forms of inner wall thickenings. It includes: annular, reticulate, spiral and scalariform

Cohesion-Tension Theory for Transpiration Pull

Transpiration is the driving force of the ascent of sap in the tracheary elements. As the suction force increases, it draws water from roots to leaves.

  • Water rises from the soil to the roots as roots have lower water potential than the soil
  • The existing water forms a continuous thin film over the mesophyll cells of leaves
  • Transpiration causes the evaporation of water from the intercellular spaces
  • It causes the air-water interface to withdraw into the small spaces between cellulose microfibrils and mesophyll cells
  • The continuous water evaporation exerts tension on the water column that doesn’t break due to high tensile strength. One surprising fact is this negative pressure goes as high as -2MPa
  • The resultant surface tension pulls the water from the nearby cells
  • Since the water potential from the roots to the atmosphere decreases, the water diffuses into the surrounding air, further creating a pull. This pull is known as the transpiration pull

So, we can conclude the water translocation as such: water rises from soil to roots, then cohesive and adhesive forces up to the xylem. Then the water rises in tracheary elements as a continuous water column due to transpiration and then leaves through the stomata as vapours to the atmosphere. 

Conclusion

Stomatal transpiration corresponds to 50-97% of water loss through transpiration. The plants meet their major water requirement through the ascent of water by transpiration pull, a concept given by Dixon & Jolly through their Cohesion-Tension theory and cohesion-tension-transpiration pull model.

Transpiration is the driving force for the transpiration pull. The water gets pulled from the roots to the leaves due to a negative hydrostatic pressure of around -2MPa.The continuous water column in xylary elements resists breakage due to the high tensile strength. The intermolecular hydrogen bonding between water molecules is stronger than between water and xylem walls. Thus, the stronger cohesive forces help to establish a high capillary action, and water ascends the xylem.