Absorption of Water

Prelude to Water Potential

Water is an essential component of all living organisms, comprising at least 70% or more of their total composition. Some plants, like lettuce, contain up to 95% water. In contrast, dormant organisms like seeds and buds have less than 10% water.

Water is a crucial limiting factor for crop productivity in most agricultural systems. Since Earth is often referred to as the “Water Planet,” finding water in space excites astronomers due to its role in sustaining life.

Understanding Water Potential

Definition Water potential (ΨPsiΨ) is the potential energy of water relative to pure free water under reference conditions. It describes water’s tendency to move from one region to another due to factors like:

• Osmosis
• Gravity
• Mechanical pressure
• Matrix effects (e.g., surface tension, fluid cohesion)

Water potential is measured in units of pressure (megapascal, MPa) and helps explain water movement within plants, animals, and soil.

Key Concepts About Water Potential

• Water always moves down its water potential gradient (i.e., from higher to lower water potential).
• It is typically measured as the pressure required to stop the movement of water.
• The unit of measurement is megapascal (MPa).

Components of Water Potential

The total water potential (ΨPsiΨ) is influenced by several factors:

Ψ = Ψ0 + Ψs + Ψp + Ψg + Ψv + Ψm  

Where:

• Ψ0​ → Reference correction
• Ψs→ Solute potential
• Ψp→ Pressure potential
• Ψg→ Gravitational potential
• Ψv→ Vapor pressure potential (humidity effects)
• Ψm→ Matrix potential

1. Pressure Potential (Ψp​)

• Arises from mechanical pressure within plant cells.
• Increases when water enters the cell, exerting outward pressure.
• Turgor pressure (positive pressure) is retained due to the rigid cell wall.
• Plasmolysis results in almost zero pressure potential.
• Negative pressure potential (tension) occurs when water is pulled through open systems like xylem vessels.

2. Solute Potential (Ψs​)

• Represents the effect of dissolved solutes on water potential.
• Pure water has a solute potential of zero (Ψs = 0).
• The solute potential is always negative because solutes reduce the free energy of water.
• Van ‘t Hoff equation describes solute potential:

Ψs = −miRT

Where:

• m = Solute concentration (molarity)
• i= Van ‘t Hoff factor (number of dissolved particles per formula unit)
• R = Ideal gas constant
• T = Absolute temperature (Kelvin)

3. Matrix Potential (Ψm​)

• Occurs when water interacts with solid surfaces (e.g., soil particles).
• Adhesive intermolecular forces between water and solids create surface tension.
• Affects water retention in soil, which is crucial for plant water uptake.
• Dry soils have strong negative matrix potentials, binding water tightly to soil particles.

4. Gravitational Potential (Ψg​)

• Water potential influenced by gravity.
• Generally ignored except in tall trees, where it significantly affects water movement from roots to leaves.

Importance of Water Potential in Plants

• Determines the direction of water flow in plant tissues.
• Regulates water uptake by roots and transpiration through leaves.
• Affects cell turgor pressure, influencing plant growth and structure.
• Plays a critical role in drought resistance and water management in agriculture.

Absorption of Water in Plants

Plants obtain water primarily through their roots, a fact known from an early age. However, it is during formal education that we understand the critical role of water in photosynthesis and other physiological processes. Most of the water absorption occurs in the younger parts of the roots, particularly in the piliferous region, located just behind the growing tip. This region is made up of numerous root hairs, which play a crucial role in water uptake.

Structures Involved in Water Absorption

Root Hairs and Their Role in Water Absorption

In higher plants, water is absorbed through root hairs, which:

• Are hair-like projections of the epidermal layer, forming the root hair zone just behind the root tips.
• Are permeable and composed of pectic substances and cellulose, which are highly hydrophilic in nature.
• Contain vacuoles filled with cell sap, helping in the movement of water.

As roots elongate, older root hairs die, and new ones develop, ensuring continuous contact with fresh water sources in the soil.

Water moves laterally through the root structure before reaching the vascular system.

Structure of Roots and Their Importance

Roots are often overlooked because they are underground, but they play a crucial role in plant growth. Understanding the structure and function of roots is important because they influence:

• Plant size and vigor
• Soil adaptation
• Response to irrigation and cultural practices
• Propagation methods

Functions of Roots

Roots serve several key functions:

1. Absorption of water and nutrients from the soil.
2. Anchoring the plant in place.
3. Providing support to the shoot system.
4. Storing food for future use.
5. Aiding in propagation in certain species.

Internal Structure of Roots

Internally, a root has three major zones:

1. Meristematic Zone (Tip of the Root)

• The root meristem is responsible for cell division and growth.
• It manufactures new cells, allowing the root to grow deeper into the soil.

2. Zone of Elongation

• Located behind the meristem, this area allows cells to grow in size by absorbing food and water.
• This elongation process pushes the root further into the soil.

3. Zone of Maturation

• The cells in this region differentiate into specific tissues like:
  • Epidermis → Responsible for water and mineral absorption.
  • Cortex → Transfers water to the vascular tissue and stores food.
  • Vascular Tissue (Xylem & Phloem) → Conducts water and nutrients throughout the plant.

External Structure of Roots

1. Root Cap

• The outermost tip of the root.
• Consists of cells that are shed as the root moves through the soil.
• Function: Protects the delicate root meristem from mechanical damage.

2. Root Hairs

• Located just behind the growing tip.
• Appear as fine, hair-like projections that increase the surface area of the root.
• Enhance the absorptive capacity of the root system.
• Lifespan: Only 1–2 days; they are easily damaged during transplantation.

Water movement mechanism in plants

Water movement in plants occurs through three main pathways:

1. Apoplastic Pathway
2. Symplastic Pathway
3. Transmembrane Pathway

1. Apoplastic Pathway: In this pathway, water moves exclusively through the cell walls of plant cells, without crossing any membranes.

• The cortex of the root receives the majority of water through this pathway because the loosely bound cortical cells offer little resistance.
• The Casparian strip present in the endodermis blocks the apoplastic movement beyond the cortex. This forces water to move into the symplastic or transmembrane pathways.

2. Symplastic Pathway

• In the symplastic pathway, water moves from one cell to another through plasmodesmata, which are cytoplasmic connections between adjacent cells.
• This pathway forms a network of cytoplasm across all connected cells, allowing water to move through the plant’s tissues.

3. Transmembrane Pathway

• Water moves through cell membranes and may also cross the tonoplast (membrane of vacuoles) in this pathway.
• This movement occurs in the roots when water moves from soil to the endodermis.
• The Casparian strip in the endodermis, made of wax-like suberin, blocks water and solute movement through cell walls, forcing water to move through cell membranes.

Mechanism of water absorption

Water absorption by plants can occur through two types of processes:

1. Active Absorption of Water
2. Passive Absorption of Water

1. Active Absorption of Water

In active absorption, root cells play an active role in water uptake, using metabolic energy released during respiration. Active absorption can be of two kinds: osmotic and non-osmotic.

Steps Involved in Active Osmotic Absorption of Water

• Imbibition of Soil Water
  • Water is imbibed into the hydrophilic cell walls of root hairs.
  • The osmotic pressure (OP) of the root hairs is higher than the soil water, so water enters the root hairs via osmotic diffusion.
  • As a result, the DPD (Diffusion Pressure Deficit) and suction pressure increase in the root hairs, while their turgor pressure rises.

• Osmotic Diffusion to Cortical Cells
• • Water from root hairs moves into adjacent cortical cells due to the osmotic pressure difference.
  • The water continues moving cell-to-cell, gradually reaching the endodermis.

• Passage Cells in Endodermis
  • Water moves into the endodermis through special thin-walled passage cells (since the other cells of the endodermis have Casparian strips that are impervious to water).
  • This process continues until water reaches the pericycle cells.

• Water Movement into Xylem
  • Water from the pericycle cells moves into the xylem (vascular tissue).
  • The turgor pressure in pericycle cells increases, and the water moves into the xylem, causing root pressure to develop.

• A) Osmotic Absorption
• Water is absorbed from the soil into the xylem of the roots following the osmotic gradient.
• Water moves via osmotic diffusion, first reaching the endodermis, then the pericycle, and finally entering the xylem, creating root pressure.

• B) Non-Osmotic Absorption
• Sometimes, water is absorbed against the osmotic gradient. This type of absorption requires metabolic energy (probably through respiration) and occurs even when the osmotic pressure (OP) of the soil water is higher than that of the root cells.

2. Passive Absorption of Water

In passive absorption, roots do not play an active role in the absorption process. The primary force driving water absorption in this case is transpiration.

Steps Involved in Passive Absorption of Water

• Transpiration Tension
  • Transpiration in the leaves creates a tension in the water column within the xylem.
  • This tension is transmitted from the xylem of the leaves to the xylem of the roots through the stem.

• Water Movement from Soil to Root
  • To maintain water supply, soil water enters the cortical cells through root hairs and moves to the xylem in the roots.
  • As the water rises through the plant, it compensates for the loss due to transpiration.

• Passive Role of Root Cells
  • In this process, the root cells remain passive. The movement of water is primarily driven by the transpiration pull in the leaves.

Key Takeaways

• Apoplastic, symplastic, and transmembrane pathways are the primary routes for water movement in plants.
• Active absorption requires energy, with osmotic and non-osmotic absorption processes, while passive absorption is driven by transpiration.
• Root pressure can develop during active absorption, pushing water upward through the xylem.
• Casparian strips in the endodermis force water to move through cell membranes, ensuring selective water uptake.