Absorption of Water In Plants – Path, Mechanism, Factors, Importance

Water absorption is a critical physiological process in plants, essential for their growth and various metabolic activities. In lower plants, such as algae and mosses, water absorption occurs across the entire plant surface through the process of osmosis. However, in higher plants, this function is specialized and occurs primarily through the root hairs.

The primary mechanism for water uptake in higher plants is via capillary action in the soil. Soil contains several types of water: runoff water, gravitational water, hygroscopic water, chemically combined water, and capillary water. Of these, only capillary water is readily accessible to plants. Epiphytic plants, which grow on surfaces like rocks, may utilize aerial roots to absorb moisture directly from the air. The total volume of water present in the soil is referred to as “holard,” while the amount of water taken up by the plant is termed “chesord,” and the water not utilized by the plant is known as “echard.”

The process of water absorption in plants involves the uptake of capillary water from the soil through root hairs and into the root xylem. This uptake is crucial for various physiological processes, including respiration, transpiration, and osmosis. Adequate water supply influences numerous plant functions, such as photosynthesis and internal water balance.

A deficiency in water can lead to adverse effects in plants, including loss of turgor pressure, leaf wilting, stomatal closure, decreased photosynthetic activity, and disorganization of protoplasm. Water absorbed by plants is generally classified into two phases: apoplastic water and symplastic water. Apoplastic water is located in the cell walls and xylem vessels, while symplastic water resides within the cell protoplast.

Historically, Renner introduced the concept of two types of water absorption—active and passive—during the early 20th century. Subsequently, two primary theories were proposed to explain active water absorption. The osmotic theory, developed by Atxins and Priestley, posits that water absorption is driven by osmotic pressure. In contrast, the non-osmotic theory, proposed by Bennet, Clark, and Thimann in 1951, suggests alternative mechanisms for water uptake. Both theories provide insights into the complex mechanisms governing water absorption in plants.

Water Absorbing Organs

Water absorption in higher plants is primarily facilitated by specialized structures known as root hairs. These root hairs are crucial for efficient water uptake and are typically found in the root hair zone, located just behind the root tips.

• Root Hairs
  • Structure and Composition: Root hairs are tubular extensions of the epidermal cells. This layer, known as the piliferous layer, is composed of hydrophilic pectic substances and cellulose. The cell walls of root hairs are permeable, allowing water to pass through easily. Inside the root hairs, vacuoles filled with cell sap further support water absorption.
  • Function: The primary function of root hairs is to increase the surface area of the root, enhancing its ability to absorb water and nutrients from the soil. As roots elongate, older root hairs die, and new ones develop to maintain contact with fresh water supplies.
• Roots
  • General Structure and Functions: Roots generally originate from the lower portion of a plant. They lack nodes and do not bear leaves or flowers directly. The main functions of roots include absorbing water and nutrients, anchoring the plant in the soil, supporting the stem, and, in some cases, storing food and aiding in propagation.
• Root Structure
  • Meristem: Located at the tip of the root, the meristem is a region of active cell division and growth. It is responsible for producing new cells that contribute to root elongation.
  • Zone of Elongation: Situated behind the meristem, this zone is where cells increase in size as they absorb nutrients and water. The growth of these cells pushes the root further into the soil.
  • Zone of Maturation: This area is where cells differentiate into specific tissues, such as the epidermis, cortex, and vascular tissues.
• Epidermis, Cortex, and Vascular Tissue
  • Epidermis: The outermost layer of the root, responsible for water and mineral absorption.
  • Cortex: Located beneath the epidermis, the cortex facilitates water movement from the epidermis to the vascular tissue and stores nutrients.
  • Vascular Tissue: Situated at the center of the root, this tissue comprises the xylem and phloem, which are involved in the conduction of water and nutrients throughout the plant.
• External Features
  • Root Cap: This protective structure covers the root tip. It consists of cells that are continuously shed as the root grows, safeguarding the meristem beneath from soil abrasion.
  • Root Hairs: Found just behind the root tip, root hairs are fine, elongated cells that significantly increase the root’s absorptive surface area. They typically have a short lifespan, often lasting only one or two days. Root hairs are particularly vulnerable during transplanting or exposure to dry conditions.

Path of Water Across Root Cells

Understanding water movement in plants is essential for comprehending their physiological processes. Water traverses plant roots via three primary pathways: the apoplastic, symplastic, and transmembrane pathways. Each pathway plays a distinct role in facilitating water movement from the soil to the plant’s vascular system.

1. Apoplastic Pathway
   • Description: The apoplastic pathway involves the movement of water through the cell walls and intercellular spaces without crossing any cell membranes.
   • Mechanism: Water moves freely through the extracellular matrix, which consists of the cell walls and the spaces between them. This pathway is predominant in the cortex, where cells are loosely packed and offer minimal resistance.
   • Limitations: The apoplastic pathway encounters a barrier at the endodermis, where the Casparian strip, composed of suberin, prevents further apoplastic movement. The Casparian strip acts as a selective barrier, ensuring that water must enter the symplastic pathway to proceed further into the root.
2. Symplastic Pathway
   • Description: The symplastic pathway entails water movement through the cytoplasm of interconnected cells via plasmodesmata, which are microscopic channels that link adjacent plant cells.
   • Mechanism: Water moves from one cell to another through these cytoplasmic connections. This pathway forms a continuous network of cytoplasmic streams that facilitate the internal distribution of water throughout the plant.
   • Advantage: The symplastic pathway allows for selective transport and communication between cells, as the plasmodesmata can regulate the movement of solutes and signaling molecules in addition to water.
3. Transmembrane Pathway
   • Description: The transmembrane pathway refers to the movement of water across cell membranes, including both plasma membranes and the tonoplast (vacuolar membrane).
   • Mechanism: Water moves through the cell membranes, bypassing the extracellular matrix. This pathway becomes significant when the apoplastic pathway is obstructed, such as by the Casparian strip. Water must then traverse the lipid bilayers of the endodermal cells, possibly crossing the tonoplast to reach the vacuole.
   • Function: This pathway ensures that water can still enter the plant’s vascular system even when the apoplastic route is blocked, facilitating the essential transport of water and nutrients.

Mechanism of water absorption

Water absorption in plants involves two primary mechanisms: active absorption and passive absorption. Each mechanism operates independently to facilitate the uptake of water from the soil and its movement through the plant. Here, we will explore these mechanisms in detail.

1. Active Absorption of Water

Active absorption is an energy-dependent process that involves the direct involvement of root cells. It requires metabolic energy, typically derived from cellular respiration, to facilitate the uptake of water. This process can be divided into the following steps:

• Imbibition: The initial stage of active absorption begins with the imbibition of soil water by the hydrophilic (water-attracting) cell walls of root hairs. The osmotic pressure (OP) of the cell sap within root hairs is generally higher than that of the surrounding soil water. Consequently, water from the soil moves into the root hairs through osmotic diffusion.
• Osmotic Gradient Creation: As water enters the root hairs, their osmotic pressure, suction pressure (SP), and diffusive pressure deficit (DPD) decrease. This results in an increase in turgor pressure within the root hairs. Adjacent cortical cells have higher osmotic pressures and DPD compared to root hairs, leading to water transfer from root hairs to these neighboring cells via osmotic diffusion.
• Movement Through Cortex and Endodermis: Water continues to diffuse through the cortex to reach the endodermis. In the endodermis, water passes through special thin-walled passage cells, as the Casparian strips in other endodermal cells block apoplastic movement.
• Transfer to Pericycle and Xylem: Water then moves from the endodermis into the pericycle cells by osmotic diffusion. As the pericycle cells become turgid, their suction pressure decreases. Water is finally drawn into the xylem vessels from the pericycle cells. This entry of water creates a pressure known as root pressure within the xylem, which helps to elevate the water column to various heights in the plant.
• Osmotic Absorption: During osmotic absorption, water moves into the xylem of the roots following an osmotic gradient. This process involves imbibition by the hydrophilic cell walls and subsequent diffusion through root tissues to the xylem.
• Non-Osmotic Absorption: In certain conditions, water can be absorbed even when the osmotic pressure of the soil water is higher than that of the root cells. This non-osmotic absorption is energy-intensive and likely involves metabolic energy from respiration.

2. Passive Absorption of Water

Passive absorption of water is a crucial physiological process in plants, primarily driven by transpiration. This process occurs when the uptake of water by the root system does not require direct metabolic activity from the root cells. Here, we provide a detailed explanation of this phenomenon.

Mechanism of Passive Absorption

1. Transpiration and Water Tension:
   • Transpiration, the process of water vapor loss from the leaves, creates a negative pressure or tension in the xylem vessels of the leaves.
   • This tension is a result of water evaporating from the stomata, leading to a continuous pull on the water column in the xylem vessels.
2. Transmission of Tension:
   • The tension generated in the leaf xylem is transmitted downward through the xylem vessels to the xylem in the stem and further to the xylem in the roots.
   • This creates a cohesive force that pulls water upward from the soil through the plant’s vascular system.
3. Water Movement from Soil to Roots:
   • As the water column in the xylem is pulled upward due to the transpiration-driven tension, a suction effect is created in the root xylem.
   • Soil water, then, moves into the root cortical cells through the root hairs by diffusion and capillary action.
4. Entry into Root Xylem:
   • The water absorbed by the cortical cells is subsequently transported to the xylem vessels in the roots.
   • This process ensures that water is continuously supplied to the transpiring surfaces of the leaves.

Characteristics of Passive Absorption

• Role of Transpiration:
  • The key driver of passive absorption is the rate of transpiration. High transpiration rates enhance the tension in the xylem and, consequently, the movement of water from the soil to the root xylem.
• Root Cell Inactivity:
  • During passive absorption, root cells do not actively participate in the uptake of water. The process relies on the physical forces generated by transpiration rather than metabolic activity within the roots.
• Water Entry Mechanism:
  • Water enters the plant through root hairs via osmosis and capillary action. This entry is facilitated by the negative pressure created in the xylem vessels due to transpiration.

Factors Affecting Water Absorption in Plants

External Factors

1. Available Soil Water
   • Capillary Water vs. Other Forms: Plants primarily absorb capillary water, which is held in films between soil particles. This form of water is readily available for uptake. Conversely, hygroscopic water, combined water, and gravitational water are less accessible to plants. Hygroscopic water adheres tightly to soil particles, combined water is chemically bound, and gravitational water drains away quickly.
   • Impact of Excess Water: An excess of water in the soil can lead to poor aeration. When soil becomes waterlogged, it reduces the oxygen availability necessary for root respiration. Consequently, root metabolic activities are impeded, leading to decreased water absorption.
2. Concentration of the Soil Solution
   • Osmotic Potential (OP): The concentration of solutes in the soil solution affects osmotic potential. Higher concentrations of solutes, such as salts, increase the osmotic pressure of the soil solution. When the osmotic potential of the soil solution exceeds that of the cell sap within root cells, the absorption of water is hindered. This is particularly evident in alkaline soils and marshy areas where high salt concentrations prevail, resulting in reduced osmotic water absorption.
3. Soil Air (Oxygen Levels)
   • Oxygen Deficiency: Water absorption is significantly affected by soil aeration. Poorly aerated soils, characterized by low oxygen levels and high carbon dioxide accumulation, impede root respiration. This deficiency in oxygen slows down the metabolic activities of roots and reduces their growth and elongation. As a result, roots are less effective in accessing water from the soil. Waterlogged soils are particularly detrimental as they are both poorly aerated and physiologically dry.
4. Soil Temperature
   • Optimal Temperature Range: The rate of water absorption is positively correlated with soil temperature up to an optimal range of 20-35°C. Within this range, the physiological processes involved in water uptake are most efficient.
   • Effects of Temperature Extremes: At temperatures exceeding this optimal range, water absorption rates decline. Similarly, at very low temperatures, around 0°C, water absorption is greatly diminished due to several factors:
     • Increased Viscosity: Both water and protoplasm become more viscous, impeding their movement.
     • Decreased Membrane Permeability: The permeability of cell membranes decreases, making it harder for water to enter the cells.
     • Reduced Metabolic Activity: Lower temperatures slow down the metabolic processes within root cells.
     • Restricted Root Growth: Cold conditions inhibit root growth and elongation, further limiting water uptake.

Internal Factors

1. Transpiration:
   • Transpiration plays a pivotal role in regulating water absorption. The rate of water absorption is directly proportional to the rate of transpiration. High transpiration rates create a negative pressure (transpiration pull) that facilitates water uptake by roots. This process ensures a continuous flow of water from the soil into the plant.
2. Root Hairs:
   • The presence and density of root hairs significantly influence water absorption. Root hairs increase the surface area available for water uptake, enhancing the efficiency of water absorption. A larger number of root hairs correlates with a higher rate of water absorption due to the expanded contact area with the soil solution.
3. Metabolism:
   • Metabolic activity within the plant, particularly root respiration, is closely related to water absorption. Factors that inhibit respiration, such as poor soil aeration or the presence of respiratory inhibitors like potassium cyanide (KCN), can reduce the rate of water absorption. Effective metabolism supports the energy needs of the root system, thereby promoting water uptake.

Importance of Absorption of Water In Plants

The absorption of water in plants is crucial for their survival and overall health. Here’s why it’s so important:

1. Nutrient Transport: Water is essential for the transport of nutrients from the soil into the plant. Through a process called transpiration, water moves through the plant’s vascular system, helping dissolve and carry nutrients from the roots to other parts of the plant.
2. Photosynthesis: Water is a key component in photosynthesis, the process by which plants convert sunlight into energy. It is used to produce glucose and oxygen, which are vital for the plant’s growth and energy.
3. Turgor Pressure: Water helps maintain turgor pressure within plant cells. This pressure keeps the cells firm and the plant upright. Without sufficient water, plants may wilt as their cells lose rigidity.
4. Temperature Regulation: Water helps regulate the plant’s temperature through the process of transpiration. As water evaporates from the plant’s leaves, it helps cool the plant down, similar to how sweating cools the human body.
5. Growth and Development: Adequate water supply is necessary for cell expansion and growth. Water is a major component of cell sap and helps in cell elongation, which is critical for the growth of roots, stems, and leaves.
6. Metabolic Functions: Water is involved in various biochemical reactions within the plant. It provides the medium for enzymatic activities and helps in the synthesis of different plant hormones.
7. Stress Resistance: Water availability affects the plant’s ability to resist stress conditions. Plants under water stress may exhibit reduced growth, lower yield, and increased susceptibility to diseases and pests.

Difference between active and passive absorption

In plant physiology, the processes of active and passive absorption are critical for understanding how plants acquire water and nutrients from their environment. These two mechanisms, although aimed at similar outcomes, differ significantly in their requirements, processes, and efficiency.

Active Absorption

1. Energy Requirement
   • Active absorption necessitates the use of energy. Specifically, it relies on the expenditure of adenosine triphosphate (ATP) to facilitate the movement of ions and nutrients into the plant roots. This energy is crucial because it drives the transport processes against concentration gradients.
2. Driving Force
   • This type of absorption is primarily created by an osmotic gradient, which is established through the active transport of ions. The osmotic gradient induces water uptake into the root cells by osmosis.
3. Location and Mechanism
   • Active absorption predominantly occurs at the root hair cells. These cells are specialized structures that extend from the root’s epidermis, enhancing the surface area for nutrient and water uptake.
4. Pathway
   • The absorption process usually follows the symplastic pathway. In this pathway, water and solutes move through the cytoplasm of cells, connected by plasmodesmata, which are microscopic channels that traverse the cell walls.
5. Association with Respiration
   • The efficiency of active absorption is closely linked to cellular respiration, as respiration provides the necessary ATP for the active transport mechanisms.
6. Rate of Absorption
   • The rate of active absorption tends to be slower compared to passive absorption. This slower rate is due to the time required for energy-intensive processes and the establishment of osmotic gradients.

Passive Absorption

1. Energy Requirement
   • Passive absorption, in contrast, does not require direct energy input. This process relies on natural physical forces, such as concentration gradients, rather than cellular energy expenditure.
2. Driving Force
   • Passive absorption is driven by transpiration pull. Transpiration creates a negative pressure within the plant’s vascular system, which facilitates the movement of water and dissolved nutrients from the soil into the roots.
3. Location and Mechanism
   • This type of absorption predominantly occurs through stomata-containing organs, with a primary focus on the leaves. Stomata are pores on the leaf surface that regulate gas exchange and contribute to water movement.
4. Pathway
   • The process typically follows the apoplastic pathway. In this pathway, water and solutes move through the spaces between cell walls and along the cell wall matrix, bypassing the cell membranes.
5. Association with Transpiration
   • The rate and efficiency of passive absorption are closely correlated with transpiration rates. High transpiration rates increase the suction force that draws water into the plant.
6. Rate of Absorption
   • Passive absorption generally occurs at a faster rate compared to active absorption. The process benefits from the continuous movement of water driven by transpiration and does not require the additional step of energy input.

Aspect	Active Absorption	Passive Absorption
Energy Requirement	Requires energy; relies on adenosine triphosphate (ATP) to move ions and nutrients against gradients.	Does not require direct energy; relies on natural physical forces like concentration gradients.
Driving Force	Created by an osmotic gradient established through active transport of ions.	Driven by transpiration pull, creating negative pressure in the vascular system.
Location and Mechanism	Predominantly occurs at root hair cells, which extend from the root’s epidermis to enhance surface area.	Predominantly occurs through stomata-containing organs, primarily leaves, regulating gas exchange.
Pathway	Typically follows the symplastic pathway, moving through cytoplasm of cells connected by plasmodesmata.	Typically follows the apoplastic pathway, moving through spaces between cell walls and wall matrix.
Association with Respiration/Transpiration	Closely linked to cellular respiration for ATP supply.	Closely correlated with transpiration rates, which increase the suction force.
Rate of Absorption	Tends to be slower due to energy-intensive processes and establishment of osmotic gradients.	Generally faster due to continuous water movement driven by transpiration, without energy input.