Absorption of water in plants is a vital process which is very important for the growth of plants and other metabolic activities. Water absorption in lower plants takes place by the process of osmosis through the whole plant body. In higher plants, the mechanism of water absorption is through the root hairs.
Plants mainly absorb “Capillary water” from the soil. There are five types of water that are found in the soil namely: Runway water, gravitational water, hygroscopic water, chemically combined water and capillary water. Among runway water, gravitational water, hygroscopic water, chemically combined water only the capillary water is useful for the plant.
There are some epiphytic plants which grow on the substratum like rock, soil and other plants which absorbs water by their aerial roots.
The total amount of water present in the soil refers to as “Holard”. The amount of total water consumed by the plant refers to as “Chesord”. Water which is not consumed by the plant refers as “Echard”.
Content: Absorption of water In Plants
- Types of water absorption in plants
- Role of root hairs in water absorption
- Mechanism of water absorption in plants
- Factors affecting water absorption in plants
Absorption of water in plants is a biological process which can define as the uptake of the capillary water by the root hairs of a plant from the soil to the root xylem by many ways like respiration, transpiration and osmosis.
Firstly, the two types i.e. active and passive water absorption was introduced by the scientist Renner in 1912-1915.
After the types of water absorption, two theories were introduced to know more about the concept of active absorption of water.
The osmotic theory was given by two scientists Atxins and Priestley.
The non-osmotic theory was given by Bennet, Clark and Thimann in 1951.
Types of water absorption in plants
There are two types of water absorption occurs in a plant namely:
- Active absorption of water
- Passive absorption of water
Active absorption of water
This type of water absorption requires the expenditure of metabolic energy by the root cells to perform the metabolic activity like respiration.
Active absorption in plant occurs in two ways namely: Osmotic and non-osmotic absorption of water.
- Osmotic active absorption of water: In this type, the water absorption occurs through osmosis where the water moves into the root xylem across the concentration gradient of the root cell. The osmotic movement is due to the high concentration of solute in the cell sap and low concentration of the surrounding soil.
- Non-osmotic active absorption of water: In this type, the water absorption occurs where the water enters the cell from the soil against the concentration gradient of the cell. This requires the expenditure of metabolic energy through the respiration process. Hence, as the rate of respiration increases, the rate of water absorption will also increase. Auxin is a growth regulatory hormone which increases the rate of respiration in plants which in turn also increase the rate of water absorption.
Passive absorption of water
This type of water absorption does not require the use of metabolic energy. The absorption occurs by metabolic activity like transpiration.
Passive absorption is the type where the water absorption is through the transpiration pull. This creates tension or force that helps in the movement of water upwards into the xylem sap. Higher is the transpiration rate, higher is the absorption of water.
Role of root hairs in water absorption
The root contains some tubular, hair-like and unicellular structures refers as “Root hair”. In the root system, where the root hair develops refers to as “Root hair zone”.
The zone of root hair is the only region that participates in water absorption activity. Root hair zone is the water permeable region. Root hairs are the outgrowths which arise from the epidermal layer refers as “Piliferous layer”.
The cell wall of root hair consists of a double layer membrane. Pectin is present in the outer layer and cellulose is present in the inner layer of the cell wall. Under the cell wall, there is a cytoplasmic-membrane which is selectively permeable. The cytoplasmic membrane will allow specific substances to pass across the cell concentration gradient.
Inside the cytoplasmic membrane, there are root cells, nucleus and a vacuole or cell sap are present. Soil aggregates surround the root hair which contains water, from where the root hair carries the water into the root xylem by different mechanisms, from which osmosis is most common.
Mechanism of water absorption in plants
The movement of water from the soil to the root xylem occurs by the following stages through the root hairs:
- In the first step, there is an absorption of water by the root hairs of the plant through the process of osmosis. The soil has high water concentration than the cell sap, therefore water will move from the high concentration to the low concentration following the process of osmosis through the cytoplasmic membrane of the root hairs.
- After entering into the root hair, the water will cross the epidermis or piliferous layer of the root system.
- Then, the water will move from the epidermis to the root cortex.
- From the root cortex, the water will then move to the endodermis consists of suberic and passive cells where passive cells help in the diffusion of water further into the root cell.
- Then water moves from the pericycle to the root xylem i.e. perixylem and metaxylem. Therefore, water will store in the xylem root system which the plant body uses to perform various metabolic activities and most importantly for its growth.
There are two kinds of factors involve which influences the activity of water absorption.
- Extrinsic factors
- Intrinsic factors
Extrinsic factors are the external factors or the environmental factors which includes the following:
Soil water: Soil carries five different types of water, out of which the capillary water is useful for the biological activity of the plant.
The concentration of soil solution: The concentration of soil must be less. If there is a high concentration of soil then it will refer as “Physiologically dry soil”. Highly concentrated or dry soil makes the water absorption difficult.
Soil air: There should be space between the soil particles for the proper air supply. If the quantity of oxygen is less, then the quantity of carbon dioxide will increase, which leads to the anaerobic respiration.
Soil temperature: The optimum temperature is 20- 35 degrees Celsius.
Intrinsic factors involve some internal factors that occur inside the cell which includes the following:
It involves the metabolic activities like respiration, transcription and the number of root hairs which directly influences the rate of water absorption.
Therefore water absorption in plants is through the root hairs which carry water present in the soil and forms a zone refers as “Root hair zone”. The root hairs absorb water through their wall which is water-loving “Hydrophilic” in nature. Therefore due to the high permeability of root hairs to the water will help in uptake water either through osmosis or transpiration.