Water Absorption in Plants:  Water uptake into plants is a biological process in which plants take capillary water from the soil through the root hairs into the xylem of the roots during various plant processes such as breathing, transpiration, and penetration. Water supply is an important factor that directly or indirectly affects the activity of all plants, such as photosynthesis and internal water balance.

 Plants have the ability to absorb water directly from roots, stems, leaves, flowers, etc. to the entire surface. However, since water is mainly available in the soil, only the underground root system specializes in absorbing water. The roots are often widespread and grow rapidly on the ground.

Roots Of Plants: In the roots, the most efficient area of ​​fluid intake is the root hair zone. Each root hair zone has thousands of root hairs. Root hair specializes in water absorption. They are tubular by-products with a length of 501,500 μm (0.051.5 mm) and a width of 10 nm.

Each root hair has a central vacuole filled with osmotically active extracellular fluid and peripheral cytoplasm. The walls are thin and permeable, with pectic substances in the outer layer and cellulose in the inner layer. Root hairs penetrate the capillaries and bind to soil particles by pectin binding, absorbing water from the capillaries.

How the root water moves: 

 There are two waterways, apoplast and symplast, from the root hair to the xylem in the root.

• Apoplast: Here, water flows from the root hairs to the xylem through the walls of the intervening cells, without passing through the membrane or cytoplasm. This path has the least resistance to water movement. However, it is destroyed by the presence of impermeable lignosuperin-Casparian strips on the walls of endoderm cells.

• Symplast: Water passes through the protoplasm from cell to cell. It does not penetrate the vacuole. The cytoplasm of adjacent cells is connected by a bridge called plasmodesmata. Water must cross the plasma membrane (cell membrane) at least at one point to enter the symplast. Also called a transmembrane pathway. Symplast movement is supported by a single-cell cytoplasmic flux. However, it is slower than the movement of the apoplast.

Types of Water Absorption in Plants: Plants usually absorb water in two ways.

1. Active absorption of water
2. Passive absorption of water

1) Active absorption of water: Active absorption is the uptake of water by the roots with the help of adenosine triphosphate, which is produced by the respiration of the roots. It is called active absorption because the root cells are actively involved in this process. According to Jenner, active absorption occurs in low-sweat, water-rich plants, with 4% of total water intake taking place in this process. Active absorption is carried out by two theories. Active osmotic water uptake and active non-osmotic water uptake. No energy is required for this process. Active uptake is important for plants.

Active absorption of plants occurs in two ways: 

• Osmotic absorption 
• Non-osmotic absorption of water

  2) Passive absorption of water: Passive intake does not require metabolic energy. It occurs with the help of metabolic activities such as sweating. Here, transpiration creates tension or tension along the water column that extends from the leaves to the roots. Therefore, when water evaporates from the stomata of the leaves, pulling it will allow the water to enter the root xylem. This is how to absorb water. 

• Root hairs play a passive role. Therefore, absorption is called “through the roots” rather than “through the roots”. 
• It usually occurs when there is a lot of sweating due to the opening and closing of the pores. Sweat is the driving force behind water absorption.
• In passive absorption, water can move through the free space of the roots or apoplasts (cell wall + intercellular space). 
• Passive absorption is caused by the stress caused to the xylem cells of the leaves by rapid transpiration or evaporation. 
• This tension travels through the xylem of the stem to the xylem of the roots, where the water rises and reaches the surface where it evaporates. 
• As a result, soil moisture penetrates the cortical cells through the root hairs.

Ascent of SAP: The water absorbed by the roots is distributed to all parts of the plant (the excess is lost by transpiration). In order to reach the top of the plant, water needs to flow up through the stem. This rise in water is called the rise in sap.

Theories of Ascent of sap: 

• Root Pressure Theory
• Vital theories
• Physical Theories

Conclusion: 

The uptake of water into plants occurs through the root hairs that transport the water present in the soil, forming a zone called the root hair zone. Root hairs absorb water through the walls. This is “hydrophilic”, which naturally prefers water. Therefore, the high permeability of the root hairs aids in the uptake of water by either osmosis or transpiration.