Introduction
Plant-water relations are concerned with how plants regulate the hydration of their cells, including water collection from the soil, transportation throughout the plant, and evaporation from the leaves.Plant water status is commonly expressed as ‘water potential,’ which has pressure units, is always negative, and is the algebraic sum of the hydrostatic and osmotic pressure of water in simple form.Evaporation of water from the leaves is mainly controlled by stomata and is unbalanced by the flow of water from the soil through the plants to the leaves, causing the plants to wilt.The interactions between plants and water, including hydration of plant cells and water transport within a plant. The most abundant constituent of all physiologically active plant cells is water.It acts as a dissolution medium for substances. Plants consume a large amount of water on a daily basis, and a significant amount is lost through transpiration.
Water Potential
The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant.The major tissues responsible for this movement are the phloem and xylem. Water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects and the potential of water is measured in pascals.
Osmosis
Osmosis is the net movement of a solvent, such as water, by a semipermeable membrane from a less concentrated solution to a more concentrated one, bringing the concentrations on both sides of the membrane to the same level.
Hypertonic → used to denote the solution with higher concentration or more solute.
Hypotonic → used to denote the solution with lower concentration or less solute.
Isotonic → if both solutions have equal concentrations they were said to be isotonic
Again, two processes are tangled: endosmosis and exosmosis.
Endosmosis is the process by which water diffuses by a semipermeable membrane when the medium of surrounding is less concentrated whereas exosmosis is the process by which water diffuses outward through a semipermeable membrane when the surrounding medium is more concentrated.
Plasmolysis
Plasmolysis is the process by which water exits the cell and the cell membrane shrinks away from the cell wall.This happens when the cell is immersed in a hypertonic solution (which has more solutes). Water is lost first from the cytoplasm, then from the vacuole. When the cell is immersed in an isotonic solution, there is no net movement of water; however, when immersed in a hypotonic solution, water moves into the cell and exerts turgor pressure on its walls.
Imbibition
Hydrophilic substances in cell walls and storage tissues, such as polysaccharides and proteins, attract dipolar water. Consecutively, water molecules bind to the charged surfaces.As a result, the imbibant swells in volume; this phenomenon is known as imbibition and the pressure generated as a result of imbibition, i.e., in the form of swelling force, is known as imbibition pressure.Some energy is lost during this process, which is known as imbibitional energy.In many cases, the imbibition force generated by water imbibition is extremely high (ranges from 1000 to 10000 bars). The same method can be used to break down large boulders in queries. This method is still used today.
Functions of Water in Plants
Among the various functions of water in plants are the following:
1.Because many enzymes are dissolved in cell water, it acts as a medium (and sometimes a substrate) for biochemical reactions in cells.
2.Structural support water provides the turgor pressure that gives many cells their shape; as a result, many tissues will lose their structure and wilt when water availability is inadequate.
3.Solute transport between organs via xylem and phloem vessels evaporative cooling of leaves during transpiration.
4.Cell enlargement- Turgor pressure provides the physical force required to expand cells during growth.
Absorption of Water by Plants
Plants absorb water through their entire surface, including their roots, stems, and leaves. However, roots absorb the majority of the water. The root hair zone are the area of young roots where the most absorption occurs. These root hairs are delicate structures that are constantly replaced by new ones at a rate of 100 million per day on average.
Water in the roots moves by two pathways. They can be classified as
1) Apoplast pathway
2) Symplast pathway
Apoplast pathway
It is one of two major pathways for water transport in plants, the other being the symplastic pathway, and it is one of the most important. In apoplastic transport, water and minerals are transported upward through the apoplast and into the xylem at the root of the plant. Some of the ions that enter the body through the roots do not make it to the xylem, which is where they should be. Water moves exclusively through the cell wall in this pathway, with no involvement of membranes. The apoplast pathway carries the majority of the water. The cortex of the root does not oppose such water movement.
Symplast pathway
Water molecules are transported from cell to cell via the plasmodesmata. The plasmodesmata network connects all cells’ cytoplasm. The Casparian strip is the boundary between the cortex and the endodermis. It consists of a wax-like substance called suberin, which blocks water and solutes molecules through the cell wall of the endodermis. Water is now forced to pass through the cell membranes of different cells, forming a transmembrane pathway.
Mechanism of Water Absorption
Water can be absorbed by two methods:
- Active Absorption- It is defined as the process by which the roots actively seek out and absorb water on their own. It occurs when transpiration is low and the amount of water in the soil is high, which is a combination of the two. The absorption of water is aided by the root cells, which are active participants in this process.
- Passive Absorption: According to passive absorption, the root hair cells do not participate actively in the water absorption process. During the process of water absorption, the root hair cells remain passive. It usually takes place when the rate of transpiration is at its highest.
Conclusion
Water is absorbed as a result of root activity. Water absorption is aided by energy in the form of ATP, which is released as a result of root cell metabolic activities such as respiration. Even when the concentration of cell sap is lower than that of soil water, absorption occurs against a concentration gradient. Osmosis is used for passive absorption. Passive absorption occurs along the concentration gradient, which occurs when the concentration of cell sap is greater than that of soil water. When the transpiration rate is high or the soil is dry, water is absorbed. Water deficit is created in transpiring cells as a result of the high rate of transpiration. Rapid transpiration removes water and lowers turgor pressure in root living cells. The suction force created as a result is transmitted to the root xylem. To compensate for the water deficit, it draws water from the surrounding root cells.