Loss of water by evaporation is considered as Transpiration and it takes place through the stomata present in the leaves. Apart from the water loss, exchange of oxygen and carbon-di-oxide also occurs through these pores called stomata. Stomata usually remain open during the day and closed during the night. The opening and closing of stomata are due to the change in the turgidity of the guard cell. The inner wall of the guard cell near the stomatal aperture is elastic and thick. The guard cells flanking the stomatal aperture bulge towards the thin outer walls due to increase in turgidity. The inner walls are forced to form a crescent shape. The stomatal opening is supported by the arrangement of microfibrils in the walls of the guard cell. The stomata are opened easily by arranging the cellulose microfibrils radially rather than longitudinally. If the guard cells lose turgidity, water is lost easily and the inner walls that are elastic will retain the original shape. The lost turgor creates flaccid guard cells and leads to the closure of stomata.
The dorsiventral leaf of all dicotyledons are found to have more stomata below the surface of the leaf. The isobilateral leaf of monocotyledons are known to have equal number of stomata on both the surfaces of the leaf. The external factors that influence transpiration are light, wind speed, temperature, and humidity. Transpiration is affected by certain plant factors like stomata number, stomata distribution, number of opened stomata, canopy structure, and plant water levels, etc.
Stomatal Aperture with Guard cells
The movement of xylem sap due to transpiration is based mainly on certain physical properties of water such as surface tension, cohesion and adhesion. Cohesion represents attraction between water molecules. Adhesion represents attraction of water molecules to the polar surfaces of the tracheary elements. Surface tension represents the attachment of water molecules with each other in liquid phase to be higher than the attachment between water molecules in gas phase. The above water properties provide high tensile strength, which is the force that is used to resist the pulling force. Tensile strength also enhances the capillarity or the ability of water to rise through very thin tubes. The movement of water in tracheids and vessels in plant showed capillary movement due to the tiny diameter.
Water is very much essential in the process of photosynthesis. The xylem vessels that extend from the roots to the leaf veins aid in the transport of necessary water. The question is that what could be the force that is functioning in the transport of essential water into the parenchyma cells of the leaf? The thin continuous film of water that is flowing in the capillary path is pulled by the force of transpiration towards the leaf from the xylem vessels.
In the atmosphere, lower concentration of water vapour in comparison to the intercellular spaces and substomatal cavity allows the diffusion of water into the surrounding environment. This diffusion creates a “PULL”. The investigation and experimental evaluation has revealed that transpiration force can generate pressures enough to drag a column of water of the size of xylem approximately to the height of 130 meters.
The below diagram illustrates the leaf water movement.
Water movement is active or passive? When the water moves against the ground in the stem, it needs some energy for moving up.
The ions in the soil move towards the vascular tissues of the roots in active transport. Due to the change in the potential gradient, water also moves and enhances the xylem pressure. This positive pressure in the xylem is called as root pressure, which is responsible for movement of water to a certain height in the stem.
Let us see how the root pressure functions. A small soft stem is chosen for the test when there is lots of moisture in the atmosphere. The stem is cut at its base during the early part of the day which ends up in the release of a few drops of solution oozed out from the stem. The water drops coming out of the stem is due to the root pressure. If any rubber tube is fixed under the stem then the exudates can be gathered and rate of exudates can be measured. The ingredients of the exudates also can be evaluated.
The root pressure can be observed during the nights and even in the morning when the evaporation is less. The edges of the grass blades and leaves exudes water droplets from the vein openings of many herbs. This type of water loss is known as guttation. The water transport process can at best be stimulated by root pressure. Root pressure itself is not solely responsible for the movement of water to the top of the tall trees. Root pressure aids in establishing the continuation of chain of water molecules in the xylem which might frequently be broken due to intensive tension formed by the transpiration pull. Most of the plants have the water movement aided by transpiration pull rather than by root pressure.
Though there is no specific circulatory system in the plants, water movement through the xylem can be faster and can reach even upto 15 meters of height in an hour. There was a big question regarding this aspect for many years. People were wondering whether the water is reaching the plant top by a ‘push’ or ‘pull’. Many research studies have proved that water was ‘pulled’ towards top and it was due to the transpiration process in the leaves. This model was known as cohesion-tension transpiration pull of water. The force that is responsible for this ‘pull’ is called transpiration. It is observed that only about one percent of water that is absorbed into the plant leaves are used for plant growth and photosynthesis, and the rest is evaporated through the stomata by a process called transpiration.
The plant absorbs water through the roots which are anchored in the soil. The water that is added to the soil will be transported through the roots. The main function of water and mineral absorption is done by the root hairs that are present at the tip of the root. The root hairs are the extensions to the epidermal cells of the root which enhance the surface area of absorption. The root hairs absorb the minerals mostly by the process of diffusion. The water that is taken by the root hairs will move further into the deep layers of the root by two separate ways such as symplast and apoplast pathways.
Apoplast system of water movement
This pathway involves movement of water through adjacent walls of the cells right from epidermis to the inner xylem vessels. The water movement does not occur through the casparian strips region of the endodermis. The movement of water through apoplastic pathway happens through the intercellular spaces and the cell walls. The continuous water flow that is maintained in this pathway is called apoplast. The water movement through the apoplast does not depend on the membrane of the cell as it occurs due to the presence of gradient. The apoplastic transport of water that creates the apoplast is not considered as the obstacle to the mass flow of the water. The water that evaporates into the atmosphere or into the intercellular spaces will generate certain pressure in the apoplast. The mass transport of water occurs because of the cohesive and adhesive features of water. Apoplast is the mass continuous water flow.
Symplast system of water movement
The protoplasts are interconnected in the symplast system of water movement. The water body in the neighbouring cells are connected through the cytoplasmic strands of each of the cells which happens due to the presence of plasmodesmata. The symplast way of water movement involves water transport through the cell cytoplasm via the plasmodesmata. The movement of water enters into the cells through the membranes of the cell which is observed to be a little slow. Symplastic water movement is observed to happen down the potential gradient. The symplastic water movement might be supported by cytoplasmic streaming. The chloroplast movement along with the movement of water is clearly observed in hydrilla leaf.
The movement of water in the roots takes place through the apoplast as the cortical cells are packed loosely. The cortical cells that are loosely arranged allow the movement of water without any resistance. The layer that is present as innermost region of the cortex is endodermis. Endodermis has the walls covered by suberized matrix which is called as casparian strip. When the movement of water cannot happen through the casparian strips, the water is made to travel through the membranes of the cells. The water movement occurs through the symplast and ultimately reach the xylem by crossing the cell membrane. Hence, the movement of water in the root hairs through the endodermis is symplastic. Symplast is the way of water movement through the xylem vessels.
In the xylem vessels, water moves across the cells or through the cells. In the young roots, xylem vessels receive water directly. The tracheids are non living cells and form the apoplast. There are some more structures that help in the transport of water or in absorption of minerals or water. The symbiotic association of the root system with the fungus is termed as mycorrhiza. The mycorrhiza occupy the entire young root as a fungal network and enter into the root cells. The hyphae of the fungus will spread onto a large area and absorb the water and mineral ions from the soil, which cannot be done by the root. The fungus and root mutually provide benefit. The fungus provides minerals and water to the root while the root provides nitrogen containing compounds and sugars to the mycorrhizal growth. Some plants like Pinus are known to have obligate association with the mycorrhizae.
There is a small experiment that demonstrates the movement of water in the plant. A twig bearing white flowers is cut at one end and is placed in the colored water for few hours. The white flowers will turn into the color of water and the region in the twig where the mark is present will indicate that the colored water is taken in by the plant. This experiment explains that plant transports water through the vascular bundles and especially the Xylem. The mechanism of movement of water up into the plant from the soil has to be understood now.
The movement of water up into the plant cannot take place just by diffusion as diffusion in general is very slow and it will be beneficial for the movement of molecules for short distances. The molecule moves across the plant cell in about 2.5 seconds. In the large plants, the minerals and water have to move long distances. The site of the availability of the minerals and their storage in the plant parts are very far from each other. So, diffusion or active transport of the molecules will not be sufficient.
The transport of substances to long distances is very essential to make the water and substances to move long distances at fast rate. The food, water and minerals move as a mass flow or bulk flow system. The minerals and water move to long distances as a mass flow from one point to another due to the difference in pressure. Usually in a mass flow, the substances are seen to be passing in the flowing river which is moving at the equal speed. Mass flow is distinct from the diffusion where the substances move individually based on the individual concentration gradients. The positive and negative hydrostatic pressure gradients help the conduction of mass flow.
The movement of substances through the vascular tissues in the bulk manner is known as translocation. There are highly advanced vascular tissues called xylem and phloem that are concerned with the translocation of water, minerals, hormones, and organic nitrogen from the roots of the plant to the top parts of the plant. The organic and inorganic solutes are translocated from the leaves of the plants to other parts of the plant.
Imbibition is one type of diffusion that features absorption of water by the solids similar to the process of formation of colloids. The absorption of water makes the solids to enhance their volumes. The typical examples for imbibition are water absorption by seeds and water absorption by the dry wood. The wood swells and the pressure that is generated by the swollen wood is utilized by the man in the pre-historic period to break the boulders and rocks. It is also understood that the pressure created due to imbibition is responsible for the seedling development from the soil. The seedlings will not be able to come out into the environment and adjust to it in the absence of the imbibition pressure.
Imbibiton can also be compared with diffusion as the movement of water occurs towards the concentration gradient. The seeds which do not possess water inside will be able to absorb water as there is a water potential gradient between the seed and the water. The proper affinity that exists between the absorbing substance and the liquid that is absorbed will determine the intensity of imbibition.
The movement of water in between the plant cells happens according to the solution concentration that exists in the surrounding. The osmotic pressure in the cytoplasm of the plant cell is equilibrated by the solution external to the cell. Now, the external solution is termed as isotonic. If the concentration of the external solution is lower than that of the cytoplasm then it is termed as hypotonic. If the external solution concentration is higher than that of the cytoplasm then it is termed as hypertonic. The plant cells bulge by water intake from the hypotonic surroundings and shrinks by the expulsion of water to the hypertonic surroundings.
When water moves out of the plant cell then the process is called as plasmolysis. The cell membrane will get separated from the cell when the cell shrinks from the cell wall during the plasmolysis. The shrinking of the plant cell occurs when it is placed in the hypertonic solution which has higher concentration of solute than that is present in the protoplasm of the cell. The movement of water occurs from the cytoplasm and then from the vacuole. The water that is taken away from the cell by diffusion into the external fluid makes the protoplast to shrink away from the cell wall and the cell state is called as ‘plasmolysed’. The water movement occurs usually from the region of higher water potential (inside cell) to the region of lower water potential (outside cell). When the movement of water does not take place from the cell into an isotonic solution then the two regions are said to be in equilibrium. The cells are called as flaccid.
When the plant cell is kept in hypotonic solution, the water moves from the solution into the cell creating a pressure inside the cell on the cell wall. This pressure is called as turgor pressure. The pressure that is created by the protoplasts on the cell wall due to the water movement into the cell is called as pressure potential ᵠp. The cell does not break up as the cell wall is very much rigid. The growth and extention of the cells happen due to the turgor pressure that is created by the entry of water.
The plant cell possesses a cell wall as well as a cell membrane. Cell wall has permeability to the solute molecules and water and so is not an obstacle to the movement of solute molecules. The cells of the plants have a large vacuole at the center consisting of the vacuolar sap. The sap of the vacuole offers the solute potential of the cell. The cell membrane and the vacuolar membrane called the tonoplast are majorly involved in the molecular movement on either side of the membrane of the cell.
Osmosis is the process that refers to the diffusion of water through the membrane that is differently permeable or semipermeable. The net rate of osmosis is dependent on the concentration gradient and pressure gradient. The movement of water from the region of its higher chemical potential to the region of its lower chemical potential till the equilibrium is maintained is called osmosis. The water potential of the two regions should reach the equilibrium.
Potato osmometer: This is a simple experiment to understand and visualize the osmosis process in potato. The potato tuber with a cavity created in it is filled with sugar solution. This tuber with sugar solution when placed inside the water will allow the movement of water into the sugar solution.
Some interesting experiments on Osmosis
In the above diagram, there are two chambers A and B. The water potential of A is higher than that of B while the solute potential of B is greater than that of A. Water will move from A to B as the water concentration in A is more than in B. The chemical potential of water is higher in A than in B. So, Osmosis occurs from A towards B.
The above diagram explains the experiment in which the funnel consists of sucrose solution separated from the pure water taken in the beaker by a semipermeable membrane.
The egg can be used for making the semipermeable membrane. The yolk and albumin present in the egg can be removed from the egg. The shell can be located in the dilute hydrochloric acid solution for some time. The membrane is left intact after the egg shell is dissolved. The pure water will move into the funnel and enhances the solution level in the funnel. The movement towards inside the funnel will continue until the equilibrium between the beaker and funnel is reached.
The pressure that can be applied externally from the funnel will be able to prevent the entry of water into the funnel. The pressure that prevents the water to diffuse into the funnel through the membrane is the osmotic pressure which is the measure of the solute concentration.
To study the process of movement of water, the concept of water potential is considered as fundamental. Two other components that actually determine the water potential are pressure potential and solute potential. The molecules of water consist of kinetic energy. The molecules of the gases and liquid move randomly which is represented as constant and rapid motion. If a system has a higher concentration of water in it, then it has higher water potential. Therefore, we can understand that highest water potential can be seen in pure water. The system with higher water potential is also known to have higher kinetic energy.
If two liquids are in contact with each other, the water molecules move randomly from one system to the other. The net movement of water molecules will happen from the system that has higher kinetic energy to the system with lower kinetic energy. The movement of water will happen from the system that has higher water potential into the system with lower water potential. The movement of the substance from higher solute concentration into the lower solute concentration region is called as diffusion. The water potential is identified with the Greek symbol known as Psi or ᵠ. The units of measurement used for water potential are Pascals also known as pressure units. The water potential of pure water at standard temperature and no pressure is measured as zero.
If pure water is mixed with any substance, the concentration of water reduces and the water potential in it decreases. Any solution will have water potential lower than that of pure water. The evaluation of reduced water potential with the addition of solute in pure water can be done with solute potential. Solute potential is represented by ᵠs which has negative value every time. If the solute molecules are more in pure water, then the solute potential value will be less measured as a more negative value. The pure water that is applied with pressure which is higher than the atmospheric pressure will enhance its water potential. The water potential will trigger the movement of water from one side to another. The plant system will develop pressure inside the cell when the water moves into the cell by diffusion. The pressure will develop inside the cell against the cell wall and the cell is said to be turgid. The turgidity increases the pressure potential. The pressure potential created by the water column in plant xylem is a negative potential while normally pressure potential is positive. The pressure potential exists in xylem during the transport of water into the stem from the soil. The symbol used for denoting pressure potential is ᵠp. The cellular water potential is the combination of pressure and solute potential. The relation among them is given as ᵠw = ᵠp ¬¬+ ᵠs
All the physiological activities in the plant utilize water. Water is considered as an important constituent of all the living organisms. Water is an important solvent for many substances. Protoplasm in the cell consists of water in which many molecules are dissolved. Some of the fruits like watermelon are known to consist of 92 percent of water. About 10 to 15 percent of the fresh weight of the herbs seems to represent the dry material. The presence of water in plants varies based on the plant parts. The woody portion of the plant comprises of comparatively meager amounts of water than the soft portion of the plants. The seed of the plants appears dry though they possess some water in them. Water helps to retain life in the seed and helps it in respiration.
The land plants consume large amount of water every day while much of the water will be evaporated into the atmosphere through the leaves. This process of evaporation of water from the leaves into the environment is called as transpiration. It is estimated that an adult corn plant is found to be able to absorb nearly three liters of water every day. The mustard plant is known to consume an equal amount of water as that of its weight in a 5-hour duration. As water is necessary for plant growth, it is considered as the important factor essential for the productivity and growth of the plant.
Some of the transport proteins carry out the diffusion when two molecules are moving through it together. Moving of molecules across the membrane in one direction is called as symport. If the molecules move in the opposite direction, then it is called as antiport. If one molecule is moving in a different direction than the other molecules, then the movement process is called as uniport.
If the molecules are transferred through the membrane across the concentration gradient with the use of energy then the process is called as Active transport. The membrane proteins are involved in carrying out the active transport. The proteins in the membrane play important role in the active transport and passive transport. Pumps are the membrane proteins which make use of energy to transport the substances against the membrane of the cell. These transport proteins or channels will transfer the substances from their low concentration side to their high concentration side.
The rate of transport will be maximum when the transporting proteins are made use of and are saturated. The membrane protein is very much specific towards the substance that is transferred across the membrane which is the characteristic of an enzyme. Certain inhibitors work on these proteins and make them non-functioning by reacting with the side chains of the proteins.