Mechanism of water transport

Plants unlike animals do not possess any metabolically active pump, like the heart to carry the fluid in the vascular system. The movement of water in passive manner occurs by pressure and by the gradient in the chemical potential. Another way of water movement that occurs in plants is called as cohesion-tension mechanism. Here, the water movement is caused by the absorption and transportation of water bulk, driven by the negative pressure that is created by the transpiration or evaporation of water from the leaves. The forces created by hydrogen bonding are called as “cohesive” and the water movement is due to the cohesive nature of the water movement during transpiration.

The significant tension in the water columns of the plant is sustained by the hydrogen bonds. This tension is considered to be helpful in the movement of water to 100m above the soil surface. The cohesive-tension is generated by transpiration. The evaporation inside the leaves occurs from moist cell wall surfaces surrounded by the air space network. At the interface of the air and water, menisci are formed. The apoplastic water present in the cell wall capillaries is connected with the air present in the sub-stomatal cavity. The sun’s energy used for breaking the hydrogen bonds between the molecules helps in the evaporation of water from the menisci. The surface tension in the water at menisci pulls away the water molecules, to substitute the molecules that are lost due to the evaporation. This surface tension or force that is transmitted through the water columns into the roots will stimulate the water influx from the soil. The continuous water transport pathway is otherwise called as Soil Plant Atmosphere Continuum (SPAC) by the scientists.

The water movement in the plants is carried out by cohesive tension mechanism which is primarily suggested by Stephen Hales. The movement of solute across the semi-permeable membrane is dependent on the water movement as per the chemical potential of water, by the process of osmosis. The water movement between the cells and plant compartments is governed mainly by the osmosis. In the transpiration deficiency, the movement of water into roots is dominated by osmotic forces. The osmotic forces are manifested as guttation and root pressure, which are usually observed in lawn grass. Guttation is the process where the water droplets are accumulated at the leaf margins when the evaporation is low. The root pressure occurs when the solutes are accumulated at higher concentrations in the root xylem than in the other tissues of root. The root water influx is driven by chemical potential gradient across the root and into the xylem. The plants where the transpiration occurs very rapidly, do not contain root pressure. The root pressure is considered to be playing the main role in filling the non-functional xylem especially after winter.

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Transport and uptake of mineral nutrients

The carbon and oxygen for the plant is available from the carbon-dioxide present in the atmosphere. The hydrogen for the plants is obtained from the water and the mineral nutrients are obtained from the soil.

The plant roots cannot absorb mineral nutrients in the passive manner from the soil unlike they take up water. Minerals cannot be taken passively by the plants as they are present as ions or charged particles in the soil. These charged particles cannot get transported across cell membranes. The mineral concentration in the soil is found to be lower compared to that of root. Hence, the minerals have to enter into the epidermal cells of the root through active absorption which means energy in the form of ATP is essential for the entry of minerals into the plant. The gradient of water potential in roots is caused by the active uptake of ions. Osmotic intake of water is also done by the active process. The charged ions also move into the epidermal cells in the passive manner.

The ions are absorbed from the soil through both the active as well as passive transport. The proteins present in the membrane of the root hair cells pump the ions from the soil into the epidermal cell cytoplasm. The endodermal plasma cell membrane consists of transport proteins that are usually present in all other cells. Some of the solutes cross the membrane transport proteins of the endodermal cells while some solutes are not transported. The screening of solute types that enter into the xylem and the quantity of them is monitored by the endodermal cells. The single direction active transport of ions occurs due to the suberin layer of the root endodermis.

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Transpiration and Photosynthesis

The various advantages of transpiration are

–          Generates transpiration pull that helps in the absorption and transport of water in plants.

–          Provides water for photosynthesis.

–          The minerals are transported from the soil in the various parts of the plants.

–          The leaf surface is cooled by the process of evaporation.

–          The cell turgidity is enhanced which maintains the shape and structure of the plants.

 The plants active in photosynthetic process are known to have never-ending requirement for water. The photosynthesis is managed by the water present in the plant and the available amount water can be reduced by transpiration. The humidity present in the rainforests is considered to be part of the process of recycling of water from plant root to leaves and to the atmosphere and in turn to the soil.

The water loss due to transpiration is regulated by the evolutionary process by the emergence of C4 plants possessing C4 photosynthetic system. These plants are evolved to maximize the CO2 availability and reduce the loss of water. The carbon fixation in C4 plants is considered as more efficient than that of C3 plants. For fixing the same amount of carbon-dioxide, C4 plants were found to be losing half the amount of water that is lost from C3 plants. 

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PLANT TRANSPIRATION

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

                       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.

transpiration in leaf

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Upward movement of water in the plant

Water movement is active or passive? When the water moves against the ground in the stem, it needs some energy for moving up.

Root Pressure

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.

Transpiration Pull

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. 

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Absorption of water in plants

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.

water absorption by roots
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.

symplast and apoplast

 

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Long distance movement of water in the plant

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.

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