Role of macro and micronutrients

Based on the criteria of essentiality, some of the elements are considered to be very much essential for the growth and metabolism of the plant. These elements are further classified into two groups. They are micronutrients and macronutrients.

Essential elements participate in the metabolic processes of the plant cell-like regulating the osmotic concentration of the cell sap, permeability of the cell membrane, buffering action, electron transport systems, and enzymatic activity. These elements form a major part of the macromolecules and co-enzymes.

The various functions of the mineral elements are given here:

Nitrogen

It is absorbed as nitrate, nitrite, and ammonia. Nitrogen is taken by all parts of the plant including the meristematic tissues and metabolically active cells. Nitrogen is present in nucleic acids, proteins, hormones, and vitamins.

Phosphorus

It is absorbed in the form of phosphate ions. Phosphorus is part of the cell membranes, nucleic acids, proteins, nucleotides and is required in all the phosphorylation reactions.

Potassium

It is absorbed as potassium ions and is utilized mostly in meristematic tissues, leaves, root tips, and buds. The anion-cation balance is maintained in the cells by the potassium. It is used in the protein synthesis, closure and opening of stomata, in the maintenance of cell turgidity and in the activation of enzymes.

Calcium

It is absorbed in the form of calcium ions. It is utilized by the meristematic and differentiating tissues. Calcium is used in the cell wall synthesis in the form of calcium pectate in the middle lamella. It is also used in the formation of the mitotic spindle. Calcium is used in the normal functioning of the cell membranes. It helps in the activation of the enzymes and plays an important role in maintaining the metabolic activities.

Magnesium

This divalent cation is involved in the activation of the enzymes of respiration, photosynthesis and in the synthesis of DNA and RNA. Magnesium is part of the chlorophyll structure and it helps in the maintenance of the structure of the ribosome.

Sulphur

It is absorbed in the form of sulfate and is present in two amino acids called cysteine and methionine. Sulfur is also the major constituent of coenzymes, ferredoxin, and vitamins.

Ferrous

It is the major constituent of proteins like cytochromes and ferredoxin that are associated with the transfer of electrons. It is used in the chlorophyll synthesis.

Manganese

It is absorbed in the form of manganous ions. It activates the enzymes associated with respiration, photosynthesis and nitrogen metabolism. Manganese is involved in the water splitting to liberate oxygen at the time of photosynthesis.

Zinc

It is absorbed as zinc ions. It stimulates various enzymes like carboxylases. This enzyme is required in the auxin synthesis.

Copper

It is absorbed as cupric ions. It is associated with the plant metabolism. Iron is involved in the enzymes of the redox reactions.

Boron

It is absorbed as borate ion. Boron is necessary for the uptake of calcium and its utilization, pollen germination, cell differentiation, cell elongation and carbohydrate translocation.

Molybdenum

Plants absorb it in the form of molybdate ions. It is associated with the functioning of nitrogenase and nitrate reductase which participates in nitrogen metabolism.

Chlorine

It is absorbed in the form of chloride. It helps in the determination of solute concentration and anion-cation balance in the cells.

Symptoms of plants deficient in essential elements

If the essential elements are supplied in limited quantities, the growth of plant will be reduced. The concentration of essential elements below which the growth of the plant is reduced is known as the critical concentration of the element. If the element is present in a lower concentration than its critical concentration then it is said to be deficient.

If every element has a specific structural and functional role in plants, the plants show morphological changes in the absence of that particular element. These morphological changes identify certain deficiencies of elements and are called as deficiency symptoms. The deficiency symptoms differ from one element to another and they vanish when the mineral element that is deficient is supplied to the plant. If the deficiency of that element continues then it would lead to the death of the plant. The plant parts that show the deficiency symptoms depends on the mobility of the element in the plant. If the element is actively mobilized inside the plant and is exported to the younger parts of the plant tissue, the deficiency symptoms are seen earlier in older tissues.

For example, the deficiency symptoms of potassium, nitrogen, and magnesium are seen initially in senescent leaves. Older leaves get separated making these elements mobilized to younger leaves. The young tissues are initially made to experience the deficiency symptoms when the element is immobile and not transported out of the mature organs.

Sulfur and calcium are the elements that are part of the structural component of the cell and so are not released from the organs. This aspect of the mineral nutrition of the plants is of higher significance and importance to agriculture and horticulture.

The deficiency symptoms that are generally appearing in plants include necrosis, chlorosis, premature fall of buds and leaves, and ceasing of cell division. Chlorosis is the chlorophyll loss leading to yellowing of leaves, which is caused from the deficiency of elements like Fe, S, Mn, Mg, Zn, Mo, N,  and K. Necrosis is another symptom of the leaf that occurs due to the deficiency in Mg, Ca, K and Cu. Reduction in the levels of N, Mo, S, and K stops the division of cells. Some elements such as Mo, N, and S delay the flowering process when their levels in plants are low. Therefore, deficiency of elements can lead to several symptoms caused by the deficiency of various different elements. Same symptoms might be caused due to the deficiency of a single element out of many. To learn about the deficient element, the symptoms developed in all parts of the plant have to be studied and compared with the standard symptoms that are already given in the standard tables. It is also vital to know that different parts of the plant will show different symptoms in response to the same element.

Essential mineral elements required by plants

Minerals enter into plants through roots. Plants were found to be consisting of more than 60 elements. Selenium and gold are also accumulated by some plants. Plants surviving near nuclear test sites will absorb even radioactive strontium. How to check whether the plant is really in need of all the minerals. How to know about the essential minerals for the plants.

Criteria for essentiality

The reasons for the elements to be considered essential for the plant growth are mentioned below.

§  The normal growth and reproduction of the plant is supported and the plants depend on the elements to complete their life cycle and prepare the seeds for further propagation.

§  The requirement of the element is specific and cannot be replaced by another element.

§  This element is involved in the plant metabolism directly.

Based on the above-mentioned features, a few elements were found to be completely essential for the plant metabolism and growth. These elements are further widely divided into two categories, like macronutrients and micronutrients.

Macronutrients

These are generally present in the plant tissues in large amounts, like 10mmole per kg of dry matter. The macronutrients include oxygen, hydrogen, nitrogen, carbon, sulfur, calcium, potassium, magnesium, and phosphorus. Of all these elements, carbon, oxygen, and hydrogen are mainly obtained from water and carbon-di-oxide, while the other elements are taken from the soil as mineral nutrition.

Micronutrients

These are called trace elements as they are necessary in small amounts. These nutrients include copper, manganese, iron, molybdenum, nickel, boron, chlorine and zinc.

Apart from the above-mentioned nutrients, there are some more useful elements such as selenium, sodium, cobalt, and silicon which are used by the higher plants. Based on the relevant functions of the essential elements, they are classified into four categories. They are:

§  Essential elements that form part of the biomolecules and that are seen as structural part of the cell, like carbon, hydrogen, nitrogen, and oxygen.

§  Essential elements that are part of the components of the chemical substances in plants, like phosphorus in ATP and magnesium in chlorophyll.

§  Essential elements that act as catalysts inactivating or inhibiting the enzymes. For example, Mg2+ activates Ribulose bis-phosphate carboxylase oxygenase and phosphoenolpyruvate carboxylase, which are critical enzymes in photosynthetic carbon fixation processes. Nitrogenase is activated by molybdenum, alcohol dehydrogenase is activated by zinc during the process of nitrogen fixation. There are several other biochemical pathways that involve minerals in the process.

§  Essential elements can also change the osmotic potential of the cell. Stomatal opening and closure are influenced by potassium. Minerals show the impact on determining the water potential in the cell.

Pressure flow or mass flow hypothesis

The process of translocation of sugars from the source to the sink is known as a pressure flow hypothesis. Glucose is synthesized by photosynthesis and it is changed into sucrose. Sucrose moves into the companion cells and then into the sieve tube cells through the active transport process. The increase of sugar content in the source region makes the phloem to become hypertonic. The water moves from the adjacent xylem into the phloem through the process of osmosis. Once the osmotic pressure is increasing in the source region, the phloem sap moves to the region of lower osmotic pressure. The osmotic pressure of the sink has to be lower than that of the source.

The phloem sap moves out of the phloem into the cells that make use of the sugars by the active transport process. The sugars at the sink region are converted into energy, cellulose and starch. When the sugars are moved out from the source, the osmotic pressure reduces at that region and hence the water moves out of the phloem.

To conclude, the sugars move from the phloem after they are loaded into the sieve tubes by the active transport. The phloem is loaded to create the water potential gradient that can facilitate the movement of sap.

The phloem comprises of long sieve tubes that have sieve plates at the end of each cell. The sieve plates possess small holes. The cytoplasmic strands traverse through the sieve plate holes and form continuity in those filaments. The pressure flow starts when the hydrostatic pressure in the sieve tubes increases and the movement of sap occurs in the phloem. At the sink region, the sugars in the phloem are actively transported out of it, which get transformed into complex carbohydrates. The solute removal creates a higher water potential in the phloem, while the water moves out of the phloem into the nearest xylem vessels.

An experiment called ‘girdling’ helps in identifying the plant tissues that are involved in the transport of food materials. The bark that exists till the phloem layer towards inside, in the trunk region is removed. This stops the food movement downwards. After some time, it can be observed that the part of the stem that is above the removed region will be swollen. This simple experiment illustrates that the phloem is responsible for the transport of food and the food transport occurs in one direction itself, down towards the roots. This experiment can be easily performed by everyone.

Transport and uptake of mineral nutrients

The carbon and oxygen for the plant are 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 a 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 the 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 a 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.

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 a 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 a 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 airspace 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 from 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 a 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 the 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.

Phloem Transport – Flow from Source to Sink

The primary food source commonly called as sucrose is transported by the vascular tissue phloem from the source to the sink. The source is the plant part which can synthesize the food, like leaves. The sink is the plant part that requires the food and stores the food. The source and sink are reversed based on the season or the needs of the plant. The sugars stored in the roots would be made to move as the source of food in the spring while the tree buds act as sink receiving the food from the roots. The photosynthetic apparatus development and growth of the plant parts occurs by the energy generated from the food that is received by the sink. The positions of source and sink are variable and hence the movement of food through phloem can be bi-directional (upwards or downwards). The movement of water in the xylem is always in the upward direction. The flow of water in transpiration is towards up and the movement of phloem sap occurs in both the directions, depending on the source of the sugar and sink that can make use, store and remove the sugars. The major constituents of phloem sap are water and sucrose while the hormones, amino acids, and sugars are also translocated through the phloem.

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 the photosynthetic process are known to have a 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. 

MINERAL NUTRITION OF PLANTS

The basic requirements of all the plants are macromolecules, such as carbohydrates, proteins, fats, minerals, and water. These elements help in the growth and development of plants. This article discusses the various methods involved in the growth and development of the plants and the conditions that aid in monitoring the necessity of these methods.

Methods to analyze the mineral requirements of plants

A well-known botanist in 1860 called Julius Von Sachs could demonstrate that plants can be grown in a well-defined nutritious environment in the absence of the soil. This technique of growing plants in a nutrient solution is called “Hydroponics”. From then onwards various methods were developed and improved to find out the essential mineral nutrients for the plant growth. All these methods focused basically on growing the plants in a defined mineral solution in the complete absence of soil. Mineral nutrient salts and purified water were essential for running these methods.

Series of experiments were conducted in which plant roots were immersed in the nutrient solution. Each of the elements was added and removed alternately to check the necessity of that particular element in the growth of the plant. Gradually, a mineral solution that is appropriate for the optimal plant growth was designed and developed. The essential elements for the plant were recognized by this method. Hydroponics is a technique that is used commercially in the production of vegetables, like lettuce, tomato, and seedless cucumber. It has also been found that nutrient solutions have to be aerated to support optimal growth of the plant.

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 c

reates flaccid guard cells and leads to the closure of stomata.

The dorsiventral leaf of all dicotyledons is found to have more stomata below the surface of the leaf. The isobilateral leaf of monocotyledons is known to have an 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.

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 the liquid phase to be higher than the attachment between water molecules in the 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, a lower concentration of water vapor 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 have 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.

Upward movement of water in the plant

Is water movement 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 the 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 exude 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 the 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 up to 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 the 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 the 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. 

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.

Apoplast system of water movement
This pathway involves the movement of water through adjacent walls of the cells right from the 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 a 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 neighboring cells is 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 an innermost region of the cortex is endodermis. Endodermis has the walls covered by the suberized matrix which is called as the 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 the absorption of minerals or water. The symbiotic association of the root system with the fungus is termed as mycorrhiza. The mycorrhiza occupies 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 an obligate association with the mycorrhizae.

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 move long distances at a fast rate. The food, water and minerals move as a mass flow or bulk flow system. The minerals and water move 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.

What is imbibition?

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. 

 Imbibition 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.

Plasmolysis

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 a 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 the 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 extension of the cells happen due to the turgor pressure that is created by the entry of water.

DIFFUSION and OSMOSIS

The plant cell possesses a cell wall as well as a cell membrane. Cell wall has the permeability of 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

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 eggshell 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.

Water Potential

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 the 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 the 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. The solute potential is represented by ᵠs which have 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 a 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

Plant Water Relations

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 a 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.

Transport in Plants

Plants ranging from small plants to tall trees have a mechanism of transport of water from the roots till their leaves. The movement of water occurs either by utilizing or not utilizing metabolic energy. It is learned that plants have to move water along long distances than the animals. In plants, the food that is prepared by photosynthesis has to reach the root tips that are situated deep inside the soil while the water in the soil has to reach the tip of the stem. Water has to get transferred from one cell to another through the membrane. The transport mechanism in the plants is easily understood if we have the basic knowledge of the plant cell structure, its anatomy, process of diffusion, ions and chemical potential.
The materials that have to be transported in the flowering plants are organic substances, mineral nutrients, growth regulators of plant and water. The studies on transport mechanism in plants have revealed that the above substances are moved inside the plant through small distances by diffusion, active transport accompanied by cytoplasmic streaming. Long distance transport is done by vascular system constituting xylem and phloem. This method of long-distance transport is called as translocation.
The water and minerals in plants with roots travel through xylem in one direction that is from roots to stem and leaves. The organic substances and mineral nutrients travel in both the directions. The organic substances that are synthesized in the leaves will be transported to all other parts of the plant body along with the storage parts. The minerals are taken up by the roots from the soil and are transported to the leaves, stems and other parts of the plant. If any part of the plant is dead then the transported nutrients to those parts will be taken back and redirected to the growing regions of the plant. Apart from the transport of water, minerals, and organic nutrients, plants also transport growth regulators, hormones, and chemical stimuli.

Means of Transport

Diffusion

Movement of substances by diffusion is known as passive and it is transferred from one cell to the other cell. Diffusion also involves the transfer of substances in the intercellular space of the leaf to its outside. Passive diffusion does not involve any energy. The molecules in a diffusion process move from higher concentration region to that of lower concentration in a random fashion in the case of passive diffusion. Diffusion takes place very slowly and it is found to occur in the living tissue. Diffusion is very important in plants as it is the only way to gaseous drive inside the plant. The rate of diffusion is influenced by the factors like concentration gradient, pressure, temperature and permeability of the membrane.

Facilitated diffusion

A gradient has to be present for diffusion to occur. The rate of diffusion depends on the substance size and the smaller sized substances will move quickly. The diffusion of the substances through the membrane is based on the solubility of the substance in the lipids, which are the important ingredients of the membrane. The substance that is able to get dissolved in lipids will be able to transfer through the membrane rapidly. The hydrophilic molecules which are able to dissolve properly in water will not be able to get diffused through membrane properly. Therefore, the transfer of water-soluble substances has to be facilitated through the membrane as the membrane is made of lipids. The proteins present in the membrane facilitate the molecules to pass through the membrane. Even for the facilitated diffusion, there should be a concentration gradient that exists across the membrane.

In the case of facilitated diffusion, the proteins inside the membrane aid in the transfer of substances and the transport process do not involve energy. The transport of molecules from their lower concentration to their higher concentration cannot be classified as facilitated diffusion and it will not need energy input. The transport of molecules will become maximized when all the proteins in the membrane are used up.

The facilitated diffusion is very specific. Specific substances are chosen by the cell for its uptake in this form of diffusion. The inhibitors that stop the proteins to transport the substances will also show the impact on facilitated diffusion. The proteins are found to create channels in the membrane which are small while some are larger. A few of the protein channels works by controlling mechanisms. The channels called prions are present in the membranes of mitochondria, plastids, and bacteria which are known to allow larger molecules to pass through them. The aquaporins are protein molecules present in the membrane that will rotate and leave the molecules inside the cell.