Turgor pressure

Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall.[1]

It is also called hydrostatic pressure, and defined as the pressure measured by a fluid, measured at a certain point within itself when at equilibrium.[2] Generally, turgor pressure is caused by the osmotic flow of water and occurs in plants, fungi, and bacteria. The phenomenon is also observed in protists that have cell walls.[3] This system is not seen in animal cells, seeing how the absence of a cell wall would cause the cell to lyse when under too much pressure.[4] The pressure exerted by the osmotic flow of water is called turgidity. It is caused by the osmotic flow of water through a selectively permeable membrane. Osmotic flow of water through a semipermeable membrane is when the water travels from an area with a low-solute concentration, to one with a higher-solute concentration. In plants, this entails the water moving from the low concentration solute outside the cell, into the cell's vacuole.[5]

Mechanism

Turgor pressure on plant cells diagram

Osmosis is the process in which water flows from an area with a low solute concentration, to an adjacent area with a higher solute concentration until equilibrium between the two areas is reached.[6] All cells are surrounded by a lipid bi-layer cell membrane which permits the flow of water in and out of the cell while also limiting the flow of solutes. In hypertonic solutions, water flows out of the cell which decreases the cell's volume. When in a hypotonic solution, water flows into the membrane and increases the cell's volume. When in an isotonic solution, water flows in and out of the cell at an equal rate.[4]

Turgidity is the point at which the cell's membrane pushes against the cell wall, which is when turgor pressure is high. When the cell membrane has low turgor pressure, it is flaccid. In plants, this is shown as wilted anatomical structures. This is more specifically known as plasmolysis.[7]

Turgid
A turgid and flaccid cell

The volume and geometry of the cell affects the value of turgor pressure, and how it can have an effect on the cell wall's plasticity. Studies have shown how smaller cells experience a stronger elastic change when compared to larger cells.[3]

Turgor pressure also plays a key role in plant cell growth where the cell wall undergoes irreversible expansion due to the force of turgor pressure as well as structural changes in the cell wall that alter its extensibility.[8]

Turgor pressure in plants

Turgor pressure within cells is regulated by osmosis and this also causes the cell wall to expand during growth. Along with size, rigidity of the cell is also caused by turgor pressure; a lower pressure results in a wilted cell or plant structure (i.e. leaf, stalk). One mechanism in plants that regulate turgor pressure is its semipermeable membrane, which only allows some solutes to travel in and out of the cell, which can also maintain a minimum amount of pressure. Other mechanisms include transpiration, which results in water loss and decreases turgidity in cells.[9] Turgor pressure is also a large factor for nutrient transport throughout the plant. Cells of the same organism can have differing turgor pressures throughout the organism's structure. In higher plants, turgor pressure is responsible for apical growth of things such as root tips[10] and pollen tubes.[11]

Dispersal

Transport proteins that pump solutes into the cell can be regulated by cell turgor pressure. Lower values allow for an increase in the pumping of solutes; which in turn increases osmotic pressure. This function is important as a plant response when under drought conditions[12] (seeing as turgor pressure is maintained), and for cells which need to accumulate solutes (i.e. developing fruits).[13]

Flowering and reproductive organs

It has been recorded that the petals of Gentiana kochiana and Kalanchoe blossfeldiana bloom via volatile turgor pressure of cells on the plant's adaxial surface.[11] During processes like anther dehiscence, it has been observed that drying endothecium cells cause an outward bending force which led to the release of pollen. This means that lower turgor pressures are observed in these structures due to the fact that they are dehydrated. Pollen tubes are cells which elongate when pollen lands on the stigma, at the carpal tip. These cells grow rather quickly due to increases turgor pressure. These cells undergo tip growth. The pollen tube of lilies can have a turgor pressure of 0–21 MPa when growing during this process.[14]

Seed dispersal

Mature squirting cucumber
Mature squirting cucumber fruit

In fruits such as Impatiens parviflora, Oxalia acetosella and Ecballium elaterium, turgor pressure is the method by which seeds are dispersed.[15] In Ecballium elaterium, or squirting cucumber, turgor pressure builds up in the fruit to the point that aggressively detaches from the stalk, and seeds and water are squirted everywhere as the fruit falls to the ground. Turgor pressure within the fruit ranges from .003 to 1.0 MPa.[16]

Growth

Tree growing out of rock in Coire Earb - geograph.org.uk - 853941
Tree roots penetrating rock

Turgor pressure's actions on extensible cell walls is usually said to be the driving force of growth within the cell.[17] An increase of turgor pressure causes expansion of cells and extension of apical cells, pollen tubes, and in other plant structures such as root tips. Cell expansion and an increase in turgor pressure is due to inward diffusion of water into the cell, and turgor pressure increases due to the increasing volume of vacuolar sap. A growing root cell's turgor pressure can be up to 0.6 MPa, which is over three times that of a car tire. Epidermal cells in a leaf can have pressures ranging from 1.5 to 2.0 MPa.[18] As plants can operate at such high pressures, this can explain why they can grow through asphalt and other hard surfaces.[17]

Turgidity

Turgidity is observed in a cell where the cell membrane is pushed against the cell wall. In some plants, their cell walls loosen at a quicker rate than water can cross the membrane, which results in a cell with lower turgor pressure.[3]

Stomata

Stomata opened and closed unlabelled
Open stomata on the left and closed stomata on the right

Turgor pressure within the stomata regulates when the stomata can open and close, which has a play in transpiration rates of the plant. This is also important because this function regulates water loss within the plant. Lower turgor pressure can mean that the cell has a low water concentration and closing the stomata would help to preserve water. High turgor pressure keeps the stomata open for gas exchanges necessary for photosynthesis.[9]

Mimosa pudica

Sismonastia de la Mimosa pudica
Mimosa pudica

It has been concluded that loss of turgor pressure within the leaves of Mimosa pudica is responsible for the reaction the plant has when touched. Other factors such as changes in osmotic pressure, protoplasmic contraction and increase in cellular permeability have been observed to affect this response. It has also been recorded that turgor pressure is different in the upper and lower pulvinar cells of the plant, and the movement of potassium and calcium ions throughout the cells cause the increase in turgor pressure. When touched, the pulvinus is activated and exudes contractile proteins, which in turn increases turgor pressure and closes the leaves of the plant.[19]

Function in other taxa

As earlier stated, turgor pressure can be found in other organisms besides plants and can play a large role in the development, movement, and nature of said organisms.

Fungi

Shaggy Ink Caps busting through asphalt
Shaggy ink caps bursting through asphalt due to high turgor pressure

In fungi, tugor pressure has been observed as a large factor in substrate penetration. In species such as Saprolegnia ferax, Magnaporthe grisea and Aspergillus oryzae, immense turgor pressures have been observed in their hyphae. The study showed that they could penetrate substances like plant cells, and synthetic materials such as polyvinyl chloride.[20] In observations of this phenomenon, it is noted that invasive hyphal growth is due to turgor pressure, along with the coenzymes secreted by the fungi to invade said substrates.[21] Hyphal growth is directed related to turgor pressure, and growth slows as turgor pressure decreases. In Magnaporthe grisea, pressures of up to 8 MPa have been observed.[22]

Protists

Some protists do not have cell walls and cannot experience turgor pressure. These few protists are ones that use their contractile vacuole to regulate the quantity of water within the cell. Protist cells avoid lysing in solutions by utilizing a vacuole which pumps water out of the cells to maintain osmotic equilibrium.[23]

Animals

Turgor pressure is not observed in animal cells because they lack a cell wall. In organisms with cell walls, the cell wall prevents the cell from lysing from high-pressure values.[1]

Diatoms

In Diatoms, the Heterokontophyta have polyphyletic turgor-resistant cell walls. Throughout these organisms' life cycle, carefully controlled turgor pressure is responsible for cell expansion and for the release of sperm, but not for things such as seta growth.[24]

Cyanobacteria

Gas-vaculate cyanobacterium are the ones generally responsible for water-blooms. They have the ability to float due to the accumulation of gases within their vacuole, and the role of turgor pressure and its effect on the capacity of these vacuoles has been observed in varying scientific papers.[25][26] It is noted that the higher the turgor pressure, the lower the capacity of the gas-vacuoles in different cyanobacterium. Experiments used to correlate osmosis and turgor pressure in prokaryotes have been used to show how diffusion of solutes into the cell have a play on turgor pressure within the cell.[27]

Measurements

When measuring turgor pressure in plants, many things have to be taken into account. It is generally stated that fully turgid cells have a turgor pressure value which is equal to that of the cell and that flaccid cells have a value at or near zero. Other cellular mechanisms taken into consideration include the protoplast, solutes within the protoplast (solute potential), transpiration rates of the plant and the tension of cell walls. Measurement is limited depending on the method used, some of which are explored and explained below. Not all methods can be used for all organisms, due to size and other properties. For example, a diatom doesn't have the same properties as a plant, which would place constrictions on what could be used to infer turgor pressure.[28]

Units

Units used to measure turgor pressure are independent from the measures used to infer its values. Common units include bars, MPa, or newtons per square meter. 1 bar is equal to .1 MPa.[29]

Methods

Water potential equation

Turgor pressure can be deduced when total water potential, Ψw, and osmotic potential, Ψs, are known in a water potential equation.[30] These equations are used to measure the total water potential of a plant by using variables such as matric potential, osmotic potential, pressure potential, gravitational effects and turgor pressure.[31] After taking the difference between Ψs and Ψw, the value for turgor pressure is given. When using this method, gravity and matric potential are considered to be negligible, since their values are generally either negative or close to zero.[30]

Pressure-bomb technique

Pressurebomb
Diagram of a pressure bomb

The pressure bomb was developed by plant physiologist Lan Wang and colleagues in order to test water movement through plants. The instrument is used to measure turgor pressure by placing a leaf (with stem attached) into a closed chamber where pressurized gas is added in increments.[32] Measurements are taken when xylem sap appears out of the cut surface and at the point which it doesn't accumulae or retreat back into the cut surface.[33]

Atomic force microscope

Atomic force microscopes use a type of scanning probe microscopy (SPM). Small probes are introduced to the area of interest, and a spring within the probe measures values via displacement.[34] This method can be used to measure turgor pressure of organisms. When using this method, supplemental information such as continuum mechanic equations, single force depth curves and cell geometries can be used to quantify turgor pressures within a given area (usually a cell).

Pressure probe

This machine was originally used to measure individual algal cells, but can now be used on larger-celled specimens. It is usually used on higher plant tissues but wasn't used to measure turgor pressure until Hüsken and Zimmerman improved on the method.[35] Pressure probes measure turgor pressure via displacement. A glass micro-capillary tube is inserted into the cell and whatever the cell exudes into the tube is observed through a microscope. An attached device then measures how much pressure is required to push the emission back into the cell.[33]

These are used to accurately quantify measurements of smaller cells. In an experiment by Weber, Smith and colleagues, single tomato cells were compressed between a micro-manipulation probe and glass to allow the pressure probe's micro-capillary to find the cell's turgor pressure.[36]

Theoretical speculations

Negative turgor pressure

It has been observed that the value of Ψw decreases as the cell becomes more dehydrated,[30] but scientists have speculated whether this value will continue to decrease but never fall to zero, or if the value can be less than zero. There have been studies[37][38] which show that negative cell pressures can exist in xerophytic plants, but a paper by M. T. Tyree explores whether this is possible, or a conclusion based on misinterpreted data. In his paper, he concludes that by miscategorizing "bound" and "free" water in a cell, researchers that claimed to have found negative turgor pressure values were incorrect. By analyzing the isotherms of apoplastic and symplastic water, he shows that negative turgor pressures cannot be present within arid plants due to net water loss of the specimen during droughts. Despite his analysis and interpretation of data, negative turgor pressure values are still used within the scientific community.[39]

Tip growth in higher plants

A hypothesis formed by M. Harold and his colleagues suggests that tip growth in higher plans is amoebic in nature, and isn't caused by turgor pressure as is widely believed, meaning that extension is caused by the actin cytoskeleton in these plant cells. Regulation of cell growth is implied to be caused by cytoplasmic micro-tubules which control the orientation of cellulose fibrils, which are deposited into the adjacent cell wall and results in growth. In plants, the cells are surrounded by cell walls and filamentous proteins which retain and adjust the plant cell's growth and shape. As explained in the paper, lower plants grow through apical growth, which differs since the cell wall only expands on one end of the cell.[40]

References

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Acid growth

Acid growth refers to the ability of plant cells and plant cell walls to elongate or expand quickly at low (acidic) pH. The cell wall needs to be modified in order to maintain the turgor pressure. This modification is controlled by plant hormones like auxin. Auxin also controls the expression of some cell wall genes. This form of growth does not involve an increase in cell number. During acid growth, plant cells enlarge rapidly because the cell walls are made more extensible by expansin, a pH-dependent wall-loosening protein. Expansin loosens the network-like connections between cellulose microfibrils within the cell wall, which allows the cell volume to increase by turgor and osmosis. A typical sequence leading up to this would involve the introduction of a plant hormone (auxin, for example) that causes protons (H+ ions) to be pumped out of the cell into the cell wall. As a result, the cell wall solution becomes more acidic. It was suggested by different scientist that the epidermis is a unique target of the auxin but this theory has been disapproved over time. This activates expansin activity, causing the wall to become more extensible and to undergo wall stress relaxation, which enables the cell to take up water and to expand. The acid growth theory has been very controversial in the past.

Aphanizomenon

Aphanizomenon is an important genus of cyanobacteria that inhabits freshwater lakes and can cause dense blooms. Studies on the species Aphanizomenon flos-aquae have shown that it can regulate buoyancy through light-induced changes in turgor pressure. It is also able to move by means of gliding, though the specific mechanism by which this is possible is not yet known.

Appressorium

An appressorium is a specialized cell typical of many fungal plant pathogens that is used to infect host plants. It is a flattened, hyphal "pressing" organ, from which a minute infection peg grows and enters the host, using turgor pressure capable of punching through even Mylar.Following spore attachment and germination on the host surface, the emerging germ tube perceives physical cues such as surface hardness and hydrophobicity, as well as chemical signals including wax monomers that trigger appressorium formation. Appressorium formation begins when the tip of the germ tube ceases polar growth, hooks, and begins to swell. The contents of the spore are then mobilized into the developing appressorium, a septum develops at the neck of the appressorium, and the germ tube and spore collapse and die. As the appressorium matures, it becomes firmly attached to the plant surface and a dense layer of melanin is laid down in the appressorium wall, except across a pore at the plant interface. Turgor pressure increases inside the appressorium and a penetration hyphae emerges at the pore, which is driven through the plant cuticle into the underlying epidermal cells.

Cytorrhysis

Cytorrhysis is the permanent and irreparable damage to the cell wall after the complete collapse of a plant cell due to the loss of internal positive pressure (hydraulic turgor pressure). Positive pressure within a plant cell is required to maintain the upright structure of the cell wall. Desiccation (relative water content of less than or equal to 10%) resulting in cellular collapse occurs when the ability of the plant cell to regulate turgor pressure is compromised by environmental stress. Water continues to diffuse out of the cell after the point of zero turgor pressure, where internal cellular pressure is equal to the external atmospheric pressure, has been reached, generating negative pressure within the cell. That negative pressure pulls the center of the cell inward until the cell wall can no longer withstand the strain. The inward pressure causes the majority of the collapse to occur in the central region of the cell, pushing the organelles within the remaining cytoplasm against the cell walls. Unlike in plasmolysis (a phenomenon that does not occur in nature), the plasma membrane maintains its connections with the cell wall both during and after cellular collapse.Cytorrhysis of plant cells can be induced in laboratory settings if they are placed in a hypertonic solution where the size of the solutes in the solution inhibit flow through the pores in the cell wall matrix. Polyethylene glycol is an example of a solute with a high molecular weight that is used to induce cytorrhysis under experimental conditions. Environmental stressors which can lead to occurrences of cytorrhysis in a natural setting include intense drought, freezing temperatures, and pathogens such as the rice blast fungus (Magnaporthe grisea).

Drought rhizogenesis

Drought rhizogenesis is an adaptive root response to drought stress. New emerging roots are short, swollen, and hairless, capable of retaining turgor pressure and resistant to prolonged desiccation. Upon rewatering, they are capable of quickly forming an absorbing root surface and hair growth. This rhizogenesis has been called a drought tolerance strategy for after-stress recovery.

Leaf sensor

A leaf sensor is a phytometric device (measurement of plant physiological processes) that measures water loss or the water deficit stress (WDS) in plants by real-time monitoring the moisture level in plant leaves. The first leaf sensor was developed by LeafSens, an Israeli company granted a US patent for a mechanical leaf thickness sensing device in 2001. LeafSen has made strides incorporating their leaf sensory technology into citrus orchards in Israel. A solid state smart leaf sensor technology was developed by the University of Colorado at Boulder for NASA in 2007. It was designed to help monitor and control agricultural water demand. AgriHouse received a National Science Foundation (NSF) STTR grant in conjunction with the University of Colorado to further develop the solid state leaf sensor technology for precision irrigation control in 2007.

Nastic movements

Nastic movement are directional responses to stimuli (e.g. temperature, humidity, light irradiance), and are usually associated with plants. The movement can be due to changes in turgor or changes in growth. Decrease in turgor pressure causes shrinkage while increase in turgor pressure brings about swelling. Nastic movements differ from tropic movements in that the direction of tropic responses depends on the direction of the stimulus, whereas the direction of nastic movements is independent of the stimulus's position. The tropic movement is growth movement but nastic movement may or may not be growth movement. The rate or frequency of these responses increases as intensity of the stimulus increases. An example of such a response is the opening and closing of flowers (photonastic response), movement of euglena, chlamydomonas towards the source of light. They are named with the suffix "-nasty" and have prefixes that depend on the stimuli:

Epinasty: downward-bending from growth at the top, for example, the bending down of a heavy flower.

Hyponasty

Photonasty: response to light

Nyctinasty: movements at night or in the dark

Chemonasty: response to chemicals or nutrients

Hydronasty: response to water

Thermonasty: response to temperature

Seismonasty: response to shock

Geonasty/gravinasty: response to gravity

Thigmonasty/seismonasty/haptonasty: response to contactThe suffix may come from Greek νάσσω = "I press", ναστός = "pressed", ἐπιναστια = "the condition of being pressed upon".

Osmosis

Osmosis () is the spontaneous net movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides. It may also be used to describe a physical process in which any solvent moves across a selectively permeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations. Osmosis can be made to do work. Osmotic pressure is defined as the external pressure required to be applied so that there is no net movement of solvent across the membrane. Osmotic pressure is a colligative property, meaning that the osmotic pressure depends on the molar concentration of the solute but not on its identity.

Osmosis is a vital process in biological systems, as biological membranes are semipermeable. In general, these membranes are impermeable to large and polar molecules, such as ions, proteins, and polysaccharides, while being permeable to non-polar or hydrophobic molecules like lipids as well as to small molecules like oxygen, carbon dioxide, nitrogen, and nitric oxide. Permeability depends on solubility, charge, or chemistry, as well as solute size. Water molecules travel through the plasma membrane, tonoplast membrane (vacuole) or protoplast by diffusing across the phospholipid bilayer via aquaporins (small transmembrane proteins similar to those responsible for facilitated diffusion and ion channels). Osmosis provides the primary means by which water is transported into and out of cells. The turgor pressure of a cell is largely maintained by osmosis across the cell membrane between the cell interior and its relatively hypotonic environment.

Plant cell

Plant cells are eukaryotic cells present in green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids with the capability to perform photosynthesis and store starch, a large vacuole that regulates turgor pressure, the absence of flagella or centrioles, except in the gametes, and a unique method of cell division involving the formation of a cell plate or phragmoplast that separates the new daughter cells.

Plasmolysis

Plasmolysis is the process in which cells lose water in a hypertonic solution. The reverse process, deplasmolysis or cytolysis, can occur if the cell is in a hypotonic solution resulting in a lower external osmotic pressure and a net flow of water into the cell. Through observation of plasmolysis and deplasmolysis, it is possible to determine the tonicity of the cell's environment as well as the rate solute molecules cross the cellular membrane.

Pressure Flow Hypothesis

The Pressure Flow Hypothesis, also known as the Mass Flow Hypothesis, is the best-supported theory to explain the movement of sap through the phloem. It was proposed by Ernst Munch, a German plant physiologist in 1930.

A high concentration of organic substances, particularly sugar, inside cells of the phloem at a source, such as a leaf, creates a diffusion gradient (osmotic gradient) that draws water into the cells from the adjacent xylem. This creates turgor pressure, also known as hydrostatic pressure, in the phloem. Movement of phloem sap occurs by bulk flow (mass flow) from sugar sources to sugar sinks. The movement in phloem is bidirectional, whereas, in xylem cells, it is unidirectional (upward). Because of this multi-directional flow, coupled with the fact that sap cannot move with ease between adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be flowing in opposite directions.

Pulvinus

A pulvinus (pl. pulvini) is a joint-like thickening at the base of a plant leaf or leaflet that facilitates growth-independent (nyctinastic and thigmonastic) movement. Pulvini are common, for example, in members of the bean family Fabaceae (Leguminosae) and the prayer plant family Marantaceae.Pulvini may be present at the base or apex of the petiole or where the leaflets of a compound leaf are inserted into the rachis. They consist of a core of vascular tissue within a flexible, bulky cylinder of thin-walled parenchyma cells. A pulvinus is also sometimes called a geniculum.Pulvinar movement is caused by changes in turgor pressure leading to a contraction or expansion of the parenchyma tissue. The response is initiated when sucrose is unloaded from the phloem into the apoplast. The increased sugar concentration in the apoplast decreases the water potential and triggers the efflux of potassium ions from the surrounding cells. This is followed by an efflux of water, resulting in a sudden change of turgor pressure in the cells of the pulvinus. Aquaporins on the vacuole membrane of pulvini allow for the efflux of water that contributes to the change in turgor pressure. The process is similar to the mechanism of stomatal closure.

Common examples for pulvinar movements include the night closure movement of legume leaves and the touch response of the sensitive plant (Mimosa pudica). Nyctinastic movements (sleep movements) are controlled by the circadian clock and light signal transduction through phytochrome. Thigmonastic movements (touch response) appear to be regulated through electrical and chemical signal transduction spreading the stimulus throughout the plant.

Stomatal conductance

By definition, stomatal conductance, usually measured in mmol m⁻² s⁻¹, is the measure of the rate of passage of carbon dioxide (CO2) entering, or water vapor exiting through the stomata of a leaf. Stomata are small pores on the top and or bottom of a leaf that are responsible for taking in CO2 and expelling water vapour.

The rate of stomatal conductance, or its inverse, stomatal resistance, is directly related to the boundary layer resistance of the leaf and the absolute concentration gradient of water vapor from the leaf to the atmosphere. It is under direct biological control of the leaf through the use of guard cells, which surround the stomatal pore (Taiz/Zeiger 1991). The turgor pressure and osmotic potential of guard cells is directly related to the stomatal conductance.

Stomatal conductance is a function of stomatal density, stomatal aperture, and stomatal size.

Stomatal conductance is integral to leaf level calculations of transpiration (E). Multiple studies have shown a direct correlation between the use of herbicides and changes in physiological and biochemical growth processes in plants, particularly non-target plants, resulting in a reduction in stomatal conductance and turgor pressure in leaves.

Subsporangial vesicle

A subsporangial vesicle is a vesicle which is below the sporangium on a fungus.

Is often used in the turgor-building and release to launch the sporangium from the stalk of the fungus using this spore-dispersal method.

e.g. The subsporangial vesicle in Pilobolus fills with fluid, thereby creating turgor pressure that is released, launching the sporangium out towards the light, with the intent of landing on a plant.

Suction pressure

In refrigeration and air conditioning systems, the suction pressure' (also called the low-side pressure) is the intake pressure generated by the system compressor while operating. The suction pressure, along with the suction temperature and the wet bulb temperature of the discharge air are used to determine the correct refrigerant charge in a system.

If some solute is dissolved in water, its diffusion pressure decreases. The difference between diffusion pressure of pure water and solution is called diffusion pressure deficit (DPD).

When a plant cell is placed in a hypotonic solution, water enters into a cell by osmosis and as a result turgor pressure develops. The cell membrane becomes stretched and the osmotic pressure of the cell decreases. As the cell absorbs more and more water its Total Pressure increases and Osmotic Pressure decreases. When a cell is fully turgid, its OP is equal to TP and DPD is zero. Turgid cells cannot absorb any more water. Thus, with reference to plant cells, the DPD can be described as the actual thirst of a cell for water and can be expressed as DPD=OP-TP.

When DPD is zero, entry of water will stop. Thus it is DPD that tends to equate and represents the water-absorbing ability of a cell, it is also called suction force (SF) or suction pressure (SP).

DPD is directly proportional to the height of the plant, tree or organism. DPD is governed by two factors i.e. turgor pressure and osmotic pressure.

The formula is DPD = OP - TP. Turgor pressure can also be denoted as wall pressure in some cases.

The term diffusion pressure deficit (DPD) was coined by B.S Meyer in 1938. Originally DPD was described as suction pressure by Renner (1915). It is a reduction in the diffusion pressure of water in solution or cell over its pure state due to the presence of solutes in it and forces opposing diffusion. Diffusion pressure of water is maximum and its theoretical value is 1236 atm.

DPD of a solution is equal to its osmotic pressure i.e. DPD = OP(of solution). The DPD of a cell is

influenced by both osmotic pressure and wall pressure (turgor pressure) which opposes the endosmotic entry of water, i.e. DPD = OP - wall pressure. DPD is directly proportional to the concentration of the solution. DPD decreases with dilution of the solution.

The actual pressure with which a cell absorbs water is called "suction pressure".

Vacuole

A vacuole () is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain cases they may contain solids which have been engulfed. Vacuoles are formed by the fusion of multiple membrane vesicles and are effectively just larger forms of these. The organelle has no basic shape or size; its structure varies according to the requirements of the cell.

Vital theory

According to the vital force theory, the conduction of water up the xylem vessel is a result of vital action of the living cells in the xylem tissue. These living cells are involved in ascent of sap. Relay pump theory and Pulsation theory support the active theory of ascent of sap.

Emil Godlewski (senior) (1884) proposed Relay pump or Clamberinh force theory (through xylem parenchyma) and Jagadish Chandra Bose(1923) proposed pulsation theory (due to pulsatory activities of innermost cortical cells just outside endodermis).

Jagadish Chandra Bose suggested a mechanism for the ascent of sap in 1927. His theory can be explained with the help of galvanometer of electric probes. He found electrical ‘pulsations’ or oscillations in electric potentials, and came to believe these were coupled with rhythmic movements in the telegraph plant Codariocalyx motorius (then Desmodium). On the basis of this Bose theorized that regular wave-like ‘pulsations’ in cell electric potential and turgor pressure were an endogenous form of cell signaling. According to him the living cells in the inner lining of the xylem tissue pump water by contractive and expulsive movements similar to the animal heart circulating blood.

This mechanism has not been well supported, and in spite of some ongoing debate, the evidence overwhelmingly supports the cohesion-tension theory for the ascent of sap.

Wilting

Wilting is the loss of rigidity of non-woody parts of plants. This occurs when the turgor pressure in non-lignified plant cells falls towards zero, as a result of diminished water in the cells. Wilting also serves to reduce water loss, as it makes the leaves expose less surface area. The rate of loss of water from the plant is greater than the absorption of water in the plant. The process of wilting

modifies the leaf angle distribution of the plant (or canopy) towards more erectophile conditions.

Lower water availability may result from:

drought conditions, where the soil moisture drops below conditions most favorable for plant functioning;

the temperature falls to the point where the plants vascular system can not function;

high salinity, which causes water to diffuse from the plant cells and induce shrinkage;

saturated soil conditions, where roots are unable to obtain sufficient oxygen for cellular respiration, and so are unable to transport water into the plant; or

bacteria or fungi that clog the plant's vascular system.Wilting diminishes the plant's ability to transpire and grow. Permanent wilting leads to plant death. Symptoms of wilting and blights resemble one another.

In woody plants, reduced water availability leads to cavitation of the xylem.

Wilting occurs in plants such as balsam and holy basil. Wilting is an effect of the plant growth inhibiting hormone, abscisic acid.

With cucurbits, wilting can be caused by the squash vine borer.

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