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Transpiration is the process of
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

water
movement through a
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
and its
evaporation Evaporation is a type of that occurs on the of a as it changes into the gas phase. The surrounding gas must not be saturated with the evaporating substance. When the molecules of the liquid collide, they transfer energy to each other bas ...

evaporation
from aerial parts, such as
leaves A leaf (plural leaves) is the principal lateral appendage of the vascular plant plant stem, stem, usually borne above ground and specialized for photosynthesis. The leaves, stem, flower and fruit together form the shoot system. Leaves are ...

leaves
,
stems
stems
and
flower A flower, sometimes known as a bloom or blossom Image:Cerisier du Japon Prunus serrulata.jpg, Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, trees (genus ''Prunus'') and of some other plan ...

flower
s. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth and metabolism. The remaining 97–99.5% is lost by transpiration and
guttation'' Guttation is the exudation of drops of xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water Water is an Inorganic compound, inorgani ...
. Leaf surfaces are dotted with pores called
stomata File:LeafUndersideWithStomata.jpg, The underside of a leaf. In this species (''Tradescantia zebrina'') the guard cells of the stomata are green because they contain chlorophyll while the epidermal cells are chlorophyll-free and contain red pigme ...

stomata
(singular "stoma"), and in most plants they are more numerous on the undersides of the foliage. The stomata are bordered by
guard cell Guard cells are specialized plant cell ''The Plant Cell'' is a monthly peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ( peers). It functions as a form of self ...
s and their stomatal accessory cells (together known as stomatal complex) that open and close the pore. Transpiration occurs through the stomatal apertures, and can be thought of as a necessary "cost" associated with the opening of the stomata to allow the diffusion of
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
gas from the air for
photosynthesis Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

photosynthesis
. Transpiration also cools plants, changes
osmotic pressure
osmotic pressure
of cells, and enables
mass flow Mass flow, also known as “mass transfer” and “bulk flow”, is the movement of fluids down a pressure or temperature gradient,Moyes & Schulte (2008). Principles of Animal Physiology. Pearson Benjamin Cummings. San Francisc, CA. particularly i ...
of
mineral nutrients In the context of nutrition, a mineral is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms ...
and water from roots to
shoot In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Anci ...

shoot
s. Two major factors influence the rate of water flow from the soil to the roots: the hydraulic conductivity of the soil and the magnitude of the pressure gradient through the soil. Both of these factors influence the rate of bulk flow of water moving from the roots to the stomatal pores in the leaves via the xylem. Mass flow of liquid water from the roots to the leaves is driven in part by
capillary action Capillary action (sometimes capillarity, capillary motion, capillary effect, or wicking) is the ability of a liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the ...
, but primarily driven by
water potential Water potential is the potential energy In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors. Common types of potential e ...
differences. If the water potential in the ambient air is lower than the water potential in the leaf airspace of the stomatal pore, water vapor will travel down the gradient and move from the leaf airspace to the atmosphere. This movement lowers the water potential in the leaf airspace and causes evaporation of liquid water from the mesophyll cell walls. This evaporation increases the tension on the water menisci in the cell walls and decrease their radius and thus the tension that is exerted on the water in the cells. Because of the cohesive properties of water, the tension travels through the leaf cells to the leaf and stem xylem where a momentary negative pressure is created as water is pulled up the xylem from the roots. As evaporation occurs at the leaf surface, the properties of adhesion and cohesion work in tandem to pull water molecules from the roots, through xylem tissue, and out of the plant through stomata. In taller plants and trees, the force of gravity can only be overcome by the decrease in hydrostatic (water) pressure in the upper parts of the plants due to the
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration In chemistry Chemistry is the study of the properties and behavior of . It is a that covers ...

diffusion
of water out of stomata into the
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
. Water is absorbed at the roots by
osmosis Osmosis (, ) is the spontaneous net movement or diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration In chemistry Chemistry is the ...

osmosis
, and any dissolved mineral nutrients travel with it through the
xylem Xylem is one of the two types of transport tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North Ame ...

xylem
. The cohesion-tension theory explains how leaves pull water through the xylem. Water molecules stick together, or exhibit cohesion. As a water molecule evaporates from the surface of the leaf, it pulls on the adjacent water molecule, creating a continuous flow of water through the plant.


Etymology

We can see the history of the word transpiration when we break it down into trans, a Latin noun which means "across," and spiration, which comes from the Latin verb spīrāre, meaning "to breathe." The tion suffix adds the meaning "the act of," so we can see transpiration is, literally, "the act of breathing across," which clearly identifies vapor emission from plant leaves. Transpiration is vital to the earth’s ecology, and is being reduced by our decimation of the forests.


Capillary action

Capillary action Capillary action (sometimes capillarity, capillary motion, capillary effect, or wicking) is the ability of a liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the ...
is the process of a
liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical objects, ...

liquid
flowing in narrow spaces without the assistance of, or even in opposition to, external forces like
gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

gravity
. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper and plaster, in some non-porous materials such as sand and liquefied
carbon fiber Carbon fiber reinforced polymer (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to ...

carbon fiber
, or in a
biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referred to as the "building blocks of life". The s ...

biological cell
. It occurs because of
intermolecular force An intermolecular force (IMF) (or secondary force) is the force that mediates interaction between molecules, including the Electromagnetism, electromagnetic forces of attraction or repulsion which act between atoms and other types of neighboring pa ...

intermolecular force
s between the liquid and surrounding solid surfaces. If the diameter of the tube is sufficiently small, then the combination of
surface tension Surface tension is the tendency of liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of physi ...

surface tension
(which is caused by cohesion within the liquid) and
adhesive forces
adhesive forces
between the liquid and container wall act to propel the liquid.


Regulation

Plants regulate the rate of transpiration by controlling the size of the stomatal apertures. The rate of transpiration is also influenced by the evaporative demand of the atmosphere surrounding the leaf such as boundary layer conductance,
humidity Humidity is the concentration of water vapor, water vapour present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, d ...

humidity
,
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

temperature
, wind and incident sunlight. Along with above ground factors, soil temperature and moisture can influence stomatal opening, and thus transpiration rate. The amount of water lost by a plant also depends on its size and the amount of water absorbed at the roots. Transpiration accounts for most of the water loss by a plant by the leaves and young stems. Transpiration serves to evaporatively cool plants, as the evaporating water carries away heat energy due to its large
latent heat of vaporization The enthalpy of vaporization (symbol ), also known as the (latent) heat of vaporization or heat of evaporation, is the amount of energy (enthalpy Enthalpy is a property of a thermodynamic system A thermodynamic system is a body of matter a ...
of 2260 kJ per litre. File:Transpiration Temperature Graph.svg, The effect of temperature on the transpiration rate of plants. File:Transpiration WindVelocity Graph.svg, The effect of wind velocity on the transpiration rate of plants. File:Transpiration Humidity Graph.svg, The effect of humidity on the transpiration rate of plants. During a growing season, a leaf will transpire many times more water than its own weight. An acre of corn gives off about 3,000–4,000 gallons (11,400–15,100 liters) of water each day, and a large oak tree can transpire 40,000 gallons (151,000 liters) per year. The transpiration ratio is the ratio of the mass of water transpired to the mass of dry matter produced; the transpiration ratio of
crops A crop is a plant that can be grown and harvested extensively for profit or subsistence. Crops may refer either to the harvested parts or to the harvest in a more refined state. Most crops are cultivated in agriculture Agriculture is the ...

crops
tends to fall between 200 and 1000 (''i.e.'', crop plants transpire 200 to 1000 kg of water for every kg of dry
matter In classical physics Classical physics is a group of physics theories that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major ...
produced). Transpiration rates of plants can be measured by a number of techniques, including
potometer
potometer
s,
lysimeter A lysimeter (from Greek λύσις (loosening) and the suffix ''-meter'') is a measuring device which can be used to measure the amount of actual evapotranspiration Evapotranspiration (ET) is the sum of water evaporation and transpiration from a ...

lysimeter
s, porometers, photosynthesis systems and thermometric sap flow sensors. Isotope measurements indicate transpiration is the larger component of
evapotranspiration Evapotranspiration (ET) is the sum of water evaporation Evaporation is a type of that occurs on the of a as it changes into the gas phase. The surrounding gas must not be saturated with the evaporating substance. When the molecules of ...

evapotranspiration
. Recent evidence from a global study of water stable isotopes shows that transpired water is isotopically different from groundwater and streams. This suggests that soil water is not as well mixed as widely assumed.
Desert A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to the processes of ...

Desert
plants have specially adapted structures, such as thick cuticles, reduced leaf areas, sunken stomata and
hairs Hair is a protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Phy ...

hairs
to reduce transpiration and conserve water. Many
cacti A cactus (plural cacti, cactuses, or less commonly, cactus) is a member of the plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living thi ...

cacti
conduct
photosynthesis Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

photosynthesis
in
succulent In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the An ...

succulent
stems, rather than leaves, so the surface area of the shoot is very low. Many desert plants have a special type of photosynthesis, termed
crassulacean acid metabolism Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation Carbon fixation or сarbon assimilation is the process by which inorganic carbon (particularly in the form of carbon dioxide Carbon dioxide (chemical form ...
or CAM photosynthesis, in which the stomata are closed during the day and open at night when transpiration will be lower.


Cavitation

To maintain the pressure gradient necessary for a plant to remain healthy they must continuously uptake water with their roots. They need to be able to meet the demands of water lost due to transpiration. If a plant is incapable of bringing in enough water to remain in equilibrium with transpiration an event known as
cavitation Cavitation is a phenomenon in which the static pressure of the liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, ...

cavitation
occurs. Cavitation is when the plant cannot supply its xylem with adequate water so instead of being filled with water the xylem begins to be filled with water vapor. These particles of water vapor come together and form blockages within the xylem of the plant. This prevents the plant from being able to transport water throughout its vascular system. There is no apparent pattern of where cavitation occurs throughout the plant's xylem. If not effectively taken care of, cavitation can cause a plant to reach its permanent wilting point, and die. Therefore, the plant must have a method by which to remove this cavitation blockage, or it must create a new connection of vascular tissue throughout the plant. The plant does this by closing its stomates overnight, which halts the flow of transpiration. This then allows for the roots to generate over 0.05 mPa of pressure, and that is capable of destroying the blockage and refilling the xylem with water, reconnecting the vascular system. If a plant is unable to generate enough pressure to eradicate the blockage it must prevent the blockage from spreading with the use of pit pears and then create new xylem that can re-connect the vascular system of the plant. Scientists have begun using
magnetic resonance imaging Magnetic resonance imaging (MRI) is a medical imaging Medical imaging is the technique and process of imaging Imaging is the representation or reproduction of an object's form; especially a visual representation (i.e., the formation of a ...
(MRI) to monitor the internal status of the xylem during transpiration, in a non invasive manner. This method of imaging allows for scientists to visualize the movement of water throughout the entirety of the plant. It also is capable of viewing what phase the water is in while in the xylem, which makes it possible to visualize cavitation events. Scientists were able to see that over the course of 20 hours of sunlight more than 10 xylem vessels began filling with gas particles becoming cavitated. MRI technology also made it possible to view the process by which these xylem structures are repaired in the plant. After three hours in darkness it was seen that the vascular tissue was resupplied with liquid water. This was possible because in darkness the stomates of the plant are closed and transpiration no longer occurs. When transpiration is halted the cavitation bubbles are destroyed by the pressure generated by the roots. These observations suggest that MRIs are capable of monitoring the functional status of xylem and allows scientists to view cavitation events for the first time.


See also

* Antitranspirant – a substance to prevent transpiration *
Canopy conductance Canopy conductance, commonly denoted g_c, is a dimensionless quantity In dimensional analysis In engineering and science, dimensional analysis is the analysis of the relationships between different physical quantities by identifying their base q ...
*
Ecohydrology Ecohydrology (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approxim ...
*
Eddy covariance The eddy covariance (also known as eddy correlation and eddy flux) technique is a key atmospheric measurement technique to measure and calculate vertical turbulent fluxes within planetary boundary layer, atmospheric boundary layers. The method an ...
flux (aka eddy correlation, eddy flux) *
Hydrology (agriculture)Agricultural hydrology is the study of water balance The law of water balance states that the inflows to any water system or area is equal to its outflows plus change in storage during a time interval. In hydrology Hydrology (from Greek: wikt: ...
*
Latent heat flux Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system A thermodynamic system is a body of matter and/or radiation, confined in space by walls, with defined permeabi ...
*
Perspiration Perspiration, also known as sweating, is the production of fluids secreted by the sweat gland Sweat glands, also known as sudoriferous or sudoriparous glands, , are small tubular structures of the skin Skin is the layer of usually soft, f ...
*
Soil plant atmosphere continuum The soil-plant-atmosphere continuum (SPAC) is the pathway for water moving from soil File:Stagnogley.JPG, Surface-water-Gley soil, gley developed in glacial till, Northern Ireland. Soil is a mixture of organic matter, minerals, gases, liquids, ...
*
Stomatal conductance By definition, stomatal conductance, usually measured in mmol m−2 s−1, conditions the net molar flux of carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical propor ...
* Transpiration stream *
Turgor pressure Turgor pressure is the force within the cell that pushes the plasma membrane cell membrane vs. Prokaryotes A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In b ...
* Water Evaluation And Planning system (WEAP)


References


External links


USGS The Water Cycle: Evapotranspiration
{{Authority control Hydrology Plant physiology