Editing Xylem (section)

== Main function – upwards water transport ==
The xylem, vessels and tracheids of the roots, stems and leaves are interconnected to form a continuous system of water-conducting channels reaching all parts of the plants. The system transports water and soluble mineral nutrients from the roots throughout the plant. It is also used to replace water lost during [[transpiration]] and photosynthesis. Xylem [[plant sap|sap]] consists mainly of water and inorganic ions, although it can also contain a number of organic chemicals as well. The transport is passive, not powered by energy spent by the [[Vessel elements|tracheary]] elements themselves, which are dead by maturity and no longer have living contents. Transporting sap upwards becomes more difficult as the height of a plant increases and upwards transport of water by xylem is considered to limit the maximum height of trees.<ref>{{cite journal|last1=Koch|first1=George W.|last2=Sillett|first2=Stephen C.|last3=Jennings|first3=Gregory M.|last4=Davis|first4=Stephen D.|title=The limits to tree height|journal=Nature|date=2004|volume=428|issue=6985|pages=851–854|doi=10.1038/nature02417|pmid=15103376|bibcode=2004Natur.428..851K|s2cid=11846291}}</ref> Three phenomena cause xylem sap to flow:
* '''[[Pressure flow hypothesis]]''': Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a [[Water potential|solute pressure]] differential versus the xylem system carrying a far lower load of [[solute]]s—water and minerals. The phloem pressure can rise to several MPa,<ref>{{Cite journal|last1=Knoblauch|first1=Michael|last2=Knoblauch|first2=Jan|last3=Mullendore|first3=Daniel L.|last4=Savage|first4=Jessica A.|last5=Babst|first5=Benjamin A.|last6=Beecher|first6=Sierra D.|last7=Dodgen|first7=Adam C.|last8=Jensen|first8=Kaare H.|last9=Holbrook|first9=N. Michele|date=2016-06-02|title=Testing the Münch hypothesis of long distance phloem transport in plants|journal=eLife|language=en|volume=5|pages=e15341|doi=10.7554/eLife.15341|issn=2050-084X|pmc=4946904|pmid=27253062 |doi-access=free }}</ref> far higher than atmospheric pressure. Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure.
* {{anchor|Transpirational pull}}'''[[Transpiration]]al pull''': Similarly, the [[evaporation]] of [[water]] from the surfaces of [[mesophyll]] cells to the atmosphere also creates a negative pressure at the top of a plant. This causes millions of minute [[Meniscus (liquid)|menisci]] to form in the mesophyll cell wall. The resulting [[surface tension]] causes a negative pressure or [[tension (physics)|tension]] in the xylem that pulls the water from the roots and soil.{{cn|date=January 2025}}
* '''[[Root pressure]]''': If the water potential of the root cells is more negative than that of the [[soil]], usually due to high concentrations of solute, water can move by [[osmosis]] into the root from the soil. This causes a positive pressure that forces sap up the xylem towards the leaves. In some circumstances, the sap will be forced from the leaf through a [[hydathode]] in a phenomenon known as [[guttation]]. Root pressure is highest in the morning before the opening of stomata and allow transpiration to begin. Different plant species can have different root pressures even in a similar environment; examples include up to 145 kPa in ''[[Vitis riparia]]'' but around zero in ''[[Celastrus orbiculatus]]''.<ref>{{cite journal|journal=American Journal of Botany|year=2000|volume=87|pages=1272–78|title=Root pressure and specific conductivity in temperate lianas: exotic ''Celastrus orbiculatus'' (Celastraceae) vs. Native ''Vitis riparia'' (Vitaceae)|author=Tim J. Tibbetts|author2=Frank W. Ewers|doi=10.2307/2656720|pmid=10991898|issue=9|jstor=2656720|doi-access=free}}</ref>

The primary force that creates the [[capillary action]] movement of water upwards in plants is the adhesion between the water and the surface of the xylem conduits.<ref>Cruiziat, Pierre and Richter, Hanno. [http://4e.plantphys.net/article.php?ch=&id=99 Plant Physiology] {{webarchive|url=https://web.archive.org/web/20081228144231/http://4e.plantphys.net/article.php?ch=&id=99 |date=2008-12-28 }}. Sinauer Associates.</ref><ref>{{cite book|title=Plant solute transport|url=https://archive.org/details/plantsolutetrans00yeoa|url-access=limited|publisher=Blackwell Publishing|year=2007|isbn=978-1-4051-3995-3|editor-last=Anthony R. Yeo|location=Oxford UK|page=[https://archive.org/details/plantsolutetrans00yeoa/page/n240 221]|editor-last2=Timothy J. Flowers}}</ref> Capillary action provides the force that establishes an equilibrium configuration, balancing gravity. When transpiration removes water at the top, the flow is needed to return to the equilibrium.{{cn|date=January 2025}}

Transpirational pull results from the evaporation of water from the surfaces of [[cell (biology)|cell]]s in the [[leaves]]. This evaporation causes the surface of the water to recess into the [[wikt:pore|pore]]s of the [[cell wall]]. By [[capillary action]], the water forms concave menisci inside the pores. The high surface tension of water pulls the [[wikt:Concavity|concavity]] outwards, generating enough [[force]] to lift water as high as a hundred meters from ground level to a [[tree]]'s highest branches.

Transpirational pull requires that the vessels transporting the water be very small in diameter; otherwise, [[cavitation]] would break the water column. And as water [[evaporates]] from leaves, more is drawn up through the plant to replace it. When the water pressure within the xylem reaches extreme levels due to low water input from the roots (if, for example, the soil is dry), then the gases come out of solution and form a bubble&nbsp;– an [[embolism]] forms, which will spread quickly to other adjacent cells, unless [[pit (botany)|bordered pits]] are present (these have a plug-like structure called a torus, that seals off the opening between adjacent cells and stops the embolism from spreading). Even after an embolism has occurred, plants are able to refill the xylem and restore the functionality.<ref name="Nardini2011">{{cite journal|last1=Nardini|first1=Andrea|last2=Lo Gullo|first2=Maria A.|last3=Salleo|first3=Sebastiano|title=Refilling embolized xylem conduits: Is it a matter of phloem unloading?|journal=Plant Science|volume=180|issue=4|year=2011|pages=604–611|issn=0168-9452|doi=10.1016/j.plantsci.2010.12.011|pmid=21421408|bibcode=2011PlnSc.180..604N }}</ref>

=== Cohesion-tension theory ===
The ''cohesion-tension theory'' is a theory of [[intermolecular attraction]] that explains the process of water flow upwards (against the force of [[gravity]]) through the xylem of plants. It was proposed in 1894 by [[John Joly]] and [[Henry Horatio Dixon]].<ref>{{cite journal |last1=Dixon |first1=Henry H. |last2=Joly |first2=J. |title=On the ascent of sap |journal=Annals of Botany |date=1894 |volume=8 |pages=468–470 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015001235830;view=1up;seq=566}}</ref><ref>{{cite journal |last1=Dixon |first1=Henry H. |last2=Joly |first2=J. |title=On the ascent of sap |journal=Philosophical Transactions of the Royal Society of London, Series B |date=1895 |volume=186 |pages=563–576 |url=https://babel.hathitrust.org/cgi/pt?id=uva.x001659730;view=1up;seq=609|doi=10.1098/rstb.1895.0012 |doi-access=free }}</ref> Despite numerous objections,<ref>{{cite journal|author=Tyree, M.T.|year=1997|title=The Cohesion-Tension theory of sap ascent: current controversies|journal=Journal of Experimental Botany|volume=48|issue=10|pages=1753–1765|doi=10.1093/jxb/48.10.1753|doi-access=free}}</ref><ref>{{cite journal|author1=Wang, Z. |author2=Chang, C.-C. |author3=Hong, S.-J. |author4=Sheng, Y.-J. |author5=Tsao, H.-K. |year=2012|title=Capillary Rise in a Microchannel of Arbitrary Shape and Wettability: Hysteresis Loop|journal=Langmuir|volume=28|issue=49|pages=16917–16926|doi=10.1021/la3036242|pmid=23171321}}</ref> this is the most widely accepted theory for the transport of water through a plant's vascular system based on the classical research of Dixon-Joly (1894), Eugen Askenasy (1845–1903) (1895),<ref>{{cite journal|last=Askenasy|first=E.|title=Ueber das Saftsteigen|trans-title=On the ascent of sap|journal=Botanisches Centralblatt|year=1895|volume=62|pages=237–238|language=de|url=https://babel.hathitrust.org/cgi/pt?id=uc1.c079536710;view=1up;seq=269}}</ref><ref>{{cite journal |last1=Askenasy |first1=E. |title=Ueber das Saftsteigen |journal=Verhandlungen des Naturhistorisch-medizinischen Vereins zu Heidelberg (Proceedings of the Natural History-Medical Society at Heidelberg) |date=1895 |volume=5 |pages=325–345 |url=https://www.biodiversitylibrary.org/item/43534#page/349/mode/1up |series=2nd series |trans-title=On the ascent of sap |language=de}}</ref> and Dixon (1914,1924).<ref>{{cite book|last=Dixon|first=H|title=Transpiration and the ascent of sap in plants|year=1914|publisher= Macmillan and Co.|location=London, England, UK|url=https://archive.org/stream/transpirationasc00dixo#page/n5}}</ref><ref>{{cite book|last=Dixon|first=H|title=The transpiration stream|year= 1924|publisher=University of London Press, Ltd|location=London|pages=80}}</ref>

Water is a [[polar molecule]]. When two water molecules approach one another, the slightly negatively charged [[oxygen]] atom of one forms a [[hydrogen bond]] with a slightly positively charged [[hydrogen]] atom in the other. This attractive force, along with other [[intermolecular force]]s, is one of the principal factors responsible for the occurrence of [[surface tension]] in liquid water. It also allows plants to draw water from the root through the xylem to the leaf.{{cn|date=January 2025}}

Water is constantly lost through transpiration from the leaf. When one water molecule is lost another is pulled along by the processes of cohesion and tension. Transpiration pull, utilizing [[capillary action]] and the inherent surface tension of water, is the primary mechanism of water movement in plants. However, it is not the only mechanism involved. Any use of water in leaves forces water to move into them.{{cn|date=January 2025}}

[[Transpiration]] in leaves creates tension (differential pressure) in the cell walls of mesophyll cells. Because of this tension, water is being pulled up from the roots into the leaves, helped by [[Cohesion (chemistry)|cohesion]] (the pull between individual water molecules, due to hydrogen bonds) and [[adhesion]] (the stickiness between water molecules and the [[hydrophilic]] cell walls of plants). This mechanism of water flow works because of [[water potential]] (water flows from high to low potential), and the rules of simple [[diffusion]].<ref>{{cite book|last=Campbell|first=Neil|title=Biology|year=2002|publisher=Pearson Education, Inc.|location=San Francisco, CA|isbn=978-0-8053-6624-2|pages=[https://archive.org/details/biologyc00camp/page/759 759]|url=https://archive.org/details/biologyc00camp/page/759}}</ref>

Over the past century, there has been a great deal of research regarding the mechanism of xylem sap transport; today, most plant scientists continue to agree that the ''cohesion-tension theory'' best explains this process, but multiforce theories that hypothesize several alternative mechanisms have been suggested, including longitudinal cellular and xylem [[osmotic pressure]] [[gradient]]s, axial potential gradients in the vessels, and gel- and gas-bubble-supported interfacial gradients.<ref>{{cite journal|last=Zimmerman|first=Ulrich|title=What are the driving forces for water lifting in the xylem conduit?|journal=Physiologia Plantarum|year=2002|doi=10.1034/j.1399-3054.2002.1140301.x|pmid=12060254|volume=114|issue=3|pages=327–335|bibcode=2002PPlan.114..327Z }}</ref><ref name=tyree1997>{{cite journal|last=Tyree|first=Melvin T.|title=The cohesion-tension theory of sap ascent: current controversies|journal=Journal of Experimental Botany|volume=48|issue=10|pages=1753–1765|year=1997|doi=10.1093/jxb/48.10.1753|doi-access=free}}</ref>

=== Measurement of pressure ===
[[Image:Pressurebomb.svg|upright=1.3|thumb|A diagram showing the setup of a [[pressure bomb]]]]

Until recently, the differential pressure (suction) of transpirational pull could only be measured indirectly, by applying external pressure with a [[pressure bomb]] to counteract it.<ref>[https://web.archive.org/web/20090918110039/http://bugs.bio.usyd.edu.au/learning/resources/plant_form_function/external_sites/PAP/Lab08_WaterPot/08Lab_14.html The pressure of the water potential of the xylem in your plant's stem can be determined with the Scholander bomb.] bio.usyd.edu.au</ref> When the technology to perform direct measurements with a pressure probe was developed, there was initially some doubt about whether the classic theory was correct, because some workers were unable to demonstrate negative pressures. More recent measurements do tend to validate the classic theory, for the most part. Xylem transport is driven by a combination<ref>{{Cite web|url=https://www.nature.com/scitable/knowledge/library/water-uptake-and-transport-in-vascular-plants-103016037|title=Water Uptake and Transport in Vascular Plants|last=Andrew J. McElrone, Brendan Choat, Greg A. Gambetta, Craig R. Brodersen|date=2013|website=The Nature Education Knowledge Project}}</ref> of transpirational pull from above and [[root pressure]] from below, which makes the interpretation of measurements more complicated.