Editing Sap (section)

== Types of sap ==
Saps may be broadly divided into two types:  xylem sap and phloem sap.

===Xylem sap===
Xylem sap (pronounced {{IPAc-en|ˈ|z|aɪ|l|ə|m}}) consists primarily of a watery solution of [[plant hormone|hormones]], [[dietary mineral|mineral elements]] and other [[nutrient]]s. Transport of sap in xylem is characterized by movement from the [[root]]s toward the [[leaf|leaves]].<ref name=marschner1983>{{cite book|last=Marschner|first=H|chapter=General Introduction to the Mineral Nutrition of Plants|year=1983|doi=10.1007/978-3-642-68885-0_2|title=Inorganic Plant Nutrition|pages=5–60|isbn=978-3-642-68887-4|publisher=Springer|series=Encyclopedia of Plant Physiology|volume=15 A}}</ref>

Over the past century, there has been some controversy regarding the mechanism of xylem sap transport; today, most plant scientists 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|year=1997|doi=10.1093/jxb/48.10.1753|volume=48|issue=10|pages=1753–1765|doi-access=free}}</ref>

Xylem sap transport can be disrupted by [[cavitation]]—an "abrupt phase change [of water] from liquid to vapor"<ref name=sperry1994/>—resulting in air-filled xylem conduits. In addition to being a fundamental physical limit on tree height, two environmental stresses can disrupt xylem transport by cavitation: increasingly negative xylem pressures associated with [[moisture stress|water stress]], and freeze-thaw cycles in temperate climates.<ref name=sperry1994>{{cite journal |last=Sperry |first=John S. |author2=Nichols, Kirk L. |author3=Sullivan, June E |author4=Eastlack, Sondra E. |title=Xylem Embolism in ring-porous, diffuse-porous, and coniferous trees of Northern Utah and Interior Alaska |journal=Ecology |year=1994 |volume=75 |issue=6 |pages=1736–1752 |doi=10.2307/1939633 |jstor=1939633 |bibcode=1994Ecol...75.1736S |url=http://bioweb.biology.utah.edu/sperry/publications/Sperry%20et%20al.%201994%20Ecology.pdf |access-date=2018-12-18 |archive-date=2017-08-10 |archive-url=https://web.archive.org/web/20170810100919/http://bioweb.biology.utah.edu/sperry/publications/Sperry%20et%20al.%201994%20Ecology.pdf |url-status=dead }}</ref>

===Phloem sap===
Phloem sap (pronounced {{IPAc-en|ˈ|f|l|oʊ|ɛ|m}}) consists primarily of [[sugar]]s (mainly [[sucrose]]<ref>{{cite journal |last1=Broussard |first1=Louis |last2=Abadie |first2=Cyril |last3=Lalande |first3=Julie |last4=Limami |first4=Anis M. |last5=Lothier |first5=Jérémy |last6=Tcherkez |first6=Guillaume |title=Phloem Sap Composition: What Have We Learnt from Metabolomics? |journal=International Journal of Molecular Sciences |date=7 April 2023 |volume=24 |issue=8 |pages=6917 |doi=10.3390/ijms24086917 |doi-access=free |pmid=37108078 |pmc=10139104 |issn=1422-0067}}</ref>), hormones, and mineral elements dissolved in water. It flows from where [[carbohydrate]]s are produced or stored (sugar source) to where they are used (sugar sinks).{{cn|date=June 2020}} The [[pressure flow hypothesis]] proposes a mechanism for phloem sap transport,{{cn|date=June 2020}} although other hypotheses have been proposed.<ref name=turgeon2009/> Phloem sap is thought to play a role in sending informational signals throughout vascular plants. According to ''[[Annual Review of Plant Biology]]'',{{quote|Loading and unloading patterns are largely determined by the [[Electrical resistivity and conductivity|conductivity]] and number of [[plasmodesmata]] and the position-dependent function of [[solute]]-specific, [[plasma membrane]] [[transport proteins]]. Recent evidence indicates that mobile proteins and [[RNA]] are part of the plant's long-distance communication signaling system. Evidence also exists for the directed transport and sorting of [[macromolecules]] as they pass through plasmodesmata.<ref name=turgeon2009>
{{cite journal |last=Turgeon|first=Robert |author2=Wolf, Shmuel  |title=Phloem Transport: Cellular Pathways and Molecular Trafficking |journal=Annual Review of Plant Biology |year=2009 |doi=10.1146/annurev.arplant.043008.092045 |pmid=19025382 |volume=60 |pages=207–21 |issue=1|bibcode=2009AnRPB..60..207T }}</ref>}}

[[File:Common jassid nymph feeding.jpg|thumb|right|[[Leafhopper]]s feeding on sap, attended by ants]]
Many insects of the [[Order (biology)|order]] [[Hemiptera]] (the half-wings), feed directly on phloem sap, and make it the primary component of their diet.  Phloem sap is "nutrient-rich compared with many other plant products and generally lacking in toxins and feeding deterrents, [yet] it is consumed as the dominant or sole diet by a very restricted range of animals".<ref name=douglas2006/>  This apparent paradox is explained by the fact that phloem sap is physiologically extreme in terms of animal digestion, and it is hypothesized that few animals take direct advantage of this because they lack two adaptations that are necessary to enable direct use by animals.  These include the existence of a very high ratio of [[Non-essential amino acid|non-essential]]/[[Essential amino acid|essential]] [[amino acids]] in phloem sap for which these adapted Hemiptera insects contain [[symbiotic]] [[microorganism]]s which can then provide them with essential amino acids; and also insect "tolerance of the very high sugar content and [[osmotic pressure]] of phloem sap is promoted by their possession in the gut of [[sucrase]]-[[wikt:transglucosidase|transglucosidase]] activity, which transforms excess ingested sugar into long-chain [[oligosaccharides]]."<ref name=douglas2006>
{{cite journal |last=Douglas |first=A.E. |title=Phloem-sap feeding by animals: problems and solutions |journal=Journal of Experimental Botany |year=2006 |doi=10.1093/jxb/erj067 |volume=57 |issue=4 |pages=747–754 |pmid=16449374 |doi-access=free }}</ref>
A much larger set of animals do however consume phloem sap by proxy, either "through feeding on the [[Honeydew (secretion)|honeydew]] of phloem-feeding hemipterans. Honeydew is physiologically less extreme than phloem sap, with a higher essential/non-essential amino acid ratio and lower osmotic pressure,"<ref name=douglas2006/> or by feeding on the [[biomass]] of insects that have grown on more direct ingestion of phloem sap.