Sap is afluid transported in thexylem cells (vessel elements or tracheids) orphloemsieve tube elements of aplant. These cells transport water andnutrients throughout the plant.
Sap is distinct fromlatex,resin, orcell sap; it is a separate substance, separately produced, and with different components and functions.
Insect honeydew is called sap, particularly when it falls from trees, but is only the remains of eaten sap and other plant parts.[1]
Saps may be broadly divided into two types: xylem sap and phloem sap.
Xylem sap (pronounced/ˈzaɪləm/) consists primarily of a watery solution ofhormones,mineral elements and othernutrients. Transport of sap in xylem is characterized by movement from theroots toward theleaves.[2]
Over the past century, there has been some controversy regarding the mechanism of xylem sap transport; today, most plant scientists agree that thecohesion-tension theory best explains this process, but multiforce theories that hypothesize several alternative mechanisms have been suggested, including longitudinal cellular and xylemosmotic pressuregradients, axial potential gradients in the vessels, and gel- and gas-bubble-supported interfacial gradients.[3][4]
Xylem sap transport can be disrupted bycavitation—an "abrupt phase change [of water] from liquid to vapor"[5]—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 withwater stress, and freeze-thaw cycles in temperate climates.[5]
Phloem sap (pronounced/ˈfloʊɛm/) consists primarily ofsugars (mainlysucrose[6]), hormones, and mineral elements dissolved in water. It flows from wherecarbohydrates are produced or stored (sugar source) to where they are used (sugar sinks).[citation needed] Thepressure flow hypothesis proposes a mechanism for phloem sap transport,[citation needed] although other hypotheses have been proposed.[7] Phloem sap is thought to play a role in sending informational signals throughout vascular plants. According toAnnual Review of Plant Biology,
Loading and unloading patterns are largely determined by theconductivity and number ofplasmodesmata and the position-dependent function ofsolute-specific,plasma membranetransport proteins. Recent evidence indicates that mobile proteins andRNA are part of the plant's long-distance communication signaling system. Evidence also exists for the directed transport and sorting ofmacromolecules as they pass through plasmodesmata.[7]
Many insects of theorderHemiptera (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".[8] 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 ofnon-essential/essentialamino acids in phloem sap for which these adapted Hemiptera insects containsymbioticmicroorganisms which can then provide them with essential amino acids; and also insect "tolerance of the very high sugar content andosmotic pressure of phloem sap is promoted by their possession in the gut ofsucrase-transglucosidase activity, which transforms excess ingested sugar into long-chainoligosaccharides."[8]A much larger set of animals do however consume phloem sap by proxy, either "through feeding on thehoneydew 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,"[8] or by feeding on thebiomass of insects that have grown on more direct ingestion of phloem sap.
Maple syrup is made from reducedsugar maple xylem sap.[9] The sap often is harvested from the sugar maple,Acer saccharum.[10]
In some countries (e.g.,Lithuania,Latvia,Estonia,Finland,Belarus,Russia) harvesting the early spring sap ofbirch trees (so called "birch juice") for human consumption is common practice; the sap can be used fresh orfermented.[11]
Certain palm tree sap can be used to makepalm syrup.[citation needed] In theCanary Islands they use theCanary Island date palm while in Chile they use theChilean wine palm to make their syrup calledmiel de palma.[citation needed]
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