Plant sap

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Multiple cross sections of a flowering plant stem showing primary and secondary xylem cells and phloem sieve tubes[1]

Plant sap, or simply sap, is a fluid transported in xylem cells (tracheids or vessel elements) or phloem sieve tube elements of a plant. The xylem cells transport water and nutrients throughout the plant.

Plant sap is not to be confused with latex, resin or cell sap; it is a separate substance, separately produced, and with different components and functions.[2]

Types of plant sap[edit]

Sap droplets of Sansevieria trifasciata

Plant saps may be broadly divided into two types: xylem sap and phloem sap.

Xylem sap[edit]

Xylem sap consists primarily of a watery solution of hormones, mineral elements and other nutrients. Transport of sap in xylem is characterized by movement from the roots toward the leaves.[3]

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 gradients, axial potential gradients in the vessels, and gel- and gas-bubble-supported interfacial gradients.[4][5]

Xylem sap transport can be disrupted by cavitation—an "abrupt phase change [of water] from liquid to vapor"[6]—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 water stress, and freeze-thaw cycles in temperate climates.[6]

Phloem sap[edit]

Phloem sap consists primarily of water, with sugars, hormones, and mineral elements dissolved in it. It flows from where carbohydrates are produced or stored to where they are used.[2]

The pressure flow hypothesis proposes a mechanism for phloem sap transport.[2] although other hypotheses have been proposed.[7] Phloem sap is also thought to play a role in sending informational signals throughout vascular plants. "Loading and unloading patterns are largely determined by the 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."[7]

Leafhoppers feeding on plant sap, attended by ants

A large number of animals, all of the single insect order Hemiptera, 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 of non-essential/essential amino acids in phloem sap for which these adapted Hemiptera insects contain symbiotic microorganisms 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-transglucosidase activity, which transforms excess ingested sugar into long-chain oligosaccharides."[8] A much larger set of animals do however consume phloem sap by proxy, either "through feeding on the 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,"[8] or by feeding on the biomass of insects that have grown on more direct ingestion of phloem sap.

Human uses[edit]

Maple syrup is made from reduced sugar maple xylem[citation needed] sap. The sap often is harvested from the Sugar Maple, Acer saccharum.[9]

In some countries (e.g., Russia, Latvia, Estonia or Finland) harvesting the early spring sap of birch trees (so called "birch juice") for human consumption is common practice; the sap can be used fresh or fermented and contains xylitol.[10]

Preparations made from the plant sap of Aloe vera,[11] are widely used for their purported soothing, moisturizing, and healing properties.[12][13] Aloe vera gel is also used as an ingredient in commercially available lotions, yogurt, beverages, and some desserts,[14] Note however that so-called aloe products such as "aloes", "aloe gel" etc. are not generally true sap. They are largely distinct substances, different from each other and different from the true phloem and cellular saps. They also are produced in different cellular structures, much as other distinct materials such as latex and resin are produced in special vessels in various other species of plants.[15]

See also[edit]

References[edit]

  1. ^ Winterborne J, 2005. Hydroponics – Indoor Horticulture
  2. ^ a b c Aslam Khan (1 January 2001). Plant Anatomy And Physiology. Gyan Publishing House. ISBN 978-81-7835-049-3. Retrieved 6 April 2013. 
  3. ^ Marschner, H (1983). General introduction to the mineral nutrition of plants. doi:10.1007/978-3-642-68885-0_2. 
  4. ^ Zimmerman, Ulrich (2002). "What are the driving forces for water lifting in the xylem conduit?". Physiologia 114 (3): 327–335. doi:10.1034/j.1399-3054.2002.1140301.x. PMID 12060254. 
  5. ^ Tyree, Melvin T. (1997). "The cohesion-tension theory of sap ascent: current controversies". Journal of Experimental Botany. doi:10.1093/jxb/48.10.1753. 
  6. ^ a b Sperry, John S.; Nichols, Kirk L., Sullivan, June E, Eastlack, Sondra E., (1994). "Xylem Embolism in ring-porous, diffuse-porous, and coniferous trees of Northern Utah and Interior Alaska". Ecology 75 (6): 1736–1752. 
  7. ^ a b Turgeon, Robert; Wolf, Shmuel (2009). "Phloem Transport: Cellular Pathways and Molecular Trafficking". Annual Review of Plant Biology 60: 207–21. doi:10.1146/annurev.arplant.043008.092045. PMID 19025382. 
  8. ^ a b c Douglas, A.E. (2006). "Phloem-sap feeding by animals: problems and solutions". Journal of Experimental Botany. doi:10.1093/jxb/erj067. 
  9. ^ Morselli, Mariafranca; Whalen, M Lynn (1996). "Appendix 2: Maple Chemistry and Quality". In Koelling, Melvin R; Heiligmann, Randall B. North American Maple Syrup Producers Manual. Bulletin 856. Ohio State University. Archived from the original on 29 April 2006. Retrieved 20 September 2010. 
  10. ^ Suzanne Wetzel; Luc Clement Duchesne; Michael F. Laporte (2006). Bioproducts from Canada's Forests: New Partnerships in the Bioeconomy. Springer. pp. 113–. ISBN 978-1-4020-4992-7. Retrieved 6 April 2013. 
  11. ^ Rajendran, A. (2007). "Evaluation of Therapeutic Efficacy of Aloe vera Sap in Diabetes and Treating Wounds and Inflammation in Animals". Journal of Applied Scienes Research 3 (11): 1434–1436. 
  12. ^ Boudreau MD, Beland FA (2006). "An Evaluation of the Biological and Toxicological Properties of Aloe Barbadensis (Miller), Aloe Vera". Journal of Environmental Science and Health Part C 24: 103–154. 
  13. ^ Kunkel. G. Plants for Human Consumption. Koeltz Scientific Books 1984 ISBN 3-87429-216-9
  14. ^ Reynolds, T. (2004) Aloes: The Genus Aloe. CRC Press
  15. ^ Marloth, Rudolf. The Flora of South Africa" 1932 Pub. Capetown: Darter Bros. London: Wheldon & Wesley.

External links[edit]