Ascent of sap

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The ascent of sap in the xylem tissue of plants is the upward movement of water and minerals from the root to the aerial parts of the plant. The conducting cells in xylem are typically non-living and include, in various groups of plants, vessel members and tracheids. Both of these cell types have thick, lignified secondary cell walls and are dead at maturity. Although several mechanisms have been proposed to explain how sap moves through the xylem, the cohesion-tension mechanism [1] has the most support. Although cohesion-tension has received criticism due to the apparent existence of large negative pressures in some living plants, experimental and observational data favor this mechanism. [2] [3]

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Theories of sap ascent

One early theory that has recently been revisited is the one presented by Jagadish Chandra Bose in 1923. In his experiment, he used his invention called a galvanometer (made of an electric probe and copper wire) and inserted it into the cortex of the Desmodium plant. After analyzing the findings his experiment, he saw that there were rhythmic electric oscillations. He concluded that plants move sap through pulses or a heartbeat. Many scientists discredited his work and claimed that his findings were not creditable. These scientists believed that the oscillations he recorded was an action potential across the cell wall. Modern-day scientists hypothesized that the oscillations that were measured in Bose's initial experiment was a stress response due to presence of sodium in the water. The results of this modern-day experiment showed that there were no rhythmic electric oscillations present in the plant. Despite not being able to replicate the oscillations that Bose recorded, this study believes that the presence of sodium played a role in his findings. Furthermore, plants do not have a pulse or heartbeat. [4]

An alternative theory based on the behavior of thin films has been developed by Henri Gouin, a French professor of fluid dynamics. [5] The theory is intended to explain how water can reach the uppermost parts of the tallest trees, where the applicability of the cohesion-tension theory is debatable. [6]

The theory assumes that in the uppermost parts of the tallest trees, the vessels of the xylem are coated with thin films of sap. The sap interacts physically with the walls of the vessels: as a result of van der Waals forces, the density of the film varies with distance from the wall of a vessel. This variation in density, in turn, produces a "disjoining pressure", whose value varies with distance from the wall. (Disjoining pressure is a difference in pressure from that which prevails in the bulk of a liquid; it is due to the liquid's interaction with a surface. The interaction may result in a pressure at the surface that is greater or less than that which prevails in the rest of the liquid.) As a tree's leaves transpire, water is drawn from the xylem's vessels; hence, the thickness of the film of sap varies with height within a vessel. Since the disjoining pressure varies with the thickness of the film, a gradient in the disjoining pressure arises during transpiration: the disjoining pressure is greater at the bottom of the vessel (where the film is thickest) and less at the top of the vessel (where the film is thinner). This spatial difference in pressure within the film results in a net force that pushes the sap upwards towards the leaves.

Xylem structure

Xylem tissue is one of two types of vascular tissue found in plants, and is composed of dead cells. It is used mainly for the transport of water, along with some small nutrients. The meristem of the stem creates cells which make up the cambium and pro-cambium. These cells then produce a highly branched poly-phenolic protein, called lignin, in a very high concentration. The cells then perform apoptosis, and the actual xylem tube begins to form. Being made of dead cells allows the xylem tube to function more efficiently by reducing friction, and by eliminating interactions xylem sap may have with living cells. It allows for a smooth and fast suction of water, and provides just enough friction to keep the column from rupturing. The rigid structure of the lignin protein gives a sturdy structure to the tube, and even provides some structure and support for the plant. Xylem mainly functions to transport water from the roots to the rest of the plant, however it also transports some nutrients, such as amino acids, small proteins, ions, and some other vital nutrients. [7]

Phloem structure

The phloem is the living portion of the vascular system of a plant, and serves to move sugars and photosynthate from source cells to sink cells. Phloem tissue is made of sieve elements and companion cells, and is surrounded by parenchyma cells. The sieve element cells work as the main player in transport of phloem sap. When fully matured, they have no nucleus, and only a handful of organelles. This allows them to be highly specified, and very efficient at transport, since they are not taking any of the solutes they are transporting. These cells are connected to form the full tube by their plasmodesmata. From here, the solutes traveling through the phloem can move either as a symplast, or apoplast. The loading and unloading of phloem sap is done mainly by pressure flow, and relies on loading of the cells and unloading of the cells happening at the same time to maintain the turgor pressure of the system. [8]

Sap components

There are two different types of sap that are in a plant. [9] These types are xylem and phloem sap, both differing in their compositions. Sap that is transported in the phloem is mainly made of water. The second most abundant substance is sucrose. [10] One study found that the rice plant Oryza sativa had a sucrose concentration of 570 nm, [11] but the sucrose concentration is unique to each organism. Another important component of phloem sap is nitrogen. Nitrogen is usually not being transported in its ionic form. [10] Instead it is incorporated in amino acids such as glutamate and aspartate. [10] Hormones, inorganic ions, RNA, and proteins are found in the phloem sap as well. [10] [11]

Xylem sap is mostly made of water. This is because one of the main roles of xylem is to transport water and inorganic nutrients throughout the plant. [12] Water is not the only thing that makes up xylem sap though. Xylem sap contains long-distance signaling hormones, proteins, enzymes, and transcription factors. One study found that proteins transported in this sap can be as large as 31 kDa. [12]

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<span class="mw-page-title-main">Plant cell</span> Type of eukaryotic cell present in green plants

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<span class="mw-page-title-main">Xylem</span> Water transport tissue in vascular plants

Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of the xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word xylem is derived from the Ancient Greek word ξύλον (xylon), meaning "wood"; the best-known xylem tissue is wood, though it is found throughout a plant. The term was introduced by Carl Nägeli in 1858.

<span class="mw-page-title-main">Phloem</span> Sugar transport tissue in vascular plants

Phloem is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as photosynthates, in particular the sugar sucrose, to the rest of the plant. This transport process is called translocation. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Ancient Greek word φλοιός (phloiós), meaning "bark". The term was introduced by Carl Nägeli in 1858. Different types of phloem can be distinguished. The early phloem formed in the growth apices is called protophloem. Protophloem eventually becomes obliterated once it connects to the durable phloem in mature organs, the metaphloem. Further, secondary phloem is formed during the thickening of stem structures.

<span class="mw-page-title-main">Vascular plant</span> Phylum of plants with xylem and phloem

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<span class="mw-page-title-main">Root pressure</span> Transverse osmotic pressure within the cells of a root system

Root pressure is the transverse osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves.

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Sap is the fluid transported in xylem cells or phloem sieve tube elements of a plant.

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The vascular cambium is the main growth tissue in the stems and roots of many plants, specifically in dicots such as buttercups and oak trees, gymnosperms such as pine trees, as well as in certain other vascular plants. It produces secondary xylem inwards, towards the pith, and secondary phloem outwards, towards the bark.

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<span class="mw-page-title-main">Sap</span> Fluid transported in xylem cells or phloem sieve tube elements of a plant

Sap is a fluid transported in xylem cells or phloem sieve tube elements of a plant. These cells transport water and nutrients throughout the plant.

<span class="mw-page-title-main">Girdling</span> Removal of the bark from around the entire circumference

Girdling, also called ring-barking, is the circumferential removal or injury of the bark of a branch or trunk of a woody plant. Girdling prevents the tree from sending nutrients from its foliage to its roots, resulting in the death of the tree over time, and can also prevent flow of nutrients in the other direction depending on how much of the xylem is removed. A branch completely girdled will fail and when the main trunk of a tree is girdled, the entire tree will die, if it cannot regrow from above to bridge the wound. Human practices of girdling include forestry, horticulture, and vandalism. Foresters use the practice of girdling to thin forests. Extensive cankers caused by certain fungi, bacteria or viruses can girdle a trunk or limb. Animals such as rodents will girdle trees by feeding on outer bark, often during winter under snow. Girdling can also be caused by herbivorous mammals feeding on plant bark and by birds and insects, both of which can effectively girdle a tree by boring rows of adjacent holes.

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<span class="mw-page-title-main">Vascular tissue</span> Conducting tissue in vascular plants

Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue: the vascular cambium and the cork cambium. All the vascular tissues within a particular plant together constitute the vascular tissue system of that plant.

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According to the vital force theory, the conduction of water up the xylem vessel is a result of vital action of the living cells in the xylem tissue. These living cells are involved in ascent of sap. Relay pump theory and Pulsation theory support the active theory of ascent of sap.

The pressure flow hypothesis, also known as the mass flow hypothesis, is the best-supported theory to explain the movement of sap through the phloem. It was proposed by Ernst Münch, a German plant physiologist in 1930. A high concentration of organic substances, particularly sugar, inside cells of the phloem at a source, such as a leaf, creates a diffusion gradient that draws water into the cells from the adjacent xylem. This creates turgor pressure, also known as hydrostatic pressure, in the phloem. Movement of phloem sap occurs by bulk flow from sugar sources to sugar sinks. The movement in phloem is bidirectional, whereas, in xylem cells, it is unidirectional (upward). Because of this multi-directional flow, coupled with the fact that sap cannot move with ease between adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be flowing in opposite directions.

<span class="mw-page-title-main">Plant stem</span> Structural axis of a vascular plant

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<span class="mw-page-title-main">Transpiration</span> Process of water moving through a plant parts

Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. It is a passive process that requires no energy expense by the plant. Transpiration also cools plants, changes osmotic pressure of cells, and enables mass flow of mineral nutrients. When water uptake by the roots is less than the water lost to the atmosphere by evaporation plants close small pores called stomata to decrease water loss, which slows down nutrient uptake and decreases CO2 absorption from the atmosphere limiting metabolic processes, photosynthesis, and growth.

Nasuia deltocephalinicola was reported in 2013 to have the smallest genome of all bacteria, with 112,091 nucleotides. For comparison, the human genome has 3.2 billion nucleotides. The second smallest genome, from bacteria Tremblaya princeps, has 139,000 nucleotides. While N. deltocephalinicola has the smallest number of nucleotides, it has more protein-coding genes (137) than some bacteria.

References

  1. ↑ Henry H. Dixon and J. Joly (1895) "On the Ascent of Sap", Philosophical Transactions of the Royal Society of London. B, 186 : 563–576.
  2. ↑ Xylem Structure and the Ascent of Sap, 2nd ed. 2002. by Melvin T. Tyree and Martin H. Zimmermann ( ISBN   3-540-43354-6) Springer-Verlag
  3. ↑ "The Cohesion-Tension Theory" by Angeles G, Bond B, Boyer JS, Brodribb T, Brooks JR, Burns MJ, Cavender-Bares J, Clearwater M, Cochard H, Comstock J, Davis SD, Domec J-C, Donovan L, Ewers F, Gartner B, Hacke U, Hinckley T, Holbrook NM, Jones HG, Kavanagh K, Law B, López-Portillo J, Lovisolo C, Martin T, Martínez-Vilalta J, Mayr S, Meinzer FC, Melcher P, Mencuccini M, Mulkey S, Nardini A, Neufeld HS, Passioura J, Pockman WT, Pratt RB, Rambal S, Richter H, Sack L, Salleo S, Schubert A, Schulte P, Sparks JP, Sperry J, Teskey R, Tyree M. New Phytologist, Vol. 163:3, pp. 451–452. (2004) https://eurekamag.com/research/035/842/035842571.php
  4. ↑ Das, Supriyo Kumar; Dutta, Debasish; Naskar, Saranya; Palchaudhury, Snigdha; Gayen, Rabindranath; Dey, Abhijit (October 25, 2018). "Revisiting the physiology of ascent of sap in plants: legendary experiment of J.C. Bose". Current Science. 115 (8): 1451–1453. ISSN   0011-3891.
  5. ↑ See:
    • Henri Gouin (October 2008) "A new approach for the limit to tree height using a liquid nanolayer model," Continuum Mechanics and Thermodynamics, 20 (5) : 317-329. Available on-line at: Arxiv.org
    • Henri Gouin (2011) "Liquid-solid interaction at nanoscale and its application in vegetal biology," Colloids and Surfaces A, 383 : 17–22. Available on-line at: Arxiv.org
    • Henri Gouin (2012) "The nanofluidics can explain ascent of water in tallest trees". Available on-line at: Arxiv.org
    • Henri Gouin (2014) "The watering of trees. Embolization and recovery in xylem microtubes." Available on-line at: Arxiv.org
  6. ↑ See:
    • Tyree M.T. (1997) "The cohesion-tension theory of sap ascent: current controversies," Journal of Experimental Botany, 48 : 1753-1765.
    • Koch, W.; Sillett, S.C.; Jennings, G.M.; Davis, S.D. (2004) "The limit to tree height," Nature, 428 : 851-854.
  7. ↑ Růžička, Kamil; Ursache, Robertas; Hejátko, Jan; Helariutta, Ykä (2015). "Xylem development - from the cradle to the grave". New Phytologist. 207 (3): 519–535. doi: 10.1111/nph.13383 . PMID   25809158.
  8. ↑ Knoblauch, Michael; Oparka, Karl (2012). "The structure of the phloem - still more questions than answers". The Plant Journal. 70 (1): 147–156. doi: 10.1111/j.1365-313X.2012.04931.x . PMID   22449049.
  9. ↑ "What is sap?". Texas A&M Agrilife Extension. April 29, 2022.
  10. 1 2 3 4 Taiz, Lincoln; Zeiger, Eduardo; Moller, Ian Max; Murphy, Angus (2018). Fundamentals of Plant Physiology. United States of America: Oxford University Press. pp. 277–278. ISBN   9781605357904.
  11. 1 2 Hayashi, Hiroaki; Chino, Mitsuo (1990). "Chemical Composition of Phloem Sap from the Uppermost Internode of the Rice Plant". Plant Cell Physiol. 31(2) (5): 247–251. doi:10.1007/s00425-011-1352-9. PMID   21246215. S2CID   12111902 – via National Library of Medicine.
  12. 1 2 Krishnan, Hari B.; Natarajan, Savithity S.; Bennett, John O.; Sicher, Richard C. (January 19, 2011). "Protein and metabolic composition of xylem sap from field-grown soybeans (Glycine max)" . Planta. 233 (5): 921–931. doi:10.1007/s00425-011-1352-9. PMID   21246215. S2CID   12111902 – via Springer Link.
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