Pressure Flow Hypothesis
Encyclopedia
The Pressure Flow Hypothesis, also known as the Mass Flow Hypothesis, is the best-supported theory to explain the movement of food through the phloem
. It was proposed by Ernst Münch
, a German
plant physiologist
in 1930.
A high concentration of organic substance inside cell
s of the phloem at a source, such as a leaf
, creates a diffusion gradient
(osmotic gradient) that draws water into the cells. Movement occurs by bulk flow (mass flow); phloem sap moves from sugar sources to sugar sinks by means of turgor pressure, also known as hydrostatic pressure. A sugar source is any part of the plant that is producing or releasing sugar. During the plant's growth period, usually during the spring, storage organs such as the root
s are sugar sources, and the plant's many growing areas are sugar sinks. The movement in phloem is bidirectional, whereas, in xylem cells, it is unidirectional (upward).
s are dormant, the leaves
are sources, and storage organs are sinks. Developing seed
-bearing organs (such as fruit
) are always sinks. 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.
solute molecules into it. This causes water to move into the sieve-tube element by osmosis
, creating pressure that pushes the sap down the tube. In sugar sinks, cells actively transport solutes out of the sieve-tube elements, producing the exactly opposite effect.
and other larger oligosaccharides. Now they are unable to move back, but can proceed through wider plasmodesmata into the sieve tube element.
The symplastic phloem loading is confined mostly to plants in tropical rain forests and is seen as more primitive. The actively-transported apoplastic phloem loading is viewed as more advanced, as it is found in the later-evolved plants, and particularly in those in temperate and arid conditions. This mechanism may therefore have allowed plants to colonise the cooler locations.
Organic molecule
s such as sugars, amino acid
s, certain hormone
s, and even messenger RNA
s are transported in the phloem through sieve tube element
s.
Phloem
In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
. It was proposed by Ernst Münch
Ernst Munch
Ernst Münch was a German plant physiologist who proposed the Pressure Flow Hypothesis in 1926.He worked in Aschaffenburg and Munich with Robert Hartig. He worked in a number of fields including forest pathology, resin production, and fungi...
, a German
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
plant physiologist
Plant physiology
Plant physiology is a subdiscipline of botany concerned with the functioning, or physiology, of plants. Closely related fields include plant morphology , plant ecology , phytochemistry , cell biology, and molecular biology.Fundamental processes such as photosynthesis, respiration, plant nutrition,...
in 1930.
A high concentration of organic substance inside cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s of the phloem at a source, such as a leaf
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
, creates a diffusion gradient
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
(osmotic gradient) that draws water into the cells. Movement occurs by bulk flow (mass flow); phloem sap moves from sugar sources to sugar sinks by means of turgor pressure, also known as hydrostatic pressure. A sugar source is any part of the plant that is producing or releasing sugar. During the plant's growth period, usually during the spring, storage organs such as the root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s are sugar sources, and the plant's many growing areas are sugar sinks. The movement in phloem is bidirectional, whereas, in xylem cells, it is unidirectional (upward).
Developement
After the growth period, when the meristemMeristem
A meristem is the tissue in most plants consisting of undifferentiated cells , found in zones of the plant where growth can take place....
s are dormant, the leaves
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
are sources, and storage organs are sinks. Developing seed
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
-bearing organs (such as fruit
Fruit
In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...
) are always sinks. 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.
Mechanisms
While movement of water and minerals through the xylem is driven by negative pressures (tension) most of the time, movement through the phloem is driven by positive hydrostatic pressures. This process is termed translocation, and is accomplished by a process called phloem loading and unloading. Cells in a sugar source "load" a sieve-tube element by actively transportingActive transport
Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...
solute molecules into it. This causes water to move into the sieve-tube element by osmosis
Osmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...
, creating pressure that pushes the sap down the tube. In sugar sinks, cells actively transport solutes out of the sieve-tube elements, producing the exactly opposite effect.
Evidences
There are different evidences that support the hypothesis. Firstly, there is an exudation of solution from the phloem when the stem is cut or punctured by the mouthparts of an aphid, a classical experiment demonstrating the translocation function of phloem, indicating that the phloem sap is under pressure. Secondly, concentration gradients of organic solutes are proved to be present between the sink and the source. Thirdly, when viruses or growth chemicals are applied to a well-illuminated (actively photosynthesising) leaf, they are translocated downwards to the roots. Yet, when applied to shaded leaves, such downward translocation of chemicals does not occur, hence showing that diffusion is not a possible process involved in translocation.Criticisms
Opposition or criticisms against the hypothesis are often voiced, on the other hand. Some argue that mass flow is a passive process while sieve tube vessels are supported by companion cells. Hence, the hypothesis neglects the living nature of phloem. Moreover, it is found that amino acids and sugars (examples of organic solutes) are translocated at different rates, which is contrary to the assumption in the hypothesis that all materials being transported would travel at uniform speed. Bi-directional movements of solutes in translocation process as well as the fact that translocation is heavily affected by changes in environmental conditions like temperature and metabolic inhibitors are two defects of the hypothesis.Other theories
Some plants appear not to load phloem by active transport. In these cases a mechanism known as the polymer trap mechanism was proposed by Robert Turgeon. In this case small sugars such as sucrose move into intermediary cells through narrow plasmodesmata, where they are polymerised to raffinoseRaffinose
Raffinose is a trisaccharide composed of galactose, fructose, and glucose. It can be found in beans, cabbage, brussels sprouts, broccoli, asparagus, other vegetables, and whole grains. Raffinose can be hydrolyzed to D-galactose and sucrose by the enzyme α-galactosidase , an enzyme not found in the...
and other larger oligosaccharides. Now they are unable to move back, but can proceed through wider plasmodesmata into the sieve tube element.
The symplastic phloem loading is confined mostly to plants in tropical rain forests and is seen as more primitive. The actively-transported apoplastic phloem loading is viewed as more advanced, as it is found in the later-evolved plants, and particularly in those in temperate and arid conditions. This mechanism may therefore have allowed plants to colonise the cooler locations.
Organic molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s such as sugars, amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s, certain hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
s, and even messenger RNA
Messenger RNA
Messenger RNA is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein...
s are transported in the phloem through sieve tube element
Sieve tube element
Sieve tubes are mainly to transport sugars and nutrients up and down the plant. In plant anatomy, sieve vascular tissue tube elements, also called sieve tube members, are a type of elongated sclerenchyma cells in phloem tissue. The ends of these cells are connected with other sieve elements, and...
s.