Root pressure

	Email		Print
	Bookmark		Link

Root pressure

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

Root pressure occurs in the xylem

Xylem

In vascular plants, xylem is one of the two types of transport tissue, phloem being the other. The word "xylem" is derived from classical Greek language ????? , "wood", and indeed the best known xylem tissue is wood, though it is found throughout the plant....
of some vascular plant

Vascular plant

Vascular plants are those plants that have lignin tissue for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the ferns, clubmosses, flowering plants, conifers and other gymnosperms....
s when the soil moisture level is high either at night or when transpiration

Transpiration

Discussion

Ask a question about 'Root pressure'

Start a new discussion about 'Root pressure'

Answer questions from other users

Full Discussion Forum

Encyclopedia

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

Root pressure occurs in the xylem

Xylem

Vascular plant

Transpiration

Transpiration is the evaporation of water from the aerial parts of plants, especially leaf but also Plant stems, flowers and roots. Leaf surfaces are dotted with openings called stoma that are bordered by guard cells....
is low during the day. When transpiration is high, xylem sap is usually under tension, rather than under pressure, due to transpirational pull

Transpirational pull

Transpirational pull is the main phenomenon driving the flow of water in the xylem tissues of large plants....
. At night in some plants, root pressure causes guttation

Guttation

Guttation is the appearance of drops of xylem sap on the tips or edges of leaves of some vascular plants, such as grasses.At night, transpiration usually does not occur because most plants have their stomata closed....
or exudation of drops of xylem sap from the tips or edges of leaves. Root pressure is studied by removing the shoot of a plant near the soil level. Xylem sap will exude from the cut stem

Plant stem

A stem is one of two main structural axes of a vascular plant. The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaf, inflorescence , conifer cones or other stems etc....
for hours or days due to root pressure. If a pressure gauge is attached to the cut stem, the root pressure can be measured.

Root pressure is caused by active transport

Active transport

Root

In vascular plants, the root is the organ of a plant body that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial root or aerating ....
xylem. Without transpiration to carry the ions up the stem, they accumulate in the root xylem and lower the water potential

Water potential

Water potential is the potential energy of water relative to pure free water in reference conditions. It quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects including surface tension....
. Water then diffuses from the soil into the root xylem due to osmosis

Osmosis

Osmosis is the diffusion of a solvent through a Semipermeable membrane, from a solution of low solute concentration to a solution with high solute concentration , up a solute concentration gradient....
. Root pressure is caused by this accumulation of water in the xylem pushing on the rigid cells. Root pressure provides a force, which pushes water up the stem, but it is not enough to account for the movement of water to leaves at the top of the tallest tree

TREE

TREE was a Boston hardcore punk band formed in the summer of 1990. They were active in the Boston music scene until disbanding in 2002....
s. The maximum root pressure measured in some plants can raise water only to about 20 meters, and the tallest trees are over 100 meters tall.

Role of endodermis

The endodermis

Endodermis

Literally meaning "inner skin," endodermis is the layer of tissue deep in vascular plants.In plants, it is a thin layer of Ground tissue#Parenchyma found in roots, just outside the Vascular tissue cylinder....
in the root is important in the development of root pressure. The endodermis is a single layer of cells between the cortex

Cortex (botany)

In botany, the cortex is the outer of the plant stem or root of a plant, bounded on the outside by the Epidermis and on the inside by the endodermis....
and the pericycle

Pericycle

}The pericycle is a cylinder of parenchyma cells that lies just inside the endodermis and is the outer most part of the stele of plants.In dicots, it also has the capacity to produce lateral roots....
. These cells allow water movement until it reaches the Casparian strip

Casparian strip

In plant anatomy, the Casparian strip is a band of Cell wall material in the radial and transverse walls of the endodermis, which is chemically different from the rest of the cell wall....
, made of suberin

Suberin

Suberin is a wax substance found in higher plants. Suberin is a main constituent of cork , and is named after the Cork Oak, Quercus suber....
, a waterproof substance. The Casparian strip prevents mineral nutrient ions from moving passively through the endodermal cell walls. Water and ions move in these cell walls via the apoplast pathway. Ions outside the endodermis must be actively transported

Active transport

Active transport is the mediated process of moving particles across a biological membrane against a Concentration_gradient#In_biology . If the process uses chemical energy, such as from adenosine triphosphate , it is termed primary active transport....
across an endodermal cell membrane to enter or exit the endodermis. Once inside the endodermis, the ions are in the symplast pathway. They cannot diffuse back out again but can move from cell to cell via plasmodesmata

Plasmodesmata

Plasmodesmata are microscopic channels which traverse the cell wall of plant cells and enable transport and communication between them. Plants having plasmodesmata include the highest derived charophyceans, Charales and Coleochaetales, as well as all embryophytes, better known as land plants....
or be actively transported into the xylem. Once in the xylem vessels or tracheids, ions are again in the apoplast pathway. Xylem vessels and tracheids transport water up the plant but lack cell membranes. The Casparian strip substitutes for their lack of cell membranes and prevents accumulated ions from diffusing passively in apoplast pathway out of the endodermis. The ions accumulating interior to the endodermis in the xylem create a water potential gradient and by osmosis, water diffuses from the moist soil, across the cortex, through the endodermis and into the xylem.

Importance

Root pressure can transport water and dissolved mineral nutrients from roots through the xylem to the tops of relatively short plants when transpiration is low or zero. The maximum root pressure measured is about 0.6 megapascals

Pascal (unit)

The pascal is the SI derived unit of pressure, stress , Young's modulus and tensile strength. It is a measure of force per unit area i.e. equivalent to one newton per square meter or one joule per cubic meter....
but some species never generate any root pressure. The main contributor to the movement of water and mineral nutrients upward in vascular plants is considered to be the transpirational pull

Transpirational pull

Transpirational pull is the main phenomenon driving the flow of water in the xylem tissues of large plants....
. However, sunflower plants grown in 100% relative humidity grew normally and accumulated the same amount of mineral nutrients as plants in normal humidity, which had a transpiration rate 10 to 15 times the plants in 100% humidity. Thus, transpiration may not be as important in upward mineral nutrient transport in relatively short plants as often assumed.

Xylem vessels sometimes empty over winter. Root pressure may be important in refilling the xylem vessels. However, in some species vessels refill without root pressure.

Root pressure is often high in some deciduous trees before they leaf out. Transpiration is minimal without leaves, and organic solutes are being mobilized so decrease the xylem water potential. Sugar maple

Sugar Maple

Acer saccharum is a species of maple native to the hardwood forests of northeastern North America, from Nova Scotia west to southern Ontario, and south to Georgia and Texas....
accumulates high concentrations of sugars in its xylem early in the spring, which is the source of maple sugar. Some trees "bleed" xylem sap profusely when their stems are pruned in late winter or early spring, e.g. maple

Maple

Acer is a genus of trees or shrubs commonly known as Maple. Maples are variously classified in a family of their own, the Aceraceae, or included in the family Sapindaceae....
and elm

Elm

Elms are deciduous and semi-deciduous trees comprising the genus Ulmus, family Ulmaceae. Elms first appeared in the Miocene period about 40 million years ago....
. Such bleeding is similar to root pressure only sugars, rather than ions, may lower the xylem water potential.