Hydraulic redistribution
Encyclopedia
Hydraulic redistribution refers to the mechanism by which some vascular plant
s redistribute soil water. It occurs in vascular plants that commonly have roots in both wet soil and extremely dry soil, especially plants with both taproot
s that grow vertically down to the water table
, and lateral roots
that sit close to the surface.
pathways from the taproots to the laterals, so the loss of water from the laterals creates a pressure potential analogous to that of transpirational pull. Ground water is thus drawn up through the taproot to the laterals, only to be exuded into the surface soil.
Though often referred to as hydraulic lift, movement of water by the plant roots has been shown to occur in any direction. This phenomenon has been documented in over sixty plant species spanning a variety of plant types and over a range of environmental conditions.
s of the soil
and the atmosphere
. Therefore the driving force for moving water through a plant is the cohesive strength of water molecules and a pressure gradient from the roots to the leaves. This theory can still be applied when the boundary layer to the atmosphere is closed, e.g. when plant
stomata are closed or in senesced plants. The pressure gradient is between soil layers with different water potentials; water moves through the roots from wetter to drier soil layers in the same manner as it does when the plant is transpiring.
The implications of hydraulic redistribution seem to have an important influence on plant ecosystem
s. Whether or not plants redistribute water through the soil layers can affect plant population dynamics
, such as the facilitation of neighboring species. The increase in available daytime soil moisture can also offset low transpiration rates due to drought (see also drought rhizogenesis
) or alleviate competition for water between competing plant species. Water redistributed to the near surface layers may also influence plant nutrient availability.
Vascular plant
Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...
s redistribute soil water. It occurs in vascular plants that commonly have roots in both wet soil and extremely dry soil, especially plants with both taproot
Taproot
A taproot is an enlarged, somewhat straight to tapering plant root that grows vertically downward. It forms a center from which other roots sprout laterally.Plants with taproots are difficult to transplant...
s that grow vertically down to the water table
Water table
The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...
, and lateral roots
Lateral roots
Lateral roots extend horizontally from the primary root and serve to anchor the plant securely into the soil. This branching of roots also contributes to water uptake, and facilitates the extraction of nutrients required for the growth and development of the plant.Many different factors are...
that sit close to the surface.
Process
During hot, dry periods, the surface soil dries out so much that the lateral roots tend to exude whatever water they contain; thus the roots die unless the water is replaced. In plants that exhibit hydraulic redistribution, there are xylemXylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...
pathways from the taproots to the laterals, so the loss of water from the laterals creates a pressure potential analogous to that of transpirational pull. Ground water is thus drawn up through the taproot to the laterals, only to be exuded into the surface soil.
Though often referred to as hydraulic lift, movement of water by the plant roots has been shown to occur in any direction. This phenomenon has been documented in over sixty plant species spanning a variety of plant types and over a range of environmental conditions.
Causes
The movement of this water can be explained by the theory of water transport through a plant. This well established theory is called the cohesion-tension theory. In brief, it states that movement of water through the plant depends on having a continuous column of water, from the roots to the leaves. Water is then pulled from the roots to the leaves, through the plant system, by the difference in water potential between the boundary layerBoundary layer
In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail. In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal...
s of the soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
and the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
. Therefore the driving force for moving water through a plant is the cohesive strength of water molecules and a pressure gradient from the roots to the leaves. This theory can still be applied when the boundary layer to the atmosphere is closed, e.g. when plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
stomata are closed or in senesced plants. The pressure gradient is between soil layers with different water potentials; water moves through the roots from wetter to drier soil layers in the same manner as it does when the plant is transpiring.
Significance
The ecological importance of hydraulically redistributed water is becoming better understood as this phenomenon is more carefully examined. Water redistribution by plant roots has been found influencing crop irrigation, where watering schemes leave a harsh heterogeneity in soil moisture. The plant roots have been shown to smooth or homogenize the soil moisture. This sort of smoothing out of soil moisture is important in maintaining plant root health. The redistribution of water from deep moist layers to shallow drier layers by large trees has shown to increase the moisture available in the daytime to meet the transpiration demand.The implications of hydraulic redistribution seem to have an important influence on plant ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
s. Whether or not plants redistribute water through the soil layers can affect plant population dynamics
Population dynamics
Population dynamics is the branch of life sciences that studies short-term and long-term changes in the size and age composition of populations, and the biological and environmental processes influencing those changes...
, such as the facilitation of neighboring species. The increase in available daytime soil moisture can also offset low transpiration rates due to drought (see also drought rhizogenesis
Drought Rhizogenesis
Drought rhizogenesis is an adaptive root response to drought stress. New emerging roots are short, swollen, and hairless, capable of retaining turgor pressure and resistant to prolonged desiccation. Upon rewatering, they are capable of quickly forming an absorbing root surface and hair growth...
) or alleviate competition for water between competing plant species. Water redistributed to the near surface layers may also influence plant nutrient availability.