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Role of Water in Plants – Water Deficit and Stress; Water Potential

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Water as a Plant Constituent and its Functions

  • Water is the single most important constituent of living organisms. In plants, water has seven crucial roles.


  • Water comprises 80-90% of the fresh weight of herbaceous plants and over 59% of the fresh weight of woody plants.


  • Water is the universal solvent; more substances dissolve in the water than any other substance.
  • Because of this water is the medium in which biochemical reactants are dissolved in the cell and chemical reactions take place.
  • Cell membranes and cell walls are both very permeable to water so water can move from place to place in the plant.
  • Water forms a continues liquid throughout the plant, it fills the central portion vacuole of mature cells the walls and most intercellular     The intercellular spaces of leaves are an exception by being gas filled because of the need for carbon dioxide exchange with the air.


  • Water is a reactant in the biochemical reactions of the cell.
  • Among these is photosynthesis, where water contributes electrons ultimately used in the reductions of carbon to a carbohydrate and hydrogen protons which play a role in ATP   [adenosine triphosphate] production the oxygen evolved in photosynthesis originates in water.
  • Water is also a reactant in the hydrolysis of plant food reserves such as starch. In starch hydrolysis the elements of water are inserted b/w the glucose units of the starch polymer, converting starch into sugar.


  • Mineral absorbed from the soil are transported across the root up the stem, and throughout the plant by water movement.
  • Carbohydrates, formed in photosynthesis are also distributed through the plant by water.


  • At the time of cell division, the vacuoles of newly formed cells are scattered and small. Minerals are absorbed and deposited into these small vacuoles. This causes water to diffuse into the small vacuoles, and they enlarge creating pressure inside the cell. This pressure expands the plastic walls of the young cells, and this expansion is cell growth. Eventually, the vacuoles coalesce (merge and unite) into a central vacuole, and the walls become so thick they loose plasticity, so that at maturity the cell no longer expands, but does maintain water pressure inside the cell.


  • Mature cells retain their shape by the force of water pressing against the inside of the cell walls. This pressure keeps the cells turgid, and if the pressure is lost (eg from excess evaporation, death, or being placed in salt solutions) they may lose turgidity and become flaccid.
  • It is the turgidity of cells that gives the shape to many tissues such as leaves and annual plants that do not have a woody or other strengthening tissues.

Thermal stability:

  • More calories of heat are required to raise the temp of water than any other common substance.
  • For this reason, plants, which are mostly water, can absorb a considerable amount of heat (eg from sunlight), and only slowly gain temp.
  • Similarly, the same number of calories must be lost in order for the temp of water (for a plant) to be lowered; thus plant temp can remain about air temp during brief cold periods.
  • The high water content of plants permits them to maintain a more constant temp than that of the air.


Root Growth:

  • Root growth taps new soil, reservoirs water and minerals by growing into them. Any factor that a root growth will affect water and mineral absorption.

Path of water and nutrients across the root:

  • Nearly all of the mineral nutrients absorbed by plants are ions dissolved in the soil solution. Soil water is swept across the root in response to osmotic or transpirational pressure gradients and; carries the ions with it. Since most soil water travels the apoplast (cell walls and intercellular spaces), each cell is essentially bathed in an extension of the soil solute. This markedly increases the root surface area available for absorption.
  • When water reaches the endodermis it is forced through membranes. Although the membrane offers resistance to the passage of water, it is fairly water permeable. There are generally some unsuberized cells in the endodermis, and during rapid transpiration, water, along with its dissolved solutes, be swept into the stele through these open, unsuberized openings without being forced through a membrane.

SEE ALSO: Movement of Water Inside the Landmass


  • Membranes are impermeable to ions. It requires metabolic energy to pull ions across the membrane into the cytoplasm of the cell, but once are inside the cell, ions do not readily leak back out because of membrane’s impermeability.
  • Cells use metabolic energy to absorb ions. Much of the energy is used by ion carriers that pull ions across the barrier.  Ion carriers are special molecules that are specific that is, one carrier is designed for K+, another for Ca, etc. This allows the root to be selective, absorbing some ions more than others, and even excluding some ions, depending upon the presence and abundance of specific carriers.  For example, most plants absorb far more K* than Na+ even though there may be more Na in the soil solution.
  • Cells accumulate ions in concentrations far above that of the soil solution.
  • All of the metabolically active cortical cells participate in ion absorption, and once ions are absorbed they enter the (Recall that cytoplasmic strands pass from cell to cell through pores in the cell walls (plasmodesmata), and that this multi-cellular system of the cytoplasm is referred to as the symplast).
  • Absorbed ions move in the symplast from cell to cell, eventually crossing the endodermis, also in the symplast. Once inside the stele, ions are secreted back across the membrane to the apoplast.  In the apoplast (inside the stele) the ions are swept into the xylem and transported to the leaf.
  • If transpiration is slow, the ions may accumulate in the root, increasing the osmotic concentration of the root. the soil is moist this may result in the osmotic absorption of water, which in turn may induce exudation, root pressure, and other phenomena related to osmotic absorption.
  • When transpiration is rapid the absorbed ions are swept up xylem stream to the leaf, where the leaf absorbs ions back into the symplast by the same process that occurred in the cortical cells of the root.


  • The vacuoles of newly divided cells are small and scattered. Soon after division, the new cell begins to absorb ions, concentrating them in dispersed, small vacuoles. This lowers the ψcell by an increase in л, and soil water enters the cell in response to the gradient.
  • This increases turgor pressure P, and in young cells with thin, elastic cell walls, the vacuole expands and the cell grows.
  • During cell development, additional cellulose is laid down in the cell wall, and the wall becomes less and less elastic, until sufficient wall material accumulates, that is, it becomes rigid. At this point cell growth is complete, and the water potential component becomes more significant because P cannot be dissipated by cell expansion.


  • Most forest plants do not absorb minerals directly from the soil solution but from mycorrhizae. The strands of ectotrophic mycorrhizae (hyphae) form a mantle around the young root and penetrate the outer layers, growing to form a network through the remaining root cells. The hyphae also penetrate the soil.
  • Because hyphae are smaller in diameter and much more extensive than roots, they occupy the soil more intensively, exposing much more absorbing surface area than roots. Mycorrhizae are also more metabolically active than roots, and they liberate organic acids that dissolve soil minerals for absorption. For these reasons, mycorrhizae absorb ions, as well as water, more efficiently than roots.
  • The minerals absorbed by the mycorrhizae are carried to the root by its hyphae and then made available for root absorption bat in a higher concentration than in the soil solution.
  • The efficient mineral and water absorption system provided by mycorrhizae permit tree growth especially in soils of low fertility. Today most forests are restricted to infertile soils because forests that occurred on fertile soils in the past have been removed, and the soil converted to agricultural use.  For this reason, mycorrhizae are especially important.



  • The development of water deficits is similar to a bank savings account. Plants lose water by transpiration (withdrawals) and water is made available to plants by precipitation (deposits).
  • Soil depth and texture determine the soil capacity for moisture storage. The amount of available soil water is analogous to the current account balance.
  • Such facts as root growth and salinity determine how easy it is to make withdrawals, and the hydraulic conductivity of roots, stem, and leaves determine how rapidly transfers can be made from one account (e.g. the soil) to another (e.g. the leaf).
  • Water deficits develop whenever withdrawals (transpiration) exceed the rate at which transfers can be made to keep the leaf account with a current, positive balance.


  • Transpiration often exceeds water absorption during the day.

Transcription1 - Forestrypedia

In a 36-yr-old Scots pine forest, uptake of water over the daylight hours lagged behind transpiration by a third.’ Over a 24-h period, however, uptake balanced transpiration within 7%. Data derived from a study where uptake was estimated by radioisotope tracers injected into trees and transpiration was calculated from knowledge of canopy leaf area, stomatal conductance, and mete­orological conditions. (After Waring a al., 1980.)

  • Fig illustrates that Transpiration began at sunrise, and soon exceeded water absorption. By 9:00 am a water deficit had accumulated sufficient to increase water absorption by increasing the ψ gradient at the root-soil water interface.  At 10:00 am the accumulated water deficit had been sufficient to cause partial stomatal closure as evidenced by decreased transpiration.  Trans­piration decreased more toward sunset, but water absorption continued into the night until the deficit was removed.  The water deficit developed in the morning was removed at night.


  • The conductance to the movement of water through the stem regulated by several factors, including xylem anatomy and age.
  • As previously discussed, water transport encounters less resistance in xylem composed of vessels than of tracheids. Fig below illustrates decreased water conductance of Scots pine wood with age.

Water deficits can be induced in leaves even with adequate soil moisture if the stem or root conductance is not sufficient to supply the transpiration demand.

Conductance - Forestrypedia

Water Absorption:

  • The ability of the roots to absorb water is also affected by several factors, including soil temperature, salinity, water content, and any factor that affects root growth.
  • Poor soil aeration causes the resistance of the root to the passage of water to increase, and plants often wilt because of this, even though flooded.
  • During later stages of poor soil aeration (e.g. by flooding) roots may die, and become quite resistant to water flux, or roots may cease growth, causing the prime root region for water absorption to diminish because of tissue maturation. All of these can induce water deficit
  • Salinity affects both the permeability of the root to water and increases лsoil thus reducing the potential for water absorption. Restricted absorption may induce a water deficit.

Soil Drying Cycle:

  • Water potential changes during a soil drying cycle.
  • Water deficits accumulate over time.
  • Pre-dawn measurements of plant moisture stress (using the pressure bomb) are used to measure soil moisture stress based upon this relationship.

Stem Storage:

  • Trees have a tremendous capacity for stem storage of water, as much as 300 t ha-1, or the amount that may be transpired over 5-10 days.
  • In some cases stem water may be recharged quickly following rainstorms, but in others, the stems may not recharge water lost during the summer until autumn or winter.
  • The ability to store and use water to replace transpirational losses can significantly delay the onset of water deficits in large trees.
  • Stem water storage actually supplements soil water storage in large trees.
  • Water stored in the stem may also be used during the day by trees. The use of stored water is reflected in diameter changes of the stem.
  • Diurnal fluctuations in stem diameter are common in trees.

Water Stress:

  • As can be seen from the above discussion, the development of water deficits is a near daily occurrence in plants. Water stress develops whenever: “. . . water loss exceeds absorption long enough to cause a decrease in cell enlargement and perturbation of various essential physiological processes” (Kozlowski, 1991).
  • The usual cause for water stress is drought, but any situation of excess transpiration, inadequate absorption, and insufficient water storage in the stem and/or soil, or insufficient rate of transfer, if severe enough and prolonged enough, can cause water stress.

Image: TNAU Agritech Portal


  • Water potential expresses the free energy of water in pressure units usually megapascals [mpa]. The reference or standard state of water is taken to be pure water at ambient pressure and at the same temperature as the sample Ψ at the standard state is assigned the value of 0 making it easy to calculate the of a sample by the difference.


  • The water potential of tissues {e .g. leaves, potato disks } can be measured by the Chardakov method in which the tissue is immersed in a graded series sugar solutions of increasing If the Ψtissue is less than Ψsolution, water will leave the tissue and dilute the solution. The treated solution is then compared with a paired untreated solution by transferring a drop.  In this case, the drop should rise in the control solution, because the treated solution has become less dense.  When a drop from the treated is placed into its paired control and it neither rises nor sinks;  the  Ψtissue equals  Ψsolution which equals πsolution.
  • Water potential can also be determined by placing a soil or plant tissue sample in a small chamber with strict temperature control and determining the vapor pressure in the chamber by measuring the dew point by isopiestic psychrometry.
  • The water potential of soils may also be determined by placing the soil in pressure membrane apparatus, applying pressure to extract soil water, and measuring the amount of water that remains after extraction at various pressures.


Ψ* = P – π   – ψmatrix   – ψtemp – ψgravity

Water potential * {total} =   pressure – osmotic potential – matrix   potential – WP due to temp difference – WP due to gravity

  • The above equation includes all of the common factors that affect ψ.
  • It is observed that an increase in P increased ψ*, and that an increase in solute concentration, which caused π to become more negative, decreased ψ*.
  • Water can also be attracted to surfaces and to itself across spaces such as capillaries. This attraction is expressed as ψmatrix and also decreases ψ*.
  • Differences in temperature can also affect ψ*; cold water has less free energy than hot water. This is the reason water vapor in the air condenses on a glass of iced tea, and vapor is released from a cup of hot tea.
  • Finally, ψgravity is a special form of P taking into account the hydrostatic head created by a standing water column such as in the xylem at the base of a tall tree.
  • Not all of these factors are used in most water potential calculations.

For correction and improvements please use the comments section below.

Naeem Javid Muhammad Hassani is working as Conservator of Forests in Balochistan Forest & Wildlife Department (BFWD). He is the CEO of Tech Urdu ( Forestrypedia (, All Pak Notifications (, Essayspedia, etc & their YouTube Channels). He is an Environmentalist, Blogger, YouTuber, Developer & Vlogger.

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