Discuss about Water Absorption System in Plants: Pathways; Mechanism.

6.  Discuss about Water Absorption System in Plants: Pathways;  Mechanism.

 

Ans.

Water Absorption System in Plants: Pathways;  Mechanism and other Details.  Plants have the potentiality to absorb water through their entire surface right from root, stem * leaves, flowers, etc.  However, as water is available mostly in the soil, only the underground roo system is specialized to absorb water.  Roots are often extensive and grow rapidly in the soil.

In roots, the most efficient region of water absorption is the root hair zone.  Each root hai zone has thousands of root hairs.  Root hairs are specialized for water absorption.  They are tubula outgrowths of50 -1500? m (0. 05 - 1.5mm) length and 10nm in breadth.

Each root hair has a central vacuole filled with somatically active cell sap and a periphera cytoplasm.  The wall is thin and permeable with pectin substances in the outer layer and cellulose the inner layer.  Root hairs pass into capillary microspores, get cemented to soil particles by pectin compounds and absorb capillary water.

Pathways of Water Movements in Roots:

There are two routes of water passage from root hairs to xylem inside the root.  anonlas and symplast.

 

(i) Apoplast Pathway:
Here water passes from root hair to xylem through the walls of intervening cells without crossing any membrane or cytoplasm.  The pathway provides the least resistance to movement of water.  However, it is interrupted by the presence of impermeable lignosuberin casparian strips in the walls of endodermal cells.

(ii) Symplast Pathway:
Water passes from cell to cell through their protoplasm.  It does not enter cell vacuoles.  The cytoplasms of the adjacent cells are connected through bridges called plasmodesmata.  For entering into symplast, water has to pass through plasmalemma (cell membrane) at least at one place.  It is also called transmembrane pathway.  Symplastic movement isaided by cytoplasmic streaming of individual cells.  It is, however, slower than apoplastic movements,

Both the pathway are involved in the movement across the root.  Water flows via apoplast in the cortex.  Itenters the symplast pathway in the endodermis where walls are impervious to flow of water due to the presence of casparian strips.

Here.  only plasmodesmata are helpful to allow passage of water into pericycle from where itenters the xylem.  Mineral nutrients also have the same pathway as that of water.  However, their absorption and passage into symplast mostly occurs through active absorption.  Once inside the xylem.  the movement is purely along the pressure gradient.

Mycorrhizal Water Absorption:
In mycorrhiza a large number of fungal hyphae are associated with the young roots.  The fungal hyphae extend to sufficient distance into the soil.  They have a large surface area.  The hyphae are specialized to absorb both water and minerals.

The two are handed over to the root which provides the fungus with both sugars and N containing compounds.  Mycorrhizal association between fungus and root is often obligate.  Pinus and orchid seeds do not germinate and establish themselves intoplants without mycorrhizalassociation.

Mechanism of Water Absorption:
Water absorption is of two types, passive and active (Renner, 1912, 1915).

(1.) Passive Water Absorption:
The force for this type of water absorption originates in the acrial parts of the plant due to loss of water in transpiration.  This creates a tension or low water potential of several atmospheres in the xylem channels.  Creation of tension in the xylem channels of the plant is evident from:

(i) Anegative pressure is usually found in the xylem sap. It is because ofit that water does not spill out ifa cut is given to a shoot.

(ii) Water can be absorbed by a shoot even in the absence of the root system.

(iii) The rate of water absorption is approximately equal to the rate of transpiration.

Root hairs function as tiny osmoticsystems, each root hair hasathin permeable cell walls semipermeable cytoplasm and an osmotically actiy.  cellsap present in the central vacuole, because of the latter a root hair cell has a water potential of - Jto - 8 bars,

Water potentialofthe soil wateris.  .  / 10 - 3 bars.  As a result water of the soil passes into the root hair cell.  However, water does not pass into its vacuole.  Instead it passes into apoplast and symplastofcortical, endodermal and pericycle cellsand enter thexylem channelspassively because not the very low water potential due to tension under which water is present in them.  caused by transpiration in the nerial parns.  Agradientarwaternotential exists between root haircell.  cortical cell, endodenmal.  pericycle and xylem channels so that flow of water is not interrupted.

(2) Active Water Absorption:
It is the absorption of water due to forces present in the root.  Living cellsinactive metabolic conditions are essential for this.  Auxins are known to increase water absorption (even from hypertonic solution) while respiratory inhibitors reduce the saine.

Therefore, energy (from respiration) is involved in active water absorption.  Water absorption from soil and its inward movement may occur due toosmosis.  Passage of water from living cellsto the xylem channels can occur by:

(i) Accumulation of sugars or salts in the trachetryclements ofxylem due to either secretiort by the nearby living cellsorletthere during decay oftheir protoplasts.

(ii) Development of bioelectric potential favourable for movement of water into xylary channel.

(iii) Active pumping of water by the surrounding living. collsinto tracheary elements.

Ascent of Sap:
Sap is water with dissolved ingredients (minerals). The upward movement of water from roots towards the tips of stem branches and their kaves is called ascent of  sap. It occurs through the tracheary clements of xylem. That the ascent of sap occurs through xylem can be proved by: stain test and ringing experiment,

Theories of ascent of Sap:
Water or sap is lifted from near the root tip to the shoot tip against the  force of gravity. Sometimes toheight of 100 meters. The rate of translocation is25 - 75. cmiminutes. as nihri. Several theories have been put forward to explain the mechanism of ascent of sap.  The three main theories are vital force, root pressure and cohesion tension.

(1.) Vital Force Theory:
A common vital force theory about the ascent of sap was put forward byJ.  C.  Boso (1923), talled pulsation theory.  The theory believes that the innermost cortical cells of the root absorb water from the outer side and pump the same into rylem channels.

However, living cells do not seem to be involved in the ascent of sapas water continuests rite upward in the plant in which roots have been cut or the living cells of the stem are killed by poison and heat (Boucheric, 1840: Strasburger, 1891),

(2.) Root Pressure Theory:
The theory was put forward by Priestley (1916), Root pressure is a positive pressure that develops in the xylem sopof theroot of someplants. Ltisamanifestation of active waterabsorption. Root pressure is observed in certain season which favors optimum metabolic activity and reduce transpiration,

It is maximun during rainy season in the tropical countries arid during spring in temperate habitats. The amount ofroot pressure usually met in plantsisi - 2 bars or atmospheres. Higher values ​​(eg., 5 - 10 am) are also  oliserved occasionally. Root pressure is retarded or becomes absent underconditions of starvation, low temperature. drought and reduced availability of oxygen. There are three view points about the mechanism of root pressure development:

(I) Osmotic:
Trncheary elements ofxylem accumulate salisand sugar. High  solute concentration causes withdrawal of water from the surrounding cells as well as from the normal  pathway of water absorption.  Asa resutra positive pressure develops in the sap ofxylem.

(ii) Electro - osmotic:
Abioelectric potential exists between the xylem chamelsand surrounding cells which favor the passage of water into them.

(iii) Nonosmotics:
Differentiating xylem elements produce hormonesthat function asmetabolicsinks and cause movement of water towards them.  The living.  cellssurrounding xylem can actively pump water into: them.

Objections to Root Pressure Theory:
(I) Root pressure has not been found in all plants.  No or little root pressure has been seen in gymnosperms which have some of the tallestireesofthe world.

(ii) Roor pressure is seen only during the most favourableperiods of growth like spring or riny scason.  Atthis time thexylem sapis stronglyhypertonic to soil solution and transpiration rate islow.  In summer when the water requirements are high, the root pressure is generally absent.

(iii) Thenormally observed root pressure is gencially low which is unable to raise the sap to the top of trees.

(iv) Water continues to rise upwards even in the absence of roots.

(v) The rapidly transpiring plants donor show any root pressure.  Instead a negative pressure is observed in most of the plants.

(vi) Root pressure disappears in unfavourable environmental conditions while ascent of sap "continues uninterrupted:

(vii) Root pressure is generally observed at night when evapotranspiration is low. It may be helpful in re - involving continuous water chainsin xvlem which often break under enoma tension created by transpiration.

(3.)  Physical force theories:
These theories consider dead cells of xylem responsible for ascent of sap. Capillary they vein ochm1863, Imbibition TheoryofUnger 1868 and Cohesion - Tension Theory ofDixon and 1894 are few physical theories. Bur cohesion - tension theory (also called cohesion - tensi transpiration pull  theory) of Dixon and Joly is the most widely accepted one.

Cohesion Tension and Transpiration Pull Theory:
The theory was put forward by Dixon and Joly in 1894. It was further improved by Dixool 1914. Therefore, the theory is also  named after him as Dixon's theory of ascent of sap. Today tro of the workers believe in this theory. The main features of the theory are:

(a) Continuous Water Column:
There is a continuous column of water from root through the stem and into the leaves. The water column is present in tracheary elements.  The latter dooperate separately but formacontinual systems through their unthickened areas.

Since there are a large number of tracheary elements running together, the blockage of or ora few of them does not cause any breakage in the continuity of water column (Scholand 1957).  The column of water does not fall down under the impact of gravity because forcest transpiration provide both energy and necessary pull.  Cohesion, adhesion and surface tensionket the water in place.

(b) Cohesion or Tensile Strength:
Water molecules remain attached to one another by a strong mutual force of attractis called cohesion force.  The mutual attraction is due to hydrogen bonds formed amongst adjacen water molecules.  On account of cohesion force, the water column can bearatension or pull of upl 100atm (Mac Dougal, 1936).  Therefore, the cohesion force isalso called tensile strength.

Its theoretical value is about 15000 atm but the measured value inside the tracheary element ranges between 45 atm to207 atm (Dixon and Joly, 1894).  Water column does not further breaki connection from the tracheary clements (vessels and iracheids) because of another force calle adhesion force between their walls and water - molecules.  Water molecules are attracted to or another more than the water molecules in the gascous state.  It produces surface tensional accounts for high capillarity through tracheids and vessels.

(c) Development of Tension or Transpiration Pull:
Intercellular spaces present amongst mesophyll cells of the leaves are always saturated with water vapors.  The latter come from the wet walls of mesophyll cells.  The intercellular spaceso water vapours.  mesophyll are connected to the outside air through tornata.  Outside air is seldom saturated with water vapour.

It has lower water potential than the moist air present inside the leaf.  Therefore, water Napourn diffuse out of the leaves, The mesophyll cells.  continue to lose water to the intercellular.  Spaces, asa result curvature of meniscus holding water increases resulting in increase in surface rension and decrease in water potential, sometimes to - 30 bars.

Themesophyll cells withdraw water from the deeper cells as its molecules are held together by Tby hydrogen bonds.  The decper cells in tum obtain water from the tracheary elements.  The water Lin the tracheory elements would, therefore, come under tension.  Asimilar tension isfelt in millions of tracheary clements lying adjacent to the transpiring cells.  It causes the whole water column of the plant to come under tension.  As the tension develops due to transpiration.  it is also called Iranspiration pull.  On account of tension created by transpiration, the water column of the plant is pulled up passively from below to the top of the plant like a rope.

Evidences:
(I) The rate of water absorption and hence ascent of sapclosely follows the rate oftranspiration, Shoot attached toa tube having water and dipping in a beaker having mercury can cause the movement ofmercury into the tube showing transpiration pull.

(ii) Ine branch cut from arapidly transpiring plant, water snaps away from the cut end showing that the water column is under tension.

(iii) The maximum tension observed in water column is 10 - 20 atm.  It is sufficient to pull the water to the top of the tallest trees of even more than 130 meters in height.  The tension cannot break the continuity of water columnas.  cohesive forceofxykm sap is45 to 207 atm.
(iv) Gymnosperms are ata disadvantage in the ascent of sap because of the presence of tracheids _ instead of vessels in angiosperms.  However, tricheidalixylem is less prone to gravitation under tension.  Therefore, most of the tall trees of the world are redwoods and conifers.

Objections:
(I) The gases dissolved in sap shall fom air bubbles under tension and high temperature.  Air bubbles would break the continuity of water column and stop ascentofsap due to transpiration pull.

(ii) Atension of upto 100atm has been reported in the xylerm sap by MacDougal (1936) while the cohesive force of sap can be aslow as 45atm.

(iii) Overlapping cuts donot stop ascent of sap though they break the continuity of water column

Mineral Uptake by Roots:
Plants obtain their supply of carbon and most of their oxygen from co, of atmosphere, bydrogen from water while the rest are minerals which are picked  up individually from the soil, mineralsexist in the soil asions which cannot directly cross the cell membranes.

The concentration of ions is some 100 times more inroot interior than in the soil.  Therefore.  all minerals cannot be passively absorbed.  The movement of ions from soil to interior.  of roor is against concentration gradient and requires an active transport.  Specific ion pumps occur in the Traembrine of roon hairs.

They pump minerallions from soil to cytoplasm of epidermal cells of root hairs. Energy is provided by ATR. Respiratory inhibitors like cyanidewhich inhibitATPsynthesis. Generally reduce theion uptake. The small amount which passes into the roor  even without ATP. must be througha passive technique.

For active transport. ATPases are present over. The plasma mentibraries of root epidermal cells. They establish an electrochemical proton grndient for energy energy for movement of ions.  transport proteins present over the endodemal cells.

Endodermisallows the passage of ions inwirdly but not outwardly, Italso controlsthe quantity and type ofions to be passed intoxylem. Inward flowofions from epiblematoxylemisalong the conceritration gradient. The collectionofions in the xylemisresponsible for water potential gradient in the root that  helps in osmotic entry of water as well as  its passage to xylem.  In the xylem, minerals are carried up along with the flowofxylem solution.  In leaves, the celbabsorb the minerals selectively through membrane pumps.

Translocation of mineral lons in the plant:
Though it is generically considered that xylem transports inorganic nutrients while phloem transportsorganic nutrients, the same is not exactly inve.  inxylem sap.  nitrogen travelsasinorganic ions, as well as organic formofarmino acids and related compounds.

Small anounts of P and S are passed insolemasorganic compounds.  There is also exchange ofmaterials betweenavlemand phlocm.  Therefore.  mineralelementspassupxvlem in both inaruanics and organic form.

They reach the area of ​​their simkinanelyyoung leaves.  developing flowers, fruits and seeds, apical and lateral meristoms and individual cells for storage.  Minerals are unloaded arfine vein cndings through diffusion.  They are picked upty cell through active uptake.

There isremobilization of minerals fromcolder scnescing parts.  Nickel has aprominent role in thisactivity.  Tha senescing.  leaves send out.  many mineralshkenitrogen, sulphur.  phosphorousand potassium.  Elements involving in structural.  components are.  However, not romobiliad. Calcium.  The remobilised mineralsbecome availabletoyoung growingleavesand other sinks.