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.
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