The Absorption of Water

Lecture 2 A

The impotence of water for living

organisms

The Absorption of Water

Factor affecting passive water

absorption

Transport of water within plants

The Ascent of Sap

Functions of water:

1. is a major component of cells.

2. is a solvent for the uptake and transport of materials.

3. is a good medium for biochemical reactions.

4. is a reactant in many biochemical reactions (i.e., photosynthesis).

5. provides structural support via turgor pressure (i.e., leaves).

6. is the medium for the transfer of plant gametes (sperms swim to eggs).

7. in water, some aquatic plants shed pollen underwater.

Functions of water:

8. Offspring (propagated) dispersal (think "coconut").

9. Plant movements are the result of water moving into and out of those parts (i.E., Diurnal movements, stomatal opening, flower opening).

10. Cell elongation and growth.

11. Thermal buffer.

12. Perhaps most importantly, water has directed the evolution of all organisms. You can think of morphological features of organisms as a consequence of water availability. For example, consider organisms growing in xeric (dry), mesic (moderate) and hydric (aquatic) environments.

Acids and bases :

Water ionizes to a small degree to form a hydrogen ion (or

proton) and hydroxide ion (OH-).

In reality, two water molecules form a hydronium ion

(H30+) and a hydroxide ion (OH-)

In pure water,

[H+] = [OH-] This solution is neutral

[H+] > [OH-] Then, the solution is an acid (acidic)

[H+] < [OH-] Then the solution is a base (alkaline)

Thus:

1. An acid is a substance that increases the [H+], or as the

chemists say, is a proton donor.

eg. HCl H+ + Cl-

2. A base is a substance that increases the [OH-]; or from the

perspective of a proton, a base is a substance that

decreases the proton concentration; it is a proton acceptor.

e.g. NaOH Na+ + OH- (accepts protons to make water)

e.g. NH3 (ammonia) + H+ NH4 + (ammonium ion)

Water movement :

There are two major ways to move

molecules:

A. Bulk (or Mass) Flow

This is the mass movement of

molecules in response to a pressure

gradient.

The molecules move from high to à low

pressure, following a pressure gradient.

Water movement :

There are two major ways to move molecules:

B. Diffusion

The net, random movement of individual molecules from one area to another. The molecules move from [high] à [low], following a concentration gradient.

Another way of stating this is that the molecules move from an area of high free energy (higher concentration) to one of low free energy (lower concentration). The net movement stops when a dynamic equilibrium is achieved.

Most absorption of water occurs in the root tip

regions, and especially in the root hair zone. Older

portions of most roots become covered with

cutinized or suberized layers through which only

very limited quantities of water can pass.

Whenever the water potential in the peripheral

root cells is less than that of the soil water,

movement of water from the soil into the root

cells occurs.

The successively smaller branches of the root system of any plant

terminate ultimately in the root tips, of which there may be

thousands and often millions on a single plant.

The major functions of roots are :

1. Absorption of water and inorganic nutrients

2. Anchoring the plant body to the ground.

3. Roots also function in cytokinin synthesis, which supplies

some of shoot needs.

4. They often function in storage of food.

Primary and secondary roots

in a cotton plant

Root structure :

1- The root cap

At the tip of every growing root is a conical covering of tissue called The root cap

It usually is not visible to the naked eye.

It consists of undifferentiated soft tissue (parenchyma) with unthickened walls covering the apical meristem.

It provides mechanical protection to the meristem cells as the root advances through the soil, its cells worn away but quickly replaced by new cells generated by cell division within the meristem.

It is also involved in the production of mucigel, a sticky mucilage that coats the new formed cells. These cells contain statoliths, starch grains that move in response to gravity and thus control root orientation.

Root structure :

Root structure :

2- The epidermis

It is the outside surface of a primary root.

Recently produced epidermal cells absorb water from the surrounding environment and produce outgrowths called root hairs that greatly increase the cell's absorptive surface.

3- Root-hairs

Are very delicate and generally short-lived, remaining functional for only a few days.

However, as the root grows, new epidermal cells emerge and these form new root hairs, replacing those that die.

The process by which water is absorbed into the epidermal cells from the soil is known as osmosis. For this reason, water that is saline is more difficult for most plant species to absorb.

Root structure :

4- The cortex

Is beneath the epidermis, which comprises the bulk of the primary

root. Its main function is storage of starch.

Intercellular spaces in the cortex aerate cells for respiration.

An endodermis is a thin layer of small cells forming the innermost

part of the cortex and surrounding the vascular tissues deeper in

the root.

The tightly packed cells of the endodermis contain a substance

known as suberin in their cell walls. This suberin layer is the

Casparian strip, which creates an impermeable barrier of sorts.

Mineral nutrients can only move passively within root cell walls

until they reach the endodermis.

Root structure :

5- The vascular cylinder, or stele,

It consists of the cells inside the endodermis.

The outer part, known as the pericycle, surrounds the

actual vascular tissue.

In monocotyledonous plants, the xylem and phloem

cells are arranged in a circle around a pith or center,

in dicotyledons, the xylem cells form a central "hub"

with lobes, and phloem cells fill in the spaces between

the lobes.

Water Movement Through a Plant :

To start with the roots:

Most of the water absorption is carried out by the younger part of the roots. Just behind the growing tip of a young root is the piliferous region, made up of hundreds of projections of the epidermal tissue, the root hairs.

Root hairs can be seen very clearly in newly germinated seeds,

The root hairs are short lived being constantly replaced as new growth takes place.

Water Movement Through a Plant :

To start with the roots:

The narrow walled hairs greatly increase the surface

area over which water absorption can take place.

Water in the soil spaces is taken into the root hairs by

the process of osmosis, there being a higher water

concentration outside than within the root hair cells.

Absorption mechanism : All absorption of water occurs along gradient of decreasing

water from the medium in which the roots are growing to the root xylem.

However, the gradient is produced differently in slowly and in rapidly transpiring plants.

This results in two absorption mechanisms:

1. active absorption or osmotic absorption in slowly transpiring where roots behave as osmometers, and

2. passive absorption in rapidly transpiring plants where water is pulled in by the decreased pressure or tension produced in the xylem sap through the roots, which function as passive surfaces. It is operative in the form of root pressure, bleeding and guttation.

Root pressure:

Roots of plant absorb water from the soil.

Water is thus exuded in the xylem ducts of the root and

stem under pressure, the pressure developed inside the

roots due to absorption of water is called the root pressure.

It is believed to be a simple osmotic process, caused by

accumulation of sufficient solutes in the xylem ducts to

lower the water potential of the xylem sap below that of the

substrate.

Root pressure:

Root pressure:

Root pressure:

Root pressure:

Root pressure:

Root pressure occurs in the xylem of some vascular plants when the soil moisture level is high either at night or when transpiration is low during the day.

When transpiration is high, xylem sap is usually under tension, rather than under pressure, due to transpirational pull.

At night in some plants, root pressure causes guttation 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 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: Root pressure is caused by active transport of mineral

nutrient ions into the root xylem. Without transpiration to carry the ions up the stem, they accumulate in the root xylem and lower the water potential.

Water then diffuses from the soil into the root xylem due to osmosis. 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 trees. 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 in the root is important in the development of root pressure. The endodermis is a single layer of cells between the cortex to the outside and the pericycle.

A waterproof substance in the walls of endodermal cells, suberin, prevents mineral nutrient ions from moving passively through the endodermal cell walls. Movement of water and ions in the cell walls is the apoplast pathway.

The suberin layer is termed the Casparian strip. Ions outside the endodermis must be actively transported across an endodermal cell membrane to enter or exit the endodermis.

Root pressure:

Role of 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 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.

Passive water absorption: This is the most prevalent method of water absorption. In this

process the force concerned with this type of absorption eminates the aerial parts of the plant especially leaves and causes a tension in the xylem sap.

From the root tip to the apical portion of the plant there is a continuous column of water present in the xylem elements. These are in contact with the living cell. As a result of the active transpiration of the leaves, water is drawn from the adjacent to the intercellular spaces below the stomota and these do so from the xylem in turn. Water in the xylem ducts is put into a great tension.

This mtension decreases water potential of the xylem sap. Root hairs are present in the soil and are in touch with the water molecules to be absorbed. As a result the tension of the xylem sap can be remedied in these root hairs.

Factor affecting passive

water absorption

Plant factors

Root system

Resistance of conducting system

Environmental factors

Availability of soil water

Concentration of salts

Soil air

Transpiration

Soil temperature

1. Plant factors

Root system

• The number and length of root hairs as well as the length of

root hair zone determine the extent of water absorbed from

the soil.

• Deeper portions of the roots are less efficient in the water

uptake compared to the less deep portions.

• The continuous formation and growth of root hair facilitate

water uptake. Also metabolism of the root hair influences

the amount of water uptake.

Resistance of

conducting system

• The rate of water absorption directly depends upon the

resistance to the passage of water.

• The latter is connected with the cell wall permeability,

metabolic state of the protoplasm, nature of endodermis,

xylem vessels: their location, distribution and diameter.

2. Environmental factors

Availability of soil water

• The amount of water content of the soil influences the

rate of the water absorption.

• Soil having poor aeration, low metabolism affect water

uptake.

Concentration of salts

• If the soil water has enormous quantities of minerals

dissolved in it, this will increase the osmotic pressure

of the soil.

Soil air

• The amount of aeration of soil greatly influences the

water absorption.

• Water logged soil has less amount of dissolved oxygen.

• Also higher CO2 is detrimental to the absorption of

water.

2. Environmental factors

Transpiration

• Water uptake is closely linked with the rate of

transpiration.

• Since transpiration causes tension through the water

loss.

• Therefore high rate of transpiration causes increased

water absorption.

Soil temperature

• Cold soils are physiologically dry.

• Low temperature affects root metabolism especially its

permeability and its elongation.

• At temperatures between 15-25oc the absorption of

water is maximal.

Most plants secure the water they need from their roots.

The path taken is: soil -> roots -> stems -> leaves .

Less than 1% of the water reaching the leaves is used in

photosynthesis and plant growth. Most of it is lost in

transpiration.

However, transpiration does serve two useful functions:

• It provides the force for lifting the water up the stems.

• It cools the leaves.

Water and minerals enter the root by separate paths which

eventually converge in the stele.

The Pathway of Water

The Pathway of Water

Soil water enters the root through its epidermis. It appears

that water then travels in both

1. The cytoplasm of root cells — called the symplast — that is,

it crosses the plasma membrane and then passes from

cell to cell through plasmodesmata.

2. In the nonliving parts of the root — called the apoplast —

that is, in the spaces between the cells and in the cells

walls themselves. This water has not crossed a plasma

membrane.

The Pathway of Water

However, the inner boundary of the cortex, the endodermis,

is impervious to water because of a band of suberized

matrix called the casparian strip.

Therefore, to enter the stele, apoplastic water must enter

the symplasm of the endodermal cells. From here it can

pass by plasmodesmata into the cells of the stele.

Once inside the stele, water is again free to move between

cells as well as through them. In young roots, water enters

directly into the xylem vessels and/or tracheids. These are

nonliving conduits so are part of the apoplast.

The Pathway of Water

Once in the xylem, water with the minerals that have been deposited in it (as well as occasional organic molecules supplied by the root tissue) move up in the vessels and tracheids.

At any level, the water can leave the xylem and pass laterally to supply the needs of other tissues.

At the leaves, the xylem passes into the petiole and then into the veins of the leaf.

Water leaves the finest veins and enters the cells of the spongy and palisade layers. Here some of the water may be used in metabolism, but most is lost in transpiration.

The upward movement of water from the root to the

top of the plant is called as ascent of sap.

The water uptake takes place through the roots

and the leaves transpire most of this water.

Theories of Ascent of Sap

Vital Force Theories

Physical Force Theories

Root pressure theory

Imbibitional force

Capillary rise

Cohesion-tension theory

Theories of Ascent of Sap

The intimate association of vessels and tracheids with living

cells (xylem parenchyma and xylem ray cells) has tempted

many workers to suggest that upward translocation of water

is brought about in some manner by the living cells of the

stem.

1-Vital theories

a- Root pressure theory :

It has already explained before.

b- Imbibitional force :

Water rises by imbibtion through the thick walls of the xylem cells, as well as of the sclernchyma of the phloem.

The forces of imbibition seem adequate for carrying water to any required distance.

This imbibitional force works with the other forces to aid in the ascent of sap.

2- Physical theories

c- Capillary rise :

The water moves through the lumina of tracheids

and vessel by capillarity.

The height to which water can rise in small

tracheids (0.02-mm width) is about 150 cm,

while in larger vessels (0.5-mm width) the height

would be only 6 cm.

Thus capillarity in the usual sense does not

operate in plants.

2- Physical theories

c- Capillary rise :

2- Physical theories

d- Cohesion-tension theory

In 1895, the Irish plant physiologists Dixon and Joly proposed that

water is pulled up the plant by tension (negative pressure) from

above.

As we have seen, water is continually being lost from leaves by

transpiration. Dixon and Joly believed that the loss of water in the

leaves exerts a pull on the water in the xylem ducts and draws more

water into the leaf.

But even the best vacuum pump can pull water up to a height of

only 34 ft or so.

This is because a column of water that high exerts a pressure (~15

lb/in2) just counter balanced by the pressure of the atmosphere.

2- Physical theories

d- Cohesion-tension theory

How can water be drawn to the top of a sequoia (the tallest

is 370 feet high)?

Taking all factors into account, a pull of at least 270 lb/in2

is probably needed.

The answer to the dilemma lies the cohesion of water

molecules; that is the property of water molecules to cling

to each through the hydrogen bonds they form.

2- Physical theories

d- Cohesion-tension theory

When water is confined to tubes of very small bore, the

force of cohesion between water molecules imparts great

strength to the column of water.

Tensions as great as 3000 lb/in2 are needed to break the

column, about the value needed to break steel wires of the

same diameter. In a sense, the cohesion of water molecules

gives them the physical properties of solid wires.

Because of the critical role of cohesion, the transpiration-

pull theory is also called the cohesion theory.

2- Physical theories

2- Physical theories d- Cohesion-tension theory