Xylem Transport Water and Minerals from Roots to Shoots

Chapter 9WATER IN PLANTS

Plants absorb water and minerals through their roots and transport them to the leaves and stems for metabolic use, e. g. photosynthesis.

Xylem transport water and minerals from roots to shoots.

Phloem transport sugars from where they are produced or stored to where they are needed for growth and metabolism.

SIMPLE DIFFUSION

Diffusion is movement of molecules down the concentration gradient from the area of high concentration to the area of low concentration using the kinetic energy of the molecules. This results in even scattering of molecules throughout the environment.

The greater the difference in concentration between two areas of the environment, the faster diffusion will occur.

Dialysis is the diffusion of a substance through a membrane.
Osmosis is the diffusion of water, a solvent, through a membrane from the region of high water concentration to the region of low water concentration.
Osmotic pressure of a solution is the tendency of water to move from the area of high concentration to the area of low concentration.

Much of the traffic across the membrane occurs by diffusion. This is called passive transport.

The concentration gradient represents potential energy and drives diffusion.

Diffusion is unaffected by the presence of other substances.

A solution with high solute concentration (e.g. salt) has in effect low water concentration and high osmotic pressure.

Isotonic solutions have the same osmotic pressure. E.g. the cell has the same solute concentration as its environment.

Hypertonic solutions have higher solute concentration than other solution. E.g. the environment has greater concentration than the cell, so the environment is hypertonic to the cell. The cell loses water and becomes plasmolyzed (plasmolysis).

Hypotonic solutions have lower solute concentration than other solution. E.g. the environment has lower concentration than the cell, so the environment is hypotonic to the cell. The cell swells.

Turgor pressure is the pressure caused by the cell of plants against the cell wall when the cell swells with water.

Cells lacking walls are isotonic to their environment or have adaptations for osmoregulation, the control of water balance.

In a hypertonic solution, these cells lose water, shrivel and die. They become plasmolyzed.
In a hypotonic solution, these cells will take in water, swell and eventually burst.

Cells with walls are those of plants, fungi and some protists.

In a hypertonic solution, these cells will also become plasmolyzed.
In a hypotonic solution, the cells will swell until they begin to exert pressure against the cell wall. The cell wall will exert a backpressure that will counteract the pressure of the swollen cell. At this point, the volume will not increase any more and water will leave the cell in the same amount as it enters. The cell is turgid (very firm) and has reached a point of equilibrium.

OSMOSIS

Osmosis is the passive transport of water across a membrane.

Water will move across a cell membrane in the hypotonic (low solute concentration) → hypertonic direction (high solute concentration).

Remember that high solute concentration means relatively low water concentration

In plants, the presence of the cell wall that limits the expansion of the cell adds another factor that affects osmosis.

The combine effect of solute concentration and pressure makes what is called the water potential, represented by the letter psi, ψ.

Water moves across the membrane from the region of high water potential to that of low water potential.

Water potential is measured in megapascals, MPa.

1 MPa = 10 atmospheres or 14.5 pounds/inch2.

Examples:

Your lungs exert less than 0.1 MPa.
A car tire is usually inflated to about 0.2 MPa.
Water pressure in home plumbing is about 0.25 MPa.
Plant cells exist at about 1 MPa.

Water potential for pure water in an open container is standardized a 0 MPa.

Adding solutes lowers the water potential because the water molecules surrounding the solute have less freedom of movement due to intermolecular attractions.

Any solution at atmospheric pressure has a negative water potential.

There is an inverse relationship of ψ to solute concentration (osmotic potential).

Plant cells have a cell wall, which adds another factor affecting osmosis: physical pressure.

Increasing the pressure on water increases the ψ.

There is direct relationship of ψ to physical pressure.

Water potential equation

Water potential equals the combine pressure and solute concentration (osmotic potential).

ψ = ψp + ψs

ψp= Pressure potential

ψs = Solute potential; it is proportional to the number of dissolved solute molecules. It is

also called osmotic potential.

Water potential determines the direction of movement of water.

Free water, water that is not bound to solutes or surfaces, moves from the area of higher water potential to the area of lower water potential.

Adding solutes always lowers the water potential and the solute potential (ψs) is negative.

A plant cell placed in pure water will absorb water and become turgid.

Turgor pressure will increase and the cell will push against the rigid wall. The partially elastic wall will push back increasing the ψp until it becomes great enough to offset the tendency of water to enter.

A cell placed in a hypertonic solution will become plasmolyzed.

There are water channels that allow the flux of water in and out of the cell. These transport proteins are called aquaporins.

Aquaporins affect only the rate at which the water flows. They do not affect the concentration gradient or the direction of the water flow.

Imbibition is the passive absorption of water by a substance.

Colloidal materials and macromolecules (e. g. cellulose) develop electrical charges when they are wet.

The charged colloids attract water molecules, which adhere to the internal surfaces of the materials.

ACTIVE TRANSPORT

Hydrophilic substances enter the cell through transport proteins thus avoiding contact with the hydrophobic core of the membrane.

These transport proteins span the membrane.

Carrier proteins change shape in a way that pushes the hydrophilic substance to the other side of the membrane.

Carrier-mediated active transport.

The cell spends energy from ATP to move ions or molecules across the membrane against the concentration gradient.
Ions bind to the transmembrane protein, the pump.
Phosphate group is transferred from ATP to the transport protein.
Transport protein undergoes a conformational change and ions are released to the other side of the membrane.

WATER AND ITS MOVEMENT THROUGH THE PLANT

Water and minerals that have been transferred to the xylem are transported upwards in the xylem sap.

Plants lose a large amount of water by transpiration, the loss of water vapor from leaves and other aerial parts of the plant.

An average maple tree loses an average of 200 liter/hour in the summer time or about 53 gallons/hour.
A hardwood tree uses 450 liters (120 gallons) of water to produce 0.45 kg (1 pound) of wood.
A mature corn plant transpires 15 liters (4 gallons) per week.

Water makes about 90% of the weight of young cells.

Metabolism takes place in water.

Water is needed for gas exchange.

Water maintains cell turgor and helps to support herbaceous plants.

Evaporation of water from plant parts cools the plant.

Roots have many solutes dissolved in their cells, which lowers their water potential in relation to the soil in which they grow.

Water moves in from the soil into the roots by osmosis.

When soil is very dry, its water potential is very low.

Unless the soil is extremely dry, roots have a lower water potential (very negative) than the soil and water tends to move by osmosis from the soil into the roots.

1. Root pressure

Cells in the root pump ions into the root stele.

The endodermis prevents these ions from leaking back into the cortex.

The water potential in the stele is lowered and water flows in from the root cortex generating a positive pressure that forces fluid up the xylem. This pressure is called root pressure.

Root pressure pushes water from the root up the stem.

Not strong enough to push the up tall plants.
It is very low or non-existent during the summer months.
Movement of water is greatest in the summer months when root pressure is the lowest.

Guttation is the release of water droplets through small openings on leaves of plants.

Guttation is the result of root pressure.

Root pressure can force the water up a few meters only. It is not the main mechanism that brings water to the top of the plant but it contributes.

Many plants do not generate root pressure at all.

2. Cohesion-Tension Theory

Also known as the Transpiration-Cohesion Theory.

Water is constantly being lost through the stomata. This water is replaced with water vapor from the mesophyll cells.

As water evaporates, a meniscus is formed by the remaining water in the cell wall spaces, attracted by adhesion to the hydrophilic wall.

Cohesive forces also operate on the surface of the water film.

These two forces, adhesion and cohesion, create the meniscus that has a negative force.

This negative pressure draws water out of the xylem through the mesophyll, and toward the cells and surface film bordering the air spaces near stomata.

There is a gradient in water potential from the atmosphere down to the soil.
The atmosphere has very negative water potential.
Leaves have higher water potential than the atmosphere and lose water to it.
Stems have higher water potential than the leaves; the roots higher than the stem; and the soil higher than the roots.

The gradient creates a pull of the column of water in the xylem due to the hydrogen bonds that exist between the water molecules (cohesion).

Adhesion of the water molecules to the xylem walls maintains an unbroken column of water.

The walls of the vessels and tracheids are hydrophilic and increase the adhesion of water molecules.

The transpiration pull is transmitted from the leaves to the root tips and even into the soil solution.

The plant does not spend any of its energy in bringing the water up to the top.

Solar energy drives transpiration by causing water to evaporate from the moist walls of mesophyll cells and by maintaining a high humidity in the air spaces with a leaf.

The introduction of an air bubble that breaks the column would result in a temporary or permanent blockage of the water flow up the xylem.

CONTROL OF TRANSPIRATION

Stomata regulate transpiration and gas exchange.

The control of transpiration is greatly influenced by the water vapor concentration of the atmosphere.

Leaves have a high surface area-to-volume ratio. This ratio facilitates the uptake of CO2 needed in photosynthesis and the release of the waste product, O2.

Photosynthesis consumes CO2 and produces O2. Both gases diffuse in and out of the leaf respectively through the stomata.

The spongy mesophyll of the leaf increases the surface area exposed to CO2 but also increases the surface area of evaporation.

The internal surface area of the leaf may be 10 to 30 times greater than the external surface area.

Effects of transpiration on wilting and leaf temperature

A leaf may lose more than its weight in water every day.

Water may move in the xylem as fast as 75 cm/min, about the speed of a second hand moving around the clock.

Water loss is a trade-off for allowing CO2 to enter the leaf.

Transpiration also results in evaporative cooling, which can lower the temperature of a leaf by as much as 10-15ºC compared to the surrounding air.

The transpiration-to-photosynthesis ratio evaluates how efficiently a plant uses water.

It is the amount of water lost per gram of CO2 assimilated into organic material by photosynthesis.

For many plants species this ratio is 600:1 or 600 g of water are lost for each gram of CO2 incorporated into carbohydrate.

For C4 plants, the ratio is 300:1

Transpiration also brings mineral to all parts of the plant and helps in cooling the plant.

Stomata: major pathway of water loss.

A plant loses 90% of the water through the open stomata.

The waxy cuticle covering most of the leaf surface prevents evaporation.

The stoma is the opening located between two kidney or dumbbell-shaped guard cells.

Dumbbell shape in monocots.
Kidney shape in dicots.

The guard cells are suspended over an air chamber by subsidiary epidermal cells.

Guard cells control the diameter of the stoma by changing shape.

When the guard cells become turgid, the stoma opens. When flaccid, the stoma closes.

Potassium ion mechanism.

The changes in turgor pressure in the guard cells are the result of the reversible uptake and loss of K+.

Light triggers an influx of K+ into the guard cells.
It occurs through active transport; ATP required. A proton pump is probably involved and K+ move through channels driven by a membrane potential.
Osmotic pressure decreases and water moves into the guard cells.
The increase turgidity of the cells causes a change in shape and the stoma opens.
Most of the K+is stored in the central vacuole. The tonoplast plays a role here.
Regulation of aquaporins may also be involved by varying the permeability of the membranes to water.

Opening of the stomata is most pronounced in blue light, and to a lesser extent in red light.

Light  proton pump moves H+  K+ transported into the cell  water diffuses into

out of the guard cell through specific K channels the guard cells

guard cells change shape and open the stoma.

The stoma may close by a reversal of the process when light decreases.

Loss of turgidity closes the stoma.

Stimuli to open and close the stomata:

Stomata are open or closed according to the physiological needs of the plant.

Photosynthesis: depletion of CO2. A low concentration of CO2 in the leaf induces stomata to open even in the dark.

Transpiration: loss of water causes loss of turgor throughout the plant.
The hormone abscisic acid is produced in response to water deficiency and causes the guard cells to close.
Increase in temperature increases cellular respiration and CO2 production.

In mesophytes, the stomata are usually open during the day and closed at night.

CAM plants do the reverse.

Light triggers the intake of K+ by the guard cells.

There are blue-light receptors in the membrane of the guard cells that trigger ATP-powered proton pumps which in turn promotes the uptake of K+ ions.

An internal clock causes the stomata to open and close periodically.

Cycle of 24 hours is called Circadian rhythms.

Xerophytes have leaf adaptations that reduce the rate of transpiration.

Small, thick leaves or reduced to spines.
Thick cuticle
Highly reflective leaves and hairy leaves that trap a boundary of water.
Stomata are concentrated on the lower, shady leaf surface, in pits: sunken stomata.
CAM pathway of photosynthesis. The stomata open during the night to incorporate CO2 into organic acids.

TRANSLOCATION OF PHLOEM SAP

Sucrose is the main sugar translocated in the phloem.

Sugars moves from the source where it is being produced, to the sink, where the sugars are being utilized or stored.

Sucrose manufactured in mesophyll cells can travel via the symplast to sieve-tube members.

In some species, sucrose leaves the symplast and travels through the apoplast and is actively incorporated into the sieve-tube members or by the companion cells that then pass the sucrose to the sieve tubes through plasmodesmata.

In some plants, companion cells have many ingrowths of their walls, enhancing transfer of solutes between apoplast and symplast. These cells are called transfer cells.

In maize and many other plants, phloem loading requ9res active transport because sucrose concentrations in sieve-tube members are two to three times higher than in mesophyll. The loading is done through proton pumps and cotransport mechanisms.

The process of unloading at the sink end varies between the species and organs of the plant.

Pressure flow hypothesis

Phloem sap flows from source to sink at rates as great as 1 m/hr, much too fast to be accounted for by either diffusion or cytoplasmic streaming.

This theory postulates that sugar moves in the phloem by means of a pressure gradient that exists between the source, where sugar is loaded into the sieve tube members, and the sink, where sugar is removed from the phloem.

Sucrose and other carbohydrates areactively loaded into the sieve tubes at the source by a chemiosmotic mechanism.

It requires ATP.

ATP supplies energy to pump protons out of the sieve tube members into the apoplast.
Creates proton gradient.
The gradient drives the uptake of sucrose into the symplast through channels by the cotransport of protons back into the sieve tube members.

As a result water moves into the sieve tubes by osmosis increasing the hydrostatic pressure in the sieve tubes that forces water to flow in the sieve tubes.

Sugar is actively or passively unloaded from the sieve tube into tissues at the sink.

As a result water leaves the sieve tubes at the sink decreasing the hydrostatic pressure inside the sieve tubes.

A gradient is created between the sources and sinks which drives the flow within the sieve tubes.

Other substances transported in the phloem are hormones, ATP, amino acids, inorganic ions, viruses and complex organic molecules like sugar-alcohol compounds.