Lecture 4

PLANT-WATER

RELATIONSHIPS

Study of water movement from soil to roots to shoot and until it is

transpired on the leaf

From Holy Quran

“

“Tidakkah engkau memerhatikan bahawa Allah menurunkan hujan dari langit , lalu di alirkan menjadi mata air dibumi , kemudian Dia tumbuhkan dengan air itu tanam-tanaman yang berbagai jenis dan warna, kemudian tanam-tanaman itu bergerak segar higgga sesuatu masa yang tertentu

Selepas itu engkau melihatnya berupa kuning. Kemudian Dia menjadikan hancur bersepai sesungguhnya segala sesuatu yang tersebut itu mengandungi peringatan kepada orang yang berakal sempurna”

Surah al-Dzumar (ayat 21)

The Water Transport From Root to

Atmosphere

How water move up a giant tree?

Role of Water in Plant Growth and Development

� transports minerals through the soil to the roots

� principal medium for the chemical and biochemical processes

� water provides physical support for plants

� acts as a solvent for dissolved sugars and minerals transported throughout the plant.

� evaporation within intercellular spaces provides the cooling mechanism that allows plants to maintain the favorable temperatures necessary for metabolic processes.

Chemical Properties of Water

� Tendency of water molecules to stick together because of the hydrogen bond – cohesion.

� Cohesive properties due to intermolecular attraction of water and maintains the xylem water column.

Terms Related to Water Movements

Into Living Cell

� Osmosis

Water movement driven by an osmotic potential gradient through a semipermeable membrane.

Drives water movement from a lower to a higher solute concn.

Remember: high solute conc. gives a low osmotic potential

Osmosis

Related Terms (cont.)

� Mass Flow

The movement of water and solute molecules together and in one direction.

Normally associated with long-distance transport systems.

The Concept of Water Potential

� Water potential (Ψ) is a measure of the free energy of water.

� Pure water (which has a high amount of free energy) is arbitrarily assigned a water potential of zero

� Units pressure: pascals (Pa) (can be positive or

negative. (1MPa=106 Pa) or bars (1 MPa=10 bars)

The Concept (cont.)

� Water movement is driven by energy level.

� Water will move from a system or area where it is at a higher free energy to a system where it is at a lower free energy.

The Concept (cont.)

� In order to predict the direction of movement of water into or out of plant, cells or tissue, we therefore need a measure for the free energy of water. --- The water potential.

Movement of Water

Water moves along gradients of water potential, from higher to lower water potential (less negative to more negative)

Let put a turgid cell in pure water

� Start with second cell.

As water is absorbed,

Osmotic Pot= Turgor Pot.

As Water is lost,

Ψ decreases, protoplast no

longer presses against the wall

More water lost,

either wilted or plasmolysed

Factors Affecting Plant Water Potential

Forces determining cellular water potential

� Amount of solutes:

Increasing concentrations of low-molecular-solutes in plant cells will lower/decrease the free energy (water potential), therefore osmotic potential has a negative value.

Osmotic potential (Ψs or Ψπ).

Plant Ψ (cont.)

� turgor pressure (ψp) in plant cell

Forces determining cellular water

potential (cont.)

� In plant cells, the cell wall exerts a hydrostatic pressure, the turgor pressure (wall pressure) on the protoplast.

Turgor pressure (Ψp) in plant cell increases free energy and raises water potential. Positive value.

Loss of turgor = wilting (in the field)

Forces determining cellular

water potential (cont.)

� Surfaces of macromolecules (e.g. cellulose) exerts an attractive force on water-matric potential (Ψm); lowers the free energy; usually a minor component.

Cell with high water content, Ψm insignificant

contribution. Tissues of low water content, such as imbibing

seeds Ψm controls water uptake.

What is semipermeable membrane?

� In a cell, two membranes involve; Plasma membrane (plasmalemma) which

surrounds the cell. Tonoplast which surrounds the vacuole. � Water channel proteins termed

Aquaporins, the membrane proteins make water-specific channels play important roles in controlling speed of water movement into or out of the living cell.

Total Ψ of A Cell/Plant

Ψplant = ψs + ψp + ψm

or Ψ= ΠΠΠΠ + P + γγγγ

Where, P = hydrostatic pressure or turgor pressure

ΠΠΠΠ = osmotic pressure

γγγγ = matric potential

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Factors Affecting Water Potential and Its Reference State

� Factors that can change the

potential energy of soil

water include:

� Adsorption of water onto

soil particles

� Solutes dissolved in the soil

water

� Elevation of soil water in the

earth's gravitational field

� Applied pressure (both

positive and negative).

� Reference State of Soil

Water:

� Pure (no dissolved solutes),

� Free (free of adsorptive

forces),

� water at a specified

temperature and

� Specified Elevation exposed to

Atmospheric Pressure.

Relative Water Potential

Root to Atmosphere

Water Pathways

Transpiration and Plant Water Uptake

� Transpiration is lost of water vapor from a plant's surface, especially through the stomates.

� Of the quantity of water absorbed by a plant around 90% is transpired while only 10% is used for growth.

� The driving mechanism for water uptake and transport within the plant.

� Transpiration of water from the plant creates a water potential gradient that drives water movement in the soil towards the roots.

How transpiration move the water up the plant?

� As each water molecule moves into a mesophyll cell it exerts a “pull” on the column of water within the xylem vessels all the way from the leaves to the roots. The upward “transpirational pull” on the fluid within the xylem causes a tension (negative pressure) to form.

� This tension contributes to the lowering of the water potential in the xylem. This decrease in water potential transmitted all the way from the leaves to the roots causes water to move into the root and up toward the leaves.

Water Uptake at Root Level

The Soil Water

� Soil compositions affect water movement in soil

particle type and size, and structure.

� Chemical properties of particle type (e.g. surface electrical charges) determine how water bound to soils.

� Water moves through soil by bulk flow (hydrostatic pressure), at root interface movement changes to diffusion.

Water Absorption by Roots

� Root surface area contact with soil water is maximized by growth of primary roots and development of secondary roots and roots hairs

Water Absorption by Roots

� Root hairs may constitute ≥ 50% of the root surface area.

� Water uptake (majority) occurs in the root hair region (fully elongated cells but no secondary growth) (maturation zone) and not in meristematic or elongation regions

Components of Soil Water Potential –The details

Potential Factors Affecting

ΨM = Matric Potential (capillary)

Absorption of water to soil

because of its electrical properties

ΨP = Pressure /Hydrostatic pressure

Applied pressure

ΨZ = Gravity Potential Due to elevation

ΨS = Solute Potential

Due to the presence of dissolved

solids in the water.

ΨΨΨΨ ΨΨΨΨ ΨΨΨΨ ΨΨΨΨ ΨΨΨΨ Soil M P Z S ==== ++++ ++++ ++++

SOIL WATER POTENTIAL

� Ψsoil = Total Soil Water Potential

� ΨM = Matric Potential (capillary)

� ΨP = Pressure Potential/Hydrostatic pressure

� ΨZ = Gravity Potential (elevation)

� ΨS = Solute Potential

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Gravitational potential

Gravity is the dominant force

acting on soil water when

soils are wet. Water tends to

flow downward- from a

region of more positive

potential to one of less

positive potential- under the

influence of gravity, until the

force of gravity is balanced

by that of capillarity.

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Osmotic potential

The amount of work that must be

done per unit quantity of pure

water in order to transport

reversibly and isothermally an

infinitesimal quantity of water

from a pool of pure water, at a

specified elevation and at

atmospheric pressure, to a pool

of water identical in composition

with the soil water at the point

under consideration, but in all

other respects being identical

with the reference pool.

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Solute Potential

� Due to the presence of dissolved solids in the water.

� Zero in pure water

� Negative in water containing solutes

� Only important in a system where the solutes are constrained from movement, such as across a root membrane.

� Not directly measured in the field

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Matric potential

It is also called capillary potential; it is

the amount of work that must be

done per unit quantity of pure water

in order to transport reversibly and

isothermally an infinitesimal

quantity of water, identical in

composition with the soil water,

from a pool at the elevation and the

external gas pressure of the point

under consideration, to the soil

water.

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MATRIC POTENTIAL (Capillary Potential)

� Water is attracted to soil solids because of its electrical properties.

� Water molecules are also attracted to each other because of the electrical properties.

� This attraction causes tension forces to be present in a soil which has both air and water in its pores.

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Matric Potential (capillary)

� Due to the attraction between water and soil particles

� Requires a water-air surface

� Zero in a saturated soil

� Negative in an unsaturated soil

� Helps water move into dry soil

� Matric Potential will be less negative in soil with large pore dimensions.

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Matric Potential (capillary)

� Also called Soil Moisture Tension

� Measured using a Tensiometer

� Causes water to move upward, against

the pull of gravity from the water table

into a dry surface soil layer

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Example for Soil Matric Potential

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Soil Water Release Curves

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Pressure Potential

� Due to the pressure of water depth

� Zero in an unsaturated soil

� Positive in a saturated soil

� Measured with an open tube called a

piezometer.

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Soil Water Static Profile

ΨΨΨΨT = constant at all points

ZPMT ΨΨΨΨ++++ΨΨΨΨ++++ΨΨΨΨ====ΨΨΨΨIn the Unsaturated zone:

ZTM

P

Ψ−Ψ=Ψ=Ψ 0.0

At the Water Table:

ΨΨΨΨ ΨΨΨΨΨΨΨΨ ΨΨΨΨ

M P

T Z

==== ========

0 0 0 0. .

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Example for Static Profile

� Water Table 4-feet below the surface.

� ΨZ = - 4 feet = ΨT

� 2-feet above the water table in the unsaturated zone:

(((( )))) ft.224ΨΨΨ

2Ψ4Ψ0.0Ψ

ZTM

ZTP

−−−−====−−−−−−−−−−−−====−−−−====−−−−====−−−−========

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Example for Static Profile

� Water Table 4-feet below the surface.

� ΨZ = - 4 feet = ΨT

� 2-feet below the water table in the saturated zone:

(((( )))) ft.264

640.0

ZTP

ZTM

++++====−−−−−−−−−−−−====ΨΨΨΨ−−−−ΨΨΨΨ====ΨΨΨΨ−−−−====ΨΨΨΨ−−−−====ΨΨΨΨ====ΨΨΨΨ

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Soil Water Profile Non-Static Condition

� ΨT is different at each point

� Water will move from points of higher Total Potential to points of lower Total Potential

� The same rules apply for estimating the different kinds of water potential at each point.

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Field Capacity � The soil water condition when a

wet soil has drained to

gravitational equilibrium is termed

field capacity. The inverted

triangle symbolizes a water table,

below which the soil is considered

to be saturated. Although the

water table is represented as a

surface, the real interface between

saturated and unsaturated soil is a

diffuse zone called the capillary

fringe.

Process of Water Uptake by Root

Hairs

� The Root Pressure

Develops (well watered plants with little or no transpiration) because of solute accumulation in the root xylem that lowers the osmotic potential and decreases the water potential (Ψw) resulting in water absorption.

Root pressure can only provide a modest push in the overall process of water transport.

However, root pressure is strongest when transpiration ceases, for example during the night.

Movement of Water In Root Hairs

� Water uptake by root hairs and movement through the epidermis and cortex involves intracellular and intercellular pathways.

They are:

Apoplastic – water moves along the cell wall, i.e., intercellular spaces

Transmembrane – water moves sequentially into and out of cells

Symplastic – Water moves through the plasmodesmata.

Soil to Root

Barrier of Water Movement

� Water movement through endodermal cells requires transmembrane and symplastic transport. Endodermal cells contain suberin (hydrophobic lipid polymers) in the radial cell walls, which prevents water (and solute) movement through the apoplast. This suberized cell wall barrier is the Casparian strip. Water must be in the symplast for movement.

Soil to Root

Water Movement in The Plant

� Water is transported through the xylem.

� Tracheary elements are the water conducting cells of the xylem that moves water from roots to leaves

Xylem-The Tracheary elements

Upward Movement of Water

Cohesion-Tension Theory

� Tension and cohesion is the basis for water movement upwards in the xylem – Surface tension (water-air interface) at the leaf surface creates a negative pressure that “pulls” water up through the xylem, even in trees that are 50 m tall.

� Evaporation from the leaf lowers the water potential of the leaf cells losing water and water flows in from the xylem.

Cont… � The xylem vessels are full of water and as water

leaves them a tension is set up in the columns of water. This is transmitted all the way down the stem by the cohesion of water molecules.

� Long distance water transport is very dependent on the tensile strength (cohesive properties due to intermolecular attraction) of water and maintains the xylem water column.

� The walls of xylem vessels are highly lignified in order to give them a great deal of strength to handle the forces generated in the mass flow of water within them

Cohesion-Tension Theory

(pictorial summary)

Cavitation

water evaporates from a film on the water-air

interface of cells inside the stomatal cavity.

From Leaf to Atmosphere

� After water evaporates (vapor) in the stomatal cavity, movement out of the leaf is due primarily to diffusion down the water vapor concentration gradient.

� Cuticle/waxes form a lipid layer on shoot and leaf epidermis that restricts water loss directly to the atmosphere.

� Most plant water loss (approx 95%) occurs through stomata (pores on lower side of leaf)

Water Lost to the Atmosphere

Leaf to Atmosphere (cont.)

� Water vapor movement from the leaf cavity to the atmosphere is dependent on the vapor concentration gradient (water potential gradient). Vapor concentration in the stomatal cavity is near saturation (100% R.H)

� A small increase in temperature or decrease in RH substantially lowers the water potential. Thus, create bigger water potential gradient.

The water vapour potential gradient

drives water lost from a leaf

The End