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CHAPTER 1:PLANT WATER RELATIONS
CHAPTER 1:PLANT WATER RELATIONS
INTRODUCTIONINTRODUCTION
Water fills a number of important roles in the physiology of plants; which it is unique of its physical and chemical properties
Water fills a number of important roles in the physiology of plants; which it is unique of its physical and chemical properties
WATER PROPERTIESWATER PROPERTIES
Thermal properties High specific heat, requires lots of energy to change
water temperature (4.2 J/g/ºC) Buffers rapid changes in temperature Is in liquid state over a large range in temperatures
due to the H-bond Thermal properties of water : most of reactions can
occur only in aqueous medium, contribute to temperature regulation, helping to ensure that plants do not cool down or heat up too rapidly.
Thermal properties High specific heat, requires lots of energy to change
water temperature (4.2 J/g/ºC) Buffers rapid changes in temperature Is in liquid state over a large range in temperatures
due to the H-bond Thermal properties of water : most of reactions can
occur only in aqueous medium, contribute to temperature regulation, helping to ensure that plants do not cool down or heat up too rapidly.
Solvent properties Polar solvent - dissolves important compounds: ions,
nuclei acids, charged proteins, sugars etc. Making suitable medium for the uptake and
distribution of mineral nutrients and other solutes required for growth.
Many biochemical reactions occur in water, which water and mineral taken up from the soil – through root system
Solvent properties Polar solvent - dissolves important compounds: ions,
nuclei acids, charged proteins, sugars etc. Making suitable medium for the uptake and
distribution of mineral nutrients and other solutes required for growth.
Many biochemical reactions occur in water, which water and mineral taken up from the soil – through root system
Physical propertiesPhysical properties Cohesion
due to strong mutual attraction between water molecules
Continuous column of water Has a tensile strength 0f ~30 MPa = 4,350PSI
Surface tension results because cohesion forces are stronger than
attraction to air Adhesion
is caused by waters attraction to surfaces resulting in capillary action
Transparency of water enable light to penetrate for photosynthesis
Cohesion due to strong mutual attraction between water
molecules Continuous column of water Has a tensile strength 0f ~30 MPa = 4,350PSI
Surface tension results because cohesion forces are stronger than
attraction to air Adhesion
is caused by waters attraction to surfaces resulting in capillary action
Transparency of water enable light to penetrate for photosynthesis
TYPES OF WATER MOVEMENT IN PLANT CELLSTYPES OF WATER MOVEMENT IN PLANT CELLS Diffusion
Relatively short distance Across semi-permeable membranes Temperature dependent Examples: cell-to-cell
Bulk Flow Pressure driven Response to transpiration (up) Source-sink movement (up or down) Examples: phloem & xylem
Diffusion Relatively short distance Across semi-permeable membranes Temperature dependent Examples: cell-to-cell
Bulk Flow Pressure driven Response to transpiration (up) Source-sink movement (up or down) Examples: phloem & xylem
OSMOSIS - key to movement up plantsOSMOSIS - key to movement up plants Thistle tube experiment A selective permeable is stretched across the
end pf a thistle tube containing sucrose solution and the tube in inverted in a container of pure water.
Initially water will diffuse across the membrane in response to a chemical potential.
Diffusion will continue until the force tending to drive the water into the tube is balanced by (A) the force generated by the hydrostatic
head (h) in the tube or (B) the pressure applied by the piston.
When the two forces are balanced the system has achieved the equilibrium and not further net movement of water will occur.
Thistle tube experiment A selective permeable is stretched across the
end pf a thistle tube containing sucrose solution and the tube in inverted in a container of pure water.
Initially water will diffuse across the membrane in response to a chemical potential.
Diffusion will continue until the force tending to drive the water into the tube is balanced by (A) the force generated by the hydrostatic
head (h) in the tube or (B) the pressure applied by the piston.
When the two forces are balanced the system has achieved the equilibrium and not further net movement of water will occur.
RELATE THISTLE TUBE EXPERIMENT TO WATER POTENTIAL
RELATE THISTLE TUBE EXPERIMENT TO WATER POTENTIAL Pressure potential (p) = pressure generated
by rise in water levels (positive values, push water out)
Osmotic potential (s) = pressure generated by solutes (negative values, pull water in)
Water potential () = balance of p and s At equilibrium, is 0 = p + s = 0
Pressure potential (p) = pressure generated by rise in water levels (positive values, push water out)
Osmotic potential (s) = pressure generated by solutes (negative values, pull water in)
Water potential () = balance of p and s At equilibrium, is 0 = p + s = 0
WATER POTENTIAL IN PLANT CELLSWATER POTENTIAL IN PLANT CELLS Replace thistle tube with plant cell and add a rigid cell
wall that prevents increase in cell volume (like piston pushing down)Y = p + s = usually negative
(keeps water moving into cells)
Replace thistle tube with plant cell and add a rigid cell wall that prevents increase in cell volume (like piston pushing down)Y = p + s = usually negative
(keeps water moving into cells)
In cell with solute concentration equal to solute concentration surrounding solution, the solute potential are the same and no net movement of water occurs
In cell with solute concentration equal to solute concentration surrounding solution, the solute potential are the same and no net movement of water occurs
Cell wall
0.1M solution
0.1M
Cell in a solution with a solute concentration less than of the cell, solute potentials will be different and water will move into the cell
The symbol Ψs is called osmotic potential/solute potential and is the negative of π
Thus Ψs = – π
Cell in a solution with a solute concentration less than of the cell, solute potentials will be different and water will move into the cell
The symbol Ψs is called osmotic potential/solute potential and is the negative of π
Thus Ψs = – π
p – pressure potential is the sum of turgor pressure and wall pressure
Turgor pressure is the pressure of the cell membrane on the cell wall as water moves into cell
By expanding protoplast
Wall pressure is the equal and opposite force exerted by cell wall against the cell membrane
Ψp is the same as P
p – pressure potential is the sum of turgor pressure and wall pressure
Turgor pressure is the pressure of the cell membrane on the cell wall as water moves into cell
By expanding protoplast
Wall pressure is the equal and opposite force exerted by cell wall against the cell membrane
Ψp is the same as P
Incipient plasmolysis:
Which the protoplast just fills the cell volume
Protoplast exerts no pressure against the wall; but neither it is withdrawn from the wall
Consequently, turgor pressure, (p)= 0;
water potential of the cell ( cell
= s (water potential)
Incipient plasmolysis:
Which the protoplast just fills the cell volume
Protoplast exerts no pressure against the wall; but neither it is withdrawn from the wall
Consequently, turgor pressure, (p)= 0;
water potential of the cell ( cell
= s (water potential)
Hypotonic solution
such as pure water ( = 0), water will enter the cell down the water potential gradient
small dilution of the vacuolar contents (increase in osmotic potential) and generate turgor pressure
Net movement of water into the cell will cease when the osmotic potential of the cell is balanced by its turgor pressure
Water potential of the cell = 0
Hypotonic solution
such as pure water ( = 0), water will enter the cell down the water potential gradient
small dilution of the vacuolar contents (increase in osmotic potential) and generate turgor pressure
Net movement of water into the cell will cease when the osmotic potential of the cell is balanced by its turgor pressure
Water potential of the cell = 0
Hypertonic solution
osmotic potential more negative than cell Water potential gradient favors water loss from the cell
Protoplast shrinks away from cell wall-plamolysis
Continued water loss concentrates vacuolar contents
Lowering osmotic potential Turgor pressure remains zero Water potential of the cell determined solely by its osmotic potential
Hypertonic solution
osmotic potential more negative than cell Water potential gradient favors water loss from the cell
Protoplast shrinks away from cell wall-plamolysis
Continued water loss concentrates vacuolar contents
Lowering osmotic potential Turgor pressure remains zero Water potential of the cell determined solely by its osmotic potential
Plasmolysis and WiltingPlasmolysis and Wilting
the separation of plant cell cytoplasm from the cell wall as a result of water loss
It is unlikely to occur in nature, except in severe conditions.
induced in the laboratory by immersing a plant cell in a strongly saline or sugary solution, so that water is lost by osmosis.
large vacuole in the center of the cell originally contains a dilute solution with much lower osmotic pressure than becomes smaller
Space between the cell membrane and the cell wall enlarges
Void between outer protoplast surface and the cell wall become filled with external solution
the separation of plant cell cytoplasm from the cell wall as a result of water loss
It is unlikely to occur in nature, except in severe conditions.
induced in the laboratory by immersing a plant cell in a strongly saline or sugary solution, so that water is lost by osmosis.
large vacuole in the center of the cell originally contains a dilute solution with much lower osmotic pressure than becomes smaller
Space between the cell membrane and the cell wall enlarges
Void between outer protoplast surface and the cell wall become filled with external solution
Plasmolysis
Plasmolysis can be studied in the laboratory using hypertonic solution
* Protoplast volume changed (decreased)
* Plasmodesmata are broken * Protoplast pulls away from the cell wall
* The void between outer protoplast surface (plasma membrance) and the cell wall becomes filled with external solution
Plasmolysis does not give rise to significant negative pressure (tension) on the protoplast
Plasmolysis
Plasmolysis can be studied in the laboratory using hypertonic solution
* Protoplast volume changed (decreased)
* Plasmodesmata are broken * Protoplast pulls away from the cell wall
* The void between outer protoplast surface (plasma membrance) and the cell wall becomes filled with external solution
Plasmolysis does not give rise to significant negative pressure (tension) on the protoplast
plasma membrane and the protoplasm within it contract to the center of the cell
Strands of cytoplasm extend to the cell wall because of plasma membrane-cell wall attachment points
Does not give rise to significant negative pressure (tension)
Plasmolysed cells die unless they are transferred quickly from the salt or sugar solution to water.
plasma membrane and the protoplasm within it contract to the center of the cell
Strands of cytoplasm extend to the cell wall because of plasma membrane-cell wall attachment points
Does not give rise to significant negative pressure (tension)
Plasmolysed cells die unless they are transferred quickly from the salt or sugar solution to water.
Wilting Wilting is response to dehydration in air under natural conditions
Extreme surface tension Water in small pores of cell resists the entry of air and the collapsing protoplast maintains contact with cell wall
Tend to pull the wall inward Substantial negative pressure may develop
Water potential becomes more negative as the sum of negative osmotic potential and negative pressure potential
Wilting Wilting is response to dehydration in air under natural conditions
Extreme surface tension Water in small pores of cell resists the entry of air and the collapsing protoplast maintains contact with cell wall
Tend to pull the wall inward Substantial negative pressure may develop
Water potential becomes more negative as the sum of negative osmotic potential and negative pressure potential
Water Transports in PlantsWater Transports in Plants
Water in soilWater in soil
Water movement in soil depend on soil type
Water in soil exist as Film adhering to the surface of soil particles Fill entire channel between particle
Field capacity: moisture holding capacity in soil @ water in the soil
Large in clay or high humus content of soil
Water movement in soil depend on soil type
Water in soil exist as Film adhering to the surface of soil particles Fill entire channel between particle
Field capacity: moisture holding capacity in soil @ water in the soil
Large in clay or high humus content of soil
Soil water potentialSoil water potential
Depends on 2 components;
(i) osmotic pressure of soil water; and
(ii) hydrostatic pressure (P),
depend on the water content Osmotic pressure is generally low, 0.01 MPa P is always less than or equal to zero (soil water is
under tension) e.g. Wet soil: P close to zero; Dry soil: P decreases Develop of negative pressure in soil water
Depends on 2 components;
(i) osmotic pressure of soil water; and
(ii) hydrostatic pressure (P),
depend on the water content Osmotic pressure is generally low, 0.01 MPa P is always less than or equal to zero (soil water is
under tension) e.g. Wet soil: P close to zero; Dry soil: P decreases Develop of negative pressure in soil water
Where does the negative pressure in soil water come from?Where does the negative pressure in soil water come from?
Root hairs make intimate contact with soil particles
Amplify the surface area needed for water absorption
As water absorbed by plants: Soil solution recedes into smaller
pocket More water is removed from soil,
causes the surface of soil solution develop concave menisci (curved interface between air and water), resulting in greater tension (more negative pressure)
Root hairs make intimate contact with soil particles
Amplify the surface area needed for water absorption
As water absorbed by plants: Soil solution recedes into smaller
pocket More water is removed from soil,
causes the surface of soil solution develop concave menisci (curved interface between air and water), resulting in greater tension (more negative pressure)
1. When soil is at Field Capacity water pervades all of the channels between Soil Particles.
2. Roots absorb water from their immediate environment. This creates Air pockets. This is replaced by water present in the nearest, larger channels.
3. In extremely dry soils, water is tightly bound in the smallest channels of the soil particles. It can't replace water removed by the roots & large Air Pockets are formed.
1. When soil is at Field Capacity water pervades all of the channels between Soil Particles.
2. Roots absorb water from their immediate environment. This creates Air pockets. This is replaced by water present in the nearest, larger channels.
3. In extremely dry soils, water is tightly bound in the smallest channels of the soil particles. It can't replace water removed by the roots & large Air Pockets are formed.
SPECIAL FEATURES OF ROOTSSPECIAL FEATURES OF ROOTS Amazing facts about roots
Deepest root = 5.3 m (mesquite desert shrubs, Arizona)
Study of 4-month old Rye Surface area of roots = 639 m2 Combined length = 623 km Number of root = 2500 per cm3
Rates of growth Apple tree = 1 cm per day Corn root = 6.3 cm per day
Two types of roots Fibrous Taproot
Amazing facts about roots Deepest root = 5.3 m (mesquite desert shrubs,
Arizona) Study of 4-month old Rye
Surface area of roots = 639 m2 Combined length = 623 km Number of root = 2500 per cm3
Rates of growth Apple tree = 1 cm per day Corn root = 6.3 cm per day
Two types of roots Fibrous Taproot
Root supply plants with waterRoot supply plants with water Majority of water enters within 10 cm from
root tip Water potential for root cells varies with rate
of transpiration Low transpiration = -0.13 Mpa High transpiration = -10.25 Mpa
Root hairs are specialized to take up water Grow into soil to increase surface area and
contacts with soils
Majority of water enters within 10 cm from root tip
Water potential for root cells varies with rate of transpiration Low transpiration = -0.13 Mpa High transpiration = -10.25 Mpa
Root hairs are specialized to take up water Grow into soil to increase surface area and
contacts with soils
Water absorption by rootWater absorption by root
Multiples pathways from epidermis to the endodermis of root :
Apoplast pathway: water moves exclusively through cell wall
without crossing any membranes Continuous system of cell walls and intercellular
air spaces
Multiples pathways from epidermis to the endodermis of root :
Apoplast pathway: water moves exclusively through cell wall
without crossing any membranes Continuous system of cell walls and intercellular
air spaces
Cellular pathways:Transmembrane pathway: enters a cell on one side exists the cell on the other side
Crosses at least two membrane for each cell in its path
Symplast pathway: water moves from one cell to the next (cell) via plasmodesmata
Consist of continuous network of cell cytoplasm interconnected by plasmodesmata
Water transport across root occurs through some combination of these pathways
At endodermis; water movement through apoplast blocked by casparian strip
Radial cell wall consist hydrophobic substance; suberin
Movement of water must cross plasma membrane & enter cytoplasm of endodermal cell
Cellular pathways:Transmembrane pathway: enters a cell on one side exists the cell on the other side
Crosses at least two membrane for each cell in its path
Symplast pathway: water moves from one cell to the next (cell) via plasmodesmata
Consist of continuous network of cell cytoplasm interconnected by plasmodesmata
Water transport across root occurs through some combination of these pathways
At endodermis; water movement through apoplast blocked by casparian strip
Radial cell wall consist hydrophobic substance; suberin
Movement of water must cross plasma membrane & enter cytoplasm of endodermal cell
Water Transports through XylemWater Transports through Xylem
How it moves to the tops of the tallest water?
(i) Forces required to move water to such height
(ii) Three theories have been proposed by:
Root pressure Water potential in roots generates
enough pressure to push water up to leaves
Capillarity Capillary action within small xylem cells is
sufficient to move water up to leaves Cohesion theory
Transpiration in leaves generates enough force to pull water up via its cohesive properties
How it moves to the tops of the tallest water?
(i) Forces required to move water to such height
(ii) Three theories have been proposed by:
Root pressure Water potential in roots generates
enough pressure to push water up to leaves
Capillarity Capillary action within small xylem cells is
sufficient to move water up to leaves Cohesion theory
Transpiration in leaves generates enough force to pull water up via its cohesive properties
Root pressure prominent in well hydrated plants under humid conditions (little transpiration)
Root pressure exhibit phenomenon of guttation
Exudation of liquid from leave, i.e. xylem sap from hydathodes (located near terminal tracheid of the bundle end around margin of leaves)
Small plant at night, RH (relative humidity) air =100%
Transpiration is suppressed
Root pressure prominent in well hydrated plants under humid conditions (little transpiration)
Root pressure exhibit phenomenon of guttation
Exudation of liquid from leave, i.e. xylem sap from hydathodes (located near terminal tracheid of the bundle end around margin of leaves)
Small plant at night, RH (relative humidity) air =100%
Transpiration is suppressed
GUTTATION
TRANSPIRATION
Is a cooling process Is defined as the loss of water from the plant in the form of water vapor
The driving force for transpiration is the gradient in water vapor density
90% water vapor lost through leaves Small amount through lenticels in the bark of young twigs and branches
Outer surface of leaves of vascular plant are covered with multi-layered waxy deposit called cuticle (component of cuticle is cutin)
Cuticular waxes are very hydrophobic; they offer high resistance to diffusion of water and water vapor
TRANSPIRATION
Is a cooling process Is defined as the loss of water from the plant in the form of water vapor
The driving force for transpiration is the gradient in water vapor density
90% water vapor lost through leaves Small amount through lenticels in the bark of young twigs and branches
Outer surface of leaves of vascular plant are covered with multi-layered waxy deposit called cuticle (component of cuticle is cutin)
Cuticular waxes are very hydrophobic; they offer high resistance to diffusion of water and water vapor
Transpiration may be considered a 2 stage process:
(i) the evaporation of water from the moist cell
walls into substomatal air space (ii) the diffusion of water vapor from
substomatal space into atmosphere
Water evporates from the inner surface of epidermal cells known as peristomal evaporation
Diffusion of water vapor through the stomatal pores known as stomatal transpiration; 90 – 95 % water loss from leaves; and through cuticle known as
cuticular transpiration ; 5-10 %
Transpiration may be considered a 2 stage process:
(i) the evaporation of water from the moist cell
walls into substomatal air space (ii) the diffusion of water vapor from
substomatal space into atmosphere
Water evporates from the inner surface of epidermal cells known as peristomal evaporation
Diffusion of water vapor through the stomatal pores known as stomatal transpiration; 90 – 95 % water loss from leaves; and through cuticle known as
cuticular transpiration ; 5-10 %
Factors influence transpirationFactors influence transpiration
(i) Effect of Humidity
Actual water content of air Express as relative humidity: the ratio of the actual
water content of air to maximum amount of water can be held by air at that temperature
Vapor pressure in the substomatal leaf spaces will be the saturation vapor pressure at the leaf temperature
Even in rapidly transpiring leaf, RH grater than 95% Vapor pressure of atmospheric air depends on both
humidity and temperature
(i) Effect of Humidity
Actual water content of air Express as relative humidity: the ratio of the actual
water content of air to maximum amount of water can be held by air at that temperature
Vapor pressure in the substomatal leaf spaces will be the saturation vapor pressure at the leaf temperature
Even in rapidly transpiring leaf, RH grater than 95% Vapor pressure of atmospheric air depends on both
humidity and temperature
Water content of air = % RH
Rates of Transpiration
% RH atmosphere Hot cool
Water content of air = % RH
Rates of Transpiration
% RH atmosphere Hot cool
(ii) Effect of Temperature Temperature modulates transpiration rate through its effect on vapor pressure
10 C increase in temperature with water content of the atmosphere remains constant
⇨ vapor pressure will increase ⇨ rate of evaporation increase , thus will increase transpiration
Leaf exposed to full sun reach temperatures 5-10 oC higher than ambient temperature
⇨ vapor pressure gradient increase
Transpiration may occur even when RH of atmosphere is 100%
(ii) Effect of Temperature Temperature modulates transpiration rate through its effect on vapor pressure
10 C increase in temperature with water content of the atmosphere remains constant
⇨ vapor pressure will increase ⇨ rate of evaporation increase , thus will increase transpiration
Leaf exposed to full sun reach temperatures 5-10 oC higher than ambient temperature
⇨ vapor pressure gradient increase
Transpiration may occur even when RH of atmosphere is 100%
(iii) Wind Wind speed has an effect on transpiration because, it modification the effective length of the diffusion path for exiting water molecule
Water vapor molecules exiting the leaf diffuse through epidermal layer and boundary layer
The thickness of boundary layer will decrease rate of diffusion and rate of transpiration
Wind speed increase (high wind speed) :⇨ it tends to cool the leaf ⇨ cause sufficient to close the stomata ⇨ have less of an effect on transpiration rate ⇨ thickness of boundary layer and diffusion path decrease
(iii) Wind Wind speed has an effect on transpiration because, it modification the effective length of the diffusion path for exiting water molecule
Water vapor molecules exiting the leaf diffuse through epidermal layer and boundary layer
The thickness of boundary layer will decrease rate of diffusion and rate of transpiration
Wind speed increase (high wind speed) :⇨ it tends to cool the leaf ⇨ cause sufficient to close the stomata ⇨ have less of an effect on transpiration rate ⇨ thickness of boundary layer and diffusion path decrease
How to measure transpiration?How to measure transpiration? Lysimeter = weight loss method
Seal pot so only route is via transpiration Weigh over time to calculate loss
Gas exchange method Place plant/leaf in transparent chamber Measure air-moisture content through chamber
Water accounting (only method for ecosystems) Measure inputs (rainfall) Measure outputs (runoff, drainage, storage) Calculate transpiration
Lysimeter = weight loss method Seal pot so only route is via transpiration Weigh over time to calculate loss
Gas exchange method Place plant/leaf in transparent chamber Measure air-moisture content through chamber
Water accounting (only method for ecosystems) Measure inputs (rainfall) Measure outputs (runoff, drainage, storage) Calculate transpiration
The strongest principle of growth lies in human choice.
George Elliot
The strongest principle of growth lies in human choice.
George Elliot
