Water Plant

CHAPTER 1: PLANT WATER RELATIONS

INTRODUCTION 

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Water fills a number of important roles in the physiology of plants; which it is unique of its physical and chemical properties

WATER 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.

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Solvent properties

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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 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

TYPES 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

OSMOSIS - 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.

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 

WATER 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) Ψ =  p +  s = usually negative  (keeps water moving into cells) §

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In cell with solute concentration equal to solute concentration surrounding solution, the solute potential are the same and no net movement of water occurs

0.1M

Cell wall 0.1M solution

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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

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The symbol Ψs is called osmotic potential/solute potential and is the negative of π Thus Ψs = – π

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Ψ p – pressure potential is the sum of turgor pressure and wall pressure

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Turgor pressure is the pressure of the cell membrane on the cell wall as water moves into cell

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By expanding protoplast

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Wall pressure is the equal and opposite force exerted by cell wall against the cell membrane

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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, (ψ p)= 0; water potential of the cell ( ψ potential)

cell

=ψ

s (water

Hypotonic solution   

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such as pure water (Ψ = 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

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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 Wilting   

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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 

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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

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plasma membrane and the protoplasm within it contract to the center of the cell

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Strands of cytoplasm extend to the cell wall because of plasma membrane-cell wall attachment points

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Does not give rise to significant negative pressure (tension)

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Plasmolysed cells die unless they are transferred quickly from the salt or sugar solution to water.

Wilting   

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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 Plants

Water in soil 

Water movement in soil depend on soil type

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Water in soil exist as  Film adhering to the surface of soil particles  Fill entire channel between particle 

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Field capacity: moisture holding capacity in soil @ water in the soil

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Large in clay or high humus content of soil

Soil water potential    

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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?     

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.

SPECIAL 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

Root 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

Water absorption by root 

Multiples pathways from endodermis of root :

epidermis

to

the

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Apoplast pathway: water moves exclusively through cell wall without crossing any membranes Continuous system of cell walls and intercellular air spaces 

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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 Xylem 

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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

GUTTATION 

Root pressure prominent in well hydrated plants under humid conditions (little transpiration)

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Root pressure exhibit phenomenon of guttation

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Exudation of liquid from leave, i.e. xylem sap from hydathodes (located near terminal tracheid of the bundle end around margin of leaves)

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Small plant at night, RH (relative humidity) air =100%

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Transpiration is suppressed

TRANSPIRATION      

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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

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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

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Water evporates from the inner surface of epidermal cells known as peristomal evaporation

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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 transpiration (i) Effect of Humidity

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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

(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

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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

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Water vapor molecules exiting the leaf diffuse through epidermal layer and boundary layer

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The thickness of boundary layer will decrease rate of diffusion and rate of transpiration

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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? § 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