WATER Plants' most important chemical most often limits productivity.

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WATER• Plants' most important chemical• most often limits productivity

• Often >90%% of a plant cell’s weight

• Often >90%% of a plant cell’s weight• Gives cells shape

• Often >90%% of a plant cell’s weight• Gives cells shape• Dissolves many chem

WATER• Dissolves many chem• most biochem occurs in water• Source of e- for PS

WATER• most biochem occurs in water• Source of e- for PS• Constantly lose water due to PS (1000 H2O/CO2)

WATER• most biochem occurs in water• Source of e- for PS• Constantly lose water due to PS• Water transport is crucial!

WATER• Water transport is crucial!• SPAC= Soil Plant Air Continuum• moves from soil->plant->air

WATER Formula = H2O Formula weight = 18 daltons Structure = tetrahedron, bond angle 104.5˚

WATER Structure = tetrahedron, bond angle 104.5˚ polar :O is more attractive to electrons than H

+ on H- on O

WaterPolarity is reason for water’s properties water forms H-bonds with polar molecules

Hydrophilic = polar moleculesHydrophobic = non-polar molecules

Properties of water1) Cohesion = water H-bonded to water

-> reason for surface tension

Properties of water1) Cohesion = water H-bonded to water

-> reason for surface tension-> why water can be drawn from roots to leaves

Properties of water1) Cohesion = water H-bonded to water2) Adhesion = water H-bonded to something else

Properties of water1) Cohesion = water H-bonded to water2) Adhesion = water H-bonded to something else• Cohesion and adhesion are crucial for water movement in plants!

Properties of water1) Cohesion = water H-bonded to water2) Adhesion = water H-bonded to something else• Cohesion and adhesion are crucial for water movement in plants!• Surface tension & adhesion in mesophyll creates force that draws water through the plant!

Properties of water1) Cohesion = water H-bonded to water2) Adhesion = water H-bonded to something else3) high specific heat• absorb heat when break H-bonds: cools leaves

Properties of water1) Cohesion = water H-bonded to water2) Adhesion = water H-bonded to something else3) high specific heat• absorb heat when break H-bonds• Release heat when form H-bonds

Properties of water1) Cohesion = water H-bonded to water2) Adhesion = water H-bonded to something else3) high specific heat4) Ice floats

Properties of water1) Cohesion = water H-bonded to water2) Adhesion = water H-bonded to something else3) high specific heat4) Ice floats5) Universal solvent

Properties of water1) Cohesion = water H-bonded to water2) Adhesion = water H-bonded to something else3) high specific heat4) Ice floats5) Universal solvent•Take up & transport nutrients dissolved in water

Properties of water5) “Universal” solvent•Take up & transport nutrients dissolved in water•Transport organics dissolved in water

Properties of water1) Cohesion = water H-bonded to water2) Adhesion = water H-bonded to something else3) high specific heat4) Ice floats5) Universal solvent6) Hydrophobic bonds

Properties of water1) Cohesion = water H-bonded to water2) Adhesion = water H-bonded to something else3) high specific heat4) Ice floats5) Universal solvent6) Hydrophobic bonds7) Water ionizes

pH[H+] = acidity of a solutionpH = convenient way to measure aciditypH = - log10 [H+]pH 7 is neutral: [H+] = [OH-]

-> at pH 7 [H+] = 10-7 moles/l

pHPlants vary pH to control many processes!

Water movementDiffusion: movement of single molecules down ∆[ ] due to random motion until [ ] is even•Driving force?

Water movementDiffusion: movement of single molecules down ∆[ ] due to random motion until [ ] is even • Driving force: lowers free energy•∆G = ∆H- T∆S

Water movementDiffusion: movement of single molecules down ∆[ ] due to random motion until [ ] is even Bulk Flow: movement of groups ofmolecules down a pressure gradient

Water movementDiffusion: movement of single molecules down ∆[ ] due to random motion until [ ] is even Bulk Flow: movement of groups ofmolecules down a pressure gradient• Independent of ∆ [ ] !

Water movementDiffusion: movement of single molecules down ∆[] due to random motion until [ ] is even Bulk Flow: movement of groups of molecules down a pressure gradient•Independent of ∆[ ] !•How water moves through xylem

Water movementDiffusion: movement of single molecules down [] due to random motion until [ ] is even Bulk Flow: movement of groups of molecules down a pressure gradient•Independent of ∆ [ ] !•How water moves through xylem•How water moves through soil and apoplast

Water movementBulk Flow: movement of groups of molecules down a pressure gradient•Independent of ∆ [ ] !•How water moves through xylem•Main way water moves through soil and apoplast•Very sensitive to radius of vessel: increases as r4

Water movementDiffusion: movement of single molecules down ∆[] due to random motion until [ ] is even Bulk Flow: movement of groups of molecules down a pressure gradient•Independent of ∆[ ] !•How water moves through xylem•Main way water moves through soil and apoplast•Very sensitive to radius of vessel: increases as r4

Osmosis: depends on bulk flow and diffusion!

Water movementOsmosis: depends on bulk flow and diffusion!water crosses membranes but other solutes do notwater tries to even its [ ] on each side

Water movementOsmosis: depends on bulk flow and diffusion!water crosses membranes but other solutes do notwater tries to even its [ ] on each sideother solutes can’t: result is net influx of water

Water movementOsmosis: depends on bulk flow and diffusion!•Moves through aquaporins, so rate depends on pressure and [ ] gradients!

Water movementOsmosis: depends on bulk flow and diffusion!•Moves through aquaporins, so rate depends on pressure and [ ] gradients!• Driving force = water's free energy (J/m3 = MPa)

Water potentialDriving force = water's free energy = water potential w

• Important for many aspects of plant physiology

Water moves to lower its potential

Water moves to lower its potentialDepends on:

1. [H2O]: s (osmotic potential)

Water potentialWater moves to lower its potentialDepends on:

2. Pressure : p

• Turgor pressure inside cells

Water potentialWater moves to lower its potentialDepends on:

2. Pressure : p

• Turgor pressure inside cells• Negative pressure in xylem!

Water potentialWater moves to lower its potentialDepends on:• [H2O]: s (osmotic potential)

• Pressure p

• Gravity g

w = s +p + g

Water potentialWater moves to lower its potentialDepends on:• [H2O]: s (osmotic potential)

• Pressure p

• Gravity g

w = s +p + g

w of pure water at sea level

& 1 atm = 0 MPA

Water potentialw = s +p + g

w of pure water at sea level & 1 atm = 0 MPA

s (osmotic potential) is always negative

s (osmotic potential) is always negative• If increase [solutes] water will move in

p (pressure potential) can be positive or negative

• Usually positive in cells to counteract s

Water potentialp (pressure potential) can be positive or negative

• Helps plants stay same size despite daily fluctuations in w

• p in xylem is negative, draws water upwards

• p in xylem is negative, draws water upwardsg can usually be ignored, but important for tall trees

Water potentialMeasuring water potential

Water potentialMeasuring water potentials (osmotic potential) is “easy”• Measure concentration of solution in equilibrium with

cells g (gravity potential) is easy: height above ground• -0.01 Mpa/m

cells g (gravity potential) is easy: height above ground

P (pressure potential) is hard!• Pressure bomb = most common technique

cells g (gravity potential) is easy: height above ground

P (pressure potential) is hard!• Pressure bomb = most common techniqueOthers include pressure transducers, xylem probes

Measuring water potentialP (pressure potential) is hard!• Pressure bomb = most common techniqueOthers include pressure transducers, xylem probesTherefore disagree about H2Otransport in xylem

WATER Plants' most important chemical most often limits productivity.

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