Gas exchange and stomatal physiology- LECTURE 1

Leaf strucutre

• Waxy impermeable cuticle

  • made of cutin

• Upper epidermis (adaxial)

• mesophyll cell tissue

• Lower epidermis (abaxial)

With stomata pores across the leaf surfaces

Advantages of this strucutre

1. Prevent water loss

   • essential to allow plants to become terrestrial

2. But does not limit gas exchange

   • stomata are controllable values to allow gas exchange

     • CO2, O2 and water vapour

3. Also allows evaporative cooling!

4. Transpiration!

OVERALL: can easily balance water loss

Balance of stomata

Good for:

• CO2 in

• O2 out

• H2O out→

  • evaporative cooling

  • transpiration

Bad for:

• too much water loss!

Therefore need a balance between these→ must be controlled!!

Importance of stomata

• 120Gt of carbon

• twice atmospheric water vapour content

  • all exchanged through stomatal pores

• Terristrial plants fix 15% of the atmospheric pool of carbon each year

  • through photosynthesis

Stomatal form

• stomtal pore

• flanked by a pair of guard cells (GC)

  • ‘flanked’→ on each of the sides

Types of guard cells

(most) Dicots

• ‘sausage -shaped’ GC

Monocots (grasses)

• dumb-bell shaped

  • with an array of flanking subsidiary cells (SC)

Over several hundred million years

• stomata have remained relatively unchanged in basic form

Special case stomata→ Commelina communis

• sausage shaped guard cells

and

• subsidiary cells

Stomatal pore opening and closing depends on

Respond to changing envirionment

1. soil water availability

2. evaporative demand

   • air vapour pressure

3. Co2 concentrations

4. light intensity

What drives the opening process

• Guard cell turgor

  • generated vis accumulation os osmotically active solutes

  • causes water to flow into the guard cells from surrounding tissues

    • phiw=phis +phip

Recap of water potential

→ Water potential: water moves fown gradient of free energy

• converted into pressure

• always negative (unless hot or under pressure)

→ solute potential

• addition of solutes to pure water reduced delta G

• water wraps themselves around hydration shell around ions

• numerically equivalent to osmotic pressure +pi

→ Hydrostatic or turgor pressure

• positive tugour exerted within cells

• to provide rigidity

• difference between water and solute potential

Also additional factors that defines the free energy status of water on pores and cell wall matrices

In dry soils or cell walls

• Matric potential phiM

How do changes in turgor pressure in GC cause opening?

Due to ultrastructure of cell wall

• cell wall thickening

• cellulose microfibril orientation

→ causes GCs to become rounder with increased pressure

• → Open aperture

Antagonism between guard cells and epidermal cells

1. Guard cells become turgid

2. Become rounder and open stomata

3. Pop up and out of the epidermal plant

Easy to see they have popped up and out compared to epidermal cells surrounding

Mechanical Advantage

• Guard cell expansion is limited by the

• antagonism of adjacent epidermal cell turgor

• whereas Subsidiary Cells act as ion reservoirs

  • reciprocate turgor loss with GC turgor gain, improving stomatal sensitivity and rapidity.

What do subsidiary cells do?

Helps overcome mechanical advantage imposed by epidermal cell turgor

1. Reciprocal exchange of solutes between GCs and SCs

2. allow a “push-pull” system

3. Potentially improves sensitivity to the enviornment

4. allows grass stomata to react faster

How does this work (simplified)?

e.g

When opening

1. guard cells can expand

2. into relaxed subsidiary cells when opening

When closing

1. Subsidiary cells push

2. guard cells shut

Compared to just suasage shaped cells

• More efficient control of pushing the stomatal in and out

• don’t have to pop up and out as much

  • pushed and pulled together!

  • improves sensitivity to environment

    • don’t have to overcome the epidermal cells and really push out the guard cells like before?

Mechanism for stomatal opening

1. Activation of plasma membrane “P-type H+-ATPase”

   • pumps H+ out of cell into apoplast

2. Creates hyperpolarisation on the outside

3. Causes Passive uptake of K+

4. To maintain electroneutralilty, causes:

   • uptake of Cl- and H+ via a symporter

   • synthesis of malate^2- in the cytosol

     • Co2→OAA→malate^2-

5. Solute accumulation causes water to flow in

   • opens the stoma

Concentrations of K+ and Cl- transporter

1. 350mM K+

2. 90mM Cl-

Needed to drive the opening process

→ can be worked out using nernst equation:

• blah blah blah

How to close stomata

Need to remove solutes, and allow water to leave

• → using Ca²⁺ signalling! (see lectures of other stuff this is used for, wounding etc)

Mechanism for stomatal closure

1. Ca2+ activates slow (S-type) ansion channel

2. Mediates Cl- efflux

3. Membrane depolarisation

4. Leads to K+ efflux

5. Ca2+ signalling also reduces acitivity of H+ ATPase

   • → so stops the opening process too!

6. Malate2- concentrations also decrease

7. Less solute inside→ water is drawn out→ closes

Do Ca2+ contribute to the solute potential??

NO

• Concentrations are so low

• it is used as a signalling agent only

What regulates stomatal pore opening/closing

1. Light

2. Water availability→ ABA

   • in response to stress

     • drought

     • salinity

3. CO2

1. Regulation by Light→ blue light signalling

Receptor:

• phototropin

Response

• promotes stomatal opening

Mediated by

• P type H+-ATPase on the plasma membrane

1. Regulation by Light→ blue light signalling- in absence of light

• C-terminal auto inhibitory domain of the ATP-ase inactivates the pump

• by blocking the catalytic domain

1. Regulation by Light→ blue light signalling- in response to blue light

1. Phototropin mediates phosphorylation of C-terminal domain

2. Domain is displaced from catalytic site

3. 14-3-3 regulatory protein binds to phosphorylated domain

4. Makes sure it stays phosphorylated

   • keeps the proton pump activated

5. So proton pump can power the opening of the stomata etc

2. Regulation to drought→ why needed?

• drought→ plant growth and productivity severely impaired

• plants evolved to increase drought tolerance

  → One of the first responses:

  • close stomata so decrease water loss

2. Regulation to drought→ how detect drought?

Not really understood

• Channel proteins and receptor like kinases involved?

2. Regulation to drought→ How response mediated?

Abscisic Acid (ABA)

• accumulates in plant tissues

  • inhibits stomatal opening and promotes stomatal closure

2. Regulation to drought→ How ABA stomatal closure works

1. ABA binds to cytosolic receptor PYR1

2. Activates a MAP kinase phosphorylation cascade

3. Leads to release of internal and external pools of Ca2+

   • from vacuole and endoplasmic reticulum

4. Ca2+ activates or reinforces stomatal closure mechanism

   • from before: i.e removing solutes and water from GCs

Latest evidence suggests

ABA triggers Ca2+ transients

• temporary localised changes in [Ca2+]

• well controlled

  • i.e pulses

Magnitude and frequency of these pulses

Reflect the rate of the closing response is transduced

Subsidary cells enable…

Faster closing of stomata

→ recent genetic evidence for the role of subsidiary cells in maize leaves

• during stomata response to ABA

Control of Water use→ where is ABA syntehsised

In plant plastids from carotenoid precursors in

1. Leaf chloroplasts

2. Root plastids

May provide sources to trigger GC responses

Plastids

• membrane-bound organelles

• differentiate into chloroplasts in leaves,

• but may contain pigments in flowers or metabolite (starch) storage and mobilisation in roots

  • plastids are sites of ABA synthesis.

Control of water use→ where is ABA released in response to water deficits?

1. Root senses water deficits

2. ABA released from root plastids de novo

   • Into transpiration stream

   → To GC complexes in leavesduring evaporation

   • closes stomata

3. BUT also made locally in leaves

   → even before transport from the roots

   • Plant prepares itself already!!

     • Ca2+ fast signalling??

Why important to understand signalling→ commercial use

e.g Grapevines

Irrigated on both sides:

• Top growth excessive

• fruit quality reduced by shading

INSTEAD: Irrigating on alternate sides of plant:

• ABA made on one half→ carried to leaves

• top growth is reduced

• fruit quality is maintained

Why useful?

• reduces overall demand for irrigation

  → useful in warming drying climate

Stomatal opening mechanisms can be hijacked by pathogens

e.g Fusicoccum

1. produces toxin: fusicoccin

2. activates P-type H+-ATPase

3. causes stomatal opening

   • even when plant doesn’t want to

4. Causes plant wilting

opens them similar to blue light signalling through phototropin

Environmental factors not only affect stomatal aperture but also…

Stomatal development

Young leaves develop:

1. environmental conditions sensed by plants

2. number and size of stomata are adjusted

• cannot be changed once finished development programme

Plants that grow under high light and suffient water sonditions

Develop

1. higher stomatal density

2. larger stomata

Because Co2 exchange in the limiting factor

When plant grows in high CO2 concentrations

• fewer stomata develop

  → do not need to open as much to get to the CO2

  → Have less stomata to decrease water loss

Water is the limiting factor

E.g Commelina Communis: High humidity, sufficient water

• Large stomata

• High Stomatal density

To maximise the Co2 uptake→ the limiting factor

E.g Succulent plants: Semi arid habitats, water limitation

• small stomata

• low stomatal density

because water is limiting factor→ need to limit its loss

(got sufficient amounts of CO2 (hopefully))