ABA and stomatal closure

What do guard cells do

Surround the stomata to control the entry of carbon dioxide into leaves and can respond to environmental stimuli in order to control the internal environment of the leaf

What do stomata open in response to

• increasing light intensity

• Low carbon dioxide concentration within air spaces within the leaf

What do stomata close in response to

• darkness

• High carbon dioxide concentration in leaf air spaces

• Low humidity

• High temperature

• Water stress - when high rates of transpiration are not matched by high water uptake from the soil

What is the disadvantage of closed stomata

Photosynthesis rates decreases

What advantage is there to closing the stomata

Water is retained within the leaf.

What can be said from this graph

The stomata close at regular intervals even when kept in constant light or dark - they have natural circadian rhythms.

How do stomata open?

• guard cells either side of a stomach open when they gain water by osmosis (turgid) and close when they lose water by osmosis (flaccid)

• As the two guard cells are joined at the top and the bottom, when the cells become turgid, the guard cells move apart at their centre and the pore opens.

How are the guard cell walls adapted to be effective for the opening/closing of stomata

have unevenly thickened cell walls - inside wall is very thick and quite inflexible but outside wall is thinner and easier to bend

How is cellulose used in guard cells to help them be better adapted to their function

Cellulose is arranged in hoops around the cells preventing them from expanding width-ways, so any changes in size affect the length

What is the process by which stomatal opening is controlled

• H+ ions actively pumped out of guard cells via proton pumps in the cell surface membrane

• Causes K+ channels in cell surface membrane so K+ diffuse into cell down electrochemical gradient and Cl- ions also diffuse in

• Extra Cl- and K+ in cell lowers water potential causing water to move in through aquaporins

• Increases turgidity of cell so stomata opens

What does ABA stand for

Abscisic acid

What is the role of ABA and when is it released

• ‘stress hormone’

• Under conditions of water stress ABA is released and causes stomatal closure to reduce transpiration

How does ABA affect closure of stomata by binding to receptors

• binds to receptors on cell surface membrane of guard cells

• Inhibits proton pumps

• Stops H+ leaving cell

• Causes closure of K+ channel which were allowing K+ into cell

• Solute conc in cytoplasm decreases, water potential becomes higher than epidermal cells and water moves out of cell

• Guard cells become flaccid and stomata close.

How does ABA affect closure of stomata by Ca2+ stimulation

• ABA stimulates Ca2+ ions to move into cytosol from vacuole through tonoplast

• Ca2+ acts as second messenger

  • Opening of channel proteins in cell surface membrane allowing Cl- out of cell

  • Opening of K+ channels which allow K+ to diffuse out of cell, down electrochemical gradient

• Solute conc in cytoplasm decreases, water potential becomes higher than epidermal cells and water moves out of cell

• Guard cells become flaccid and stomata close.