#116 Homeostasis in plants

Stomata have daily rhythms of opening and closing and also respond to changes in environmental conditions to

- allow diffusion of CO­_2­

- regulate water loss by transpiration

Stomata open due to:	Stomata close due to:
high light intensity low concentration of CO­2­	darkness high concentration of CO­2­  low humidity high temperature water stress

Opening and closing of stomata

1. ATP powers proton pumps to actively transport H⁺ out of cell
2. There is a low concentration of H⁺ and negative charge inside the cell --> K⁺ channels open --> K⁺ diffuse in
3. High concentration of K⁺ inside the cell decreases water potential
4. Water moves in via osmosis
5. Water entry increases the volume of the guard cell, causing it to expand --> open

Structure of stomata

Each stomatal pore is surrounded by 2 guard cells. Guard cells:

• open when turgid (gain water)
• close when flaccid (lose water)

Abscisic acid and stomatal closure

Abscisic acid (ABA) is a stress hormone that is secreted in response to difficult environmental conditions such as very high temperatures or much reduced water supplies. ABA triggers the closure of stomata to reduce transpiration and prevent water loss.

ABA binds to cell surface receptors 

• inhibits proton pumps: stop H⁺ pumped out
• stimulates movement of Ca²⁺ through the cell surface membrane and tonoplast

Ca²⁺ acts as a 2nd messenger to activate channel proteins to open that allow negatively charged ions to leave the guard cell. This in turn

• opens channel proteins that allow  K⁺ to leave the cell
• closes channel proteins that allow K⁺ to enter the cell
• --> net movement: K⁺ leaves cell

Loss of ions = higher water potential inside cell = water passes out by osmosis = guard cells become flaccid --> stomata close

Syllabus 2016-2018 14.2  Homeostasis in plants Stomatal aperture is regulated in response to the requirements for uptake of carbon  dioxide for photosynthesis and conserving water. a)   explain that  stomata have daily rhythms of opening and closing and also respond to changes in environmental conditions to allow diffusion of carbon  dioxide and regulate water loss by transpiration b)   describe the structure and function  of guard cells and explain the mechanism by which they  open  and close  stomata c)   describe the role of abscisic acid in the closure of stomata during times of water stress (the role of calcium  ions as a second messenger should  be emphasised)