ABA and Stomatal Closure
Q.1. How does abscisic acid function to stimulate the closure of stomata?
Ans: Abscisic acid (ABA) is a plant hormone that is synthesized by any cells in a plant that contain chloroplasts or amyloplasts, especially in stress conditions. The presence of large concentrations of abscisic acid in leaves stimulates stomata to close, reduces the rate of transpiration, and conserves water inside the plant.
To close the stomata, the proton pumps stop working and the potassium ions flow out of the cells. This raises the water potential inside the cells, so water passes out by osmosis. The guard cells become flaccid and this closes the stoma.
Q.2. Describe the mechanism of stomatal opening.
Ans: In general, guard cells open during the day and close at night although this rhythm persists in continuous light and in the continuous dark. The cell surface membranes of guard cells contain proton pumps that actively transport hydrogen ions out of the cells. this stimulates the inward movement of potassium ions down their electrochemical gradient. The potassium ions decrease the water potential of the guard cells so water enters by osmosis, and the cells become turgid and open the stoma.
Q.3. Explain when it is an advantage for plants to close their stomata.
Ans: Closing of the stomata has significant effects on the plant. It reduces the uptake of CO2 for photosynthesis and reduces the rate of transpiration. As transpiration is used for cooling the plant and also for maintaining the transpiration stream that supplies water and mineral ions to the leaves, stomatal closure only occurs when reducing the loss of water vapour and conserving water is the most important factor. In conditions of water stress, the hormone abscisic acid (ABA) is produced in plants to stimulate stomatal closure.
Q.4. How do stomata respond to changes in environmental conditions?
Ans: When stomata are open, leaves gain CO2 for photosynthesis, but tend to lose much water in transpiration.
Stomata close in response to:
a) darkness
b) high CO2 concentrations in the air spaces in the leaf
c) Low humidity
d) high temperature
e) water stress
Stomata open in response to:
a) increasing light intensity
b) low CO2 concentrations in the air spaces in the leaf
Q.5. Rearrange the following statements to make a flow diagram of the mechanism of opening a stoma.
1. volume of guard cell increases
2. H+ transported out of guard cells
3. water enters guard cells by osmosis
4. K+ diffuses into guard cells
5. guard cells curve to open stoma
6. water potential of guard cells falls
7. K+ channels open
Ans:
2 , 7, 4, 6, 3, 1, 5
Q.10. Abscisic acid (ABA) is a weak acid. Its structure can be represented as ABA -H. It dissociates into positively charged H+ ions (protons) and negatively charged ABA -ions as shown:
The following observations have been made by scientists:
* light stimulates proton (H+ ion) uptake into the grana of chloroplasts
* ABA - H can diffuse into and out of chloroplasts, but ABA - cannot.
This information is summarised in the diagram below.
a) Using all the information provided, predict what happens to the pH of the stroma in the light.
b) i) When light shines on the chloroplast, dissociation of ABA- H is stimulated. Explain why this happens.
ii) Explain the effect that this will have on the diffusion of ABA -H into or out of the chloroplast.
Ans:
b) i) Concentration of protons in the stomata decreases; shifts equilibrium to the right.
ii) Increases diffusion into the chloroplast; as the concentration of ABA - H decreases; so maintaining a concentration gradient into the chloroplast.
c) ABA stimulates closure of stomata; less water vapour is lost.