Question

Stomatal opening is affected by
(a) Nitrogen concentration, carbon dioxide concentration and light
(b) Carbon dioxide concentration, temperature, and light
(c) Nitrogen concentration, light, and temperature
(d) Carbon dioxide concentration, nitrogen concentration, and temperature

Answer 1

Hint: Stomata functions as turgor operated valves. Whenever the osmotic concentration of guard cells decreases, guard cells become flaccid and hence get closed.

Complete answer:
There is an accumulation of potassium ions in the guard cells during the stomatal opening which plays a critical role in stomatal movement.
Opening of stomata in light:
- In light, the starch in guard cells is incompletely oxidized into phosphoenolpyruvate. It is later converted into organic acids mostly malic acid.
- Malic acid dissociates into malate ions and protons in guard cells. H+ in the guard cells are transported to epidermal cells and potassium ions from the epidermal cells get into the guard cells through the hydrogen potassium ion exchange pump in the plasma membrane.
- The process of ion exchange requires ATP; thus, it is an active process.
- Increased potassium ions and Malate ions form potassium malate and store it in vacuoles of guard cells increasing their osmotic concentration. Hence, water enters the guard cells by endosmosis.
- Turgor pressure of guard cells increases due to endosmosis and stomata are opened.
- Blue light induces maximum opening of the stomata and in its absence, the stomata remain closed.
Closing of stomata in dark:
- As carbon dioxide is not utilized in photosynthesis during the night, Hence its concentration in the substomatal cavity increases.
- An inhibitor hormone Abscisic acid functions in the presence of carbon dioxide, it inhibits potassium ions uptake by changing the diffusion and permeability of guard cells for positive ions.
- The potassium ions are transported back to epidermal or subsidiary from guard cells. Thus, the stomata close.
Temperature:
- The higher the temperature, the more is the rate of transpiration, thus more is the opening as it results in a high vapor pressure gradient.
So, the correct answer is ‘Carbon dioxide concentration, temperature, and light’.

Note:
- Light also affects the rate of transpiration by increasing the temperature. Thus, temperature and light go hand to hand.
- Stomata are responsible for 50- 97% of total water transpired.