When plants are stressed by dry conditions, they secrete the hormone abscisic acid (ABA) and in response the stomata close, even in the light. ABA is known to trigger the release of calcium ions that serve as second messengers in the guard cells and cause massive opening of potassium (K ) channels. Which of the following would be part of the mechanism by which ABA triggers stomata to close?

A. K+ ions flow out of guard cells and water moves into guard cells in response
B. K+ ions flow into guard cells and water moves into guard cells in response
C. K+ ions flow out of guard cells and water moves out of guard cells in response
D. K+ ions flow into guard cells and water moves out of guard cells in response

C. K+ ions flow out the the guard cells and water flows out the cells.

Explanation:

The turgidity of the guard cells allows the opening and closing of the stomata. This is especially necessary when it comes to water conservation in plants. In order for the cells to become turgid or flaccid, there must be the influx and out ward movement of water via osmosis.

In order for this the occur the osmotic pressure of the cellular environment must change and this is done by changing the solute concentration.

The pumping in of K+ ions out the cell allows the solute concentration to decrease and this in turn encourages the movement of water across a concentration gradient through a semi-permiable membrane.

The water leaves the guard cells and they become flaccid. This causes the stomata to be covered.

K+ ions flow out of the guard cells and water moves out of the guard cells in response

Explanation:

When the Stomatal opens and closes it is occurs as a result of the absorption and removal of water into and from the guard cells respectively. K + ions have a huge role to play in this.

When it opens huge amounts of potassium ions accumulate in the guard cells which increases the solute potential and hence the water potential is dropped. This causes water from neighbouring cells to enter the guard cell and they become turgid or swollen. In this condition, the stomatal pore is opened. This is the opposite phenomenon of opening when it closes as water is removed from the guard cells and they become flaccid. K + ions move out of the cell. This causes the stomatal pore to close. This occurs in the absence of light or when rates of photosynthesis are low. In this condition the stomatal pore is closed.