Question

According to sugar concentration, during daytime guard cells
A) Bulge inwards
B) Bulge outwards,
C) No bulging
D) All of the above

Answer 1

Hint: The guard cells are cells that open and close the stomata to regulate water loss by transpiration as well as the movement of gases; Turgidity is the main mechanism that regulates this function.

Complete Answer:
- Guard cells do close when the turgidity decreases. This is both due to transpiration induced loss of water, as well as fewer osmolytes like potassium and sucrose. However, these osmolytes are more involved in increasing rather than decreasing the water pressure. Option A is incorrect.
- Opening and closing of the stomatal apertures does depend on the concentration of various osmolytes, one important one being sugar or sucrose. During the daytime turgidity causes the guard cells bulge outwards from the stomatal opening as the inner wall is very thin. Turgidity will increase if the sugar concentration is high as sugar will pull in more water by osmosis. Option B is correct.
- Increased sucrose concentration does cause the guard cells to bulge outwards. Guard cells do become flaccid and close the stomata in response to water loss, rather than to sucrose concentration. Option C is incorrect.
- As only one of the above options was correct, option D is incorrect. Concentrations will cause the guard cells to either bulge or relax, depending on the situation.

Hence the correct answer is option B.

Note: Stomatal opening is regulated by more than one factor. Peak stomatal opening is associated with high concentrations of sucrose as a result of photosynthesis. Light also regulates stomatal opening, causing the guard cells to open. Water loss has the opposite effect, causing the guard cells to become flaccid and relax.