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Homeostasis: Question 5

Syllabus 14.2

Multiple choice A2 1 mark

During a period of drought, a plant's leaves show a reduced stomatal aperture (the pore of each stoma is much less open) compared with the same species growing in well-watered soil.

Which statement correctly describes the role of guard cells and abscisic acid (ABA) in bringing about this closure of the stomata?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall how guard cell turgor controls stomatal aperture

A stoma opens or closes because of changes in the turgor (internal water pressure) of the two guard cells surrounding it. Guard cell walls are unevenly thickened, with a thicker wall on the side next to the pore; when guard cells are turgid, this uneven thickening makes them bow outward into a more curved shape, opening the pore. When guard cells lose turgor (become flaccid), they straighten again and the pore closes.

Step 2: Recall how ABA brings about stomatal closure

During water stress, ABA (abscisic acid) is produced in the roots (where a soil water deficit is first detected) and in the leaves, and is transported to the guard cells. There, it binds to receptors on the guard cell membrane and triggers efflux of potassium ions (and accompanying anions) out of the guard cells. As solutes leave, the guard cells’ water potential rises (becomes less negative), so water follows the solutes out of the cells by osmosis. This water loss causes the guard cells to become flaccid, straighten, and the stomatal pore to close, reducing water loss by transpiration at the cost of restricting carbon dioxide uptake for photosynthesis.

Step 3: Evaluate each option

  • A: correct, describes ABA acting on guard cell membrane receptors to cause solute efflux, a rise in water potential, osmotic water loss, and closure of the pore.
  • B: incorrect, this describes the opposite process, the mechanism of stomatal opening (proton pumping driving potassium ion influx, lowering water potential, drawing water in by osmosis), not closure.
  • C: incorrect on two counts. ABA is not synthesised only in the guard cells (it is produced more widely, including in the roots, in response to water deficit rather than light intensity), and water movement out of guard cells during closure is by osmosis, not active transport.
  • D: incorrect. ABA does not permanently damage or break down the guard cell wall; its effect on turgor is temporary and reversible, so stomata can reopen once water stress eases.

Final answer

  • ABA causes potassium ions (and other solutes) to leave the guard cells, raising their water potential so water leaves by osmosis; the resulting loss of turgor closes the stomatal pore, option A.