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Difficulty: Very hardHormonal Coordination in Plants and Animals

During periods of severe drought stress, plants utilize abscisic acid (ABA) to minimize transpirational water loss through stomatal regulation. What is the correct sequential order of the physiological events in ABA-mediated stomatal closure, starting from initial drought detection to the final closure of the stomatal pore?

  1. 1Water deficit in roots triggers the synthesis of abscisic acid (ABA), which is transported via xylem vessels to the leaf tissues.
  2. 2Abscisic acid binds to specific plasma membrane receptors on guard cells, causing an influx of calcium ions (Ca2+Ca^{2+}) into the cytosol.
  3. 3Elevated cytosolic calcium levels activate anion efflux channels, leading to membrane depolarization.
  4. 4Membrane depolarization activates voltage-gated potassium ion (K+K^+) channels, driving efflux of potassium ions and anions from the guard cells.
  5. 5The rapid loss of inorganic ions increases the internal water potential of guard cells relative to adjacent epidermal cells, causing water to leave by osmosis and reducing turgor pressure to close the stomatal pore.

Answer

The correct sequence starts with drought-induced synthesis and xylem transport of abscisic acid (ABA) from roots to leaves, followed by ABA binding to guard cell receptors which raises cytosolic calcium levels. Next, elevated calcium activates anion efflux channels to depolarize the membrane, which opens voltage-gated potassium efflux channels for rapid ion loss. Finally, solute exit increases guard cell water potential, causing osmotic water loss, turgor loss, and stomatal closure.
Stomatal closure by abscisic acid (ABA) follows a specific cascade: 1) ABA is synthesized in roots during drought and transported via xylem to leaves. 2) ABA binds guard cell plasma membrane receptors, elevating cytosolic calcium (Ca2+Ca^{2+}). 3) Calcium activates anion efflux channels, depolarizing the plasma membrane. 4) Membrane depolarization opens voltage-gated potassium (K+K^+) efflux channels, causing rapid ion exit. 5) Loss of solutes increases guard cell water potential, causing osmotic water loss, turgor loss, and stomatal closure.

Step-by-Step Solution

1
Identify the signal perception and hormone transport phase
Water deficit in roots induces ABA synthesis, which travels through xylem to leaves.
Hormonal regulation begins with stimulus perception and hormone release into the transport tissue.
2
Identify receptor binding and second messenger activation
ABA binds to guard cell receptors, opening channels for cytosolic Ca2+Ca^{2+} influx.
Hormones act on target guard cells by binding receptors and activating intracellular signals.
3
Determine initial channel activation and electrical membrane change
Cytosolic Ca2+Ca^{2+} opens anion channels, causing anion efflux and membrane depolarization.
Increased intracellular calcium ion concentration triggers membrane potential changes.
4
Determine major ion efflux driven by membrane potential
Depolarization opens voltage-gated K+K^+ channels, resulting in massive K+K^+ outflow.
Membrane depolarization is the trigger required for opening voltage-gated potassium channels.
5
Link solute loss to osmotic water movement and cell turgor changes
Solute efflux raises guard cell water potential, driving osmotic water loss and turgor reduction that closes the stoma.
Stomatal movement is mechanically governed by osmotic water flow in response to ion concentration gradients.

Key Concept

Abscisic Acid Signal Transduction and Osmotic Regulation of Stomatal Movement
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Topics

Hormonal Coordination in Plants and Animals

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