explain that stomata have daily rhythms of opening and closing

Homeostasis in Plants – Stomatal Daily Rhythms

1. Homeostatic Principle of Stomatal Control

Stomata act as a negative‑feedback system that keeps two opposing variables within optimal limits:

• Leaf water potential (Ψ_leaf) – when Ψ_leaf falls (e.g., because of high transpiration), guard‑cell turgor drops, the pore closes and water loss is reduced, allowing Ψ_leaf to recover.
• Intercellular CO₂ concentration (C_i) – when C_i falls during photosynthesis, guard cells swell, the pore opens and CO₂ influx increases, raising C_i and the photosynthetic rate (A).

This feedback links stomatal aperture to both the hydraulic (Ψ) and carbon (C) status of the leaf, exemplifying the definition of homeostasis in the Cambridge Homeostasis (Topic 14) syllabus.

2. Structure of a Stomatal Complex

• Guard cells – a pair of kidney‑shaped cells that change turgor to open or close the pore.
• Subsidiary cells – assist guard cells in many monocots and some dicots.
• Stomatal pore – the actual opening for gas exchange.

3. Light‑Driven Opening (Blue‑Light Pathway)

1. Blue‑light photoreceptors (phototropins) are activated.
2. Phototropins phosphorylate and activate plasma‑membrane H⁺‑ATPases.
3. H⁺ is pumped out of guard cells, creating an electrochemical gradient.
4. Inward‑rectifying K⁺ channels (e.g., KAT1) open; K⁺ (and, to a lesser extent, Cl⁻) enter the cells.
5. Solute accumulation lowers guard‑cell water potential (Ψ_gc); water flows in osmotically, guard cells swell, and the pore opens.

4. Hormonal Control – Detailed ABA Signalling

Drought or low soil water potential raises abscisic acid (ABA) in roots and leaves. The core cascade in guard cells is:

1. ABA binds to PYR/PYL/RCAR receptors.
2. The ABA‑receptor complex inhibits PP2C phosphatases.
3. Inhibition of PP2C releases SnRK2 kinases, which become active.
4. Active SnRK2 phosphorylates and opens outward‑rectifying K⁺ channels (e.g., GORK) and anion channels (SLAC1).
5. K⁺ and Cl⁻ exit the guard cells; water follows, turgor falls and the stomatal pore closes.

This pathway can override the light‑driven opening, ensuring water conservation under stress.

5. Circadian (Daily) Rhythm of Stomatal Aperture

Even when light, temperature and humidity are kept constant, many species exhibit an internal ≈ 24 h rhythm driven by the plant’s circadian clock. A typical pattern is:

• Pre‑dawn (≈ 04:00–06:00) – Stomata largely closed.
• Morning (≈ 06:00–10:00) – Rapid opening as light intensity rises.
• Mid‑day (≈ 10:00–14:00) – Partial closure to limit water loss under high temperature and vapour‑pressure deficit (VPD).
• Afternoon (≈ 14:00–18:00) – Re‑opening if light remains high and water is available.
• Evening (≈ 18:00–20:00) – Closure as light fades.
• Night (≈ 20:00–04:00) – Stomata remain closed.

6. Environmental Factors Modulating the Rhythm

• Light quality & intensity – Blue light is the most effective stimulus; red light influences stomata indirectly via photosynthetic feedback.
• Temperature – High temperature increases transpiration demand, often causing midday closure.
• Humidity (VPD) – Low humidity (high VPD) promotes closure to conserve water.
• Water status (soil moisture) – Drought → ↑ ABA → stomatal closure, overriding light signals.

7. Quantifying Gas Exchange and Water‑Use Efficiency

Stomatal conductance (g_s) links CO₂ uptake and water loss:

\$\$

A \;=\; gs \, (Ca - C_i)

\$\$

where A is the photosynthetic rate (µmol m⁻² s⁻¹), C_a the ambient CO₂ concentration and C_i the intercellular CO₂ concentration.

Intrinsic water‑use efficiency (iWUE) is then:

\$\$

\text{iWUE} \;=\; \frac{A}{g_s}

\$\$

The instantaneous transpiration rate (E) can be expressed as:

\$\$

E \;[\text{mm day}^{-1}] \;=\; g_s \;[\text{mol m}^{-2}\text{s}^{-1}] \times \text{VPD} \;[\text{kPa}] \times 0.044

\$\$

The factor 0.044 converts mol m⁻² s⁻¹ · kPa to mm day⁻¹ (latent heat of vapourisation and molar volume of water).

8. Typical Daily Stomatal Conductance (g_s)

9. Significance for Plant Homeostasis

• CO₂ optimisation – Opening synchronises with periods of photosynthetically active radiation, maximising carbon gain (high A).
• Water‑use efficiency – Closure during high evaporative demand or drought conserves soil water and maintains leaf Ψ, improving iWUE.
• Integration with the transpiration stream – Stomatal conductance determines the driving force for water movement from roots to leaves, linking hydraulic and photosynthetic processes.

10. Suggested Classroom Activity

1. Grow two identical seedlings under a controlled 12 h light / 12 h dark cycle.
2. Using a portable porometer, record stomatal conductance (g_s) every 2 hours for 48 hours.
3. Plot g_s against time and compare the observed curve with the typical pattern shown above.
4. Repeat the experiment after raising the ambient temperature by 5 °C or reducing relative humidity, and discuss how the curve shifts.