explain that stomata have daily rhythms of opening and closing

Homeostasis in Plants – Stomatal Daily Rhythms

1. Introduction

Plants maintain internal stability (homeostasis) by regulating gas exchange and water loss through specialised pores called stomata. One key aspect of this regulation is the daily (circadian) rhythm of stomatal opening and closing.

2. Structure of a Stomatal Complex

• Guard cells – paired kidney‑shaped cells that change shape to open or close the pore.
• Subsidiary cells – support guard cells in many species.
• Stomatal pore – the opening through which CO₂ enters and water vapour exits.

3. Mechanism of Opening and Closing

Opening and closing are driven by osmotic changes in guard cells:

1. Light activates photoreceptors → pumps H⁺ out of guard cells.
2. H⁺ gradient drives K⁺ uptake via inward‑rectifying channels.
3. K⁺ accumulation lowers water potential, water enters, guard cells swell → pore opens.
4. In darkness or under stress, K⁺ is released, water leaves, guard cells become flaccid → pore closes.

4. Daily (Circadian) Rhythm of Stomatal Aperture

Even in constant environmental conditions, many plants exhibit an internal \overline{24} h rhythm of stomatal behaviour, known as a circadian rhythm. The typical pattern is:

• Pre‑dawn (≈ 04:00–06:00) – Stomata are 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.
• 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.

5. Factors Modulating the Rhythm

The circadian pattern can be altered by external cues:

• Light quality and intensity – Blue light is especially effective at opening stomata.
• Temperature – High temperature increases transpiration demand, often causing midday closure.
• Humidity (VPD) – Low humidity (high \cdot PD) promotes closure to conserve water.
• Water status – Soil drought triggers abscisic acid (ABA) production, overriding the light‑driven opening.

6. Quantifying Transpiration and Stomatal Conductance

The rate of water loss (transpiration, \$E\$) can be expressed as:

\$E = g_s \times \cdot PD\$

where \$gs\$ is stomatal conductance (mol m⁻² s⁻¹) and \$VPD\$ is the vapour pressure deficit (kPa). Daily changes in \$gs\$ reflect the rhythmic opening and closing of stomata.

7. Typical Daily Stomatal Conductance

8. Significance for Plant Homeostasis

By timing stomatal aperture, plants balance two competing needs:

• Maximising CO₂ uptake for photosynthesis.
• Minimising water loss to avoid dehydration.

The circadian rhythm ensures that stomata are open when photosynthetic light is available, yet closed during periods of high evaporative demand or low light, contributing to overall water‑use efficiency.

9. Suggested Classroom Activity

1. Grow two identical seedlings under controlled light/dark cycles.
2. Measure stomatal conductance with a portable porometer at 2‑hour intervals for 48 h.
3. Plot \$g_s\$ against time and compare the observed pattern with the typical rhythm shown above.
4. Discuss how changes in temperature or humidity would modify the curve.