explain that stomata respond to changes in environmental conditions by opening and closing and that regulation of stomatal aperture balances the need for carbon dioxide uptake by diffusion with the need to minimise water loss by transpiration
Cambridge A-Level Biology 9700 – Homeostasis in Plants: Stomatal Regulation
Homeostasis in Plants – Stomatal Regulation
Learning Objective
Explain how stomata respond to changes in environmental conditions by opening and closing, and how the regulation of stomatal aperture balances the need for carbon dioxide uptake by diffusion with the need to minimise water loss by transpiration.
1. Introduction to Stomata
Stomata are microscopic pores on the epidermis of leaves and young stems. Each stoma is surrounded by a pair of specialised guard cells that control its opening and closing. The primary functions are:
Facilitating the diffusion of CO₂ into the leaf for photosynthesis.
Allowing water vapour to escape (transpiration), which drives the uptake of mineral nutrients and cools the leaf.
2. Environmental Signals that Influence Stomatal Aperture
Guard cells integrate a range of external and internal cues. The most important signals are:
Light intensity – blue light activates photoreceptors that promote opening.
Leaf water status – low water potential (drought) leads to closure.
Air humidity and vapour pressure deficit (VPD) – high \cdot PD encourages closure to limit water loss.
Temperature – extreme heat can cause closure to conserve water.
Endogenous hormones – abscisic acid (ABA) is a key signal for drought‑induced closure.
3. Mechanism of Stomatal Opening
The opening process is driven by the active transport of ions into the guard cells, creating an osmotic gradient that draws water in, swelling the cells and pulling the stomatal pore open.
Blue light activates plasma‑membrane H⁺‑ATPases.
Proton extrusion hyperpolarises the membrane, allowing K⁺ influx through inward‑rectifying K⁺ channels.
Cl⁻ and malate⁻ follow to maintain charge balance.
Water enters by osmosis, increasing turgor pressure.
4. Mechanism of Stomatal Closure
Closure is essentially the reverse of opening, often accelerated by the hormone abscisic acid (ABA).
Cl⁻ and malate⁻ exit to maintain electroneutrality.
Loss of solutes reduces osmotic potential, water leaves the guard cells, and turgor falls.
The guard cells become flaccid, and the pore closes.
5. Balancing CO₂ Uptake and Water Loss
Stomatal aperture (a) determines the rates of both CO₂ diffusion into the leaf and water vapour diffusion out of the leaf. These fluxes can be expressed by Fick’s law:
\$\$
J = -D \frac{\Delta C}{\Delta x}
\$\$
where J is the flux, D the diffusion coefficient, and ΔC/Δx the concentration gradient. For CO₂, the gradient is between the external atmosphere and the mesophyll; for water vapour, it is between the leaf interior (saturated) and the surrounding air.
Because the diffusion pathways for CO₂ and H₂O are the same, any change in aperture simultaneously affects both processes. The plant must therefore find an optimum aperture that maximises photosynthetic carbon gain while minimising transpiration loss.
6. Practical Example – Response to Drought
When soil water becomes scarce:
Root cells detect low water potential and synthesise ABA.
ABA travels via the xylem to guard cells.
Guard cells lose K⁺ and Cl⁻, water follows, and turgor drops.
Stomata close, reducing transpiration and conserving water.
Photosynthetic rate declines because CO₂ entry is limited, illustrating the trade‑off.