Homeostasis in plants

Stomata (singular: stoma) are pores in the epidermis of leaves and young stems. They are the only sites of significant gas exchange in the plant: CO₂ enters the leaf, O₂ leaves it, and water vapour escapes (transpiration). Stomata occur mainly on the lower epidermis of leaves (~100-500 per mm² in most plants), but in some species (e.g. aquatic plants) they are on the upper epidermis.

Each stoma is bordered by a pair of guard cells — specialised epidermal cells that can change shape to open or close the pore. The aperture of the pore is regulated minute-by-minute according to environmental conditions, balancing the plant's need for CO₂ (for photosynthesis) against its need to conserve water.

Why dynamic control matters. A plant cannot have permanently open stomata — it would lose water faster than the roots could supply it, and the leaves would wilt. Nor can it have permanently closed stomata — it would have no CO₂ for photosynthesis, and would starve. The guard-cell system provides the dynamic control needed to navigate this trade-off.

Guard cells are perhaps the most sophisticated single-cell systems in a plant. They sense light, CO₂, humidity, temperature and hormone signals (ABA), integrate these inputs, and respond by physically bending to open or close a pore — all in the space of a few minutes.

Guard cells are highly specialised compared with the rest of the epidermis. Key structural features:

1. Pair of kidney- or sausage-shaped cells. Two guard cells lie side by side; the gap between them is the stoma. They are joined at both ends, so when they bend outward they pull apart in the middle, opening the pore.

2. Asymmetric cell wall. The inner wall (next to the pore) is thick and less elastic; the outer wall is thinner and more elastic. This asymmetry is the key structural feature for opening: when the cell becomes turgid, the thin outer wall stretches but the thick inner wall does not, so the cell bows outward.

3. Radial cellulose microfibrils. Cellulose microfibrils run radially around the cell — perpendicular to its long axis — like the hoops of a barrel. This arrangement prevents the cell from widening when turgid (the microfibrils resist circumferential expansion) but allows it to lengthen. The combination of radial microfibrils + asymmetric wall thickness converts isotropic swelling into the specific bending motion that opens the stoma.

4. Contain chloroplasts. Unlike most other epidermal cells, guard cells contain chloroplasts. The chloroplasts:

• Store starch, which is converted to malate as a counterion during opening.
• Carry out a low level of photosynthesis (more important for signalling than for ATP supply).
• Contain phytochrome and other light-responsive pigments contributing to light sensing.

5. Many mitochondria. Guard cells have a high density of mitochondria — they need a lot of ATP for the H⁺ ATPase pumps that drive stomatal movements.

6. Receptors and signal transduction machinery. Guard cells have:

• Phototropin (blue-light receptor) in the plasma membrane.
• PYR/PYL receptors for ABA.
• A complete Ca²⁺-based signalling cascade for both opening and closing.

Most surrounding epidermal cells, by contrast, are flat, irregular, tightly packed, transparent and lack chloroplasts. They form a protective cuticle-covered seal across the leaf surface.

Stomatal opening at dawn is driven by a cascade of events that turn a light signal into a physical pore opening:

Step 1 — Light absorption. At dawn, blue light (peak ~470 nm) is absorbed by phototropin photoreceptors in the guard cell plasma membrane. Red light absorbed by chloroplasts also contributes, partly by reducing internal CO₂ as photosynthesis starts.

Step 2 — H⁺ ATPase activation. Phototropin activation triggers a signalling cascade that switches on H⁺ ATPase pumps in the guard cell plasma membrane. These pumps use ATP to actively transport H⁺ out of the guard cell into the apoplast.

Step 3 — Membrane hyperpolarisation. As positively-charged H⁺ leaves the cell, the inside becomes more negative (hyperpolarised). The apoplast becomes more acidic.

Step 4 — K⁺ uptake. Hyperpolarisation opens voltage-gated K⁺ channels in the plasma membrane. K⁺ moves into the guard cell down its electrochemical gradient by facilitated diffusion. Cl⁻ may also enter through anion channels to balance charge.

Step 5 — Malate synthesis. Starch stored in chloroplasts is broken down into malate (a 4-carbon dicarboxylic acid). Malate stays in the cytoplasm and vacuole and serves as an additional anionic counterion to K⁺.

Step 6 — Water potential falls. Accumulation of K⁺ (positive), Cl⁻ (negative) and malate (negative) inside the vacuole lowers the water potential of the guard cell — it becomes more negative than that of neighbouring epidermal cells.

Step 7 — Water uptake by osmosis. Water moves from neighbouring epidermal cells down the water potential gradient into the guard cells through aquaporins. The guard cells swell and become turgid.

Step 8 — Bending and opening. Because the inner wall is thick and the outer wall is thin, and because radial microfibrils prevent sideways swelling, the turgor pressure causes the cell to bend outward. The two guard cells bow apart in the middle, opening the stoma.

The pore can now allow CO₂ to diffuse in for photosynthesis. Water vapour also diffuses out — but this is acceptable as long as water supply from the roots can keep pace.

Key idea. Notice that water uptake is passive (osmotic) — but it is driven by the active uptake of K⁺ which is itself driven by the H⁺ ATPase pump powered by ATP. The whole process is reversed at dusk (or earlier if water is scarce, see ABA below).

Stomata close in response to two main signals: darkness (passive reversal of the opening mechanism) and water stress (active closure mediated by abscisic acid, ABA). The ABA pathway is more important for survival and is the main focus of the syllabus.

Step 1 — Detecting drought. When soil water is low, or transpiration exceeds water uptake, root cells lose turgor. They synthesise abscisic acid (ABA) — a sesquiterpenoid hormone. Leaf mesophyll cells can also synthesise ABA when leaf turgor falls.

Step 2 — Transport to leaves. ABA dissolves in xylem sap and is transported upward in the xylem from roots to leaves in the transpiration stream. This is a remarkably elegant signalling mechanism — the same flow that is causing water loss carries the hormone that will stop it.

Step 3 — Binding to receptors. ABA in the leaf apoplast binds to PYR/PYL receptors on (and inside) guard cells. Binding initiates a Ca²⁺-based signalling cascade involving the protein phosphatase PP2C and the kinase SnRK2.

Step 4 — Inhibition of H⁺ pumps + opening of anion channels. ABA signalling has two effects on the guard cell membrane:

• Inhibits the H⁺ ATPase pumps that normally drive opening.
• Opens slow anion channels (SLAC1) and rapid anion channels (R-type), allowing Cl⁻ and malate to flow OUT of the guard cell.

Step 5 — Membrane depolarisation. Loss of anions makes the membrane less negative inside (depolarisation).

Step 6 — K⁺ efflux. Depolarisation opens outward K⁺ channels (different from the inward channels that operate during opening). K⁺ flows out of the cell down its electrochemical gradient.

Step 7 — Water potential of guard cell rises. With K⁺, Cl⁻ and malate now leaving, the guard cell's water potential rises (becomes less negative).

Step 8 — Water leaves by osmosis. Water moves from guard cell to surrounding cells down the water potential gradient. The guard cells become flaccid.

Step 9 — Stoma closes. With turgor lost, the bent guard cells straighten and the two cells come back together, closing the pore. Transpiration is sharply reduced.

Trade-off. Stomatal closure conserves water but also stops CO₂ entering, so photosynthesis halts. The plant grows more slowly — a survival trade-off.

Other roles of ABA. ABA has multiple effects beyond stomatal closure:

• Promotes seed dormancy (prevents premature germination).
• Promotes leaf abscission (loss) and dormancy in deciduous plants — hence the name 'abscisic acid'.
• Activates drought-tolerance gene expression (e.g. proteins that protect cellular structures during desiccation).
• Inhibits stem elongation in seedlings.

Stomata serve a dual function — they let CO₂ in for photosynthesis, but they also let water vapour out. The two cannot be separated structurally. This creates a fundamental trade-off that has shaped plant evolution.

The numbers. A typical C3 plant loses ~500 g of water per gram of CO₂ fixed. This ratio reflects:

• A large water vapour concentration gradient (leaf interior ~100% relative humidity vs atmosphere ~50% RH).
• A small CO₂ concentration gradient (atmosphere ~0.04% vs leaf interior ~0% when photosynthesis is active).
• Water vapour and CO₂ have similar molecular sizes, so they diffuse at similar rates per unit concentration gradient — but the water gradient is far larger.

How plants manage the trade-off — temporal control. Stomata open when conditions favour photosynthesis (daylight, adequate water, moderate temperature) and close when water loss is dangerous (drought, very high temperature, very low humidity, night). The ABA system provides rapid closure in emergencies.

Diurnal pattern. In a well-watered C3 plant on a sunny day:

• Dawn: stomata open (blue-light signal).
• Mid-morning: maximum aperture; rapid photosynthesis.
• Midday: partial closure if heat stress (the 'midday stomatal depression').
• Afternoon: re-opening.
• Dusk: closure.
• Night: closed.

C4 plants (e.g. maize, sugarcane, sorghum). C4 plants use PEP carboxylase in mesophyll cells to fix CO₂ initially into a 4-carbon intermediate (oxaloacetate → malate). The malate is transported to specialised bundle sheath cells, where CO₂ is released at high concentration around Rubisco. This:

• Eliminates photorespiration.
• Allows stomata to be open for less time per unit CO₂ fixed.
• Is particularly efficient in hot, dry, high-light conditions.

CAM plants (cacti, pineapple, succulents). Crassulacean Acid Metabolism plants invert the timing:

• Night: stomata open (lower T, higher humidity → less water loss). CO₂ is fixed by PEP carboxylase into malate, which is stored in the vacuole.
• Day: stomata closed. Malate is decarboxylated to release CO₂ internally; the Calvin cycle uses this with light-reaction ATP/NADPH.

CAM plants conserve water spectacularly well but grow slowly — well-adapted to deserts.

Xerophyte structural adaptations. Plants of dry habitats (xerophytes) reduce water loss with:

• Thick waxy cuticle (reduces cuticular water loss).
• Sunken stomata in pits (reduce the water vapour concentration gradient by trapping humid air).
• Stomatal hairs / trichomes (further trap moist air).
• Rolled leaves (marram grass — Ammophila) — stomata are on the inner surface of the roll, trapping humid air inside.
• Reduced leaves (cacti have spines instead; photosynthesis in green stems).
• Deep root systems (e.g. acacia trees) to reach groundwater.

Hydrophytes (water plants) have the opposite problem — they have abundant water but limited gas exchange — and have stomata on the upper leaf surface (water lily) or no stomata at all (submerged plants exchange gases through the cuticle).