Plant Physiology – Transpiration & Stomatal Regulation

Vocabulary flashcards covering key terms related to plant transpiration, stomatal physiology, blue-light signaling, and associated regulatory mechanisms.

Transpiration

Loss of water as water vapour from aerial parts of a plant, mainly leaves, via stomata, cuticle or lenticels.

Stomatal Transpiration

Water vapour loss through stomatal pores, accounting for about 90 % of total transpiration.

Cuticular Transpiration

Evaporation of water through the waxy cuticle, contributing roughly 5–10 % of total transpiration.

Lenticular Transpiration

Water vapour loss through lenticels in woody stems, contributing 1–5 % of total transpiration.

Stomata

Microscopic epidermal pores surrounded by guard cells that regulate gas exchange and most water loss.

Guard Cells

Specialised epidermal cells flanking a stoma that alter their turgor to open or close the pore.

Cuticle

Hydrophobic, waxy layer covering aerial plant tissues that restricts water loss; thicker in arid-adapted plants.

Lenticel

Lens-shaped, loosely packed region in bark or stems that remains open for gas exchange and minor transpiration.

Hydathode

Specialised pore at leaf tips or margins that exudes xylem sap during guttation.

Guttation

Night-time exudation of liquid xylem sap from hydathodes driven by positive root pressure.

Transpiration Pull

Upward force generated by leaf evaporation that draws water and minerals through xylem.

Anti-transpirant

Substance applied to foliage to decrease the rate of transpiration.

Phenylmercury Acetate (PMA)

Chemical anti-transpirant that partially blocks stomata to reduce water loss.

Abscisic Acid (ABA)

Plant hormone that induces stomatal closure, acting as a natural anti-transpirant.

Film-forming Anti-transpirant

Material such as silicon oil or wax that forms a clear film allowing gas diffusion but limiting water vapour escape.

Transpiration Ratio

Mass of water lost to mass of CO₂ fixed (mol H₂O / mol CO₂), indicating water-use efficiency.

Atmospheric Humidity

Environmental water vapour content; higher humidity lowers transpiration rate.

Temperature Effect on Transpiration

Higher air temperature speeds evaporation and increases transpiration; lower temperature does the opposite.

Wind Effect on Transpiration

Moderate wind removes moist boundary layers and raises transpiration; very strong wind may induce stomatal closure.

Light Effect on Transpiration

Illumination opens stomata and supplies energy for evaporation, so transpiration is generally greater in daylight.

Soil Water Availability

Adequate soil moisture sustains transpiration; drought lowers leaf water potential and closes stomata.

Atmospheric Pressure Effect

Lower pressure reduces external vapour pressure, enhancing transpiration; higher pressure suppresses it.

Stomatal Complex

Functional unit consisting of a stoma, its guard cells and any subsidiary cells.

Subsidiary Cells

Epidermal cells adjacent to guard cells (especially in grasses) that assist stomatal movement.

Turgor Pressure

Internal water pressure within guard cells that determines stomatal aperture.

Photosynthesis in Guard Cells

Light-driven carbon fixation within guard-cell chloroplasts that contributes osmolytes for stomatal opening.

Blue Light Response

Specific stimulation of guard cells by blue wavelengths that triggers proton pumping and stomatal opening.

Phototropins (Phot1 & Phot2)

Blue-light receptor kinases initiating guard-cell signalling for stomatal opening.

Potassium Ion Influx Theory

Model in which guard-cell K⁺ uptake, balanced by malate/Cl⁻, lowers water potential and opens stomata.

Zeaxanthin

Xanthophyll formed from violaxanthin in strong light; acts as auxiliary blue-light sensor in guard cells.

Xanthophyll Cycle

Reversible conversion of violaxanthin ↔ antheraxanthin ↔ zeaxanthin that modulates light harvesting and signalling.

Violaxanthin

High light (Violoxanthin to zeaxanthin by deepoxidase), low light (zeaxanthin to Violoxanthin by epixidase)

Antheraxanthin

Intermediate xanthophyll formed during the conversion of violaxanthin to zeaxanthin.

Proton Pump (H⁺-ATPase)

Plasma-membrane enzyme that extrudes H⁺, hyperpolarises guard cells and drives K⁺ uptake during blue-light opening.

Fusicoccin

Fungal toxin that permanently activates H⁺-ATPase, causing continuous stomatal opening and lethal water loss.

DCMU

Herbicide blocking photosynthetic electron transport; partially inhibits light-stimulated stomatal opening.

DTT (Dithiothreitol)

Reducing agent that prevents violaxanthin → zeaxanthin conversion, inhibiting blue-light-induced opening.

CCCP

Protonophore that dissipates pH gradients, blocking blue-light-activated proton pumping in guard cells.

Boundary Layer Resistance (rb)

Diffusive resistance to water vapour movement presented by the still air layer adjacent to a leaf surface.

Leaf Stomatal Resistance (rs)

Resistance to gas flow through stomata, governed by guard-cell aperture.

Mesophyll Cells

Photosynthetic leaf cells exchanging CO₂ and water vapour with substomatal cavities.

Hydrophobic

Water-repelling property of cuticular waxes that slows water movement across the leaf surface.

Starch-Sugar Conversion Theory

Hypothesis that diurnal conversion of guard-cell starch to osmotically active sugars drives opening in light and closure in dark.

Transpirational Cooling

Temperature reduction in leaves produced by evaporative water loss through transpiration.

Thermoregulation in Plants

Maintenance of favourable leaf temperature, partly via transpiration-driven evaporative cooling.

Osmotic Potential

Water-potential component determined by solute concentration; lower osmotic potential in guard cells draws in water.

Transpiration

Loss of water vapour from aerial parts of a plant, mainly leaves, driving water movement from roots to atmosphere.

Stomatal Transpiration

Water‐vapour loss through stomata; accounts for ~90 % of total transpiration.

Cuticular Transpiration

Water‐vapour loss through the waxy cuticle; 5–10 % of total transpiration, slowed by thicker cuticles.

Lenticular Transpiration

Water‐vapour loss through lenticels in woody stems; contributes 1–5 % of transpiration.

Stomata

Microscopic pores in epidermis bordered by guard cells enabling gas exchange and major water loss.

Guard Cells

Specialized epidermal cells flanking each stoma; change turgor to open or close the pore.

Cuticle

Hydrophobic waxy layer on above-ground plant surfaces that limits water movement outward.

Lenticel

Lens-shaped porous region on woody stems permitting gas exchange; always open.

Transpirational Pull

Upward movement of water and minerals generated by water loss from leaves.

Thermoregulation (Plants)

Cooling of plant surfaces via evaporative water loss during transpiration.

Turgor Pressure

Pressure of cell contents against the cell wall; drives guard-cell movement.

Relative Humidity

Atmospheric moisture content; high humidity lowers transpiration, low humidity raises it.

Light (Effect on Transpiration)

Opens stomata and increases evaporation; darkness usually closes stomata.

Wind

Moderate wind removes boundary layer, raising transpiration; very strong wind can close stomata.

Anti-transpirant

Substance applied to reduce transpiration rate (e.g., PMA, ABA, silicone oils).

Transpiration Ratio (TR)

Mass of water lost per mass of CO₂ assimilated (mol H₂O / mol CO₂).

Guttation

Nighttime exudation of xylem sap droplets at leaf tips/edges via hydathodes due to root pressure.

Hydathode

Specialized epidermal pore through which guttation fluid is released.

Stomatal Complex

Guard cells plus subsidiary cells and the central pore (stoma).

Subsidiary Cell

Epidermal cell adjacent to guard cell that aids stomatal function, especially in grasses.

Kidney-shaped Stomata

Typical guard-cell form in dicots and many monocots (non-grass); elliptical outline.

Dumbbell-shaped Stomata

Guard-cell form characteristic of grasses; flanked by subsidiary cells.

Potassium Ion Influx Theory

Daytime K⁺ influx with malate formation lowers guard-cell water potential, causing stomatal opening.

Starch–Sugar Conversion Theory

Daytime photosynthesis converts starch to sugars in guard cells, increasing osmotic pressure and opening stomata.

Phototropin

Blue-light receptor kinase (phot1/phot2) that initiates stomatal opening by activating H⁺-ATPase.

Blue-Light Response (Stomata)

Rapid, reversible stomatal opening triggered by blue light via phototropin-mediated proton pumping.

Xanthophyll Cycle

Light-dependent conversion of violaxanthin ⇌ antheraxanthin ⇌ zeaxanthin; involved in photoprotection and guard-cell signaling.

Zeaxanthin

De-epoxidized xanthophyll produced in high light; acts as blue-light sensor in guard cells and enables green-light reversal.

Violaxanthin De-epoxidase (VDE)

Thylakoid enzyme converting violaxanthin to zeaxanthin under high light.

BLUS1

Guard-cell protein phosphorylated by activated phototropins; participates in blue-light signal transduction to H⁺-ATPase.

PP1c–PRSL1 Complex

Protein phosphatase module regulated by BLUS1 that modulates a kinase controlling H⁺-ATPase activation.

H⁺-ATPase (Plasma Membrane)

Proton pump hyperpolarizing guard-cell membrane; drives K⁺ uptake and stomatal opening.

14-3-3 Protein

Regulatory protein that binds phosphorylated H⁺-ATPase, stabilizing it in an active state.

Abscisic Acid (ABA)

Stress hormone promoting stomatal closure; antagonizes blue-light signaling through phosphatidic acid.

Cytokinin (Guard Cells)

Phytohormone that can promote K⁺ uptake and stomatal opening.

DCMU

Herbicide inhibiting photosynthetic electron transport; partially blocks light-stimulated stomatal opening.

Dithiothreitol (DTT)

Reducing agent that inhibits zeaxanthin formation and blocks blue-light–induced stomatal opening.

Fusicoccin

Fungal toxin that irreversibly activates H⁺-ATPase, causing persistent stomatal opening and plant wilting.

CCCP

Protonophore uncoupler that dissipates pH gradients; abolishes blue-light–induced medium acidification.

Green-Light Reversal

Closure of blue-light opened stomata upon green-light exposure; requires zeaxanthin but not phototropins.

Boundary Layer Resistance (rb)

Resistance to water vapour movement across the still air layer adjacent to leaf surface.

Leaf Stomatal Resistance (rs)

Resistance to vapour diffusion through stomatal pores.

Transpiration Cooling

Temperature reduction of leaves due to latent heat loss via evaporating water.

Root Pressure

Positive hydrostatic pressure in xylem that can drive guttation when transpiration is low.

Water Potential (Ψ)

Chemical potential of water; decline in guard-cell Ψ draws water in, increasing turgor.

Endosmosis

Water influx into a cell due to lower internal water potential.

Exosmosis

Water efflux from a cell when internal water potential is higher than external.

Phot1/Phot2 Double Mutant

Plant lacking both phototropins; shows minimal blue-light–induced stomatal opening.

npq1 Mutant

Arabidopsis mutant deficient in zeaxanthin; lacks green-light reversal of blue-light stomatal response.

Film-Forming Anti-transpirant

Spray (e.g., silicone oil, wax) forming permeable film that lets gases but not water vapour diffuse.