Table of Contents
Transpiration is the loss of water vapor from plants, mainly through stomata in the leaves. While this process is essential for water transport and cooling, it also risks excessive water loss. Plants therefore need to regulate transpiration continuously and finely in response to both internal needs and external conditions.
This chapter focuses on how plants regulate transpiration, especially by controlling stomatal aperture, but also through structural and developmental adaptations.
Levels of Regulation
Transpiration is regulated on several levels:
- Short-term, reversible regulation
- Seconds to hours
- Mainly via opening and closing of stomata
- Involves guard cell physiology and signal transduction
- Medium-term regulation
- Days to weeks
- Adjustment of leaf angle, leaf rolling, temporary shedding of leaves or older organs
- Changes in stomatal sensitivity to signals
- Long-term, developmental regulation
- Weeks to years
- Leaf number, size, thickness
- Stomatal density and distribution
- Root-to-shoot ratio
- Structural traits like cuticle thickness, leaf hairs
Most of the rapid and precise control happens through stomatal regulation, which is therefore central to this chapter.
Stomatal Regulation: Guard Cells as Valves
Basic Principle
Stomata consist of two guard cells that control the size of the stomatal pore. Changes in the turgor pressure (internal water pressure) of guard cells determine whether the pore is open or closed:
- Opening: Guard cells take up solutes, water follows osmotically, cells swell and bend apart → pore widens.
- Closing: Solutes are released, water leaves the cells, turgor drops → cells become flaccid, pore narrows or closes.
This regulation is highly dynamic and can respond within minutes to changes in light, humidity, CO₂, or signals from the roots.
Osmotic Mechanisms in Guard Cells
Guard cell turgor is controlled primarily by the movement of ions and small molecules:
- Uptake of K⁺ ions (and often Cl⁻ or malate²⁻ as counterions) increases the osmotic potential inside guard cells.
- Water enters the guard cells by osmosis, increasing their volume.
- For closure, K⁺ and other solutes are exported back to surrounding cells, water follows, and turgor decreases.
Key processes:
- Ion transporters and channels in the guard cell plasma membrane regulate K⁺ and anion flow.
- Proton pumps (H⁺-ATPases) create electrochemical gradients that drive the uptake or release of ions.
- Organic acids (e.g., malate) and sugars can also contribute to osmotic adjustment during stomatal movements.
The biochemical details of these transport systems are part of cellular-level physiology; here the key point is that guard cell turgor is an actively regulated, energy-dependent process.
Environmental Factors Influencing Stomatal Behavior
Stomata integrate multiple external signals. These signals often act simultaneously and can reinforce or oppose each other. The most important environmental factors are light, CO₂ concentration, air humidity, and temperature.
Light
Light is a primary signal for stomatal opening, because photosynthesis requires CO₂ uptake.
- Blue light is especially effective in inducing stomatal opening.
- Guard cells possess specific photoreceptors (blue-light receptors).
- Blue light activates H⁺-ATPases, hyperpolarizes the membrane, and promotes K⁺ uptake.
- Red light can also promote stomatal opening, mainly indirectly by increasing photosynthesis in the mesophyll and thus altering internal CO₂ levels.
Typical patterns:
- Daytime (light): stomata generally open to allow CO₂ uptake.
- Night (darkness): stomata usually close or narrow because photosynthesis ceases, and CO₂ uptake is no longer needed.
There are important exceptions:
- Some plants (e.g., certain desert species with CAM metabolism) open their stomata at night and close them during the day. This special adaptation is linked to their overall metabolism and is treated in more detail in the context of photosynthesis.
CO₂ Concentration
CO₂ around and within the leaf is a key signal:
- Low internal CO₂ (caused by high photosynthetic activity) tends to open stomata:
- The plant "demands" more CO₂ for photosynthesis.
- High internal CO₂ (e.g., in darkness, or low photosynthetic activity) tends to close stomata:
- CO₂ uptake is less needed, and water can be conserved.
CO₂ thus provides a feedback loop between photosynthesis and water loss.
Humidity and Vapor Pressure Deficit
The driving force for transpiration is the difference in water vapor concentration between saturated air spaces inside the leaf and usually drier outside air. This difference is often expressed as vapor pressure deficit (VPD).
- High air humidity (low VPD):
- Transpiration rate is low.
- Stomata can remain relatively open without excessive water loss.
- Low air humidity (high VPD):
- Transpiration rate increases strongly.
- Many plants respond by partially closing stomata to limit water loss.
This response is critical for preventing rapid dehydration under dry, windy, or hot conditions.
Temperature
Temperature influences transpiration in two ways:
- Physical effect:
- Warmer air can hold more water vapor, often increasing the vapor pressure deficit and thus the potential rate of transpiration.
- Warmer leaves also increase evaporation from internal water surfaces.
- Biological effect:
- At moderate temperatures, increased temperature can stimulate photosynthesis (up to a point), indirectly favoring stomatal opening.
- At high temperatures, water loss risk increases sharply, and stomata tend to close to avoid overheating and dehydration.
Temperature effects are therefore context-dependent: moderate warming can favor opening, extreme heat promotes closure.
Wind and Air Movement
Wind removes the saturated boundary layer of air around the leaf and replaces it with drier air:
- Mild air movement can enhance gas exchange and cooling, sometimes tolerated with open stomata.
- Strong wind greatly increases transpiration, and many plants respond by partial stomatal closure.
Wind also interacts with other factors (temperature and humidity) to affect overall transpiration rate.
Internal Signals and Hormonal Control
Environmental signals are not always detected directly by guard cells. Water status in roots and leaves, and growth regulators, strongly influence stomatal behavior.
Water Status and Root–Shoot Signaling
When soil dries:
- Roots experience water stress before leaves.
- Even before leaves wilt, stressed roots can send chemical signals to the shoot that reduce stomatal opening.
This allows the plant to anticipate water shortage and reduce water loss early.
Abscisic Acid (ABA) – The Drought Hormone
The best-studied hormonal regulator of transpiration is abscisic acid (ABA):
- ABA is synthesized mainly in roots and leaves under water deficit.
- It is transported via the xylem to leaves and accumulates especially in guard cells.
- ABA triggers a cascade leading to:
- Opening of ion channels that cause efflux of K⁺ and anions.
- Loss of guard cell turgor.
- Stomatal closure.
Important features:
- ABA action is rapid (minutes to hours) and can override some environmental opening signals (e.g., light).
- Even localized root drying on part of the root system can increase ABA and reduce stomatal conductance, while the plant is still overall hydrated.
- This conserves water under drought and salinity stress, but reduces CO₂ uptake and can limit growth.
ABA is also involved in adjusting stomatal sensitivity over longer times, helping plants acclimate to chronically dry environments.
Other Internal Factors
Beyond ABA, other internal conditions modify stomatal behavior:
- Leaf water potential and turgor in epidermal and mesophyll cells:
- Direct dehydration of guard cells and surrounding cells leads to stomatal closure.
- Nutritional status:
- Long-term deficiencies (e.g., nitrogen) may reduce leaf area and stomatal density.
- Developmental and circadian (day–night) rhythms:
- Many species exhibit regular daily patterns of stomatal conductance, even under constant conditions, governed by an internal circadian clock.
These factors fine-tune stomatal behavior so that water loss is coordinated with photosynthetic capacity and growth.
Structural and Morphological Adaptations Affecting Transpiration
While stomatal movements regulate transpiration on short time scales, plants also possess structural features that influence baseline water loss.
Cuticle
The cuticle is a waxy layer covering epidermal cells:
- Thicker cuticles reduce cuticular transpiration (water loss through the epidermis outside stomata).
- In some dry-habitat plants (xerophytes), the cuticle is especially thick and often covered with additional wax layers, reflecting light and reducing heat absorption.
Stomatal Density and Distribution
During leaf development, plants can adjust:
- Stomatal density (number of stomata per unit area).
- Position of stomata (only lower surface, both surfaces, or sunken in pits).
Typical patterns:
- Many shade or mesic plants concentrate stomata on the lower leaf surface to reduce direct exposure to sun and air currents.
- Some xerophytes have sunken stomata in small pits or grooves, often protected by hairs; this traps moist air and lowers transpiration.
- Plants in wet or aquatic environments may have abundant stomata on the upper surface, reflecting their different constraints.
These distribution patterns are long-term developmental decisions rather than short-term regulation.
Leaf Form and Anatomy
Leaf traits that influence transpiration include:
- Leaf size and shape:
- Small, narrow, or dissected leaves reduce the leaf boundary layer and can decrease overheating.
- Leaf thickness (succulence):
- Succulent leaves store water, allowing stomata to remain closed for long periods.
- Leaf hairs (trichomes):
- Create a humid microclimate near the surface, reducing transpiration.
- Leaf rolling and folding:
- Some grasses and other species can roll or fold leaves under drought or intense light, reducing exposed surface area and trapping humid air.
These traits are partly reversible (e.g., leaf rolling) and partly fixed by development (e.g., leaf thickness).
Leaf Shedding and Seasonal Strategies
At longer time scales, plants control transpiration by adjusting how much leaf area they maintain:
- Seasonal leaf shedding (deciduous trees) reduces water loss during unfavorable seasons (cold winters, dry periods).
- Drought-induced leaf abscission:
- Some species shed older leaves during drought, reducing total transpiring surface.
- Leaf phenology:
- Timing of leaf flush and senescence is coordinated with available water and temperature regimes.
These strategies represent a higher-level regulation of transpiration at the whole-plant and life-history scale.
Trade-Off: CO₂ Uptake vs. Water Conservation
Stomatal regulation is always a compromise between:
- Maximizing CO₂ uptake for photosynthesis and growth.
- Minimizing water loss to avoid dehydration and loss of turgor.
Key consequences:
- When water is plentiful and conditions are mild, plants tend to keep stomata relatively open, favoring growth.
- Under drought, high temperature, or strong evaporative demand, plants close stomata more, conserving water but limiting photosynthesis.
- Species differ in their strategy:
- Some are “water spenders” (open stomata longer to maximize growth when water is available, but risk stress during drought).
- Others are “water savers” (tight stomatal control, lower growth but higher survival under chronic stress).
This balance is central to plant ecology and to agricultural practices, because it affects water-use efficiency—how much biomass or yield a plant produces per unit of water lost through transpiration.
Summary of Main Regulatory Mechanisms
Regulation of transpiration involves:
- Rapid control via stomatal movements:
- Driven by ion transport and guard cell turgor.
- Influenced by light, CO₂ concentration, humidity, temperature, wind.
- Modulated by internal signals, especially ABA under water deficit.
- Intermediate adjustments:
- Changes in stomatal sensitivity to signals.
- Leaf movements (rolling, folding, orienting).
- Long-term structural adaptations:
- Cuticle thickness and composition.
- Stomatal density and distribution.
- Leaf anatomy (size, thickness, hairs, sunken stomata).
- Seasonal and drought-induced changes in leaf area.
Through the integration of these mechanisms, plants maintain water balance while allowing sufficient gas exchange for photosynthesis and growth, constantly adapting to variable environmental conditions.