Lab 5 - Transpiration

Transpiration

The process of water vapor loss from plant parts, primarily leaves, through small openings called stomata.

Where does most transpiration occur?

On the undersides of leaves where most stomata are located.

Stomata

Microscopic pores in the epidermis of leaves and stems that allow gas exchange and water vapor release.

Guard cells

Specialized epidermal cells that flank each stoma; they open and close the stomatal pore to control water loss and gas exchange.

How many guard cells surround each stoma?

Two.

What causes stomata to open?

Guard cells absorb water and become turgid, causing the pore to open.

What causes stomata to close?

Guard cells lose water and become flaccid, closing the pore.

Primary purpose of transpiration

Cools the plant and drives the upward movement of water (and minerals) from roots to shoots.

What does transpiration drive?

The transpiration-cohesion-tension mechanism that pulls water upward through xylem.

Why is transpiration essential?

It maintains water and nutrient flow, cools the plant, and maintains internal water balance.

Main issue caused by transpiration

Plants lose significant amounts of water daily and must regulate this loss to avoid dehydration.

ADAPTATIONS TO REDUCE WATER LOSS:

1. Guard cells → Regulate stomatal opening to prevent excess water loss.

2. Thickened cuticle → Waxy layer on epidermis that reduces evaporation.

3. Reduced leaves → Smaller leaves or spines minimize surface area for water loss (e.g., cacti).

4. Stomata on stems → Some plants move stomata to stems instead of leaves to limit evaporation.

5. Modified photosynthetic pathways → C4 and CAM photosynthesis reduce need for open stomata during hot days.

Cuticle

A protective waxy layer covering the epidermis that helps prevent water loss.

Reduced leaves

Adaptation found in xerophytes where leaves are reduced in size or transformed into spines.

C4 photosynthesis

Pathway in which CO₂ is fixed into 4-carbon compounds; allows stomata to remain partially closed under high light/heat.

CAM photosynthesis

Crassulacean Acid Metabolism; stomata open at night and close during the day to conserve water.

Why do CAM plants open stomata at night?

To reduce water loss by allowing CO₂ uptake when evaporation rates are lowest.

Example of plants with CAM photosynthesis

Cacti, succulents, pineapples.

Example of plants with C4 photosynthesis

Corn, sugarcane, crabgrass.

What instrument measures the rate of transpiration?

Potometer.

What does a potometer measure?

The rate of water uptake by a plant cutting, indicated by movement of a water column in a capillary tube.

How is transpiration rate calculated?

By measuring the distance moved by a water bubble over a specific time period.

What does faster water movement in a potometer indicate?

Greater rate of transpiration and water uptake.

Why is a potometer only an indirect measurement?

It measures water uptake, not actual water loss, since some water is used in metabolism.

FACTORS AFFECTING TRANSPIRATION RATE:

1. Light intensity → Increases transpiration by opening stomata for photosynthesis.

2. Temperature → Higher temperatures increase evaporation and transpiration rate.

3. Humidity → High humidity decreases transpiration; low humidity increases it.

4. Wind speed → Wind removes humid air near leaf surface, increasing transpiration.

5. Type of plant → Xerophytes have adaptations to limit transpiration; mesophytes lose more water.

How does light affect stomata?

Causes guard cells to take in K⁺ ions and water, becoming turgid and opening the pore.

How does temperature affect water movement?

Heat increases kinetic energy of water molecules, speeding evaporation and diffusion.

How does humidity influence transpiration?

The drier the air (low humidity), the greater the water vapor gradient, increasing transpiration.

How does wind influence transpiration?

Wind reduces boundary layer resistance by moving saturated air away from the leaf surface.

Why does high humidity slow transpiration?

The air is already saturated with water vapor, reducing diffusion of water from leaves.

Why does low humidity increase transpiration?

It increases the vapor pressure gradient between leaf interior and outside air.

What plant is used to observe stomata?

Privet (Ligustrum).

Where are stomata observed on the privet leaf?

On the lower epidermis.

Why are most stomata found on the underside of leaves?

It reduces exposure to sunlight and wind, minimizing water loss.

How do guard cells differ from other epidermal cells?

They contain chloroplasts and can change shape through osmotic pressure changes.

What does it mean if stomata are closed during observation?

The leaf was likely dehydrated or exposed to high light or heat prior to sampling.

Transpiration stream

Continuous movement of water from roots to leaves through xylem, driven by evaporation at leaf surfaces.

Cohesion-tension theory

Explains how water moves upward through xylem due to cohesive forces between water molecules and tension from transpiration.

Why does transpiration cool plants?

Evaporative cooling occurs as water vapor leaves the leaf surface, absorbing heat energy.

How much water can a large tree lose daily?

Up to hundreds of liters through transpiration.

Why is regulation of transpiration critical?

Excessive water loss leads to wilting and reduced photosynthesis efficiency.

What environmental conditions increase transpiration the most?

High temperature, low humidity, bright light, and strong wind.

What environmental conditions decrease transpiration the most?

Low light, high humidity, and low temperature.

Why might plants close stomata even during the day?

To prevent water loss when soil moisture is low or temperature is too high.

What happens to photosynthesis when stomata close?

CO₂ entry decreases, reducing photosynthetic rate.

How does leaf size affect transpiration?

Larger leaves have more stomata and greater surface area, increasing transpiration rate.

How does leaf orientation affect transpiration?

Vertical leaves reduce sunlight exposure and heat load, lowering transpiration.

What kind of plants have the lowest transpiration rates?

Xerophytes (desert plants) due to thick cuticles, reduced leaves, and CAM metabolism.

What kind of plants have high transpiration rates?

Mesophytes and hydrophytes with thin cuticles and abundant stomata.