1. Xylem and Water Transport

Xylem and Water Transport

Plants need a continuous supply of water and mineral ions from the soil.

The tissue mainly responsible for transporting these substances upward through the plant is called xylem.

Xylem carries:

• water
• dissolved mineral ions

from the roots to the stems and leaves.

This movement is essential for:

• photosynthesis
• maintaining cell turgor
• transporting minerals
• cooling the plant
• supporting growth

What Is Xylem?

Xylem is a plant transport tissue.

Its main functions are to:

• transport water from roots to leaves
• transport dissolved mineral ions
• provide structural support

Xylem is found in:

• roots
• stems
• leaves

It forms a continuous pathway through the plant.

Water can therefore move from the soil all the way to the leaves.

---

Structure of Xylem Vessels

Xylem contains long tubes called xylem vessels.

These vessels are formed from specialized cells joined end to end.

As the cells mature:

• they lose their cytoplasm
• their end walls break down
• the cells die
• a continuous hollow tube remains

This hollow structure allows water to move with very little resistance.

---

Important Features of Xylem Vessels

Xylem vessels are well adapted for transporting water.

They have:

• hollow interiors
• no cytoplasm
• no end walls between vessel cells
• thick walls
• lignin in their walls
• small openings called pits

Each feature supports the function of the tissue.

---

Hollow Interior

Because mature xylem vessels contain no cytoplasm, the centre of the vessel is hollow.

This creates an open pathway for water.

Therefore:

hollow vessel → less resistance → efficient water movement

---

No End Walls

The cells forming xylem vessels are connected end to end.

Their end walls disappear during development.

This creates one long continuous tube.

Water can therefore move upward without repeatedly crossing cell membranes.

---

Lignin

Xylem walls contain a strong substance called lignin.

Lignin:

• strengthens the vessel
• prevents the vessel from collapsing
• helps support the plant

Water in xylem can be under tension as it is pulled upward.

Without strong walls, the vessels could collapse.

Lignin may form:

• rings
• spirals
• continuous thickened areas

along the vessel wall.

---

Pits in Xylem

Small thinner areas called pits occur in the walls of xylem vessels.

Pits allow water to move sideways between:

• neighbouring xylem vessels
• xylem and surrounding tissues

This is useful because water does not always need to move only upward.

---

How Water Enters the Roots

Water usually enters a plant through specialized cells called root hair cells.

Root hair cells are found near the tips of young roots.

They have long extensions that reach between soil particles.

These extensions greatly increase the surface area available for absorption.

---

Water Enters by Osmosis

Water enters root hair cells mainly by osmosis.

Osmosis is the net movement of water across a partially permeable membrane from an area of higher water concentration to an area of lower water concentration.

The soil solution often has a higher water concentration than the inside of the root hair cell.

Therefore:

soil → root hair cell

Water crosses the cell membrane by osmosis.

---

Mineral Ions Enter the Roots

Plants also need mineral ions such as:

• nitrate ions
• magnesium ions
• potassium ions
• phosphate ions

Some mineral ions are absorbed by active transport.

Active transport can move ions from:

lower concentration → higher concentration

This requires energy from cellular respiration.

Once dissolved in water, these mineral ions can be carried upward in the xylem.

---

From Root Hair Cells to Xylem

After entering a root hair cell, water moves across the root toward the xylem.

A simplified pathway is:

soil → root hair cell → root tissues → xylem

Water can move:

• through cell walls
• through cytoplasm
• from cell to cell by osmosis

Eventually, it reaches the xylem vessels in the centre of the root.

---

Movement of Water Up the Plant

Once water enters the xylem, it moves upward through:

• roots
• stems
• leaves

This upward flow of water is called the transpiration stream.

The main driving force is water loss from the leaves.

---

What Is Transpiration?

Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through the stomata of leaves.

Water evaporates from moist cell surfaces inside the leaf.

The water vapour then diffuses out through the stomata.

As water leaves the leaf, more water is pulled upward through the xylem.

---

The Transpiration Stream

The overall pathway is:

soil → roots → xylem → stem → leaves → atmosphere

Water enters the roots.

Water enters the xylem.

Water travels upward through the stem.

Water moves into leaf tissues.

Water evaporates inside the leaf.

Water vapour exits through stomata.

This continuous movement is called the transpiration stream.

---

Transpiration Pull

When water evaporates from the leaf, it creates a pulling force.

This force is called transpiration pull.

Water molecules are attracted to one another.

This attraction is called cohesion.

Because water molecules stick together, water forms a continuous column inside the xylem.

When water is lost from the top of the column, the rest of the water is pulled upward.

Therefore:

transpiration → pull → upward movement of water

---

Cohesion and Adhesion

Two properties of water help xylem transport.

Cohesion

Cohesion is the attraction between water molecules.

It helps water remain as a continuous column.

Adhesion

Adhesion is the attraction between water molecules and the walls of the xylem vessel.

It helps water remain in contact with the vessel walls.

Together, cohesion and adhesion help support water movement through the plant.

---

Water Movement in a Leaf

Water arriving in a leaf moves from the xylem into surrounding cells.

Some water is used in photosynthesis.

Much of it evaporates from the surfaces of mesophyll cells.

The water vapour enters air spaces in the leaf.

It then diffuses out through stomata.

---

Why Water Transport Is Essential

Water transport is necessary for several important plant processes.

These include:

• photosynthesis
• maintaining turgor
• mineral transport
• cooling
• growth

Without an effective water transport system, a large plant could not survive.

---

Water and Photosynthesis

Water is a reactant in photosynthesis.

A simplified equation is:

carbon dioxide + water → glucose + oxygen

Light energy and chlorophyll are required.

Water absorbed by the roots is transported through xylem to the leaves.

Without enough water, photosynthesis decreases.

---

Water and Turgor

Plant cells often contain large vacuoles filled with cell sap.

When water enters plant cells by osmosis, the cells become turgid.

Turgor pressure pushes the cell contents against the cell wall.

This helps:

• support leaves
• support young stems
• keep the plant upright

If too much water is lost, cells become flaccid and the plant may wilt.

---

Water and Mineral Transport

Mineral ions absorbed by roots dissolve in water.

They are then transported through the xylem.

For example:

nitrate ions are needed to make amino acids and proteins.

magnesium ions are needed to make chlorophyll.

Therefore, transporting water also allows the plant to distribute essential minerals.

---

Water and Cooling

Evaporation requires energy.

When water evaporates from leaves during transpiration, heat energy is removed.

This can help cool the plant.

Transpiration therefore has a cooling effect similar to sweating in animals.

---

Factors Affecting Transpiration

Several environmental factors can change the rate of transpiration.

Important factors include:

• temperature
• wind speed
• humidity
• light intensity

---

Temperature

Higher temperature usually increases transpiration.

At higher temperatures:

• water evaporates more quickly
• water molecules have more kinetic energy

Therefore:

higher temperature → faster evaporation → increased transpiration

---

Wind Speed

Wind can remove humid air from around a leaf.

This maintains a strong concentration gradient for water vapour.

Therefore:

greater wind speed → greater transpiration

---

Humidity

Humidity describes the amount of water vapour in the air.

High humidity means the air already contains a lot of water vapour.

This reduces the concentration gradient between the inside and outside of the leaf.

Therefore:

higher humidity → lower transpiration

---

Light Intensity

Stomata often open in the light to allow carbon dioxide to enter for photosynthesis.

Open stomata also allow more water vapour to escape.

Therefore, increasing light intensity often increases transpiration.

---

Xylem Structure and Function

The structure of xylem is closely matched to its function.

This is another example of the biological principle:

structure supports function

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Xylem and Phloem

Xylem is one of the two major transport tissues in plants.

The other is phloem.

Do not confuse the two systems.

---

Worked Example: Root Water Uptake

A root hair cell is surrounded by moist soil.

The soil has a higher water concentration than the root hair cell.

How does water enter?

Water moves across the cell membrane by osmosis.

Therefore:

soil → root hair cell by osmosis

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Worked Example: Xylem Structure

Why do mature xylem vessels lack cytoplasm?

Without cytoplasm, the inside of the vessel is hollow.

This provides a low-resistance pathway for water.

Therefore:

no cytoplasm → hollow tube → efficient transport

---

Worked Example: Lignin

Why does xylem contain lignin?

Water in the xylem can be under tension due to transpiration pull.

Lignin strengthens the walls and prevents the vessels from collapsing.

It also helps support the plant.

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Worked Example: A Hot Day

Predict what happens to transpiration on a hot, dry, windy day.

High temperature increases evaporation.

Dry air creates a steep water-vapour concentration gradient.

Wind removes humid air from around the leaf.

Therefore, transpiration will usually increase.

If water loss becomes greater than water uptake, the plant may wilt.

---

Worked Example: Wilting

A plant loses water faster than its roots can replace it.

What happens?

Plant cells lose water.

Their vacuoles shrink.

Turgor pressure decreases.

Cells become flaccid.

Leaves and stems may droop.

The plant wilts.

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Common Misconceptions

Xylem transports sugar.

Incorrect. Xylem mainly transports water and mineral ions. Phloem transports sugars.

Water enters roots by active transport.

Water mainly enters root hair cells by osmosis. Some mineral ions may enter by active transport.

Xylem vessels are living tubes full of cytoplasm.

Mature xylem vessel cells are dead and hollow.

Water is pushed upward only by the roots.

Root processes can contribute, but transpiration pull is the major force responsible for water movement in many plants.

All water absorbed by a plant is used in photosynthesis.

Only a small proportion is used directly in photosynthesis. Much of the water eventually leaves through transpiration.

Transpiration is useless water loss.

Although excessive water loss can be harmful, transpiration helps drive water and mineral transport and can cool the plant.

Did You Know?

Very tall trees can transport water from roots to leaves more than 100 metres above the ground.

They do not use a mechanical pump like a heart.

Instead, water is pulled upward through extremely narrow xylem vessels as evaporation from leaves creates tension in the continuous water column.

This makes xylem one of the most impressive transport systems in biology.

Key Terms

Xylem – Plant tissue that transports water and dissolved mineral ions.

Xylem vessel – A long hollow tube made from dead specialized cells.

Lignin – A strong substance that reinforces xylem walls.

Root hair cell – A specialized root cell with a long extension that increases absorption surface area.

Osmosis – Net movement of water across a partially permeable membrane from higher to lower water concentration.

Active transport – Movement of substances against a concentration gradient using energy.

Transpiration – Loss of water vapour from the aerial parts of a plant.

Transpiration stream – Continuous movement of water from roots through xylem to leaves.

Transpiration pull – Pulling force created as water evaporates from leaves.

Cohesion – Attraction between water molecules.

Adhesion – Attraction between water molecules and another surface.

Stoma – A pore in the leaf surface that allows gas exchange and water vapour loss.

Turgor pressure – Pressure produced when water-filled plant cells push against their cell walls.

Key Takeaways

• Xylem transports water and dissolved mineral ions from roots toward leaves.
• Xylem vessels are long, hollow tubes formed from dead cells.
• Mature xylem vessels have no cytoplasm and no complete end walls.
• Their walls contain lignin, which strengthens the vessels and prevents collapse.
• Water enters root hair cells mainly by osmosis.
• Mineral ions may enter roots by active transport.
• Water moves from root tissues into the xylem.
• Water moves upward through the plant in the transpiration stream.
• Water loss from leaves creates transpiration pull.
• Cohesion helps keep water molecules together as a continuous column.
• Water reaches leaves, where some is used in photosynthesis.
• Transpiration also helps transport minerals and cool the plant.
• Water maintains turgor pressure, helping support plant tissues.
• Temperature, wind, humidity, and light can affect transpiration rate.
• Xylem structure is closely adapted to its transport function.
• Efficient water transport is essential for plant growth and survival.

2. Transpiration

3. Factors Affecting Transpiration

What Is Transpiration?

Transpiration is the loss of water vapour from the leaves and other aerial parts of a plant.

Most transpiration occurs through tiny openings in leaves called:

stomata

Water moves from the roots, through the plant, and eventually into the atmosphere.

A simplified pathway is:

soil → roots → xylem → leaves → stomata → atmosphere

Transpiration is therefore part of the continuous movement of water through a plant.

---

The Transpiration Stream

Water absorbed by the roots moves upward through tubes called:

xylem vessels

This movement of water through the plant is called the:

transpiration stream

The transpiration stream helps transport:

• water
• dissolved mineral ions

from the roots to the leaves and other parts of the plant.

---

How Does Water Leave a Leaf?

Water arrives in the leaf through the:

xylem

It moves into cells in the leaf.

Some water evaporates from moist cell surfaces into the air spaces inside the leaf.

Water vapour then diffuses through the stomata into the surrounding atmosphere.

The sequence is:

xylem → mesophyll cells → evaporation → leaf air spaces → stomata → atmosphere

---

Evaporation and Diffusion

Two important processes are involved in transpiration.

Evaporation

Liquid water changes into:

water vapour

Diffusion

Water vapour moves from an area of:

higher water-vapour concentration

to an area of:

lower water-vapour concentration

The air spaces inside a leaf are usually humid.

If the surrounding air is drier, water vapour diffuses out through the stomata.

---

Stomata

Stomata are microscopic pores in the epidermis of leaves.

A single pore is called a:

stoma

Each stoma is surrounded by:

guard cells

Guard cells can change shape and influence whether the stomatal pore is:

• open
• partly open
• closed

Stomata allow gases to move between the leaf and atmosphere.

---

Why Do Plants Have Stomata?

Plants need carbon dioxide for:

photosynthesis

Carbon dioxide enters through stomata.

However, when stomata are open, water vapour can also escape.

This creates an important biological trade-off:

open stomata → CO₂ enters more easily, but water can also be lost

Plants therefore have mechanisms that help regulate stomatal opening.

---

What Is Transpiration Rate?

Transpiration rate describes how quickly a plant loses water through transpiration.

For example, we could measure:

water lost per hour

or use another measurement that estimates the rate of water loss.

Transpiration rate is affected by environmental conditions.

Major factors include:

• temperature
• humidity
• wind speed
• light intensity

---

Temperature

In general:

higher temperature → higher transpiration rate

Why?

At higher temperatures, water molecules have greater average kinetic energy.

This generally increases the rate of:

evaporation

from moist surfaces inside the leaf.

More water vapour becomes available in the leaf air spaces and can diffuse out through open stomata.

---

Temperature and Particle Movement

At a higher temperature:

water molecules have greater average kinetic energy

↓

evaporation generally increases

↓

more water vapour enters leaf air spaces

↓

water vapour diffuses through stomata

↓

transpiration rate generally increases

This connects transpiration to the particle model.

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Temperature Example

Imagine two identical plants.

Plant A is kept at:

20°C

Plant B is kept at:

30°C

All other conditions are kept approximately the same.

We would generally predict:

Plant B has the higher transpiration rate.

This is because the higher temperature promotes evaporation.

---

Extremely High Temperatures

The relationship is not always as simple as:

hotter = unlimited increase in transpiration

Under hot, dry conditions, a plant may begin losing water faster than its roots can replace it.

The plant may respond by closing some stomata.

Therefore, under severe water stress:

stomatal closure can reduce transpiration

This is an example of how biological systems can modify simple physical relationships.

---

Humidity

Humidity describes the amount of water vapour in the air.

Humidity strongly affects the concentration gradient for water vapour between:

inside the leaf

and:

the surrounding air

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Low Humidity

Low humidity means the surrounding air is relatively dry.

Inside the leaf:

high water-vapour concentration

Outside the leaf:

lower water-vapour concentration

This creates a relatively steep concentration gradient.

Therefore:

low humidity → steeper water-vapour gradient → faster diffusion → higher transpiration

---

High Humidity

When humidity is high, the surrounding air already contains substantial water vapour.

The difference between the leaf and surrounding air becomes smaller.

Therefore:

high humidity → smaller water-vapour gradient → slower diffusion → lower transpiration

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Comparing Humidity

Imagine two identical plants.

Plant A:

30% relative humidity

Plant B:

90% relative humidity

If other conditions are the same, Plant A would generally lose water more rapidly.

Why?

Because the surrounding air is much drier.

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Humidity and Diffusion

Remember:

diffusion occurs down a concentration gradient

A larger difference in water-vapour concentration generally produces faster net diffusion.

Therefore:

dry air

creates a larger gradient.

humid air

creates a smaller gradient.

This is why humidity has such a strong influence on transpiration.

---

Wind Speed

Wind also affects transpiration.

In general:

higher wind speed → higher transpiration rate

To understand why, we need to consider the air immediately surrounding the leaf.

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The Boundary Layer

As water vapour leaves a leaf, the air immediately next to the leaf becomes more humid.

This creates a thin region of relatively still, humid air called the:

boundary layer

The boundary layer reduces the water-vapour concentration gradient between the leaf and the surrounding air.

This can slow further water loss.

---

What Does Wind Do?

Moving air removes some of the humid air surrounding the leaf.

Therefore:

wind removes humid boundary layer

↓

drier air replaces it

↓

water-vapour concentration gradient increases

↓

diffusion increases

↓

transpiration increases

---

Still Air

In still conditions, humid air can accumulate around the leaf.

Therefore:

still air → thicker humid boundary layer → smaller gradient → slower transpiration

---

Windy Air

In windy conditions, the humid boundary layer is continually disturbed or removed.

Therefore:

windy air → thinner boundary layer → steeper gradient → faster transpiration

---

Wind Speed Example

Plant A is placed in still air.

Plant B is placed in front of a gently operating fan.

Everything else is kept constant.

We would generally predict:

Plant B loses water faster.

The fan removes humid air from around the leaves.

---

Very Strong Wind

Extremely strong wind can sometimes produce more complicated responses.

For example, strong wind may:

• cause stomata to close
• damage leaves
• increase water stress

Therefore, the simple relationship:

more wind → more transpiration

is most useful when comparing moderate environmental changes while other factors remain controlled.

---

Light Intensity

Although the main targets emphasize temperature, humidity, and wind, another important environmental factor is:

light intensity

In many plants:

greater light intensity → greater stomatal opening → increased transpiration

Stomata commonly open in light because carbon dioxide is required for photosynthesis.

Open stomata also allow more water vapour to escape.

---

Light and Transpiration

The general relationship is:

increased light

↓

stomata tend to open

↓

greater pathway for water vapour diffusion

↓

transpiration generally increases

At night, stomata of many plants partially or mostly close.

This generally reduces transpiration.

There are exceptions among plants with different photosynthetic adaptations.

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The Four Major Factors

A useful summary is:

These relationships describe general trends, assuming other conditions remain reasonably similar.

---

A Simple Memory Pattern

Conditions that tend to increase transpiration are:

Hot + Dry + Windy + Bright

Conditions that tend to decrease transpiration are:

Cool + Humid + Still + Dark

This is a useful starting point for predictions.

---

Predicting Transpiration

Consider a plant on a:

hot, dry, windy afternoon

Temperature:

high

Humidity:

low

Wind:

high

All three factors tend to:

increase transpiration

Therefore, we predict:

a high rate of water loss

---

Example 2: Cool and Humid

A plant is placed in:

cool, humid, still air

Temperature:

low → decreases evaporation

Humidity:

high → decreases the water-vapour gradient

Wind:

low → allows a humid boundary layer to remain

Prediction:

low transpiration rate

---

Example 3: Hot but Humid

Suppose conditions are:

high temperature + high humidity

Temperature tends to:

increase transpiration

Humidity tends to:

decrease transpiration

The factors are working in opposite directions.

Without more information, we cannot always determine exactly how large the overall change will be.

This is an important scientific idea:

multiple variables can influence the same process simultaneously.

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Example 4: Cool but Windy

Conditions:

low temperature + high wind speed

Lower temperature tends to:

decrease transpiration

Wind tends to:

increase transpiration

Again, the factors act in opposite directions.

We should avoid assuming that one factor always determines the final result.

---

Environmental Factors Interact

In nature, temperature, humidity, wind, and light do not normally change independently.

For example, a sunny afternoon may involve:

• increased light
• increased temperature
• decreased relative humidity
• increased wind

Several factors may therefore increase transpiration simultaneously.

This can produce rapid water loss.

---

Why Does Transpiration Matter?

Transpiration is not simply wasted water.

It contributes to several important processes.

Water transport

Transpiration helps maintain movement of water through the xylem.

Mineral transport

Mineral ions dissolved in water are transported from roots toward other plant tissues.

Cooling

Evaporation of water from leaves can remove thermal energy.

Water supply

Water transported to leaves is needed for processes including photosynthesis.

---

Transpiration and Cooling

Evaporation requires energy.

When water evaporates from leaf surfaces, thermal energy is transferred.

This can help cool the leaf.

This is similar to the cooling effect of:

sweating in humans

Both involve evaporative cooling.

---

The Problem of Excessive Water Loss

Plants require water for:

• photosynthesis
• transport
• maintaining cell turgor
• biochemical reactions
• growth

If transpiration becomes too rapid:

water loss > water absorption

The plant may become water stressed.

---

Turgor Pressure

Plant cells contain large vacuoles.

When cells contain sufficient water, internal pressure helps keep tissues firm.

This is called:

turgor pressure

If too much water is lost:

cells lose water

↓

turgor decreases

↓

leaves and stems may wilt

---

Wilting

Wilting occurs when plant tissues lose enough turgor that leaves and stems become less rigid.

Wilting can reduce:

• leaf exposure to sunlight
• stomatal opening
• photosynthesis

Temporary wilting may be reversible if water becomes available.

Prolonged severe water shortage can damage tissues.

---

Plants Can Reduce Water Loss

Plants have several mechanisms and adaptations that reduce excessive transpiration.

These can include:

• closing stomata
• waxy cuticles
• reduced leaf area
• hairs on leaves
• sunken stomata
• rolled leaves

These features are particularly important in plants living in dry environments.

---

The Waxy Cuticle

Leaves are covered by a protective layer called the:

cuticle

The cuticle contains waxy substances that reduce water loss directly through the epidermis.

Plants adapted to dry environments often have particularly thick waxy cuticles.

---

Sunken Stomata

Some plants have stomata located in small depressions.

These are called:

sunken stomata

Water vapour can accumulate inside the depression.

This creates a more humid microenvironment.

Therefore:

sunken stomata → more humid trapped air → smaller concentration gradient → reduced transpiration

---

Leaf Hairs

Leaf hairs can trap relatively still air near the leaf surface.

This helps maintain a:

humid boundary layer

Therefore:

leaf hairs → reduced air movement → reduced water-vapour gradient → reduced transpiration

This is similar to the effect of still air.

---

Reduced Leaf Area

Some plants living in dry environments have very small leaves.

Others have leaves modified into:

spines

Smaller leaf area means:

less surface area for water loss

This helps reduce transpiration.

---

Desert Plant Adaptations

Plants adapted to dry conditions are called:

xerophytes

Possible adaptations include:

• thick waxy cuticles
• reduced leaves
• sunken stomata
• leaf hairs
• water-storage tissues
• extensive root systems

These adaptations help plants survive where water is limited.

---

Measuring Transpiration

Scientists can investigate transpiration using an instrument called a:

potometer

A potometer measures the rate at which a plant shoot takes up water.

Water uptake is used as an estimate of transpiration rate.

It is not exactly the same as directly measuring water loss because some absorbed water is used by the plant.

---

Bubble Potometer

A common potometer contains:

• leafy shoot
• water-filled tubing
• capillary tube
• air bubble
• scale

As the shoot takes up water, the air bubble moves along the capillary tube.

Faster bubble movement indicates:

faster water uptake

which usually indicates:

higher transpiration rate

---

Calculating Water Uptake

Suppose an air bubble moves:

24 mm in 6 minutes

Average movement rate:

24 ÷ 6 = 4 mm/min

If another condition produces:

8 mm/min

the water uptake is faster under the second condition.

This can be used to compare environmental factors.

---

Investigating Temperature

To investigate temperature:

Independent variable:

temperature

Dependent variable:

rate of water uptake

Control variables might include:

• plant species
• leaf area
• light intensity
• wind speed
• humidity
• duration of experiment

Only one factor should intentionally change.

---

Investigating Wind Speed

A fan can be used to investigate the effect of air movement.

For example:

Trial 1:

fan off

Trial 2:

fan at low speed

Trial 3:

fan at higher speed

Prediction:

As wind speed increases, water uptake will generally increase because humid air is removed from around the leaves.

---

Investigating Humidity

Humidity can be changed by altering the moisture in the air surrounding the plant.

Prediction:

higher humidity → lower transpiration rate

because the water-vapour concentration gradient between the leaf and atmosphere becomes smaller.

---

Investigating Light Intensity

A lamp can be used to investigate light intensity.

However, there is an experimental problem.

A lamp can also:

heat the plant

Therefore, changing light intensity may accidentally change:

temperature

A good investigation must control this variable.

---

Designing a Fair Test

When investigating one factor:

change one variable

measure one outcome

keep other important factors constant

For example, when investigating wind speed:

Change:

wind speed

Measure:

water uptake

Keep constant:

• temperature
• humidity
• light intensity
• plant species
• approximate leaf area

This improves the validity of the investigation.

---

Reliability

A single measurement may be affected by random variation.

Scientists can improve reliability by:

• repeating measurements
• conducting multiple trials
• calculating a mean
• identifying unusual results

For example:

Trial 1 = 4.2 mm/min

Trial 2 = 4.4 mm/min

Trial 3 = 4.3 mm/min

Mean:

(4.2 + 4.4 + 4.3) ÷ 3

= 4.3 mm/min

---

Graphing Transpiration Data

Experimental results can be represented using graphs.

For example:

x-axis: temperature

y-axis: transpiration rate

A graph allows us to:

• identify trends
• compare conditions
• detect anomalies
• make predictions

---

Predicting a Temperature Graph

Within a normal physiological range, we would generally expect:

temperature increases → transpiration rate increases

The graph would therefore show an overall:

positive relationship

However, at extreme temperatures, stomatal closure or plant stress may alter the pattern.

---

Predicting a Humidity Graph

As humidity increases:

transpiration rate generally decreases

Therefore, we expect an overall:

negative relationship

High humidity reduces the water-vapour concentration gradient.

---

Predicting a Wind-Speed Graph

As moderate wind speed increases:

transpiration rate generally increases

The graph should show an overall positive relationship.

Wind removes humid air from the boundary layer surrounding the leaf.

---

Applying the Ideas: Greenhouses

Greenhouse growers can control environmental conditions such as:

• temperature
• humidity
• ventilation
• light
• irrigation

If a greenhouse becomes:

very hot + dry

plants may lose water rapidly.

Increasing humidity or reducing excessive temperature can reduce water loss.

Understanding transpiration therefore has practical importance in agriculture.

---

Applying the Ideas: Irrigation

Farmers need to consider environmental conditions when deciding how much water crops require.

Hot, dry, windy conditions can increase water loss.

Therefore, crops may require more water during periods of high transpiration.

This connects plant biology to:

• agriculture
• weather
• water management
• food production

---

Applying the Ideas: Indoor Plants

Imagine placing a houseplant beside:

a heater

The surrounding air may become:

• warmer
• drier

Both conditions can increase transpiration.

Therefore, the plant may require more frequent watering than the same plant in cooler, more humid conditions.

---

Applying the Ideas: Windy Conditions

A plant on an exposed balcony experiences strong air movement.

Wind removes the humid boundary layer around the leaves.

Therefore:

wind → increased concentration gradient → increased transpiration

The plant may lose water more rapidly than a similar plant in a sheltered location.

---

Connecting Transpiration to Diffusion

Transpiration provides an excellent biological example of diffusion.

Inside the leaf:

high water-vapour concentration

Outside the leaf:

usually lower water-vapour concentration

Therefore:

water vapour diffuses out

The size of this gradient is influenced by:

humidity and wind

---

Connecting Transpiration to Evaporation

Temperature affects transpiration partly because it affects:

evaporation

Higher temperatures increase the average kinetic energy of water molecules.

This increases the likelihood that molecules escape from liquid water into the gas phase.

Therefore:

temperature → evaporation → transpiration

---

Connecting Transpiration to Osmosis

Water entering plant roots moves through tissues partly by:

osmosis

Water then enters the xylem and travels upward.

Therefore, plant water transport connects several processes:

osmosis → xylem transport → evaporation → diffusion

---

Connecting Transpiration to Photosynthesis

Plants need carbon dioxide for photosynthesis.

Carbon dioxide enters through:

stomata

But open stomata also allow:

water vapour to escape

Therefore, plants face a trade-off:

CO₂ uptake ↔ water conservation

This is one of the major challenges faced by land plants.

---

Common Misconception: Plants Lose Water Only When It Is Hot

Plants can transpire under many environmental conditions.

Temperature affects the rate of transpiration, but it is not the only factor.

Transpiration is also influenced by:

• humidity
• wind
• light
• stomatal opening
• leaf structure

---

Common Misconception: High Humidity Increases Transpiration

High humidity generally:

decreases transpiration

because the air surrounding the leaf already contains substantial water vapour.

This reduces the concentration gradient.

Remember:

Dry air → faster water loss

Humid air → slower water loss

---

Common Misconception: Wind Stops Water Loss

Wind generally increases transpiration because it removes the humid boundary layer around the leaf.

Therefore:

wind → drier air beside leaf → steeper gradient → increased water loss

---

Common Misconception: Transpiration Is Completely Harmful

Water loss can become harmful if excessive.

However, transpiration also contributes to:

• water movement
• mineral transport
• leaf cooling

Therefore, transpiration is a normal and important plant process.

---

Common Misconception: Potometers Directly Measure Transpiration

A potometer measures:

water uptake

It does not directly measure water leaving the leaf.

Because most water absorbed by a cut shoot is eventually lost through transpiration, water uptake provides a useful estimate.

But scientifically:

water uptake ≠ exactly the same as water loss

---

Challenge: Predict the Highest Transpiration

Four identical plants are placed under different conditions.

Plant A: cool, humid, still

Plant B: warm, humid, still

Plant C: warm, dry, windy

Plant D: cool, dry, still

Which would probably have the highest transpiration rate?

Plant C

because it combines:

• higher temperature
• low humidity
• high wind speed

All three factors generally increase transpiration.

---

Challenge: Predict the Lowest Transpiration

Which plant would probably have the lowest transpiration rate?

Plant A

because:

• cool temperature reduces evaporation
• high humidity reduces the concentration gradient
• still air maintains the humid boundary layer

---

Did You Know?

A mature tree can move a surprisingly large amount of water from the soil into the atmosphere through transpiration.

Across entire forests, transpiration contributes significantly to the movement of water into the atmosphere.

This means plant transpiration is not only important for individual plants.

It also contributes to the:

water cycle

---

Key Terms

• Transpiration: Loss of water vapour from the aerial parts of a plant, mainly through stomata.
• Transpiration rate: Rate at which water is lost through transpiration.
• Transpiration stream: Movement of water through a plant from roots through xylem toward the leaves.
• Xylem: Vascular tissue that transports water and mineral ions.
• Stoma: Microscopic pore in the epidermis of a leaf.
• Stomata: Plural of stoma.
• Guard cells: Specialized cells that control stomatal opening.
• Evaporation: Change of liquid water into water vapour.
• Diffusion: Net movement of particles from higher concentration toward lower concentration.
• Humidity: Amount of water vapour present in the air.
• Relative humidity: Amount of water vapour present relative to the maximum possible at that temperature.
• Concentration gradient: Difference in concentration between two regions.
• Boundary layer: Thin layer of relatively still air surrounding a surface such as a leaf.
• Turgor pressure: Pressure of cell contents against the cell wall resulting from water inside the cell.
• Wilting: Loss of rigidity in plant tissues due to reduced turgor.
• Cuticle: Waxy protective layer covering the epidermis of leaves and young stems.
• Xerophyte: Plant adapted to dry environments.
• Potometer: Apparatus used to estimate transpiration by measuring water uptake.
• Independent variable: Variable deliberately changed in an investigation.
• Dependent variable: Variable measured in response to the independent variable.
• Control variable: Variable kept constant to make an investigation fair.

---

Key Relationships

Temperature:

↑ temperature → ↑ evaporation → generally ↑ transpiration

Humidity:

↑ humidity → ↓ water-vapour concentration gradient → ↓ transpiration

Wind:

↑ wind → boundary layer removed → ↑ concentration gradient → generally ↑ transpiration

Light:

↑ light → stomata tend to open → generally ↑ transpiration

Dry conditions:

low humidity → steep gradient → rapid diffusion of water vapour

Water stress:

water loss > water uptake → reduced turgor → wilting

Plant water pathway:

soil → roots → xylem → leaves → evaporation → stomata → atmosphere

---

Key Takeaways

• Transpiration is the loss of water vapour from the aerial parts of a plant.
• Most transpiration occurs through stomata in leaves.
• Water moves from the soil through the roots and xylem to the leaves.
• Water evaporates from moist surfaces inside the leaf.
• Water vapour then diffuses through stomata into the atmosphere.
• Transpiration rate is affected by environmental conditions.
• Temperature, humidity, wind speed, and light intensity are important environmental factors.
• Higher temperatures generally increase evaporation and therefore increase transpiration.
• High humidity generally decreases transpiration.
• Low humidity creates a larger water-vapour concentration gradient and increases water loss.
• Wind generally increases transpiration by removing the humid boundary layer surrounding a leaf.
• Still air allows humid air to remain near the leaf and reduces transpiration.
• Increased light often increases transpiration because stomata tend to open.
• A useful general rule is hot + dry + windy + bright = higher transpiration.
• Cool + humid + still + dark = lower transpiration.
• Environmental factors can work together or oppose one another.
• Extreme environmental conditions can cause stomatal closure, making relationships more complex.
• Transpiration contributes to water and mineral transport through the plant.
• Evaporation during transpiration can help cool leaves.
• Excessive water loss can cause cells to lose turgor and plants to wilt.
• Plants can reduce water loss using stomatal closure and structural adaptations.
• Xerophytes may have thick cuticles, sunken stomata, leaf hairs, or reduced leaves.
• A potometer measures water uptake and can be used to estimate transpiration rate.
• Experiments investigating transpiration should change one environmental factor while controlling the others.
• Repeated measurements improve the reliability of experimental results.
• Transpiration connects several biological processes, including osmosis, xylem transport, evaporation, diffusion, gas exchange, and photosynthesis.
• The central idea is: environmental conditions change evaporation and diffusion, which changes how quickly a plant loses water.

4. Phloem and Translocation

5. Water Balance in Plants

What Is Water Balance?

Plants continuously:

take in water

and:

lose water

Water enters mainly through the roots and is lost mainly through transpiration from the leaves.

A plant maintains water balance when its water uptake is sufficient to replace the water it loses while supporting normal cellular functions.

A simplified relationship is:

water uptake ↔ water loss

If water loss becomes much greater than water uptake, the plant experiences:

water stress

---

Why Do Plants Need Water?

Water is essential for almost every aspect of plant life.

Plants need water for:

• photosynthesis
• transport of mineral ions
• maintaining cell shape
• maintaining turgor pressure
• cooling through transpiration
• chemical reactions
• growth
• movement of substances between cells

Water is therefore not simply something plants absorb from the soil. It is a fundamental part of their structure and metabolism.

---

Water and Photosynthesis

Water is a reactant in photosynthesis.

The simplified equation is:

carbon dioxide + water → glucose + oxygen

Therefore, plants need a continuous water supply to support photosynthesis.

Water absorbed by roots is transported to leaves through:

xylem

---

Water and Mineral Transport

Mineral ions in soil dissolve in water.

Plants absorb mineral ions such as:

• nitrate
• magnesium
• potassium
• phosphate

These substances can then be transported through the plant.

Therefore:

water acts as a transport medium

for many dissolved substances.

---

Water and Plant Cells

Plant cells contain a large central:

vacuole

The vacuole contains:

cell sap

When water enters a plant cell by osmosis, the vacuole expands.

The cell contents push against the:

cell wall

This produces:

turgor pressure

---

Turgor Pressure

Turgor pressure is the pressure produced when the contents of a plant cell push against its cell wall.

Turgor helps:

• support leaves
• support young stems
• maintain cell shape
• keep plant tissues firm

A plant containing many turgid cells appears:

firm and upright

---

Turgid Cells

A plant cell with sufficient water is described as:

turgid

Water enters the cell by:

osmosis

The vacuole expands.

The cell membrane and cytoplasm push outward against the cell wall.

The rigid cell wall prevents the cell from bursting under normal conditions.

Therefore:

water enters → vacuole expands → turgor pressure increases → cell becomes firm

---

Why Don't Plant Cells Burst Easily?

Animal cells do not have cell walls.

Plant cells do.

The rigid cell wall resists expansion as water enters.

This creates pressure inside the cell while limiting further expansion.

The cell wall therefore provides:

structural support

and helps plant cells remain turgid.

---

Water Uptake by Roots

Most water enters plants through:

root hair cells

Root hairs increase the surface area available for absorption.

Water moves from the soil into root cells and eventually reaches:

xylem vessels

---

Water Moves by Osmosis

Osmosis is the net movement of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential.

In suitable soil conditions:

soil water → root hair cell

Water can then move through root tissues toward the xylem.

---

The Water Pathway Through a Plant

A simplified pathway is:

soil

↓

root hairs

↓

root tissues

↓

xylem

↓

stem

↓

leaves

↓

mesophyll cells

↓

leaf air spaces

↓

stomata

↓

atmosphere

Water therefore moves continuously through the:

soil–plant–atmosphere system

---

Water Loss Through Transpiration

Most water lost by plants leaves through:

transpiration

Transpiration is the loss of water vapour from aerial parts of a plant, mainly through stomata.

Water evaporates from moist surfaces inside the leaf.

Water vapour then diffuses through the stomata.

---

Water Balance

The plant's water status depends on the relationship between:

water absorbed by roots

and:

water lost through transpiration

If:

water uptake ≈ water loss

the plant can generally maintain adequate hydration.

If:

water uptake < water loss

the plant begins to lose water overall.

---

When Water Loss Exceeds Water Uptake

Suppose a plant is exposed to:

• high temperature
• low humidity
• strong wind
• intense sunlight

These conditions can increase transpiration.

If roots cannot absorb water rapidly enough:

water loss > water uptake

↓

cells lose water

↓

turgor pressure decreases

↓

tissues become less rigid

↓

plant wilts

---

What Is Wilting?

Wilting is the drooping of leaves and stems caused largely by loss of turgor in plant cells.

Wilting is particularly noticeable in:

• leaves
• young stems
• non-woody plants

These structures depend heavily on turgor for support.

---

How Wilting Happens

The sequence is:

transpiration continues

↓

water loss exceeds water uptake

↓

plant cells lose water

↓

vacuoles become smaller

↓

turgor pressure decreases

↓

cells become flaccid

↓

leaves and stems droop

This visible drooping is:

wilting

---

Turgid vs Flaccid Cells

Turgid cell

• large vacuole
• sufficient water
• strong pressure against cell wall
• firm tissue

Flaccid cell

• reduced water content
• smaller vacuole
• reduced turgor pressure
• less rigid tissue

A wilted plant contains many cells with reduced turgor.

---

Wilting Can Help Reduce Water Loss

Wilting is a sign of water stress, but drooping leaves can sometimes reduce further water loss.

A wilted leaf may:

• expose less surface area to direct sunlight
• experience lower temperatures
• have altered air movement around it

Plants may also close their stomata during water stress.

Together, these responses can reduce transpiration.

---

Temporary Wilting

Wilting does not always mean a plant is dying.

Imagine a plant on a hot afternoon.

Transpiration may temporarily exceed water absorption.

The plant wilts.

Later:

• temperature decreases
• transpiration decreases
• roots continue absorbing water

Cells may regain turgor.

The plant becomes upright again.

This is:

temporary wilting

---

Permanent Wilting

If water shortage becomes severe or continues for too long, the plant may be unable to recover fully.

This can lead to:

• prolonged loss of turgor
• reduced photosynthesis
• cellular damage
• leaf death
• reduced growth
• eventually death of the plant

Severe water stress is therefore much more serious than brief temporary wilting.

---

Stomata Help Control Water Balance

Plants can regulate water loss by controlling:

stomata

Each stoma is surrounded by:

guard cells

Guard cells can alter the size of the stomatal opening.

When stomata close:

less water vapour escapes

Therefore:

stomatal closure → reduced transpiration

---

The Stomatal Trade-Off

Closing stomata conserves water.

However, it creates another problem.

Plants need:

carbon dioxide

for photosynthesis.

Carbon dioxide normally enters through stomata.

Therefore:

closed stomata → less water loss

but also:

closed stomata → less CO₂ entry

This can reduce photosynthesis.

---

Plants Must Balance Two Needs

Plants therefore face a fundamental challenge:

obtain CO₂

while:

preventing excessive water loss

Open stomata:

more CO₂ enters + more water escapes

Closed stomata:

less water escapes + less CO₂ enters

Water balance is therefore closely connected to:

photosynthesis

---

How Plants Detect Water Stress

When water becomes limited, plants can produce chemical signals.

One important plant hormone is:

abscisic acid (ABA)

During water stress, ABA contributes to changes in guard cells that promote:

stomatal closure

This reduces water loss.

The response can be summarized:

water shortage → ABA signaling → stomata close → transpiration decreases

---

Environmental Conditions Affect Water Balance

Water balance changes with environmental conditions.

Important factors include:

• temperature
• humidity
• wind speed
• light intensity
• soil water availability

The plant must continually respond to these changing conditions.

---

Temperature

Higher temperatures generally increase evaporation.

Therefore:

higher temperature → greater evaporation → increased transpiration

This can increase the risk of water stress.

If temperatures become very high, plants may respond by closing stomata.

---

Humidity

Low humidity means the surrounding air is relatively dry.

This creates a large water-vapour gradient between the leaf and atmosphere.

Therefore:

low humidity → increased transpiration

High humidity generally reduces water loss.

---

Wind

Wind removes humid air from around the leaf.

Therefore:

increased wind → boundary layer removed → increased transpiration

Windy conditions can therefore increase the plant's need for water.

---

Soil Water Availability

Even if transpiration is relatively low, a plant can experience water stress if:

very little water is available in the soil

Dry soil makes it harder for roots to obtain water.

Therefore, plant water balance depends on both:

water loss

and:

water availability

---

Drought

A drought is a prolonged period of unusually low water availability.

During drought:

• soil water decreases
• water uptake becomes more difficult
• plants experience water stress
• stomata may close
• photosynthesis may decrease
• growth may slow
• leaves may wilt
• some leaves may be shed

Plants have evolved many responses and adaptations to survive water shortages.

---

Short-Term Responses to Water Shortage

Plants can respond quickly to changing water availability.

Short-term responses may include:

• stomatal closure
• reduced leaf orientation toward sunlight
• temporary wilting
• reduced growth

These responses help reduce immediate water loss.

---

Long-Term Responses

If water shortage continues, plants may make longer-term changes.

These can include:

• increased root growth relative to shoot growth
• reduced production of new leaves
• leaf shedding
• smaller leaves
• changes in metabolism

Some species are much better adapted to prolonged drought than others.

---

Root Growth and Water Availability

When surface soil becomes dry, some plants invest more resources in root growth.

Deeper or more extensive roots can increase access to water.

Different plants may have:

deep roots

or:

widely spreading shallow roots

depending on their environment.

Both strategies can improve water collection under particular conditions.

---

Water-Conserving Adaptations

Plants living in dry environments often have structural adaptations that reduce water loss.

Plants adapted to dry environments are called:

xerophytes

Examples of water-conserving adaptations include:

• thick waxy cuticles
• reduced leaf area
• sunken stomata
• leaf hairs
• rolled leaves
• water-storage tissues
• specialized stomatal behaviour
• extensive root systems

---

Thick Waxy Cuticle

The outer surface of a leaf is covered by a:

cuticle

The cuticle contains waxy substances.

A thick waxy cuticle reduces evaporation directly through the leaf surface.

Therefore:

thicker cuticle → reduced water loss

---

Reduced Leaf Area

Large leaves provide a large surface area for:

• light absorption
• gas exchange

but also provide more surface through which water can be lost.

Plants in dry environments may have:

• small leaves
• narrow leaves
• needle-shaped leaves
• spines

Therefore:

smaller leaf area → less potential surface for water loss

---

Spines

In plants such as many cacti, leaves are highly reduced and modified into:

spines

Photosynthesis occurs mainly in the:

green stem

Reducing leaves dramatically decreases the surface area available for transpiration.

Spines may also provide:

• protection from herbivores
• some shading

---

Sunken Stomata

Some xerophytes have stomata located in small pits or depressions.

These are:

sunken stomata

Water vapour can accumulate in the depression.

This creates a humid microenvironment.

Therefore:

sunken stomata → humid trapped air → smaller gradient → reduced transpiration

---

Leaf Hairs

Some plants have hairs around their stomata or across their leaf surfaces.

Leaf hairs trap:

still, humid air

This increases the thickness of the boundary layer.

Therefore:

leaf hairs → reduced air movement → smaller water-vapour gradient → reduced transpiration

---

Rolled Leaves

Some plants can roll their leaves during dry conditions.

Rolling the leaf can trap:

humid air

around stomata.

It can also reduce the surface directly exposed to:

• sunlight
• wind

Therefore, rolled leaves can reduce water loss.

---

Water Storage

Some plants store water in thick:

• leaves
• stems
• roots

Plants with specialized water-storage tissues are often called:

succulents

Stored water can be used during periods when soil water is unavailable.

---

Cacti

Cacti demonstrate several water-conserving adaptations.

These may include:

• thick water-storing stems
• reduced leaves or spines
• thick waxy surfaces
• extensive root systems
• specialized stomatal behaviour

These adaptations work together rather than independently.

---

CAM Plants

Some plants living in dry environments use a specialized photosynthetic strategy called:

CAM photosynthesis

Many CAM plants open their stomata mainly:

at night

rather than during the hottest part of the day.

Night air is usually:

• cooler
• often more humid

Therefore, opening stomata at night can reduce water loss.

---

Why Opening Stomata at Night Helps

During a hot day:

high temperature + low humidity → potentially rapid water loss

At night:

lower temperature + often higher humidity → slower water loss

Therefore:

night-time stomatal opening → improved water conservation

Plants such as many cacti and succulents use CAM metabolism.

---

Plants in Wet Environments

Not all plants need strong water-conservation adaptations.

Plants living where water is abundant may have:

• larger leaves
• thinner cuticles
• less extensive root systems
• less need to reduce transpiration

Adaptations therefore reflect the environment in which a plant lives.

---

Xerophytes and Hydrophytes

Plants adapted to dry conditions are:

xerophytes

Plants adapted to living in or around abundant water are:

hydrophytes

Their structures can be very different because they face different environmental challenges.

---

Comparing Water Availability

Dry environment

Major challenge:

prevent excessive water loss

Useful adaptations:

• thick cuticle
• reduced leaves
• sunken stomata
• water storage

Wet environment

Water conservation is less challenging.

Plants may invest more heavily in:

• large leaf surfaces
• gas exchange
• rapid growth

---

Water Balance and Photosynthesis

Water shortage can reduce photosynthesis in two major ways.

First:

water itself is required for photosynthesis

Second:

stomata close during water stress

When stomata close:

less CO₂ enters

Therefore:

water shortage → stomatal closure → less CO₂ → reduced photosynthesis

---

Water Balance and Growth

Growth requires:

• cell division
• cell expansion
• protein synthesis
• energy
• water

Water is particularly important for cell expansion.

When water is scarce, plants often reduce growth.

This conserves resources and decreases the production of new tissues that would lose additional water.

---

Water Balance and Mineral Uptake

Mineral ions are transported through the plant in water.

If water uptake decreases:

mineral transport may also decrease

This can affect:

• growth
• chlorophyll production
• protein synthesis
• enzyme function

Water shortage can therefore affect plant nutrition as well as hydration.

---

Too Much Water Can Also Be a Problem

Water balance is not only about drought.

Excessive soil water can also harm plants.

If soil becomes waterlogged, water fills spaces that normally contain:

air

Roots may receive insufficient:

oxygen

---

Waterlogging

Root cells require oxygen for:

aerobic respiration

If waterlogged soil contains too little oxygen:

respiration decreases

↓

less ATP becomes available

↓

active transport can be disrupted

↓

root function may decline

Therefore:

more water is not always better.

---

Overwatering

A plant that receives excessive water may show:

• yellowing leaves
• reduced growth
• root damage
• wilting

It may seem strange that an overwatered plant can wilt.

However, damaged roots may become less effective at absorbing water.

Therefore:

wet soil does not automatically mean healthy water uptake.

---

Responding to Changing Water Availability

Plants continuously adjust to their environment.

When water is abundant:

• cells remain turgid
• stomata can remain open when appropriate
• photosynthesis can proceed efficiently
• growth can continue

When water becomes scarce:

• stomata may close
• growth may slow
• leaves may wilt
• root growth may change
• leaves may be shed

This demonstrates:

homeostasis-like regulation in plants

although plants regulate their internal conditions differently from animals.

---

Scenario 1: Hot Afternoon

A plant experiences:

• high temperature
• intense sunlight
• low humidity

Prediction:

transpiration increases

If water uptake cannot match water loss:

turgor decreases

The plant may:

temporarily wilt and close stomata

---

Scenario 2: Rain After Drought

Rain increases soil water availability.

Roots can absorb more water.

Water moves into cells.

Vacuoles expand.

Turgor increases.

Therefore:

water uptake → increased turgor → leaves become firm again

provided the tissues have not been permanently damaged.

---

Scenario 3: Strong Wind

Wind removes humid air surrounding leaves.

This increases the water-vapour concentration gradient.

Therefore:

wind → increased transpiration

If soil water is limited:

water stress becomes more likely

The plant may respond by closing stomata.

---

Scenario 4: High Humidity

High humidity reduces the difference in water-vapour concentration between:

leaf air spaces

and:

the atmosphere

Therefore:

transpiration decreases

Water balance becomes easier to maintain, assuming other conditions remain suitable.

---

Scenario 5: Dry Soil

Suppose temperature and humidity remain unchanged, but soil water decreases.

Root water uptake becomes more difficult.

Therefore:

water uptake decreases

Even without an increase in transpiration, the plant can develop:

water stress

This demonstrates why water balance depends on both sides of the equation.

---

Scenario 6: Waterlogged Soil

The soil contains excessive water.

At first this might appear beneficial.

However:

water fills soil air spaces

↓

root oxygen availability decreases

↓

root respiration is affected

↓

root function declines

↓

water and mineral uptake may decrease

The plant may eventually show signs of stress.

---

Investigating Water Loss

Plant water loss can be investigated using a:

potometer

A potometer measures:

water uptake

which can be used to estimate transpiration rate.

Environmental variables can then be changed to investigate their effects.

---

Measuring Plant Mass

Another method is to measure changes in the mass of a plant.

For example:

Initial mass:

425 g

Mass after 2 hours:

417 g

Mass lost:

425 − 417 = 8 g

If evaporation directly from the soil is prevented, much of this decrease can be attributed to water lost through transpiration.

---

Calculating Water Loss Rate

Suppose a plant loses:

12 g of water in 3 hours

Average rate:

12 ÷ 3

= 4 g/hour

If another plant loses:

6 g in 3 hours

Rate:

6 ÷ 3

= 2 g/hour

The first plant has approximately twice the water-loss rate.

---

Investigating Leaf Adaptations

Students could compare leaves from plants living in different environments.

Look for:

• leaf size
• leaf thickness
• waxy surfaces
• hairs
• stomatal position
• water-storage tissue

Then ask:

How might each feature affect water loss?

This connects observable structures to their functions.

---

Common Misconception: Wilting Means the Plant Is Dead

Wilting means that tissues have lost:

turgor

If water becomes available before severe damage occurs, cells can regain water.

Therefore:

wilting can be reversible.

---

Common Misconception: Plants Should Always Have Open Stomata

Open stomata allow:

CO₂ uptake

but also:

water loss

Plants must balance these competing requirements.

During severe water stress, conserving water may become more important than maximizing photosynthesis.

---

Common Misconception: More Water Is Always Better

Plants need water, but roots also need:

oxygen

Excessive water can create waterlogged soil and interfere with root respiration.

Healthy plants require an appropriate balance of:

water + air + nutrients

around their roots.

---

Common Misconception: Cactus Spines Are Only for Protection

Spines can discourage herbivores.

However, they also represent greatly reduced leaves.

Reduced leaf area helps decrease:

transpiration

Therefore, cactus spines can contribute to water conservation.

---

Common Misconception: Plants Only Respond to Water Shortage by Wilting

Plants can respond before severe wilting occurs.

Responses include:

• stomatal closure
• hormonal signaling
• reduced growth
• altered root growth
• leaf orientation changes
• leaf shedding

Wilting is only one visible part of the response.

---

Connecting Water Balance to Osmosis

Osmosis explains how water affects individual plant cells.

When water enters:

vacuole expands → turgor increases

When water leaves:

vacuole shrinks → turgor decreases

Therefore:

osmosis at the cellular level → support at the whole-plant level

---

Connecting Water Balance to Transpiration

Transpiration causes water to leave the plant.

Water must therefore continually be replaced.

The balance is:

root absorption → xylem → leaves → transpiration

If this flow remains adequate, the plant maintains hydration.

If loss becomes too rapid:

water stress develops

---

Connecting Water Balance to Xylem

Xylem carries water from roots toward leaves.

Therefore, water balance depends on an effective:

root–xylem–leaf pathway

Damage to:

• roots
• xylem
• leaves

can affect the plant's ability to regulate water.

---

Connecting Water Balance to Phloem

Phloem transport also depends partly on water.

Water enters phloem during pressure-flow processes.

Therefore, severe water stress can influence not only:

transpiration and photosynthesis

but also the movement of:

sugars around the plant

Plant systems are interconnected.

---

Did You Know?

Some desert plants can survive extremely long dry periods by combining several different adaptations.

A cactus may simultaneously use:

• a thick waxy surface
• water-storage tissue
• reduced leaves
• widespread roots
• night-time stomatal opening

No single adaptation explains its success.

It is the combination of adaptations that allows it to survive severe water limitation.

---

Key Terms

• Water balance: Relationship between water entering and leaving a plant.
• Water stress: Condition in which water availability is insufficient to meet the plant's requirements.
• Turgor pressure: Pressure produced when cell contents push against the cell wall.
• Turgid: Firm state of a plant cell containing sufficient water.
• Flaccid: Condition of a plant cell with reduced turgor pressure.
• Wilting: Drooping of plant tissues caused largely by reduced turgor.
• Osmosis: Net movement of water across a partially permeable membrane down a water-potential gradient.
• Transpiration: Loss of water vapour from aerial parts of a plant.
• Stoma: Microscopic pore involved in gas exchange and transpiration.
• Guard cell: Specialized cell controlling stomatal opening.
• Xylem: Tissue transporting water and mineral ions through a plant.
• Cuticle: Waxy protective covering that reduces water loss.
• Drought: Prolonged period of unusually low water availability.
• Xerophyte: Plant adapted to dry conditions.
• Hydrophyte: Plant adapted to environments with abundant water.
• Succulent: Plant containing specialized tissues for water storage.
• ABA: Abscisic acid, a plant hormone involved in responses to water stress.
• Waterlogging: Saturation of soil with water, reducing air spaces and oxygen availability.
• CAM: Photosynthetic adaptation in which stomata typically open mainly at night.
• Potometer: Apparatus used to estimate transpiration through measurements of water uptake.

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Key Relationships

Normal water movement:

soil → roots → xylem → leaves → atmosphere

Water balance:

water uptake ↔ water loss

Water stress:

water loss > water uptake → cells lose water → turgor decreases

Wilting:

water loss → smaller vacuoles → reduced turgor → flaccid cells → drooping tissues

Recovery:

water uptake → osmosis into cells → increased turgor → firmer tissues

Stomatal response:

water shortage → ABA signaling → stomata close → transpiration decreases

Photosynthesis trade-off:

stomata close → less water loss + less CO₂ uptake

Dry-environment adaptation:

reduced evaporation + reduced air movement + reduced exposed leaf area → improved water conservation

Waterlogging:

excess soil water → less root oxygen → reduced aerobic respiration → impaired root function

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Key Takeaways

• Plants must maintain a balance between water uptake and water loss.
• Water enters mainly through roots and is transported through xylem.
• Most water loss occurs through transpiration from leaves.
• Water is required for photosynthesis, transport, growth, cellular reactions, and maintaining cell structure.
• Water entering plant cells creates turgor pressure.
• Turgor pressure helps support leaves and young stems.
• Turgid cells contain sufficient water and are firm.
• When cells lose water, their vacuoles shrink and turgor decreases.
• Loss of turgor causes plant tissues to become less rigid.
• Wilting occurs when leaves and stems droop because cells have lost turgor.
• Temporary wilting can be reversed if water becomes available soon enough.
• Prolonged severe water stress can cause tissue damage and plant death.
• Plants can reduce water loss by closing stomata.
• Stomatal closure conserves water but also reduces carbon dioxide uptake.
• Reduced carbon dioxide uptake can decrease photosynthesis.
• The hormone ABA is involved in plant responses to water stress.
• High temperatures, low humidity, and wind can increase water loss.
• Low soil-water availability can cause water stress even when transpiration has not increased.
• Plants can respond to drought by reducing growth, closing stomata, altering root growth, or shedding leaves.
• Xerophytes are adapted to environments where water is limited.
• Thick waxy cuticles reduce evaporation.
• Small leaves and spines reduce surface area for water loss.
• Sunken stomata and leaf hairs trap humid air and reduce transpiration.
• Rolled leaves can reduce exposure to dry moving air.
• Succulent tissues allow some plants to store water.
• Extensive root systems improve access to limited water.
• CAM plants can reduce water loss by opening stomata mainly at night.
• Too much water can also harm plants because waterlogged soil contains less oxygen for roots.
• Water balance connects osmosis, transpiration, xylem transport, photosynthesis, respiration, phloem transport, and plant growth.
• Plants constantly adjust their physiology to changing environmental conditions.
• The central idea is: plants must obtain enough water to support their cells while controlling how quickly that water is lost to the environment.