Transport in Plants
5. Water Balance in Plants
Learning outcomes
- I can explain the importance of maintaining water balance in plants.
- I can describe what happens when plants lose too much water.
- I can explain the process of wilting.
- I can identify adaptations that help plants conserve water.
- I can analyze how plants respond to changing water availability.
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.
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
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.