1. Water Balance in Plants
http://www.botany.hawaii.edu/facult
y/Webb/BOT470/PlantWatMove/Wa
terBalancePlants.htm

2. Importance of Water to Plants
Water is the major abundant constituent of all
the plant cells that are physiologically active.
The water present in the plants is not static;
constitutes a part of the hydrodynamic system :
process of transpiration and evaporation
water absorption
translocation across the plant

3. Soil Types
Soil plays an important role in water retention
and availability : sand and clay
Sand: large particles (1mm) ;
wide spaces (channels) in between
with a relatively low surface area
Water moves rapidly through sand channels such
that only a thin film remains attached to the sand
particles. The channel is largely filled with air.

4. Clay : composed of minute particles (2 microns)
with very narrow spaces between
large proportion of surface area
The presence of organic matter in clay soils leads to the
formation of solid "crumbs". These improve soil
aeration and water penetration. Water is retained by
the narrow channels of clay particles and it is held more
tightly than other soils.
Field Capacity – the moisture-holding capacity of soil
Sandy soil has a low Field Capacity while clay soils have
a much greater Field Capacity.

5. • Soil Water does not usually contain a lot of dissolved
materials. Consequently, it has a High Water Potential.
• Saline soils are an obvious exception to this rule.
• The Hydrostatic Pressure in wet soil is near ZERO!
• As soils dry their water potential decreases.
Physical properties of water and their interactions with soil
particles:
* Water evaporates first from the center of any meniscus
that forms in soil channels.
* As soil dries, water is replaced by air.
* The soil particles are lined with a thin coat of water that
clings to their surface. This surface coat of water is held
tenaciously by the adhesive properties of water.
* The smaller channels hold water better than larger ones.

6. The adhesive properties of water greatly reduce the water
potential and can make it unavailable for plants.
• This may reach -2.0 MPa (MegaPascals). Remember that
the water potential of Pure Water is 0! Consequently,
water potentials in nature are usually negative.

7. When soil is at Field Capacity , Roots Absorb water from the immediate
water pervades all of the environment, creating air pockets. This
channels between soil particles. Is replaced by water present in the
nearest larger channels.
In extremely dry soils, water is tightly bound in the
smallest channels of the soil particles. It
can't replace water removed by the roots & large
air pockets are formed.

8. Soil Water moves by Bulk Flow
Plants deplete the local levels of water that are
immediately adjacent to absorbing roots. This
lowers the proximal water potential.
If more distal areas of soil have greater water potential, water
will flow through soil channels towards the roots. This is possible
because the soil channels form an interconnected system. This is
an example of Bulk Flow because water and everything dissolved
in it moves from an area of high water potential to one of low
water potential.

9. Soil Hydraulic Conductivity
- represents the ability of water to move through soil
Sandy soil would have high
Hydraulic Conductivity (HC) while
clay soil would have a low HC. This
is largely due to the diameter of the
soil channels. The amount of water
in the soil also affects (HC). Wet
soils have high HC while dry soils
have low HC. Air replaces water in
soil channels and blocks the flow of
water.

10. • It is possible to measure the Hydraulic Conductance of
Roots.
• This decreases at low temperatures or after exposure
to inhibitors of respiration.
• This indicates that there are physiological mechanisms
which help to regulate these processes.
• Roots grown in waterlogged solid have poor Hydraulic
Conductance.
• This is due to anaerobic conditions which inhibit
respiration.
• Consequently, plants that grow in swamps and bogs
may have Xeromorphic (Dry Form) leaf and stem traits.
• These environments have been called "Physiological
Desserts".

11. In extremely dry soils the Permanent Wilting
Point (PWP) may occur. This is the water
potential at which plant cells loose their turgor
pressure and can't regain it even when
transpiration ceases.
Wilting is the visible symptom of PWP. This
means that the water potential of soil water is
less than that in the roots.

12. Roots and Water Absorption
Root Hairs –increase
the absorptive area
of the roots ; 60% of
the total surface area

13. Movement of Water in Roots:
1. Apoplastic – via the cell until the endodermis
2. Symplastic – across the cells through the cell
membrane
Water can enter the Symplast of the Root Hair and
pass from one cell to the next via Plasmodesmata

14. • Water can stay in the Symplast until it reaches
the Xylem or it may pass from the Endodermis
into the Apoplast of the Pericycle.
• The water conducting cells (Tracheary
Elements) of the Xylem are part of the
Apoplast because they do not have intact
Protoplasts. In most cases they are free of
debris and can be treated like pipes. However,
Pits make the lateral walls uneven and this
affects the passage of water through
Tracheary Elements.

15. Water Transport Through the Xylem
Types of Tracheary Elements:
1. Tracheids- found in Gymnosperms;have
overlapping end walls which are connected by
pits ( regions with thin and porous primary walls
and with no secondary walls).
2. Vessel Members /Elements- form Vessels which are
constructed like a series of tube; have larger openings
(Perforation Plates) on their end-wall; have less
resistance than Pits and facilitate longitudinal
transport. The most advanced Vessel Members have
No Endwalls (Simple Perforation Plates). They are
generally wider than Tracheids and are more
specialized for water transport.

16. Tracheids
There may be intact remnants of the Primary
Wall & Middle Lamella between opposite Pits.
This has been called the "Pit Membrane“.
This is an unfortunate name because it can be
confused with a true biological membrane like
the Plasmalemma or Tonoplast. The "Pit
Membrane" contains cellulose & middle lamella
and is generally very porous.

17. Pit Membranes from the Tracheids of Gymnosperms like Pine have a
thick, impermeable center which is called the Torus. This acts like a
valve and can open or close lateral transport between Tracheids.

18. Vessel Elements
SEM images of Vessel Members (Elements):
Note the uneven lateral walls which reflect the
presence of Pits.

19. Vessel Members from Oak showing lateral Pits and Simple
Perforation Plates (Open End Wall).

20. * The thick lignified walls of Tracheary Elements
prevent their collapse under the tension that
develops during water translocation.
* It has been calculated that the amount of
pressure required to move water to the top of a
100 m tree is approximately 3 MegaPascals
(MPs). In order to do this a Negative Pressure or
Tension must develop in the Xylem.
* Root Pressure is usually less than 0.1 MPs. This
is clearly insufficient to move water to the top of
a tall tree.

21. * The pulling force for water translocation comes from
Transpiration in the Leaves.
* The prevailing theory that is used to describe this
phenomenon is called the "Cohesion-Tension theory of
sap ascent".
* Leaf Anatomy & Transpiration
• Water is brought to leaves in the xylem that is present
in the veins. Most cells are no more than 0.5 mm
away from a minor vein. Water is transferred to the
Protoplasts & Walls of Mesophyll Cells. Water
evaporates from the cell walls until the atmosphere
inside the leaf is saturated with water molecules.
• You should recall that soil water is held tenaciously
within the capillaries between minute soil particles.

22. • Plant Cell Walls are made of Cellulose Strands.
The capillaries between these are microscopic.
• Consequently, they dramatically lower the
water potential of water molecules associated
with them.
• As a leaf dries, the strong tension that
develops in the cell walls is sufficient to
provide the 3 MPs that are needed to pull
water to the top of the tree.

23. Diagram of a typical dicot leaf: The Apoplastic route of water
molecules from the Xylem to the Mesophyll Cell Walls is
illustrated by the blue lines.

24. Physiological Dilemna
• Leaves need to open their stomata to let CO2
diffuse inside because CO2 levels are higher in
the outside atmosphere than inside the leaf.
However, CO2 levels in the atmosphere are far
lower than the concentration of water
molecules.
• Water molecules are far more concentrated
inside the leaf than outside. Consequently, when
stomata are open, water molecules rapidly pass
through the Stomatal Pore to the outer
atmosphere.

25. Plants have developed physiological means to
control stomatal opening and closing.
1. Epidermal Trichomes which increase the
Boundary Layer moderate Transpiration rates.
2. Stomatal aperture- Temporal regulation
3. Regulating leaf temperatue by the
presence of wax, etc.,
4. Guard cell anatomy - thickened inner radial
walls which are not completely joined; Cellulose
Fibrils (CF) in the Guard cell Walls have a Radial
orientation as seen from above

27. Factors which affect closing /opening
of stomata:
• CO2 concentration, light intensity and
color, temperature & relative humidity.

28. Various Processes that Affect the Water Balance in Plants
1. Transpiration and the mechanism of Stomata:
Types of transpiration:
a. cuticular transpiration
b. stomatal transpiration- contributes
approximately 90% of the foliar transpiration
2. Osmosis or the Cellular water relations
Cytoplasmic lining of cell wall – semi-permeable
membrane
3. Translocation of water : ascent of sap’ mechanism & Root
pressure
4. Absorption of water - occurs in the root tip regions, zone of
root hair : water potential in the root cells at the periphery is
lower than that of the soil water

29. Root Pressure
• Root pressure is a positive hydrostatic
pressure that develops in roots. When a lawn
is extremely well watered & the relative
humidity is high, Guttation can occur.
Guttation produces dew-like drops of water
that emerge from the tips of some grasses &
other plants. Modified Stomata called
Hydathodes are the sites of water exudation.
The driving force for this is Root Pressure. This
may help to distribute important minerals
when trnapiration rates are low.

30. • Guttation occurs when the soil and atmosphere are saturated
with water. Water secretion occurs through modified Stoma called
Hydathodes. Root Pressure provides the motive force for this
process.

31. Soil Hydraulic Conductivity and Water Potential
Conductivity measures the ease with which
water moves through the soil.
As water content (and hence
the water potential) decreases,
the hydraulic conductivity
decreases drastically.