Plant
and
Water
Relation
Lina Riza N. Montero
WATER

Makes up approximately 90 % of a plant's mass
and performs many functions:
1.
2.
3.
4.
5.
6.
Required for seed germination.
Serves as part of the plant's structure.
Carries minerals into and through the plant.
Transports photosynthates and other biochemicals through
the plant,
Cools the plant by evaporation
Involved in photosynthesis.
The Importance of Water
 Physiological aspects
BISC 367
Movement of water in plants
 Molecular diffusion
 Water moves from an area of high free energy to area of low
free energy


i.e. down a conc. gradient
Described by FICKS LAW
Js = -Ds dcs/dx
Js = flux density for s (mol m-2 s-1)
Ds = diffusion coefficient
dcs/dx = difference in water conc. over distance x
BISC 367
Movement of water in plants
 Bulk flow
 Movement of water in response to a pressure gradient


Analogous to water flowing in a pipe
Affected by:
Radius of pipe (r)
 Viscosity of liquid (h)
 Pressure gradient dyp/dx


Described by POISEUILLE’S equation:
vol. flow rate (m3 s-1) = (pr4/8h)(dyp/dx)
BISC 367
Movement of water into a plant cell
occurs by osmosis
 2 mechanisms:
 Diffusion across the
membrane
 Bulk flow across
aquaporins (water filled
pores)
BISC 367
Movement of water into a plant cell
occurs by osmosis
 Water uptake is driven by a free energy gradient
composed of:


Concentration gradient
Pressure gradient
Free energy gradient for water movement is referred to
as a Water Potential Gradient
BISC 367
Water Potential
 Water potential (Yw) is equivalent to the free energy
of water & influenced by:



Concentration (or activity)
Pressure
Gravity
 Yw is the free NRG of water per unit volume (J m-3)
 Divide chem. pot. of water (J mol-1) by the partial molal vol.
(m3 mol-1)
 Units equivalent to pressure (Pa)
BISC 367
Characteristics:
•Water is the universal solvent
•One of nature's most stable compounds,
•Water molecule is not symmetrical (creates a
dipole)
This phenomenon of polarity
creates an attraction
between water molecules.
Water molecules can also
attract or be attracted by
Cations, such as Na+, K+, and
Ca++, or
 Anions or clay colloids in the
soil.

SOIL WATER
Availability
Even though water is present in the soil, it
sometimes is not available to the plant.
The pore spaces are always filled with water, air, or
a mixture of both.
When the pore spaces are filled with water, the soil is
said to be saturated. Saturation is an unhealthy
condition for plants if it lasts too long because the
oxygen needed for respiration is missing.
 When the pore spaces are filled mostly with air, the soil
is too dry → -ve effect on plant growth.

To move onto land, plants needed a waxy cuticle to avoid unsupportable water loss.
However, depending on diffusion to move water through the plant body, as in nonvascular plants, severely limits plant size. The non-vascular plants all are short.
Further, plant reproduction depends on the male gamete “swimming” to the female in
those non-vascular plants.
Non-vascular plants all live in moist environments where that swim is possible.
Early vascular plants could increase in stature, and began the evolution of vascular
tissue…
Water in the Soil
 The main driving forces
for water flow from the
soil through the plant to
the atmosphere include:
 Differences in:



[H2O vapor]
Hydrostatic pressure
Water potential
 All of these act to allow
the movement of water
into the plant.
Water absorption from soil
 Water clings to the surface
of soil particles.
 As soil dries out, water moves
first from the center of the
largest spaces between
particles.
 Water then moves to smaller
spaces between soil particles.
 Root hairs make intimate
contact with soil particles –
amplify the surface area for
water absorption by the
plant.
Water Moves through soil by bulk
flow
 Bulk flow:
 Concerted movement of groups of molecules en masse,
most often in response to a pressure gradient.
 Dependant on the radius of the tube that water is
traveling in.

Double radius – flow rate increases 16 times!!!!!!!!!!
 This is the main method for water movement in
Xylem, Cell Walls and in the soil.
 Independent of solute concentration gradients – to
a point

So different from diffusion
Water Moves through soil by bulk
flow
 In addition, diffusion of water vapor accounts for
some water movement.
 As water moves into root – less in soil near the root


Results in a pressure gradient with respect to neighboring regions of
soil.
yp near the root and a higher yp in the
So there is a reduction in
neighboring regions of soil.
 Water filled pore spaces in soil are interconnected,
water moves to root surface by bulk flow down the
pressure gradient
Water Moves through soil by bulk
flow
 The rate of water flow depends on:
 Size of the pressure gradient
 Soil hydraulic conductivity (SHC)

Measure of the ease in which water moves through soil
 SHC varies with water content and type of soil
 Sandy soil high SHC


Large spaces between particles
Clay soil low SHC

Very small spaces between particles
Water Moves through soil by bulk
flow
 As water moves from soil into root the spaces fill
with air

This reduces the flow of water
 Permanent wilting point

y
At this point the water potential ( w) in soil is so low that
plants cannot regain turgor pressure
There is not enough of a pressure gradient for water to flow to the
roots from the soil
 This varies with plant species

Water transport processes
 Moves from soil, through plant, and to atmosphere
by a variety of mediums




Cell wall
Cytoplasm
Plasma membranes
Air spaces
 How water moves depends on what it is passing
through
Water across plant membranes
 There is some diffusion of
water directly across the bilipid membrane.
 Auqaporins: Integral
membrane proteins that form
water selective channels –
allows water to diffuse faster

Facilitates water movement in
plants
 Alters the rate of water flow
across the plant cell membrane
– NOT direction
Water uptake in the roots
 Root hairs increase surface
area of root to maximize
water absorption.
 From the epidermis to the
endodermis there are three
pathways in which water can
flow:
 1: Apoplast pathway:
 Water moves exclusively
through cell walls without
crossing any membranes

The apoplast is a continuous
system of cell walls and
intercellular air spaces in plant
tissue
Water uptake in the roots
 2: Transmembrane
pathway:
 Water sequentially enters a
cell on one side, exits the
cell on the other side,
enters the next cell, and so
on.
 3: Symplast pathway:
 Water travels from one cell
to the next via
plasmodesmata.

The symplast consist of the
entire network of cell
cytoplasm interconnected by
plasmodesmata
Water uptake in the roots
 At the endodermis:
 Water movement through
the apoplast pathway is
stopped by the Casparian
Strip

Band of radial cell walls
containing suberin , a wax-like
water-resistant material
 The casparian strip breaks
continuity of the apoplast
and forces water and solutes
to cross the endodermis
through the plasma
membrane

So all water movement across
the endodermis occurs through
the symplast
Water transport through xylem
 Compared with water movement across root tissue
the xylem is a simple pathway of low resistance
 Consists of two types of tracheary elements.


Tracheids
Vessile elements – only found in angiosperms, and
some ferns
 The maturation of both these elements involves the
death of the cell. They have no organelles or
membranes

Water can move with very little resistance
Water
transport
through
xylem
 Tracheids: Elongated spindleshaped cells –arranged in
overlapping vertical files.

Water flows between them via pits –
areas with no secondary walls and thin
porous primary walls
 Vessel elements: Shorter &
wider. The open end walls provide
an efficient low-resistance
pathway for water movement.
 Perforation plate forms at each
end – allow stacking end on to
form a larger conduit called a
vessel

At the end there are no platescommunicate with neighboring vessels
via pits
Water transport through xylem
 Water movement through xylem needs less




pressure than movement through living cells.
However, how does this explain how water moves
from the roots of a tree up to 100 meters above
ground?
Cohesion-tension theory:
Relies on the fact that water is a polar molecule
Water is constantly lost by transpiration in the
leaf. When one water molecule is lost another is
pulled along. Transpiration pull, utilizing capillary
action and the inherent surface tension of water, is
the primary mechanism of water movement in
plants.
Water transport through xylem
 Plants can get embolisms too!
 Air bubbles can form in xylem
 Air can be pulled through
microscopic pores in the xylem cell
wall
 Cold weather allows air bubbles to
form due to reduced solubility of
gases in ice
 Once a gas bubble has formed it
will expand as gases can not
resist tensile forces

Called Cavitation
Water
transport
through
xylem
 Such breaks in the water
column are not unusual.
 Impact minimized by several
means
Gas bubbles can not easily pass
through the small pores of the
pit membranes.
 Xylem are interconnected, so
one gas bubble does not
completely stop water flow

 Water can detour blocked
point by moving through
neighboring, connected
vessels.
Water transport through xylem
 Gas bubbles can also be
eliminated from the xylem.
At night, xylem water pressure
increases and gases may simply
dissolve back into the solution
in the xylem.
 Many plants have secondary
growth in which new xylem
forms each year. New xylem
becomes functional before old
xylem stops functioning


As a back up to finding a way around
gas bubbles.
Water evaporation in the leaf
affects the xylem
 The tensions needed to pull
water through the xylem
are the result of
evaporation of water from
leaves.
 Water is brought to leaves
via xylem of the leaf
vascular bundle, which
branches into veins in the
leaf.
 From the xylem, water is
drawn in to the cells of the
leaf and along the cell wall.
Water evaporation in the leaf
affects the xylem
 Transpiration pull, which
causes water to move up the
xylem begins in the cell walls of
leaf cells
 The cell wall acts as a capillary
wick soaked with water.
 Water adheres to cellulose and
other hydrophilic wall
components.
 Mesophyll cells within leaf are
in direct contact with
atmosphere via all the air
spaces in the leaf
Water evaporation in the leaf
affects the xylem
 So, negative pressure exists in
leaves- cause surface tension
on the water
As more water is lost to the
atmosphere – the remaining
water is drawn into the cell
wall
As more water is removed
from the wall the pressure
of the water becomes more –
ve
This induces a motive force to
pull water up the xylem
Water movement from leaf to
atmosphere
 After water has
evaporated from the cell
surface of the intercellular
air space diffusion takes
over.
 So: the path of water





Xylem
Cell wall of mesophyll cells
Evaporated into air spaces of
leaf
Diffusion occurs – water vapor
then leaves via stomatal pore
Goes down a concentration
gradient.
Water Vapor diffuses quickly in air
 Diffusion of water out of the leaf is very fast

Diffusion is much more rapid in a gas than in a liquid
 Transpiration from the leaf depends on two
factors:
 ONE

Difference in water vapor concentration between leaf
air spaces and the atmosphere
Due to high surface area to volume ratio
 Allows for rapid vapor equilibrium inside the leaf

 TWO

The diffusional resistance of the pathway from leaf to
atmosphere
Water Vapor diffuses quickly in air
 The diffusional resistance of the pathway from
leaf to atmosphere
 Two components:
 The resistance associated with diffusion through the
stomatal pore.

Leaf stomatal resistance (rs)
 Resistance due to a layer of unstirred air next to the
leaf surface

Boundary layer resistance
Boundary layer resistance
 Thickness of the layer is
determined by wind speed.
 Still air – layer may be so thick
that water is effectively
stopped from leaving the leaf
 Windy conditions – moving air
reduces the thickness of the
boundary layer at the leaf
surface
 The size and shape of leaves
influence air flow – but the
stomata itself play the most
critical role leaf transpiration
Stomatal control
 Almost all leaf transpiration
results from diffusion of water
vapor through the stomatal pore

Remember the way cuticle?
 Provide a low resistance
pathway for diffusion of gasses
across the epidermis and cuticle
 Regulates water loss in plants
and the rate of CO2 uptake

Needed for sustained CO2
fixation during photosynthesis
Stomatal control
 When water is abundant:
 Temporal regulation of stomata is
used:


OPEN during the day
CLOSED at night
 At night there is no photosynthesis,
so no demand for CO2 inside the leaf
 Stomata closed to prevent water loss
 Sunny day - demand for CO2 in leaf is
high – stomata wide open
 As there is plenty of water, plant
trades water loss for photosynthesis
products
Stomatal control
 When water is limited:

Stomata will open less or even remain
closed even on a sunny morning

Plant can avoid dehydration
 Stomatal resistance can be
controlled by opening and
closing the stomatal pores.
 Specialized cells – The Guard
cells
Stomatal guard cells
 There are two main
types
 One is typical of
monocots and grasses
Dumbbell shape with
bulbous ends
 Pore is a long slit

 The other is typical of
dicots

Kidney shaped - have an
elliptical contour with pore
in the center
Summary
 Water is the essential medium of life.
 Land plants faced with dehydration by water loss to
the atmosphere
 There is a conflict between the need for water
conservation and the need for CO2 assimilation
This determines much of the structure of land plants
 1: extensive root system – to get water from soil
 2: low resistance path way to get water to leaves – xylem
 3: leaf cuticle – reduces evaporation
 4: stomata – controls water loss and CO2 uptake
 5: guard cells – control stomata.

Plant water transport
 How can water move from
the ground
all the way
to the top
of a 100 m
tall redwood
tree?
Water transport in plants:
 The same way we drink soda
from a straw!
 Water’s great
cohesive forces (molecules
sticking to each other)
and adhesive forces
(attaching to walls of xylem cells)
Transpiration-cohesion Theory
for water transport in the xylem
 Evaporation of water in the leaves (through
stomates) generates the ‘sucking force’ that
pulls adjacent water molecules up the leaf
surface
Water transport (cont.)
 Like a long chain, water molecules pull each other
up the column.
 The column goes from roots  leaves.
 What’s amazing is that the
water moves up by using the sun’s
evaporative energy…
 Plants control transpiration by opening/closing
stomata
Sugar translocation
 1. Sugars made in leaf mesophyll cells (source)
diffuse to phloem cells in the vascular bundles.
 2. Companion cells load dissolved sugars into the
phloem STM using energy (ATP).
 3. Water moves into cells with high sugar
concentration.
 4. Osmotic water flow generates a high hydraulic
pressure that moves dissolved sugars through the
phloem to the rest of the plant (sink).
Pressure flow in phloem
 Sugars made in the
leaves are loaded into
companion cells and into
phloem STM.
 Water (from xylem)
moves in by osmosis,
creating pressure flow
down the phloem.