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Water Balance in Plants
An organism usually exists in a state of
gradients
• s gradient keeps sugar flowing down the phloem
to supply the needs of the various cells in the root.
• p gradient : without these pressure differences,
bulk flow would cease, the shoot would wilt and
the root would starve.
•  gradient from the soil water to the xylem
provides the potential differences needed to bring
water and minerals in from the soil to be lifted up
the xylem.
SOIL WATER
The type of water in soil affects the ability of
plant to obtain water :
1 – bound water, locked in particle –
unavailable to plants
2 – unbound water, avalailable to plants:
• gravitational water(1) : the water that drains from the soil by gravity.
Move to lower soil levels.
• When all gravitational water has drained, the soil is saturated and to be at
field capacity.
• Most of water is held by surface tension in small pores in the soil :
capillary water(2), which is easily absorbed by plant
• When the tension is equal with the ability to extrach it, the remaining
water in soil reach permanent wilting point. This is the lower limit of soil
moisture that can support plant growth
• Hygroscopic water is all remaining unbound water which is held tightly by
small soil particles. Plants cannot extract hygroscopic water from the soil.
The Path of Water into Root
How water and mineral enter the root
The path water and mineral into
the root
• Root (hairs) epidermal cells take up water and nutrients
• Active uptake of mineral into root cells  s cells 
• Water osmosis enter between cells (apoplast) or through cells and
cells membrane (symplast)
• Endodermis : one layer cells with casparian strip in cell wall
(apoplast changes to symplast) to regulate the quantity and type of
minerals and ions reach the xylem
• Further transport through symplast has to pass through selective
membrane : control of which minerals/ions reach root xylem
• Active uptake of minerals is facilitated by transpiration. Low
transpiration will be supported by root pressure
Transmembrane
pathway
Casparian strip: a waxy layer in
the endodermis cells to
regulate the quantity and
type of minerals and ions
reach the xylem
Symplastic route


In order for water and minerals
to reach the stele (xylem) the
highly regulated (cytoplasmic)
must be taken.
The symplastic route involves
special openings between
adjacent cell walls called
plasmodesmata.
Apoplastic route



Casparian strip




a waxy layer in the endodermis
cells
to regulate the quantity and
type of minerals and ions reach
the xylem
Pericycle,
Stele :xylem, phloem


in the nonliving parts of the
root — that is, in the spaces
between the cells and in the
cells walls themselves.
This water has not crossed a
plasma membrane.
Therefore, to enter the stele,
apoplastic water must enter the
symplasm of the endodermal
cells. From here it can pass by
plasmodesmata into the cells of
the stele.
Water and mineral normally can
travel through the porous cell
walls of the root cortex.
The apoplastic route is blocked
by the casparian strip.
The water potentials of apoplast and symplast for a
given tissue are the same but the two components of
water potential are different

In the symplast :
- hydrostatic pressure is
positive (turgor pressure),
and
- the osmotic pressure is the
larger of the two
components because of
the dissolved solutes in
the cytoplasm.
•
In the apoplast :
- Hydrostatic pressure is
negative. The apoplast is
under tension.
- osmotic pressure is small
because there are fewer
dissolved solutes in the
apoplastic water.
Cells involve in pathway of water
Xylem vessels
•
•
•
•
no cell contents (dead)
form continuous tubes
lignin fibres strengthen the cell walls
so do not collapse when pressure
inside falls
Water Movement in Xylem :
1.
2.
•
3.
4.
5.
Transpiration : the pulling of water up through the xylem of a plant utilizing
the energy of evaporation and the tensile strength of water.
Cohesion is the attractive force between molecules of the same substance.
Water's cohesive force within xylem give it a tensile strength.
A combination of adhesion, cohesion, and surface tension allow water to
climb the walls of small diameter tubes like xylem. This is called capillary
action.
Adhesion is the attractive force between water molecules and other
substances. Because both water and cellulose are polar molecules there is a
strong attraction for water within the hollow capillaries of the xylem.
Tension. This pulling force is created by the surface tension which develops
in the leaf's air spaces.
Root pressure. It occurs when no transpiration
Capillarity
Water rises up narrow tubes due to the adhesive
forces between the water molecules and the wall
of the tube
Xylem vessels are very
narrow
Water rises higher in narrower tubes
Transpiration pathway through a plant
 the xylem  the petiole
 the veins of the leaf 
the finest veins  the
cells of the spongy and
palisade layers.
 Here some of the water
may be used in
metabolism, but most is
lost in transpiration.
The purposes of transpiration :
• supplies water for photosynthesis
• transports minerals from the soil to all parts of
the plant
• cools leaf surfaces some 10 to 15 degrees by
evaporative cooling
• maintains the plant's shape and structure by
keeping cells turgid
Transpiration depends on two major factors:
1.
2.
a)
b)
Difference in water vapor concentration
Diffusional resistance (r)
Cl  Ca
Leaf stomatal resistance (rs)
Transpiration flow =
Rs  Ra
Boundary layer resistance (rb)
Transpiration from the Leaf
http://www.jochemnet.de/fiu/BSC1011/BSC1011_9/sld004.htm
Guard cell and water transport
• The physical structure of guard cells : A stoma is a
physical gap between two special epidermal cells called
guard cells.
• If the plant  water deprivation  it will wilt. To
compensate the guard cells become flaccid and the
stoma is closed.
• When the pair of guard cells are turgid -- full of water -they bow in such a way as to increase the gap -- stoma -between them. WHY ????
Guard Cell Physiology
Radially oriented cellulose microfibril
•
The structure of guard cells explains why they bow apart when
turgid.
– The two guard cells are fused at their ends.
– The inner cell walls which form the stoma are
thicker than the outer walls.
– Cellulose microfibrils are oriented radially rather
than longitudinally.
 Microfibrils are made of cellulose and are oriented
transversely to the long axis of the cell resulting in cell
expansion in the direction of its long axis because the
cellulose reinforcement offers the least resistance at right
angles to its orientation.
The sequence of events which result in stomatal opening
• The immediate cause is an increase in turgor pressure - water
enters the central vacuole by osmosis
• Turgor pressure increases because of a negative water
potential due to an influx of potassium ions (K+). The cell
becomes hypertonic to its environment
• The reversible uptake of K+ ions takes place because of the
membrane potential created when H+ are actively pumped
out of the cell - consuming ATP. The cell's interior becomes
negative compared to the surroundings.
The stoma is closed at
night when the large
central vacuole is
isotonic, even
hypotonic to
surrounding fluids. K+
ions are outside of the
cell, and H+ ions by and
large remain attached
to the weak organic
acids within the cell.
Blue light is absorbed by a
membrane protein
which somehow causes
an increase in the
activity of proton
pumps which use ATP
to transport H+ out of
the cell.
With H+ on the outside K+
readily diffuse into the
cell to compensate for
the negative electrical
potential. The
hypertonic conditions
within the cell attract
water molecules and
the stoma opens as
turgor pressure
increases.
Guard cell transport
• GC membrane-transport processes during stomatal opening
(left) and stomatal closing (right).
• Transporters in a conducting state are green; in a non
conducting state, red; conducting state not specified, blue.
• Note, as indicated, that one icon may represent several
transporters having different properties. The traffic-control
icon by a transporter indicates inhibition (red, upper) or
activation (green, lower); effectors act directly and
indirectly. The roles of some transporters are not assigned
with certainty.
stomatal opening
• It is initiated by H+ extrusion (1), which hyperpolarizes the PM
and acidifies the apoplast.
• These effects increase the driving force for K+ uptake and
activate the voltage-regulated K+-in channel (2).
• Cl- (3) and suc (4) may also be taken up.
• H+ pumping into the vacuole (5, 6) provides for K+ antiport (7)
and Cl- uptake (8).
• GC solute increase leads to H2O uptake and therefore an
increase in GC volume and aperture size.
Stomatal closing
• It is initiated by activation of the A- channel (9), which
depolarizes the PM.
• This effect increases the driving force for K+ efflux and
activates the voltage-regulated K+-out channel (10).
• Several channels (11) provide for solute release from the
vacuole.
• Internal and external ABA receptors (12) cause the release of
Ca2+ through cellular messengers (e.g., IP3); ABA also causes
Ca2+ influx (13).
• ABA also induces stomatal closure by Ca2+-independent
means. Compartments are not to scale.
An organism usually exists in a state of
gradients so that flows can occur, both
between and around its cells.
Microfibrils
• In guard cells, microfibrils are made of
cellulose and are oriented transversely to the
long axis of the cell resulting in cell expansion
in the direction of its long axis because the
cellulose reinforcement offers the least
resistance at right angles to its orientation.
• Keunikan dari sel penjaga adalah serat halus sellulosa
(cellulose microfibril) pada dinding selnya tersusun melingkari
sel penjaga,
• Pola susunan ini dikenal sebagai miselasi Radial (Radial
Micellation). Karena serat sellulosa ini relatif tidak elastis,
maka jika sel penjaga menyerap air mengakibatkan sel ini tidak
dapat membesar diameternya melainkan memanjang.
• Akibat melekatnya sel penjaga satu sama lain pada kedua
ujungnya memanjang akibat menyerap air maka keduanya
akan melengkung ke arah luar. Kejadian ini yang menyebabkan
celah stomata membuka.
The sequence of events which result in stomatal opening
• The immediate cause is an increase in turgor pressure - water
enters the central vacuole by osmosis
• Turgor pressure increases because of a negative water potential due
to an influx of potassium ions (K+). The cell becomes hypertonic to
its environment
• The reversible uptake of K+ ions takes place because of the
membrane potential created when H+ are actively pumped out of
the cell - consuming ATP. The cell's interior becomes negative
compared to the surroundings.