Chapter 32, pages 699-701
I. Water Economy in the Plant
A. Uses of water in the plant
1. Metabolism
2. Retained in plant cells
3. Transpiration - 98% of all water is used here
B. Examples of transpiration
1. Definition - the movement
of water through the plant from roots to leaves during the
photosynthetic
process
2. Single corn plant in
Kansas used 196 liters of water between May 5 and
September
8
3. Red maples in a growing season - 3 inches of water
4. Alfalfa - 900 units of water/unit of dry matter produced
5. Millet - 248 units of water/unit of dry matter produced
C. Pathway of transpiration
1. Bulk (liquid not vapor or molecular water) occurs in soil and is called soil water
2. Root hairs absorb water and pass it through epidermis of root to cortex
3. The cortex passes it to the endodermis
4. The endodermis controls
the solute absorption and passes the water into the
xylem
of the root
5. The water is carried up the xylem of the stem to the xylem of the leaf
6. Xylem of the leaf releases the water to the mesophyll of the leaf
7. Water in the cellulose fibrils of the leaf evaporates into the substomatal chambers
8. When stomata open, the water vapor inside of the leaf diffuses out into atmosphere
II. Transport in Xylem (Or how does water get to the top of a 300 foot redwood tree?)
A. Direction of movement
1. Water moves toward low water potential
2. Water moves toward a rising solute concentration
3. Water moves toward where water is being removed
B. Sinks for water
1. In regions of photosynthesis
C. Cohesion and Tension Theory
1. How it works:
a.
Matrix
potential decreases as the cell walls of the
spongy
parenchyma begins to
dry when the
stomata
open
b. The water moves from the cell to the cell wall cellulose
c. Water moves from adjacent cells to replace the water in the cells
d.
The
xylem
supplies the water to these cells that are losing water to the
cellulose
cell walls in the spongy parenchyma
e.
The decrease in
water
potential from the loss of water in the leaves is
transmitted into the
xylem
so that the water in the xylem is under
tension
all
the way down to the roots
f.
This
water
potential amounts to about 30
bars
of
tension
depending upon the
tree height
g.
The taller the tree, the lower the water potential in the leaves needs
to be in
order to pull the water up to the top of the tree from the
roots
2. Water pulled (under tension) not pushed
a.
Water has enough
tensile
strength in columns the size of
vessels
to be lifted to
300 meters
b.
Gas bubbles destroy tensile strength if they enter the system. With
gas
bubbles a column of water can only be raised 9.75 meters
at sea level
c.
Too much tension causes
cavitation
(spontaneous bubbles) in the
vessels
and
tracheids
d. If cavitation occurs in vessels or tracheids
(1) Bubbles are contained within the cell and won't move
because the
pit
pairs
will not let them through
(2) When the extreme tension stops (usually at night)
the tension on the water
is released and the bubbles are
reabsorbed by the water and the complete
water column is restored
D. Root Pressure
1. Xylem sap is pushed
up the stem as a result of
osmotic
pressure in the xylem cells
of the
root
2. Examples of where it occurs in the plant:
a. Water bleeds out of newly cut tree stump
b. Water is pushed out the tips of leaves (guttation, see below)
3. How it works:
a.
The
osmotic
potential in the root decreases as
active
transport takes up
minerals from the soil
b.
With decreased osmotic potential in the roots the
water
potential drops and the
plant begins to uptake water by
osmosis
and push it up the xylem
c.
It bleeds out the stump or out the leaf tips as the pressure is
transmitted
up the
stem
E. Guttation
1. When root pressure in small plants pushes water out the tips of leaves
2.
Hydathodes
- The special structures (pressure relief valves) that allow the water
from
root pressure to pass through
3. Usually happens
in the mornings when the
humidity
is 100% and no
transpiration
is
occurring
F. How roots come in contact with water
1. Roots only grow in moist soil, and will not penetrate dry soil
2. Important to water plants deeply and widely enough to include a good root ball
3. Weeds transpire water out of soil and thus deplete soil moisture
G. Too much fertilizer makes the soil hypertonic to the plant and dehydrates it
H. Saline water can do the same thing if too strong
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