Ascent of Sap
I. The Problem
A. How do you raise the water column in a plant without
having the column of water cavitate?
B. Root Pressure: occurs when there is: (1) ample moisture is present
in the soil, (2) humidity is high, (3) transpiration is
low. Droplets of water exude from hydathodes creating the
phenomenon of guttation. Root pressure is not found in
conifers.
C. Capillarity is the interaction between surface contact
of a liquid and a solid. This interaction distorts the liquid
surface from a planar shape to a concave shape (meniscus).
This distortion caused the rise of liquids in small tubes.
The liquid is able to wet the side of the tube and pull itself up.
Liquids can rise higher in tubes of small diameter (like a cell!).
II. Ascent of Sap: Cohesion Mechanism
A. Three basic elements of the cohesion theory for the ascent
of sap: the driving force, hydration (adhesion), and the
cohesion of water.
1. Driving Force: the gradient of water potentials from the
soil through the plant to the atmosphere. These water potentials
decrease (become more negative) going from the soil to the
atmosphere
a. The xylem cell's small diameter and thick walls prevent them
from collapseing
b. The xylem cell's of the leaf and stem have low osmotic potentials
c. The xylem cell walls have a great hydration property, especially
in leaves
d. WATER POTENTIAL: Y = YP Ys
Y = water potential (negative)
YP = pressure potential (positive)
Ys = osmotic or solute potential (negative)
2. Adhesion: hydrogen bonding between water and cell walls.
3. Cohesion: hydrogen bonding between the water molecules.
Cohesive forces of water are so great that when it is pulled,
by osmotic and evaporative forces at the top of a tall tree,
the pull extends through the trunk and the roots
into the soil. Thus the water column in a whole tree is a
vertical pipe that would normally cavitate. But cavitation does
not hinder the flow of the sap because of the highly specialized
anatomy of the xylem cells.
III. Xylem Anatomy
A. Vascular Tissues in the Stem
1. Vascular bundles are "open" in dicots and "closed" in
monocots. "Open" refers to the fact that dicot stems can
increase in girth because they contain a layer
of cambial cells. Monocots lack this cambial layer and
thus contain "closed" vascular bundles that are surrounde
in a layer of cells called the bundle sheath.
Monocot vascular bundles are scattered in the stem while
Dicot vascular bundles are found in a ring. In woody
plants the xylem makes up the wood and the phloem makes
up the bark as well as the cortex and cork.
2. Xylem: tracheids, vessel elements, fibers, xylem
parenchyma
a. Secondary wall: cellulose, lignin, hemicelluloses that
cover the primary wall
b. Pits: round thin places in the secondary wall
(1) Simple pits: small round hole in the secondary wall.
(2) Bordered pits: secondary walls extend over the
center of the pit and the primary walls are swollen
in the center of the pit to form a torus. The primary
wall around the torus appears porous. The torus acts
as a valve, closing when pressure on one side is
greater than pressure on the other side.
3. Tracheid cells have tapered ends which overlap to form
long files of cells. These tracheid cells have numerous pits.
Tracheid diameters are often in the range of 10-25 mm.
4. Vessel elements: have perforation plates at the end walls.
Allow rapid movement of water. Vessel elements are
aligned so that they form long tubes called vessels.
Vessel elements are typically 40-80 mm.
5. The rate of sap flow is reduced by friction (adhesion) between
the fluid and the sides of the capillary tube. The
sap molecules touching the sides of the xylem wall do not
move, while the molecules in the center of the tube
move the farthest in a given time.
6. Measurements have shown that flow through large vessels is
much more rapid, for a given pressure gradient, than it
is through tracheids and small vessels.
7. There is a much greater chance that cavitation will occur
in a large vessel than in a small one or in a tracheid,
producing an embolism.
8. Some vessel elements with spiral thickenings can elongate
and grow while conducing water under tension. They grow
as the surrounding cells (with contents under pressure)
grow and pull them along, their spiral thickenings
expanding like coiled springs. Such growth occurs a the
base of most grass leaves.
9. Apical Meristems: Primary xylem, cambium. Secondary
xylem: spring wood summer wood
B. Vascular Tissues: Root Anatomy
1. Pericycle: layer of living cells
2. Stele: vascular tissues
3. Endodermis: radial and transverse cells walls have
Casparian Strips or bands that are impregnated with
suberin. Impermeable to water. Tangential walls (the
inside and outside walls parallel to the surface of the
root) are not impregnated with these substances. Suberin
lamella sometimes found on inner tangential wall
4. Cortex, epidermis, root hairs.
5. Water with its dissolved substances enters through the
epidermis and then moves freely through the cortical
cells, both protoplast (symplast) and cells walls
(apoplast) but that it cannot pass through the Casparian
strips around the endodermal cells.
6. Just inside the epidermis, of a cell layer, an exodermis
also has cells with Casparian strips and that typically
has a suberin lamella.
7. Hypodermis: a layer of cells just below the epidermis
with no wall modifications. Exodermis: hypodermis with
Casparian bands.
8. Most ion uptake and probably water uptake occurs in the
epidermis, even in roots without an exodermis. Movement
of ions through the cortex may be mostly from protoplast
to protoplast (symplastic), although water can move
freely through the membranes and thus may move in the
cell walls (apoplast) as well.
9. Root Cap and Root Hairs: much water enters through the
root hairs and their associated epidermal cells: the
region of a young root where xylem vessels are mature and
resistance is low.
C. Apoplast-Symplast Concept
1. Apoplast: interconnecting walls and water-filled xylem
elements. "Dead" part of the plant. Endodermal and
exodermal walls with Casparian strips are apoplast.
2. Symplast: "living" part of the plant, cytoplasm of all
the cells. Protoplast of adjoining cells are connected
through the plasmodesmata.
D. Anatomical Basis of Root Pressure
1. Roots in contact with soil solution. Ions diffuse into
root via apoplast of epidermis. If root have an
exodermis, ions must move into the symplast of the
epidermis, otherwise ions remain in the apoplast of the
cortex until they reach the endodermis. Ions pass across
the cell membranes from the apoplast into the symplast in
an active process that requires cellular reparation.
Increase in the concentration of ions inside the cells to
levels higher than those outside. Ions move freely into
the pericycle and other living cells within the stele.
Plasmodesmata, velocity of movement inward could be
increased by cytoplasmic streaming.
2. Once inside the stele, ions are actively pumped out of the
symplast and into the apoplast. Buildup in concentration
of solutes within the apoplast tin the stele to a level
higher than that of the soil solution. Osmotic potential
in the stele is more negative than the osmotic potential
of the soil. Water passes through protoplasts of the
endodermal and exodermal layers, then layers act as
differentially permeable membranes, and the root becomes
as osmotic system.
IV. Driving Force: A Water-Potential Gradient
A. Atmospheric Water Potential
1. Great capacity that dry air has for water vapor
2. Rapid drop in the water potential of increasingly dry air
3. temperature has a marked effect n relative humidity
B. Role of Osmosis into Living Cells
1. Living cells obtain water from the apoplast. Water
potential in protoplast must be slightly more negative
than that in the surrounding walls.
2. Cohesion theory says that water in the apoplast,
particularly in the xylem, is under tension or negative
pressure.
3. Osmotic potential of water in the apoplast must be less
negative (higher) then that of water in the living cells
4. Driving force would be the negative water potentials
within living cells, established by the very negative
osmotic potentials and the relatively low positive
pressures in the cells.
C. Role of Cell-Wall Hydration
1. When the water potential in a drying plant drops to some
critical level, cavitation and the resulting embolisms
occur in the transporting elements
2. Plants are constructed in such a way that cavitation
occurs when water potential reaches a critical low level
in a manner that allows refilling of the tracheids or
vessels if conditions are right. when the pressure in
the air spaces on the outside of the pit membrane exceeds
the tension inside by that amount, the meniscus will be
pulled through the pore allowing a minute quantity of air
to enter. The air would lead to vacuum boiling, and the
air and water vapor would immediately and explosively
expand to fill the tracheid or vessel.
3. The vessel would be filled with water vapor at its vapor
pressure for the prevailing temperature. The pressure in
the cell would be positive but quite low. Such pressures
would immediately seal all the pores, because a water
meniscus could never be pulled through such a pore by
such a small pressure difference across the cell wall.
The torus in bordered pits might also contribute to the
sealing. When conditions allowed the vapor pressure in
the xylem t increase above that of the vapor pressure of
water for the particular temperature, the cells might
refill by condensation of vapor and solution of the
minute amount of air.
D. Soil Water
1. Gravitational water: soil saturated with water, the
excess water moves down through the pore spaces.
2. Soil that contains all the capillary water it can retain
against gravity is said to be wet to field capacity.
3. When soils are not able to hold enough water to overcome
wilting in a plant, the amount of soil water is said to
be at the permanent wilting percentage. The minimum
plant water potential is apparently determined by the
osmotic potential of plant cells and the designed leakage
in the xylem. This minimum plant water potential
determines the permanent wilting point.
4. The amount of water in the soil at field capacity and at
the permanent wilting percentage depends strongly upon
soil texture, soil structure, and the amount and kind of
soil organic matter.
a. Soil Texture: size of the primary particles that make
up the soil
Clay: extremely small particles in the colloidal size
range with much hydrophilic surfaces.
Silt: intermediate in size between the two (best)
Sand: large, little holding capacity
b. Organic materials improve water holding capacity
Humus: soil organic material
c. Soil Structure depends on Particle Size: All soils are
mixtures of sand, silt, and clay.
Clay: extremely small particles in the colloidal size
range with much hydrophilic surfaces. Can hold more
water that is available to plants
Loam: intermediate in-between the two (best) - high
organic content. 10-25% clay and the rest about equal
parts of sand and silt
Sand: large, little holding capacity
V. Tension in the Xylem: Cohesion
A. Does Water Have a High Enough Tensile Strength?
water can sustain tensions of up to -3.0 MPa without
cavitating
B. What are the flow velocities? Are the water columns really
continuous?
Water columns in the xylem are continuous, and observed flow
velocities in stems strongly agree. When velocities are
measured at different points along the trunk, increase fist
at higher points along the trunk in the morning as
transpiration begins. Peak velocities (at noon) range from
1- 6 mh-1 for narrow-vessel trees to 16- 45 m h-1 for trees
with wide vessels. Slowest velocities occur in conifers.
C. Are the columns really under tension?
Tensions always observed varied from a few tenths of a
megapascal below zero to more negative than -8.0 MPa
Forest and freshwater species have tensions with the least
negative values, and desert and seashore plants, growing
in soils that are likely to be salty, have more negative
values. Water in the xylem of land plants in summer must
nearly always be under tension. Tensions normally exist
in their conductive xylem tissues, but their turgid leaf
cells have osmotic potentials even more negative than
those of the xylem sap with its negative pressure.
Without the differentially permeable membranes around the
living cells and the highly negative osmotic potentials of
the sap and cytosol inside, the high tensions in the xylem
system would lead to collapse (wilting) of the living
tissue. Without osmosis, plants would collapse.
D. What if the columns cavitate?
When dye is inserted at a point in a trunk it spreads
circumferentially around the trunk as it moves up and down,
mostly within a single growth ring. The spread amounts to
about 1-20, which equals about 17-35 mm per meter of trunk.
Water entering the xylem of any root is spread throughout
the entire trunk until it reaches virtually all the
branches that form the crown. If a given root is damaged,
no part of the crown suffers from lack of water.
E. What about air excluded from solution in the stem by
freezing?
1. Air blockage occurs when some trees are frozen. Blocked
pathways are replaced or restored in such trees.
2. "Throw-away" method. Large and efficient vessels that a
single growth increment of the trunk is sufficient to
provide the crown with water. Vessels are formed before
leaves emerge in the spring, the vessels of previous
years are not used. Root pressure in the spring, which
is quite clearly seen in grapevines.
3. Imagine a northern tree thawing in the spring. As the ice
melts, the tracheids become filled with liquid containing
the many bubbles of air that had been forced out by
freezing. Melting continues and transpiration begins,
tension begins to develop in the xylem. A large bubble
reaches a critical point at which it expands explosively
under tension as water turns to vapor in a fraction of a
second. this is confined to the tracheid in which it
occurs because of its anatomy. but it sends a shock wave
to surrounding tracheids, driving their small air bubbles
back into solution. The tracheid in which bubble
expansion occurred would be vapor-locked and forever lost
to sap movement. About 10% of the tracheids were indeed
filled with vapor, but the remaining 90% are ample to
handle sap movement.
VI. Xylem Anatomy: A fail-safe system
A. Plants, especially tall trees, are apparently designed to
allow sufficient flow in response to the pressure gradients
to prevent cavitation in the transport elements to bypass
cells that do become vapor-locked, and sometimes even to
restore such vapor-locked cells.
B. Wall sculpturing in vessel elements and the various kinds of
performation plates might keep bubbles that do form from
coalescing; small bubbles are much easier to dissolve than
large ones. Highest resistance to water flow occurs at the
leaf trace or in the base of the petiole. water stress
builds during drought, cavitation occurs first in the
leaves, so they may wilt and die; but the water system in
the trunk remains relatively intact.
C. Tyloses seal against water loss and pathogen invasion. Gums
or resins are secreted into nonfunctional xylem cells.
D. Measure the circumference of the tree; the circumference
contracts slightly during periods of high transpiration and
expands at night or during rain.