Transport
of Water and Minerals in Plants
Most plants secure the water and minerals they need from their roots.
The path taken is: Soil -> roots -> stems -> leaves
The minerals (e.g., K+, Ca2+) travel dissolved in
the water (often accompanied by various organic molecules supplied by
root cells).
Less than 1% of the water reaching the leaves is used in photosynthesis
and plant growth. Most of it is lost in transpiration.
Link
to Transpiration
However, transpiration does serve two useful functions:
It provides the force for lifting the water up the stems.
It cools the leaves.
Water and minerals enter the root by separate paths which eventually
converge in the stele.
Link
to diagram of root structure.
The Pathway of Water
Soil water enters the root through its epidermis.
It appears that water then travels in both
the cytoplasm of root cells — called the symplast — that
is, it crosses the plasma membrane and then passes from cell to cell
through plasmodesmata.
In the nonliving parts of the root — called the apoplast
— that is, in the spaces between the cells and in the cells walls
themselves. This water has not crossed a plasma membrane.
However, the inner boundary of the cortex, the endodermis, is
impervious to water because of a band of suberized matrix called the casparian
strip. 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.
Once inside the stele, water is again free to move between cells as well
as through them.
In young roots, water enters directly into the xylem vessels
and/or tracheids [link
to views of the structure of vessels and tracheids]. These are
nonliving conduits so are part of the apoplast.
Once in the xylem, water with the minerals that have been deposited in
it (as well as occasional organic molecules supplied by the root tissue)
move up in the vessels and tracheids.
At any level, the water can leave the xylem and pass laterally to supply
the needs of other tissues.
At the leaves, the xylem passes into the petiole and then into the veins
of the leaf. Water leaves the finest veins and enters 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 Pathway of Minerals
Minerals enter the root by active
transport into the symplast of epidermal cells and move toward and
into the stele through the plasmodesmata connecting the cells.
They enter the water in the xylem from the cells of the pericycle (as
well as of parenchyma cells surrounding the xylem) through specialized
transmembrane channels.
What Forces Water
Through the Xylem?
Observations
The mechanism is based on purely physical forces because the
xylem vessels and tracheids are lifeless.
Roots are not needed. This was demonstrated over a century ago
by a German botanist who sawed down a 70-ft oak tree and placed the base
of the trunk in a barrel of picric acid solution. The solution was
drawn up the trunk, killing nearby tissues as it went.
However, leaves are needed. When the acid reached the leaves
and killed them, the upward movement of water ceased.
Removing a band of bark
from around the trunk — a process called girdling — does not
interrupt the upward flow of water. Girdling removes only the phloem,
not the xylem, and so the foliage does not wilt. (In due course,
however, the roots — and thus the entire plant — will die because the
roots cannot receive any of the food manufactured by the leaves.)
Transpiration-Pull
In 1895, the Irish plant physiologists H. H. Dixon and J. Joly proposed
that water is pulled up the plant by tension (negative pressure)
from above.
As we have seen, water is continually being lost from leaves by
transpiration. Dixon and Joly believed that the loss of water in the
leaves exerts a pull on the water in the xylem ducts and draws more
water into the leaf.
But even the best vacuum pump can pull water up to a height of only 34
ft or so. This is because a column of water that high exerts a pressure
(~15 lb/in2) just counterbalanced by the pressure of the
atmosphere. How can water be drawn to the top of a sequoia (the tallest
is 370 feet high)? Taking all factors into account, a pull of at least
270 lb/in2 is probably needed.
The answer to the dilemma lies the cohesion of water molecules;
that is the property of water molecules to cling to each through the
hydrogen bonds they form.
Link
to discussion of hydrogen bonding in water.
When ultrapure water is confined to tubes of very small bore, the force
of cohesion between water molecules imparts great strength to the column
of water. It has been reported that tensions as great as 3000 lb/in2
are needed to break the column, about the value needed to break steel
wires of the same diameter. In a sense, the cohesion of water molecules
gives them the physical properties of solid wires.
Because of the critical role of cohesion, the transpiration-pull theory
is also called the cohesion theory.
Some support for the theory
If sap in the xylem is under tension, we would expect the column
to snap apart if air is introduced into the xylem vessel by puncturing
it. This is the case.
If the water in all the xylem ducts is under tension, there
should be a resulting inward pull (because of adhesion) on the walls of
the ducts. This inward pull in the band of sapwood in an actively
transpiring tree should, in turn, cause a decrease in the
diameter of the trunk.
The graph shows the results of obtained by D. T. MacDougall when he made
continuous measurements of the diameter of a Monterey pine. The
diameter fluctuated on a daily basis reaching its minimum when
the rate of transpiration reached its maximum (around noon)
The rattan vine may climb as high as 150 ft on the trees of the
tropical rain forest in northeastern Australia to get its foliage into
the sun. When the base of a vine is severed while immersed in a basin of
water, water continues to be taken up. A vine less than 1 inch in
diameter will "drink" water indefinitely at a rate of up to 12
ml/minute.
If forced to take water from a sealed container, the vine does so
without any decrease in rate, even though the resulting vacuum becomes
so great that the remaining water begins to boil spontaneously. (The
boiling temperature of water decreases as the air pressure over the
water decreases, which is why it takes longer to boil an egg in Denver
than in New Orleans.)
Transpiration-pull enables some trees and shrubs to live in
seawater. Seawater is markedly hypertonic
to the cytoplasm in the roots of the coastal mangrove, and we might
expect water to leave the cells resulting in a loss in turgor
and wilting. In fact, the remarkably high tensions (on the order of
500–800 lb/in2) in the xylem can pull water into the plant
against this osmotic
gradient. So mangroves literally desalt seawater to meet their
needs.
Problems with the theory
When water is placed under a high vacuum, any dissolved gases come out
of solution as bubbles (as we saw above with the rattan vine). This is
called cavitation. Any impurities in the water enhance the
process. So measurements showing the high tensile strength of water in
capillaries require water of high purity — not the case for sap in the
xylem.
So might cavitation break the column of water in the xylem and thus
interrupt its flow? Probably not so long as the tension does not greatly
exceed 270 lb/in2.
By spinning branches in a centrifuge, it has been shown that water in
the xylem avoids cavitation at negative pressures exceeding 225 lb/in2.
And the fact that sequoias can successfully lift water 358 ft (109 m) —
which would require a tension of 270 lb/in2 — indicates that
cavitation is avoided even at that value.
However, such heights may be approaching the limit for xylem transport.
(The tallest tree ever measured, a Douglas fir, was 413 ft. High.)
Measurements close to the top of the tallest living sequoia (370 ft
high) show that the high tensions needed to get water up there have
resulted in:
Smaller stomatal
openings, causing
lower concentrations of CO2 in the needles, causing
reduced photosynthesis, causing
reduced growth (smaller cells and much smaller needles).
(See Koch, G. W. Et al., Nature, 22 April 2004.)
Root Pressure
When a tomato plant is carefully severed close to the base of the stem,
sap oozes from the stump. The fluid comes out under pressure which is
called root pressure.
Root pressure is created by the osmotic
pressure of xylem sap which is, in turn, created by dissolved
minerals and
sugars
that have been actively transported into the apoplast of the stele.
One important example is the sugar maple when, in very early spring, it hydrolyzes
the starches stored in its roots into sugar. This causes water to
pass by osmosis through the endodermis and into the xylem ducts. The
continuous inflow forces the sap up the ducts.
Although root pressure plays a role in the transport of water in the
xylem in some plants and in some seasons, it does not account for most
water transport.
Few plants develop root pressures greater than 30 lb/in2,
and some develop no root pressure at all.
The volume of fluid transported by root pressure is not
enough to account for the measured movement of water in the xylem of
most trees and vines.
Those plants with a reasonably good flow of sap are apt to have
the lowest root pressures and vice versa.
The highest root pressures occur in the spring when the sap is
strongly hypertonic
to soil water, but the rate of transpiration is low. In summer, when
transpiration is high and water is moving rapidly through the xylem,
often no root pressure can be detected.
So although root pressure may play a significant role in water transport
in certain species (e.g., the coconut palm) or at certain times, most
plants meet their needs by transpiration-pull.
Link
to discussion of food transport in the phloem.
Welcome&Next
Search
25 January 2010
of Water and Minerals in Plants
Most plants secure the water and minerals they need from their roots.
The path taken is: Soil -> roots -> stems -> leaves
The minerals (e.g., K+, Ca2+) travel dissolved in
the water (often accompanied by various organic molecules supplied by
root cells).
Less than 1% of the water reaching the leaves is used in photosynthesis
and plant growth. Most of it is lost in transpiration.
Link
to Transpiration
However, transpiration does serve two useful functions:
It provides the force for lifting the water up the stems.
It cools the leaves.
Water and minerals enter the root by separate paths which eventually
converge in the stele.
Link
to diagram of root structure.
The Pathway of Water
Soil water enters the root through its epidermis.
It appears that water then travels in both
the cytoplasm of root cells — called the symplast — that
is, it crosses the plasma membrane and then passes from cell to cell
through plasmodesmata.
In the nonliving parts of the root — called the apoplast
— that is, in the spaces between the cells and in the cells walls
themselves. This water has not crossed a plasma membrane.
However, the inner boundary of the cortex, the endodermis, is
impervious to water because of a band of suberized matrix called the casparian
strip. 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.
Once inside the stele, water is again free to move between cells as well
as through them.
In young roots, water enters directly into the xylem vessels
and/or tracheids [link
to views of the structure of vessels and tracheids]. These are
nonliving conduits so are part of the apoplast.
Once in the xylem, water with the minerals that have been deposited in
it (as well as occasional organic molecules supplied by the root tissue)
move up in the vessels and tracheids.
At any level, the water can leave the xylem and pass laterally to supply
the needs of other tissues.
At the leaves, the xylem passes into the petiole and then into the veins
of the leaf. Water leaves the finest veins and enters 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 Pathway of Minerals
Minerals enter the root by active
transport into the symplast of epidermal cells and move toward and
into the stele through the plasmodesmata connecting the cells.
They enter the water in the xylem from the cells of the pericycle (as
well as of parenchyma cells surrounding the xylem) through specialized
transmembrane channels.
What Forces Water
Through the Xylem?
Observations
The mechanism is based on purely physical forces because the
xylem vessels and tracheids are lifeless.
Roots are not needed. This was demonstrated over a century ago
by a German botanist who sawed down a 70-ft oak tree and placed the base
of the trunk in a barrel of picric acid solution. The solution was
drawn up the trunk, killing nearby tissues as it went.
However, leaves are needed. When the acid reached the leaves
and killed them, the upward movement of water ceased.
Removing a band of bark
from around the trunk — a process called girdling — does not
interrupt the upward flow of water. Girdling removes only the phloem,
not the xylem, and so the foliage does not wilt. (In due course,
however, the roots — and thus the entire plant — will die because the
roots cannot receive any of the food manufactured by the leaves.)
Transpiration-Pull
In 1895, the Irish plant physiologists H. H. Dixon and J. Joly proposed
that water is pulled up the plant by tension (negative pressure)
from above.
As we have seen, water is continually being lost from leaves by
transpiration. Dixon and Joly believed that the loss of water in the
leaves exerts a pull on the water in the xylem ducts and draws more
water into the leaf.
But even the best vacuum pump can pull water up to a height of only 34
ft or so. This is because a column of water that high exerts a pressure
(~15 lb/in2) just counterbalanced by the pressure of the
atmosphere. How can water be drawn to the top of a sequoia (the tallest
is 370 feet high)? Taking all factors into account, a pull of at least
270 lb/in2 is probably needed.
The answer to the dilemma lies the cohesion of water molecules;
that is the property of water molecules to cling to each through the
hydrogen bonds they form.
Link
to discussion of hydrogen bonding in water.
When ultrapure water is confined to tubes of very small bore, the force
of cohesion between water molecules imparts great strength to the column
of water. It has been reported that tensions as great as 3000 lb/in2
are needed to break the column, about the value needed to break steel
wires of the same diameter. In a sense, the cohesion of water molecules
gives them the physical properties of solid wires.
Because of the critical role of cohesion, the transpiration-pull theory
is also called the cohesion theory.
Some support for the theory
If sap in the xylem is under tension, we would expect the column
to snap apart if air is introduced into the xylem vessel by puncturing
it. This is the case.
If the water in all the xylem ducts is under tension, there
should be a resulting inward pull (because of adhesion) on the walls of
the ducts. This inward pull in the band of sapwood in an actively
transpiring tree should, in turn, cause a decrease in the
diameter of the trunk.
The graph shows the results of obtained by D. T. MacDougall when he made
continuous measurements of the diameter of a Monterey pine. The
diameter fluctuated on a daily basis reaching its minimum when
the rate of transpiration reached its maximum (around noon)
The rattan vine may climb as high as 150 ft on the trees of the
tropical rain forest in northeastern Australia to get its foliage into
the sun. When the base of a vine is severed while immersed in a basin of
water, water continues to be taken up. A vine less than 1 inch in
diameter will "drink" water indefinitely at a rate of up to 12
ml/minute.
If forced to take water from a sealed container, the vine does so
without any decrease in rate, even though the resulting vacuum becomes
so great that the remaining water begins to boil spontaneously. (The
boiling temperature of water decreases as the air pressure over the
water decreases, which is why it takes longer to boil an egg in Denver
than in New Orleans.)
Transpiration-pull enables some trees and shrubs to live in
seawater. Seawater is markedly hypertonic
to the cytoplasm in the roots of the coastal mangrove, and we might
expect water to leave the cells resulting in a loss in turgor
and wilting. In fact, the remarkably high tensions (on the order of
500–800 lb/in2) in the xylem can pull water into the plant
against this osmotic
gradient. So mangroves literally desalt seawater to meet their
needs.
Problems with the theory
When water is placed under a high vacuum, any dissolved gases come out
of solution as bubbles (as we saw above with the rattan vine). This is
called cavitation. Any impurities in the water enhance the
process. So measurements showing the high tensile strength of water in
capillaries require water of high purity — not the case for sap in the
xylem.
So might cavitation break the column of water in the xylem and thus
interrupt its flow? Probably not so long as the tension does not greatly
exceed 270 lb/in2.
By spinning branches in a centrifuge, it has been shown that water in
the xylem avoids cavitation at negative pressures exceeding 225 lb/in2.
And the fact that sequoias can successfully lift water 358 ft (109 m) —
which would require a tension of 270 lb/in2 — indicates that
cavitation is avoided even at that value.
However, such heights may be approaching the limit for xylem transport.
(The tallest tree ever measured, a Douglas fir, was 413 ft. High.)
Measurements close to the top of the tallest living sequoia (370 ft
high) show that the high tensions needed to get water up there have
resulted in:
Smaller stomatal
openings, causing
lower concentrations of CO2 in the needles, causing
reduced photosynthesis, causing
reduced growth (smaller cells and much smaller needles).
(See Koch, G. W. Et al., Nature, 22 April 2004.)
Root Pressure
When a tomato plant is carefully severed close to the base of the stem,
sap oozes from the stump. The fluid comes out under pressure which is
called root pressure.
Root pressure is created by the osmotic
pressure of xylem sap which is, in turn, created by dissolved
minerals and
sugars
that have been actively transported into the apoplast of the stele.
One important example is the sugar maple when, in very early spring, it hydrolyzes
the starches stored in its roots into sugar. This causes water to
pass by osmosis through the endodermis and into the xylem ducts. The
continuous inflow forces the sap up the ducts.
Although root pressure plays a role in the transport of water in the
xylem in some plants and in some seasons, it does not account for most
water transport.
Few plants develop root pressures greater than 30 lb/in2,
and some develop no root pressure at all.
The volume of fluid transported by root pressure is not
enough to account for the measured movement of water in the xylem of
most trees and vines.
Those plants with a reasonably good flow of sap are apt to have
the lowest root pressures and vice versa.
The highest root pressures occur in the spring when the sap is
strongly hypertonic
to soil water, but the rate of transpiration is low. In summer, when
transpiration is high and water is moving rapidly through the xylem,
often no root pressure can be detected.
So although root pressure may play a significant role in water transport
in certain species (e.g., the coconut palm) or at certain times, most
plants meet their needs by transpiration-pull.
Link
to discussion of food transport in the phloem.
Welcome&Next
Search
25 January 2010
