in Transport Plants - 1
During the past few weeks, we examined the structure of the higher plant body,
with occasional references to the functions of plant systems. In the next few days we
shall look at how plants are adapted for resource acquisition and to:
Maintain water balance and transport water throughout the plant
Transport nutrients and solutes to cells and tissues
Obtain nutrients for growth and survival from their substrate and from the
atmosphere
Regulate growth and developmental activities
o Chemical signals
o Environmental signals
Respond to environmental challenges
As we look at these subjects, keep in mind how other organisms with which you are
more familiar accomplish these same functions.
Resource Acquisition
Vascular Plant Transport
The plant vascular system functions to transport water and minerals throughout the
plant. In addition, plant structure and function is optimized for the process of
photosynthesis to efficiently obtain water, CO2 and light energy in the
photosynthetic regions of the plant, primarily leaves, a subject briefly addressed in
Biology 211, and reviewed here.
Shoot Structure and Light Acquisition
As discussed earlier, stems elevate the plant above ground and serve for the
attachment of leaves. Phyllotaxy, the arrangement of leaves on the stem, is typically
optimized for maximum light capture for photosynthesis, and the typical plant has a
spiral phyllotaxy, with leaf primordia placement chemically controlled by the shoot tip
meristem.
Transport in Plants - 2
The leaf area index, the ratio of the upper leaf area to the area of the plant, is one
way to measure the plant's ability to capture light energy.
Root Structure and Water and Mineral Acquisition
In a similar fashion, branching roots, with root tips having root hairs to increase
surface area for absorption, maximize the root's ability to take advantage of local
resource regions in their substrate. A significant advance in land plants was the coevolution with mycorrhizae fungi, discussed previously, which increases surface area
for resource extraction for both plant and fungus.
Transport in Plants
Our plant ancestors were aquatic filamentous or laminar (sheet like) organisms whose
cells came into contact with their environmental medium, which also supported the
plant body. Water and needed nutrients were readily available to individual cells, and
little if any differentiation of plant tissues occurred.
Migration to land and differentiation of the plant body into tissues and organs required
systems to transport needed materials to cells in different parts of the plant from the
region where they were obtained. In addition, land plants needed structural support.
Both support and conduction are accomplished with secondary vascular tissue in
woody plants. As studied previously, herbaceous plants use water pressure to
maintain turgor.
Plant transport involves three activities:
Movement of water, gases and solutes into and out of individual cells
throughout the plant, from the environment into the plant (generally from soil
or other substrate into roots), and from the plant out into the environment
Localized transport of materials from cell to cell in tissue regions, such as
loading solutes into phloem sieve tubes
Distance transport of water and solutes through the vascular tissues of the
plant. The tallest trees (Pacific Coast Redwood and Australian Eucalyptus) need
to be able to transport water and solutes to heights of over 300 feet.
To start our discussion, it will be valuable to review a bit about membranes and
membrane transport mechanisms.
Transport in Plants - 3
Review of Membrane Transport Mechanisms
The plasma membrane is selectively permeable. Materials that enter and leave a cell
must pass through the membrane. The direction of water movement is dependent on
the gradient between a cells internal and external environment.
Although membranes are permeable to water, small membrane transport proteins,
called aquaporins, also facilitate the diffusion of water, particularly across the
central plant vacuole tonoplast.
Most solutes, even if permeable, do not diffuse readily, and rely on transport proteins
or membrane protein transport channels for facilitated diffusion. Recall that transport
proteins have a selective binding site for the solute to attach on one side of the
membrane and then carry the solute through the membrane releasing it on the other
side. Channels allow free passage of specific substances, and some channels are
gated to control solute passage. K+ channels are common in the membranes of plant
cells, and K+ plays a vital role in opening and closing of stomata (see later).
Plant cells also rely on active transport. The H + proton pump is a common use of
active transport. The cell creates a proton gradient by pumping H+ out of the cell,
which is a source of potential energy (for the passive diffusion of the H+ back into
the cell through the membrane), and creates the membrane potential (the
differential charge between the interior, more negative charge, and external more
positive charge), a second source of potential energy.
Proton Gradient and Membrane Potential
Membrane Potential favors diffusion of
+
ions
The membrane potential facilitates the movement of positively charged ions into
cells. The ion movement is passive but is a consequence of the active movement of
H+ through the proton pump that maintains the membrane potential. The proton
pump moves many minerals into cells in quantities greater than what is found in the
environment.
Transport in Plants - 4
Negative ion movement can be coupled to H+ flow along the H+ gradient. The
negative charges are attracted to the H+ and can be moved against their gradient, in a
process called cotransport.
Cotransport of negative ions
Cotransport of neutral solutes
Cotransport is also used to move neutral solutes such as sucrose against a gradient.
As H+ moves down its gradient it is coupled to a substance being moved against its
gradient. Sugar is loaded and unloaded from phloem sieve tubes by cotransport. Use
of proton pumps for solute movement is a variant of the general process of
chemiosmosis, which generates ATP in photosynthesis and cell respiration.
To add to the information about membranes and membrane transport, we also need a
brief diversion on water potential before discussing the transport of water in cells and
throughout the plant body.
Water in? Water out? The Water Potential ( ) Discussion
Ultimately, whether water is gained or lost by cells is a consequence of osmosis
(although aquaporins also play a role in water uptake). With the exception of cells
that have thickened secondary walls, plant cells need to have adequate water in their
cells to maintain a physical pressure on the cell wall for turgor.
There are two factors involving water and osmosis in plant cells:
solute concentration
turgor pressure.
These two factors combined comprise the water potential, which is known as
(psi). In plants water always moves from greater water potential to a lesser water
potential. Water potential ( ) in plants is measured in some arcane thing called
megapascals (Mpa) which is equal to about 10 atmospheres of pressure (that
famous 15 lb/in2 or 1 kg/cm2)
How can we use this information to discuss how water moves in plants?
Pure water has a water potential of 0 Mpa ( = 0 Mpa).
Any solution will have a negative water potential relative to pure water (which
explains why water moves from a greater water potential to a lesser water potential).
With osmosis, pure water will pass through a membrane into a solution whose water
potential is negative.
Transport in Plants - 5
But for a plant its not this simple. Plants have that physical pressure against the
cell wall (called the pressure potential) that affects water potential. So to determine
the movement of water into and out of a plant cell, one needs to have that
information, too.
True water potential equals the sum of the solute (osmotic)
physical pressure of the wall, or
=
s
+
potential plus the
p
What might seem really tricky here for plants is that one can also have a negative
pressure or tension (a suction pressure such as we use when we suck beverages
through a straw). This means that although s is always negative, p can be
positive or negative, although in plants it is always positive, and that affects
whether the water potential will be positive or negative.
Solute and wall pressure effects on water movement
Adding solutes reduces
water potential
Adding pressure increases water potential
Negative pressure
decreases water potential
Having said a lot, all that we really need to remember here is that water moves from a
greater to a lesser water potential and thats important for plants. And when we
measure water potential in plants, as we move from soil, to roots to stem to leaves
we get a more negative water potential.
Transport in Plants - 6
To Summarize:
When the water potential inside the cell is higher than the environment, water will
leave cells and plasmolysis results. This phenomenon is frequently observed in
plants left in hot sunny windows and not watered adequately. It can be observed on
the cellular level, too.
Plasmolysis and Water Potential
Turgor and Water Potential
When the cells water potential is less than the environment, water will move into the
cell until the water potential is equalized. The water pressure inside the cell results in
turgor, which we have previously discussed, and is important to plant structure and
strength for primary growth tissues. Most plant cells achieve turgor for their normal
functioning state. The plant central vacuole helps maintain turgor. The vacuole
holds water and other substances that help maintain proton gradients within the
cytosol. Maintaining water balance is critical to plants, just as it is to animals.
Water Balance, Water Movement and Transpiration Water Loss
Plants are 80-90% water (wet weight). Soil and atmosphere usually contain a much
lower proportion of water (winter and spring in Puget Sound excepted). Most plants
present large surface areas to their surroundings; both the root and leaf surface areas
are large: roots to absorb water and nutrients, leaves for exposure to sun for
photosynthesis. Plants consume and lose much more water than we do, about 17
times as much.
Transport in Plants - 7
Transpiration
Plants are especially vulnerable to water loss because they need porous surfaces for
gas exchange. To a significant extent, water enters plants through roots, travels
through the vascular system and departs through open stomata in leaves.
Most water lost from plants is by the process of transpiration, the evaporation of
water from the plant through open stomata and other plant surfaces. Transpiration
also plays a role in the movement of water throughout the plant, as we shall discuss in
a bit.
Transpiration loss is significant a mature maple tree can evaporate more than 50
gallons of water a day. In cornfields, as much as 90% or more of the water absorbed
by the roots is lost by transpiration.
Naturally, plants have a number of ways to conserve water and minimize transpiration
losses:
Epidermal cells on above-ground structures have a cuticle layer (cutin) to
prevent water loss; the walls of cork cells contain impermeable suberin.
Plant cells have vacuoles to accumulate water, and cell walls to help maintain
turgor. (This works better at preventing excess water than it does at preventing
dehydration.)
Many cells and tissues need not be maintained because theyre dead (saves
energy as well as water needed for metabolic functions)
Xeromorphic plants frequently have anatomical adaptations to minimize
transpiration loss as well as physiological adaptations such as the CAM
metabolism discussed with photosynthesis
Most transpiration loss is through stomata. Guard cells open and close so that
water loss by transpiration is minimized when plants cant use CO 2 for
photosynthesis. (The mechanism for stomatal operation will be discussed in a bit.)
Even so, taking in sufficient water for metabolic needs and to counter transpiration
loss is a challenge that plants must meet and physicists can explain.
Transport in Plants - 8
Moving Water the Mechanisms
We need to answer the following in our discussion of water movement mechanisms:
1. How does water enter and move through the plant when plants have no pumps?
2. How does a plant move water upward as much as 300 feet above the ground
against the force of gravity?
Water and other substances move into and within the plant root in a variety of ways:
Apoplastic movement is movement of water through intercellular spaces and
cell walls
Transmembrane movement moves water from cell to cell via membrane
transport
Symplastic movement is movement of water through plasmodesmata from cell
to cell, once water has entered a cell (Transmembrane and symplastic
movement are sometimes considered to be the same.)
Root hairs increase the surface area for absorption, and soil particles with their
surrounding water films adhere to root hairs to promote a diffusion gradient. Root
hairs may also use active transport for uptake. Mycorrhizae associates absorb both
water and minerals and transfer them into the roots.
Once through the root cortex, substances pass into the cells of the endodermis
(protected by the casparian strip) and from root stele parenchyma cells into xylem
tissue for upward movement throughout the plant. Dilute minerals also move with
water. In spring, even very dilute solutes can move with water through the xylem.
The substances that move through xylem are called sap.
Transport in Plants - 9
Moving through Xylem The CohesionTension Theory
Although early researchers attempted to explain water movement by some
mysterious "pumping" action within cells, no pumps have been found in plants. In
1893, Eduard Strasberger demonstrated that leaves were critical to water movement.
He took trees of 20-meter height, cut them at the base of the trunk and placed them
in a toxic copper sulfate solution. The solution progressively moved up through the
severed trunk until it reached the leaves and the leaves died. Loss of roots did not
affect movement of the solution.
Additional research further demonstrated that positive root pressure could not explain
water movement. Although root pressure is about 0.1 0.2MPa (or 1 2
atmospheres) (and is a positive force in water movement during some circumstances
discussed later), a positive pressure would have to be sustained throughout the xylem
of the trunk, and most xylem is actually under a tension, a phenomenon that helps to
explain how water appears to move upward throughout the plant.
Ultimately it was been demonstrated that the loss of water through transpiration,
discussed earlier, plays a significant role in water movement throughout the xylem.
Water lost by transpiration creates a negative water potential in cells that
exerts a pull on the H2O in cell walls that is connected (by cohesion) to H2O in
xylem resulting in a strong tension the in xylem.
As water evaporates out of the stomata, the film of water that coats mesophyll
cells diminishes. Primary cell walls have adhesive properties; the remaining
water is attracted to the walls (cellulose is very hydrophilic think of the paper
towel commercials), resulting in even less water and the water potential of the
mesophyll cells decreases.
Transport in Plants - 10
Water molecules also tend to cohere (stick to each other -the surface tension
property of water). This second force tends to make water molecules take on a
concave shape that resists the increasing surface area. The two forces
together (adhesion and cohesion) generate a negative water pressure that
literally pulls water in the leaf out of the xylem.
This transpiration pull can be exerted throughout the xylem down to the roots
making the root water potential negative, too.
o When transpiration rate is high, the osmotic gradient across the
endodermis decreases as dissolved ions are drawn upward in the xylem.
The depletion of water in the root hair zone can draw water from
surrounding soil areas.
This combination of forces (transpiration and cohesion) is sufficient to move
water upward against the forces of gravity.
In addition, the diameter of vessels and tracheids promotes cohesion.
It also means that a plant disadvantage, water lost by transpiration, can be
turned around to do something beneficial for the plant.
For all of this to work, one must have an unbroken column of water. Air bubbles in
the column dramatically affect water movement. A rupture of the water column is
called cavitation and vessels affected will no longer function. The air or water vapor
blockage that forms after the cavitation is called an embolism.
Transport in Plants - 11
In case you were curious about if this idea of sucking water up the tree holds water,
there are ways of measuring all of this.
Pressure bombs measure the hydrostatic pressure within the xylem. When a
twig is cut, water recedes into the interior. The twig is put into a pressure
chamber and gas pressure is applied until water appears at the cut end of the
twig. That pressure can be measured. It is a positive pressure that equates to
the tension within the xylem.
Thermocouples have been used to measure the velocity of movement of small
amounts of heated xylem. Xylem movement is always detected earlier in
twigs than in the trunk of trees.
The negative transpiration pressure is reflected by a narrowing of the diameter
of the tree as xylem cells narrow in diameter from the tension generated.
In addition, the rate of xylem movement varies. Virtually no movement occurs at
night, when there is no transpiration. In the daytime, the rate of xylem movement
depends on environmental conditions such as temperature, wind velocity, and light
intensity, as well as solutes in the sap. Studies have shown that the rate of xylem
movement increases with K+ concentration in the sap, and slows when K + diminishes.
Transport in Plants - 12
Transpiration is also be responsible for some water and dissolved ion movement from
soil into the roots. When transpiration rate is high, the osmotic gradient across the
endodermis decreases as dissolved ions are drawn upward in the xylem. The depletion
of water in the root hair zone can draw water from surrounding soil areas.
Although the transpiration-cohesion explanation for water movement through xylem is
"widely" accepted, many questions remain and water movement through xylem is an
active area of research in plant physiology. For example, some vessel walls are lipid
lined, so no water adhesion occurs in such vessels. Others question the accuracy of
pressure bombs and other mechanisms for measuring water potential. In addition, the
rate of water movement in xylem is not uniform within "equivalent" regions of plants.
Transpiration-cohesion is surely a force in water movement through xylem. As
techniques improve, as with all science, better explanations may result.
Controlling Transpiration The Stomatal Mechanism
Transpiration is, in large part, a consequence of the process of photosynthesis. Plants
must have access to CO2 from the atmosphere. CO2 diffuses into plants through open
stomata. There may be as many as 12,000 20,000 stomata per cm2 in the leaf
epidermis. (As an aside, stomatal density is affected by CO2 concentration in some plants.
Stomatal density has decreased in one woodland area of England that has been studied since
1927 corresponding to an increase in ambient CO2 levels.) At the same time water is lost
through the stomata by transpiration. Since it is important to conserve as much water
as possible, plants have mechanisms to open and close their stomata to minimize
water loss during non-photosynthetic times and/or when transpiration exceeds the
ability of the plant to metabolically function. (This latter affects photosynthesis.)
How stomata work
The mechanical operation of stomata is a phenomenon of turgor, osmotic balance and
active transport, helped by the structure of the guard cells. Guard cells are typically
bean shaped with radially oriented cellulose microfibrils in their walls that limit
stretching to one direction when water is absorbed by the cell. The pair of guard cells
are attached at their ends, so when they stretch, they become distorted. This
distortion results in a shape that causes a gap between the guard cell pairs inner
walls. This gap is the stoma. When guard cells lose turgor, they shrink, and the
collapsed cells force the inner walls of the guard cell pair together, closing the stoma.
Transport in Plants - 13
To produce these changes in turgor, a ratio of potassium K+ and H2O is maintained
within guard cells that is different in daytime than nighttime. In addition, in contrast
to other epidermal cells, guard cells contain chloroplasts, and the process of
photosynthesis is used to maintain turgor when stomata are open.
Daytime
1. K+ moves into guard cells in response to presence of light through membrane
channels using the proton gradient generated by hydrogen proton pumps. H + is
actively pumped out of guard cells at the time stomata are opening. The
pumps, which are activated by blue-light receptors in the guard cell
membrane, create a negative water potential in the cell. (More solutes = more
negative water potential).
2. The increasing + charge (K+ may be 4 8 time higher within the cell) is
countered in some plants by the active transport of Cl- into the cell. The Cluptake is coupled to a proton pump generated by the release of H+ from a
number of organic acids within the guard cells. The acids also help increase the
balance.
3. K+ concentration remains much higher in guard cells than in surrounding
epidermal cells when stomata are open.
4. H2O moves by osmosis into cell in response to K+ solute concentration,
increasing the cell volume (a turgor phenomenon) swelling the guard cells which
open forming the stoma pore. The water is stored in the vacuole.
5. Photosynthesis solute products, along with the H+/ K+ pump maintain the
osmotic gradient needed for turgor, keeping the stomata open during
photosynthetic hours.
Transport in Plants - 14
Nighttime, water deficit or some other harmful conditions*
1. K+ leaches out of guard cells passively.
2. H2O follows.
3. Guard cells lose turgor and close.
K+ and water movement out of guard cell
Things that affect stomatal opening and closing
Stomatal Opening
As stated, blue light receptors in the guard cells activate the H+/K+ pumps.
Photosynthesis maintains a high solute concentration for turgor and also
provides a source of ATP for the proton pumps to function. Guard cell
chloroplasts respond to red light.
Low CO2 in the mesophyll, which occurs when photosynthesis starts, can trigger
stomatal opening. Artificially low CO2 levels even in the absence of light can
trigger stomatal opening.
Stomata opening may also be under the influence of circadian rhythms.
Stomatal Closing
Abscisic acid produced in roots and translocated through xylem monitors water
concentration in the plant (root and leaf mesophyll cells) and when there is a
water deficit, serves as a signal for a transduction pathway (that has a Ca ++
secondary messenger) which results in ion (K+ especially, but possibly Cl- and
malate) movement from the guard cells, closing stomata within minutes.
High CO2 levels detected in guard cells will promote stomatal closure
High temperatures also trigger stomatal closure (but may be related to the CO 2
concentrations at higher temperature).
Some chemicals, such as ozone, also promote stomatal closure.
Plants can respond to long-term water deficit by dropping leaves, reducing
transpiration, common in many desert plants.
The genetic model plant, Arabidopsis, when grown in high CO2 concentrations,
produces fewer stomata per leaf, validating the observations of the English
woodland.
Transport in Plants - 15
Other Water Movements
Before we leave our discussion of movement of water in plants, lets look at a few
other water phenomena.
Imbibition
Water can be absorbed rapidly into cells by means other than osmosis. Certain
molecules, especially starch and cellulose, attract water molecules when they are
wet because of surface charges (+ and -). Since water is polar, it is attracted to the
surfaces of these molecules, and large amounts of water can be taken into cells in this
manner. Imbibition is very important for the process of germination, causing the
seeds to swell rapidly with the uptake of water.
Imbibition
Force of Imbibition
Positive Root Pressure and Guttation
Roots actively move nutrient ions from the soil all of the time, and when transpiration
is low, the increasing concentration of ions within the cortex cells creates a water
potential gradient for moving water into the roots. This results in a positive pressure.
Simple diffusion pressure in roots moves H2O upward often forcing the H2O to be
exuded from vein tips in leaves, a phenomenon called guttation. The special leaf tip
cells are called hydathodes. Guttation is limited. Positive root pressure is soon
matched by the atmospheric pressure, and in daylight, transpiration rates rapidly
exceed any positive pressure generated.
Root Pressure
Guttation
Transport in Plants - 16
Movement of Solutes
While some solutes, including many minerals, are moved through xylem, most organic
solutes, especially sucrose, are transported, or translocated, in phloem. Phloem is also
the vehicle for much of the chemical communication that occurs within the plant.
Phloem is responsible for the translocation of many signal molecules, including
hormones and various RNA molecules that activate systemic changes such as defense
responses and induction of flowering shoots. Molecules produced in one area are
moved through phloem to their target destination. Electrical signals also move
through phloem, including electrical signals that activate some rapid turgor responses.
Phloem Movement
The discovery of phloem movement is credited to Malpighi who recorded that when
one rings a tree, the tree dies by lack of nourishment below the ring. Although this
was noted a long time ago, learning how phloem transports solutes was inhibited
because access to the phloem tissue was difficult. (The cells collapsed and quit
functioning when manipulated.)
Ultimately, the common aphid was used as a research tool. Aphids normally penetrate
into phloem to feed, and their actions do not stop the phloem activity in the plant.
Aphids merely divert the flow into (and sometimes through) the aphid body.
Sucrose has a high osmotic potential (hydrostatic pressure). While sucrose, like any
substance, can move by diffusion, the normal rate of phloem movement is much
faster than simple diffusion.
Movement in phloem is always in a direction of more concentrated solute to less
concentrated, or from where you have solutes (mostly sugars from photosynthesis)
to where you need the solutes. Phloem can also be used to move solutes from
storage areas to where solutes are needed. This pressure-flow gradient between
the source of solutes and the sink, the location to which the solutes are being
moved, explains how solutes are moved through phloem.
Transport in Plants - 17
Solutes move through leaf mesophyll or storage parenchyma cells both symplastically
and apoplastically to the vascular tissue. Solutes are then actively secreted, or
loaded, from the source into a sieve tube. Transfer companion cells do this.
Proton pumps, discussed earlier, use cotransport to transfer the sugars, which may
accumulate concentrations two to three times the surrounding areas.
Once the sugars are in the sieve tubes, movement is facilitated by osmotic
potential (turgor increases the hydrostatic pressure in the cells). The presence
of sugar in a sieve tube attracts water.
Water moves into the sieve tube from adjacent xylem cells increasing the
pressure in the sieve tube
Increased pressure forces the solutes into the next cell of the sieve tube. This
mechanism is called pressure flow, a type of bulk flow.
Active transport (cotransport with proton pumps) is again used at the sink, to move
the solutes into the cells where they will be needed or stored.
Transport in Plants - 18
Other Phloem Transport
Revisiting the Symplast and Plasmodesmata
When cell wall structure is discussed in biology, plasmodesmata, membrane channels
between adjacent plant cells are described. Earlier in this section, the role of
plasmodesmata in symplast movement of water in roots was also mentioned.
In recent years, research on plasmodesmata had resulted in more information on these
intercellular connections and their structure and function. The plasmodesmata are
critical to the overall communication that maintains localized symplast domains within
the plant.
Environmental changes are rapidly transmitted through the dynamic symplast,
particularly in relation to plant transport, and plasmodesmata appear to change in
response to a number of signals.
Plasmodesmata may open and close rapidly in response to turgor changes, pH and/or
Ca++ concentrations affecting symplast movement. The number of plasmodesmata
can change during development and functioning of the cell. Leaf cells have many
plasmodesmata during development because they need solutes for cell maturation.
Once mature, photosynthesis supplies the solutes and the number of plasmodesmata
decreases.
Research on mutant strains of Arabidopsis demonstrated that signals to initiate root
hair development are normally transmitted cell-to-cell via plasmodesmata, or in this
case, inhibition, since cessation of the signal molecule initiates root hair development.
Recent studies on plant viral infections have shown the viral proteins can dilate
plasmodesmata facilitating viral movement from cell to cell. The viral proteins are
mimicking plant proteins that normally regulate plasmodesmata diameter.
Investigating plasmodesmata and the symplast is just one area of active research on
how plants communicate and the relationship of structure to function.