LIQUID TRANSPORT SYSTEMS IN PLANTS
Plants transport water and dissolved materials throughout their
bodies. Water and dissolved minerals move upward in xylem from roots to
leaves. Water and dissolved carbohydrates move up and down in the
phloem.
Principal terms:
GUARD CELLS: specialized epidermal cells that border a stoma
PHLOEM: the tissue in plants that transports water and dissolved
carbohydrates
PRESSURE-FLOW HYPOTHESIS: a hypothesis accounting for the movement
of carbohydrates in the phloem
STOMA: an opening in the epidermis that controls movement of gases
into and out of leaves
TRANSPIRATION: the evaporation of water from leaves of plants
TRANSPIRATION-COHESION HYPOTHESIS: a hypothesis accounting for the
movement of water and dissolved minerals in the xylem
XYLEM: the tissue in plants that transports water and dissolved
minerals from roots to leaves
Summary of the Phenomenon
-------------------------
Water is the most abundant compound in plant cells: It accounts for
85-95 percent of the weight of most plants and even makes up 5-10
percent of the weight of "dry" seeds. More than 95 percent of the water
gathered by a plant, however, evaporates back into the atmosphere, often
within only a few hours after being absorbed. This evaporation of water
from a living plant is called transpiration. Most transpiration is from
leaves.
Plants transpire huge amounts of water. On a warm, dry day, an
average-size maple tree transpires more than 200 liters per hour, while
herbaceous plants transpire their own weight in water several times per
day. A corn plant transpires almost 500 liters of water during its four
month growing season. If humans required an equivalent amount of water,
each would have to drink approximately 40 liters of water per day.
Leaves are the primary photosynthetic organs of most plants. The
rate of gas exchange for photosynthesis depends, among other things, on
the amount of surface area available for exchange and evaporation. The
loose internal arrangement of cells in leaves produces a large internal
surface area for transpiration--an area that may be more than two
hundred times greater than the leaf's external surface area. The
internal surface area of a leaf is connected with the atmosphere via an
extensive system of intercellular spaces that occupies as much as 70
percent of a leaf's volume. Stomata (sing. stoma) are pores that link
the internal surface area of a leaf with the atmosphere. Leaves have
many stomata; for example, a typical leaf of a squash plant has more
than 80 million stomata. Leaves also have an efficient system of veins
(vascular tissue) for distributing water to their internal evaporative
surface. One square centimeter of leaf may have as many as six thousand
outlets of vascular tissue.
Water lost via transpiration must be replaced by water absorbed
from the soil by the plant's roots. The movement of this water to the
leaves for evaporation is very rapid. Water molecules may move as fast
as 75 centimeters per minute, which is roughly equivalent to the speed
of the tip of a second hand sweeping around a wall clock.
Water and its dissolved minerals move from roots to leaves in
xylem. The two kinds of cells in xylem that carry water are tracheids
and vessel elements. Both of these cell types are hollow and dead at
maturity, and therefore they contain no structures that retard water
flow. Both of these cell types also have thick cell walls and can
therefore withstand the fluctuations in pressure associated with water
flow caused by transpiration. Tracheids are usually long (up to 10
millimeters) and thin (10-15 micrometers in diameter), and overlap one
another. Their walls have numerous thin areas that link adjacent
tracheids into long, water-conducting chains. Vessel elements are
shorter and much wider than tracheids. They are stacked end-to-end, and
the walls separating adjacent vessel elements are often wholly or
partially dissolved. Because of their larger diameter and dissolved
walls, water moves faster in vessels than in tracheids. This increased
flow rate in vessels may help explain why angiosperms dominate today's
landscapes: Flowering plants, such as cotton and grasses, contain
tracheids and vessels, while gymnosperms, such as pines, contain only
tracheids. In woody plants, the xylem that transports water and
dissolved minerals makes up the wood of the trunk.
Water movement through plants requires no metabolic energy. Rather,
water flows passively from one place to another. Water and dissolved
minerals are pulled up through the xylem. The driving force for this
movement is the transpiration of water from the leaves. This hypothesis
was formulated more than a century ago, and today is known as the
transpiration-cohesion hypothesis of water movement.
This theory states that solar-driven transpiration of water dries
the cell walls of mesophyll cells of leaves; the loss of water from the
cell wall then causes water from neighboring cells to enter the leaf
cell. Cells bordering tracheids and vessels replace their water with
water from the xylem. This loss of water from xylary elements creates a
negative pressure, thereby lifting the water column up the plant. The
water column does not break, because water molecules cohere strongly.
The negative pressure created in the xylem by transpiration extends all
the way down to the tips of roots, even in the the tallest trees. The
tension in the root xylem causes water to flow passively from the soil,
across the root cortex, and into the xylem of the root. This water is
then pulled up the xylem to leaves to replace water lost via
transpiration.
Transpiration is affected by atmospheric humidity, the
concentration of carbon dioxide in the leaf, wind, air temperature,
soil, and light intensity. Transpiration is greatest in plants growing
in moist soil on a sunny, dry, warm, and windy day. In these conditions,
in fact, transpiration often exceeds the plant's ability to absorb
water. As a result, many plants wilt at midday, despite the fact that
the soil in which they are growing may contain much water.
Almost all the factors that affect transpiration do so by
influencing the opening or closing of stomata. For example, decreasing
the internal concentration of carbon dioxide in a leaf causes stomata to
open and therefore increases transpiration. Stomata regulate gas
exchange between the atmosphere and a plant and are a key adaptation to
life on land. Stomata overlie air chambers, which are connected to an
extensive system of intercellular spaces that link mesophyll cells to
the atmosphere.
Stomata occur throughout the plant kingdom. In angiosperms, they
occur on all aboveground organs, including leaves, stems, petals,
stamens, and carpels. Open stomata occupy less than 1-2 percent of a
leaf's area. A stomatal complex consists of two guard cells and adjacent
epidermal cells called subsidiary cells, all of which surround a pore.
Guard cells and stomata have several distinguishing features. For
example, guard cells of dicotyledons are crescent-shaped, while in most
grasses they are shaped like dumbbells. Most leaves have 1,000 to
100,000 stomata per square centimeter of leaf area. Plants in dry,
bright environments such as deserts often have smaller and more numerous
stomata than plants growing in wet, shaded environments. When wide open,
stomatal pores are usually 3-12 micrometers wide and 10-40 micrometers
long.
Guard cells control the size of a stomatal pore by changing shape--
their unusually elastic walls buckle outward when stomata open and sag
inward when stomata close. Guard cells pump positively charged potassium
ions in and positively charged hydrogen ions out during stomatal
opening. Similarly, stomata close when positive potassium ions are
pumped out of the guard cells. This causes water to leave the guard
cells and the pore to close. Epidermal cells surrounding the guard cells
store the potassium ions when stomata are closed. Stomatal opening is
primarily caused by radial micellation of guard cells by cellulose
microfibrils arranged much like the belts in a belted tire. These
microfibrils are inelastic; they restrict radial expansion of guard
cells while allowing increases in length. When the guard cells lengthen
because of an influx of water, they bow apart and form the stomatal
pore.
Transpiration has several beneficial effects on plants. For
example, transpiration cools plants. Transpiration can cool a leaf to as
much as 10-15 degrees Celsius below air temperature. On a larger scale,
the heat absorbed by the evaporation of hundreds of liters of water per
day from large trees can make the interior of a forest 10-15 degrees
Celsius cooler than the surrounding countryside. Each large tree of a
forest has the cooling capacity of approximately five ten-thousand-watt
air conditioners. Transpiration also moves solutes in plants; for
example, most minerals move from roots to shoots in the transpiration
stream.
Sugars and other organic substances move in sieve tubes of the
phloem. Sieve tube members are arranged end-to-end and are associated
with files of companion cells. Companion cells and sieve elements
function as a single unit. Sieve tube members are tiny cylinders
approximately 40 micrometers in diameter and 1,200 micrometers long. The
protoplasts of sieve tube members are connected by sievelike areas
called sieve plates, each of which has numerous sieve pores. In woody
plants, the phloem makes up the innermost layers of the bark.
Peak rates of solute transport in phloem may exceed 2 meters per
hour. As a result, as much as 20 liters of sugary sap can be collected
per day from severed stems of sugar palm. A sieve element 0.5 millimeter
long empties and fills every two seconds, thereby delivering
approximately 5 to 10 grams of sugar per hour per square centimeter of
phloem area to sites of sugar storage or utilization.
Solutes move through the phloem via pressure flow. In 1926, German
plant physiologist Ernst Munch proposed the pressure-flow hypothesis,
which states that a turgor pressure gradient drives the unidirectional
mass flow of solutes and water through sieve tubes of the phloem.
According to this model, solutes move passively through sieve tubes
along a pressure gradient in a fashion analogous to the movement of
water through a garden hose. Sucrose produced at a source is actively
loaded into a sieve tube. This loading causes water to enter the sieve
tube via osmosis from the xylem. The influx of water into sieve tubes
carries sucrose to a sink, which is an area where sucrose is unloaded
for use or storage. Removing sucrose at the sink causes water also to
move out of the sieve tube. The influx of water at the source and the
efflux of water at the sink create a pressure-driven flow of fluids in
the phloem. Sugars in the cell wall are loaded into sieve tubes by
companion cells. These companion cells often have numerous cell-to-cell
channels called plasmodesmata. Their cell walls and cell membranes also
have elaborate infoldings that provide a large surface area for
transporting sugars from the cell wall into the sieve tube. The loading
of sieve tubes requires metabolic energy in the form of adenosine
triphosphate (ATP), and is driven indirectly by a proton gradient.
Phloem transport is affected by temperature, light, and the nutritional
status of the plant.
More than 90 percent of the solutes in sieve tubes are
carbohydrates. In most plants, these carbohydrates are transported as
sucrose (table sugar). The concentration of sucrose may be as high as 30
percent, thereby giving the phloem sap a syrupy thickness. A few plant
families also transport other sugars, such as raffinose and stachyose.
These sugars are similar and consist of sucrose attached to one or more
molecules of D-galactose. Like sucrose, all these sugars are nonreducing
sugars. This is important, since nonreducing sugars such as sucrose are
less reactive and less labile to enzymatic breakdown than are reducing
sugars such as glucose and fructose. Sieve tubes also contain ATP and
nitrogen-containing compounds. More than a dozen different amino acids
occur in sieve tubes of some plants. Sieve tubes also transport
hormones, alkaloids, viruses, and inorganic ions such as potassium ions.
Methods of Study
----------------
Biologists use radioactive tracers to follow sugars in plants. For
example, radioactive carbon dioxide is rapidly incorporated into sugars
by the photosynthetic cells of the leaves. Soon thereafter, the
radioactive sugars can be detected in the cell walls between
photosynthetic cells and veins. These results suggest that sugars made
by photosynthetic cells are pumped into the cell wall, where they are
later absorbed by adjacent companion cells. Radioactive sugars cannot be
detected between adjacent chlorenchyma cells, however, suggesting that
sugar movement between photosynthetic cells is through cell-to-cell
channels rather than through the cell wall.
For decades, biologists wondered what moves in the sieve tubes of
the phloem. One seemingly easy way to determine the content of a sieve
tube would be to collect phloem sap as it oozes from a wounded stem.
There are problems with this approach, however, since the wounds
necessary to rupture a sieve tube kill other cells. When these cells are
killed, their contents mix with and contaminate the exudate from the
phloem. Also, cutting releases the pressure in sieve tubes, and the
release of pressure causes water to enter the cell via osmosis.
Biologists needed a microneedle that could be inserted into a single
sieve tube without a sudden release of pressure. Such a microneedle was
discovered in 1953 by two insect physiologists, J. S. Kennedy and T. E.
Miller. The scientists were studying the feeding habits of aphids, a
group of insects that obtain their food from the sieve tubes of phloem.
Kennedy and Miller noted that aphids are natural phloem-probes that tap
into individual sieve tubes with a needle-like mouthpart called a
stylet. Since the contents of sieve tubes are under positive pressure,
the tube's contents surge into the aphid--all that the aphid must do is
sit calmly and enjoy its sugary meal. This rush of phloem sap is often
overwhelming; excess fluid forced into the aphid is secreted by the
aphid as a drop of sugary honeydew, which eventually drops onto
underlying plants, sidewalks, and cars. The researchers set out to
ensure that the aphid did not alter the sap's contents. They did this by
first allowing the aphid to insert its stylet into a sieve tube member.
When the feeding started, the aphid was anesthetized with a gentle
stream of carbon dioxide. The stylet was then cut from the aphid's body,
leaving only the open-ended stylet, inserted into a sieve tube member.
The positive pressure in the sieve tube pushes droplets of sugary sap
through the severed stylet for several days at rates of approximately 1
cubic millimeter of phloem exudate per hour.
Context
-------
The survival of all plants depends on their ability to transport
water and dissolved materials throughout their bodies. Diffusion is
usually adequate for this movement inside individual cells. Indeed,
small molecules can diffuse across a 50-micrometer-wide cell in less
than one second. Yet diffusion is inadequate for transport in
multicellular plants. Multicellular plants must absorb and transport
large amounts of water and dissolved minerals from the soil to their
leaves, which may be tens of meters away from the soil. Multicellular
plants must also have a system for transporting the sugars produced in
leaves to distant sites for storage and utilization. Thus,
multicellularity was largely responsible for the evolution of xylem and
phloem, the two long-distance transport systems in plants. These systems
are very successful because deficiencies in internal transport seldom
limit plant growth.
Bibliography
------------
Evert, R. F. "Sieve Tube Structure in Relation to Function." BIOSCIENCE
32 (1982): 789-795. An excellent article discussing the
structural adaptations of sieve tubes for transporting water and
dissolved carbohydrates. Includes excellent electron micrographs.
Kaufman, Peter B., and Michael L. Evans. PLANTS: THEIR BIOLOGY AND
IMPORTANCE. New York: Harper & Row, 1989. A well-written
textbook. Chapters 10, "Plant Water Relations," and 11, "Transport
of Organic Solutes and Plant Mineral Nutrition," contain good
discussions of the discoveries underlying scientists' knowledge of
how liquids move in plants. Includes an extensive glossary,
illustrations, and selected references.
Martin, E. S., M. E. Donkin, and R. P. Stevens. STOMATA. London: Edward
Arnold, 1983. This small book describes the structure and
function of stomata, whose opening and closing control
transpiration in plants. Includes illustrations and an index.
Steward, F. C., James F. Sutcliffe, and John E. Dale. PLANT PHYSIOLOGY:
A TREATISE: WATER AND SOLUTES IN PLANTS. Vol. 9. New York: Academic
Press, 1986. A college-level treatment of how water and solutes
move in plants. Includes representative data and many references to
the landmark studies. Complex articles are cited in a detailed
bibliography.
Zimmermann, M. H. "Piping Water to the Treetops." NATURAL HISTORY 91
(1982): 6-13. This article describes how plants move water to
their uppermost leaves. An excellent account of the structural and
functional adaptations of xylem for moving water and dissolved
minerals in plants.
Zimmermann, M. H. XYLEM STRUCTURE AND THE ASCENT OF SAP. New York:
Springer-Verlag, 1983. A college-level book that describes how
the structure of xylem is ideally suited for water movement in
plants. Includes illustrations and a detailed bibliography.