25.3^25.3. Transport Mechanisms in Plants^462^470^,,^21042^21224%
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25.3
Transport Mechanisms in Plants

Flowering plants are well adapted to living in a terrestrial environment. Their leaves, which carry on photosynthesis, are positioned to catch the rays of the sun because they are held aloft by the stem (Fig. 25.9). Carbon dioxide enters leaves at the stomata, but water, the other main requirement for photosynthesis, is absorbed by the roots. Water must be transported from the roots through the stem to the leaves.

FIGURE 25.9
Plant transport system.
Vascular tissue in plants includes xylem, which transports water and minerals from the roots to the leaves, and phloem, which transports organic nutrients oftentimes in the opposite direction. Notice that xylem and phloem are continuous from the roots through the stem to the leaves, which are the vegetative organs of a plant.
Reviewing Xylem and Phloem Structure

Vascular plants have a transport tissue, called xylem , that moves water and minerals from the roots to the leaves. Xylem contains two types of conducting cells: tracheids and vessel elements. Tracheids are tapered at both ends. The ends overlap with those of adjacent tracheids (see Fig. 24.6). Pits located in adjacent tracheids allow water to pass from cell to cell. Vessel elements are long and tubular with perforation plates at each end (see Fig. 24.6). Vessel elements placed end to end form a completely hollow pipeline from the roots to the leaves. Xylem, with its strong-walled, non-living cells, gives trees much-needed internal support.

The process of photosynthesis results in sugars, which are used as a source of energy and building blocks for other organic molecules throughout a plant. Phloem is the type of vascular tissue that transports organic nutrients to all parts of the plant. Roots buried in the soil cannot possibly carry on photosynthesis, but they still require a source of energy in order to carry on cellular metabolism. Vascular plants are able to transport the products of photosynthesis to regions that require them and/or that will store them for future use. In flowering plants, the conducting cells of phloem are sieve-tube members , each of which typically has a companion cell (see Fig. 24.7). Companion cells can provide proteins to sieve-tube members, which contain cytoplasm but have no nucleus. The end walls of sieve-tube members are called sieve plates because they contain numerous pores. The sieve-tube members are aligned end to end, and strands of cytoplasm within plasmodesmata extend from one cell to the other through the sieve plates. In this way, sieve-tube members form a continuous sieve tube for organic nutrient transport throughout the plant.

Determining Xylem and Phloem Function

Knowing that vascular plants are structured in a way that allows materials to move from one part to another does not tell us the mechanisms by which they move. Plant physiologists have performed numerous experiments to determine how water and minerals rise to the tops of very tall trees in xylem and how organic nutrients move in the opposite direction in phloem. It would be expected that these processes are mechanical in nature and based on the properties of water because water is a large part of both xylem sap and phloem sap , as the watery contents of these vessels are called. In living systems, water molecules diffuse freely across plasma membranes from the area of higher concentration to the area of lower concentration. Botanists favor describing the movement of water in terms of water potential: Water always flows passively from the area of higher water potential to the area of lower water potential. As can be seen in the Science Focus below, the concept of water potential has the benefit of considering water pressure in addition to osmotic pressure.

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The Concept of Water Potential
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otential energy is stored energy due to the position of an object. A boulder placed at the top of a hill has potential energy. When pushed, the boulder moves down the hill as potential energy is converted into kinetic (motion) energy. Once it's at the bottom of the hill, the boulder has lost much of its potential energy.

Water potential is defined as the energy of water. Just like the boulder, water at the top of a waterfall has a higher water potential than water at the bottom of the waterfall. As illustrated by this example, water moves from a region of higher potential to a region of lower water potential.

In terms of cells, two factors usually determine water potential, which in turn determines the direction in which water will move across a plasma membrane. These factors concern differences in:

  1. Water pressure across a membrane

  2. Solute concentration across a membrane

Pressure potential is the effect that pressure has on water potential. With regard to pressure, it is obvious that water will move across a membrane from the area of higher pressure to the area of lower pressure. The higher the water pressure, the higher the water potential. The lower the water pressure, the lower the water potential, and the more likely it is that water will flow in that direction. Pressure potential is the concept that best explains the movement of sap in xylem and phloem.

To fully explain the movement of water into plant cells, the concept of osmotic potential is also required. Osmotic potential takes into account the effects of solutes on the movement of water. The presence of solutes restricts the movement of water because water tends to interact with solutes. Indeed, water tends to move across a membrane from the area of lower solute concentration to the area of higher solute concentration. The lower the concentration of solutes (osmotic potential), the higher the water potential. The higher the concentration of solutes, the lower the water potential and the more likely it is that water will flow in that direction.

Not surprisingly, increasing water pressure will counter the tendency of water to enter a cell because of the presence of solutes. A common situation exists in plant cells. As water enters a plant cell by osmosis, water pressure will increase inside the cell—a plant cell has a strong cell wall that allows water pressure to build up. When will water stop entering the cell? When the pressure potential inside the cell increases and balances the osmotic potential outside the cell.

Pressure potential that increases due to the process of osmosis is often called turgor pressure. Turgor pressure is critical, since plants depend on it to maintain the turgidity of their bodies (Fig. 25A). The cells of a wilted plant have insufficient turgor pressure, and the plant droops as a result.

FIGURE 25A
Water potential and turgor pressure.
Water flows from an area of higher water potential to an area of lower water potential. a. The cells of a wilted plant have a lower water potential; therefore, water enters the cells. b. Equilibrium is achieved when the water potential is equal inside and outside the cell. Cells are now turgid, and the plant is no longer wilted.

Chemical properties of water are also important in movement of xylem sap. The polarity of water molecules and the hydrogen bonding between water molecules allow water to fill xylem cells.

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Water Transport

Figure 25.5 traces the path of water from the root hairs to the xylem. As you know, xylem vessels constitute an open pipeline because the vessel elements have perforation plates separating one from the other (Fig. 25.10a, b ). The tracheids, which are elongated with tapered ends, form a less obvious means of transport, but water can move across the end and side walls of tracheids because of pits, or depressions, where the secondary wall does not form (Fig. 25.10c ).

FIGURE 25.10
Conducting cells of xylem.
Water can move from vessel element to vessel element through perforation plates (a and b). Vessel elements can also exchange water with tracheids through pits. c. Tracheids are long, hollow cells with tapered ends. Water can move into and out of tracheids through pits only.

Water entering root cells creates a positive pressure called root pressure . Root pressure, which primarily occurs at night, tends to push xylem sap upward. Root pressure may be responsible for guttation [L. gutta, drops, spots] when drops of water are forced out of vein endings along the edges of leaves (Fig. 25.11). Although root pressure may contribute to the upward movement of water in some instances, it is not believed to be the mechanism by which water can rise to the tops of very tall trees. After an injury or pruning, especially in spring, some plants appear to “bleed” as water exudes from the site. This phenomenon is the result of root pressure.

FIGURE 25.11
Guttation.
Drops of guttation water on the edges of a strawberry leaf. Guttation, which occurs at night, may be due to root pressure. Root pressure is a positive pressure potential caused by the entrance of water into root cells. Often guttation is mistaken for early morning dew.
Cohesion-Tension Model of Xylem Transport

Once water enters xylem, it must be transported to all parts of the plant. Transporting water can be a daunting task, especially for some plants, such as redwood trees, which can exceed 90 m (almost 300 ft) in height.

The cohesion-tension model of xylem transport, outlined in Figure 25.12 describes a mechanism for xylem transport that requires no expenditure of energy by the plant and is dependent on the properties of water. The term cohesion refers to the tendency of water molecules to cling together. Because of hydrogen bonding, water molecules interact with one another and form a continuous water column in xylem, from the leaves to the roots, that is not easily broken. In addition to cohesion, another property of water called adhesion plays a role in xylem transport. Adhesion refers to the ability of water, a polar molecule, to interact with the molecules making up the walls of the vessels in xylem. Adhesion gives the water column extra strength and prevents it from slipping back.

 
FIGURE 25.12
Cohesion-tension model of xylem transport.
Tension created by evaporation (transpiration) at the leaves pulls water along the length of the xylem–from the roots to the leaves.
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The Leaves.

When the stomata of a leaf are open, the cells of the spongy layer are exposed to the air, which can be quite dry. Water then evaporates as a gas or vapor from the spongy layer into the intercellular spaces. Evaporation of water through leaf stomata is called transpiration . At least 90% of the water taken up by the roots is eventually lost by transpiration. This means that the total amount of water lost by a plant over a long period of time is surprisingly large. A single Zea mays (corn) plant loses somewhere between 135 and 200 liters of water through transpiration during a growing season. An average-sized birch tree with over 200,000 leaves will transpire up to 3,700 liters of water per day during the growing season.

The water molecules that evaporate from cells into the intercellular spaces are replaced by other water molecules from the leaf veins. Because the water molecules are cohesive, transpiration exerts a pulling force, or tension, that draws the water column through the xylem to replace the water lost by leaf cells.

Note that the loss of water by transpiration is the mechanism by which minerals are transported throughout the plant body. Also, evaporation of water moderates the temperature of leaf tissues.

There is an important consequence to the way water is transported in plants. When a plant is under water stress, the stomata close. Now the plant loses little water because the leaves are protected against water loss by the waxy cuticle of the upper and lower epidermis. When stomata are closed, however, carbon dioxide cannot enter the leaves, and many plants are unable to photosynthesize efficiently. Photosynthesis, therefore, requires an abundant supply of water so that stomata remain open, allowing carbon dioxide to enter.

Plant Transpiration

The Stem.

The tension in xylem created by evaporation of water at the leaves pulls the water column in the stem upward. Usually, the water column in the stem is continuous because of the cohesive property of water molecules. The water molecules also adhere to the sides of the vessels. What happens if the water column within xylem breaks? The water column “snaps back” down the xylem vessel away from the site of breakage, making it more difficult for conduction to occur. Next time you use a straw to drink a soda, notice that pulling the liquid upward is fairly easy, as long as there is liquid at the end of the straw. When the soda runs low and you begin to get air, it takes considerably more suction to pull up the remaining liquid. When preparing a vase of flowers, you should always cut the stems under water to preserve an unbroken water column and the life of the flowers.

The Roots.

In the root, water enters xylem passively by osmosis because xylem sap always has a greater concentration of solutes than do the root cells. The water column in xylem extends from the leaves down to the root. Water is pulled upward from the roots due to the tension in xylem created by the evaporation of water at the leaves.

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Opening and Closing of Stomata

Each stoma , a small pore in leaf epidermis, is bordered by guard cells . When water enters the guard cells and turgor pressure increases, the stoma opens; when water exits the guard cells and turgor pressure decreases, the stoma closes. Notice in Figure 25.13 that the guard cells are attached to each other at their ends and that the inner walls are thicker than the outer walls. When water enters, a guard cell's radial expansion is restricted because of cellulose microfibrils in the walls, but lengthwise expansion of the outer walls is possible. When the outer walls expand lengthwise, they buckle out from the region of their attachment, and the stoma opens.

FIGURE 25.13
Opening and closing of stomata.
a. A stoma opens when turgor pressure increases in guard cells due to the entrance of K+ followed by the entrance of water. b. A stoma closes when turgor pressure decreases due to the exit of K+ followed by the exit of water.

Since about 1968, it has been clear that potassium ions (K+) accumulate within guard cells when stomata open. In other words, active transport of K+ into guard cells causes water to follow by osmosis and stomata to open. Also interesting is the observation that hydrogen ions (H+) accumulate outside guard cells as K+ moves into them. A proton pump run by the hydrolysis of ATP transports H+ to the outside of the cell. This establishes an electrochemical gradient that allows K+ to enter by way of a channel protein (see Fig. 25.5b ).

What regulates the opening and closing of stomata? It appears that the blue-light component of sunlight is a signal that can cause stomata to open. Evidence suggests that a flavin pigment absorbs blue light, and then this pigment sets in motion the cytoplasmic response that leads to activation of the proton pump. Similarly, there could be a receptor in the plasma membrane of guard cells that brings about inactivation of the pump when carbon dioxide (CO2) concentration rises, as might happen when photosynthesis ceases. Abscisic acid (ABA), which is produced by cells in wilting leaves, can also cause stomata to close (see page 480). Although photosynthesis cannot occur, water is conserved.

If plants are kept in the dark, stomata open and close just about every 24 hours, just as if they were responding to the presence of sunlight in the daytime and the absence of sunlight at night. This means that some sort of internal biological clock must be keeping time. Circadian rhythms (a behavior that occurs nearly every 24 hours) and biological clocks are areas of intense investigation at this time. Other factors that influence the opening and closing of stoma include temperature, humidity, and stress.

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Plants Can Clean Up Toxic Messes
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hytoremediation uses plants—many of them common species such as poplar, mustard, and mulberry—that have an appetite for lead, uranium, and other pollutants. These plants' genetic makeups allow them to absorb and to store, degrade, or transform substances that kill or harm other plants and animals. “It's an elegantly simple solution to pollution problems” says Louis Licht, who runs Ecolotree, an Iowa City phytoremediation company.

The idea behind phytoremediation is not new; scientists have long recognized certain plants' abilities to absorb and tolerate toxic substances. But the idea of using these plants on contaminated sites has just gained support in the last decade. Different plants work on different contaminants. The mulberry bush, for instance, is effective on industrial sludge; some grasses attack petroleum wastes; and sunflowers (together with soil additives) remove lead. The plants clean up sites in two basic ways, depending on the substance involved. If it is an organic contaminant, such as spilled oil, the plants or microbes around their roots break down the substance. The remainders can either be absorbed by the plant or left in the soil or water. For an inorganic contaminant such as cadmium or zinc, the plants absorb the substance and trap it. The plants must then be harvested and disposed of, or processed to reclaim the trapped contaminant.

Poplars Take Up Excess Nitrates

Most trees planted along the edges of farms are intended to break the wind. But a mile-long stand of spindly poplars outside Amana, Iowa, is involved in phytoremediation.

The poplars act like vacuum cleaners, sucking up nitrate-laden runoff from a fertilized cornfield before this runoff reaches a nearby brook—and perhaps other waters. Nitrate runoff into the Mississippi River from Midwest farms, after all, is a major cause of the large “dead zone” of oxygen-depleted water that develops each summer in the Gulf of Mexico.

Before the trees were planted, the brook's nitrate levels were as much as ten times the amount considered safe. But then Licht, a University of Iowa graduate student, had the idea that poplars, which absorb lots of water and tolerate pollutants, could help. In 1991, Licht tested his hunch by planting the trees along a field owned by a corporate farm. The brook's nitrate levels subsequently dropped more than 90%, and the trees have thrived.

Canola Plants Take Up Selenium

Canola plants (Brassica rapa and B. napa), meanwhile, are grown in California's San Joaquin Valley to soak up excess selenium in the soil to help prevent an environmental catastrophe like the one that occurred there in the 1980s.

Back then, irrigated farming caused naturally occurring selenium to rise to the soil surface. When excess water was pumped onto the fields, some selenium would flow off into drainage ditches, eventually ending up in Kesterson National Wildlife Refuge. The selenium in ponds at the refuge accumulated in plants and fish and subsequently deformed and killed waterfowl, says Gary Bañuelos, a plant scientist with the U.S. Department of Agriculture who helped remedy the problem. He recommended that farmers add selenium-accumulating canola plants to their crop rotations (Fig. 25B). As a result, selenium levels in runoff are being managed. Although the underlying problem of excessive selenium in soils has not been solved, says Bañuelos, “this is a tool to manage mobile selenium and prevent another unlikely selenium-induced disaster.”

FIGURE 25B
Canola plants.
Scientist Gary Bañuelos recommended planting canola to pull selenium out of the soil.
Mustard Plants Take Up Uranium

Phytoremediation has also helped clean up badly polluted sites, in some cases at a fraction of the usual cost. Edenspace Systems Corporation of Reston, Virginia, just concluded a phytoremediation demonstration at a Super-fund site on an Army firing range in Aberdeen, Maryland. The company successfully used mustard plants to remove uranium from the firing range, at as little as 10% of the cost of traditional cleanup methods. Depending on the contaminant involved, traditional cleanup costs can run as much as $1 million per acre, experts say.

Limitations of Phytoremediation

Phytoremediation does have its limitations, however. One of them is its slow pace. Depending on the contaminant, it can take several growing seasons to clean a site—much longer than conventional methods. “We normally look at phytoremediation as a target of one to three years to clean a site,” notes Edenspace's Mike Blaylock. “People won't want to wait much longer than that.”

Phytoremediation is also only effective at depths that plant roots can reach, making it useless against deep-lying contamination unless the contaminated soils are excavated. Phytoremediation will not work on lead and other metals unless chemicals are added to the soil. In addition, it is possible that animals may ingest pollutants by eating the leaves of plants in some projects.

Despite its shortcomings, experts see a bright future for this technology because, for one reason, the costs are relatively small compared to those of traditional remediation technologies. Traditional methods of cleanup require much energy input and therefore have higher cost. In general, phytoremediation is a low-cost alternative to traditional methods because less energy is required for operation and maintenance. Phytoremediation is a promising solution to pollution problems but, says the EPA's Walter W. Kovalick, “it's not a panacea. It's another arrow in the quiver. It takes more than one arrow to solve most problems.”

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Organic Nutrient Transport

Not only do plants transport water and minerals from the roots to the leaves, but they also transport organic nutrients to the parts of plants that need them. This includes young leaves that have not yet reached their full photosynthetic potential; flowers that are in the process of making seeds and fruits; and the roots, whose location in the soil prohibits them from carrying on photosynthesis

Role of Phloem

As long ago as 1679, Marcello Malpighi suggested that bark is involved in translocating sugars from leaves to roots. He observed the results of removing a strip of bark from around a tree, a procedure called girdling . If a tree is girdled below the level of the majority of leaves, the bark swells just above the cut, and sugar accumulates in the swollen tissue. We know today that when a tree is girdled, the phloem is removed, but the xylem is left intact. Therefore, the results of girdling suggest that phloem is the tissue that transports sugars.

Radioactive tracer studies with carbon 14 (14C) have confirmed that phloem transports organic nutrients. When 14C-labeled carbon dioxide (CO2) is supplied to mature leaves, radioactively labeled sugar is soon found moving down the stem into the roots. It's difficult to get samples of sap from phloem without injuring the phloem, but this problem is solved by using aphids, small insects that are phloem feeders. The aphid drives its stylet, which is a sharp mouthpart that functions like a hypodermic need le, between the epidermal cells, and sap enters its body from a sieve-tube member (Fig. 25.14). If the aphid is anesthetized using ether, its body can be carefully cut away, leaving the stylet. Phloem can then be collected and analyzed by a researcher. By the use of radioactive tracers and aphids, it is known that the movement through phloem can be as fast as 60–100 cm per hour and possibly up to 300 cm per hour.

FIGURE 25.14
Acquiring phloem sap.
Aphids are small insects that remove nutrients from phloem by means of a needle-like mouthpart called a stylet. a. Excess phloem sap appears as a droplet after passing through the aphid's body. b. Micrograph of stylet in plant tissue. When an aphid is cut away from its stylet, phloem sap becomes available for collection and analysis.
Pressure-Flow Model of Phloem Transport

The pressure-flow model is a current explanation for the movement of organic materials in phloem (Fig. 25.15). Consider the following experiment in which two bulbs are connected by a glass tube. The first bulb contains solute at a higher concentration than the second bulb. Each bulb is bounded by a differentially permeable membrane, and the entire apparatus is submerged in distilled water.

 
FIGURE 25.15
Pressure-flow model of phloem transport.
At a source, sugar (pink) is actively transported into sieve tubes. Water (blue) follows by osmosis. A positive pressure causes phloem contents to flow from source to a sink. At a sink, sugar is actively transported out of sieve tubes and cells use it for cellular respiration. Water exits by osmosis. Some water returns to the xylem, where it mixes with more water absorbed from the soil. Xylem transports water to the mesophyll of the leaf. Most water is transpired, some is used for photosynthesis, and some reenters phloem by osmosis.

Distilled water flows into the first bulb because it has the higher solute concentration. The entrance of water creates a positive pressure, and water flows toward the second bulb. This flow not only drives water toward the second bulb, but it also provides enough force for water to move out through the membrane of the second bulb—even though the second bulb contains a higher concentration of solute than the distilled water.

In plants, sieve tubes are analogous to the glass tube that connects the two bulbs. Sieve tubes are composed of sieve-tube members, each of which has a companion cell. It is possible that the companion cells assist the sieve-tube members in some way. The sieve-tube members align end to end, and strands of plasmodesmata (cytoplasm) extend through sieve plates from one sieve-tube member to the other. Sieve tubes, therefore, form a continuous pathway for organic nutrient transport throughout a plant.

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At the Source (e.g., leaves).

During the growing season, photosynthesizing leaves are producing sugar. Therefore, they are a source of sugar. This sugar is actively transported into phloem. Again, transport is dependent on an electrochemical gradient established by a proton pump, a form of active transport. Sugar is carried across the membrane in conjunction with hydrogen ions (H+), which are moving down their concentration gradient (see Fig. 25.5). After sugar enters sieve tubes, water follows passively by osmosis.

In the Stem.

The buildup of water within sieve tubes creates the positive pressure that accounts for the flow of phloem contents.

At the Sink (e.g., roots).

The roots (and other growth areas) are a sink for sugar, meaning that they are removing sugar and using it for cellular respiration. After sugar is actively transported out of sieve tubes, water exits phloem passively by osmosis and is taken up by xylem, which transports water to leaves, where it is used for photosynthesis. Now, phloem contents continue to flow from the leaves (source) to the roots (sink).

The pressure-flow model of phloem transport can account for any direction of flow in sieve tubes if we consider that the direction of flow is always from source to sink . For example, recently formed leaves can be a sink, and they will receive sucrose until they begin to maximally photosynthesize.

25.3 Check Your Progress
  1. Explain why water is under tension in stems.

  2. Explain the significance of the cohesion and adhesion properties of water to water transport.

  3. Explain how sugars move from source to sink in a plant.

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Connecting the Concepts

The land environment offers many advantages for plants, such as greater availability of light and carbon dioxide for photosynthesis. (Water, even if clear, filters out light, and carbon dioxide concentration and rate of diffusion is less in water.) The evolution of a transport system was critical, however, for plants to make full use of these advantages. Only if a transport system is present can plants elevate the leaves so that they are better exposed to solar energy and carbon dioxide in the air. A transport system brings water, a raw material of photosynthesis, from the roots to the leaves and also brings the products of photosynthesis down to the roots. Roots lie beneath the soil, and their cells depend on an input of organic food from the leaves to remain alive. An efficient transport system allows roots to penetrate deeply into the soil to absorb water and minerals.

The presence of a transport system also allows materials to be distributed to those parts of the plant body that are growing most rapidly. New leaves and flower buds would grow rather slowly if they had to depend on their own rate of photosynthesis, for example. Height in vascular plants, due to the presence of a transport system, has other benefits aside from elevation of leaves. It is also adaptive to have reproductive structures located where the wind can better distribute pollen and seeds. Once animal pollination came into existence, it was beneficial for flowers to be located where they are more easily seen by animals.

Clearly, plants with a transport system have a competitive edge in the terrestrial environment.