Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Plant Transport

I.  Water Potential in Plants - A Review (click here for a review of diffusion, osmosis and membranes)
The movement of water from one place to another in a plant depends on its water potential (Yw), which is essentially a measure of the energy state of water.  Thus, water always moves from higher Y to a lower Yw .  Pure water is defined as having a potential of zero.  The units of water potential are given in pressure units - megapascals (MPa).

Water potential is influenced by two important factors, solutes and pressure.  Solutes, symbolized by (Yπ) always decrease the water potential while pressure (Yp) is usually positive.  We can express the impact of these factors in the equation:    Yw = Yπ + Yp.

Pressure is usually only a factor in plant tissues because plant cells have a wall.  The wall allows the cell contents to develop positive pressures.  Since animal cells lack a wall, pressure is not an important consideration and animal physiologists rarely consider pressure.

Now, consider an osmometer which is a device used to measure osmosis. In its simplest form it is constructed of a glass tube attached to a semi-permeable membrane. The system is filled with water and then immersed in a beaker with water. At equilibrium, the height of water in the tube will be at the level of the water in the beaker. At equilibrium, for every water molecule that diffuses osmotically into the membrane another diffuses out. Now, what will happen if we put some sucrose inside of the membrane sac? There will be a net movement of water from the beaker into the membrane sac. Water will move into the sac (from high to low energy, or from low to high solute concentration). As the water moves in, what happens to the column of water? � right, it moves up the tube. As it does, the water pressure inside the cell increases. Water stops moving in when the pressure inside the cell balances out the tendency for the water to move into the cell.  Let's try to demonstrate this:

 Beaker Inside membrane sac Water movement from beaker to Sac Height of water column Initially, let's fill the beaker and membrane sac with water.  The water potential inside the sac and in the beaker are the same so there is no net movement of water molecules in either direction.  There is no change in the height of the water in the tube. Yw = 0 Yπ = 0 P = 0 Yw = 0 Yπ = 0 P = 0 water moves equally between beaker and membrane no change Now, let's put sugar inside the membrane sac.  The sugar lowers the osmotic potential (let's arbitrarily say Yπ = -0.5MPa.).  This lowers the water potential so water moves into the sac from the beaker and the column of water begins to move up the tube. Yw = 0 Yπ = 0 P = 0 Yw = -0.5 MPa Yπ = -0.5 MPa P = 0 water begins to move into the membrane moves upward As the water moves up the tube, the pressure in the system increases.  The osmotic potential also increases because the sugar is being diluted. Yw = 0 Yπ = 0 P = 0 Yw = -0.1 MPa Yπ = -0.45 MPa P = 0.35 MPa water continues to move into the sac; the rate slows down as the water potential difference between sac and beaker become smaller upward movement continues The water potential inside the sac eventually reaches zero when the pressure and osmotic potential increase.  Note that the pressure changes more than osmotic potential. Yw = 0 Yπ = 0 P = 0 Yw = 0 Yπ = -0.4 MPa P = 0.4 MPa no net movement of water column stops moving

Spuds McSaupe � Potatoes and osmosis - Now, let's meet "Spuds" and study his experiment.

As a review: Consider three beakers, one filled with water, a second with a dilute sugar solution and the third with a concentrated sugar solution. Place a potato core in each of the beakers and allow them to incubate for awhile. In the beaker filled with water we will observe that the potato core swells up and becomes more turgid. The pressure has increased inside the cells because water has moved from the solution (higher Yw ) into the potato (lower Yw ).  The pressure on the membrane is called tonicity.  Thus the potato has greater tonicity (or is hypertonic) than the water in the solution. Conversely, we can envision that the tonicity of the solution is less or hypotonic.

Now consider the core in the concentrated solution. Water will move out of the potato (which now has a higher Y ) and into the solution (lower Yw ).  As water leaves, the core shrivels and becomes limp.  The membrane pressure decreases (it is hypotonic relative to the solution in the beaker). In the dilute sugar solution that has the same water potential as the potato core, there will be no change in the core � it will neither gain no loose water. It is said to be isotonic.

II. Water Uptake � from soil to the root

1. Root anatomy - epidermis, cortex, endodermis, casparian strip, stele, phloem, xylem, pericycle, root cap, root hair.

2. Apoplast vs. symplast � the apoplast refers to the "non-living" regions of the plant such as the cell walls and spaces between cells. The symplast is the "living" areas. The symplast is continuous throughout the plant via plasmodesmata.

3. Region of Water Absorption � most of the water is absorbed near the tip of the root, by the root hairs. The further from the tip, the less water that is absorbed by the root.

4. Root Formation � lateral roots develop from the pericycle, which is a meristematic tissue; film loop

5. Route of Water Movement � there are three routes water can follow: (a) Apoplastic � the water (and dissolved minerals) move via the apoplast from soil through cortex. However, water must enter the stele via the symplast because of the Casparian strip. Once inside, water leaks back out and enters the apoplast (xylem) where it is transported to the apex of the plant. This is the major route of transport; (b) Symplastic: Transmembrane � in other words, the water moves from cell-to-cell crossing membranes as it goes.  When water moves across the membrane is moves primarily through protein channels called aquaporins; and (c) Symplastic: Plasmodesmata � the water moves from cell-to-cell via the plasmodesmata.

6. Ion movement - ions move primarily through symplast via active transport

7. Root as osmometer - analogy. Allows for the development of root pressures in the stem. These can be measured and are about 0.2 - 0.3 MPa.

8. Guttation and hydathodes � excess water pressure is relieved through "pressure release" valves called hydathodes. This process is termed guttation and occurs under conditions during which little transpiration occurs (i.e., at night, high humidity) and the stomata are closed.

III. Water Transport: From roots to leaves

1. What is the transport tissue for water? Xylem. Evidence comes from various tracer studies where xylem is loaded with dyes. I�ll bet you�ve done the classic celery stalk dipped in food coloring experiment.

2. In which cells does water move? Vessels & Tracheids. There are four major types of cells in the xylem: (a) tracheids - long, tapered ends, thick secondary wall; (b) vessel elements, - shorter, ends attached, stacked end-to-end to form a long pipe called a vessel; (c) fibers - long and skinny which thick secondary wall, mostly for support; and (d) parenchyma - alive, thin, store starch and other materials, lateral transport. The primary water transport cells are tracheids and vessels. Note that gymnosperms only have tracheids whereas angiosperms have both. Both tracheids and vessels are dead at maturity ("suicide" cells) and have pits, thin circular regions, in the walls.

3. How is Water Moved to the top of trees? Hypotheses......

1. Root Pressure.  Is water moved to the tops of trees by a "push from the bottom" pump? � NO
This type of pump would be root pressure. Recall that root pressures only generate 0.2-0.3 MPa.  Since it requires at least 3 MPa to move water to the top of a tall tree (you'll have to take my word for this), root pressure doesn't have nearly enough power!

2. Capillary Action.  Is water moved to the tops of trees by "capillary action" � NO.
Capillary action is the movement of water up a thin tube due to the cohesive and adhesive properties of water. Essentially the meniscus "pulls" the water up the tube. Without worrying about the derivation of the equation, the height to which a column of water can move is inversely related to the radius of the pipe and is mathematically expressed as: h = 14.87/r (where r = radius in μm; and h = height in meters). Let's look at some actual data:

 Table 1: Capillary Heights of Water Movement Tube Radius (μm) Column Height (m) 10 1.4877 40 (tracheid) 0.37 100 (vessel) 0.148

Vessels and tracheids are too wide to support movement very high.  Obviously capillarity cannot be responsible for water movement to the top of a redwood tree.

3. Cohesion-Tension Theory.   Is water pulled to the top of a tree from above? YES!
According to this hypothesis, water is drawn up and out of the plant by the force of transpiration. Because of the cohesive/adhesive properties of water, as one water molecule evaporates at the opening it pulls the other molecules and sends this pull all the way down the column. This is similar to jumpers parachuting from a plane while holding hands. If this model is correct then, it must adhere to the following:

• The system must have little resistance. The vessels and tracheids are hollow at maturity. Imagine how difficult it would be to move water through a clogged pipe.
• The columns of water must be continuous from the leaves to the soil. If not, it would be analogous to having a chain with a single broken link � it would be impossible to pull anything below the break. The tracheids and vessels form a continuous column from roots to leaves. If there are gaps in individual cells, the water is routed around the gap. By the way, this is one reason why you don't want to go outside and beat on the trunk of a tree on a hot sunny day...it could cause enough of the columns to break that the plant will have a difficult time transporting water.
• There must be sufficient pulling force. Even though ca. 3 MPa are required to move water to the top of a tall tree, the water potential gradient from soil to air is considerably steeper (on the order of -100 MPa.)
• The tensile strength of water must be able to withstand the pull. In other words, the columns of water must not snap as they are being pulled. This was demonstrated by an elegant experiment in which water was centrifuged in Z-shaped tubes. Water has a very high tensile strength, equivalent to a similar-sized column of steel, which is more than sufficient to withstand the pulling forces.
• The xylem should be under a tension. This can be demonstrated: (a) cut a stem and the water will snap up into the top and accumulate at the cut surface on the bottom; (b) dendrometer - this device is essentially a band wrapped around a tree hooked to a pressure transducer. As the tree transpires the diameter of the tree is measured. These experiments show that the diameter of the stem is smallest during the day when transpiration is occurring and largest at night. Imagine putting your finger on the end of a straw and then sucking on the other end. The straw will get thinner (collapse) as you apply tension to the air in the straw - just like a plant stem; and (c) film loop - water is sucked up into a tree trunk when punctured with a knife.
• Tracheids and vessels must be able to withstand tensions without imploding. Hence the reason that they have thick cell walls with circular thickenings. It's no surprise that wood is hard.

IV. Translocation in Phloem.

A. Difficult to study.
Phloem is difficult to study in plants because: (1) the transport cells/tissue in plants are small (microscopic) in comparison to the transport structures in animals; (2) there is a very rapid response of the phloem to wounding (contents under pressure); and (3) transport in plants is intracellular (vs. extracellular in animals).

B. Phloem is the transport tissue for photosynthates (photoassimilates = organic materials).
Radiotracer studies in which leaves are briefly exposed to 14C-labeled carbon dioxide show that radioactive photosynthates are localized in the phloem.

C. Aphids provided the first big breakthrough.
Kennedy & Mittler (1953) noted that phloem feeding aphids could be used to tap directly into phloem. Aphids stick their stylet into phloem cells, but the phloem doesn�t seal itself in response. Aphids don�t suck! The stylet is hollow like a syringe and the phloem contents are forced into the aphid (thus the phloem is under pressure) and the excess is forced out the anus (honeydew). Physiologists collect the honeydew and identify its composition. Even better, after anaesthetizing the aphid with CO2 the body is severed from the stylet leaving a miniature spile tapped directly into the phloem.

D. Phloem Content
Phloem is rich in: (1) carbohydrates that make up 16-25% of sap. (2) nitrogen containing compounds like amines/amides (0.04-4%) such as asparagine, glutamine, aspartic acid, citrulline, allantoin and allantoic acid. These are transport forms of "nitrogen"; (3) ATP, hormones and an assortment of other organic materials; and (4) inorganic substances including magnesium and potassium.

E. Direction of Phloem Transport.
Girdling experiments (removing the bark of a woody plant) showed the accumulation of material above the girdle and that carbohydrates were not translocated below the girdle. Thus, plants transport substances in phloem downward toward the roots. However, sophisticated girdling experiments, using tracers like 32P, 13C and 14C demonstrate that substances in the phloem are transported downward towards the roots or upwards to the shoot meristem. Conclusion - phloem transports organic materials from sites of production (called a source) to a site of need (called a sink). Thus, the typical direction of transport is downward from the primary source (leaves) to the major sink (roots), but can go either way.

F. Cell types.

1. Sieve tube members or sieve elements - are joined end to end to make a sieve tube. These are called sieve cells in gymnosperms. At maturity, these cells: (a) are alive, (b) have a functional plasma membrane and therefore are osmotically active/responsive; (c) no tonoplast or vacuole; (d) no nucleus, thus no DNA-directed protein synthesis, (e) few mitochondria or plastids; (f) the ER is primarily beneath plasma membrane and it is mostly smooth. Sieve elements are joined by sieve plates that have numerous pores.

2. Companion cells (angiosperms; albuminous cells - gymnosperms). These cells have a dense cytoplasm, mitochondria, nucleus, golgi, ER, chloroplasts - the standard goodies. Although their function is not well understood, they can be considered "nurse cells" to the sieve tube members. These cells are derived from the same cambial initial cell as the sieve tube members. Some companion cells have inward growths on the wall to increase the S/V ratio. Thus, they seem important in transferring material into/out of the sieve cells. Such cells are called transfer cells. Transfer cells are not found in all species.

3. Parenchyma cells. These are vacuolated, storage cells. They help in lateral conduction and may help in transferring material to/from sieve cells. Transfer cells are specialized parenchyma cells.

4. Fibers - primarily for support.

G. Mechanism for phloem transport

1. Requirements. The model must account for: (1) speed of transport. The process is much faster than simple diffusion. For example, a conservative estimate of phloem flow rate is 15 g cm-2 hr-1. If the rate was based solely on diffusion is would be predicted to be 200 ug cm-2 hr-1; (2) bidirectional flow - recall that substances can be transported down or up in the phloem; and (3) pressures in the phloem.

2. Pressure flow (or Bulk Flow) hypothesis of Munch.
This is the best model that fits the data. Phloem transport is analogous to the operation of a double osmometer (see diagram, not included). If solute is added to bulb A water potential decreases osmotic uptake of water pressure increases bulk flow of water and solute to bulb B pressures increases in bulb B water potential in B greater than in beaker osmotic flow of water into the beaker water returns to side A via the connection. This system could be maintained indefinitely if there was a mechanism to remove solute (sucrose) at the end (sink) and a mechanism to add solute (source). Sinks include young leaves, roots, developing fruits. Sources include mature leaves, cotyledons, endosperm, and bulbs and storage roots in spring.

3. If this model works then.....
• Sieve tubes should be continuous pipes...they are (anatomical studies).
• Sieve tubes should provide minimal resistance to flow. In other words they shouldn�t be clogged by p-protein. This is true in specimens that were rapidly prepared. This was a concern in early experiments until rapid fixation techniques because the phloem always appeared clogged up in TEM pictures. Further, this explains why sieve tube members have few "typical" cellular structures - they would "get in the way".
• The phloem should be under pressure, as the aphid experiments suggest. It is. In fact, mini pressure gauges can be attached to a severed aphid stylet and the pressure can be measured and it varies from 0.1-2.5 MPa. Further, there should be a pressure gradient from source to sink (driving force for movement). There is.
• There should be a gradient of osmotic potential (this is the component of water potential due to dissolved solutes) from source to sink.  There is. The source region of the phloem has considerably lower osmotic potential than the sink regions.
• Sieve elements must have a membrane (for development of pressure gradients) - they do.
• There must be a mechanism to load solutes from the source into sieve cells. This process must be active since the solutes (usually sucrose) are being loaded against a concentration gradient. Evidence - respiratory inhibitors block the process. The loading mechanism should be: (a) selective - it should only load the materials that are transported. This is supported by radiotracer studies; abraded leaves have been shown to only load materials that are normally transported; (b) provide a mechanism to transport sucrose across the membrane - the sucrose/proton cotransport system. According to this model, protons are pumped out of the sieve cells into the apoplast by a membrane bound ATPase the proton concentration increases in the apoplast pH decreases K+ is brought into the sieve cell to balance the charge the proton gradient provides the driving force for transporting sucrose against a gradient the sucrose and protons bind to a carrier protein in the membrane and are released in the sieve tube member.
• There must be a mechanism to unload solute at the sink. Sucrose is unloaded into the apoplast in some tissues (i.e., ovules) and into the symplast of others (growing/respiring tissues like young leaves, meristems).