Chapter 11 Notes

•The dominant process in water relations of the whole plant is the absorption of large quantities of water from the soil, its translocation through the plant, and its eventual loss to the surrounding atmosphere as water vapor.

• Of all the water absorbed by plants, less than 5 percent is actually retained for growth and even less is used biochemically.

• The balance passes through the plant to be lost as water vapor, a phenomenon known as transpiration.

Water and Its Movement Through The Plant

• More than 90% of the water entering a plant passes into leaf air spaces and then evaporates through the

stomata into the

atmosphere (Transpiration).

– Usually less than 5% of

– water escapes through

– the cuticle.

Cohesion - Tension Theory

• When the negatively charged end of one water molecule comes close to the positively charged end of another water molecule, weak hydrogen bonds hold the molecules together.

– Water molecules adhering to capillary walls, and each other, create a certain amount of tension.

Cohesion - Tension Theory

• When water transpires, the cells involved develop a lower water potential than the adjacent cells.

– Creates tension on water columns, drawing water from one molecule to another, throughout the entire span of xylem cells.

•. Nowhere is transpiration more evident than in crop plants, where several hundred kilograms of water may be required to produce each kilogram of dry matter and excessive transpiration can lead to significant reductions in productivity.

•The quantitative importance of transpiration has been indicated by a variety of studies over the years.

•In his classic 1938 physiology textbook, E. C. Miller reported that a single maize plant might transpire as much as 200 liters of water over its lifetime-approximately 100 times its own body weight.

• Extrapolated to a field of maize plants, this volume of water is sufficient to cover the field to a depth of 38 cm over the course of a growing season.

• A single, 14.5 m open-grown silver maple tree may lose as much as 225 liters of water per hour.

• In a deciduous forest, such as that found in the southern Appalachians of the United States, one-third of the annual precipitation will be absorbed by plants, only to be returned to the atmosphere as vapor.

• Whether there is any positive advantage to be gained by transpiration is a point for discussion, but the potential for such massive amounts of water loss clearly has profound implications for the growth, productivity, and even survival of plants.

• Were it not for transpiration, for example, a single rainfall might well provide sufficient water to grow a crop.

• As it is, the failure of plants to grow because of water deficits produced by transpiration is a principal cause of economic loss and crop failure across the world.

• Thus, on both theoretical and practical grounds, transpiration is without doubt a process of considerable importance..

• TRANSPIRATION IS DRIVEN BY DIFFERENCES IN VAPOR PRESSURE

• Transpiration is defined as the loss of water from the plant in the form of water vapor.

• Although a small amount of water vapor may be lost through small openings (called lenticels) in the bark of young twigs and branches, the largest proportion by far (more than 90%) escapes from leaves.

Transpiration

• Transpiration is the loss of water from a plant by evaporation

• Water can only evaporate from the plant if the water potential is lower in the air surrounding the plant

• Most transpiration occurs via the leaves

• Most of this transpiration is via the stomata.

Regulation of Transpiration

• Changes in turgor pressure occur when osmosis and active transport between the guard cells and other epidermal cells cause shifts in solute concentrations.

– When photosynthesis is not occurring in the guard cells, potassium ions leave, and the stomata close.

• An increase in potassium ions causes a lowering of the water potential and osmosis leading to turgid guard cells.

Regulation of Transpiration

• Stomata of most plants are open during the day and closed at night.

– Stomata of many desert plants open only at night.

• Conserves water, but makes carbon dioxide inaccessible during the day.

• Humidity plays an inverse role in transpiration rates.

– High humidity reduces transpiration, while low humidity accelerates it.

Regulation of Transpiration

• If a cool night follows a warm, humid day, water droplets may be produced through hydathodes at the tips of veins of some plants (Guttation).

How Transpiration is Measured

How Transpiration is Measured

6 Environmental Factors Affecting Transpiration

• Relative humidity:- air inside leaf is saturated (RH=100%). The lower the relative humidity outside the leaf the faster the rate of transpiration as the Y gradient is steeper

• Air Movement:- increase air movement increases the rate of transpiration as it moves the saturated air from around the leaf so the Y gradient is steeper.

3. Temperature:- increase in temperature increases the rate of transpiration as higher temperature

– Provides the latent heat of vaporisation

– Increases the kinetic energy so faster diffusion

– Warms the air so lowers the Y of the air, so Y gradient is steeper

4. Atmospheric pressure:- decrease in atmospheric pressure increases the rate of transpiration.

5. Water supply:- transpiration rate is lower if there is little water available as transpiration depends on the mesophyll cell walls being wet (dry cell walls have a lower Y). When cells are flaccid the stomata close.

6. Light intensity :- greater light intensity increases the rate of transpiration because it causes the stomata to open, so increasing evaporation through the stomata.

Intrinsic Factors Affecting the Rate of Transpiration.

• Leaf surface area

• Thickness of epidermis and cuticle

• Stomatal frequency

• Stomatal size

• Stomatal position

• The outer surfaces of a typical vascular plant leaf are covered with a multilayered waxy deposit called the cuticle.

• The principal component of the cuticle is cutin.

• The cutin network is embedded in a matrix of cuticular waxes.

• Because cuticular waxes are very hydrophobic, they offer extremely high resistance to diffusion of both liquid water and water vapor from the underlying cells.

• The cuticle thus serves to restrict evaporation of water directly from the outer surfaces of leaf epidermal cells and protects both the epidermal and underlying mesophyll cells from potentially lethal desiccation.

• The integrity of the epidermis and the overlying cuticle is occasionally interrupted by small pores called stomata (sing. stoma).

• Each pore is surrounded by a pair of specialized cells, called guard cells.

• These guard cells function as hydraulically operated valves that control the size of the pore

• Stomata are located such that, when open, they provide a route for the exchange of gases (principally carbon dioxide, oxygen, and water vapor) between the internal air space and the bulk atmosphere surrounding the leaf.

• Because of this relationship, this space is referred to as substomatal space.

• The cuticle is generally impermeable to water and open stomata provide the primary route for escape of water vapor from the plant.

. The Cellulose Fibrils (CF) in the Guard cell Walls have a Radial orientation as seen from above. This has an important bearing on their function. The spaces between the CF are small near the inner radial walls but are wide near the outer radial walls.

. A cell with parallel CFs would enlarge evenly. If the CF were close together there would be little enlargement. Cell enlargement would be greater if the CF were more widely spaced. The CFs in Guard Cells have an asymmetric organization because they are closely spaced at the inner radial wall but widely spaced at the outer radial wall

. If a Guard Cell enlarges, the outer radial walls can expand but the inner radial walls can not. As the outer radial walls enlarge, the inner radial walls are pulled apart. This opens the Stomatal Pore. If the Guard Cells shrink, the Stomatal Pore would close.

. Turgor Pressure regulates the opening and closing of the Stomatal Pore. There are many factors which regulate this process. These include CO2 concentration, light intensity and color, temperature & relative humidity.

Following a dark period, stomata open in response to light. Light induces an influx of Potassium ions from adjacent cells to the Guard Cells.

. It also stimulates production of an organic acid (malate) and accumulation of sucrose. This lowers the Water Potential of the Guard Cells & this causes an influx of Water. This increases the Turgor Pressure of the Guard Cells. The Inner Radial Walls can expand due to the loose organization of their Cellulose Fibrils.

.

The Inner Radial Walls can not expand due to the tight organization of their Cellulose Fibrils and the overall thickness of these walls. They are deformed by the volume increase of the Guard cells, and they pull apart to form the Stomatal Pore. Guard Cells can increase their volume by 40 - 100%!

Movement of Water Through the Stomata

Increase in stomatal frequency increases the rate of transpiration

The guard cells control the opening and closing of the stomata

Regulating Stomatal Opening:-the potassium ion pump hypothesis

Regulating Stomatal Opening:-the potassium ion pump hypothesis

• Transpiration may be considered a two-stage process:

• (1) the evaporation of water from the moist cell walls into the substomatal air space and

• (2) the diffusion of water vapor from the substomatal space into the atmosphere.

• The diffusion of water vapor from the substomatal space into the atmosphere is relatively straightforward.

• Once the water vapor has left the cell surfaces, it diffuses through the substomatal space and exits the leaf through the stomatal pore.

• Diffusion of water vapor through the stomatal pores, known as stomatal transpiration, accounts for 90 to 95 percent of the water loss from leaves.

• The remaining 5 to 10 percent is accounted for by cuticular transpiration.

• Although the cuticle is composed of waxes and other hydrophobic substances and is generally impermeable to water, small quantities of water vapor can pass through.

• The contribution of cuticular transpiration to leaf water loss varies considerably between species.

• It is to some extent dependent on the thickness of the cuticle.

• Thicker cuticles are characteristic of plants growing in full sun or dry habitats, while cuticles are generally thinner on the leaves of plants growing in shaded or moist habitats.

Adaptations to Reduce Water Loss in Xerophytes

• Thick waxy cuticle to reduce evaporation

• Reduced leaf area e.g.needles

• Hairy leaves:- the hairs trap a layer of saturated air

• Sunken stomata:- the pits above the stomata become saturated

• Rolled leaves:- this reduces the area exposed to the air and keeps the stomata on the inside so increasing the water vapour inside the roll

. Sectional View of a Stoma stained with Toluidine Blue: The Guard Cells are recessed (sunken) & the Epidermis of the Subsidiary Cells have formed arch-like extensions which create a cavity outside the Stoma. This would create a microenvironment which would shield the Stoma from air currents & reduce the rate of water loss.

Adaptation to Increase Water Uptake in Xerophytes

• Deep extensive root system to maximise water uptake

• Accumulation of solutes in the root system to reduce the Y, so making the Y gradient from the soil to the root cells steeper

• Some very shallow roots to absorb dew which condenses on the soil at night

Graph to show stomatal opening over 24 hours

24h Cycle of Stomatal Opening and Closing

• TRANSPIRATION CAN BE MEASURED BY WEIGHT LOSS AND GAS EXCHANGE

• How is transpiration measured? Two principal methods are commonly used: weight loss and gas exchange.

• The weight-loss method can be demonstrated by sealing a well-watered potted plant to prevent evaporation through the pot or from the soil surface.

• The potted plant can then be weighed at intervals and any weight loss attributed to loss of water via transpiration through the shoot.

• The weight-loss method, also known as the lysimeter method, has been scaled up for agricultural field studies by constructing large containers filled with soil (perhaps several cubic meters), mounted on weighing devices usually buried in the ground.

• In such cases, records must be kept of water input (rainfall, irrigation) and evaporation from the soil as well.

• The lysimeter method is generally considered most reliable and accurate for field studies, but lysimeters are expensive to construct and are not usually considered portable.

• The gas-exchange method, often used in conjunction with experiments on photosynthesis, involves sealing a leaf or branch in a transparent chamber with a flowing air stream.

• Transpiration can be estimated as the difference in water content of the air entering the chamber and the air leaving the chamber.

• Temperature, carbon dioxide content, and other parameters can also be measured, coupling measurements of transpiration with stomatal opening and the rate of photosynthesis.

• This method has also been scaled up for field studies by enclosing entire trees or other large plants within a sealed plastic canopy.

• Gas-exchange methods, whether on a small scale in the laboratory or in large scale field measurements, are usually limited to short-term studies.

• Chambers and measuring systems can be made quite portable and a number of commercial instruments are now available for field studies.

• Transpiration in large-scale natural ecosystems is difficult to measure and is most commonly estimated indirectly.

• The investigator essentially calculates a water balance sheet, taking into account inputs (rainfall) and outputs such as soil storage, drainage, runoff, and so forth.

• The difference between measured inputs and outputs is taken as a measure of transpiration

• THE DRIVING FORCE OF TRANSPIRATION IS DIFFERENCES IN VAPOR PRESSURE

• The driving force for transpiration is the difference in water potential between the substomatal air space and the external atmosphere.

• However, because the problem is now concerned with the diffusion of water vapor rather than liquid water, it will be more convenient to think in terms of vapor systems.

• Because we are now accustomed to dealing with the components of water potential in pressure units, it will be more consistent for us to use vapor pressure (expressed as kilopascals, kPa) in our discussion.

• We can then say that when a gas phase has reached equilibrium and is saturated with water vapor, the system will have achieved its saturation vapor pressure.

• The vapor pressure over a solution at atmospheric pressure is influenced by both solute concentration and temperature.

• Temperature has a significant effect on vapor pressure.

• This is due to the effect of temperature on the average kinetic energy of the water molecules.

• As the temperature of a volume of water or an aqueous solution increases, the proportion of molecules with sufficient energy to escape the fluid surface also increases.

• This in turn will increase the concentration of water molecules in the vapor phase and, consequently, the equilibrium vapor pressure.

• An increase in temperature of about 12°C will nearly double the saturation vapor pressure.

• According to Fick's law of diffusion, molecules will diffuse from a region of high concentration to a region of low concentration, or, down a concentration gradient.

• Because vapor pressure is proportional to vapor concentration, water vapor will also diffuse down a vapor pressure gradient, that is, from a region of high vapor pressure to a region of lower vapor pressure.

•

• In principle, we can assume that the substomatal air space of a leaf is normally saturated or very nearly saturated with water vapor.

• This is because the mesophyll cells that border the air space present a large, exposed surface area for evaporation of water.

• On the other hand, the atmosphere that surrounds the leaf is usually unsaturated and may often have a very low water content.

• These circumstances create a gradient between the high water vapor pressure in the interior of the leaf and the lower water vapor pressure of the external atmosphere.

• This difference in water vapor pressure between the internal air spaces of the leaf and the surrounding air is the driving force for transpiration.

• THE RATE OF TRANSPIRATION

• IS INFLUENCED BY ENVIRONMENTAL FACTORS

• The rate of transpiration will naturally be influenced by factors such as humidity and temperature, and wind speed, which influence the rate of water vapor diffusion between the substomatal air chamber and the ambient atmosphere.

• The rate of transpiration will be governed in large measure by the magnitude of the vapor pressure gradient between the leaf and the surrounding air.

_• T~e_leaf-e_air

• E= vapor pressure

• T= rate of Transpiration

• Resistance is encountered by the vapor molecules as they pass through the intercellular spaces, which are already saturated with water vapor, and the stomatal pores.

• WHAT ARE THE EFFECTS OF HUMIDITY?

• Humidity is the actual water content of air, which, as noted earlier, may be expressed either as vapor density (g m-3) or vapor pressure (kPa).

• In practice, however, it is more useful to express water content as the relative humidity (RM. Relative humidity is the ratio of the actual water content of air to the maximum amount of water that can be held by air at that temperature.

• Expressed another way, relative humidity is the ratio of the actual vapor pressure to the saturation vapor pressure.

• Relative humidity is most commonly expressed as RH X 100, or percent relative humidity.

• The effects of humidity and temperature on the vapor pressure of air are illustrated in Table 11.1.

• Air at 50 percent RH by definition contains one-half the amount of water possible at saturation.

• Its vapor pressure is therefore one-half the saturation vapor pressure.

• Note also that a 10°C rise in temperature nearly doubles the saturation vapor pressure.

• Relative humidity and temperature also have a significant effect on the water potential of air.

• WHAT IS THE EFFECT OF TEMPERATURE?

• Temperature modulates transpiration rate through its effect on vapor pressure, which in turn affects the vapor pressure gradient

• In the first example (A), assuming an ambient temperature of 10°C and a relative humidity of 50 percent, the leaf-to-air vapor pressure gradient is 0.61 kPa.

• This might be a typical situation in the early morning hours. As the sun comes up, the air temperature will increase.

•

• A 10°C increase in temperature assuming the water content of the atmosphere remains constant, will increase the leaf-to-air vapor pressure gradient and, consequently, the potential for transpiration, by a factor of almost 3.

• Note that in this example it is assumed that leaf temperature is in equilibrium with the atmosphere.

• This is not always the case. A leaf exposed to full sun may actually reach temperatures 5°C to 10°C higher than that of the ambient air.

• Under these circumstances, the vapor pressure gradient may increase as much as sixfold!

• As long as the stomata remain open and a vapor pressure gradient exists between the leaf and the atmosphere, water vapor will diffuse out of the leaf.

• This means transpiration may occur even when the relative humidity of the atmosphere is 100 percent.

• This is often the case in tropical jungles where leaf temperature and, consequently, saturation vapor pressure is higher than the surrounding atmosphere.

• Because the atmosphere is already saturated, the water vapor condenses upon exiting the leaf, thereby giving substance to the popular image of the steaming jungle.

• WHAT IS THE EFFECT OF WIND?

• Wind speed has a marked effect on transpiration because it modifies the effective length of the diffusion path for exiting water molecules.

• This is due to the existence of the boundary layer.

• Before reaching the bulk air, water vapor molecules exiting the leaf must diffuse not only through the thickness of the epidermal layer (i.e., the guard cells), but also through the boundary layer.

• The thickness of the boundary layer thus adds to the length of the diffusion path.

• According to Fick's law, this added length will decrease the rate of diffusion and, hence, the rate of transpiration.

Moving Air Removes the Boundary Layer of Water Vapour From the Leaf

The Effect of Wind Speed on the Rate of Transpiration

• The thickness of the boundary layer is primarily

• a function of leaf size and shape,

• the presence of leaf hairs (trichomes),

• and wind speed.

• With increasing wind speed, the thickness of the boundary layer and, consequently, the length of the diffusion path decreases.

• Boundary layer thickness can also be influenced by a variety of plant factors.

• Boundary layers are thicker over larger leaves and leaf shape may influence the wind pattern.

• Leaf pubescence, or surface hairs, helps to maintain the boundary layer, and thus reduce transpiration, by breaking up the air movement over the leaf.

• Given our (my?) discussion of transpiration, it is clear that plant water relations reflects the acquisition of water from the soil through the constant loss of water through the leaves. This apparent conundrum may lead one to question whether transpiration offers any positive advantage to plants.

• This question is addressed in Box 11.1-p226

• What is transpiration? Give three environmental factors which will increase transpiration rate.

• Explain how potassium ions are moved into the guard cells in light, and how this affects the guard cells and stomata.

• Give three adaptations a xerophyte may have to reduce transpiration and explain how they do this.

• Plants close their stomata at night and some also close their stomata around mid day. Explain why this is advantageous to the plant

• Transpiration is the loss of water from a plant by evaporation

• Higher temperature, increased air movement, lower humidity

• Potassium ions are pumped into the guard cells by active transport

• against the concentration gradient

• this lowers the water potential inside the guard cells

• water is drawn in by osmosis

• from the surrounding cells which have a higher water potential/down the water potential gradient

• guard cells swell and become turgid

• guard cells bend

• causing the stomata to open Any three from:-

• Thick waxy cuticle on leaves reduces evaporation

• Curled leaves reduce evaporation by trapping humid air inside the curl so reducing the water potential gradient

• Reduced leaf area, e.g. spines, reduces the area from which evaporation can occur

• Hairy leaves -trap a layer of humid air around the leaf,so reducing the water potential gradient

• Sunken stomata – moist air trapped above stomata, so reducing the water potential gradient

• Stomata closed at night when there is no light for photosynthesis, so reducing water loss by evaporation/transpiration via the stomata

• Closing stomata at mid day, which is the hottest part of the day, is an advantage in hot dry environments, as transpiration is reduced.

• Break

• WATER CONDUCTION OCCURS VIA TRACHEARY ELEMENTS

• The distinguishing feature of vascular plants is the presence of vascular tissues, the xylem and phloem, which conduct water and nutrients between the various organs.

• Vascular tissues begin differentiating a few millimeters from the root and shoot apical meristems and extend as a continuous system into other organs such as branches, leaves, flowers, and fruits.

• In organs such as leaves, the larger veins subdivide into smaller and smaller veins such that no photosynthetic leaf cell is more than a-few cells removed from a small vein ending.

• Xylem tissue is responsible for the transport of water, dissolved minerals, and, on occasion, small organic molecules upward through the plant from the root through the stem to the aerial organs.

• Phloem, on the other hand, is responsible primarily for the translocation of organic materials from sites of synthesis to storage sites or sites of metabolic demand.

• Xylem consists of fibers, parenchyma cells, and tracheary elements (Fig. 11.5).

• Fibers are very elongated cells with thickened secondary walls.

• Their principal function is to provide structural support for the plant.

• Parenchyma cells provide for storage as well as the lateral translocation of solutes.

• The tracheary elements include both tracheids and vessel elements (Fig. 11.6).

• Tracheary elements are the most highly specialized of the xylem cells and are the principal water-conducting cells.

• Tracheids and vessels are both elongated cells with heavy, often sculptured, secondary cell walls.

• Their most distinctive feature, however, is that when mature and functioning, both tracheids and vessels form an interconnected network of nonliving cells, devoid of all protoplasm.

• The hollow, tubular nature of these cells together with their extensive interconnections facilitates the rapid and efficient transport of large volumes of water throughout the plant.

• Although their principal function is to conduct water, the thickened secondary walls of tracheids also contribute to the structural support of the plant.

• The movement of water between tracheids is facilitated by interruptions, known as pit pairs, in the secondary wall (Fig. 11.7).

• During the development of tracheids, regions that are to become pit pairs avoid the deposition of secondary wall material.

• Vessels are very long tracheary elements made up of individual units, known as vessel members, which are arranged end-to-end in longitudinal series.

• At maturity, the end walls of the vessel members have dissolved away, leaving openings called perforation plates.

. The most advanced Vessel Members have No Endwalls (Simple Perforation Plates). They are generally wider than Tracheids and are more specialized for water transport.

Lateral Pits and Simple Perforation Plates

• THE ASCENT OF XYLEM SAP IS EXPLAINED BY COMBINING TRANSPIRATION WITH THE COHESIVE FORCES OF WATER

• The tallest-standing trees are generally found growing in the rainforests along the Pacific coast of the northwestern United States and southwestern British Columbia.

• The best known are the redwoods (Sequoia sempervirens) of northern California, some of which exceed 110 m in height.

• Individual specimens of Douglas fir (Pseudotsuga menziesii) have been reported in excess of 100 m and a sitka spruce (Picea sitchensis) measuring 95 m has been located in the Carmanah Valley of Vancouver Island.

• In Australia, there have been reports of Eucalyptus trees measuring more than 130 m in height

• The length of vessels in maple is generally 4 cm or less, but some may reach lengths of 30 cm.

• In oak, on the other hand, vessel lengths up to 10 m have been recorded.

• However, because of extensive branching of the vascular system and the large number of lateral connections between overlapping tracheary elements, the xylem constitutes a single continuous, interconnected system of waterconducting conduits between the extremes of the plant-from the tip of the longest root to the outermost margins of the highest leaf.

• Vessels are considered evolutionarily more advanced than tracheids.

• For example, xylem tissue in the gymnosperms, considered evolutionarily more primitive than the angiosperms, consists entirely of tracheids.

• Although tracheids do occur in angiosperms, the bulk of the water is conducted in vessels.

• Also because of their larger size, vessels are considerably more efficient than tracheids when it comes to conducting water.

• The impact of this relationship can be seen by comparing the relative volume flow rates for a 40-µm-diameter (r = 20 µm) tracheid and a 200-µm-diameter (r = 100 µm) vessel.

• Although the relative diameter of the vessel is 5 times that of the tracheid, its relative volume flow rate will be 625 (i.e., 54) times that of the tracheid.

• The high rate of flow in the larger vessels occurs because the flow rate of water is not uniform across the conduit.

• The flow rate of molecules near the conduit wall is reduced by friction, due to adhesive forces between the water and the conduit wall.

• As the diameter of the conduit increases, the proportion of molecules near the wall and consequently subject to these frictional forces will decrease.

• Put another way, the faster-moving molecules in the center of the conduit constitute a larger proportion of the population and the overall rate of flow increases accordingly.

• As roots take up mineral ions from the soil, the ions are transported into the stele where they are actively deposited in the xylem vessels.

• The accumulation of ions in the xylem lowers the osmotic potential and, consequently, the water potential of the xylem sap.

• In response to the lowered water potential, water follows, also passing from the cortical cells into the stele through the membranes of the endodermal cells.

• Since the Casparian band prevents the free return of water to the cortex, a positive hydrostatic pressure is established in the xylem vessels.

• In a sense, the root may be thought of as a simple osmometer in which the endodermis constitutes the differentially permeable membrane, the ions accumulated in the xylem represent the dissolved solute, and the xylem vessels are the vertical tube.

• So long as the root continues to accumulate ions in the xylem, water will continue to rise in the vessels or exude from the surface when the xylem vessels are severed.

• The question to be answered at this point is whether root pressure can account for the rise of sap in a tree.

• The answer is probably no, for several reasons.

• To begin with, xylem sap is not as a rule very concentrated and measured root pressures are relatively low.

• Values in the range of 0.1 to 0.5 MPa are common, which are no more than 16 percent of that required to move water to the top of the tallest trees.

• In addition, root pressure has not been detected in all species

• However, root pressure could serve to fill vessels in small, herbaceous plants and in some woody species in the spring when sap moves up to the developing buds.

• WATER RISE BY CAPILLARITY IS DUE TO ADHESION AND SURFACE TENSION

• If a glass capillary tube (i.e., a tube of small diameter) is inserted into a volume of water, water will rise in the tube to some level above the surface of the surrounding bulk water.

• This phenomenon is called capillary rise, or simply capillarity.

• Capillary rise is due to the interaction of several forces.

• These include adhesion between water and polar groups along the capillary wall, surface tension (due to cohesive forces between water molecules), and the force of gravity acting on the water column

• The calculated rise of water in a capillary tube is inversely proportional to the radius of the tube.

• In a large tracheid or small vessel, with a diameter of 50 pm (r = 25 µm), water will rise to a height of about 0.6 m.

• For a large vessel (r = 200 µm), capillarity would account for a rise of only 0.08 m.

• On the basis of these numbers, capillarity in tracheids and small vessels might account for the rise of xylem sap in small plants, say less than 0.75 m in height.

• However, to reach the height of a 100 m tree by capillarity, the diameter of the capillary would have to be about 0.15 µm-much smaller than the smallest tracheids.

• Clearly capillarity is inadequate as a general mechanism for the ascent of xylem sap.

• THE COHESION THEORY BEST EXPLAINS THE ASCENT OF XYLEM SAP

• The most widely accepted theory for movement of water through plants is known as the cohesion theory. This theory depends on there being a continuous column of water from the tips of the roots through the stem and into the mesophyll cells of the leaf. The theory is generally credited to H. H. Dixon, who gave the first detailed account of it in 1914.

• What is the driving force?

• According to the cohesion-tension theory, the driving force for water movement in the xylem is provided by evaporation of water from the leaf and the tension or negative pressure that results.

• Water covers the surfaces of the mesophyll cells as a thin film, adhering to cellulose and other hydrophillic surfaces.

• As water evaporates from this film, the air-liquid interface retreats into the small spaces between cellulose microfibrils and the angular junctions between adjacent cells.

• This creates very small curved surfaces or microscopic menisci (Fig. 11.14).

• As the radii of these menisci progressively decrease, surface tension at the air-water interface generates an increasingly negative pressure, which in turn tends to draw more liquid water toward the surface.

• Because the water column is continuous, this negative pressure, or tension, is transmitted through the column all the way to the soil.

• As a result, water is literally pulled up through the plant from the roots to the surface of the mesophyll cells in the leaf.

• How is the integrity of the water column maintained?

• The ability to resist breakage of the water column is a function of the tensile strength of the water column.

• Tensile strength is a measure of the maximum tension a material can withstand before breaking.

• Tensile strength is expressed as force per unit area, where the area for the purpose of our discussion is the cross-sectional area of the water column.

• Tensile strength is yet another property of water attributable to the strong intermolecular cohesive forces, or hydrogen bonding, between the water molecules.

• When the water column is under tension, there is a tendency for these gases to come out of solution.

• Submicroscopic bubbles first form at the interface between the water and the walls of the tracheid or vessel, probably in small, hydrophobic crevices or pores in the walls.

• These small bubbles may redissolve or they may coalesce and expand rapidly to fill the conduit.

• This process of rapid formation of bubbles in the xylem is called cavitation

• The resulting large gas bubble forms an obstruction, called an embolism in the conduit.

• The implications of embolisms with respect to the cohesion theory are quite serious, because a conduit containing an embolism is no longer available to conduct water.

• In 1966 J. A. Milburn and R. P. C. Johnson introduced an acoustic method for detecting cavitation in plants.

• In laboratory experiments with glass tubes, the rapid relaxation of tension that follows cavitation produces a shockwave that can be heard as an audible click.

• Milburn and Johnson found that similar clicks could be "heard" in plant tissue by using sensitive microphones and amplifiers. Each click is believed to represent formation of an embolism in a single vessel element

• Milburn and Johnson studied cavitation in waterstressed leaves of castor bean (Ricinus communis).

• Water stress was introduced by detaching the leaf from the plant and permitting it to wilt.

• As the leaf wilted, the number of clicks occurring in the petiole was recorded.

• A total of 3000 clicks were detected, which is approximately equal to the number of vessels that might be expected in such a petiole.

• Cavitation could be prevented by adding water to the severed end of the petiole.

• Various methods that either increased or decreased transpiration from the leaf resulted in a corresponding increase or decrease in the number of clicks.

• These results indicate a reasonably straightforward relationship between cavitation and tension in the xylem, which appears to support the cohesion theory.

• They further suggest that cavitation is readily induced by water stress, a condition that herbaceous plants might be expected to encounter on a daily basis.

• Clearly, the effect of cavitation and embolisms on long-term survival of plants would be disastrous if there were not means for their removal or for minimizing their effects.

• The principal mechanism for minimizing the effect of embolisms is a structural one.

• The embolism is simply contained within a single tracheid or vessel member.

• In those tracheary elements with bordered pit pairs, the embolism is contained by the structure of the pit membrane (Fig. 11. 1 5A).

• A difference in pressure between the vessel member containing the embolism and the adjacent water-filled vessel causes the torus to press against the pit border, thus preventing the bubble from being pulled through.

• At the same time, surface tension prevents the bubble from squeezing through the small openings in the perforation plates between successive vessel members

• 11.15 Diagram to illustrate how water flow bypasses embolisms in tracheids and vessels. In tracheids (A), the pressure differential resulting from an embolism causes the torus to seal off bordered pits lining the affected tracheary element. In vessels (B), the bubble may expand through perforation plates, but will eventually be stopped by an imperforate end wall. In both tracheids and vessels, surface tension prevents the air bubbles from squeezing through small pits or capillary pores in the side walls. Water, however, continues to move around the blockage by flowing laterally into adjacent conducting elements

• Water, however, will continue to flow laterally through available pits, thus detouring around the blocked element by moving into adjacent conduits.

• In addition to bypassing embolisms, plants may also avoid long-term damage by repairing the embolism.

• This can happen at night, for example, when transpiration is low or absent.

• Reduced tension in the xylem water permits the gas to simply redissolve in the xylem solution.

• An alternative explanation, particularly in herbaceous species, is that air may be forced back into solution on a nightly basis by positive root pressure.

• WATER LOSS DUE TO TRANSPIRATION MUST BE REPLENISHED

• Uptake of water from the soil by the roots replenishes the water lost as a consequence of leaf transpiration.

• This establishes an integrated flow of water from the soil, through the plant, and into the atmosphere, referred to as the soil-plant-atmosphere continuum.

• The concept of a soil-plant-atmosphere continuum reinforces the observation that plants do not exist in isolation, but are very interdependent with their environment.

• SOIL IS A COMPLEX MEDIUM

• In order to understand interactions between roots and soil water, a review of the nature of soils would be helpful.

• Soil is a very complex medium, consisting of a solid phase comprised of inorganic rock particles and organic material, a soil solution containing dissolved solutes, and a gas phase generally in equilibrium with the atmosphere.

• The inorganic solid phase of soils is derived from parent rock that is degraded by weathering processes to produce particles of varying size.

Soil Properties

• Texture

– Definition: relative proportions of various sizes of individual soil particles

– USDA classifications

• Sand: 0.05 – 2.0 mm

• Silt: 0.002 - 0.05 mm

• Clay: <0.002 mm

– Textural triangle: USDA Textural Classes

– Coarse vs. Fine, Light vs. Heavy

– Affects water movement and storage

• Structure

– Definition: how soil particles are grouped or arranged

– Affects root penetration and water intake and movement

• The clay particles in a soil combine to form complex aggregates that, in combination with sand and silt, determine the structure of a soil.

• Soil structure in turn affects the porosity of a soil and, ultimately, its water retention and aeration.

• Porosity, or pore space, refers to the interconnected channels between irregularly shaped soil particles.

• Pore space typically occupies approximately 40 percent to 60 percent of a soil by volume.

• Two major categories of pores-large pores and capillary pores-are recognized.

• Although there is no sharp line of demarcation between large pores and capillary pores-the shape of the pore is also a determining factor-water is not readily held in pores larger than 10 to 60 µm diameter.

. When soil is at Field Capacity water pervades all of the channels between Soil Particles.

• When a soil is freshly watered, such as by rain or irrigation, the water will percolate down through the pore space until it has displaced most, if not all, of the air.

• The soil is then saturated with water.

• Water will drain freely from the large pore space due to gravity.

• The water that remains after free (gravity) drainage is completed is held in the capillary pores.

• At this point, the water in the soil is said to be at field capacity.

•

• Under natural conditions, it might require two to three days for a loam soil to come to field capacity following a heavy rainfall.

• The relative proportions of large and capillary pore space in a soil can be estimated by determining the water contents of the soil when freshly watered and at field capacity.

• Water content, expressed as the weight of water per unit weight of dry soil, may be determined by drying the soil at 105°C.

• It should not be surprising that a sandy soil, with its coarse particles, will have a relatively high proportion of large pores.

• A sandy soil will therefore drain rapidly, has a relatively low field capacity, and is well aerated.

• The pore space of a clay soil, on the other hand, consists largely of capillary pores.

• Clay soils hold correspondingly larger quantities of water and are poorly aerated.

• A loam soil represents a compromise, balancing water retention against aeration for optimal plant growth.

• As the water content of the soil decreases, either by evaporation from the soil surface or because it is taken up by the roots, the air-water interface will retreat into the capillary spaces between the soil particles.

• Because water adheres strongly to the soil particles, the radius of the meniscus decreases and pressure becomes increasingly negative.

. In extremely dry soils, water is tightly bound in the smallest channels of the soil particles. It can't replace water removed by the roots & large Air Pockets are formed.

• Thus as the soil dries and its water potential declines, plants may experience difficulty extracting water from the soil rapidly enough to balance losses by transpiration.

• Under such conditions, plants will lose turgor and wilt.

• If transpiration is reduced or prevented for a period of time (such as at night, or by covering the plant with a plastic bag), water uptake may catch up, turgor will be restored, and the plants will recover.

• Eventually, however, a point can be reached where the water content of the soil is so low that, even should all water loss by transpiration be prevented, the plant is unable to extract sufficient water from the soil and the loss of turgor is permanent.

• ROOTS ABSORB AND TRANSPORT WATER

• Roots have four important functions.

• Roots (1) anchor the plant in the soil;

• (2) provide a place for storage of carbohydrates and other organic molecules;

• (3) are a site of synthesis for important molecules such as alkaloids and some hormones; and

• (4) absorb and transport upward to the stem virtually all the water and minerals taken up by plants.

• The effectiveness of roots as absorbing organs is related to the extent of the root system

• THE PERMEABILITY OF ROOTS TO WATER VARIES

• The permeability of roots to water varies widely with age, physiological condition, and the water status of the plant.

• Water uptake starts about 0.5 cm from the tip and may extend down the root as far as 10 cm (Fig. 11.16).

• Little water is absorbed in the meristematic zone itself, presumably because the protoplasm in this zone is dense and there are no differentiated vascular elements to carry the water away.

• The region over which water appears to be taken up most rapidly corresponds generally with the zone of cell maturation.

• This is the region where vascular tissue, in particular the xylem, has begun to differentiate.

• RADIAL MOVEMENT OF WATER THROUGH THE ROOT INVOLVES TWO POSSIBLE PATHWAYS

• Once water has been absorbed into the root hairs or epidermal cells, it must traverse the cortex in order to reach the xylem elements in the central stele.

• In principle, the pathway of water through the cortex is relatively straightforward.

• There appear to be two options: water may flow either past the cells through the apoplast of the cortex or from cell to cell through the plasmodesmata of the symplast.

• It does both

• SUMMARY

• Large amounts of water are lost by plants through evaporation from leaf surfaces, a process known as transpiration.

• Transpiration is driven by differences in water vapor pressure between internal leaf spaces and the ambient air.

• A variety of factors influence transpiration rate, including temperature, humidity, wind, and leaf structure.

• Water is conducted upward through the plant primarily in the xylem, a tubelike system of tracheary elements including tracheids and vessels.

• The principal driving force for water movement in the xylem is transpiration and the resulting tension in the water column.

• The water column is maintained because of the high tensile strength of water.

• Water lost by transpiration is replenished by the absorption of water from the soil through the root system.

• 1. Explain why transpiration rate tends to be greatest under conditions of low humidity, bright sunlight, and moderate winds.

• 2. Describe the anatomy of xylem tissue and explain why it is an efficient system for the transport of water through the plant.

• 3. Trace the path of water from the soil, through the root, stem, and leaf of a plant, and into the atmosphere.

• 4. Explain how water can be moved to the top of a 100 m tree, but a mechanical pump can lift water no higher than about 10.3 m. What prevents the water column in a tree from breaking? Under what conditions might the water column break, and, if it does break, how is it reestablished?

• 5. Many farmers have found that fertilizing their fields during excessively dry periods can be counterproductive, as it may significantly damage their crops. Based on your knowledge of the water economy of plants and soils, explain how this could happen.

• 6. Does transpiration serve any useful function in the plant?

• 7. Explain the relationships between field capacity, permanent wilting percentage, and available water. Even though permanent wilting percentage is based on soil weight, it is often said to be a property of the plants. Explain why this might be so.

• 8. Describe the path followed by water from the soil, through the plant and into the atmosphere. Where in the path are the important resistances to water movement?

• 9. Describe the casparian strip and its function. What is the most important substance in the casparian strip, from the point of view of its function?

• 10. What is the driving force for the movement of water from the soil to the top of a tree and into the atmosphere?

• 11. Describe root pressure and indicate when it might occur.

• 12. How does air relative humidity affect the transpiration rate of a leaf? If the air surrounding a transpiring leaf gets warmer, how will such a change affect the transpiration rate of the leaf?

• Study Questions

• 1. Describe the path followed by water from the soil, through the plant and into the atmosphere. Where in the path are the important resistances to water movement?

• 2. Describe the casparian strip and its function. What is the most important substance in the casparian strip, from the point of view of its function?

• 3. What is the driving force for the movement of water from the soil to the top of a tree and into the atmosphere?

• 4. Describe root pressure and indicate when it might occur.

• 5. How does air relative humidity affect the transpiration rate of a leaf? If the air surrounding a transpiring leaf gets warmer, how will such a change affect the transpiration rate of the leaf?