Download Properties of Water and the SoilPlantAtmosphere

PBIO*3110 – Crop Physiology Lecture #19 Fall Semester 2008 Lecture Notes for Tuesday 11 November Water Relations I: Properties of Water and the Soil­Plant­Atmosphere Continuum What are the unique properties of the water molecule, and how do these relate to the functions of water in the physiology of crop plants? Why does water move through plants, from the soil to the atmosphere? Learning Objectives 1. Know the important physical / chemical properties of water that affect its major functional roles in plant physiology. 2. Be able to calculate rates of transpiration based on simple diffusion theory. 3. Understand the physical nature of the different components of “water potential”, and the role they play in the movement of water in the soil­plant­atmosphere continuum. Introduction With this lecture, we begin our discussion of crop water relations. We begin by considering the role of water in plants, and some of the basic chemical and physical properties of water. Understanding these basic properties will critical to our subsequent discussion of water movement through the soil­plant­atmosphere continuum. In future lectures, we will consider how plants exert active control over this flow of water, and the potential implications of plant "water use strategies" for crop productivity. We will also consider the topics of drought stress and water use efficiency in detail.
1 Functions of Water in Plants It has been said that the study of plant physiology is, for the most part, the study of plant water relations. This is understandable, given the central role water plays in a large number of plant processes. Among the many functions of water in plants are the following:
· it serves as a medium (and sometimes substrate) for biochemical reactions in cells, since many enzymes are dissolved in the cell water
· structural support – water provides the “turgor pressure” that gives many cells their shape; thus, many tissues will lose their structure and wilt when water availability is inadequate
· cell enlargement – turgor pressure provides the physical force needed to expand cells during growth
· transport of solutes between organs, via the xylem and phloem vessels
· evaporative cooling of leaves during transpiration Next to light, water availability is probably the single most important environmental factor affecting plant growth. Accordingly, plants have evolved with complex physiological strategies for regulating water use, including, but not limited to, minutes time­scale regulation of stomatal apertures in response to sudden changes in environmental conditions. In cropping situations worldwide, water deficits constitute the single largest cause of crop failure. Properties of Water PROPERTY 1: Dipolarity Water is a strongly dipolar molecule. The two hydrogen atoms are not attached to the oxygen atom in a straight line. Instead, the two oxygen­hydrogen covalent bonds are at an angle of approximately 105° to one another. Because the electrons associated with the covalent bonds are, on average, closer to the oxygen nucleus than to the hydrogen nuclei, the molecule is left with a slight negative charge near the O end, and a slight positive charge at the two H ends. In liquid water, these charge differences cause a tendency for adjacent molecules to align themselves such that a H of one molecule is in close proximity to the O of another molecule. The electrical interactions that produce this tendency are called hydrogen bonds. All of the other physiologically important properties of water arise as a consequence of this basic property of diploarity.
2 The dipolar nature of water. In the liquid state, hydrogen bonding tends to cause adjacent molecules of water to align themselves with the O of one molecule in close proximity to an H of another molecule. (From Salisbury and Ross, 1992) PROPERTY 2: Liquid at Physiological Temperature Because the hydrogen bonds of water are stronger than the Van der Waals attractive forces that act between molecules of non­polar liquids, water has a higher boiling point than many substances with much higher molecular weights. For example, ethane, with a MW of 30, is a gas at room temperature, while water (MW = 18) is a liquid. It is water’s strongly dipolar nature that causes it to remain in a liquid state at physiological temperatures, even though it has a very low molecular weight. PROPERTY 3: Adhesion and Cohesion The hydrogen bonds between molecules of liquid water are responsible for the phenomenon of surface tension. This bonding between adjacent, identical molecules is termed cohesion. Cohesion between water molecules is thought to be critical for the maintenance of continuous columns of water in the xylem of tall plants, so that the water can effectively be "pulled" from the soil to the leaves. Hydrogen bonds also cause adhesion of water to other polar molecules and charged surfaces, including soil particles and the protein and polysaccharide constituents of cell walls. Thus, water has a tendency to wet surfaces. This tendency has important implications for plant water relations, as we shall see.
3 PROPERTY 4: High Latent Heat of Evaporation Because water molecules adhere to one another so strongly, an unusually large amount of heat energy is required to convert water from its liquid state to its gaseous state. At normal pressure, 2452 J are required to convert 1 g of liquid water at 20°C to water vapor at 20°C. This very high latent heat of evaporation is important in regulating the temperatures of leaves; evaporation of water in the substomatal cavity results in significant cooling of the leaf tissue when transpiration rates are high. We will consider this phenomenon further in a future discussion of the energy balance of leaves. PROPERTY 5: Incompressibility Water, like all liquids, is essentially incompressible. As such, the laws of hydraulics are relevant to many plant processes. For example, tugor pressure resulting from the elastic cell wall pressing against the incompressible cell contents is largely responsible for the structural rigidity of herbaceous tissues. Also, the turgor pressure of cells provides the force to drive cell expansion during growth. Evaporation of Water from Leaves An understanding of how and why water evaporates from leaves requires a familiarity with the basic concepts underlying kinetic theory and the law of diffusion (Fick’s law). A brief description of each of these is provided below. Kinetic Theory and Evaporation of Water Kinetic theory states that at temperatures above absolute zero (0 K), elementary particles such as molecules are in constant motion. In a volume of water, for example, the average kinetic energy of the water molecules (i.e., their average velocity) increases as the temperature of the water rises. As the molecules move about, they collide with each other; in fact, each molecule experiences several billion such collisions per second. When two molecules collide, they affect each other’s direction of travel and velocity. If the collision is not perfectly symmetrical, one of the molecules will lose kinetic energy, while the other will gain kinetic energy. Because the collisions are not symmetrical, there is always a statistical distribution of kinetic energies (or velocities) among the molecules; at any given moment, some are moving very quickly, others are moving more slowly. The equations for liquids have not been determined, but the distributions of molecular velocities for a gas at two different temperatures are shown in the Figure below as an example (liquids are thought to behave similarly). From this diagram, it can be seen that at higher temperatures a larger proportion of the molecules will have velocities in the extreme upper
4 region. In the case of water, it is these few molecules with high energies that will be able to overcome the hydrogen bonds with adjacent molecules and "break free" into the gaseous state (i.e. evaporate). At higher temperatures, a greater proportion of the molecules will exceed this critical energy threshold, and evaporation will be more rapid. The distribution of molecular velocities for an ideal gas at two different temperatures: 0°C and 10°C (top), or 0°C and 100°C (bottom). The top panel shows an enlargement of the extreme upper ends of the two curves. (From Salisbury and Ross, 1992) Evaporative Cooling It is important to note that when a molecule of water evaporates, it takes with it its (relatively high) kinetic energy. Therefore, the average kinetic energy of the rest of the system decreases. This is the same as saying the temperature of the system decreases. This decrease in the temperature of the remaining liquid water as some molecules enter the gaseous phase is the basis of evaporative cooling of leaves. The loss of heat per gram of water evaporated is the latent heat of evaporation mentioned above. The latent heat of evaporation is relatively high for water because so much energy is required to break the hydrogen bonds that retain the molecules in the liquid state. Only very energetic molecules enter the gaseous state, and thus the drop in average kinetic energy per gram of water evaporated is relatively high.
5 Vapour Pressure and Condensation The process just described can also operate in reverse; that is, molecules of water vapour also collide with each other and with other molecules in the atmosphere. Therefore, the molecules of water in the atmosphere also have a statistical distribution of kinetic energies. Those with low kinetic energies will occasionally condense and re­enter the liquid phase. At higher temperatures, condensation events are rare, since very few of the molecules have energies below the critical threshold. However, if the water vapour concentration in the atmosphere increases, the total concentration of slowly moving water molecules will increase, and condensation events will become more common. Of course, if the temperature declines, condensation rates will also be increased. At some particular vapour concentration, the rate of evaporation is equal to the rate of condensation. At this point, the atmosphere is saturated with vapour. Rather than measuring vapour in units of concentration (e.g., g water m ­3 ), it is usually measured in terms of pressure. Since all gases in a mixture are assumed to exert the same pressure per unit of concentration (recall the Universal Gas Laws), pressure units are equivalent to concentration units. Therefore, the saturation vapour pressure is a much more common term than the saturation vapour concentration. It should be clear from this discussion that the saturation vapour pressure of air (i.e., the amount of water vapour the air can hold) is related to the air temperature. At higher temperatures, the saturation vapour pressure is higher. If air at a given temperature contains a saturating amount of water vapour, water will condense out of the air as it is cooled. This is the process that results in dew formation on cool mornings that follow humid evenings. The saturation vapour pressure (in kPa) of air at standard pressure (100 kPa) may be calculated using the following equation: where t is the temperature in °C.
6 Vapour pressure as a function of air temperature and relative humidity, at standard pressure. The amount of water vapour the air can hold at saturation (100% RH) increases with temperature. Relative Humidity and Dew Point "Relative humidity" (RH) is a term used to describe the amount of water vapour in air, relative to the maximum amount of vapour the air could hold at that temperature. The figure above indicates that air at 20°C and standard atmospheric pressure has a saturation vapour pressure of 2.34 kPa. Thus, air at 20°C with a vapour pressure of 1.75 kPa has a relative humidity of 75% (i.e. 1.75 / 2.34). "Dew point" is a term that refers to the temperature at which water would condense out of the air, given its current vapour pressure. For instance, the air at 20°C and 1.75 kPa of water vapor described above has a dew point of approximately 15.5°C. This can be determined by examining the figure; at 100% RH and 1.75 kPa water vapor, air must have a temperature of 15.5°C. Diffusion and Transpiration The evaporation of water from leaves can be described according to diffusion theory, just as we previously described the inward diffusion of CO2 into photosynthesizing leaves. Recall that Fick’s Law of Diffusion states that the rate of diffusion of a substance across a distance is directly proportional to the concentration gradient, as
7 where J is the flux of the substance, D c is the difference in concentration, D x is the distance over which the diffusion occurs, and D is the diffusion coefficient. In the specific case of leaf gas exchange, the value of D x can not be practically determined, and so D and D x are combined into a single parameter, g, the conductance. Thus, Alternatively, the conductance (g) can be replaced by a resistance to flux (r). Resistance is the inverse of conductance, so An Example Consider the case of leaf transpiration, as presented in the following figure: Schematic representation of evapotranspiration from a leaf. The transpiration rate can be calculated from the information provided (see text).
8 The concentration gradient, Dc, is determined by the difference in water vapour concentration inside the leaf and outside the leaf. If the ambient air temperature is 25°C, atmospheric pressure is 100 kPa and RH is 60%, then ambient vapour pressure is 1.91 kPa. The ambient vapour concentration is then equal to 1.91 kPa / 100 kPa = 0.0191 kPa kPa ­1 (or mol mol ­1 ). The leaf is warmer than the air in this example, at 30°C. Generally, the air inside the leaf is presumed to be water­saturated. Therefore, the vapour pressure inside the leaf is estimated as 4.26 kPa, and the vapour concentration in the leaf is 0.0426 mol mol ­1 . The conductance to water vapour flux out of the leaf is determined by the mean stomatal aperture, as well as the turbulence characteristics of the air close to the leaf surface. In this example, the conductance is given, as 0.2 mol m ­2 leaf s ­1 . The evapotranspiration rate (E) is the same as the flux of water out of the leaf (J). Therefore, E can be calculated as Thus, E = (0.0426­0.0191) x 0.2 mol m ­2 s ­1 E = 0.00470 mol m ­2 s ­1 E = 4.70 mmol m ­2 s ­1 . Movement of Liquid Water in the Soil­Plant­Atmosphere Continuum (SPAC) So, we have just seen how the rate of transpiration – loss of water from leaves as vapour – is determined by physical parameters: leaf temperature, air temperature and relative humidity, and the leaf conductance to water vapour. This process occurs according to the law of diffusion ­ that is, net flux of molecules in response to a concentration gradient. Within the rest of the plant, however, water movement can not be adequately described in terms of diffusion. Instead, water moves through plants primarily as bulk flow, in response to gradients of water potential. In this section, we will describe the concept of water potential, and consider the components of water potential that determine water movement in the soil­plant­atmosphere continuum (SPAC).
9 The Components of Water Potential Water potential is a measure of the potential energy of water; that is, its ability to do work. It is generally measured in units of pressure (usually MPa). Water moves via bulk flow from regions of high water potential to regions of low water potential. In other words, water flows down a water potential gradient. We will see that in plants under normal conditions, a water potential gradient exists between the soil (high water potential) and the leaves (low water potential). Thus, water moving up a plant from the roots to the leaves can also be seen as moving down a thermodynamic potential gradient. Water potential is most often measured in pressure units. By convention, pure water at standard pressure is assigned a water potential of 0 MPa. In all physiologically realistic scenarios, water potential in plants can not exceed this level; thus, water potentials of plant tissues are always negative, ranging from barely less than 0 MPa to as low as, say, ­3.0 MPa. Water potential is usually designated by the Greek capitol letter psi (Y). The overall water potential of any plant tissue is determined by a number of constituent components:
· · · · Solute potential (or, osmotic potential) ­ YS
Pressure potential ­ YP
Matric potential ­ YM
Gravitational potential ­ YG Each of these components is described in more detail below. Solute Potential (Y S) The osmotic (or solute) potential is always negative. It is determined by the concentration of solutes dissolved in water. In an osmometer (see figure), water can be separated from a salt solution by a membrane that allows water molecules to pass through, but excludes salt ions. In such a system, water will flow from the "pure water side" to the "solution side", down the water potential gradient. All plant cells have a negative osmotic potential, since no cells contain pure water. However, osmotic potentials vary between cell types. For example, the least negative osmotic potentials are found in xylem vessels, since the xylem sap is quite dilute (i.e., closest to pure water).
10 In an osmometer, water will flow across the membrane from the pure water side (Ys = 0) to the salt solution side (Ys < 0), down the water potential gradient. Pressure can balance solute potential in an osmometer, so that net flux of water from the water side to the solution side is zero. (From Kramer, 1983) Pressure Potential (YP) Pressure potential can be either positive or negative, or zero. In turgid cells, the cell walls exert pressure against the cell contents, and there is positive pressure inside the cell. This positive pressure can balance the negative osmotic potential, such that the overall water potential of the cell approaches zero. As cells lose water, the pressure potential declines, and the osmotic potential also declines, since the cell solutes become more concentrated as water is lost. In a flaccid cell, the pressure potential is 0 MPa, and the water potential is approximately equivalent to the osmotic potential, as shown in the following figure. It is important to realize that pressure can be used to balance osmotic potential in artificial systems as well. For example, it is possible to apply a balancing pressure against the solution side of an osmometer, such that the osmotic potential is exactly balanced (i.e., water potential on the solution side is 0 MPa, the same as the pure water side). In this case, there will be no net flow of water across the membrane. This example also helps to illustrate why osmotic potential can be meaningfully expressed in terms of pressure units.
11 Water potential (MPa) 2 Pressure potential 1 0 Total water potential ­1 ­2 ­3 Solute potential ­4 1 0.9 0.8 0.7 0.6 Relative water content A Hoefler­Thoday diagram illustrating the relationships between total water potential, pressure potential, solute potential and relative water content for a single cell. On the x­axis is the cell water content, as a fraction of its water content at complete saturation. The dotted line below zero turgor represents the possibility of negative turgor (tension) in some cells. In xylem vessels of transpiring plants, pressure potential is generally negative. That is, the water column in the xylem is under tension, rather than pressure. It is this tension that allows water to be "pulled" from the roots to the leaves. The integrity of the water column is maintained by hydrogen bonding between adjacent water molecules (cohesion, as discussed above). Matric Potential (YM) Matric potential arises from adhesion of water molecules to solid surfaces, such as cell wall proteins and polysaccharides, by hydrogen bonding. Like osmotic potential, matric potential is always negative. Matric potential is largely responsible for imbibition of water by seeds, as wetting of starch and other seed components causes water to flow in down the water potential gradient. Matric potential is also the major component of water potential in soils, as water adheres to the surfaces of soil particles.
12 Gravitational Potential (Y G) As water is raised vertically above some reference level, its gravitational potential increases. In other words, all other things being equal, water flows down a gravitational potential gradient from high places to low places. Although gravity dominates water behaviour in familiar, macroscopic systems, it is almost irrelevant on the microscopic scale of the plant vascular system. Gravitational potential increases by 0.01 MPa per m of height, which is a small percentage of the overall water potential gradient from soil to leaf, except in very tall trees. For our discussions of typical field crops, we can safely ignore the effects of gravity on water movement. Therefore, water potential of any plant tissue can be described as the sum of the solute, pressure and matric potentials:
Y = YS + YP + YM.
The Water Potential Gradient from Soil to Atmosphere The flow of water through the soil­plant­atmosphere continuum can be seen as flow "downhill", that is, flow towards increasingly negative water potentials. What is the actual magnitude of the water potential gradient? Leaf and Atmospheric Water Potentials The atmosphere is the final acceptor of water from the transpiration stream, and must therefore have the lowest water potential of the entire system. Air at 100% relative humidity theoretically has a water potential of 0 MPa, which is higher than the leaf water potential of any crop plant (­ 0.5 to perhaps ­3.0 MPa). However, as RH declines, the water potential decreases very rapidly. For air at 20°C and 98% RH, Y = ­2.73 MPa. If RH declines to 75%, Y = ­38.8 MPa. Clearly, a steep water potential gradient exists between leaves and the atmosphere under normal conditions. In practice, however, leaf­to­air water potential gradients can not be easily used to predict transpiration rates. The calculations based on diffusion theory (previous section) are much more appropriate for this part of the system. Soil and Root Water Potentials The water potential of soil is a function of both its volumetric water content, and its structure / texture. In all soils, a certain amount of water is held so firmly by matric forces that it is unavailable for uptake by plants. Additional water, which is available to plants, may be held in small capillary pores and on colloid surfaces by cohesion to "bound" water. As the soil dries, its water potential drops. When Ysoil declines below Yroot, the plant can not extract any additional water from the soil. This water content is termed the wilting point.
13 In the figure below, the wilting point is indicated as correlating to a water potential of ­1.5 MPa, but obviously this is a function of the specific plant species under consideration; drought­tolerant species may be able to extract water from soils with much lower Y. The soil water potential increases as the soil gets wetter. However, at a certain point, the soil can hold no more water against the force of gravity, and any additional water drains out of the soil. The water content at which free drainage begins is termed the field capacity of the soil. The relationship between soil water content and soil water potential for two different soils (from Kramer, 1983). See text for definitions of field capacity and wilting point. The next figure shows how wilting point and field capacity vary by soil texture. Fine textured soils such as clays have more colloid surface area per unit soil volume, and therefore hold more water at both the wilting point and field capacity than do sandy soils.
14 The relationship between soil texture, field capacity, wilting point and available water. (From Kramer, 1983) The Water Potential Gradient from Root to Leaf Given the above examples, a typical soil water potential might be ­0.05 MPa, and almost all of this is attributable to YM. A typical leaf water potential in a rapidly­transpiring plant might be ­ 1.0 MPa. As we have seen, Yleaf is determined by the balance between YS (negative) and YP (positive). In the connecting vascular tissue, the water potential must be somewhere between the leaf value and the soil value. The xylem sap is quite dilute (see Table in Lecture #17), so YS is not a major component of the total water potential in the xylem. Instead, the negative potential of xylem vessels arises primarily from negative YP (tension). Resistance to the Flow of Water through Plants Recall that the rate of diffusion of water vapor from leaves is determined by
· the vapour pressure concentration gradient between the leaf and the atmosphere, and
· the resistance to diffusion, which depends on the stomatal aperture, as well as the leaf boundary layer conductance. Similarly, the flow of water through plants is determined by
15 · the water potential gradient from soil to leaf, and
· the resistance to flow. The plant can be considered as a system of differences in water potential across various resistances in different plant organs. Together, these differences and resistances determine the flow of water. This is analogous to the flow of current being determined by electrical potential differences and resistances in an electrical circuit. Organs of the plant that store substantial amounts of water (i.e., any succulent tissue) can be viewed as capacitors in the system. For instance, when soils dry to the wilting point, water from stem and leaf tissue may contribute to the transpiration stream. Resistance to water flow in plants occurs at several points, and can be considered analogous to resistance to flow of current in an electrical circuit. Water stored in plant tissues also provides "capacitance" to the system. (From Kramer, 1983). The flow between any two points is equal to the potential difference between those two points, divided by the resistance of the flow path in that region. Thus, when there is a flow of water from soil to the leaves, it must be true that Ysoil > Yroots > Ystem > Yleaves. Some resistances to water flow are variable. Most importantly, plants actively regulate the stomatal aperture to control transpiration. In the next lecture, we will see how plants sense a variety of environmental parameters to "make decisions" about water use in the field.
16