Chapter 4 - Water Relations
Contents - Class Homepage
Importance and Uses of Water in Plants Cell Water Relations
Water Uptake into Tall Trees Drought Tolerance Mechanisms

When you have successfully completed this section you will:

1.  Understand the importance of water in tree distribution and growth.

2.  Understand what water potential is and how it largely governs water movement in plants.

3.  Be able to explain the ascent of water to the tops of tall trees.

4.  Know the mechanisms that trees use to cope with drought.

Water is considered by most plant biologists and forest scientists to be the single most important environmental factor influencing plant growth and distribution. This is particularly true in the forest regions of the United States where the amount of available water controls site productivity on a majority of sites.

Matching species to site water relations is critical for optimizing inherent site productivity. Here longleaf pine (left) is greatly outperforming loblolly pine (right) on the very dry sandhills of the southeastern U.S. Both trees are the same age, but longleaf pine is much better adapted to growing on dry, deep sands.

Another example of this can be seen in the mountains surrounding Blacksburg in the Ridge and Valley Physiographic Province of Virginia. On these sites, upland slopes that face south and southwest receive significantly more direct solar radiation, causing them to be warmer and drier. Consequently these sites have low productivity and are often dominated by pine species (primarily Virginia pine, Table-mountain pine and pitch pine) which are generally more tolerant to drought. The best forest sites typically occur on north and northeast lower slopes. These sites receive less direct solar radiation resulting in a cooler and moister site which is better for tree growth.

Water availability may vary on a local scale as well as on a regional scale.  This red spruce may suffer frequent water shortages, while its neighbors are unaffected.

Water can have a profound influence on cell division and stem elongation. After transplanting and losing most of its original root system, this Douglas-fir was water stressed. The new growth (formed as fixed growth the year prior to transplanting) actually had more needles (stem units) but they did not elongate due to the water stress.

The alder on the left received only half the water that the alder on the right received.  Growth has been drastically reduced.  Particularly notice the small leaves.  The larger leaves on the droughted plant were formed before the tree was water stressed.

Too little water for plant growth is obviously a problem.  What would be the impact of too much water?  Trees like this baldcypress often withstand prolonged periods of flooding.

Water is the largest component of plants. Actively growing tissue (leaves, root tips) can be 80 to 90 percent water. Woody parts of trees are a much lower percent of water ranging between 45 and 60 percent water by weight. Water serves as the solvent which transports minerals and dissolved carbohydrates throughout the plant. Because of its unique chemical properties water is an excellent overall solvent and is therefore able to dissolve a great many chemical substances. Water is also a reactant in many chemical reactions in the plant. Probably the most significant of these is photosynthesis, where water serves as the source of electrons. The oxygen we breathe every day is a result of this reaction in photosynthesis. Another important function of water is that it maintains turgidity (or pressure) in plant tissue. Water is literally what leaves are "inflated" with. One of the first visible signs of a lack of water is wilted (or "deflated") leaves. This "turgor pressure" is necessary for cell enlargement, growth and even maintenance of form in some plants.

Water movement from cell to cell in plants occurs along gradients of "water potential". Water potential is actually a measure of the free energy of water. As you may recall, reactions progress naturally from states of high energy to lower energy. Water movement is no exception; water moves from regions of high to low free energy or from regions of high to low water potential. For the sake of convenience, plant biologists have defined pure water as having a water potential of zero. Typically water potential is expressed in units of pressure, with the most common units being bars or megapascals (MPa). One MPa is equal to 10 bars. A bicycle tire inflated to 70 pounds per square inch (PSI) is at 4.8 bars or 0.48 MPa. We will use bars in all of our discussion from this point on.

Water potential in plant cells has several components. The most important are osmotic potential and turgor (pressure) potential. These two potentials sum up and together equal total water potential. Osmotic potential is due to the presence of dissolved solutes (e.g. sugars, salts) in the water. When a solute is dissolved in water it lowers the osmotic potential. Since pure water (nothing dissolved in it) has a water potential of zero, adding salt or some other solute will result in a negative water potential. An osmometer demonstrates this principle very well. Turgor (or pressure) potential results when pressure is applied to the water. For example, if a tank of water is pressurized, its water potential can be raised above zero. Living plant cells typically have positive turgor potential, and osmotic and turgor potential often work to balance each other . When you see wilted leaves they have zero turgor potential. In the dead xylem of trees, water often has a negative turgor pressure and we say it is "under tension".

These wilted sugar maple leaves have nearly zero turgor potential.

This is a U-tube osmometer.  On the right is a solution containing a dissolved solute, on the left is pure water.  The solutions are separated by a differentially permeable membrane which only allows pure water to pass.  A cell membrane would work similarly.  In the above example, water would flow from left to right since the water potential of the solution is lower than that of pure water (zero bars).  Pressure could be applied to the solution on the right which would prevent the flow of water.

So we now know that water moves in plants from high to low regions of water potential. Pure water has a water potential defined as zero. Adding solutes (e.g. salts or sugars) lowers water potential by lowering the osmotic potential. Turgor potential is the result of pressure (positive or negative) being applied to the water. To complete the picture we need to add a selectively permeable membrane to the system. Cells are enclosed in membranes which typically allow water to pass freely, but exclude the free passing of ions (dissolved salts) and sugars. That is, the membranes are selectively permeable. Cell membranes can thus hold back dissolved solutes and as a result, cells can vary greatly in their osmotic potentials.

For example, let's assume one cell, call it "Cell A" is filled with pure water (osmotic and water potential equal to zero) . An adjacent cell, "Cell B", has salts and sugars dissolved in its water resulting in a osmotic potential of -5 bars and a total water potential of -5 bars. The two cells are separated by a selectively permeable membrane. In this example we assumed that the movement of water did not dilute the osmotic potential of cell B and that cell A's water potential did not change. In this case water will move from cell A into cell B (from higher to lower water potential). Because plant cells have rigid cell walls, turgor potential (pressure) will build up in cell B until it reaches positive 5 bars, at which point the total water potential in cell B will equal zero (plus 5 turgor minus 5 osmotic). The rigid cell wall allows plants to maintain quite high turgor potentials. Without it the membranes would rupture at very low turgor potentials.

Plant biologists have adapted the use of abbreviations when discussing plant water relations.
       Total Water Potential   = y
       Osmotic Potential        = p
       Turgor potential           = R

Osmotic and turgor potential sum to equal the total water water potential:
       y = p + R

Any theory proposed for water uptake into plants must be able to account for uptake into the tallest trees. In the eastern U.S., trees routinely reach 60 to 80 feet in height with some species reaching well over 100 feet. Trees (Douglas-fir, redwood) in the western U.S. reach well over 300 feet tall (a 30 story building!) with occasional specimens reaching 350 feet. Just how water gets to the top of these trees has intrigued scientist for ages. Some of the first theories proposed suggested "pulsating pump-like " cells in the stems. Most of these "pump" theories were disproved by the observation that tree stems could still pull up poisonous liquid which would, of course, kill any living pumping cells. It was recognized that once the leaves were killed the uptake of the sap slowed dramatically or stopped. Other scientists observed the ascent of water in stems killed by poisons, steam and other techniques.

Another early theory was that water rose as a result of pressure generated in the stems much like water rises in a tall building. Observation however does not support this theory. Generally, except for some very specific situations (i.e. sugar maple sap flow in early spring) substantial positive pressures do not exist in large trees. For example, a tree 300 feet tall would require over 132 psi (over 9 bars) of pressure just to support the column of water let alone cause flow through the stem. This means that if the stem on that tree were cut water would spray out at a pressure of 132 psi (a typical house faucet has just 40 psi!). In fact , water is normally under tension (negative pressure) in the stems of trees and to get water out great pressure needs to be applied. A tool called a "pressure chamber" is often used by plant scientist to measure this tension.

What is currently believed to be the mechanism of water ascent to the tops of trees? Most researchers currently accept a mechanism known as the "cohesion-tension theory". This theory proposes that water is actually "pulled" up trees by the action of transpiration (evaporation from leaf surfaces). This, of course, results in the water columns being stretched and placed under considerable tension much like pulling on a rubber band. For the water columns to continue their pull they must not break or snap when stretched. Most fluids could not handle the tension or stretching necessary for water uptake. Water however, has many unique physical and chemical properties. One of these properties is cohesion or how one water molecule clings to another. Water has very high cohesive forces and when confined in small tubes (like the xylem of trees) it can be subjected to very high tensions before the columns break or cavitate. In fact, if the cohesive forces of water are lowered, for example by adding soap to the solution, the water columns break quite easily and flow is disrupted.

So, as one water molecule evaporates from the leaf (transpiration) another is pulled in and so on down the stem. The tension in the xylem lowers the water potential which allows the tree to pull water in from the soil, unless of course the soil is at a more negative water potential. The drier the soil the more tension that is required to pull water in from the soil. When the soil is moist (water potential close to zero) water flows easily into the root and up the stem. As the soil dries (water potential becoming more and more negative) the tree has more difficulty drawing in water. This eventually results in a lack of water and decreased growth.

The cohesion-tension theory stipulates a driving force and a continuous column of water that is contained in small vessels.

The evaporation of water from leaf surfaces (transpiration) can be shown to pull water up a tube. The greater the transpiration, the more quickly the water rises.

Cohesion of water molecules allows water to be "pulled" up the tube. Complete hydration of the water column is necessary for this mechanism to function efficiently.

The lower the humidity in the air, the more rapid the transpiration rate. Other environmental factors which tend to increase drying will increase transpiration.

The drier the soil, the more difficult it is to pull in water.

When water loss from the crown exceeds absorption from the soil, the xylem water potential becomes more negative (greater tension).

Scientist can measure stem sap flow using instruments that pulse the stem with heat. The flow of sap dissipates the heat. The faster the sap flow the more quickly the heat is dissipated. The foil insulation (l) keeps the heat from dissipating to the air.

Plant scientists routinely utilize a tool known as a pressure chamber to measure the water potential in the xylem of woody plants. Stem segments, with a small portion protruding out of the chamber, are placed inside a sealed chamber which is then pressurized. Since water in the xylem is under tension, when pressure is applied that is equal to but opposite in sign to the tension in the stem, the water will appear at the cut end of the segment protruding out of the chamber. Too much pressure and water will spray out of the cut end, too little pressure and no water will appear. This balance pressure is assumed to be numerically equal (but opposite in sign) to the tension in the xylem. Typically the drier the soil the more pressure is required to push water out of the stem sample.

Pressure chamber operation is really quite simple.  The first step in using a pressure chamber is to feed a freshly cut stem through an airtight stopper.

Next, secure the top of the chamber to the base and begin to apply pressure.  A hand lens is usually needed to watch for the first signs of water as the pressure climbs.  The pressure reading at which water appears is the "water potential".

Just as a pressure chamber can be used to measure a plant's water potential, a tensiometer can be used to measure soil water potential. The tensiometer is buried in the soil and the tension required to pull water through the porous ceramic cup at its end is measured. This tension equals the soil's water potential. Irrigation systems can be tied directly to tensiometer readings and come on automatically at predetermined soil water potentials.

Drought is a meteorological event which can be defined as a period without rainfall of sufficient duration that plant growth is impacted negatively. Most plants, and particularly tree species since they are so long-lived, are exposed to drought during their lifetime. In order to minimize the impact, and during severe drought, survive, plants must have mechanisms in place to cope with the drought.

At this point we should differentiate between the terms adaptation and acclimation. Some plants are better adapted to cope with a drought. Others aren't very drought hardy but they can be acclimated to perform better. Adaptations are characteristics which are heritable or passed on from generation to generation. They are genetically based and the parent will pass on the adaptation to their offspring. An example of this could be the potential of a particular species or genotype to produce a deep and wide spreading root system. Regardless of where the tree is planted it will produce a deep and wide spreading root system.

An acclimation on the other hand is a modification of a characteristic in response to the environment. An example of this is the ability of a species to change the morphology of its root system in response to the environment. The root system is deep and wide spreading when the tree is planted on a dry site, but shallow when planted on a wet site. When an athlete trains for an upcoming event they are acclimating their physiology and morphology in such a way that they optimize their performance. Similarly, high elevation mountain climbers must acclimate their physiology to low oxygen atmospheres. The differences between these two terms, adaptation and acclimation, can begin to fade when we consider the fact that the ability to acclimate can be an adaptation. Think about that one for a while!

The overall ability of a tree to survive a drought depends on many morphological, physiological and phenological characteristics. The mechanisms are categorized here, but it needs to be emphasized that these categories are not mutually exclusive and its is the interaction of many factors that result in the overall ability to cope with a drought.

Drought Avoider - Active life cycle occurs when water is available

Drought Tolerator - Growth occurs when drought can be expected (most trees)

Dessication Postponement - Mechanisms which slow water loss or increase uptake (most trees)

Dessication Tolerance - Ability to withstand desiccation and recover when water is again available.

First, some plants can be classified as drought avoiders. These are plants that complete their entire active life cycle during a period where drought does not occur. Typically these are plants that grow in dessert regions with well defined rainy seasons. During the rainy season the seeds sprout, the plant matures, flowers, and develops new seed before the next often prolonged dry season. Obviously tree species we are dealing with do not avoid drought.

This now dead desert legume is a great example of a drought avoider. It grows in desert washes just after the rainy season completing its entire life cycle before dying and leaving only its seeds. The seeds actually get scarified during the flooding of the washes while tumbling over and through the rocks.

Most tree species fall into the category of drought tolerators. These are plants which have portions of their active life cycles during periods when drought can be expected. Drought tolerance can be broken down into two subcategories - desiccation postponement and desiccation tolerance. Desiccation postponement mechanisms prevent the loss of water out of the plant or increase the rate of water uptake into the plant, in this way they postpone desiccation during a drought. Many tree species utilize avoidance mechanisms. These mechanisms might involve:
1) deep, wide spreading root systems,
2) rapid stomatal closure during the onset of drought,
3) smaller leaf size,
4) metabolic adaptations to avoid water loss,
5) water storage in plant organs (e.g. trunk) and in extreme cases
6) leaf abscission to prevent further water loss.  All of these mechanisms can occur as adaptations and many as acclimations.

Creosote bush is a great example of a drought tolerator. It is known to live for thousands of years in the very dry and hot deserts of the southwestern U.S. It has tiny, wax coated leaves and a deep root system

Desiccation tolerance is the ability of plants to desiccate but still survive. Most tree species do not do this well. if leaves and other living tissues actually desiccate they typically become damaged or even killed. However, some plants such as grasses and mosses can become severely desiccated and upon rehydration resume growth. Tree species however will often osmotically adjust in response to a drought. Osmotic adjustment is an acclimation where the plant lowers its osmotic potential in response to a drought and in this way maintain turgor despite a lower water potential. In a way the plant is desiccating as indicated by the lower water potential but at the same time it maintains its turgor.