Lecture 3:
Transpiration

 

Transpiration

Transpiration means to perspire and is common within plants. This is loss of water vapor through leaves and/or stems. Most transpiration occurs through the stomata. Why do plants lose such large quantities of water to transpiration? Do you know the answer?

To answer this question, let us look again at the function of the leaf. The chief function of the leaf is for photosynthesis, which is the source of all food for the entire plant body. The necessary energy for photosynthesis comes from sunlight. Therefore, for a maximum amount of photosynthesis to occur, a plant must have a maximum amount of surface area able to reach the sunlight.

In order for CO2 to enter the plant cell, it must go into solution. Why? Because cell membranes are almost impervious to gaseous CO2. Thus, there must be contact with a moist cell surface. Wherever water is exposed to air, evaporation occurs. Plants have developed a number of special adaptations for limiting evaporation. All the adaptations cut down the supply of CO2.

 

Regulation of Transpiration

Transpiration is extremely costly to the plant, especially when the water supply is limited. A number of special adaptations exist that minimize water loss while optimizing the gain of CO2.

    1. The cuticle and the stomata - leaves are covered by a cuticle that makes the leaf largely impervious both to water and carbon dioxide. By far the largest amount of water transpired by a higher plant is lost through the stomata. Stomatal transpiration involves two processes. The first is evaporation of water from cell wall surfaces bordering the intercellular spaces, or air spaces of the mesophyll tissue. The second is diffusion of water into the atmosphere by way of the stomata. Closing the stomata prevents the loss of water vapor from the leaf and prevents the entry of carbon dioxide into the leaf.

    2. Humidity - water is lost much more slowly into air already laden with water vapor.

    3. Air currents - a breeze cools your skin on a hot day because it blows away the water vapor that has accumulated near the skin surfaces, and so accelerates the rate of evaporation the same as plants.

 

 

Measuring Transpiration

Transpiration can be defined as the process by which water is lost from plants to the atmosphere. It is the evaporation of water from plants and can be thought of as plants "breathing". While you cannot see transpiration taking place in the environment you can measure it by capturing the water loss of a plant inside a plastic bag placed around its leaves.

 

During a growing season, a leaf will transpire many times more water than its own weight. For example a large oak tree can transpire 40,000 gallons of water per year. About 10 percent of the earth's atmospheric moisture can be attributed to plant transpiration. The rest is supplied by evaporation and the water cycle.

Transpiration is a biological process necessary for plant life which uses about 90% of the water absorbed by the roots of the plant. Only about 10% of the water taken up is used for chemical reactions and tissue formation in the plant.

 

 

How Transpiration Occurs

Water is lost from the stomata of the plant. Stomata are pores found in the epidermis of the underside of leaves. They are located on the lower surface of leaves to reduce water loss due to minimized solar radiation. The moist air in these spaces has a higher water potential than the outside air, and water tends to evaporate from the leaf surface. The stomata act as pumps which pull water and nutrients from the roots through the rest of the plant to the leaves in a phenomenon known as transpirational pull.

 

Transpirational pull drives water flow in the plant. Water is absorbed by the root hairs of a plant and is passed through vascular tissues into the xylem where it is transported to the leaves and stomata. Vascular tissue is made ofm ore than one cell type and in plants consists of the xylem and phloem. These carry water and nutrients throughout the plant along vascular bundles of cells arranged end to end to form long, narrow conduits.

 

In the vascular tissue water molecules form a column. The uppermost molecule turns to water vapor and is transpired through the stomata. As a water vapor droplet evaporates the column of water forms a concave meniscus. The high surface tension of water pulls the hollow formation outwards generating force. The force provides enough pull to lift water through the vascular tissue of the plant to the leaf surface. In large trees, water may be lifted hundreds of feet from the roots to the canopy.

 

The actions of the stomata are closely related to the hydration of the plant. The stomata pores are regulated by surrounding guard cells which regulate the rate of transpiration. When guard cells become turgid they cause stomata to open allowing water to evaporate. When transpiration exceeds the absorption of water by a plant's roots a loss of turgor occurs and the stomata close. Guard cells loose water and become flaccid. This also occurs when the plant has become dehydrated or when the plant is not photosynthesizing such as at night. If a flaccid state continues the plant will wilt and eventually die. The shape of guard cells changes depending on the level of potassium which relates to the water potential of the cell. The rate of transpiration can be directly related to whether the stomata are open or closed.

 

 

When Transpiration Occurs

Transpiration occurs during photosynthesis when the stomata open for the passage of carbon dioxide gas. Carbon dioxide is a necessary component of photosynthesis that the plant must get from their environment. Water transported to the leaves is converted to a gas. As carbon dioxide is allowed into the leaf, water vapors escape through evaporation to the atmosphere. Plants lack membranes that are permeable to carbon dioxide and impermeable to water making transpiration an inevitable consequence of photosynthesis.

 

 

Why Transpiration Occurs

There are several reasons why plants utilize transpiration. The direct effect of transpiration is to regulate the temperature of the plant and to provide water for photosynthesis. It also serves to move nutrients and sugars through the vascular tissues of the plant. Transpiration also helps to regulate turgor pressure in the plant's vascular tissues.

Plants sweat through transpiration. The water that dissipates into the atmosphere pulls excess heat with it away from the plant. This reduces overheating and cools the leaves. Water is one of the substances needed for photosynthesis and must be pumped from the roots of the plant. The "engine" pulling water and nutrients up the plant is transpiration. Nutrients are absorbed from the soil and moved throughout the plant's cells by way of transpiration. The minerals distributed during this process are necessary for biosynthesis in the leaves.

 

 

Environmental Effects

There are many environmental factors that can affect the rate of transpiration. I will address five of the most important here; light, temperature, humidity, wind, and soil water.

 

Light stimulates the opening of the stomata at daybreak. As the stomata opens to allow photosynthesis to occur, the transpiration rate increases. With light comes heat. The leaf can be heated by the temperature of the environment and also by the heat released during photosynthesis. Transpiration provides a cooling mechanism for the plant to release excess heat in the leaves and maintain internal temperature necessary for biological and chemical processes to occur. Transpiration occurs more quickly at higher temperatures due to increased evaporation. Summer tends to be a time of decreased transpiration in plants because of increased temperature. A difference of 10°C can lead to three times the amount of transpiration in a leaf.

In dry climates transpiration is increased. Water is forced to diffuse more rapidly into the air due to the concentration difference between the environments outside and inside the plant. Low humidity creates a vapor gradient between the plant and the air. In dry air, there is a lack of water, forcing water to be pulled from the plant to the atmosphere increasing transpiration. Therefore, in humid climates, transpiration is less effected by diffusion

On windy days the moisture present in the air is swept away from the leaf causing it to transpire more. On calmer days, the humidity rate can rise causing a decrease in transpiration. The amount of wate rin the soil also plays a major role in the rate of transpiration. The plant must have a continuous supply of water to be able to transpire. If adequate water cannot be absorbed by the roots and carried up the xylem, the rate of transpiration will decrease. A lack of water supply will also decrease the rate of photosynthesis and the overall health of the plant.

 

 

Hazard

Transpiration can be hazardous to plants if there is a higher rate of transpiration than rate of moisture absorption through the roots. This is called moisture stress or plant stress. This often happens to houseplants in the winter months when we increase the ambient temperature. Furnaces typically create dry heat which results in a warm, dry environment. Even well watered plants may wilt if the plant cannot adapt it transpiration rate.

 

 

Plant Adaptations

As discussed above, environmental factors can play a large role in the rate of transpiration. Plants in hot arid environments have found ways of limiting their water loss to avoid dehydration. Some of the adaptations desert plants use are: the absence of leaves, stomata that can open and close or that only open at night, C4 photosynthesis, special water storage capabilities, alternative root structures, and periods of dormancy.

 

Xerophytes are plants that have adapted by altering their physical structure. These plants exhibit several adaptations which allow them to survive in harsh climates. Xerophytic plants, such as cacti, do not have leaves but instead depend on chlorophyll in the outer tissue of their skin to conduct photosynthesis. By eliminating leaves or greatly reducing leaf size, transpiration is reduced. The waxy surface of their skin seals in moisture and produces food for the plant.

Some desert plants have smaller stomata or a lower frequency of stomata pores. Fewer, smaller stomata reduce the opportunity for water loss.

Many desert plants open their stomata at night when water loss is lessened due to cooler temperatures and lack of solar radiation. Closing stomata during the day limits the uptake of carbon dioxide and therefore the rate of photosynthesis. This is usually related to a slower growth rate.

Adaptations in the physiological pathways of some desert plants reflect a change in the way carbon dioxide is absorbed from the atmosphere. In C4 plants carbon dioxide gas is quickly captured, allowing the stomata to be closed more often than in plants not adapted for this. In C4 plants carbon is fixed into a four carbon compound as malate or aspartate acid. This is a very efficient means of photosynthesizing..

Succulent plants store water in specialized tissues. This allows plants to survive dehydration in arid climates by providing a means to rehydrate from their stored water sources.

 

Phreatophytes have adapted to arid environments by growing long roots which allow them to access moisture deep below the surface of the soil and reach the water table. The roots of mesquite trees have been recorded as long as 80 feet. These are considered tap roots. Other plants have radial root systems which fan out to quickly absorb moisture during rare rainfalls. Radial root systems typically consist of fibrous roots spread out relatively near the soil surface. Tall sagebrush has an extensive root system with roots that radiate out up to 25 meters from the plant. Some desert plants combine the two systems and have a set of radial roots and one deep tap root. An example of this is the creosote bush.

Another way plants cope with the perils of an arid environment is to become dormant during the winter or duing times of drought. This is true of annuals and ephemerals which produce seeds that wait to germinate during optimum conditions. Plants that use this technique may remain dormant for years. Plants in the Lilicate and Lomatium families store energy for use during dormant periods. Although the parts of the plant above the ground including the leaves and the stalk desiccate in the heat, nutrients are stored in the root system, allowing the plant to survive. Annuals germinate, grow, and produce their seeds or pollen during time of rainfall. Many annuals grow during the spring so they can take advantage of spring thaws and mild temperatures. By summer, they have seeded themselves and have withered.