Lecture 3:



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.


Part 2: Environmental Effects