Unit

Plant Science

Problem Area

Managing Plant Growth

Lesson

Transpiration in Plants

Student Learning Objectives

1. Describe the transpiration process. 2. Identify the factors that affect

transpiration. 3. Explain water movement caused at the

root level.

Terms

Adhesion Cohesion Diffusion Evapotranspiration Guard cell Guttation Hydathodes Osmosis

Plasmolysis Root pressure Mesophyll Stomata Transpiration Transpiration stream Turgor Xylem

What is the transpiration process?

Transpiration is the process of water loss from plants through stomata. Transpiration is the final step in a continuous water pathway that starts from the soil, into plant roots and ends as it passes into the atmosphere.

A. Water enters the root hairs by osmosis. Osmosis is the diffusion of water through a differentially permeable membrane. Water will pass through the cells of the plant or between the cells within the cell walls. Filling the xylem, a tube-like network of cells, water is distributed throughout the plant.

B. Minerals in water are transported throughout the plant through the plant tissue. Minerals are transported through the plant through diffusion. Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration.

C. The upward movement of water from the roots to the leaves is known as the transpiration stream. As water is lost form the outer leaf tissues, water then moves in the water deficient cells from adjacent cells. This osmotic ripple effect occurs backwards from the leaves to the roots. Differences in osmotic pressure between cell layers drive the continuous uptake of water from the soil. Cohesion is the attraction between like molecules. An example of cohesion is the attraction of two water molecules.

Adhesion is the attraction of unlike molecules, for example, the attraction between water and plant tissues. The cohesion-tension theory suggests that water rises in plants due to its adhesion to capillary walls in plants and to its cohesion to itself.

D. Stomata are small openings on the leaves of a plant. Located on the lower epidermis of a plant, stomata allow water molecules to evaporate. This evaporation occurs when the moisture of the surrounding air is less moist than the mesophyll of the plant. The air, which surrounds the plant, is usually less moist than the leaf mesophyll. The spongy mesophyll allows for the exchange of gases in photosynthesis and transpiration. If outside atmosphere is less than 100 percent humid, water will be lost by evaporation from the interior of the leaf.

E. Water lost from between individual leaf cells is lost to the outside environment. The water lost from between leaf cells is replaced by water evaporating from the miniscule spaces within the cellulose walls of the leaf cells. This water is in turn replenished by water drawn through the cell walls of the xylem tubes, which extend through the veins of the leaf. As cells lose water content, they lose their turgidity.

Turgor is a measure of cell firmness. This water loss is associated with a process known as plasmolysis. Plasmolysis is the shrinkage of the protoplasm from the cell wall.

F. Transpiration results are a massive amount of water loss from a plant each day. More than ninety percent of the water entering a plant evaporates into the atmosphere. A mature corn plant will transpire about fifteen liters of water per week.

G. Guard cells encircle the stomata and have the ability to close the stomata when transpiration is occurring too rapidly. This process helps plants survive in drought conditions and optimizes photosynthesis in favorable conditions. The closing of the guard cells close in part to turgor pressure.

What factors affect the process of transpiration in plants?

A. Light has a major effect on plants. In a majority of plants, the stomata are open during the day and closed at night. When light intensity is low, transpiration is low.

B. Water availability has a major effect on plants. Plants begin to wilt during times of water stress. During water stress, low turgor pressure leads to closing of the stomata. Closing of the stomata will end photosynthesis due to a lack of carbon dioxide entering the plant. This slows the plant’s metabolism.

Evaporation losses through transpiration and from the soil are known as evapotranspiration. Higher evapotranspiration results in greater water stress in plants.

C. As temperature rises, transpiration rates will increase. Transpiration rates may double with an increase from 68 degrees F to 86 degrees F. Plant respiration will also increase with an increase in temperature until carbon dioxide levels reach a certain point, then the stomata close ending transpiration and photosynthesis.

D. Wind and humidity are closely related in their effects on transpiration. High humidity decreases transpiration, due to a slow-down of diffusion and evaporation. Wind sweeps away water molecules as they diffuse from leaf surfaces, thus reducing humidity at the leaf/air surface and increasing transpiration. Wind speeds greater than 15 mph lead to stomatal closure.

Increasing the potassium ion concentration in the guard cells leads to an increase in osmotic flow of water into the guard cells. The resulting increase in turgor pressure of the guard cells causes the stomata to open and transpiration to rise.

What process of water movement originates at the root level?

A. Plant roots move minerals into plants with the use of the xylem. Minerals become concentrated in the xylem tubes, and water follows by osmosis. The continual pumping of mineral ions during the night, when transpiration is low, causes guttation.

B. Guttation is observable in the early morning as drops of water formed around a plant’s leaf margin. Excess water that moved into the plant by osmosis during the night has been pushed out special openings in the leaf margins called hydathodes. This fluid pressure buildup is called root pressure. Root pressure is not significant enough to move water through trees or a large woody plant.

Review/Summary

What is the transpiration process? What factors affect the process of

transpiration in plants? What process of water movement

originates at the root level?