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?
