Plant Science

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Unit

• Plant Science

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Problem Area

• Managing Plant Growth

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Lesson

• Transpiration in Plants

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Student Learning Objectives

• 1. Describe the transpiration process.
• 2. Identify the factors that affect
  transpiration.
• 3. Explain water movement caused at the root
  level.

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Terms

• Adhesion
• Cohesion
• Diffusion
• Evapotranspiration
• Guard cell
• Guttation
• Hydathodes
• Osmosis

• Plasmolysis
• Root pressure
• Mesophyll
• Stomata
• Transpiration
• Transpiration stream
• Turgor
• Xylem

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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.

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• 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.

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• 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.

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• 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.

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• 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.

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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 plants metabolism.

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• 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.

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• 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.

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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 plants 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.

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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?