Plant Nutrition and Transport

1
Chapter 21

• Plant Nutrition and Transport

2
Root Functions

• Water and ions move from the soil into the root.
• Most water and minerals enter through tiny root
  hairs.
• Cells in the endodermis are waterproof,
  preventing water molecules from seeping between
  cells.

3
Roots

• Roots have a simpler structure
• than stems

• root cap protective end
• root hairs - epidermal cells one cell thick ?
  surface area for absorption
• cortex ground tissue specialized for storage
• endodermis innermost cylinder which regulates
  material entering the vascular tissues

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Vascular tissue

• There are two types of vascular tissue

• Xylem which consists of two types of tissue
• tracheids - narrow, elongated, thick walled cells
• vessel elements - wider, perforations in ends
• both are dead at maturity

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Vascular Tissues

• Phloem - consists of two types of tissues
• sieve tube members - conducting cells with pores
• companion cells - contain the organelles for the
  STM
• both are alive at maturity

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Stem Functions - Transport in Plants

• Xylem is responsible for movement of water and
  minerals in the plant.
• This transport involves several processes, the
  most important is transpiration.
• Transpiration is the loss of water vapor from a
  plant through its stomata.

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Transpiration

• Stomata allow gas exchange but water also
  escapes.
• Guard cells regulate the rate of transpiration by
  opening or closing stomata.
• The loss of water by transpiration creates
  suction - water extends in a unbroken column
  through the stems down to the roots.
• gt90 of water that enters a plant is lost through
  transpiration.

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• Tension-Cohesion Theory
• Transpiration from leaves (creates negative
  pressure pulls water up)
• Adhesion to xylem walls
• Cohesion between water molecules (so column does
  not break)
• Capillary action up xylem
• Osmosis/root pressure from soil to root

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Stem Functions - Transport in Plants

• Translocation is the movement of sugars through a
  plant.
• In a plant, sugar moves from where it is made or
  stored to where it is needed.
• The source is the part of the plant that provides
  sugar (usually leaf, stem)
• The sink is the part of the plant to which the
  sugar is delivered (often roots, fruit)

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Translocation...

• Flow may be in either direction but it is always
  source to sink
• For example, in the spring, starch stored in the
  roots over the winter, is converted back to sugar
  and moves upward in the vascular tissue to buds
  on the stems. In sugar maple trees, farmers tap
  these trees, collect the sap and produce maple
  syrup.

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Translocation...

• Pressure-Flow Hypothesis
• sugar from source enters phloem first passively,
  then by active transport.
• as sugar concentration in phloem ?, water enters
  from xylem (osmosis) to equalize pressure.

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Translocation...

• pressure builds and pushes sugar ? (since it is
  dissolved in the water, it flows with it!)
• sugar moves from phloem to sink first passively
  then by active transport
• water follows and returns to xylem