Structure function relationship

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Structure function relationship Botany
130 Lectures 15
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Water movement in the leaf
Water movement is controlled primarily by
stomatal movements.
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Water movement in the leaf
Stomata compromise between the need to acquire
carbon dioxide and the need to save water. They
are rarely totally open or totally closed, most
of the time they are somewhere in between.
Compromise
Open
Closed
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How stomata work (kidney-shaped guardcells)
K and either Cl- or malate- enter the guard cells
The increased osmotic pressure leads to water
uptake, increased hydrostatic pressure, and
swelling.
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How stomata work (grasses)
Guard cells Subsidiary cells
K and Cl- enter the guard cells
The increased osmotic pressure leads to water
uptake, increased hydrostatic pressure, and
swelling.
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Water movement in the leaf
Water travels apoplastically from the xylem to
the substomatal cavities.
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Water moves in the xylem

• Moves from region of high potential (usually the
  roots) to low potential (the leaves)
• The water potential of the atmosphere is very low
• The water potential drop through the plant is
  small
• Evaporation of water inside the leaves is the
  first step
• Develops a pull that is transmitted through the
  xylem to the roots

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Water moves in the xylem

• The Dixon cohesion theory of sap ascent
• Water evaporation at the cell surfaces inside the
  leaves creates a tension (pull) on the water.
• Because of cohesion, the water sticks together.
• Therefore, the pull is transmitted to the roots.

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Water moves in the xylem

• Surface tension of water is very important here
• Consider

After some water loss
At full hydration
There is an increase in the surface area
creating tension (pull)
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Water moves in the xylem
At full hydration
After some water loss
Adhesion Cohesion
There is an increase in the surface area
creating tension (pull)
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Water moves in the xylem
The surface tension is pulling on the water. The
pull exists as long as the surface is more than
the minimum (flat across). This pull is
transmitted by cohesion all the way down to the
roots. Therefore, the xylem is under tension
(negative pressure).
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Fig. 31-7
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Because of the tension on the xylem, trees
get measurably skinnier during the day when
tranpiration is occurring.
Fig. 31-11
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A pressure bomb for determining how much a twig
is pulling on the water in the xylem.
Fig. 31-9
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Water moves in the xylem
The water in the xylem is under tension. Cohesion
makes it act like a string, transmitting the
transpirational pull to the roots. Can the
string break? Yes. The xylem can cavitate.
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Water moves in the xylem
The water in the xylem is under tension. Cohesion
makes it act like a string, transmitting the
transpirational pull to the roots. Can the
string break? Yes. The xylem can cavitate.
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Water moves in the xylem
The xylem can cavitate. If one vessel cavitates,
the cavitation will spread. Because the xylem is
under tension, the pressure is less than zero and
even a pure vacuum has more pressure than the
water inside the xylem.
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Where the water has to go through a small
passage, the cavitation can be stopped, and water
can bypass the cavitated vessel.
In order for the air to get into the water-filled
cell, there would have to be an increase in the
surface area of the water as a bubble started to
form.
Fig. 31-8
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Water moves in the xylem
Cavitations do occur regularly, especially
during droughts. These can be heard as they
form. Listening to stems is one method being
used to assess the need for irrigation.
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Fig. 31-7
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Fig. 31-17
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Water movement in the roots

• Water uptake occurs primarily in very young roots
• Waterlogging can suffocate roots, stopping growth
• Waterlogged plants can sometimes wilt for lack of
  water
• To get from the soil to the xylem of the root,
  water must traverse several cell layers
• Does the water go through or around the cells?
• Symplastic or apoplastic?

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Water movement in the roots is primarily
apoplastic up to the endodermis
Fig. 31-13
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Fig. 31-12
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Root pressure

• By far, most of the water that moves through the
  xylem moves by cohesion/tension
• BUT, roots can also pump water by active
  transport of solutes (ions) into roots
• The increased osmotic pressure causes water to
  move in

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Root pressure
Sometimes the pressure forces water out of the
tips of leaves, a process called guttation.
Ions such as potassium (K) are pumped into
roots. The increased osmotic pressure causes
osmosis. This increases the hydrostatic pressure
inside the xylem
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Fig. 31-15
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Fig. 31-16
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Root pressure
Root pressure can refill cavitated xylem
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Root pressure
Root pressure can refill cavitated xylem
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Phloem transport (page 764)

• In photosynthesis, sucrose plus a few other
  compounds are made, which are then transported to
  the rest of the plant
• This transport is through the phloem
• Phloem transport is from source to sink regions
  of the plant
• This is not always up or down

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Fig. 31-19
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Fig. 31-24
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Fig. 31-24
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Phloem transport

• Phloem transport works by a complex pressure-flow
  mechanism proposed by Münch in 1927
• Consider a model of phloem

Source
Sink
Semipermeable membranes
Sucrose
H2O
Sucrose
H2O
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Phloem transport
Source
Sink
Semipermeable membranes
Sucrose
H2O
Sucrose
H2O
Phloem loading requires energy and can cause a
high osmotic pressure in the source regions.
Water then diffuses into the phloem.
Phloem unloading is believed to be passive. It
lowers the osmotic pressure, and water diffuses
out as a result
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Phloem transport
Source
Sink
Semipermeable membranes
Sucrose
H2O
Sucrose
H2O
Inside the phloem, the water entering in the
source region pushes the phloem contents to the
sink region by bulk flow, where the water leaves
the phloem. The water movement is relatively
small, specially by xylem standards.
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Phloem transport
Source
Sink
Semipermeable membranes
Sucrose
H2O
Sucrose
H2O
Any region that loads phloem is a source region,
any place that unloads is a sink. Therefore, the
physiology in each region of the plant
determines what moves where. The management of
resources is not determined by phloem function.
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Phloem loading
Companion cell
Mesophyll cells
Sieve tube (phloem proper)
Sucrose
When there are plasmadesmata, sucrose can (and
does) travel symplastically. In many plants the
last step is apoplastic.
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Phloem loading
Companion cell
Mesophyll cells
Sieve tube (phloem proper)
Verbascose (5)
Stachyose (4)
Sucrose (2)
Raffinose (3)
Some plants do not have an apoplastic step, they
are called symplastic loaders. Each
plasmadesmata acts as a one way valve, allowing
the smaller sugar through but not the
back- diffusion of the bigger sugar.