Plant Physiology

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Plant Physiology

• Water balance of plants

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Water in the soil

• The water content and the rate of water movement
  in soils depend to a large extent on soil type
  and soil structure.

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Sand

• Desert

Silt

• Under water bodies (canals)

Clay

• Traditional houses

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• Water in the soil consists of 3 parts
• 1- Gravitational water water filled in the big
  spaces/interstices of soil particles and is
  readily drained from them by gravitation.
• Gravitational water is found in the macropores.
  It moves rapidly out of well drained soil and is
  not considered to be available to plants.
• It can cause upland plants to wilt and die
  because gravitational water occupies air space,
  which is necessary to supply oxygen to the roots.
• Drains out of the soil in 2-3 days
• 2- Bound water water tightly adhered to the soil
  particles.
• This water forms very thin films around soil
  particles and is not available to the plant. The
  water is held so tightly by the soil that it can
  not be taken up by roots. 
• not held in the pores, but on the particle
  surface. This means clay will contain much more
  of this type of water than sands because of
  surface area differences. 
• Gravity is always acting to pull water down
  through the soil. However, the force of gravity
  is counteracted by forces of attraction between
  water molecules and soil particles and by the
  attraction of water molecules to each other.

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• 3- Capillary water Water filled in the small
  spaces/interstices of particles, easily get to
  the surface of water by the force of capillarity.
  
• Most, but not all, of this water is available for
  plant growth
• Capillary water is held in the soil against the
  pull of gravity 
• Forces Acting on Capillary Water
• Capillary water is held by cohesion (attraction
  of water molecules to each other) and adhesion
  (attraction of water molecule to the soil
  particle). 
• The amount of water held is a function of the
  pore size (cross-sectional diameter) and pore
  space (total volume of all pores) 

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• Field capacity
• Field capacity is the water content of a soil
  after it has been saturated with water and excess
  water has been allowed to drain away due to the
  force of gravity.
• Field capacity is large (40) for clay soils and
  soils that have a high humus content and much
  lower (3) for sandy

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Water absorption by the root

• Water Moves through the Soil by Bulk Flow
• Water moves through soils predominantly by bulk
  flow driven by a pressure gradient, although
  diffusion also accounts for some water movement.
• As a plant absorbs water from the soil, it
  depletes the soil of water near the surface of
  the roots.

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Root tipthe water absorption zone
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The overall scheme of water movement through the
plant

• 1- From soil to root epidermis
• Diffusion to the intercellular space
• Capillary movement of soil water to plant roots.
  Plant root removes water. Tension in the soil
  right around the root increases gradient flow of
  water from low tension to high. This keeps a
  source of capillary water flowing to the plant
  root.
• Osmosis to the epidermis cells

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2- From epidermis to and through cortex

• 1- Apoplast pathway water moves exclusively
  through the cell wall without crossing any
  membranes. (The apoplast is the continuous system
  of cell walls and intercellular air spaces in
  plant tissues.)
• 2- Symplast pathway water moves through the
  symplast, traveling from one cell to the next via
  the plasmodesmata (The symplast consists of the
  entire network of cell cytoplasm interconnected
  by plasmodesmata.)
• 3- Transmembrane pathway water sequentially
  enters a cell on one side, exits the cell on the
  other side. In this pathway, water crosses at
  least two membranes for each cell in its path.
• Symplast pathway and transmembrane pathway are
  two components of cellular pathway,

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Transversing endodermis

• Casparian strip?
• Casparian strip is a band of cell wall material
  deposited on the radial and transverse walls of
  the endodermis, which is chemically different
  from the rest of the cell wall. It is used to
  block the passive flow of materials, such as
  water and solutes into the stele of a plant.
• To transverse casparian strip, apoplast pathway
  does not work (blocked), only cellular pathway
  works

Stele is the central part of the root or stem
containing the tissues derived from the
procambium. These include vascular tissue, in
some cases ground tissue (pith) and a pericycle,
which, if present, defines the outermost boundary
of the stele. Outside the stele lies
the endodermis.
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• 3- From endodermis to root vessel
• apoplast pathway and cellular pathway
  (diffusion or osmosis)
• 4- From root vessel to stem vessel to leaf vessel
• apoplast pathway (mass flow)
• 5- From leaf vessel ? leaf mesophylls and
  intercellular space?stomatal cavity?stomata ?air
  (diffusion or osmosis)

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Driving Forces of Water absorption and movement

• 1- Root Pressure
• 2- Transpiration pull

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1- Root Pressure

• Solute Accumulation in the Xylem Generates Root
  Pressure
• The root absorbs ions from the dilute soil
  solution and transports them into the xylem. The
  buildup of solutes in the xylem sap leads to a
  decrease in the xylem osmotic potential (?s) and
  thus a decrease in the xylem water potential
  (?w). This lowering of the xylem ?w provides a
  driving force for water absorption.

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Guttation
Dew?

• Appearance of xylem sap drops on the tips or
  edges of leaves e.g. grasses
• Sugars, mineral nutrients and potassium

• Transpiration stops at night time due to stomata
  closing
• High soil moisture level
• Lower root water potential
• Accumulation of water in plants
• Plants will start bleeding through leaf tips and
  edges

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2-Transpiration Pull
Transpiration-cohesion theory
Transpiration is the loss of water through the
stomata in leaves. This loss of water causes an
area of low pressure within the plant and water
moves from where it is at high pressure to low
pressure. The cohesion part is what allows water
to do this against gravity.
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How do we genetically manipulate plant water
relations?
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Arabidopsis as example!!!
Mutation in MRH2 Kinesin (ARM domain-containing
kinesin-like protein) Enhances the Root Hair Tip
Growth Defect
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Stomata fail to close under scarce water
conditions
Arabidopsis PARG1 mutants
Knockout of PARG-1 gene causes Arabidopsis plants
to wilt earlier than the wild type under drought
stress
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STOMAGEN positively regulates stomatal
development.
Overexpressing
Knockout
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Further Readings

• Chapter 4, Plant Physiology by Taiz and Zeiger,
  3rd ed.