Nutrition and transport in plants

1
Nutrition and transport in plants
2
Plant macronutrients

• Nitrogen - nucleic acids, proteins, coenzymes
• Sulphur - proteins, coenzymes
• Phosphorus - nucleic acids, phospholipids,
  coenzymes, ATP
• Potassium - water balance, stomatal opening,
  protein synthesis
• Calcium - stability of walls and membranes,
  regulates many plant responses
• Magnesium - component of chlorophyll, activates
  many enzymes

3
Micronutrients - mainly cofactors

• Chlorine (ionic balancing)
• iron - cytochromes
• boron
• Manganese
• Zinc
• copper
• molybdenum

• Nickel
• The most usual symptom for deficiency of
  nutrients is chlorosis (leaves go yellow)

4
Water

• Makes up 95 of the weight of a cell.
• Plants lose a lot of water - around 300 litres
  for every Kg of carbon fixed.
• Plants must take up this water without taking up
  high concentrations of soil minerals - exclusion
  and selectivity
• Plants must have mechanisms to transport water
  efficiently up to 100m.

5
Water potential

• Water potential (Y)is the force that drives water
  movement through the plant and into the
  atmosphere.
• Pressure increases Y. Yp pressure potential
• Solutes decrease Y. Ys solute potential
• water moves down the gradient of its potential.

6
A demonstration of water potential
Yp 0 Ys -0.23Mpa Y -0.23Mpa
Pure water
0.1molar solution
1Mpa 10 atm 10kg/cm Car tyre typically pumped
up to 0.2Mpa
H20
Manometer split into 2 with a semi-permeable
membrane
7
Water relations of cells
Ys 0.4m sucrose -0.9 Mpa Y Ysoln (-0.9 Mpa)
- Ycell (-0.7Mpa) - Yp (0Mpa) -0.2Mpa Water
flows out of the cell into the solution until
Ycell Ysoln Cell becomes plasmolysed
0.4m sucrose
Flaccid cell
Ycell -0.7Mpa Yp -0Mpa
Pure water
Y Ysoln (-0 Mpa) - Ycell (-0.7Mpa) - Yp (0Mpa)
0.7Mpa Water flows into the cell from the
solution until Yp Ycell
Turgid cell
8
Ion selectivity

• Plants must take up ions selectively
• They do this by having transport proteins
• The energy for transport comes from ATP powered
  hydrogen extrusion to produce a gradient of H
  ions and an electrochemical potential difference
  between the inside and outside of the cell. Ions
  can either diffuse in or be pulled in by the
  negative charge.

9
Transport systems
Outside
H
K
H
H
H
A-
S
Cell membrane
H
K
K
Transport of neutral solutes
ATP
ADP
K
H
Inside -70mV
H pump
Cation uptake
Co-transporter
10
Short distance transport (cell to cell)

• Water and ions have 2 routes to the endodermis
• through cell walls (apoplast)
• through the cells (symplast)
• The outside of the cells in the Endodermis in the
  root is impermeable to water due to a suberised
  casparian strip. Water and ions must enter cells
  before they can cross the endodermis. ION
  SELECTIVITY

11
Routes to the endodermis
Cell wall
tonoplast
Vacuole
Plasmodesma
Plasma membrane
Cytoplasm
Trans- membrane
Symplastic
Apoplastic
12
3 Mechanisms of water movement

• Capillarity - Water will rise up capillaries,
  (like xylem vessels) but the distance is only a
  few centimetres at best.
• Root pressure - plants selectively take up ions
  and water will follow by osmosis. Not capable of
  providing the volume of flow and would result in
  toxic concentrations of ions
• Transpiration pull.

13
Transpiration pull

• The atmosphere has a very low water potential
  (-700Mpa) and cells in the leaf lose water to it.
• Water moves by osmosis from neighbouring cells
  until it reaches the bundle sheath cells.
• Bundle sheath cells take water from the xylem
• The whole column of water in the xylem moves up.
• The vacuum in the roots pulls water in.

14
Transport of solutes in the phloem

• Sugars are actively loaded into the phloem in the
  leaves, and actively removed from the phloem in
  parts of the plant like the roots that need
  sugar.
• Any sugar that leaks out is pumped back in by the
  companion cells

• Water follows by osmosis, and the difference in
  water potential between the leaves (high sucrose,
  high negative water potential) and the roots (low
  sucrose, low negative water potential) drives a
  bulk flow of the sugar solution.

15
Mass flow of solutes in the phloem
Sucrose loaded in the leaves
High Y
Shoot
Water follows by osmosis
Bulk flow of solution
Water follows by osmosis
Root
Sucrose removed
Low Y
16
Symbiotic nitrogen fixation and mycorrhizae

• Certain microorganisms like Rhizobium can form
  symbiotic associations with plants whereby the
  microbes receive organic acids in exchange for
  nitrogen fixed by the microbe.
• Fungi are very efficient at taking up minerals
  from the soil. An association between plants and
  fungi may dramatically increase mineral uptake.