Photoperiodic responses, light receptors and the biological clock

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Photoperiodic responses, light receptors and the
biological clock
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Classification according to photoperiodic control
of flowering
Short Day Plants (SDP) Flowering requires short
days (long nights)
Long Day Plants (LDP) Flowering requires long
days (short nights)
Day Neutral Plants (DNP) Flowering is not
regulated by day length
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Photoperiodism and flowering
Effect of day length on flowering and other
activities (seed germination, seed dormancy, bud
break, bud dormancy) in temperate regions of the
northern hemisphere.
Fig. 15-20, p. 252
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How does a change in day length lead to the
induction of flowering?

• For any biological organism to detect a change in
  day length, it needs
• A day light detection mechanism (the
  photoreceptors Phy and Cry)
• A biological clock (set at a 24 hr cycle) as a
  time measuring system

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Example of a circadian rhythm The circadian
oscillator controls the leaf movement rhythm in
beans
Leaf angle already starts to change before the
light of day. Leaf angle changes continue their
rhythm also in continuous dark.
Leaf angle
Leaf angle
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Circadian rhythms allow to monitor (to visualize)
the biological (circadian) clock
Without light detection (mediated by Phy and Cry
receptors) the period of the biological clock
becomes slightly longer than 24 hrs. The 24 hr
cycle of light detection allows to entrain the
clock to maintain a 24 hr cycle.
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Review
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Absorption and transport

• Osmosis and hydrostatic pressure used?

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PROTOPLAST
SOLUTION
(3) Hydrostatic pressure in cells
Concentration 0.3 molar
Concentration 0.3 molar (Isotonic)
Pressure 0 megapascals
Concentration 0.27 molar
Concentration 0 molar (Hypotonic)
Pressure 0.66 megapascals
Turgor pressure is one type of hydrostatic
pressure. Turgor pressure is the result of a
combination of osmosis and cell wall rigidity.
Concentration 0.5 molar
Concentration 0.5 molar (Hypertonic)
Pressure 0 megapascals
Fig. 3-7 (a-c), p. 36
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LIGHT
Events leading to the opening of a stoma The
production of malate and the influx of K and Cl-
powered by the electrical and pH gradients
produced by the proton pump increase the
concentration of osmotically active solutes in
the guard cells. As a result, water flows into
the cells by osmosis.
starch
malic acid
malate
plasma membrane
ATP
H
proton pump
ADP Pi
??
H
K

CI??
K
H
CI??
Fig. 11-8a, p. 170
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Root pressure is generated by an osmotic pump

• After passing the endodermis, mineral nutrients
  accumulate in the stele of the root. The
  endodermal cells provide the differentially
  permeable membrane needed for osmosis.
• Soil saturated with water
• Water tends to enter root and stele
• Builds up root pressure in xylem
• Forces xylem sap up into shoot

Fig. 11-13a, p. 178
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Mechanism of Phloem Transport
high pressure
low pressure
sieve tube
sucrose
sucrose
H2O
H2O
sucrose
sucrose
H2O
H2O
glucose
source
sink
H2O
glucose
CO2 H2O
sucrose
H2O
parenchyma
parenchyma
Fig. 11-14, p. 179
Sucrose is actively transported into the sieve
tubes at the food source region of the plant
(leaves or storage organs) and removed at the
sink regions (regions of growth or storage).
Water follows by osmosis, increasing the
hydrostatic pressure in the sieve tubes at the
source region and decreasing the pressure at the
sink region. The sieve-tube contents flow en
masse from high(source)- to low(sink)-pressure
regions.
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Absorption and transport

• Water flow through xylem compared to phloem?
• What are the similarities, what are the
  differences?

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Absorption and transport

• Do plants acidify the soil they grow in?
• Yes
• - Respiration
• - H extrusion

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Soil Formation
atmospheric gases CO2 SO2 N2O5
rock
rain
acids H2CO3 H2SO3 HNO3
wind and water erode rocks and soil
freeze-thaw produces cracks
roots crack rocks through pressure, secrete acid
Fig. 11-11, p. 175
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Active Uptake of Minerals Into Root Cells
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Differential Growth

• What is the link between turgor pressure, cell
  walls and differential growth?

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PROTOPLAST
SOLUTION
(3) Hydrostatic pressure in cells
Concentration 0.3 molar
Concentration 0.3 molar (Isotonic)
Pressure 0 megapascals
Concentration 0.27 molar
Concentration 0 molar (Hypotonic)
Pressure 0.66 megapascals
Turgor pressure is one type of hydrostatic
pressure. Turgor pressure is the result of a
combination of osmosis and cell wall rigidity.
Concentration 0.5 molar
Concentration 0.5 molar (Hypertonic)
Pressure 0 megapascals
Fig. 3-7 (a-c), p. 36
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Differential growth
a
Rate of cell elongation is higher on the a-side
of the coleoptile compared to the b-side. This
leads to differential growth increased growth
rate on one side of plant organ, results in
curvature of the organ.
b
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Plant transformation

• Agrobacterium
• Auxin
• Cytokinin
• Dedifferentiation
• Differentiation

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Transforming a plant cell by using Agrobacterium
Gene to be introduced in plant cell (for example
a gene that encodes the Luciferase protein)
Plant Cell
Agrobacterium

Modified Ti-plasmid
Nucleus
Transformed Plant Cell
Agrobacterium
Plant cell makes luciferase protein
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auxin
cytokinin
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Major signals that control plant growth and
development

• Internal signals Plant Hormones
• - AUXIN
• - CYTOKININ
• - ETHYLENE
• - ABSCISIC ACID
• - GIBBERELLIC ACID
• The plants toolbox for positive and negative
  control of physiological and developmental
  processes.

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Shade avoidance

• Shading of a plant by plants that grow above it
  leads to increased or decreased Phy activity ?

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Absorption spectra of Chlorophyll a and b
660
730
The ratio of Red (660 nm) to Far Red (730 nm)
light will be low underneath green leaves that
absorb light between 640 and 700 nm.
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The predicted properties of the receptor
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The End