Photoprotection in plants
Plant Physiology
Biology UNI PP11
Plant job description
light
sugar
CO2
O2
H2O
The good stuff?
The good stuff?
Balanced photosynthesis
High energy
compounds
Photo
Synthesis
Real
time
Harvests light
energy
Uses harvested
energy
Unbalanced photosynthesis
High energy
compounds
Synthesis
Photo
Real
time
bright light produces
lots of energy
low-capacity
biochemical pathways
use it slowly
Potential for destruction.
Photosynthetic electron flow
Thylakoid
membrane
H+
NADPH
PS II
PS I
Coupling
factor
e-
eH+
H+
H2O
ATP
e-
e-
O2
ADP
+ Pi
H2O
split
3 H+
Cytochrome complex
Water splitting complex
Too much energy in
• Biochemistry can’t use it
• Electrons pile up, high [O2]
• Nasty compounds
– O2-, 1 O2* (singlet oxygen)
– H2O2, .OH
• Oxidize chlorophyll (bleach)
• Oxidize lipids
• Can kill cells
Photoprotection
• Broadest sense
• Avoidance of excess energy into leaf
– Plant & leaf angles
– Reduce absorbtion
• Doing something harmless with the energy
• Unavoidable damage
Plant and leaf angles
• Most plants have
characteristic angles
• Upright
– Desert
– High latitude trees
– Crowded plants
• Prostrate
– Cold
– Icy winds
Leaves that move
• Adjust light interception (+/-)
Examples of avoidance
Light meets leaf
Reflected
Absorbed
Transmitted
All of the light is reflected, absorbed or transmitted.
Increasing reflection
• White top surface
– Hair
– Wax
– Air layers
• White bottom surface
– Reflect externally
– Reflect internally
Increase transmittance
• Move and turn chloroplasts
Photosystem geometry
Light harvesting complex
Reaction center
Shade and sun plants
Shade plant
Sun plant
Genetic control (capacity)
and developmental control.
May not be able to adjust
later.
Where the energy can go
• Phytochemistry (synthesis half)
• Dissipated
– Fluorescence of chlorophyll (red)
– Transferred to carotenoids or xanthophylls
• Yellow pigments
– Most carotenoids (constituitive)
– Xanthophylls (“cycle” or seasonal)
• Can go on to make harmful O2 species
Chlorophyll fluorescence
Carotenoids
• Grab excitation energy from chlorophyll
• Turns down the “volume” on chl excitation
• Beta-carotene
– Always found with chlorophyll
– Split in half to make Vitamin A
• Lutein
– Retinal protection
• Xanthophylls
Xanthophylls
• 3 kinds
– Violaxanthin (nonprotective)
– Antheroxanthin (protective)
– Zeaxanthin (protective)
• Interconvert
– Photoprotective forms in high light
– Violaxanthin in low light
– Proportional, constantly adjusted
– Cycles (shuttles) back & forth
Out in nature
• Conifers in winter
– High light, low chemistry
– High levels of photoprotective forms
• Sun plants
– Shuttle back and forth, day to night
• Shade plants
– May make & keep for day after sunfleck
• Pool sizes vary
Visualizing xanthophyll action
•
•
•
•
Light curve
High light plant
Photo-limited part
Synthesis-limited
part
• Saturation
• Reduction at high
light
• Time scale: minutes
Xanthophylls or damage?
Failure to protect
•
•
•
•
PS II rxn center vulnerable
O2 generated
No other protection
D1 protein degraded in light
–
–
–
–
Part of light harvesting complex (LHC)
Disassemble (LHC), trade proteins, reassemble
Always happening in light
More light, more degradation
• Can’t keep up? Capacity for photosynthesis drops
Problem solving at every level
•
•
•
•
Plant orientation
Leaf orientation
Leaf spectral characteristics
Protective pigments
– Constituitive
– Induced
• Repair
• Sometimes it still fails
Too much of a good thing
light
sugar
CO2
O2
H2O