Photoprotection in plants

Plant Physiology

Biology UNI PP11

Plant job description

light

H2O

CO2

sugar

O2

The good stuff? The good stuff?

Balanced photosynthesis

Photo Synthesis

High energy compounds

Harvests light energy

Uses harvested energy

Real time

Unbalanced photosynthesis

Photo Synthesis

High energy compounds

bright light produces lots of energy

low-capacity biochemical pathways

use it slowly

Real time

Potential for destruction.

Photosynthetic electron flow

ADP+ Pi ATP

H+

H+

H+

3 H+

NADPH

PS II PS I

H2O split

H2O

O2

Water splitting complex

Cytochrome complex

Thylakoid membrane

Coupling factor

e-

e-

e-

e-

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 geometryLight harvesting complex

Reaction center

Shade and sun plants

Sun plantShade 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

H2O

CO2

sugar

O2