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Photoprotection in plants

This article explores the process of photoprotection in plants, discussing how they balance photosynthesis to avoid damage from excess light energy. It covers the synthesis of high-energy compounds, real-time harvesting of light energy, and the potential for destruction when this energy is not properly utilized. The importance of photoprotection mechanisms such as leaf and plant angles, light reflection, chlorophyll fluorescence, and the role of carotenoids and xanthophylls is also explained. Furthermore, the article addresses the challenges plants face in maintaining a balance between too much light, sugar, CO2, and O2.

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Photoprotection in plants

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  1. Photoprotection in plants Plant Physiology Biology UNI PP11

  2. Plant job description light sugar CO2 O2 H2O The good stuff? The good stuff?

  3. Photo High energy compounds Synthesis Real time Harvests light energy Uses harvested energy Balanced photosynthesis

  4. Unbalanced photosynthesis Photo High energy compounds Synthesis Real time bright light produces lots of energy low-capacity biochemical pathways use it slowly Potential for destruction.

  5. Photosynthetic electron flow Thylakoid membrane ADP+ Pi H+ ATP NADPH e- e- PS II PS I Coupling factor e- e- O2 H+ 3 H+ H+ H2O split H2O Cytochrome complex Water splitting complex

  6. 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

  7. Photoprotection • Broadest sense • Avoidance of excess energy into leaf • Plant & leaf angles • Reduce absorbtion • Doing something harmless with the energy • Unavoidable damage

  8. Plant and leaf angles • Most plants have characteristic angles • Upright • Desert • High latitude trees • Crowded plants • Prostrate • Cold • Icy winds

  9. Leaves that move • Adjust light interception (+/-)

  10. Examples of avoidance

  11. Light meets leaf Reflected Absorbed Transmitted All of the light is reflected, absorbed or transmitted.

  12. Increasing reflection • White top surface • Hair • Wax • Air layers • White bottom surface • Reflect externally • Reflect internally

  13. Increase transmittance • Move and turn chloroplasts

  14. Photosystem geometry Light harvesting complex Reaction center

  15. Shade and sun plants Shade plant Sun plant Genetic control (capacity) and developmental control. May not be able to adjust later.

  16. 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

  17. Chlorophyll fluorescence

  18. 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

  19. 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

  20. 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

  21. 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?

  22. 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

  23. Problem solving at every level • Plant orientation • Leaf orientation • Leaf spectral characteristics • Protective pigments • Constituitive • Induced • Repair • Sometimes it still fails

  24. Too much of a good thing light sugar CO2 O2 H2O

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