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Light and Photosynthesis: Understanding the Role of Light in Plant Growth and Development

This article explores the relationship between light and photosynthesis, including the electromagnetic spectrum, the action spectrum for photosynthesis, the role of phytochrome, and the effects of light variation in canopies. It also discusses the importance of sunflecks and their impact on forest floor plants.

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Light and Photosynthesis: Understanding the Role of Light in Plant Growth and Development

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  1. Ch. 2 Light and Photosynthesis (Ps)

  2. Light • Radiant energy (electromagnetic radiation) • Particle + wave • Particle: torp

  3. Light • Wave: has • Wavelength inversely

  4. Light • Visible light: 380-750 nanometers (nm) (nano=10-9)

  5. Electromagnetic spectrum • T/F: Plants green because chlorophyll absorbs green light

  6. Electromagnetic spectrum • 400-700 nm: Photosynthetically Active Radiation (PAR) • Drives Ps • Green least (why leaves green) Action spectrum for photosynthesis (Ps)

  7. Radiation • Helps determine • IR: 750-106 nm. • Objects > absolute 0 emit • Gain energy from IR too Pred

  8. IR • “Infrared (IR) window” of leaves • Transmit near IR (beyond visible): little • Most visible IR window

  9. IR • Near IR transmitted through leaves (dashed line)

  10. IR • Use IR window (sense overstory)?? • First: phytochrome

  11. Phytochrome • 2 forms (interconvertible) • Prabsorbs R: converted to Pfr • Pfr absorbs FR (near IR): converted to Pr • Pfr active

  12. IR • 1) Ratio Pr:Pfr depends on ratio R:FR light • 2) R:FR influenced by leaves • Leaves remove R, let FR through “IR window”

  13. IR • Ex, red alder (Alnus rubra) • Germination sensitive to Pfr • Pfr high: • Pfr low:

  14. IR • Ex, red alder (Alnus rubra) • In open: lots R light, lots Pfr

  15. IR • Ex, red alder (Alnus rubra) • Under canopy: lots FR light, less Pfr

  16. IR • Plants may use phytochrome • 1) shaded: stimulate etiolation (spindly, less chlorophyll) • 2) info on canopy (deciduous forest) Trillium

  17. Electromagnetic spectrum • UV radiation: < 350 nm (to 1 nm)

  18. UV • Danger: Damage DNA (skin cancer!) • Pigments (ex, flavonoids) absorb • +DNA repair mechanisms Diamorpha smallii

  19. Units (Intensity) • ?

  20. Units

  21. Units • PAR: Photosynthetic Photon Flux Density (PPFD) • moles photons/sq. m/sec National Mole Day??

  22. Units PPFD • 1 mole photons = 1 Einstein • So, Einsteins/sq. m/sec National Mole Day??

  23. Light intensity variation • Canopies patchy in

  24. Light intensity variation • Phenoseason: seasonal light change + canopy change Max. lite canopy top Max. lite ground level

  25. Canopy leaf expansion Phenoseasons • Ex, Eastern Deciduous Forest understory plants • 4 strategies: • 1) Spring herbs (“Ea”): emerge early Canopy leaf drop

  26. Spring herbs Ex, Trillium

  27. Spring herbs Ex, Erythronium (trout lily)

  28. Canopy leaf expansion 2. Summer green herbs • Emerge late spring, green summer, dormant fall • “Us” Canopy leaf drop

  29. Summer green herbs • Ex, Uvularia sessilifolia (bellwort) Uvula??

  30. Summer green herbs • Ex, Uvularia sessilifolia (bellwort)

  31. Canopy leaf expansion 3. Late-summer herbs • Emerge summer, get early fall sun (Aa) • Ex, Aster acuminatus (sharp-leaved aster) Canopy leaf drop

  32. Canopy leaf expansion 4. Semievergreens • Emerge late spring, green late: “Om” • Ex, Oxalis montana (wood sorrel) Canopy leaf drop

  33. Light variation • Canopies patchy • Question: Technical name for beam sunlight on forest floor?

  34. Light variation • Canopies patchy in space • ______: beam sunlight on forest floor

  35. Sunflecks: Good • Useful (forest floor plants) • Some: 70% energy sunflecks

  36. Sunflecks: Good • Useful (forest floor plants) • Some: 70% energy sunflecks • Ability to use: • Spring herbs (Trillium) respond • Summer green herbs (Uvularia) exploit

  37. Sunflecks: Bad • Ex: shade-adapted Oxalis. Redwood forest (Oxalis oregana-redwood sorrel) • Coast redwood (Sequoia sempervirens) • Tallest tree: 368 ft. DBH 23 ft.!

  38. Sunflecks: Bad • O. oregana: Sunflecks damage Ps machinery

  39. Sunflecks: Bad • Sunflecks & drought stress • Ex, regeneration Abies magnifica (CA red fir)

  40. Sunflecks • Ex, regeneration Abies magnifica (CA red fir) • Seedling pattern from sunfleck pattern

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