Plant Ecology - Chapter 2

1. Plant Ecology - Chapter 2 Photosynthesis & Light

2. Photosynthesis & Light • Functional ecology - how the biochemistry and physiology of individual plants determine their responses to their environment, within the structural context of their anatomy and morphology

3. Photosynthesis & Light • Functional ecology - closely related to physiological ecology, which focuses on physiological mechanisms underlying whole-plant responses to their environment

4. Photosynthesis & Light • Photosynthesis is a “package deal” • How much light • Competitors • Limitations (pollution, pathogens) • Herbivores • Plants must cope with multiple items at same time

5. Process of Photosynthesis • Biochemical process to acquire energy from sun, carbon from atmosphere • 2 parts • Capture of energy (light reactions) • Storage of energy into formed organic molecules (carbon fixation)

6. Process of Photosynthesis • Reactions take place in chloroplasts • Light reactions on thylakoid membranes • Carbon fixation (Calvin cycle) within the stroma

7. Process of Photosynthesis • Light reactions involve pigment molecules • Many forms of chlorophyll • Accessory pigments (carotenoids and xanthophylls in terrestrial plants)

8. Process of Photosynthesis • Pigment molecules arranged into two molecular complexes • Photosystems I and II • Capture energy (form ATP, NADPH) plus generate oxygen

9. Process of Photosynthesis • Energy captured from light reactions powers the Calvin cycle • Captured energy ultimately stored in chemical bonds of carbohydrates, other organic molecules

10. Rates of Photosynthesis • Gross photosynthesis - total amount of carbon captured • Cellular respiration - organic compounds broken down to release energy • Net photosynthesis - gross photosynthesis minus respiration

11. Rates of Photosynthesis • Basic limiting factor - amount of light energy reaching thylakoid membranes • Darkness - loss of energy due to respiration - giving off CO2 • Low light - respiration plus some photosynthesis - giving off and taking up CO2 • Compensation point

12. Rates of Photosynthesis • Strong light - respiration plus photosynthesis - giving off and taking up CO2, up to a point • Maximum rate of photosynthesis, despite further increase in light energy

13. Rates of Photosynthesis • Different plants have different photosynthetic responses to same light intensity • Some do better under low light, others strong light • Habitat - shade vs. sun • Some can shift light compensation point to deal with changes in light availability (lots in spring, less in summer in shade)

14. Quality of Light • Light quality (availability of different wavelengths) can limit rate of photosynthesis • Blue and red wavelengths are captured preferentially • Green wavelengths are discarded (green plants)

15. Global Light Availability • Tropical latitudes - day and night equal • Polar latitudes - continuously light at midsummer, continuously dark at midwinter • Maximum sunlight energy greater in tropics than polar regions

16. Global Light Availability • Maximum sunlight energy greater at high altitudes than at sea level • Damaging UV-B radiation greater in tropics than polar regions, high elevations vs. low elevations • Biochemical protection: flavonoids to absorb, antioxidant and DNA repair enzymes

17. CO2 Uptake Limitations • CO2 diffusion from surrounding air into leaf and into chloroplast • Leaf conductance - rate at which CO2 flows into the leaf • Mostly under control of stomata

18. CO2 Uptake Limitations • Stomata open, close to maintain water balance (seconds, minutes) • Stomata change as leaf morphology, chemistry change (days, months) • Natural selection modifies (100s, 1000s of years)

19. CO2 Uptake Limitations • Controlling water loss is main reason why plants restrict their CO2 uptake • Huge amount of air required for photosynthesis - 2500 L air for each gram of glucose produced

20. CO2 Uptake Limitations • Stomata can be very dynamic, opening and closing constantly to regulate CO2 and water loss • Much variation even within same leaf • Patchy closure also common in stressed plants

21. Variation in Photosynthetic Rates: Habitats • Photosynthetic rates vary within and among habitats • Correlated with species composition, habitat preferences, growth rates

22. Variation in Photosynthetic Rates: Habitats • Photosynthetic rates may be unrelated to species distributions, populations processes • Other important components of photosynthesis: total leaf area, length of time leaves active, maintained

23. Photosynthetic Pathways • Carbon fixation done using 3 different pathways • C3 • C4 • CAM (crassulacean acid metabolism)

24. Photosynthetic Pathways • C3 and C4 named for 3-carbon and 4-carbon stable molecules first formed in these pathways • CAM named after plant family Crassulaceae where it was first discovered

25. Photosynthetic Pathways • Most plants use C3 photosynthesis, and plants that use it are found everywhere • C4 and CAM are modifications of C3, and evolved from it

26. Photosynthetic Pathways • C3: CO2 joined to 5-carbon molecule with assist from the enzyme RuBP carboxylase/oxygenase - rubisco • Rubisco probably most abundant protein on earth, but does its job very poorly

27. Photosynthetic Pathways • Rubisco inefficient at capturing CO2 • Also takes up O2 during photorespiration • O2 uptake favored over CO2 uptake as temperatures increase • Limits photosynthesis • Plants must have HUGE amounts of rubisco, especially those in warm, bright habitats, to compensate for poor performance

28. Photosynthetic Pathways • Increases in atmospheric CO2 concentrations should allow C3 plants to increase rates of photosynthesis

29. Photosynthetic Pathways • C4 photosynthesis contains additional step used for initial CO2 capture • 3-carbon PEP (phosphoenol-pyruvate) + CO2 = 4-carbon OAA (oxaloacetate) • Catalyzed by PEP carboxylate

30. Photosynthetic Pathways • PEP carboxylate only captures CO2 • Higher affinity for CO2 than rubisco • Not affected by warmer temperatures • Decarboxylation (CO2 removal) process allows standard Calvin cycle (including rubisco)

31. Photosynthetic Pathways • C4 requires special leaf anatomy • Spatial separation of C4 and C3 reactions • Rubisco exposed only to CO2, not O2 in atmosphere like in C3 plant

32. Photosynthetic Pathways • C4: Mesophyll cells for carbon fixation, bundle sheath cells for Calvin cycle - keeps O2 away from Calvin cycle • C3: Mesophyll cells for carbon fixation and Calvin cycle - allows O2 access to Calvin cycle

33. Photosynthetic Pathways • C4 plants generally have higher maximum rates of photosynthesis, and have higher temperature optima

34. Photosynthetic Pathways • C4 plants generally do not become light-saturated, even in full sunlight • Also have better nitrogen use and water use efficiencies because of reduced needs for rubisco (1/3 to 1/6)

35. Photosynthetic Pathways • Requires additional energy to run C4 pathway, but easily compensated for by photosynthetic gains at high light levels • Very successful in warm, full-light habitats, e.g., deserts

36. Photosynthetic Pathways • CAM photosynthesis - Crassulacean acid metabolism • Uses basically same biochemistry as C4, but in very different way • Rubisco found in all photosynthetic cells, not just bundle sheath cells

37. Photosynthetic Pathways • CAM uses temporal separation of light capture, carbon fixation rather than spatial separation as in C4 • CO2 captured at night, converted into organic acids

38. Photosynthetic Pathways • During daylight, organic acids broken down to release carbon, used normally in Calvin cycle • Stomata remain closed during day

39. Photosynthetic Pathways • CAM plants have thick, succulent tissues to allow for organic acid storage overnight • Tremendous water use efficiency (stomata closed during heat of day)

40. Photosynthetic Pathways • Some CAM plants not obligated to just CAM • Can use C3 photosynthesis during day if conditions are right, to achieve higher rates of photosynthesis • CAM can’t accumulate carbon as fast as C3 or C4 plants, lowering rate of photosynthesis

41. C3, C4, and CAM • C3 plants most abundant (# of species, total biomass) • More CAM species than C4 species • CAM plants less abundant than C4 in biomass, worldwide distribution

42. C3, C4, and CAM • Half of grass species are C4 • Dominate warm grassland ecosystems • Warm, bright conditions where C4 is favored

43. C3, C4, and CAM • CAM plants typically are succulents in desert habitats, or……

44. C3, C4, and CAM • Epiphytes growing on trees in tropics or subtropics • Both types experience severe water shortages

45. C3, C4, and CAM • Phenology - seasonal timing of seasonal events • C3 plants typically more springtime, vs. C4 plants being mostly summer

46. C3, C4, and CAM • C4 grasses are most common where summer temperatures are warm in N. America

47. C3, C4, and CAM • C3 grasses - cool, winter-moist • C4 grasses - warm, summer-moist

48. C3, C4, and CAM

49. Sun & Shade Leaves

50. Sun & Shade Leaves