Photosynthesis: Physiological and Ecological Considerations

1
Photosynthesis Physiological and Ecological
Considerations
2
Intro

• Larger scale
• lvs and communities vs. chloroplasts
• Understand plant productivity
• crop yields
• forestry, agronomy, botany, horticulture, food
  science, etc)
• study limiting factors (slowest step or limiting
  condition)
• 3 metabolic steps critical for optimal Ps
• RUBISCO activity, RuBP regeneration, triose-P
  metabolism

3
Light

• 3 parameters impt in measuring light
• amnt (intensity, duration, time of year, canopy)
• spectral quality (wavelength, time of year,
  canopy)
• direction (N, S, E, W, indirect, direct)
• Light measured as S
• irradiance amnt S that falls on flat area per
  unit time (W m-2)
• 1 W 1 J s-1
• photon irradiance of incident quanta (mol m-2
  s-1)
• 1 mol light 6.02 x 1023 photons
• relationship btwn quanta and S
• E hc/? where ? wavelength, c speed of
  light 3 x 108 m s-1 h Plancks
  constant 6.63 x 10-34 J s-1
• 400 nm photon has 2x more S than 800 nm photon

4
Light

• Direction of light
• light strikes obliquely or flat (perpendicular to
  surface, noon)
• flat sensors best for flat lvs
• sensing direction
• spherical sensors best for oblique
  oblique light or
  light from many
  directions (diffuse
  light, reflection)
• not sensitive to direction
• useful for non-leaf measurements
• fluence rate measurement
  from all directions
  
  (W m-2 or mol m-2 s-1)

5
PAR

• Photosynthetically active radiation (PAR)
  measure of irradiance (amnt of light) btwn
  400-700 nm (W m-2)
• when mol m-2 s-1 its called photosynthetic
  photon flux density
• PAR in direct light 400 W m-2
  2000 µmol m-2 s-1
• higher at higher altitudes

6
Energy Losses

• 1.3 kW m-2 radiant S from sun reaches Earth
• 5 S converted to carbohydrates
• why?
• outside of absorption
  spectrum (400-700
  nm)
• heat loss
• fluorescence

7
Leaf Anatomy

• Epidermal cells
• transparent to visible light
• convex focuses light to chloroplasts
• impt in low light
• Palisade cells
• photosynthetic, 2nd layer
• parallel columns, 1-3 cells deep
• high chlorophyll content
• sieve effect due to uneven distribution
  of chlorophll w/i
  cell
• light channelling light propagated through
  vacuole, transmitted to leaf interior
• Spongy mesophyll cells
• photosynthetic, 3rd layer
• irregular shape, large air spaces,
• light scattering diffusing of light ? absoption

sun leaf
shade leaf
8
Leaf Anatomy

• Other modifications ? reflection, ?
  absorption (40)
• hairs
• salt glands
• epicuticular wax

9
Leaf Movement

• Solar tracking (heliotropism)
  diurnal movement of
  lvs/flowers,
  occurs in full sun
• controlled by swelling of pulvinus
  (motor
  cells) at leaf base due to
  changes in ?s
• diheliotropism diurnal mvmt,
  keeps plant
  organs perpendicular
  to sun rays, maximize Ps
• am lvs vertical, follow the sun,
  pm lvs vertical,
  reorient at night
• paraheliotropism diurnal mvmt,
  keeps plant organs
  parallel to
  sun rays, minimize sun rays

10
Chloroplast Movement

• Chloroplast mvmt is means of controlling light
  absorbance
• low light perpendicular to light
• high light parallel to light rays
• ? absorbance 15
• move along microfilaments

11
Sun / Shade Adaptations

• Plasticity capability of growing
  in sun
  or shade
• most sp. not so plastic
• depends on location in crown/habitat
• high light sun lvs, 2 PSII1 PSI
• more RUBISCO/xanthophyll
• reddish/pale, smaller/thicker, deeper lobed
• longer palisade parenchyma
• more extensive vascular system
• thicker CW in epidermis, sun burn
• low light shade lvs, 3 PSII1 PSI
• more chlorophyll per rxn center,
  more chlorophyll b
  (antenna), ? Ps rate
• larger/thinner, shallow lobed

12
Competition for Light

• Plants compete for light
• balance canopy to allow maximum
  Ps for entire
  plant
• maximum absorption of PAR
• sunflecks patches of light in
  canopy that move
• allow short burst of Ps
• Shady environment
  more far-red (PS I)

13
Light-Response Curve

• Light-response curve graph of CO2 fixation at
  various light levels
• dark no Ps Rm - CO2 assimilation (loss)
• as light ?, CO2 assimilation ?
• light compensation pt pt where
  CO2 uptake equals
  CO2 release
• depends on sp. and development
• comp. pt sun 10-20 µmol/m/s
• comp. pt shade 1-5 µmol/m/s
• lower b/c ? R in shade plants
• above comp. pt. linear increase
• ? light ? Ps b/c light limited
• saturation pt. where ? light ? Ps
• limited by other factors

14
Light-Response Curve

• Light saturation levels lower in shade-grown
  plants
• depends on early development
• change light cond. little effect

sun
shade
15
Light-Response Curve

• Maximum quantum yield measure of photons used
  for CO2 assimilation
• C3 more efficient lt30 C
• C4 unaffected by temp.
• more efficient than C3 at gt30 C

16
Light-Response Curve

• Saturation occurs btwn 500-1000 µmol m-1 s-1
• full sun 2000 µmol m-1 s-1
• indiv. lvs dont use full sun
• allow penetration
• entire plant has greater use

17
Dissipation of Excess Energy

• Xanthophylls (carotenoid) func.
  for dissipating excess S
• zeaxanthin most effective, fluctuates
• plants grown in full sun have
  more xanthophylls

18
Dissipation of Excess Energy

• Isoprene
• bluish haze over forests, piney scent
• 5C hydrocarbon
• contributes to atmospheric levels of CO, O3, and
  methane residence time (GH effect)
• Stabilizes photosynthetic membranes at ? light
  and temp.
• absorbs 222 nm (? S)
• ? light and temp.
  ?
  isoprene syn.

19
Dissipation of Heat

• Tremendous heat load dissipated by/as
• evaporative heat loss due to evaporative
  cooling
• evaporation requires S thus pull heat from leaf
• sensible heat loss due to air
  circulation around leaf
  
• air must be cooler than leaf
• long-wave radiation
• back to atmosphere/space

20
Photosynthesis and CO2

• ? CO2 ? Ps, ? CO2 ? Ps
• Charles Keeling
• CO2 0.037 (370 ppm), H2O vapor 2, O2
  20, N2 80
• growing season in N. hemisphere

CO2 ? 1 ppm/yr due to fossil fuels
1200-2800 ppm in Cretaceous
380 ppm today
180-260 ppm
600 ppm by 2020
21
Greenhouse Effect

• Greenhouse effect warming of Earths
  climate due to trapping of
  long-wave
  radiation by atmosphere
• GH roof transmits visible light
• absorbed by plants
• excess light converted to heat and re-emitted as
  long-wave radiation
• glass doesnt transmit long-wave radiation well
• thus GH heats up
• GH gases (CO2, methane, isoprene) trap long-wave
  radiation thus Earths atmosphere heats up

22
Greenhouse Effect

• GH effect on plants
• Ps limited in C3 plants due to ? CO2
• thus ? CO2 ? Ps for C3 plants why?????
• but ? CO2 ? GH effect ? warming ????? for
  plants
• RUBISCO deactived as temp. and CO2 ?
• http//www.pubmedcentral.nih.gov/articlerender.fcg
  i?artid27241

23
(No Transcript)
24
CO2 Diffusion to Chloroplast

• CO2 diffusion depends on CO2 gradient
• diffuses as gas through stomata to wet cell
  surface
• diffuses in liquid from cell surface to
  chloroplast
• resistances to diffusion
• r rstomata rboundary layer rair space
  rmesophyll
• intercellular air space resistance
  due to
  diffusion in air space
• mesophyll resistance w/i cell,
  btwn cell
  surface and stroma
• large SA of mesophyll cells and
  intercellular spaces
  facilitate diffusion

25
Photosynthesis and CO2

• CO2 compensation pt R equal Ps
• Below CO2 comp. pt
• very low intercellular CO2
• Ps strongly limited, R unaffected
  
• thus net efflux of CO2 (R gt Ps)
• Above CO2 comp. pt
• Ps stimulated in C3
• low to med. CO2 limited by
  RUBISCO carboxylation in
  C3
• high CO2 limited by RuBP
  regeneration

26
Photosynthesis and CO2

• Stomata limit CO2 entry
• Ignoring stomatal resistance
• Ps rates saturate at much lower levels in C4
  plants than C3
• due to CO2 concentrating mechanisms in C4
• Ps rates increase across wider range
• thus C3 may benefit more
  from ? CO2
• thus C4 will benefit little
  from ? CO2
• CO2 comp. pt very low in C4
  
  b/c no photorespiration

27
C4 and CAM Plants

• CO2 at carboxylation sites is saturated in C4
  and CAM
• C4 plants need less RUBISCO
• thus less N
• maintain ? Ps rates at ? CO2
• thus can close stomata
• thus more efficient use of H2O and CO2
• C4 less efficient at using light b/c S cost of
  CO2 conc. mechanisms
• thus most shade plants are C3
• CAM plants limited by malate storage

28
Photosynthesis and Temperature

• Ps rate as func. of temp. exhibits bell curve
• Ps rxns stimulated by ? temp. to pt
• too high temp. is damaging
• At high CO2, Ps rate greatly
  affected by temp.
• limited by other rxns
• At ambient CO2, Ps rate
  affected little by temp.
• limited by RUBISCOs affinity
  for CO2

29
Photosynthesis and Temperature

• R ? as temp. ?
• CO2 fixation per photon remains
  same in C4
• b/c no photorespiration
• CO2 fixation per photon ? in C3
• thus need more S per CO2
• b/c photorespiration
• photorespiration ? why?????
• Optimal temp. for Ps depend on genetics and
  development
• species specific responses
• temp. where individual plant is grown
• do best where you were grown
• alpine vs desert