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Plant Physiology > Chapter 9: Diversity and Limitations in Photosynthesis > Flashcards

Chapter 9: Diversity and Limitations in Photosynthesis Flashcards

1
Q

how is photosynthesis limited

A

F.F. Blackman in 1905 - said photosynthesis can be limited by single factors

  • most often water, nutrients, CO2, light
  • alternative photosynthetic pathways can improve carbon assimilation
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2
Q

C4 photosynthesis evolution

A
  • common in monocots and some eudicots
  • evolved 62 times in 19 angiosperm families
  • reduces photorespiration by concentrating CO2 near rubisco
  • prominent in grasses, chenopods, and sedges
  • 1 % of plants use it
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3
Q

discovery of C4 photosynthesis

A
  • Hugo Kortschak based on work in Hawaii on sugarcane
  • Carbon labelling showed first intermediates of Calvin Cycle were 4-carbon (rather
    than 3-carbon) sugars
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4
Q

Kranz anatomy

A
  • isolates calvin cycle from atmospheric O2
  • enlarged bundle sheath (photosynthetic cells surround the vascular bundle) with lots of chloroplasts, with no mesophyll cell more than 2-3 cells away from bundle sheath
  • C4 plants only have one type of mesophyll because bundle sheath acts as a second tissue in photosynthetic processes
  • rubisco is concentrated in bundle sheath
  • things can physically be passed through bundle sheath and mesophyll
  • mesophyll allows only CO2 to enter into bundle sheath, so less O2 seeps in
  • many have plasmodesmata that connect bundle sheath and mesophyll
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5
Q

single cell C4

A
  • 2 species
  • operate C4 within a single cell be separating PEP-carboxylase reactions from the Calvin
    cycle at opposite ends of the cell
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6
Q

what is the cost of C4 photosynthesis?

A
  • Regeneration of PEP consumes 2 ATP
  • If CO2 was low and photorespiration didn’t occur, C4 plants require more quanta of light per CO2 than C3 plants for the same CO2 fixation
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7
Q

where do C4 plants thrive?

A
  • heat, drought, and saline conditions
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8
Q

why are C4 plants better at thriving in hotter climates?

A
  • Photorespiration increases with temperature so C3 plant incur a high cost at high temperature
  • Rubisco reacts quicker with O2 at high temp AND O2 becomes more soluble = major advantage for C4 which don’t photorespire as much
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9
Q

why do C4 plants thrive in dryer climates?

A
  • Carbon concentration means C4 can open stomata less for the same amount of CO2 fixation = greater water use efficiency (WUE)
  • double WUE of C3
  • maintain higher water potentials in dry soil
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10
Q

CAM

A
  • Crassulacean acid metabolism

- CO2 fixation is separated by time instead of space

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11
Q

how does CAM photosynthesis operate?

A
  • similar to the C4 cycle, PEPCase fixes HCO3 and release CO2 to calvin cycle
  • but PEPCase occurs at night when the stomata can be open at lower temperatures
  • malate is stored in the vacuole as malic acid
  • PEPCase is run by the breadown of starch in the chloroplast
  • during the day when the stomata is closed, malate is released from the vacuole and breaks down pyruvate and CO2 and the light and carbon cycles run
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12
Q

what are some benefits to CAM?

A
  • major increase in WUE - 10x more than C3 plants because stomata are open during the night
  • water resistant because of thick cuticles
  • some C3 plants can switch to CAM (faculative CAM) when stressed with heat, water, or salt
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13
Q

what are some costs to CAM?

A
  • aren’t competitive under high resource conditions - can be invasive sometimes
  • pathway is expensive because of high metabolic cost to fix malic acid and is very slow
  • have to store malate
  • limits the amount of light reactions
  • amount of malate can be limiting
  • grow slowly because water and malic limited - have to maintain turgor pressure
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14
Q

chloroplasts can move

A
  • Under low light, they spread out on the leaf surface to maximize light capture
  • Under high light they stack to self-shade to minimize photoinhibition
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15
Q

light response curve and factors that affect it

A
  • Plots CO2 fixation as a function of light intensity
  • saturation irradiance
  • light-saturated assimilation rate (A max)
  • dark respiration rate
  • quantum yield
  • light compensation point
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16
Q

saturation irradiance

A
  • light level where CO2 fixation is maximum

- sun leaves have a higher saturation irradiance

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17
Q

light-saturated assimilation rate (A max)

A
  • maximum photosynthetic rate

- sun leaves have a higher A max

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18
Q

dark respiration rate

A
  • y intercept, due to negative costs of respiration in dark

- sun leaves have a higher dark respiration rate

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19
Q

quantum yield

A
  • slope of linear portion (efficiency of the light reactions)
  • sun and shade plants have similar quantum yields
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20
Q

light compensation point

A
  • x-intercept, where photosynthetic rate equals dark respiration, after this point there is net photosynthetic gain
  • sun leaves have a higher light compensation point
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21
Q

what does high light mean?

A

high temperature and excess light energy

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22
Q

temperature response curves

A
  • plants have an optimum temperature that can be seen in this graph
  • both light and CO2 fixation are narrowly adapted to a given temp range and diminish above or blow it
23
Q

how can leaves stay in their temperature optimum?

A
  • leaf hairs, salt glands, waxes to increase reflection
24
Q

how can excess heat be given off?

A
  • long-wave radiation (reflection of light back at longer wavelengths),
  • sensible heat loss (conduction and convection)
  • evaporative cooling
25
Q

how can leaves help reduce or increase light exposure?

A
  • sun tracking = heliotropism
  • A pulvinus between lamina and petiole contains motor cells which generate
    mechanical forces by changes in turgor in response to hormones
26
Q

photoinhibition

A

reaction center of PS2 is inactivated and/or damaged by excess light

27
Q

dynamic photoinhibition

A
  • can occur daily and is reversible

- driven by the xanthophyll cycle

28
Q

xanthophyll cycle

A
  • carrotenoids
  • under high light violaxanthin is changed into antheraxanthin and into zeaxanthin, which is efficient at heat dissipation
  • high energy pigments that release alot of heat when moving from excited to ground
  • minimizes damage to photosystem but reduces photosynthetic gain - stays minimizes it can lead to chronic
29
Q

chronic photoinhibition

A
  • photoinhibition that continues to reduce photosynthetic gain
  • usually caused due to other stressors such as temperature, flooding, or drought
30
Q

shade leaves

A
  • thinner and more layers of spongy mesophyll to capture and diffuse light
  • leaves are also spread over a large are to minimize self-shading
  • higher chlorophyll content per reaction center
  • higher concentration of a to b chlorophyll
  • shade plants rely on sunflecks
31
Q

c4 photosynthesis

A
  • CO2 is fixed through a reaction with water in the form of HCO3
  • HCO3 interacts with PEPcarboxylase and turns pyruvate into malate (which is a 4 carbon intermediate)
  • malate is transferred to another tissue where enzymes decarboxylate CO2 and pyruvate to concentrate it in the second tissue
  • then normal calvin cycle occurs
  • minimizes photorespiration because rubisco is in the second tissue away from oxygen
  • malate can be substituted with aspartate
32
Q

metabolic processes are slower at colder temperatures

A
  • membranes loosen
  • rubisco may drive this decline
  • increase temp - increase rubisco, at higher temps carboxylase starts to match oxygenase slowing down photosynthesis
33
Q

latitudes on C3 v C4 plants

A
  • low lat - C4 doing better
  • high lat - C3 doing better
  • temps are typically colder at higher lats
34
Q

CO2 compensation points are analogous to what in light curves?

A
  • light compensation point
  • how much light you need to break even
  • C4 plants need virtually no CO2 to have a new CO2 fixation because they can concentrate their own CO2
35
Q

atmospheric CO2 in C3 v C4 plants

A
  • high CO2 at low temps = C3 dominate

- low CO2 at high temps = C4 dominate

36
Q

why does CAM open their stomata at night?

A
  • lower temperatures

- lower water vapor pressure deficit

37
Q

what happens when you are limited in CO2/light?

A
  • CO2 will limit rubisco activity in photosynthesis

- light will limit rubisco regeneration in photosynthesis

38
Q

how can a plant reduce stomatal resistance?

A
  • open the stomata
  • Speed up diffusion by changing the diffusion coefficient by adding more air and increasing surface area - more air = more surface area
39
Q

most light intercepted does not go to photosynthesis

A
  • 15% loss from transmission
  • 10% from heat dissipation
  • 20% is lost from metabolism
  • 5% is lost to form carbohydrates
40
Q

how can canopies limit light?

A
  • self-shading, leaves above shade those below

- other trees are out competing for sunlight

41
Q

heat in excess

A
  • light in excess means more heat

- causes stomata to close

42
Q

how can excess heat be lost?

A
  • decrease boundary layer - wind
  • evaporative cooling
  • reflective hairs
  • take on red colors
  • reduce leaf area
43
Q

PPFD

A

photon flux density

  • threshold for photosynthesis in excess light
  • light increases, O2 and CO2 increase to a point
44
Q

diaheliotropic

A
  • type of heliotropism

- leaves move horizontally for more light

45
Q

paraheliotropic

A
  • type of heliotropism

- leaves more vertically to get less sun

46
Q

can CO2 be limiting?

A
  • when stomata is closed
  • FACE = free air CO2 enrichment
  • excess CO2 can cause stomata to close but increase leaf temp
47
Q

sage

A

reflective hairs and are white to deflect white light

48
Q

agave

A
  • Blue is being reflected, which is a high energy pigment, filtering out high energy
  • Really thick cuticle to avoid water loss
  • CAM
49
Q

how is leaf anatomy specialized for light capture?

A
  • transparent epidermis w/convex cells that focus light to chloroplasts in internal tissues
  • palisade cells allow light penetration via large vacuole w/ chloroplasts pressed to cell walls and large airspace (light channeling)
  • light pass in small space between chloroplasts (sieve effect)
  • spongy mesophyll cells are honeycombed airspace and refract light (light scattering)
50
Q

what are the resistors in which CO2 diffusion through the stomata depend on?

A
  • stomatal resistance
  • intercellular airspaces
  • liquid phase resistance
51
Q

stomatal resistance

A
  • resistance that CO2 diffusion depends on
  • strongest resistance
  • modified by opening and closing stomata
52
Q

intercellular airspaces

A
  • resistance that CO2 diffusion depends on
  • most of the leaf is airspace
  • This minimizes resistance within the leaf by having most of the diffusion pathway be gas phase
53
Q

liquid phase resistance

A
  • resistance that CO2 diffusion depends on
  • moving into the chloroplasts
  • Resistance in minimized by having large internal surface area (50x the external surface) and having chloroplasts near to cell walls (minimize liquid diffusion distance)

Plant Physiology (21 decks)

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