Chapter 9: Diversity and Limitations in Photosynthesis Flashcards
how is photosynthesis limited
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
C4 photosynthesis evolution
- 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
discovery of C4 photosynthesis
- 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
Kranz anatomy
- 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
single cell C4
- 2 species
- operate C4 within a single cell be separating PEP-carboxylase reactions from the Calvin
cycle at opposite ends of the cell
what is the cost of C4 photosynthesis?
- 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
where do C4 plants thrive?
- heat, drought, and saline conditions
why are C4 plants better at thriving in hotter climates?
- 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
why do C4 plants thrive in dryer climates?
- 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
CAM
- Crassulacean acid metabolism
- CO2 fixation is separated by time instead of space
how does CAM photosynthesis operate?
- 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
what are some benefits to CAM?
- 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
what are some costs to CAM?
- 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
chloroplasts can move
- 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
light response curve and factors that affect it
- 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
saturation irradiance
- light level where CO2 fixation is maximum
- sun leaves have a higher saturation irradiance
light-saturated assimilation rate (A max)
- maximum photosynthetic rate
- sun leaves have a higher A max
dark respiration rate
- y intercept, due to negative costs of respiration in dark
- sun leaves have a higher dark respiration rate
quantum yield
- slope of linear portion (efficiency of the light reactions)
- sun and shade plants have similar quantum yields
light compensation point
- x-intercept, where photosynthetic rate equals dark respiration, after this point there is net photosynthetic gain
- sun leaves have a higher light compensation point
what does high light mean?
high temperature and excess light energy
temperature response curves
- 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
how can leaves stay in their temperature optimum?
- leaf hairs, salt glands, waxes to increase reflection
how can excess heat be given off?
- long-wave radiation (reflection of light back at longer wavelengths),
- sensible heat loss (conduction and convection)
- evaporative cooling
how can leaves help reduce or increase light exposure?
- sun tracking = heliotropism
- A pulvinus between lamina and petiole contains motor cells which generate
mechanical forces by changes in turgor in response to hormones
photoinhibition
reaction center of PS2 is inactivated and/or damaged by excess light
dynamic photoinhibition
- can occur daily and is reversible
- driven by the xanthophyll cycle
xanthophyll cycle
- 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
chronic photoinhibition
- photoinhibition that continues to reduce photosynthetic gain
- usually caused due to other stressors such as temperature, flooding, or drought
shade leaves
- 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
c4 photosynthesis
- 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
metabolic processes are slower at colder temperatures
- membranes loosen
- rubisco may drive this decline
- increase temp - increase rubisco, at higher temps carboxylase starts to match oxygenase slowing down photosynthesis
latitudes on C3 v C4 plants
- low lat - C4 doing better
- high lat - C3 doing better
- temps are typically colder at higher lats
CO2 compensation points are analogous to what in light curves?
- 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
atmospheric CO2 in C3 v C4 plants
- high CO2 at low temps = C3 dominate
- low CO2 at high temps = C4 dominate
why does CAM open their stomata at night?
- lower temperatures
- lower water vapor pressure deficit
what happens when you are limited in CO2/light?
- CO2 will limit rubisco activity in photosynthesis
- light will limit rubisco regeneration in photosynthesis
how can a plant reduce stomatal resistance?
- open the stomata
- Speed up diffusion by changing the diffusion coefficient by adding more air and increasing surface area - more air = more surface area
most light intercepted does not go to photosynthesis
- 15% loss from transmission
- 10% from heat dissipation
- 20% is lost from metabolism
- 5% is lost to form carbohydrates
how can canopies limit light?
- self-shading, leaves above shade those below
- other trees are out competing for sunlight
heat in excess
- light in excess means more heat
- causes stomata to close
how can excess heat be lost?
- decrease boundary layer - wind
- evaporative cooling
- reflective hairs
- take on red colors
- reduce leaf area
PPFD
photon flux density
- threshold for photosynthesis in excess light
- light increases, O2 and CO2 increase to a point
diaheliotropic
- type of heliotropism
- leaves move horizontally for more light
paraheliotropic
- type of heliotropism
- leaves more vertically to get less sun
can CO2 be limiting?
- when stomata is closed
- FACE = free air CO2 enrichment
- excess CO2 can cause stomata to close but increase leaf temp
sage
reflective hairs and are white to deflect white light
agave
- Blue is being reflected, which is a high energy pigment, filtering out high energy
- Really thick cuticle to avoid water loss
- CAM
how is leaf anatomy specialized for light capture?
- 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)
what are the resistors in which CO2 diffusion through the stomata depend on?
- stomatal resistance
- intercellular airspaces
- liquid phase resistance
stomatal resistance
- resistance that CO2 diffusion depends on
- strongest resistance
- modified by opening and closing stomata
intercellular airspaces
- 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
liquid phase resistance
- 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)
