Leaf level energy balance

Question 1

What non-radiative flux do you not have at leaf level?

Answer 1

Ground heat flux (G) - we dont have the energy going into the earth
- So radiative fluxes need to be balanced just by sensible heat flux (H) and latent heat flux (lamdaE)

Question 2

What is Qa?

Answer 2

Radiative forcing (Qa) = sum of absorbed solar radiation and incident longwave radiation

Question 3

What are stomata?

Answer 3

The openings on the leaf surface that govern the exchange of water and CO2
- Usually just on lower leaf surface

Question 4

What is going in and out of stomata?

Answer 4

• CO2 in - for photosynthesis
• H2O - out
• Heat - in and out

Question 5

What are the conductances within stomata?

Answer 5

• Leaf boundary layer conductance (gb) : from leaf surface to air
• Stomatal conductance (gs): from inside leaf to surface

The conductances work in series
- g = 1/r (resistance)

Question 6

How are sensible heat and latent heat exchanged in a leaf?

Answer 6

• Sensible heat is exchanged from both sides of the leaf (i.e. consider two conductances (gb) in parallel
• Latent heat is exchanged only from one side of the leaf (because stomata are typically located on one side of lead - so consisder (gb and gs) in series

Question 7

How can we approximate the size of the conductances in a leaf?

Answer 7

Using wind speed, and area of leaf

Question 8

How do the conductances relate to the fluxes?

Answer 8

The greater the conductances (g), the greater the sensible and latent heat fluxes will be
- Are proportional

Question 9

How do the conductances and fluxes work for small leaves?

Answer 9

Strong conductances = strong fluxes
- Have low boundary layer resistance
- So, has efficient heat transfer - low surface area relative to perimeter -> thin leaf boundary layer
- So strong coupling of leaf to surrounding air
- So leaf temp is similar to surrounding air
- Favoured under cold conditions
- Also similar effect for deep lobed leaves

Question 10

How do the conductances and fluxes work for large leaves?

Answer 10

Low conductance for large leaves = low fluxes
- Large boundary layer resistance - not efficient heat transfer
- Leaf creates its own climate - so leaf is decoupled from surrounding air
- High surface area relative to perimeter -> deep boundary layer
- So leaf temperature can be several degrees warmer than surrounding air
- Often found in tropics - but only when enough moisture - as leaf needs to be able to cool itself down through evapotranspiration

Question 11

How do leaf size and wind speed affect conductance and therefore the fluxes?

Answer 11

• Higher windspeed = higher conductance = higher fluxes
• Greater leaf size = lower conductance = lower fluxes

Question 12

Where are large and small leaves found?

Answer 12

• Larger leaves tend to be in tropics - due to moisture availability + no thermal constraint on leaf size
• Smaller leaves towards the poles - night time temperature is another factors that is too low to sustain large leaf growth in higher latitudes

Question 13

What are the 3 reactions happening in photosynthesis?

Answer 13

1. Light dependent reactions - conversion of light energy into chemical energy (NADPH and ATP)
2. Dark reactions (calvin cycle) - chemical energy is used to reduce CO2 to carbohydrates - e.g., sugars - catalysed by rubisco
3. Diffusion - stomata open to allow CO2 to diffuse into the leaf from the surrounding air

Any of these steps can be limiting to photosynthesis rates

Question 14

What is leaf net photosynthesis (An)?

Answer 14

An = A - Rd
- A = gross photosynthesis
- Rd = mitochondrial respiration

Question 15

What factors can be limiting to photosynthesis rates?

Answer 15

• Light
• Temperature - each plant has optimum temp
• Water availability
• CO2
• Nitrogen in leaves

Any factor can be limiting and depends on the other factors

Question 16

What is the difference between C3 and C4 plants?

Answer 16

C3 plants: CO2 is fixed into C carbon molecule in Calvin Cycle - catalysed by Rubisco in the Mesophyll cells
- But: C3 plants are prone to photorespiration - where Rubisco fixes O2 instead of CO2 - leading to a reduction in photosynthetic efficiency
- Furthermore, this is exacerbated at high temps - stomata may close to prevent water loss and CO2 will become depleted - can cause photorespiration (bad)
- ~90% of plants - e.g., wheat, rice, soybeans, most temperate plants

C4 plants: CO2 is first fixed into products that contain 4 carbon atoms - Rubisco catalyses RuBP with CO2 to generate 3 carbon molecule
- But then Calvin cycle happens in bundle sheath cells where there is low O2 - so reaction between RuBP and CO2 can occur - and reduces chance of photorespiration
- So in hot climates - C4 plants have little photorespiration and higher photosynthetic rates than C3 plants at high temps - i.e. when they need to close their stomata
- e.g., corn, sugar cane and tropical grasses

Question 17

What is photorespiration?

Answer 17

Rubisco also catalyses RuBP with O2 -> which consumes O2 and releases CO2 - reducing the overall amount of CO2 taken up during photosynthesis
- In C3 plants - when it is very hot - would be something that plants avoid

Question 18

What are the key evolutions of C4 plants?

Answer 18

• Initial fixation of CO2 is separated from rest of Calvin cycle - separates Rubisco from O2 so that it doesnt catalyse a reaction with O2 (photorespiration) rather than CO2
• Allows stomata to shut and prevent water loss without adversely affecting photosynthesis - via photorespiration
• Better for plants experiencing high light levels or temperatures
• Very efficient photosynthesis

Question 19

How will C3 and C4 plants respond differently to future climate changes?

Answer 19

• As CO2 levels rise, gradient between atmospheric conc and leaf conc increases -> more CO2 diffuses into leaf -> higher rates of photosynthesis
• So for C3 plants - higher atmospheric CO2 inhibits photorespiration in C3 plants
• C4 plants - photosynthesis rates saturate at lower atmospheric CO2 conc (than C3) because CO2 levels inside leaves are already higher

Question 20

What are FACE experiments?

Answer 20

FACE - Free-Air Carbon Dioxide Enrichment experiments
- Expose crops in an area to higher levels of CO2 in the air
- Reich et al., 2018 - found strange results - longer term response of C4 plants not expected - accumulated lots more biomass

Question 21

What does the Leaf Area Index (LAI) tell us?

Answer 21

The projected area of leaves per unit ground area - measure of density of canopy
- Can tell us about the radiative transfer in plant canopy - influences how much radition can penetrate the canopy

Question 22

How does culminative LAI change with depth into canopy?

Answer 22

• Culminative LAI increases with depth into canopy
• Therefore, downward irradiance decreases with depth into canopy - the LAI near the ground (high depth into canopy) is what we are interested in

Question 23

What methods can be used to estimate the photosynthesis of a canopy?

Answer 23

1. Production Efficiency Model - represents canopy photosynthesis in proportion to amount of radiation intercepted by canopy - uses FPAR - fraction of photosynthetically active radiation - using satellites
2. Integration on leaf photosynthesis over the light profile - explicitly scales leaf fluxes to canopy scale in proportion to LAI.
   - Can extend this by considering sun and shade areas

Question 24

How can you estimate canopy conductance?

Answer 24

Penman-Monteith extended to plant canopies
- gc governs processes within the canopy and the aerodynamic conductance (gah) governs turbulent processes above the canopy

Question 25

What environmental conditions can control NEE and GPP canopy carbon fluxes?

Answer 25

Net ecosystem exchange of carbon (NEE): clear/cloudy days
- Photosynthesis is enhanced under high light conditions as the fraction of diffuse radiation (increased cloudiness) increases due to more efficient light harvesting
- Under low light conditions - increased cloudiness reduces photosynthesis due to blocking of total downwelling photosynthetically active radiation (PAR)

Monthly gross primary productivity (GPP): evapotranspiration (ET)
- Linear relationship shows tight coupling between carbon and water fluxes - more ET, more flux of carbon into ecosystem
- Slope of this relationship is an indicator of water use efficiency (WUE)