Topic 6- Light energy

Photosynthesis Equation

6CO2 + 6H2O (sunlight is added)→ C6H12O6 + 6O2

(Light energy is converted to and stored as chemical energy in the products of photosynthesis)

Respiration and its Equation

Reverse reaction of photosynthesis (The release of energy during cellular respiration)

ATP is generated. This is the energy that fuels metabolism of the cells processes

C6H12O6 + 6O2 (energy released)→ 6CO2 + 6H2O

Productivity of leaf machinery in processing light

• how much light goes to photosynthesis

• Leaf traits (angle, thickness, stoma density, chlorophyll)

• Canopy / structure (shade tolerant vs shade intolerant)

Three other ways plants use light besides photosynthesis

1.) direction of light = direct growth and trigger changes in leaf orientation

2.) duration of light = trigger initiation of dormancy

3.) spectral quality of light = leaf shedding and seed germination

The ins and outs of light reaching the land surface

Ins- lat, time of year, time of day, angle of sun

Out- Reflected light (snow, clouds, etc) Land feature (Slope)

Spectra that drives photosynthesis of light reaching the land surface

Red (about 450 nm ) and Blue (about 680 nm) because these wavelengths are the most eddiciently captured by chlorophyll to produce sugar productions

Effect of angle on light reaching the land surface

• If the sun is high over head, light hits more directly and is more concentrated- higher energy per area

• If its low angle like sunrise, the same energy is spread over a lager surface area - less intense light

• South facing slopes get more light and are warmer

• Main picture = the angle affects how light filters through the canopy- leaves at different heights and orientations intercept different amounts of light

the angle of incoming sunlight influence light intensity- direct light produces higher productivity where was lower angles reduce energy input.

Leaf Pigments that utilize light that reach the land surface

• Chlorophyll a & f = Main photosynthetic pigments (Absorb blue and red light)

• Carotenoids = (orange and yellow pigments) absorb blue-green light and helps protect the leaf from excess sunlight by dissipating energy safely

• These pigments extend the range of light wavelengths that a plant can use for photosynthesis

All of these pigments make the “leaf machinery” more efficient at capturing light energy and converting it into chemical energy.

Small amount of absorbed light is used in photosynthesis (3%) which drives all plant biomass on Earth

Major factors of partitoning light within the canopy

• Species composition

• Canopy height

• Canopy structure

Think about that one diagram she had about tropical, conifer, and trembling aspen ecosystems

Leaf Area Index (what is it and how does it affect partitoning of light)

The one sided green area of leave per unit ground surface area (I.E. projected leaf area)

-varies greatly over time and across ecosystems

• Correlates well with productivity since leaf area determines the potential for the forest to convert light to chemical energy (seasonality is important)

-Higher LAI means more leaves, more potential for photosynthesis and productivity, but more shading below (Don’t want to much of this can reduce efficiency)

Canopy Structural Complexity (CSC)

• Newer way to evaluate forest structure and productivity

• Relates to LAI but includes CANOPY HEIGHT, OPENESS, VARIABILITY

High CSC crates diverse light environments that supports more species and higher productivity

Canopy packing and light interceptions for partitioning of light

• single species stands tend to let light onto forest floor

• Mixed species stands produce more heterogeneity in light environments and create more niches

• If crown shapes are complementary, spaces needs to be given for them to grow into one another

• Light interception predicts how much of solar radiation is reaching forest floor

• Huge range across ecosystems and seasonal ecosystems

Pine and Dogwood example for partitoning

Pine:

• chlorophyll in a pine needle receives a much smaller fraction of light

• Can maintain high net photosynthetic rate in high lights conditions of the canopy

Dogwood:

• Broad thin leaves minimize leaf shading, oriented perpendicular to direction of incident radiation

• Can efficiently in low light condition understory

How can plants manage seasonal variation in light interception

• Complete life cycle when light conditions are adequate

• Grow quickly to avoid low light condition

• Capitalize on Sunflecks

• Tolerate low light conditions

• Have leaf traits that min or max light interception

What does “tolerate low light conditions” mean

The plant has:

• Lower light compensation points

• Lower respiration rates because they invest less in pigments in the leaf

• Energy requirement lower

• “Break Even” from a carbon perspective with lower rate of photosynthesis

What does “Leaf traits that minimize or maximize light interception” mean

• Leaf angles

• Leaf anatomy

• Leaf surface props

Max:

High stomatal density

Many layers of cells that contain chlorophyll with high allocation

Solar tracking

Min:

Light Requirements for Seedlings

1.) Partial Sunlight is best (seedlings are vulnerable to high light intensity and photosynthetic machinery is not able to process high light intensity

2.) Seedlings are more vulnerable to water loss (due to greater allocation to leaves than roots at low light)

3.) Trade offs in seedlings root:shoot ratio

What are the Trade offs in root:shoot Ratio

Lowlight: Leaves > Roots. Roots are not great at taking up water or nutrients

Highlight: Roots > Leaves. Less focused on photosynthesis

High light and Water stress results look similar (long ass roots)

Low light and Water Availabilty look similar (big ass leaves)

Photoperiodism

The physiological reaction of organisms to the length of night or dark period

Photocontrol of plant growth responses

-Photoperiodism which is driven by a reversible pigment (active phytochrome cause flowering) that moves into nucleus and altars gene expression

-The length of the night period is important

-Growth Cessastion Driven

-Neightbor detection

What is the length of the night period in photocontrol important

Short day plants, the night length must exceed the critical threshold to induce flowering

Long day plants, the night length must be shorter than the critical length for flowering to occur

What does Growth Cessation Driven and Neighbor detection mean in photocontrol

Both are by red to far red detection (R/FR)

Growth cessation:

High R/FR means its in open bright unshaded conditions which indicates long days or high light

Low R/FR means its shaded or short day low light.

Neighbor Detection:

Competitive Response- rapid elongation of stems and petioles with reduced branching (herbaceous plants) because of another tree shading it.

Crown Shyness- Tops of trees avoid touching each other which indicates that phytochromes help detect nearby trees