C3, C4, and CAM plants all have the same goal,

to make carbohydrates.

What happens to the triose-phosphates made in the Calvin cycle?

1. Used to synthesize starch for storage in chloroplast.

2. Exported from chloroplast for sucrose synthesis in the cytosol.

How is starch vs. sucrose synthesis regulated?

Why is it regulated?

Triose phosphates produced in the Calvin cycle can be used for starch or sucrose synthesis.

Triose-P

Starch

Sucrose

Calvincycle

cytosol

chloroplast

Fig. 8.15Starch is synthesized in the chloroplast.

Fig. 8.14

When [Pi] is high, triose-P is exportedin exchange for Pi & used to synthesizesucrose.

If [Pi] is low, thentriose-P is retainedin chloroplast andused to synthesizestarch.

Starch vs. sucrose synthesis is regulated by level of cytosolic Pi as it affects triose-P export from chloroplast.

More on the ecological aspects of photosynthesis

(Ch. 9)

Stomatal conductance light, temperature, relative humidity, CO2]

carbon isotope discrimination

Light Leaf movements

Sun and shade leaves - anatomical and photosynthetic properties.

TemperatureLeaf energy balance

C3 vs. C4quantum yield differences

Atmospheric CO2

History of atmospheric CO2

Current trend of rising CO2

Implications for C3 & C4 photosynthesis

Fig. 4.10

Review: Stomatal aperture regulates the conductance of the diffusion pathway for CO2 entering the leaf and H2O leaving the leaf.

What factors influence stomatal conductance?

Environmental cues Effect on stomatal cond.

1. light increases as light increases

2. relative humidity increases as r.h. increases

3. temperature increases as temp. increases

Internal cues1. leaf water potential decreases as

decreases

2. internal [CO2] decreases as [CO2] increases

3. hormonal control decreases as [ABA] increases (abscisic acid, ABA)

All these cues ultimately influence the turgor pressure of the guard cell, which in turn causes the opening or closing of the stomatal pore.

Solar tracking allows leaves to increase light absorption comparedto a fixed orientation.

Diaheliotropic leaves

Fig. 9.6

lightLight and leaf movements

Light affects photosynthesis and leaf temperature

Some plants changeleaf angle to reducelight absorption.Paraheliotropic leaves

Why?

What happens to the light that strikes a leaf?

1.Absorbed85 to 90% of the PAR, 60% of total energy

2. Reflected0 to 8% of the PAR

3. Transmitted (passes through leaf)0 to 8% of PAR

Fig. 9.2

Absorption is high for PAR and decreases greatly at longer wavelengths.

Fig. 9.3

Light level attenuates (decreases) with depth in a plant canopy because each layer of leaves absorbs light.

Fig. 9.7

Sun leaf

Shade leaf

Leaf anatomy respondsto light level.

Which is the “sun”Leaf and which is the“shade” leaf?

Sun leaves Shade leaves

Thicker, Thinner, fewer more cell layers cell layers

More Rubisco Less Rubiscoper unit chlorophyll per chlorophyll

Less chlorophyll More chl perper reaction center reaction center

Light acclimation (phenotypic plasticity) vs. light adaptation.

Anatomical and biochemical differences between sun and shade leaves determine photosynthetic

properties.Sun leaf vs. shade leafSun leaf has: Higher max. photo. rate

Higher light sat’n level

Higher light compensation

point

Fig. 9.9

Physiological differences of sun and shade leaves

Fig. 9.10

Acclimation to growth lightlevel - same pattern as sun vs.shade speciesdifferences.

Light and leaf temperature.

Heat loads on leaves in the sun are large.How do leaves prevent overheating?

Mechanisms of heat dissipation by leaves

Leaves can lose heat in three main ways:

1. Emission of radiation

2. Conduction/convection

3. Evaporation

Each term can be included aspart of an “energy balance” equation.

Leaf energy balanceAt constant temperature:Energy In = Energy Out

(Radiation absorbed + Conduction/Convection + Condensation)

= (Radiation emitted + Conduction/Convection loss + Evaporation)

Leaf temperature and photosynthesis

Which is C3 and C4?

C4

C3

Why does the quantum yield of C3 plants decrease with increasing temperature? Why is the quantum yield of C4plants insensitive to temperature?

Fig 9.23

Photosynthetic responses to CO2 History of atmospheric CO2

Fig. 9.16

Current trend of rising CO2

The Mauna LoaCO2 record

Fig. 9.16

Photosynthetic response to CO2 of C3 & C4 plants

Photosynthetic response to temperature

Fig. 9.22CO2-temperatureinteraction in a C3plant.

Why does the temp.for maximum phot.increase at elevatedCO2?

2. Second approachHenry’s Law: concentration of a gas dissolved in water is proportional to the gas partial pressure (or [gas] at same total pressure) above the water.

Changing the gas partial pressure produces a proportional change in dissolved concentration.

ExamplesIf dissolved concentration is 11.68µM at 345ppm CO2, then the dissolved concentration is 2 x 11.68 if gas concentration is 2 X 345ppm.

At 250ppm CO2, dissolved CO2 is 11.68 x 250/345 = 8.46 µM.