C3, C4, and CAM plants all have the same goal, to make carbohydrates. What happens to the triose-phosphates made in the Calvin cycle? Used to synthesize starch for storage in chloroplast. Exported from chloroplast for sucrose synthesis in the cytosol. How is starch vs. sucrose synthesis regulated? Why is it regulated?

cytosol Triose phosphates produced in the Calvin cycle can be used for starch or sucrose synthesis. chloroplast Calvin cycle Starch Triose-P Sucrose cytosol

Starch is synthesized in the chloroplast. Fig. 8.15

Starch vs. sucrose synthesis is regulated by level of cytosolic Pi as it affects triose-P export from chloroplast. When [Pi] is high, triose-P is exported in exchange for Pi & used to synthesize sucrose. If [Pi] is low, then triose-P is retained in chloroplast and used to synthesize starch. Fig. 8.14

More on the ecological aspects of photosynthesis (Ch. 9) Stomatal conductance light, temperature, relative humidity, Y, [CO2] carbon isotope discrimination Light Leaf movements Sun and shade leaves - anatomical and photosynthetic properties. Temperature Leaf energy balance C3 vs. C4 quantum yield differences Atmospheric CO2 History of atmospheric CO2 Current trend of rising CO2 Implications for C3 & C4 photosynthesis

Review: Stomatal aperture regulates the conductance of the diffusion pathway for CO2 entering the leaf and H2O leaving the leaf. PP04100.jpg Fig. 4.10

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 cues 1. leaf water potential decreases as Y 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.

light Fig. 9.6 Light and leaf movements Light affects photosynthesis and leaf temperature Solar tracking allows leaves to increase light absorption compared to a fixed orientation. Diaheliotropic leaves Fig. 9.6

Some plants change leaf angle to reduce light absorption. Paraheliotropic leaves Why?

What happens to the light that strikes a leaf? Absorbed 85 to 90% of the PAR, 60% of total energy 2. Reflected 0 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 PP09030.jpg

Light level attenuates (decreases) with depth in a plant canopy because each layer of leaves absorbs light. Fig. 9.7 PP09070.jpg

Leaf anatomy responds to light level. Which is the “sun” Leaf and which is the “shade” leaf? Sun leaf Shade leaf

Sun leaves Shade leaves Thicker, Thinner, fewer more cell layers cell layers More Rubisco Less Rubisco per unit chlorophyll per chlorophyll Less chlorophyll More chl per per reaction center reaction center Light acclimation (phenotypic plasticity) vs. light adaptation.

Physiological differences of sun and shade leaves Anatomical and biochemical differences between sun and shade leaves determine photosynthetic properties. Physiological differences of sun and shade leaves Fig. 9.9 Sun leaf vs. shade leaf Sun leaf has: Higher max. photo. rate Higher light sat’n level Higher light compensation point PP09090.jpg

Fig. 9.10 Acclimation to growth light level - same pattern as sun vs. shade species differences. PP09100.jpg

Light and leaf temperature. Heat loads on leaves in the sun are large. How do leaves prevent overheating? PP09140.jpg

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 as part of an “energy balance” equation. PP09140.jpg

Leaf energy balance At 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 C4 plants insensitive to temperature? Fig 9.23 PP09230.jpg

Photosynthetic responses to CO2 History of atmospheric CO2 Fig. 9.16 PP09161.jpg

Current trend of rising CO2 Fig. 9.16 The Mauna Loa CO2 record PP09162.jpg

Photosynthetic response to CO2 of C3 & C4 plants PP09201.jpg

PP09202.jpg

Photosynthetic response to temperature Fig. 9.22 CO2-temperature interaction in a C3 plant. Why does the temp. for maximum phot. increase at elevated CO2? PP09220.jpg

2. Second approach Henry’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. Examples If 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.