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 Calvin cycle cytosol chloroplast

Fig Starch is synthesized in the chloroplast.

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

More on the ecological aspects of photosynthesis (Ch. 9) Stomatal conductance light, temperature, relative humidity,  CO 2 ] 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 CO 2 History of atmospheric CO 2 Current trend of rising CO 2 Implications for C3 & C4 photosynthesis

Fig Review: Stomatal aperture regulates the conductance of the diffusion pathway for CO 2 entering the leaf and H 2 O leaving the leaf.

What factors influence stomatal conductance? Environmental cuesEffect 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  decreases 2. internal [CO 2 ]decreases as [CO 2 ] 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 compared to a fixed orientation. Diaheliotropic leaves Fig. 9.6 light Light and leaf movements Light affects photosynthesis and leaf temperature

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

What happens to the light that strikes a leaf? 1.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

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

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

Anatomical and biochemical differences between sun and shade leaves determine photosynthetic properties. Sun leaf vs. shade leaf Sun 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 Acclimation to growth light level - same pattern as sun vs. shade species differences.

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 as part of an “energy balance” equation.

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

Photosynthetic responses to CO 2 History of atmospheric CO 2 Fig. 9.16

Current trend of rising CO 2 The Mauna Loa CO 2 record Fig. 9.16

Photosynthetic response to CO 2 of C3 & C4 plants

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

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 CO 2, then the dissolved concentration is 2 x if gas concentration is 2 X 345ppm. At 250ppm CO 2, dissolved CO 2 is x 250/345 = 8.46 µM.