Carbon Cycle, part 2 Ecophysiology of Leaves Dennis Baldocchi ESPM UC Berkeley Courtesy of Rob Jackson, Duke 3/13/2013 Outline Photosynthetic Pathways and Cycles Environmental Physiology of Photosynthesis Light Temperature CO 2 Nutrients Soil Moisture and Water Deficits Respiration 1
Basic Photosynthesis Stoichiometry 6CO 2 + 6H 2 O + (light) (C 6 H 12 O 6 ) + 6O 2 Physiological Attributes of Plants photosynthetic pathway C 3 C 4 CAM 2
Photosynthesis: A balance between Supply and Demand Biochemical limitation: carboxylation rate Light limitation Enzyme limitation Physical limitation: delivery of CO 2 to leaf Diffusion through Leaf Boundary Layer Diffusion through Stomatal Pores Potential Gradient between free Atmosphere and substomatal Cavity Critical Steps in C 3 Photosynthesis Calvin-Benson Cycle Light Reactions Chlorophyll, in chloroplasts, captures photons Light energy is used to produce chemical energy in the forms of NADPH and ATP Oxygen is produced Dark Reactions A 3-C compound, PGA, is formed at the first Carboxylation step via the reaction between CO 2 and RUBP, a C 5 compound, and its subsequent cleaving (C 6 => 2 C 3 ) The enzyme RUBISCO catalyzes the reaction between CO 2 and RUBP RUBISCO has an affinity for both CO 2 and O 2, with the later leading to photorespiration, a loss of CO 2 RUBISCO accounts for about 9 to 25% of the N in a leaf Chemical Energy (NADPH & ATP) is used to regenerate RUBP 3
C 3 Photosynthesis, The Calvin-Benson Cycle CO 2 O 2 Photorespiration, PCO CO 2 RUBP Carboxylase/ oxygenase PGA ATP ADP (CH 2 O)n NADP+ + H+ NADPH ADP ATP RUBP RUBP Regeneration PCR Cycle Triose-P ESPM 111, Ecosystem Ecology Using Solar Energy for Life Light ( Hill ) Z Reactions: PS II and Ps I 700 nm Visible solar energy (400 to 700 nm) is absorbed by pigments This energy is converted into high energy compounds, ATP and NADPH, by photosystems II and I (PS II and I) Photosystem II uses 680 nm energy to generate ATP (non-cyclic electron transport) PS I uses 700 nm solar energy to generate NADPH (cyclic electron transport). Excess energy is lost as heat or fluorescence. 8 Photons per CO 2 molecule fixed 680 nm Allen and Martin, 2007 Nature 4
C 4 leaf http://www.emc.maricopa.edu/faculty/farabee/biobk/c4leaf.gif Critical Steps in C 4 Photosynthesis The enzyme PEP Carboxylase catalyzes a reaction between CO 2 and phosophenolpyruvate (PEP) to form a C 4 compound The C 4 compound is transported into the specializes cells, the bundle sheaths, and is decarboxylated CO 2 is released into an environment and photosynthesis is completed via the C 3 cycle Photorespiration is low; RUBISCO favors CO 2 in this environment because the ratio between CO 2 :O 2 is high 5
C 4 Photosynthesis Mesophyll Bundle Sheath Mesophyll PEP Carboxylase CO 2 C 4 acids OAA CO 2 RUBP Carboxy lase CO 2 PGA (CH 2 O)n PEP C 3 acids PCR Cycle RUBP C3 vs C4 Grass Distribution Edwards et al 2010 Science 6
C3 vs C4: Isotopes and Ci/Ca C4 C3 O Leary, 1988 Bioscience Tool to Study Advance and Retreat of C3 Grasslands During Ice Age Why are CA Hot Grasslands Compose of C3 grasses? CA annual Grasslands experience cool wet winter growing season Adapted from Ehleringer et al. 1997 7
C 3 vs C 4 Photosynthesis: Light Use Efficiency, CO 2 and Temperature Ehleringer et al, Oecologia Comparative EcoPhysiology C 3 C 4 CAM: day internal [CO2] 0.7 Ca 0.4 Ca 0.5 Ca? Krantz ananatomy NO YES Succulent CAM: night carboxylase RuBP PEP RuBp PEP photorespiration YES NO Fixation product PGA C 4 acids PGA C4 acids CO2 compensation pt 30-80 ppm < 10 ppm 50 ppm < 5 ppm optimum temperature 15-30 C 25-40C 35 C Max Ps (mmol m -2 s -1 ) 14-40 18-55 6 8 Response at full sunlight saturated Linear Saturated Enhancement in low O 2 yes no yes No Quantum req. 15-22 19 Quantum yield 0.052 0.064 Carbon isotope fractionation, per mill Adapted from Jones, 1992-26 (-23 to -33) -14-30 to -10-30 to -10 8
A Resistance -Analog ( C a C c) r r r leaf mesophyll chloroplast g ( C C ) g ( C C ) g ( C C ) b a s s s i m i c Flexas et al 2008, Plant, Cell, Environ Leaf Photosynthesis A=V c-0.5v o- Rd Leaf photosynthesis (A) is a function of: Rate of carboxylation (Vc), which is the minimum of the rates associate with regeneration of RuBP by electron transport and the saturation of RUBISCO by CO 2, Rate of oxygenation (Vo, photorespiration) which is governed by the competition between CO 2 and O 2 for RUBISCO Rate of dark respiration (Rd) (all have units of mol m -2 s -1 ). 9
A-Ci Curve: C3 Leaf From Chapin et al Light Response Curve, C3 Leaf From Chapin et al 10
W c : demand limited by RUBISCO saturation 60 W j : demand limited by RuBP regeneration by electron transport Carboxylation rate ( mol m -2 s -1 ) 50 40 30 20 10 supply ~stomatal conductance (g s ) 0 0 200 400 600 800 1000 [CO 2 ] (ppm) C i Maximum rate of Carboxylation, V cmax, scales with maximum rate of electron transport, J max 350 After Wullschleger. 1993 300 250 200 Jmax 150 100 50 0 0 20 40 60 80 100 120 140 160 Vcmax 11
V cmax scales with maximum photosynthesis at saturating light and ambient CO2, P ml 100 80 Wohlfarht 1999 mountain grassland species Vcmax=2.36+1.66 Pmx r 2 =0.87 Vcmax ( mol m -2 s -1 ) 60 40 20 0 0 10 20 30 40 50 60 Pml ( mol m -2 s -1 ) Seasonality of Model Parameters: e.g. Photosynthetic Capacity Quercus alba (Wilson et al) Quercus douglasii (Xu and Baldocchi) 140 120 Live Fast, Die Young In Stressed Environments 100 V cmax 80 60 40 20 0 100 150 200 250 300 350 DOY 12
Photosynthesis and Temperature 4 Jack pine (NSA) Data of Hank Margolis Qp = >1000 mol m -2 s -1 3 A ( mol m -2 s -1 ) 2 1 0-10 0 10 20 30 40 Leaf Temperature (C) Berry and Bjorkman 1980, Ann Rev Plant Physiol Stress Physiology Adaptation is a response to long-term changes which result in inheritable genetic alterations. These alterations are stable and will remain in the population over generations. Acclimation is a response induced by an environmental change which causes a phenotypic alteration over a single generation time without any compositional change in the genetic complement Stomata close to limit water loss Temperature optimum shifts with warmer temperatures Tolerance Blue oaks are able to photosynthesize despite super severe water deficits (below -5.0 Mpa) Avoidance CA grasses adopt annual life strategy. Survive dry summer in the form of dessicated seeds Blue oaks develop deep roots that tap into the ground water Huner et al 1993 Photosyn Res 13
P s and Temperature Acclimation Berry and Bjorkman 1980, Ann Rev Plant Physiol GPP: Acclimation with Temperature 35 Temperature Optimum for Canopy CO 2 uptake (C) 30 25 20 15 10 b[0] 3.192 b[1] 0.923 r ² 0.830 5 5 10 15 20 25 30 Mean Summer Temperature (C) Analysis of E. Falge 14
Photosynthesis and Leaf Nitrogen Field and Mooney, 1986 Reich et al 1997 PNAS 15
Stomatal conductance adjusts to match photosynthetic capacity (or vice versa) Schulze et al 1994 Ann Rev Ecol Stomatal conductance and water deficits Schulze 1986 Ann Rev Plant Physiol 16
V cmax scales with water deficits Quercus douglasii 140 120 100 b[0]: 111.3 b[1]: 22.69 b[2]: 1.734 r ² 0.7532 Vcmax 80 60 40 20 0-7 -6-5 -4-3 -2-1 0 Pre-dawn Water Potential (MPa) Baldocchi and Xu, 2007 Adv Water Res Canopy Photosynthesis Sunlight, and Environmental Conditions Leaf Area Index Leaf Clumping Leaf Angle Distribution Diffuse vs Direct Light Leaf Thickness and Photosynthetic Capacity 17
WHEAT Light and Photosynthesis: Leaves, Canopies and Clumping F c (mg m -2 s -1 ) 0-5 -10-15 -20-25 -30-35 -40-45 (a) D208 Oak leaf, forest floor T leaf: 25 o C CO 2 : 360 ppm -50 0 500 1000 1500 2000 absorbed PAR (µm ol m -2 s -1 ) A ( mol m -2 s -1 ) 12 data model 10 8 6 4 2 0 0 200 400 600 800 1000 1200 1400 1600 1800 Q par ( mol m -2 s -1 ) F c ( mol m -2 s -1 ) -40-30 -20-10 0 measured 10 model: clumped leaves 0 500 1000 1500 2000 PPFD ( mol m -2 s -1 ) Dry Matter Production scales with Sunlight Monteith 1981 Phil Trans Roy Soc 18
CO 2 uptake-light Response Curve: Role of C 3 and C 4 Crops 0.25 WHEAT CORN F c /LAI (mg m -2 leaf area s -1 ) 0.00-0.25-0.50-0.75 0 250 500 750 1000 1250 1500 1750 2000 Q a (µmol m -2 s -1 ) Linear Function and High r 2 (~0.90) Baldocchi, 1994, AgForMet Canopy Light Response Curves: Effect of Diffuse Light 10 5 0-5 Temperate Broad-leaved Forest Spring 1995 (days 130 to 170) Sunny days diffuse/total <= 0.3 Cloudy days diffuse/total >= 0.7 NEE ( mol m -2 s -1 ) -10-15 -20-25 -30-35 -40 0 500 1000 1500 2000 PPFD ( mol m -2 s -1 ) Baldocchi, 1997, PCE 19
Summary Points Leaf photosynthesis involves a set of light and dark reactions. The light reactions involve the harvesting of sunlight by chlorophyll and the transfer of electrons to the produce of biochemical energy compounds adenosine triphosphate (ATP) and nicoadenomine di-phosate (NADPH) via the process called photophosphorylation. The energy contained in molecules ATP and NADPH is subsequently used in the dark reactions to drive the photosynthetic carbon reduction cycle, or Calvin Cycle. There are three photosynthetic pathways, C3 (the most common), C4 and CAM. Photorespiration, due to the competitive affinity for CO2 and O2 by the enzyme RUBISCO, is an attribute of C3 photosynthesis. This leads to its lower efficiency than C4 photosynthesis C4 plants circumvents this inefficiency by using a different enzyme to carboxylze CO2 (PEP carboxylase) and by minimizing the diffusion of O2 deep in the leaf with its unique leaf anatomy (bundle sheaths). Rates of photosynthesis achieved by a leaf is a balance between the supply and the demand for carbon dioxide. The supply is related to the CO2 concentration in the air and its diffusion through the leaf boundary layer and stomata. The demand is controlled by the Michaelis-Menton kinetic reactions that are a function of CO2 at the chloroplast. Rates of photosynthesis are a function of light, temperature, CO2, moisture, leaf nutrition, and phytotoxics. 20