Jan. 25 th, 2011 B 4730/5730 Plant Physiological Ecology Photosynthesis I
Use of Environmental Response Curves in Photosynthesis • Response curves help propose and test physiological mechanisms in ecology • Light response reveals some light dependent mechanisms – Parameters: quantum yield, saturation, compensation, curvature • CO 2 responses reveals some light independent mechanisms – Parameters: supply, demand, compensation, Rubisco, Ru. BP, CO 2 and Phosphate • Multivariate controls on response curves
C. m. shade C 3, H. v. sun C 3, Z. m. sun C 4 BL blue light, RL red light Bukhov et al 1995 J. Photo. Chem. Bio.
Pinus pinaster, Medlyn et al 2002 PCE
States of Calvin Cycle Photosynthesis • State one-Rubsico limited – CO 2 low – No Vcmax limitation • State two-Ru. BP regeneration limited – Light limited – Enzymes of Calvin cycle limited – Increases with increasing CO 2 • State three-Triose Phosphate Use (TPU) – Photosynthesis at higher rate than capacity to use products – Not CO 2 or O 2 limited • Actual A rate is which of three states is limiting – Farqhuar et al (1980) model used to determine which one
Sharkey et al. PCE 2007; incorporates newest material to Farquhar model
Farquhar Model Overview • Five parameters from Farquhar model – – – VCmax - maximum carboxylation by Rubisco J - rate of photosynthetic electron transport TPU - triose phosphate use Rd - day respiration gm-mesophyll conductance • Parameters adjusted to 25 °C for comparison – c is scaling constant = 1 at 25 C), ΔHa enthalpy of activation, ΔHd enthalpy of deactivation, ΔS entropy, R is universal gas constant, T is temperature in K
Sharkey et al. 2007
Rubisco limited Farquhar Model • • • Vcmax maximum Rubsico carboxylation Cc CO 2 partial pressure at Rubisco site Kc Michaelis constant for CO 2 O partial pressure O 2 Ko Michaelis constant for oxygenation Γ* [CO 2] at which photosynthetic CO 2 uptake compensated by photorespiration • Rd mitochondrial respiration
Ru. BP Regen and TPU Farquhar Model • J rate of electron transport • 4 electrons per oxygenation and carboxylation A=3 TPU-Rd • TPU rate of use of triose phosphates for any export of C from Calvin cycle, includes photorespiratory use of glycine and serine
Mimimimum of three limitation gives actual A and J rates. Farquhar et al. Planta 1980; Long and Bernacchi J Ex Bot. 2003
Ethier et al PCE 2006, Pseudotsuga menziesii needle ages
Filled crop, open broadleaf, crosses conifers Medlyn et al. PCE 2002
Light Dependent Reactions • Three fates of photon striking reaction center – Heat, fluorescence, photochemistry • Linear electron flux (LEF) goes from PSII to PSI to drive H+ pmf, – Cyclic flux around PSI • Low light, efficiency maximized • High light, energy dissipation maximized – redox, xanthophyll cycle, photorespiration
Baker et al. PCE 2007
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