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Jan. 25th, 2011
B4730/5730
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
• CO2 responses reveals some light independent
mechanisms
– Parameters: supply, demand, compensation,
Rubisco, RuBP, CO2 and Phosphate
• Multivariate controls on response curves
C.m. shade C3, H.v. sun C3, Z.m. sun C4
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
– CO2 low
– No Vcmax limitation
• State two-RuBP regeneration limited
– Light limited
– Enzymes of Calvin cycle limited
– Increases with increasing CO2
• State three-Triose Phosphate Use (TPU)
– Photosynthesis at higher rate than capacity to use
products
– Not CO2 or O2 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
Parameter 
e
 H a
 c 
R T

1 e
S T 



H d
R T
– 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


Cc  
A  VC max 
  Rd
 Cc  KC 1  O / KO 
*
•
•
•
•
•
•
Vcmax maximum Rubsico carboxylation
Cc CO2 partial pressure at Rubisco site
Kc Michaelis constant for CO2
O partial pressure O2
Ko Michaelis constant for oxygenation
Γ* [CO2] at which photosynthetic CO2 uptake
compensated by photorespiration
• Rd mitochondrial respiration
RuBP Regen and TPU Farquhar
Model
Cc  
A J
 Rd
*
4CC  8
*
• J rate of electron transport
• 4 electrons per oxygenation and carboxylation
A=3TPU-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