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 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 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 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