Feb. 1st, 2011B4730/5730

Plant Physiological Ecology

Photosynthesis I

Filled crop, open broadleaf, crosses conifersMedlyn 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

Baker et al. PCE 2007

Chlorophyll Fluorescence• Absorption (photochemistry), heat, and fluorescence all

occur in competition with incoming light– Light absorbed different wavelength than light fluorescing

• Light always lost, so all measurements relative – Add to 1

• Measurements made with modulated light– Allows measurement in the field with background light levels

• Kautsky effect is rise in fluorescence when leaves go from dark to light conditions– Electrons from PSII to QA, – if QA reduced; reaction center closed– fluorescence when reaction center closed

• Quenching causes fluorescence to fall after initial burst– Photochemical quenching-light induction– Nonphotochemical quenching (NPQ)-heat and zeaxanthin cycle

Measuring Fluorescence

• F rate of fluoresce– ’ indicates light acclimated (“adapted”)

measurements

• Fo background fluorescence in dark, Fo’ in light

• Fm maximal rate in dark or Fm’ in light

– Occurs when QA is maximally reduced

• Fv variable fluorescence from dark, Fv’ light– Demonstrates primary photochemistry

Interpreting Fluorescence• Fv/Fm intrinsic quantum yield of PSII• Fv’ = F’-Fm’• Quantum yield of PSII

– ΦPSII = Fv’/Fm’

• Photochemical (qP) and nonphotochemical quenching (NPQ)– qP = (F’m – F’o)/(Fm – Fo)– NPQ = 1 – (F’m – F’o)/(Fm – Fo)

• J = ΦPSII · Iabs · β · PPFD– β = 0.5 equal distribution PSII & PSI– Relative without Iabs

Nicotiana tabacum; Kramer et al Photo. Res. 2004qp puddle model PSII openqL lake model PSII open

Wang & Kellomaki New Phyt. 1997, Pinus sylvestrisa) Control N & amb CO2, b) Control N & elev CO2, c) High N & amb CO2, d) High N & elev CO2ΔF=Fq’

Fs is steady state Fluorescence (F’)Closed circle F’m amb CO2Closed triangle Fs amb CO2Open circle F’m elev CO2Open triangle Fs elev CO2

Leipner et al Env. Exp. Bot. 1999 Maize

Mesophyll conductance• Flux of CO2 to stroma (Cc) limited by gs, gm

and gb

• Recent work has shown gm to be variable and important– Low gm reduces CC underestimate VCmax

• Cc=Ci-A/gm

– Estimate gm with curve fitting

• Estime gm with fluorescence and A

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RAJ

RAJC

Ag

Hedera helix; Pons et al J Exp Bot 2009

Many species; Sc= surface area of chloroplasts exposed to intercellular airspace; Evans et al J Exp Bot 2009

Limitations to the Farquhar/Fluorescence model of

Photosynthesis• No gene expression controls

– Connection to hormones, whole plant controls– Longer time scales

• Stomatal physiology almost a black box

• Scale of leaf patchiness violates homogeneity

• Light dynamics problematic

• Respiration models are crude