8
Chlorophyll Fluorescence
A generalized description as it relates to chlorophyll fluorometry.
The following is a generalized description of the photosynthesis light reaction and the value
of chlorophyll fluorescence for investigation of plant health, plant function and plant stress.
Changes in photosystem II (PSII) fluorescence have been shown to be a sensitive test for most
types of plant stress, and reflect measurable changes of many plant functions including
photochemistry, photoprotective mechanisms, low light survival state transition mechanisms,
and heat dissipation photoinhibition mechanisms. PSII fluorescence measurements have been
found to correlate well with changes in CO
2
fixation under most conditions (for more detailed
correlation information refer to the section on “quantum photosynthetic yield of PSII”).
Reaction centers are of two types, Photosystem II (PSII), and Photosystem I (PSI). Both are
located in the thylakoid membrane of chloroplasts in higher plants. In bacteria, they are in a
membrane surrounding the cytoplasm or in more intricate constructs. All plants that produce
oxygen have both types of reaction centers.
While Photosystem I goes through a somewhat similar process to PSII, PSI fluorescence does
not vary with plant stress nor does it change as changes occur with various photosynthetic
mechanisms. Therefore PSII fluorescence is used for investigation into these areas.
Light energy utilized in photosynthesis by higher plants and algae cells is collected first by an
antenna pigment system and transferred to reaction centers where light quanta are converted
to chemical energy by chlorophylls in a protein environment. Electron transfer starts in the
reaction center when a chlorophyll molecule transfers an electron to a neighboring pigment
molecule. Pigments and protein involved in this primary electron transfer define the reaction
center. This initial electron transfer is also called charge separation.
Competing models of energy capture and transfer have existed. In the puddle model, each
reaction center possesses its own independent antenna system. In the lake model, reaction
centers share antenna. The “lake model” is considered more realistic for terrestrial plants.
PSII and PSI reaction centers also share antenna during a process called state transitions. This
process takes between ten and twenty minutes as a subset of PSII antenna detach from PSII
reaction centers and migrate to PSI reaction centers (Ruban, Johnson 2009). They can also
move back to PSII reaction centers. The process is governed by the oxidation-reduction state
of the plastoquinine pool, and it is thought to be a survival mechanism for plants to subsist in
low light conditions by balancing light levels between the two types of reaction centers
(Allen, Mullineau 2004). Light level changes and changing light quality can trigger
transitions.
During dark adaptation, higher plants and algae shift toward state 1 conditions and
cyanobacteria to state 2 conditions. (Papageorgiou G.C. Tismmilli-Michael M. Stamatakis K.
2007). State transitions should be considered when deciding on dark adaptation times and for
determination of steady state photosynthesis for quantum photosynthetic yield measurements,
and quenching measurements. State transitions affect measurements more at low light levels
than at high light levels (Lichtenthaler 1999).
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