© PSI (Photon Systems Instruments), spol. s r. o.
15
6 PRINCIPLE OF MEASUREMENT
FluorPen is a fluorometer which can measure different fluorescence parameters of plants. It is equipped with a blue LED
emitter (455 nm), optically filtered and precisely focused to deliver light intensities of up to 3,000 µmol.m
-2
.s
-1
to
measured plant tissue (Fig. 4).
Fig. 4 Schema of FluorPen FP 110/S.
To use measurements of chlorophyll fluorescence to analyse photosynthesis, researchers must distinguish between
photochemical quenching and non-photochemical quenching (heat dissipation). This is achieved by stopping
photochemistry, which allows researchers to measure fluorescence in the presence of non-photochemical quenching
alone. To reduce photochemical quenching to negligible levels, a high intensity, short flash of light is applied to the leaf.
This transiently closes all PSII reaction centers, which prevents energy of PSII being passed to downstream electron
carriers. Non-photochemical quenching will not be affected if the flash is short. During the flash, the fluorescence
reaches the level reached in the absence of any photochemical quenching, known as maximum fluorescence F
m
. The
efficiency of photochemical quenching (which is a proxy of the efficiency of PSII) can be estimated by comparing F
m
to
the steady yield of fluorescence in the light F
t
and the yield of fluorescence in the absence of photosynthetic light F
0
.
The efficiency of non-photochemical quenching is altered by various internal and external factors. Alterations in heat
dissipation mean changes in F
m
. Heat dissipation cannot be totally stopped, so the yield of chlorophyll fluorescence in the
absence of non-photochemical quenching cannot be measured. See picture below (Fig. 5).
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