Photon Systems Instruments FluorPen FP 110 - 6 Principle of Measurement

© PSI (Photon Systems Instruments), spol. s r. o.
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6 PRINCIPLE OF MEASUREMENT
FluorPen is a chlorophyll fluorometer and is used to measure different photosynthetic parameters in 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 Fluor Pen FP 110/S.
When studying photosynthesis using chlorophyll fluorescence, 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
obtained 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 result in
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). When measuring Fm it is important to dark
adapt the samples. This can be achieved by placing the sample in the dark for few minutes (the time varies with conditions)
or by using the FP110/D version of the FluorPen that has been adapted for detachable leaf clips. The leaf clips may be
placed on the leaf ahead of the measurements and once dark adaptation has been achieved the FP-110/D may be attached
to the leaf clip without exposing the leaf to light.