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4 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 (470 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).
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.