Walz MINI-PAM - ML-BURST (Menu Point 5)

CHAPTER 12 MEASUREMENTS WITH THE MINI-PAM
cause inhibition of PS II reaction centers and increase of heat
dissipation. In this respect, photoinhibition is particularly relevant.
Phenomenologically, both an increase of Fo or a decrease of Fm may
contribute to a decrease of Fv/Fm = (Fm-Fo)/Fm. While an increase
of Fo points to photodamage, a decrease of Fm reflects enhanced
nonradiative energy loss (heat dissipation), which can be viewed as
an expression of photoprotection.
12.3.2 ML-BURST (menu point 5)
Plants can differ widely with respect to their requirements for
dark-adaptation. For some indoor potted plants less than 0.1 µmol
quanta m
-2
s
-1
may already cause closure of PS II centers
accompanied by a fluorescence increase, whereas most outdoor
plants display close to minimal fluorescence yield and maximal
Fv/Fm in the steady-state at 10-40 µmol quanta m
-2
s
-1
. Even if a
sample is kept in absolute darkness, the actual fluorescence
measurement requires some excitation light. With the MINI-PAM,
under standard conditions this amounts to ca 0.15 µmol quanta m
-2
s
-
1
. It can be decreased by lowering the measuring light intensity
(menu point 50: MEAS-INT) or by applying the 'burst mode' (menu
point 5: ML-BURST).
The burst mode is particularly useful for a quick check whether
the measuring light intensity is too high or not. By intermittent dark
periods the integrated intensity is cut to 1/5, which will result in a
lowering of fluorescence yield and an increase in Fv/Fm in very light
sensitive plants. In this case, the MEAS-INT could be further
lowered via menu point 50. Application of the burst mode has two
advantages: First, there is no loss in signal/noise ratio. Second, there
is no need to repeat AUTO-ZERO via menu point 2.
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