CHAPTER 12 MEASUREMENTS WITH THE MINI-PAM
applying maximal measuring light intensity and maximal gain.
However, in this way also the background signal is increased and
AUTO-ZERO becomes very essential. For example, in an
experiment with a 1 mm² piece of a leaf at maximal gain and
measuring light intensity quite reproducibly an Fv/Fm = 0.610 to
0.630 was measured, when no offset was applied. However, when
AUTO-ZERO was properly applied, Fv/Fm = 0.795 to 0.815.
12.3.4 Fo, Fm (menu point 25)
Fo and Fm are defined as the minimal and maximal fluorescence
yields of a dark adapted sample, respectively. Knowledge of Fo and
Fm is required for determination of the quenching coefficients qP,
qN and NPQ (see section 12.3.6). Fo and Fm determination is carried
out in menu position 25 via SET. Then in menu position 26 there is
automatic reset of qP to 1.000 and of qN to 000 and in menu position
27 NPQ is reset to 0.000. With all consequent applications of
saturation pulses (via START), calculation of the quenching
coefficients will be based on these Fo, Fm values, until they are re-
determined via SET in menu position 25. As outlined in section
12.3.2, the threshold of light intensity below which a sample is dark-
adapted can vary considerably. In most plants Fm/Fo = 5 to 6, which
is equivalent to Y = Fv/Fm = 0.800 to 0.835. Such high values can be
measured only when true dark-adaptation is reached and the
measuring procedure is optimized as outlined in the preceding
sections 12.3.2 and 12.3.3.
For Fv/Fm, just as for YIELD-measurements in general, the
absolute signal amplitudes are of no concern, as long as Fo and Fm
are measured under the same conditions. However, measurements of
absolute signal amplitudes are important for full assessment of
photoinhibition (see 12.3.1) and also for calculation of the quenching
coefficients qP, qN and NPQ (see 12.3.6). It must be emphasized,
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