CHAPTER 12 MEASUREMENTS WITH THE MINI-PAM
coefficients are meaningful only, if the values of Fo and Fm were
previously measured with the same sample at the same sensitivity,
i.e. with unchanged optical parameters, measuring light intensity (see
12.3.4) and gain.
The definitions of qP and qN imply that fluorescence quenching
affects only the so-called variable fluorescence, Fm-Fo, and not Fo.
In reality, at higher levels of qN (exceeding approx. 0.4) there can be
also significant quenching of Fo, resulting in the lowered yield Fo'.
This can be estimated upon light-off, when the acceptor side of PS II
is quickly reoxidised (within 1-2 s), whereas relaxation of non-
photochemical quenching requires at least 5-10 s. Far-red light,
which mainly excites PS I, can enhance Q
A
-reoxidation and facilitate
assessment of Fo'. However, the MINI-PAM does not
feature an
intrinsic far-red light source (as e.g. the PAM-2000). Therefore, it
should be realized that the measured values of qP and qN are valid in
first approximation only, in particular when strong energy-dependent
nonphotochemical quenching is given.
Fo-quenching is of no concern for NPQ-determination. The
definition of NPQ implies a matrix model of the antenna system
(Stern-Volmer quenching). With NPQ that part of non-photochemical
quenching is emphasized which reflects heat-dissipation of excitation
energy in the antenna system. NPQ has been shown to be a good
indicator for 'excess light energy'. On the other hand, NPQ is
relatively insensitive to that part of non-photochemical quenching
which is associated with qN-values between 0 and 0.4, reflecting
mainly thylakoid membrane energization. The different responses of
qN and NPQ are illustrated in Fig. 13 in which a plot of qN vs. NPQ
is shown. In this presentation, it is assumed that no Fo-quenching
takes place. When Fo-quenching affects qN-calculation, the
relationship extends to NPQ-values exceeding 4.
74
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