CHAPTER 12 MEASUREMENTS WITH THE MINI-PAM
'quenching analysis'. For this purpose the PAM measuring principle
has been developed which allows monitoring fluorescence against
10
6
times larger background signals (see 12.2).
From the viewpoint of fluorescence emission there are two
fundamentally different types of competing de-excitation processes:
• photochemical energy conversion at the PS II centers
• non-photochemical loss of excitation energy at the antenna and
reaction center levels
By both mechanisms, the maximal potential fluorescence yield is
'quenched' and, hence, 'photochemical' and 'non-photochemical
fluorescence quenching' can be distinguished. For interpretation of
fluorescence changes, it is essential to know the relative
contributions of these two different quenching mechanisms to the
overall effect. If, for example, fluorescence yield declines, this may
be caused by
• an increase of the photochemical rate at the cost of fluorescence
and heat-dissipation
• or an increase of heat-dissipation at the cost of fluorescence and
photochemistry
These two possibilities can be distinguished by the so-called
'saturation pulse method':
With a very strong pulse of white light the electron transport
chain between the two photosystems can be quickly fully reduced,
such that the acceptors of PSII become exhausted. Hence, during the
saturation pulse photochemical fluorescence quenching becomes
zero and any remaining quenching must be nonphotochemical. It is
assumed that changes in non-photochemical quenching are too slow
to become effective within the approx. 1 second duration of a
saturation pulse.
60
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