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CHAPTER 12
illumination and longer saturation pulses can induce temperature
increases within the fluorescence standard of up to 10 °C,
corresponding to a decrease of fluorescence yield of 4 %. Actually,
the heating during strong illumination also has a small effect on leaf
measurements, as also chlorophyll fluorescence is lowered by
approximately 0.4 % per °C. Therefore the intensity and duration of
saturation pulses should not be excessive. Otherwise there would be
some underestimation of YIELD which is, however, rather small. For
example, assumed the leaf surface heats up by 5 °C, instead of
Fv/Fm = 0.833 the measured value would be 0.830.
12.3.6 qP, qN and NPQ (menu points 26 and 27)
When a leaf is illuminated, its fluorescence yield can vary
between two extreme values, Fo and Fm, which can be assessed after
dark adaptation (see section 12.3.4). Any fluorescence lowering with
respect to Fm may be caused either by enhanced photochemical
energy conversion or by increased heat-dissipation (as compared to
dark state). As was outlined in section 12.1, saturation pulse
quenching analysis allows to distinguish between these two
fundamentally different types of fluorescence quenching. In brief,
photochemical quenching can be suppressed by a pulse of saturating
light (as photochemistry is saturated), whereas non-photochemical
quenching does not change during a saturation pulse (as changes in
heat-dissipation involve relatively slow processes). The quenching
coefficients are defined as follows (with Fm' being displayed as
...M):
−
Fm F
'
=
qP
−
Fm Fo
'
qP and qN can vary between 0 and 1, whereas NPQ can assume
values between 0 and approximately 10. The displayed quenching
MEASUREMENTS WITH THE MINI-PAM
Fm Fm
=
qN
Fm Fo
−
'
NPQ
−
−
Fm Fm
'
=
Fm
'
73
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