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Cookbook check list before making Fv/Fm measurements
Fv/Fm is a normalized ratio that does not use a traceable standard. Instead, its accuracy is
determined by properly using the instrument and following the lessons learned about
plant physiology by several great researchers.
To get an accurate measurement, one has to follow tested guidelines.
1. Dark-adapt properly knowing the plant’s light history.
It takes only a few minutes
for the xanthophyll cycle and the
)
ph of the thylakoid lumen to return to a dark-adapted
state. State transitions, however; take between fifteen to twenty minutes. These times can
vary somewhat in field plants and can take slightly longer. In addition, field plants and
other plants that have been exposed to photoinhibition conditions for a number of hours,
will retain a certain amount of NPQ for up to 60 hours (Lichtenthaler 2004). This means
that even if dark adaptation is overnight, there will almost always be some residual NPQ
built into summer field measurements of Fv/Fm. For this reason it is important to only
compare samples with a similar light history. It is common for researchers to choose dark
adaptation times anywhere from twenty minutes to overnight, using pre-dawn values.
Shorter times may be used to study the effects of plant protective mechanisms. (For more
information see the section on Dark adaptation)
2. Modulation light intensity setting
Fv/Fm is (Fm-Fo)/Fm. Fo, or minimum
fluorescence is a dark adapted value made by exposing the leaf antennae to a very low
intensity modulated measuring light, that is not set high enough to drive photosynthesis.
The modulation light intensity must be set correctly for best accuracy and repeatability. If
it is set too high, it will drive photosynthesis and provide an Fo value that is too high.
The modulated light allows the measurement of pre-photosynthetic antennae
fluorescence.
3. Shade leaves vs. Sun leaves
. – The Fv/Fm ratio will be slightly higher on Sun leaves
than on shade leaves (Lichtenthaler 2004).
4
.
White light vs. red light saturation pulse.
Fv/Fm will be higher with a white
saturation pulse than a red saturation pulse. Some fluorometers use a red saturation
pulse. This not an issue for comparative measurements of plant stress, but values
measured on a fluorometer with a white saturation pulse should not be compared to
measurements of a fluorometer with a red saturation pulse.
3. Maximum Fv/Fm values vary with species.
The average maximum Fv/Fm value is
0.83 (Schreiber 2004)
4.
Field plants should only be compared to field plants
and green house plants should
be compared to green house plants due to light history. (Lichtenthaler 2004)
