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When making longer quenching and quenching relaxation parameter measurements related to
photo-inhibition and photo damage mechanisms that are common in chronic high light stress,
high heat stress, cold stress and over wintering stress, one should understand that it could take
days for full relaxation or repair of the non-photochemical quenching parameters, q
I
and
Y(NO), to pre-stress conditions. To get an accurate control value for Fm and Fo under chronic
photo-inhibition conditions (components of non-photochemical quenching parameters) it is
common to dark adapt for a full night, or 24 hours. (Maxwell and Johnson 2000) In some
cases longer times may be appropriate. Lichtenthaler (2004) One must assume that there is
some residual NPQ in all field measurement taken with plants exposed to chronic photo-
inhibition conditions, and dark adaptation times that are shorter than 60 hours. Never compare
NPQ values with samples that have different Fv/Fm readings. Fv/Fm is the yardstick for the
individual plant NPQ values (Baker 2008). Comparing NPQ values of plants with different
Fv/Fm values is like measuring samples with a ruler and dimensions that change
In Aquatic Plants Gorbunov (2001) is a good source for corals, and Consalvey (2004) is a
good source for Algae. For information regarding dark adaption for rapid light curves Rascher
2000 is a good source. Ralph (2004) describes momentary dark adaptation for Rapid Light
Curves.
The use of far-red pre-illumination that is available on some fluorometers is designed to
rapidly re-oxidize PSII by activating PSI. This can be valuable in field work, (Maxwell and
Johnson 2000), but it does not affect the relaxation of non-photo-chemical quenching
mechanisms. Consalvey (2004).
Dark adaptation can be accomplished by using dark adaptation leaf clips or cuvettes. Some
researchers use hundreds of inexpensive clips to make measurements on larger populations
quickly. Shrouds or darkened rooms may also be used.
In review, it is important to take a few things into account. Reliable dark adaptation times can
vary by species, plant light history, the fluorescence parameter of interest, and the type of
stress that needs to be measured. When dealing with a new species or an unknown photo-
history it is best to test for maximum and stable Fv/Fm at different dark adapted times for best
results. When testing for optimal dark adapting times it is important to use samples that have
been exposed to the maximum light conditions that will occur during the experiment.
