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35
Dark Adaptation – How Long is Long Enough?
Dark adaptation is a technique used in some chlorophyll fluorescence measurements to fix a
non-stressed reference point relative to various measurements (Maxwell and Johnson 2000).
Deciding where to put that reference is based on an understanding of plant mechanisms that
can affect measurements, and what one wants to measure.
Dark adaption times of twenty minutes, thirty minutes, forty minutes and sixty minutes are
common for terrestrial plants, and some researchers use pre-dawn values.
To obtain reliable modulated Fv/Fm values, decisions need to be made for control and test
measurements. The plant mechanisms listed below will lower Fm, and possibly raise Fo,
changing Fv/Fm measurements downward like other types of plant stress. One must decide
which mechanisms are of concern for specific types of plant stress measurement and dark
adapt accordingly.
Fv/Fm is affected by both photochemical and non-photochemical factors. If a leaf is dark
adapted and measured, then subjected to high light levels, then dark adapted and re-measured,
the first measurement will be higher then the second measurement. The decline in Fv/Fm
measurement may be due to a decrease in reaction centers capable of photochemistry or un-
reversed non-photochemical quenching. (Baker N.R., Oxborough K. 2004)
Papageorgiou reports that results may vary greatly depending on how long dark adaptation is
done. A few minutes of dark adaptation is enough to re-oxidize the plastoquinone pool and
the CaMn
4
OxCly cluster, while longer periods deplete respiratory substrates through
respiration in cyanobacteria and chlororespiration in higher plants and algae. Longer times
will also deplete ATP pools, and trans-membrane ion concentration gradients. Dark
adaptation also shifts higher plants and algae toward state 1 conditions and cyanobacteria to
state 2 conditions. (Papageorgiou G.C. Tismmilli-Michael M. Stamatakis K. 2007)
Rapid acting photo-protective mechanisms activated by exposure to variable light intensities
(designated in the parameters q
E
and Y(NPQ) are the xanthophyll cycle and thylakoid lumen
)
ph. They relax in a few minutes during dark adaptation. (Muller, Niyogi 2001),(Kramer D.
M., Johnson G., Kiirats O., Edwards G. (2004). According to Lichtenthaler (1999) this time is
4-6 minutes. According to Baker(2008) this time can be longer in the field.
State I – State 2 transition quenching (called q
T
) is most significant at lower light levels in
terrestrial plants and can represent more than 60% of quenching at low light levels. At high
light levels it represents about 6% of total quenching. State transition quenching relaxes in ten
to twenty minutes in terrestrial plants. (Lichtenthaler H. Burkart S 1999)
It has been shown that the effects of acute photo-inhibition caused by exposure to high light
intensities can be reversed with 20 to 30 minutes of dark adaption (Theile, Krause & Winter
1998). The reversal of chronic photo-inhibition caused by several hours of exposure starts to
relax at about 40 minutes and may take 30 to 60 hours to fully relax under dark adaptation
(Lichtenthaler H. & Babani F. 2004) (Theile, Krause & Winter 1998)