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Proper dark adaptation is very important for quenching measurements
Normally the Dark-Light Kinetic curve begins with dark adaption of the sample leaf. It is
common when making quenching measurements to dark-adapt for a full night, or even 24
hours (Maxwell and Johnson 2000).
In some cases, longer times may be appropriate. Lichtenthaler (2004) found that it could take
up to 60 hours for complete relaxation of NPQ in plants subjected to chronic photoinhibition
conditions. All non photochemical quenching measurements use Fm as a reference and some
parameters also use Fo (Schreiber 2004). For this reason it is important to ensure that one
starts with a properly dark adapted sample because all measurements are made relative to
these values. In the field, pre-dawn quenching measurements can provide a way to measure
plant stress, however it is important to understand that these values may be still be affected by
recent light history (Maxwell and Johnson 2000). Leaves with dissimilar light histories,
different Fv/Fm, or of different species should not be compared using non-photochemical
quenching parameters. (Maxwell & Johnson 2000). Baker (2008) states that only leaves with
similar Fv/Fm values should be compared. (for more details on dark adaptation please request
the Opti-Sciences dark adaptation application note).
Actinic Light Source
Most built-in fluorometer light sources used as actinic sources for quenching measurements
and light curves decline in intensity during these measurements. This is due to the fact that
heat from the internal light sources reduces light output. It can happen to halogen light
sources and to LED light sources. When this happens, the photosynthetic sample may never
really reach steady state photosynthesis, a process that takes between fifteen and twenty
minutes (Maxwell and Johnson 2000). Such light sources can produce errors in all quenching
and quantum photosynthetic yield values. It is recommended that the PAR clip be used for
quenching measurements and for the creation of quenching tests and rapid light curves.
Lake model and puddle model quenching parameters.
Understanding of the organization of antennae and reaction centers has changed over the
years. It is now understood that a single antennae does not link only to a single reaction center
as was previously described in the puddle model. Current evidence indicates that reaction
centers are connected with shared antennae in terrestrial plants. qP, the parameter that has
been used in the past to represent the fraction of PSII reaction centers that are open, is a
puddle model parameter. Dave Kramer (2004) has come up with a set of fluorescence
parameters that represent the newer shared antennae paradigm called the lake model. Others
have also come up with more simplified equations that eliminate the need for the
measurement of Fo and Fo’ and approximate the measurements made by Kramer.
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