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16. Far-red pre-illumination. Some fluorometers have the ability to pre-illuminate dark-
adapted leaves with far-red light. When this feature is used for five to ten seconds before
an Fv/Fm measurement takes place. It activates PSI, and ensures that all electrons have
been drained from PSII before the measurement of Fo. While this feature ensures that
PSII is completely re-oxidized, it does not relax the xanthophyll cycle, state transitions,
or photoinhibition. Time is still required in a darkened environment to relax all forms of
NPQ and to obtain reliable quenching values.
17. Far-red illumination. Ambient actinic sunlight contains far-red light to activate PSI.
The internal LED actinic light filters out far-red light. To approximate sun light, the far-
red light should be turned on for the entire quenching measurement.
The best experiments are ones that take these issues into account. PSI fluorescence is
involved in all measurements. It does not vary with light level or plant stress (Schreiber
2004). With this in mind, comparing samples with similar light histories allows
comparison of many types of plant stress.
The Plant Stress Guide provided by Opti-Sciences www.optisci.com , references papers
that deal with specific types of plant stress and limitations of different chlorophyll
fluorescence parameters for measuring plant stress.
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