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Definitions:
Actinic light source – This is any light source that drives photosynthesis. It may be the Sun,
or an artificial light. Higher end fluorometers contain one or more built-in artificial actinic
light sources for experimentation with specific repeatable radiation (or light) levels. The
OS1p uses a high intensity white light LED.
Dark-adapted or Dark Adaptation – This is a term that means that an area of a plant, or the
entire plant, to be measured has been in the dark for an extended period of time before
measurement. Dark adaption requirements may vary for dark-adapted tests. Dark adaption
times of twenty minutes to sixty minutes are common, and some researchers use only pre-
dawn values. Dark-adapted measurements include Fv/Fm, and non-photochemical quenching
parameters. Longer dark adaption times are common for quenching measurements. In this
case, it is common to use times of eight to twelve hours, or overnight. For a detailed
discussion of dark adaptation, refer to the section on dark adaptation, or the application note
on dark adaptation.
Far red light – is a light source that provides light above 700 nm to drive PSI, drain PSII of
electrons, and allow the rapid re-oxidation of PSII. It is used extensively for the determination
of quenching parameters in Quenching protocols, and for pre-illumination and rapid re-
oxidation of PSII in Fv/Fm measurements. Fo’ or Fod is used in determining Kramer’s
quenching parameters, as well as puddle model, qN. They require the use of far red light to
determine quenched Fo’ or Fod.
Modulated light source This is the light source that makes light adapted quantum
photosynthetic yield measurements possible along with direct measurements of Fo and Fo’ or
Fod. The modulated light source is used at an intensity range that is too low to drive
photosynthesis and yet allows fluorescence measurement of pre-photosynthetic Fo, and post
photosynthesis Fo’ or Fod. This light source is turned off and on at a particular frequency.
The frequency is adjusted automatically for optimal application usage. Intensities are adjusted
between >0 to 0.4 :mols. The intensity must be set differently for light and dark-adapted
methods. It is critical to adjust the intensity of this lamp correctly in dark-adapted protocols to
prevent driving photosynthesis in Fv/Fm, and quenching measurements. For more details see
the application note on dark adaptation.
Saturation pulse
is a short pulse of intense light designed to fully reduce a leaf’s PSII
system. For higher plants, the optimal duration of the saturation pulse is between 0.1 seconds
and 1.5 seconds (Rosenqvist and van Kooten 2006). In algae and Cyanobacteria, the optimal
saturation pulse duration is between 25ms. and 50ms. It is typically a white light that has to be
high enough to close all PSII reaction centers. On the OS1p an LED light source is used.
Opti-Sciences uses 0.8 seconds as the default saturation pulse duration for higher plants. This
duration is adjustable from 0.025 to 2.0 seconds.
Fm - is maximal fluorescence measured during the first saturation pulse after dark adaption.
Fm represents multiple turnovers of QA with all available reaction centers closed. All
available energy is channeled to fluorescence.
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