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10. The duration of the saturation pulse should be between 0.5 seconds and 1.5
seconds for higher plants, and 25 to 50 milliseconds for Phytoplankton and
cyanobacteria. The OS1p will automatically provide the optimal measurements for higher
plants, algae and cyanobacteria as long as the duration is long enough. Times outside
these ranges increase the error in Y(II) measurements. Shorter durations prevent
complete saturation of PSII regardless of the light intensity (Roseqvist & van Kooten
2006). Longer durations create a form of saturation pulse NPQ that rounds the tail end of
the pulse maximum value, and reduces the average maximum saturation pulse value
(Roseqvist & van Kooten 2006). Some fluorometers allow adjustment of this parameter,
and others are preset at the factory at either. 0.8 seconds, or 1.0 seconds for higher plants.
0.8 seconds is the default value on the OS1p and it will work well with almost all higher
plants.
11. Saturation pulse intensity. Set it to 100% it will not damage the plant.
12. PSI fluorescence - Part of the fluorescence signal contains PSI fluorescence as well
as PSII fluorescence. With RLC Y(II) and ETR, one is trying to measure variable
fluorescence of PSII in non-steady state conditions.
PSI fluorescence is not variable, but the low fluorescent signal from PSI does overlap
somewhat with PSII. This produces an a small error. This is not a problem for comparing
similar samples, because PSI fluorescence does not change with light intensity
temperature or plant stress.
PAR is photosynthetically active radiation. Radiation on the leaf is measured between
the wavelengths of 400nm to 700nm. PAR sensors and thermisters for measuring
temperature are calibrated to other instruments that are traceable to the NIST. It is
recommended that recalibration should occur every two years. Most modern sensors are
solid state, so drift is minimal. Measuring PAR is better than assuming intensity values
for different steps. ETR values can change dramatically for different PAR values.
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