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changing environmental conditions (Baker 2008). NPQ is used to determine relaxation
coefficients because it has been resurrected for Klughammer's lake model parameters. Use of
q
N
has been discredited in the lake model.
Understanding the quenching mode trace
#1 represents a condition that is normally dark-adapted (proper dark adaption is required
when making nonphotochemical quenching measurements) with the sample shielded from
any actinic light that would drive photosynthesis. The only light on at this time is the
modulated measuring light at about 0.1umols intensity. This is not enough to drive
photosynthesis but it is enough to detect and measure minimum fluorescence from the leaf
antennae. In most commercial fluorometers, 30% of the intensity of Fo in C
3
plants is the
result of fluorescence from PSI. In C
4
plants, 50% of the intensity of Fo is the result of
fluorescence from PSI. This contributes to a small error in Fv/Fm measurements and creates
an underestimation of maximum quantum efficiency
#2 shows the first saturation pulse flash. This is a very intense short lasting flash of light that
is designed to saturate PSII and close all available reaction centers. For higher plants, the
optimal time duration for a saturation pulse has been found to be between 0.5 seconds to 1.0
seconds (Rosenqvist and van Kooten 2005). For Algae and cyanobacteria the optimal duration
of the saturation pulse is shorter, 25 to 50 ms (Schreiber 1995).
This allows complete saturation without causing NPQ to underestimate Fm.
Saturation pulse
durations that are longer than recommended create a rounding of the top back of the
saturation pulse caused by a form of NPQ. However, this is not true of the OS1p. It seeks to
find the highest eight point rolling average of intensity points from the saturation pulse and
reports the value. With dark adapted samples complete closure of all PSII reaction centers can
be accomplished with minimal saturation intensity. PAR Light values of 3,000
:
mols or
higher are commonly used. Certainly values of 3000
:
mols or higher will fully saturate any
properly dark adapted sample.
The rise from Fo to Fm represents multiple turn-overs of Q
A,
the primary electron transport
receptor. PSII becomes fully reduced at Fm with all PSII reaction centers closed. All absorbed
