103
12. Saturation pulse intensity. Saturation pulse intensity is more of an issue with Y(II)
than with Fv/Fm. While shade leaves will saturate at a few hundred :mols, sun leaves
will usually saturate below 1,500:mols. However, a problem has been found when
measuring yield at high light levels. It has been discovered that at high actinic or sun
light levels, leaves resist the complete closure of all PSII reaction centers that is expected
when using a saturation pulse. Even with a 6,400 :mol saturation pulse, some reaction
centers remain open. Up to a 41% error was found in yield measurements using standard
techniques at high actinic light levels. To correct for this issue, multiple saturation flashes
are used, and the measured maximum fluorescence value for each flash is entered into a
linear regression analysis formula to determine the maximum fluorescence intensity with
an infinite saturation flash. The multiple saturation pulse approach has been shown to
work in multiple papers and posters. The resulting value has been shown to correlate well
with gas exchange carbon assimilation values. This multi-flash method is available on the
OS5p and OS1p fluorometers. (see the Multi-flash section in chapter 3 for more details).
13. The time between saturation pulses. Rosenqvist and van Kooten (2006) state that a
between one to two minutes is required for complete relaxation of saturation pulse NPQ.
If saturation pulses are not separated by this distance range, then an error caused by this
type of saturation pulse NPQ will result. It will accumulate with each saturation pulse.
When in doubt, space saturation pulses 120 seconds apart or more.
14. Overlap of PSI fluorescence -Part of the minimum fluorescence, the Fo parameter,
in Fv/Fm ((Fm – Fo)/Fm), contains PSI fluorescence as well as PSII fluorescence. With
Fv/Fm, one is trying to measure the maximum variable fluorescence of PSII in a dark-
adapted state. PSI fluorescence is not variable, but the low fluorescent signal from PSI
does overlap with PSII. This produces an error. In C
3
plants, about 30% of Fo
fluorescence is due to PSI, and in C
4
plants about 50% of Fo fluorescence is due to PSI
fluorescence. PSI produces about 6% of the fluorescence found in Fm in C
3
plants, and
about 12% in C4 plants. (Pfundle 1998). This not a problem when comparing quenching
measurements for plant stress because, PSI fluorescence does not change with light level
or plant stress.
15. PAR is photosynthetically active radiation. Radiation on the leaf is measured
Between the wavelengths of 400nm to 700 nm. 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. Since Y(II) and quenching parameter change with light
and temperature, as well as plant stress levels, there are advantages to using a shrouded
leaf and PAR Clip when making quenching measurements.
To Next Page
Loading...
