102
4. Leaves must be at steady state photosynthesis. This takes between fifteen and
twenty minutes at a new light level. (Maxwell and Johnson 2000). This is accomplished
by setting the number of saturation pulses and the time between saturation pulses. For
example, if there are ten saturation pulses spaced 120 seconds apart, the leaf will be
exposed to the actinic light for twenty minutes after dark adaptation. Since an internal
artificial light source is used, the test allows one to compare below canopy leaves as long
as the Fv/Fm values are the same. According to Klughammer (2008), the only non-
photochemical parameter that does not have to be taken at steady state photosynthesis is
Y(NO) from Hendrickson.
5. Proper test length. OSI recommends that the actinic light should be on for thirty
minutes when using the OS1p for quenching measurements. The intensity of the actinic
LED light source output changes as the heat from the lamp changes the lamp
temperature. The drop in intensity can be up to 13% of the initial lamp intensity over
several minutes. since most of the drop happens in the first ten minutes, the change is
minimized by extending the test to thirty minutes, allowing steady state photosynthesis.
6. Y(II) values vary with light level and with temperature. The higher the light level,
the lower the Y(II) value. When measuring Y(II) in the field, it is extremely important to
measure leaf irradiation or light level at the leaf, and leaf temperature. Comparing Y(II)
values taken at different light levels, and different temperature levels, introduces a
significant error, unless it is the change at different light levels, and heat levels, that is of
interest. This is commonly done with a PAR Clip.
7. Shade leaves vs. Sun leaves. – The Y(II) ratio will be higher on Sun leaves than on
shade leaves (Lichtenthaler 2004).
8. Field plants should only be compared to field plants and green houseplants should
be compared to green houseplants due to light history. (Lichtenthaler 2004)
9. Leaf orientation is not important because an artificial actinic light source is used.
10. It is common to use the youngest fully mature leaf blade for diagnosis of
deficiencies in plants (Reuter and Robinson 1997).
11. The duration of the saturation pulse should be between 0.5 seconds and 1.5
seconds for higher plants, and 25 to 50 m
illiseconds for Phytoplankton and
cyanobacteria. Times outside these ranges increase the error in Y(II) and quenching
measurements. Shorter durations prevent complete saturation of PSII regardless of the
light intensity. Longer durations create a form of saturation pulse NPQ that rounds the
tail end of the saturation pulse maximum value, and reduces the average maximum
saturation pulse value. (Schreiber 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.
To Next Page
Loading...
