opti-sciences OS1p, Руководство пользователя

Страница: 101 / 175

101
Cookbook checklist before making quenching measurements
.
NPQ and other calculated parameters such as Fv/Fm, Y(II), Y(NPQ), Y(NO) are all
normalized ratios that do not use a traceable standard. Instead, their accuracy is
determined by properly using the instrument and following the lessons learned about
their use with plant physiology by several great researchers.
To get an accurate measurement, one has to follow tested guidelines.
1. Dark-adapt properly knowing the plant’s light history. It takes only a few minutes
for the xanthophyll cycle and the
)
ph of the thylakoid lumen to return to a dark-adapted
state. State transitions, however, take between fifteen to twenty minutes. These times can
vary somewhat in field plants and can take slightly longer (Baker 2004). In addition, field
plants and other plants that have been exposed to photoinhibition conditions for a number
of hours, will retain a certain amount of NPQ for up to 30 to 60 hours (Lichtenthaler
2004). This means that even if dark adaptation is overnight, there will almost always be
some residual NPQ built into summer field measurements of Fv/Fm, and other displayed
quenching parameters. For this reason, it is important to only compare samples with a
similar light history. It is common for researchers to choose dark adaptation times
anywhere from overnight, using pre-dawn values, to twenty-four hours, Shorter times are
not normally used to study quenching (For more information, see the section on
quenching measurements in chapter one.)
2. Samples that are compared must have the same Fv/Fm values. Quenching
measurements of different samples with different Fv/Fm values should not be compared
(Baker 2008). Fv/Fm is used as the measuring standard for non-photochemical quenching
measurements, and if the measuring standard is different, the quenching values are
meaningless. Comparing values from samples with different Fv/Fm values is like
measuring items with a ruler that has dimensions that change.
3. Modulation light intensity setting Fv/Fm is (Fm-Fo)/Fm. Fo, or minimum
fluorescence is a dark-adapted value made by exposing the leaf antennae to a very low
intensity modulated measuring light, that is not set high enough to drive photosynthesis.
The modulation light intensity must be set correctly for best accuracy and repeatability. If
it is set too high, it will drive photosynthesis and provide an Fo value that is too high.
The modulated light allows the measurement of pre-photosynthetic antennae
fluorescence. Maximum fluorescence is measured when exposing a leaf to a saturation
flash with light intense enough to close all PSII reaction centers.