Walz MINI-PAM-II, Manual, Page: 42 / 130

Chapter 5 Saturation Pulse Analysis
36
5.2 Fluorescence Ratio Parameters
To quantify photochemical use and non-photochemical losses of
absorbed light energy, fluorescence ratio expressions have been
derived which use two or more of the five relative fluorescence
yields introduced above. Table 3 (page 34) compiles the fluores-
cence ratio parameters available in MINI-PAM-II and WinControl-
3. Below, these parameters will be explained briefly.
F
V
/F
M
and Y(II) Maximum and effective photochemical quan-
tum yields of PS II
The F
V
/F
M
and Y(II) estimate the fraction of absorbed quanta
used for PS II photochemistry. F
V
/F
M
corresponds to the maxi-
mum photochemical yield of PS II, Y(II) is the effective photo-
chemical yield of PS II. Measurements of F
V
/F
M
require that
samples are acclimated to darkness or dim light so that all reac-
tions centers are in the open state and non-photochemical dissi-
pation of excitation energy is minimal.
In algae and cyanobacteria, however, the dark-acclimated state
often is not showing maximal PS II quantum yield, as the PS II
acceptor pool may be reduced in the dark by stromal reductants
and, consequently, the so-called state 2 is formed exhibiting low
PS II quantum yield. In this case, preillumination with moderate
far red light should precede determinations of F
0
and F
M
.
The Y(II) value estimates the photochemical use of excitation
energy in the light. It is lowered with respect to F
V
/F
M
by partial
closure of PS II centers and various types of non-photochemical
energy losses induced by illumination.