Walz MINI-PAM-II, Manual, Page: 44 / 130

Chapter 5 Saturation Pulse Analysis
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Y(NPQ) regulated energy losses of excitation energy by heat
dissipation involving ∆pH - and zeaxanthin-dependent
mechanisms, and
Y(II) use of excitation energy for charge separation.
This concept of "complementary PS II quantum yields" is useful
to analyze the partitioning of absorbed light energy in photosyn-
thetic organisms. For instance, in the presence of strong light, a
much higher Y(NPQ) than Y(NO) indicates that excess excitation
energy is safely dissipated at the antenna level and that photo-
synthetic energy fluxes are well-regulated.
In variance, high values of Y(NO) would signify that excess exci-
tation energy is reaching the reaction centers, resulting in strong
reduction of PS II acceptors and photodamage, e.g. via formation
of reactive oxygen species.
5.3 Relative Electron Transfer Rate (ETR)
Relative electron transfer rates are calculated according to:
ETR = PAR · ETR-Factor · P
PS2
/P
PS1+2
· Y(II).
The basic idea of the ETR equation is to multiply Y(II), the effec-
tive photochemical quantum yield of PS II, by an estimate for the
photon flux density absorbed by all PS II in the sample. The latter
estimate is derived from three numbers:
(1) PAR
Quantum flux density of photosynthetically ac-
tive radiation (PAR) impinging on the sample.
(2) ETR-Factor Sample absorptance (= 1 – transmittance)
The ETR-Factor describes the fraction of incident photons ab-
sorbed by the sample. The most frequently used default value for
green leaves is 0.84 meaning that 84% of incoming light is ab-