2.3. Measurement of chlorophyll fluorescence parameters
Chlorophyll fluorescence was measured with a portable fluorescence
system (Li-Cor 6400; Li-Cor Inc., NE, USA) from the
same leaves that were previously used for photosynthetic measurements.
The initial fluorescence (Fo) was measured by switching
on the modulated irradiation of 0.6 kHz. PPFD was less than
0.1 mol m−2 s−1 on the leaf surface, which was adapted to darkness
for 30 min. The maximal fluorescence (Fm) was measured
at 20 kHz with a 1 s pulse of 6000 mol m−2 s−1 of “white light.”
The maximum quantum efficiency of PSII primary photochemistry
(Fv/Fm) was calculated as Fv/Fm = (Fm
−
Fo)/Fm. During illumination,
the steady-state fluorescence (F) was obtained. The maximal
fluorescence (Fm) was also determined by applying a saturating
pulse. Photochemical quenching (qP), the efficiency of excitation
energy capture by open PSII reaction centers (Fv/Fm), and the
quantum yield of PSII electron transport (PSII) were determined
[19]. Electron transport rate (ETR) was calculated according to
Schreiber et al. [20] using a non-photochemical quenching coefficient
(NPQ) = Fm/Fm
−
1.