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 thequantum 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.