Determination of chlorophyll fluorescence parameters
The PAM-2100 portable modulation fluorometer (PAM 2100, Walz, Germany) was used for measurement. Before measurement, leaves were pretreated in the dark for 30 min. Fo, Fm, Fv (= Fm - Fo), and Fv/Fm parameters were recorded for 15 s at a photon flux density of 4000 μmol m-2 · s-1. Additionally, we measured the steady-state fluorescence level (Fs′) under continuous illumination, the maximal fluorescence level (Fm′) induced by a saturating light pulse at the steady-state, and the minimum fluorescence level (Fo′) after exposure to far-red light for 3 s. Then, Fv′ was calculated using the formula Fv′ = Fm′-Fo′. PSII was calculated using the formula PSII = 1-(Fs′/Fm′). The ETR was calculated using the formula ETR = PAR×PSII×0.85×0.5. NPQ was calculated using the formula NPQ = (Fm/Fm′)-1. qP was calculated using the formula qP = (Fm′-Fs)/(Fm′-Fo′) (Florian et al., 2009).