*2.6. Chlorophyll a Fluorescence Determination*

Chlorophyll *a* fluorescence in PSII was measured using Junior-PAM fluorimeter (Heinz Walz, Germany). Minimum (Fo) and maximum (Fm) fluorescence rates were determined after 15 min leaf exposition in darkness. Maximum quantum efficiency of PSII Fv − Fm = (Fm − Fo)/Fm was calculated after [37]. Relative PSII operating efficiency (ΦPSII) of the light-adapted leaves was calculated as ΦPSII = (F0m − Ft)/F0m. Chlorophyll a non-photosynthetic quenching (NPQ) was calculated as NPQ = (Fm − F 0m)/F0m. Photochemical electron transport rate (ETR) was calculated as ETR = (ΦPSII)·PPFD·0.5. Fluorescence parameters were determined in 4–6 biological replicates.
