The maximum quantum yield of PSII in the samples was mea-sured after 30-min-long dark adaptation of the leaves using Fv/Fmprotocol. Fv/Fmratio is used for estimation of the maximum portionof absorbed quanta used in PSII reaction centres. Dark adaptationallows the re-oxidation of PSII and to relax non-photochemicalquenching. Minimum and maximum fluorescence (F0, and Fm,respectively) of dark-adapted leaves were measured in the sameleaves after 0.8 second for saturation pulse (35 W halogen lampwith 690 nm short pass filter) in previously dark-adapted samples.Variable fluorescence (Fv= Fm− F0) and maximum quantum yieldof PSII (Fv/Fm) were calculated by the software of the Fluorometer,moreover maximal efficiency of the photochemical process in PSII(Fv/F0) was counted.