**2.3 Chlorophyll fluorescence measurements**

Chlorophyll fluorescence was imaged, using the MINI-version of the Imaging-PAM (Heinz Walz GmbH, Effeltrich, Germany), in selected leaves attached to plants for the control and stress conditions and the measurements were made after the last night-period. The fluorometer used employs the same blue LEDs for the pulse modulated measuring light, continuous actinic illumination and saturation pulses. The minimal fluorescence yield (F0) and the maximal fluorescence yield (Fm), were measured in dark-adapted samples. F0 was measured at a low frequency of pulse modulated measuring light, while Fm was measured with the help of a saturation pulse. The maximal quantum yield of PS II was calculated as Fv/Fm = (Fm-F0)/Fm.

Light response curves were realised illuminating the selected leaves with actinic light of different intensities (20, 41, 76, 134, 205, 249, 300, 371, 456, 581,726 mol·m-2·s-1 PPFD), with 2 min illumination periods at each intensity. After each illumination periods a saturation pulse was applied to determine the relative electron transport rate, the effective PS II quantum yield of illuminated samples ((Fm'-F)/Fm') and non-photochemical quenching (Nq)), all of which were automatically calculated by the ImagingWin software. Results are shown as color-coded images of the maximal quantum yield of PS II (Fv/Fm), the fluorescence yield (F), the effective PS II quantum yield of illuminated samples ((Fm'-F)/Fm') and non-photochemical quenching (Nq)).
