2.6. Chlorophyll fluorescence parameters
After 4 weeks of all treatments, the fully expanded leaves,
located between the middle and the base of the scion’s shoots of
each plant, were used for the chlorophyll fluorescence measurements.
In vivo PSII chlorophyll fluorescence was measured by a
modulated (1.6 kHz), low intensity beam from light emitting
diodes (excitation wavelength 655 nm, detection above 700 nm)
using a portable pulse-amplitude-modulated fluorometer (Plant
Efficiency Analyzer-PEA, Hansatech 2.02; Walz, Effeltrich,
Germany). The minimum fluorescence yield (Fo) of the plants
adapted to darkness for 20 min was determined under weak red
modulated radiation. The mid-part of the front was held in the leaf
clip of the fluorometer at a standard distance from the optic fibre
probe and a weak 5s far-red (735 nm) pulse was sent to fully
oxidize the electron transport chain. The maximum fluorescence
yield (Fm) of the dark adapted plants was reached by exposed PSII
to a saturating pulse (0.8s) of white light. The difference between
Fm and Fo gave the variable fluorescence (Fv = Fm Fo). The
maximum photochemical efficiency of PSII was calculated as the
ratio of variable fluorescence to maximal fluorescence (Fv/Fm) and
represents the efficiency of open PSII in the dark adapted state.