Fluorescence parameters were determined synchronously with the gas exchange
measurements in the field using a portable pulse modulated fluorometer (FMS2,
Hansatech, England) under natural conditions according to Mateos-Naranjo et al.
(2008). Tested leaves were kept in the dark for 30 min before measurement. The Fo was obtained after excitation of the leaves with a weak measuring beam from a
light emitting diode when all PS2 reaction centers were open. The Fm was determined with a saturating pulse of white light (3000 mol m−2 s−1). The saturating
pulses were applied after darkness to determine the maximum fluorescence yield
during actinic illumination (Fm) when the primary quinone electron acceptor of PS II
(QA) was in the instantaneous maximum de oxidized state. Other chlorophyll fluorescence parameters including actual photochemical efficiency of PS II (PS II),
intrinsicefficiency of PS II (Fv/Fm), electron transport rate (ETR) and photochemical
quenching (qP) were calculated according to Kooten and Snel (1990).