Modulated Chl fluorescence measurements were made in attached leaves in the growth chamber with a PAM 2000 apparatus (H. Walz, Effeltrich, Germany). Fo (minimal Chl fluorescence yield in the dark) was measured by switching on the modulated light at 0.6 kHz; PPFD was below 0.1mol photonm−2 s−1 at leaf surface. Fm (maximal Chl fluorescence yield in the dark) was measured at 20 kHz with a 1 s pulse of 6000mol photonsm−2 s−1 of white light. The experimental protocol for the analysis of the Chl fluorescence quenching was as in Morales et al., 2000 and references therein. Fo and Fo (minimal Chl fluorescence yield after light adaptation) were measured in presence of far red (FR) light (7mol photonm−2 s−1) to fully oxidize the PSII acceptor side Dark-adapted, maximum potential PSII efficiencywas calculated as Fv/Fm, where Fv is Fm −Fo Actual (˚PSII) and intrinsic (˚exc) PSII efficiency were calculated as (F m − Fs)/F m and F v /F m , respectively. Photochemical quenching (qP) was calculated as (F m − Fs)/F
v. Non-photochemical quenching (NPQ)was calculated as (Fm/F m) − 1. Experimentswere carried out at ambient CO2 concentration, 200–250molm−2 s−1 PPFD, and at the temperature and relative humidity prevailing in the growth
chamber. All measurements were taken in leaves illuminated for 3–4 h. The experimentwas replicated 3 times and 10 measurements were made per treatment.