Chlorophyll fluorescence was analyzed with an Imaging-PAM fluorometer (Walz). Plants were first darkadapted for 20min and a saturating light pulse was applied to determine the maximum quantum efficiency of Photosystem II (PSII) (Fv/Fm)( Fv, variable fluorescence; Fm, maximum fluorescence yield in the dark-adapted state). Later, every leaf was adapted for 5 min to an actinic light of PPFD at 228lmolm2 s1 (similar to the mean growth PPFD). Then, a second saturating light pulse was used to calculate: the relative quantum efficiency of PSII electron transport (UPSII) estimated from UPSII =( Fm0 F)/Fm0( Fm0, maximum fluorescence yield in the light-adapted state; F, fluorescence yield) according to Genty et al. (1989); the coefficient of photochemical quenching (qP) estimated from qP = (Fm0 F)/(Fm0 F00)( F00, minimum fluorescence yield in the light-adapted state); and the non-photochemical coefficient (qN) estimated from qN =(FmFm0)/(FmF00)(Andrews et al., 1993).