Chlorophyll fluorescence was recorded weekly under laboratory conditions(ambient CO2 concentration) using an imaging pulse-amplitude-modulated fluorometer(Imaging PAM,Heinz-Walz GmbH,Effeltrich,Germany),equipped with 96 blue light-emitting diodes ( peak at 470 nm) used for fluorescence excitation, actinic illumination and saturation pulses. Fluorescence images (640×480 pixel)were take non the second fully expanded leaf of the third leaf level (eight plants per treatment, one measurement per plant).