The response of steady-state fluorescence (Fs) to irradiance in apple (Malus pumila Mill. cv. Tengmu No.1/Malus hupehensis Rehd.) leaf increased and decreased at light levels below and above 400 mumol(.)m(-2.)s(-1) photosynthetic photon flux density (PPFD), respectively, while the light-adapted maximal fluorescence (Fm') and minimal fluorescence (Fo') decreased constantly with the increasing PPFD, and the closure of photosystem 11 reaction center (PS 11 RC) increased continuously, reflected by the chlorophyll fluorescence parameter of (Fs-Fo')/(Fm'-Fo'). These facts indicated that decrease of Fs above 400 mumol(.)m(-2.)s(-1) PPFD was not caused by closure of PS 11 RC, but was mainly resulted from the process of light transfer from light-harvesting complex II (LHC II) to PS II RC. In the presence of N- ethylmaleimide (NEM), an inhibitor of photosynthetic state transition, Fs kept on increasing in apple leaf at light levels from 400 to 700 mumol(.)m(-1.)s(-1), which was the photosynthetic saturation irradiance of apple leaves. In addition, Fs still increased at light levels over 700 mumol(.)m(-2.)s(-1) in apple leaf pre-treated with dithiothreitol (DTT), an inhibitor of xanthophyll cycle. These changes showed that state transition and xanthophyll cycle caused a decrease of Fs in apple leaf at light levels below and above the photosynthetic saturation irradiance, respectively. When apple leaf was pre-treated with NEM, the PS II apparent rate of photochemical reaction (P-rate) and photochemical quenching (qP) decreased significantly in the light range of 600-800 mumol(.)m(-2.)s(-1), but the non-photochemical quenching (qN) existed a small increase at 600-800 mumol(.)m(-2.)s(-1) and a decrease above 800 mumol(.)m(-2.)s(-1). These phenomena suggested that state transition was mainly a photochemical and a non-photochemical process in apple leaf responding to light lower and higher than photosynthetic saturation irradiance, respectively.
