High temperature, high irradiance, low water vapour pressure, and lack of precipitation in most months characterize arid ecosystems. Under these conditions, trees such as pistachio suffer drought combined with salinity stress. In order to study the simultaneous effects of salinity and drought stresses on photosynthetic apparatus function in P. khinjuk, seedlings of the species were exposed to osmotic stress induced by a combination of salt and poly ethylene glycol (Ctrl (without any treatment), low osmotic stress (LOS), moderate osmotic stress (MOS) and high osmotic stress (HOS). Chlorophyll fluorescence yields were measured / calculated for the following parameters: F0, Fm, Fv, Fʹm, Fʹ0, Fʹm, Fʹv, Fs, F0 /Fm, Fv/ Fm, Fv/F0, Fʹv /Fʹm, &PhiPSII, ETR and Photoinhib-PSII%. A one way ANOWA showed that all the mentioned parameters were differently reacted to osmotic stress treatments without any visible symptoms of foliar phytotoxicity. The highest decrements or functional alterations in fluorescence parameters (in frame of defense mechanism or inactivation of photosynthetic apparatus) were observed at HOS.