Introduction Pistachio tree is one of the most important tree fruit crops in the arid region, especially in the central desert of Islamic Republic of Iran. Leaf gas exchange activities have an important role in plants productivity, and its study allow a direct evaluation of physiological responses of plants to environmental conditions. Photosynthesis is the basic case for gas exchange activity, growth, and biomass production by plants. Photosynthetic responses to rising global mean temperature of terrestrial plants can potentially change carbon balance and cycling of ecosystem (Rustad et al., 2001). De Herralde et al. (2003) reported that leaf photosynthetic activity can be used as a helpful means to classify tolerable plants under drought stress. Gomes- Laranjo et al. (2006) showed that there is an important correlation between irrigation and gas exchange activities, which can be suitable to find the drought resistance crop species. Wang et al. (2007) illuminated that gas exchange activities strongly change under different environmental temperatures . Flexas et al. (2001) reported the positive correlation between photosynthesis and stomatal conductance in pistachio trees. David (2002) found a positive correlation between photosynthesis and evaporation in olive trees. Moreover, it was shown that low relative water content strongly reduced photosynthesis, stomatal conductance and evaporation activities in olive trees (David, 2002). Unlike other woody species, very few studies have been done on photosynthetic activities in nut crops, especially with pistachio. There were many trials that assessed the rates of net CO2 uptake and stomatal conductance in some pistachio cultivars (Vemmos, 1994; Novello and de palma, 1995), however, there were no reports regarding to gas exchange activities at different fruit growth and development stages under ON and OFF bearing statuses of different pistachio genotypes. There are many benefits studying the plant eco-physiological characteristics that warrant plant and cultivar cultivation under special environment and reduce damages made by chilling injuries, chilling requirements, salinity and drought stress, and so no. In crops, yield is dependent on photosynthetic rate and on the allocation of assimilates to different sinks such as flowers, fruits or leaves. In fruit trees, canopy architecture makes the relationship even more complex (Pearcy and Sims, 1994). However, the leaf photosynthetic characteristics are a very good initial approach to the response of plants to environmental conditions (Dejong, 1986). Choosing the proper commercial cultivars as rootstock, scion and/or their combinations regarding to the highest yield, fruit quality, pest and disease resistance, and adaptability is the most important case of horticultural research. Considering climatic diversity in pistachio plantation; it is not easy to advise all commercial cultivars for many conditions. Thus, this investigation was conducted to evaluate the eco-physiological responses of six Iranian pistachio cultivars at different stages of fruit growth and development under ON and OFF statuses and to find the best for stress conditions.