Supplementing Si to the soil is one of the alternative strategies for overcoming the negative effects of salinity on crop yield. Therefore, a field experiment was conducted to investigate the effects of silicon application on photosynthesis characteristics of two sorghum (Sorghum bicolor L.) cultivars (CV) (Omidbakhsh and Sepideh) under three levels of salt stress (5.2, 10.5 and 23.1 dS m-1), two levels of Si (1.44 and 1.92 g.kg-1soil) application and control (0 g.kg-1 soil) arranged as a split, split plot design. Salinity significantly decreased photosynthesis rate, chlorophyll a (Cha), chlorophyll b (Chb), and dry matter accumulation (DM) of the sorghum plants, indicating that plants suffered from stress. In addition, salinity imposed a remarkable decrease on variable fluorescence (Fv) and quantum yield. At highest level of salinity supply of 1.92 g.kg-1soil Si, alleviated negative effects of salinity and increased photosynthetic rate (24%), transpiration rate (19%), quantum yield (38%), total pigments (22%) and dry matter accumulation (65%) compare with 1.44 g.kg-1soil Si application. Correlation between shoot dry weight, Cha and Chb were no significant. In the relative salt sensitive CV (Sepideh) leaf Cha, Chb content and Cha/Chb, was higher than salt tolerance CV (Omidbakhsh), however, Omidbakhsh showed higher photosynthetic rate and dry matter accumulation.

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