Abstract: The present study investigated the effect of heat stress on different stages of growth in wheat, such as germination under various temperature regimes and precocious development of wheat seedlings. The behaviors of two wheat cultivars (Triticum durum, cultivar Karim and Triticum aestivum, cultivar Salambô) were studied, particularly at the physiological, biochemical levels. Temperature stress caused a harmful effect to plant metabolism, by disrupting cellular homeostasis. A direct result of stress-induced cellular changes is the enhanced accumulation of toxic compounds in cells that include reactive oxygen species. In our investigation made on two wheat cultivars, we were found a similarity in the response to heat stress either at the germination stage or the early development (coleoptiles and first leaf). According to literatures, our results have shown that 10 or 15°C above the optimum of temperature doesn’t allow the establishment of wheat seedlings. Furthermore, reserve mobilization, seems to be damaged following heat stress by affecting the enzymes implicated in the starch breakdown and abolish giving nutrients to the wheat embryos. This was demonstrated in our work, in which we were shown that both 5 and 45°C affected the reserve mobilization but this effect is less pronounced after heat treatment of 35°C. At the photosynthetic level, heat stress generated ROS, which could damage both PS I and PS II. The detrimental effect of ROS might be alleviated by antioxidant enzymes and the sequestration of other components in plant cells. Furthermore, adaptation to temperatures changes, at the molecular level, was accompanied by the degradation of the normal proteins and the synthesis of Heat Shock Proteins (HSPs) involved in the mechanism of defence in plants.