An investigation studied the impact of spectral composition on plant responses to heat stress. Petunia × hybrida seedlings were grown under nine different light qualities, including the modifying and constant compositions of blue and red LEDs light, as well as Fluorescent and white LED lamps. After 28 days, the seedlings were exposed to hyperthermia (40 ◦C / 35 ◦C, 4 days). The results showed that the highest survival rate (100 %), and the lowest leaf chlorotic area (14 %) were observed among seedlings grown under modifying spectral compositions with a gradual increase in red light, which also showed the highest change rate of increased Chl a/ b ratio (262 %) and lowest change rate of increased total Chl/ Car ratio (45 %). Furthermore, seedlings grown under a gradual in- crease in red light showed the highest relative water content (82 %) and the lowest change rate of root water content (1.2 %). Moreover, the highest proline and lignin contents, along with the lowest electrolyte leakage, 7.22 μg g 1 FW, 104 mg g 1 FW, and 25 %, respectively, were observed in seedlings exposed to a gradual in- crease in red light after heat stress compared to other lighting treatments. Also, after heat stress, seedlings grown under a gradual increase in red light showed the highest contents of studied enzymatic antioxidants. Our investigation highlighted a link between spectral composition during the seedling stage and heat stress tolerance of examined plants. This method can potentially prepare plant material during the propagation stage against extreme temperatures

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