The present study investigated the growth attributes of Petunia × hybrida ‘Shock Wave’ to ascertain the effect of different spectrum distributions. Upon germination, seedlings were placed under eighteen lighting treatments that included modifying and constant combinations of red625nm (R) and blue476nm (B) LEDs, as well as broad-spectrum lamps such as fluorescent, high-pressure sodium, and white LED. After 28 days, randomly selected seedlings were transferred to a common environment, where they stayed for 62 days to examine the effect of light treatments during seedling stage on the flowering and branching attributes. Results indicated that the growth of the seedlings improved under modifying R: B ratios compared to constant ones, and broad-spectrum lamps. A gradual increase in blue light percentage enhanced shoot dry weight, shoot length, and leaf area. A different pattern was observed for leaf number, root dry weight, and root length, as a gradual decrease in blue light percentage was in their favor. In addition, modifying spectral distributions enhanced electricity use efficiency as they converted more electricity to plant biomass compared to constant ratios. The results showed that seedlings under a lighting treatment with a gradual increase in blue light had the earliest time to flower, while the seedlings grown under high-pressure sodium lamp showed the highest number of flowers. Also, the apical dominance affected by spectral distribution resulted in a significant difference in the number and length of lateral branches. Overall, our study demonstrated that modifying percentages of red and blue light during seedling cultivation is an efficacious alternative to the standard lighting systems, and it can be further investigated in controlled environments.

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