Journal of Plant Process and Function

Light emitting diodes (LEDs) could provide specific quality and quantity of light for normal plant growth. Melissa officinalis L. is one of the most important medicinal plants in the world that has many applications in drug and flavoring industris. We cultivated lemon balm rhizomes in pot and placed the pots in four incubators containing LED lamps with the spectrum of red (100%), blue (100%), 70% red plus 30% blue and white (100%) with the intensity of 300 µmol.m^-2.s^-1 and in greenhouse conditions as a commercial control. The highest number of stems and leaves were observed in each pot at red lights and red + blue combination and the average height of the plants was the highest in the treatment with red light. Also these LEDs increased the content of chlorophyll a, chlorophyll b, total chlorophyll and carotenoid but the ratio of chlorophyll a/b only had the highest amount at the incubator containing red LEDs. Rosmarinic acid amount in the leaves was the highest in red LED, so that it raised the amount of rosmarinic acid to 33/16 and 52/44 % compared to white LED  and samples grown in greenhouse, respectively, but the amount of this compound in plant roots was the maximum at greenhouse conditions. Since the responses of lemon balm plants grown under LED light sources were much better than the control samples, therefore, the use of artificial light sources such as LED lights is suggested as an alternative light source in the greenhouses for improving growth and raising active ingredients.