Effects of Supplemental Light Quality at The End of Day on Herb Production and Some Phytochemical Properties of Lemon Balm (Melissa officinalis L.)

Increasing day length during the short photoperiod in fall and winter is a beneficial means of increasing biomass production and altering plant morphology and phytochemistry. The objective of this study was to investigate the effects of light quality at the end of the day (EoD) on the growth and phytochemical characteristics of lemon balm. During short-day photoperiods in autumn, lemon balm (Melissa officinalis L.) seedlings were exposed to red, blue, and combined red/blue light using light-emitting diodes for 2 hours at the EoD and then compared with the control. The results showed that exposure to red light significantly increased biomass. Plants grown under blue light yielded the highest percentage of dry matter, and their leaves had the highest content of chlorophyll and flavonoids. In addition, the highest contents of carotenoids were found in the plants irradiated with blue light and later with red+blue light. The highest levels of total phenols, anthocyanins, and antioxidant activity were found in the plants grown under red light. In addition, light quality had a significant effect on essential oil content. The highest essential oil content was obtained in the red and red+blue light treatments. The light quality at the EoD significantly changed the essential oil composition. The blue light significantly increased the citronellal content but decreased the geranial and linalool content. This study provides insight into the effects of EoD light quality on plant growth and metabolite accumulation in lemon balm during a short photoperiod.