Impact of sub-radiation with LED light during cultivation of Mentha spicata on growth responses and leaf coloring

Mint is a popular herb marketed as a fresh potted plant. A dense appearance and healthy, green leaves are considered important quality traits for commercialization. However, when cultivating this plant at a high plant density, yellowing of the lower leaves may occur. To reduce the leaf yellowing, additional light applied underneath the crop canopy was evaluated for Moroccan mint (Mentha spicata 'Maroccan'). Plants were scored for overall plant morphology with special attention to chlorophyll content of upper (fully light exposed) and lower (shaded) leaves. We postulated that additional LED light (light-emitting diodes, λ_max = 440 nm) given at the level of the lower leaves would promote chlorophyll synthesis and therefore prevent yellowing of the lower leaves. The action of additional light (+19 µmol m^‑2 s^‑1 at the level of the lower leaves), given during a 16-h photoperiod, was evaluated. This light treatment significantly increased chlorophyll levels at the upper (+7%) and lower (+28%) leaves compared to control plants. Furthermore, significant increases in the number of leaves (+19%), fresh weight (+13%) and dry weight (+15%) were observed. These features add to the market value of mint as a fresh product. Furthermore, additional light was evaluated 1) only during the dark period, or 2) during the entire 24-h cycle. Plants from both treatments showed strong phototropic response of the stem orientation toward the LED light used. Leaves were heavily curled. In both treatments, chlorophyll levels of the lower leaves increased significantly, but plants from both treatments were not marketable. However, through optimization of the time that additional LED light underneath the crop was applied, these disadvantages could be eliminated.