Unlocking the potential of spectral LED lights in enhancing glandular trichome density and accumulation of bioactive specialized metabolites in spearmint.

Spearmint is a well-known aromatic plant recognized for its unique aroma, flavour, and bioactive properties. To ensure its year-round production in response to the growing market demand under rapidly changing climates, the implementation of light-emitting diodes (LEDs) in controlled environments presents a promising solution. This study explored the impact of different LED spectral lights, namely monochromatic red (R), monochromatic blue (B), combined red-blue (RB), and control warm white (WW) on the vegetative growth, glandular trichome density, key specialized metabolite contents and relevant biosynthetic gene expression levels in spearmint. Various morphometric features showed improvement under RB and B lights compared to WW, while R light resulted in overall impaired growth. Plants under RB light showed the highest density of peltate glandular trichomes (43 Nos./mm²), which exhibited strong positive correlation with maximally elevated levels of marker terpene carvone (9.2 μmol/g FM) and its precursor limonene (0.5 μmol/g FM). This suggested a possible interlinkage between trichome development and terpene biosynthesis. Additionally, rosmarinic acid content significantly increased (781.5 μg/g FM) under RB light compared to other spectral light conditions. The highest expression levels of genes related to both phenolic and monoterpene biosynthetic pathways were also obtained under RB light, explaining the enhanced production of key bioactive compounds at the transcript level. Thus, the combination of R and B lights proved advantageous for the growth of spearmint and the concurrent enrichment of major specialized metabolites. The insights gained from this research will pave ways for developing appropriate LED-based cultivation systems for commercial production of spearmint.