Exogenous epibrassinolide application improves essential oil biosynthesis and trichome development in peppermint via modulating growth and physicochemical processes

Peppermint has gained a promising status due to the presence of a high proportion of bioactive compounds, especially menthol. Due to its pharmacological efficacy, the demand for its plant-based bioactive compounds necessitates its cultivation worldwide. Brassinosteroids are polyhydroxylated sterol derivatives that regulate diverse processes and control many agronomic traits during plant growth and development. A factorial randomised pot experiment was performed in the net house to investigate the effect of 24-Epibrassinolide (EBL) on the growth, physiology, essential oil content, stomatal behaviour and trichome development of the three cultivars of peppermint. Four levels of foliage-applied EBL, viz. 0, 10(–5), 10(–6) and 10(–7) M were applied to the three cultivars of peppermint (Kukrail, Pranjal and Tushar). Among the different treatments of EBL, the application of 10(–6) M increased shoot length by 38.84, 37.59 and 36.91%, root length by 36.73, 29.44 and 33.47%, chlorophyll content by 24.20, 22.48 and 23.32%, P(N) by 32.88, 32.61 and 33.61%, EO content by 32.72, 30.00 and 28.84%, EO yield per plant by 66.66, 77.77 and 73.33% and menthol yield per plant by 127.27, 110 and 118.18% in Kukrail, Tushar and Pranjal respectively, compared with their respective control plants. Further, the 10(–6) M EBL exhibited improved trichome size and density, cellular viability and menthol content of the oil analysed from scanning electron microscopy, confocal laser scanning microscopy and GC–MS respectively as compared to the control. In conclusion, out of different levels of EBL, two sprays of 10(–6) M EBL proved effective in enhancing the morphophysiological features and productivity of mint plants, particularly for cultivar Kukrail.