Exogenous Application of Methyl Jasmonate Affects the Phytochemical Accumulation and the Antioxidant Activity in Hemp (Cannabis sativa L.) Inflorescences

Exogenous application of methyl jasmonate (MeJ) at different concentrations (0, 1, 10, and 100 µM) during flowering was studied for its impact on phytochemical profile, antioxidant activity, and biomass accumulation in hemp inflorescences of the monoecious cv. Codimono. MeJ treatments had no significant effect on CBD levels, while a 23–54% decrease in total terpene levels was observed in plants treated with 1 and 10 μM MeJ. In particular, MeJ treatments reduced β-caryophyllene and α-humulene levels by 24–43%, α-bisabolol levels by 30–40%, and α-pinene, β-pinene, and β-myrcene levels by 32–61%. By contrast, MeJ treatments had a positive effect on all other classes of phytochemicals analyzed. Plants treated with 100 μM MeJ experienced the highest increases in total flavonoid and phenolic acid levels (+42% and +50%, respectively). In particular, this treatment increased orientin, vitexin, and isovitexin levels by 36–52%, while ferulic acid level increased by 103%. Treatments with 10 and 100 µM MeJ resulted in the highest increases in total carotenoid and tocopherol levels (+41% and +33%, respectively). In particular, lutein, β-carotene, and α-tocopherol levels increased by 44%, 35%, and 36%, respectively. In line with these findings, total antioxidant activity increased by 26% following treatment with 100 μM MeJ and by 13% following the other two treatments. Interestingly, MeJ treatments did not affect plant growth and biomass accumulation in the inflorescences. This implies higher yields for those phytochemicals whose concentrations were increased by MeJ. In summary, our results indicate that hemp plants treated with 100 μM MeJ represent an interesting source of phytochemicals, fiber, and biomass. These characteristics make them suitable for multiple industrial applications and enhance both the economic and health-related value of this crop.