Green manure and growth regulators in the production of biomass, essential oil and antioxidant capacity of Origanum vulgare L. (Lamiaceae)

Abstract

Oregano (Origanum vulgare L.) is one of the most commercially important species of the Lamiaceae family, as it has been widely used in the agricultural, pharmaceutical and cosmetic industries. Among the practices used to increase biomass and essential oil production in medicinal plants are: green manuring (GM) and foliar application of plant regulators. The present study aimed to: (1) evaluate the effect of green manuring (Crotalaria juncea L.) on biomass production, essential oil chemical composition, accumulation of phenolic compounds and antioxidant activity of O. vulgare and (2) observe the influence of foliar application of plant growth regulators (PGRs) on biomass production, secondary metabolites and enzymatic antioxidant activity of O. vulgare. The green manure experiment consisted of eight treatments: control (soil); 200 g pot-1 (crotalaria root); 150, 300, 450 and 600 g pot-1 sunn hemp aerial part (Sh=leaf+stem) + 200 g pot-1 (crotalaria root); 600 g pot-1 aerial part of crotalaria (Sh); and positive control (cattle manure 300 g pot-1). For PGRs, 3 types were used (Giberellin-GA3, Indolebutyric acid-IBA, Kinetin-KIN) with 3 concentrations (25, 50, 100 mg L-1) + control. Dry weight, photosynthetic pigments, essential oil content and yield and antioxidant activity were evaluated. The GM management provided the highest dry weight averages at doses 300, 450 and 600 g (Sh+R) and 600 g pot-1 (Sh), as well as cattle manure (300 g). Treatment without GM (control) resulted in a 75% lower essential oil production compared to treatment 450 g pot-1 GM (Sh+R) and approximately 71% compared to treatments 300, 600 g GM (Sh + R) and 600 g pot-1 GM (Sh). Regarding the experiment with PGRs, the application of GA3 at a dose of 25 mg L-1 increased the dry weight of the leaf, stem, root, shoot and total dry weight in relation to the control. The highest chlorophyll a content was observed for the KIN25 dose. The gains in oil content with the KIN25 and IBA25 doses were 30.9 and 29.6%, respectively, in relation to the control and the highest average yields were observed at the lowest doses. The lowest average levels of hydrogen peroxide (H2O2) and malondialdehyde (MDA) were observed in the presence of PGRs. PGRs stimulated enzymatic antioxidant activity, signaling protection against oxidative stress. In conclusion, the use of C. juncea as GM is a low-cost resource that has a positive effect on the cultivation of O. vulgare, and its use can reduce the application of chemical fertilizers, making it an ecologically appropriate strategy for the cultivation of medicinal plants. The foliar application of PGRs at low doses may contribute to increase dry matter and chemical constituents in O. vulgare.