Enhancing sustainable sweet grass (Lippia dulcis L.) productivity and essential oil content using sunn hemp (Crotalaria juncea L.) as green manure

The application of green manure can sustain biological diversity while maintaining high soil fertility. Therefore, using green manure is a crucial practice for minimizing the environmental and ecological impact of sustainable agricultural development. Lippia dulcis is a medicinal plant used to treat coughs, colds, and bronchitis, as well as having potential antidiabetic and sweetening properties. The aim of the current study is to assess the influence of Crotalaria juncea green manure on Lippia dulcis vegetative growth and essential oil accumulation, as well as on total phenolic compounds and verbascoside levels. The experiment was conducted in the field, based on six treatments, namely: soil and sand substrate (negative control); chemical fertilizer (positive control); as well as 150; 300; 450; and 600 g of green manure in pot 10 L. Dry weights, leaf analysis, photosynthetic pigment levels, total phenolics, total flavonoids, verbascoside, as well as essential oil content, yield and chemical components were assessed. The treatment comprising 600 g of green manure recorded the most significant results for plant biomass. On the other hand, doses of 450 g and 600 g of green manure yielded the best results in terms of essential oil content and yield. In contrast, the control treatment recorded the best results for verbascoside and total flavonoids. Chemical fertilizer and 300 g green manure were the treatments that recorded the most efficient results for phenolic compounds. Using green manure instead of conventional fertilization had a positive influence on the attributes analyzed in the species Lippia dulcis, and it is a cheaper and more sustainable strategy for growing medicinal plants.