Foliar Application of Three Biostimulants in Two Time Intervals Affects Crop Yield and Chemical Parameters of Melissa officinalis L.

Lemon balm is a medicinal and aromatic plant which is commonly known for its antimicrobial, antiviral, antioxidant, antianxiety and anti-inflammatory activities. Literature reports that the intrinsic traits of this species can be greatly affected by several factors, producing relevant variations. Particularly, the crop yield and synthesis of secondary metabolites are affected by abiotic factors, such as temperature, drought and salt stresses. The application of biostimulants, complementing the conventional agronomic practices, allows to reduce the impact of stress conditions on yield and enhance the quality standards of the plant. With this in mind, a 2-year study was carried out with the aim to investigate the effects of three different biostimulant formulations such as Ascophyllum nodosum, fulvic acids, and vegetal protein hydrolysate, with two intervals of application, on morphological traits, biomass yield and chemical parameters of lemon ball. The experimental setup was a randomized complete block design with three replicates. The foliar application and application interval, as main factors, significantly affected almost all parameters in the study. In general, all biostimulants improved plant growth. The best results in terms of fresh and dry biomass were observed in plants treated with fulvic acids every 14 days. An increase of over 5 t ha−1 for fresh yield and over 1 t ha−1 for dry yield were recorded. Antioxidant activity and total phenolic content were positively affected by biostimulant and interval of application. The results highlight that few applications of biostimulants can greatly improve the yields of lemon balm and, in some cases, enhance the properties of its extracts.