## 1 Introduction  
_Melissa officinalis_ L., which is also known as lemon balm, can be easily recognized due to its lemon scent. It is a perennial herbaceous plant belonging to the Lamiaceae family [1]. The cultivation of lemon balm requires a temperature between 15 \({}^{\circ}\)C and 35 \({}^{\circ}\)C, and 500 to 600 mm rainfall per year [2]. Like other plants from this family, lemon balm contains high concentrations of phenolic acids, which is mainly caffeic acid known as the principal compound in the synthesis of rosmarinic acid. These phenolic compounds contribute to the medicinal properties of lemon balm, which is associated to its antioxidant capacity [3]. The main active constituents of _M. officinalis_ L. are volatile compounds (e.g., gerani, neral, citronellal, and geraniol), triterpenes (e.g., unsolic acid and oleanolic acid), and phenolics (e.g., caffeic acid derivates: luteolin, naringin, and hesperidin) [4]. Lemon balm herb (Melissae herba) and lemon balm leaves (Melissae folium) are the main parts of the plant used in phytotherapy as digestive antispasmodic and antiviral remedy [5]. _M. officinalis_ L. has anti-inflammatory activity, which is used in traditional medicine to treat various diseases associated with inflammation and pain. Recent studies indicate the beneficial effects of the plant: anti-bacterial, spasmolytic, memory improvement, sedative, stress, and anxiety reductant [6].  
Polyphenols are widely distributed products in the plant kingdom, which are found in fruits, vegetables, tea, and coffee [7]. They possess a wide variety of physiological effects such as anti-inflammatory and anti-allergenic but mostly antioxidant. These compounds can also be found in the bark of different tree species [8]. Among many other varieties of spruce, _Picea abies_ (L.) [PERSON] is specific to the Carpathian region (Romania). Spruce wood is commonly used in wood and the pulp industry while the bark is an important source of tannins, resins, and essential oils utilized in chemical, leathering, cosmetics, and pharmaceutical industries [9]. Likewise, beech (_Fagus sylvatica_ L.) is a common and widely used material in the wood industry, but the bark of these species are still regarded as by-products [10]. Polyphenols have a significant role in the protection of living cells against biotic (fungi, infections, diseases) and abiotic (wounds, drought, and pollution) factors. Polyphenols are known for their antioxidant capacity [11; 12]. The effects of polyphenolic extracts as antioxidant agents, plant growth regulators, or bioremediation agents have been successfully tested [12; 13; 14; 15]. On the other hand, spruce bark can be used as natural fungicide, biodegradable, and less toxic than the synthetic fungicides [16], because of its rich content in tannins [17].  
Taking into account all of this information, the aim of the study was to demonstrate the bioreactor effects of the two extracts (spruce bark polyphenolic extract-SBPE, and beech bark polyphenolic extract-BBPE), on the growth and development of _Melissa officinalis_ L. Therefore, the physiological (germination, growth, and development) and structural modifications (vascular tissue, lignification, etc.) of the treated plants were analyzed and compared to the control plants. The objectives of the study were to obtain new bioregulator products that fit into nature and enter the natural cycle of the materials, to identify the influence of the natural bark extracts on the growth and development of lemon balm plants, and to orient the research toward an ecological agriculture for sustainable development.