## 1 Introduction  
The term allelopathy is derived from the Greek words _allelon_ (one to another) and _pathos_ (to suffer), indicating a chemical inhibition of one species towards another. Allelopathy is defined as the chemical interaction of plants through the mediation of chemical compounds secreted by individual plants. Such chemical compounds are called allelochemicals. Allelochemicals are secondary metabolites which might exercise the positive or negative influence of a plant on the growth and development of other plants [1; 2; 3; 4]. Essentially, allelopathy is a biological phenomenon by which a plant, fungus or microorganism produces allelochemicals that, when released into the environment, have an inhibiting or stimulating effect on the growth and development of other organisms [5; 6; 7]. Allelochemicals are in fact secondary metabolites, and they act as biostimulators, i.e., promoters or inhibitors of plant growth and development [8; 9]. Plant species produce allelochemicals in all tissues and their release plays a significant role in natural ecosystems, in the development of plant communities [10; 11] and in agroecosystems [12]. The significance of allelopathy in agricultural production is manifested in the possibility of its application in order to improve soil properties by decomposing plant residues, to increase the diversity of crops by applying it in crop rotation, and to improve the resistance of crops to abiotic factors. However, it is especially important in the control of weeds, diseases and pests, whether directly or by developing biopesticides. The inclusion of allelopathy in integrated or ecological plant protection programs contributes to reducing the use of chemical agents, and thus the occurrence of numerous problems, such as the resistance of an increasing number of pests and pesticide residues, as well as negative effects on the environment and human health. At the same time, it makes it possible to meet the needs of the market for food produced in an environmentally friendly way [13; 14; 15; 16]. Traditionally, allelopathy was considered as a bilateral interaction between two plants or plant species. However, allelopathy is now considered as a process within the plant kingdom that can affect and be modulated by other organisms in the plant's environment [17]. Unfortunately, there are no recent studies concerning the allelopathic effects of chia, black cumin, wormwood herbs, and nettle aqueous extracts on seed germination and seedling growth of lettuce, spinach, and pepper. According to [PERSON] and [PERSON], 2015 [18], the plant allelopathy management practices applied in agriculture are determined by four the most important points: (1) a description of management practices related to allelopathy and allelochemicals in agriculture; (2) a discussion of the progress regarding the mode of action of allelochemicals and the physiological mechanisms of allelopathy, consisting of the influence on cell micro- and ultra-structure, cell division and elongation, membrane permeability, oxidative and antioxidant systems, growth regulation systems, respiration, enzyme synthesis and metabolism, photosynthesis, mineral ion uptake, and protein and nucleic acid synthesis; (3) an evaluation of the effect of ecological mechanisms exerted by allelopathy on microorganisms and the ecological environment; and (4) a discussion of existing problems and proposals for future research directions in this field to provide a useful reference for future studies on plant allelopathy. The aim of this work was to investigate the possible allelopathic influence of aqueous extracts, obtained from plant species chia (_Salvia hispanica_ L.), black cumin (_Nigella sativa_ L.), wormwood (_Artemisia absinthium_ L.), and nettle (_Urtica dioica_ L.), on the seed germination and morphometric characteristics of pepper (_Capsicum annuum_ L.), spinach (_Spinacia oleraca_ L.) and lettuce (_Lactua sativa_ L.) seedlings in laboratory conditions.