## 1 Introduction  
Effects of pre-sowing seed treatments with physical stressors, such as non-thermal or cold plasma (CP) and electromagnetic fields (EMF), are under intensive investigation due to their potential as innovative technologies applied in sustainable agriculture [1, 2, 3, 4, 5, 6]. It has been reported that such treatments can improve the agricultural performance of various crops, including stimulating germination and seedling growth [7, 8, 9, 10, 11, 12, 13, 14, 15, 16], and increasing seed yield [11, 13] and disease and stress resistance [8, 13, 14]. The response of seeds to pre-sowing treatments depends on a large variety of factors, such as technical parameters of used experimental devices and treatment conditions [1, 2, 3, 4, 5, 6], as well as plant species, seed coat structure, and seed physiological state, which depends on dormancy type, time of seed storage after harvesting, etc. [10, 15, 16]. In addition, most plant species show a continuous intra-individual variation for seed mass, seed morphology [17] causing differences in physiological (e.g., germination) traits and plant growth. Therefore, seed development is not temporally uniform in any given population, even when plants are grown in identical environments [18]. Unavoidably, seed heterogeneity enhances the scatter of the experimental results. Seed heteromorphism is considered as an adaptive plant strategy because seed morphs differ in their ecological behavior and such differences are favorable for survival of populations helping the plants cope with spatio-temporal variability of habitats (in contrast, seed uniformity is valued in agriculture). It has been demonstrated that the ability to germinate differs within a population of seeds heterogeneous by size, shape, weight, or color [18]. However, at present, little is known about impact of natural seed heteromorphism on the effects of seed treatments with CP and EMF although such information is of importance for the development of the reliable agrobiotechnologies based on plasma and EMF effects. In this respect, variation in the properties of the seed coat should also be considered. Seed coat comprises the structural barrier for seed interaction with any external factor, protects plant embryo from damaging effects, regulates gas, and water exchange, and is involved in germination control. Depending on plant species, seed coats can regulate seed dormancy [19; 20]. The seed coat's structure and coloration are particularly important for legume seeds that belong to a physical type of dormancy [21]. The presence of pigments and non-colored phenolic compounds can restrict germination since they make the seed coat harder or less permeable [22; 23; 24]. These compounds are essential for antioxidant defense (e.g., UV protection); thus, they could interact with free radicals, including those attacking seed surface upon exposure to CP [25]. It has been shown that the number of paramagnetic centers in Norway spruce seeds is increased after seed treatment with CP and EMF, indicating the production of stable organic radicals, most possibly, in a seed coat [26]. The dependence of similar CP-induced changes in radish seed EPR signal on seed color has been reported recently, indicating that the interaction of compounds in the seed coat with plasma particles may differ in seeds of different colors [27; 28]. Red clover (_Trifolium pratense_) is characterized by a considerable variation in seed color with monochrome yellow or purple as primary colors. However, the majority of seeds have an intermediate color due to various proportions of the two primary colors, so that seeds in each lot can be classified into 5 classes by color [29]. The brown color of red clover seeds was considered a hallmark of aging [30]. It has been reported that color of seed coat is an indicator of red clover seed quality and seedling growth ability. The germination parameters of bright yellow seeds were better, and their seedlings grew faster [30] and were more resistant to salt stress [31] compared to seedlings from dark purple seeds.