## 1 Introduction  
Many naturally-derived materials are recognized as biostimulants of plant growth, although their mechanism of action is still not completely understood [1]. Generally, five categories of biostimulants are distinguished: microbial inoculants, humic acids, fulvic acids, protein hydrolysates and amino acids, and seaweed extracts [2]. This paper focuses on the last category--algal extracts. Although the concept of the application of seaweed products in agriculture is still evolving, more literature on this subject has appeared, for example review articles by [PERSON] et al. (2009) [3], [PERSON] et al. (2014) [4], [PERSON] et al. (2015) [5], [PERSON] et al. 2014 [2], [PERSON] 2015 [6]. Polysaccharides (alignates, carrageenan, laminarin), micro- and macronutrients, sterols, betaines, phytohormones (cytokinins, auxins, abscisic acid, gibberellins), proteins, amino acids, and lipids are the main biologically active compounds detected in algal extracts that contribute to the plant growth promotion [3, 4, 6]. Special attention should be paid to the content of phytohormones in green macroalgae [7, 8]. Indoleacetic acid, which belongs to auxins, is responsible for the induction of elongation growth, apical dominance, and initiation of root formation; abscisic acid controls the stomatal apparatus function, gibberellic acid influences stem elongation and the initiation of seed germination, and polyamines regulate growth and development at micromolar concentrations [8]. The advantage of biostimulants is their activity (enhancement of plant growth and development) at low concentrations. This response cannot be attributed to the application of traditional plant nutrients [4]. Some authors suggest that seaweed extracts are bioactive at low concentrations--diluted as 1:1000 or more [9]. Extracts can be applied on soils, in hydroponic solutions or as foliar treatments [4; 6]. The first two application methods, increase water and low temperature stress resistance, improve modulation and promote plant growth promoting rhizobacteria. Additionally, algal extracts suppress soil borne diseases and nematodes [3; 4; 5]. Arial application of seaweed extracts (e.g., seed treatment, foliar spray) (a) affects improved shoot and root growth, higher flowering and fruit set, better yield and (b) improves the resistance to biotic (e.g., fungal, bacteria, viral pathogens, insect pests) and abiotic stress (e.g., salt, drought, freezing, chilling tolerance, as well as enhanced photosynthesis) in plants [3; 4]. Algal extracts can be obtained by different techniques, from classical solvent extraction with organic solvents, to the novel extraction techniques such as supercritical fluid extraction, as well as microwave-, enzyme- and ultrasound assisted extraction [10]. This paper focuses on algae-based products obtained with water as a solvent. Several methods of the isolation of active compounds from the algal biomass in aqueous environment can be distinguished, for example: boiling, autoclaving and homogenization (using blender) with distilled water [11]. In the literature it has been shown that such products have a positive effect on plant growth and development, for example in the works by [PERSON] et al. 2011 [12], [PERSON] and [PERSON] 2011 [13], [PERSON] et al. 2010 [14]; [PERSON] and [PERSON] 2012 [7]; [PERSON] et al. 2013 [15]; [PERSON] and [PERSON] 2009 [16]; [PERSON] et al. 2011 [17]. All the listed references concern macroalgae (_Chlorophyta_), which were used in the present study as a raw material for the production of algal bio-products. Baltic macroalgae are known to be a rich source of proteins, minerals, essential amino acids, essential fatty acids, fiber, and carbohydrates [18; 19]. In this work homogenates and filtrates were obtained from Baltic green seaweeds. The information on their production, properties and application is limited. Some of the examples are presented below.