## 1 Introduction  
Saliinity constitutes a severe constraining factor of plant growth since it is associated with a combination of osmotic, ionic and, secondarily, oxidative stress [1]. Various undesirable effects appear because of high salt concentration. Salt stress alters the structure and composition of cell plasma membrane, especially lipid and protein contents, as well as cell plasma membrane cytoplasmic viscosity and composition [2, 3]. Moreover, ion imbalance is one of the major consequences. A high concentration of Na and Cl ions, as an example, can lead to biochemical processes which can prove to be fatal for the plants [4, 5, 6]. Sodium and chloride toxicity not only induce nutritional disorders but also cause physiological drought by lowering the osmotic potential of the soil solutions [7]. Soil salinity prevents the plant from taking up water from the soil, resulting in a decline in cellular water, thus affecting cell turgor. Soil salinity also adversely affects photosynthetic activity in the plant and encourages the production of reactive oxygen species (ROS), thus reducing plant growth [8, 9]. Gas exchange is inversely related to the concentration of Na and Cl ions [10, 11], and parameterssuch as photosynthesis, stomatal conductance, and transpiration tend to be negatively influenced by saline treatments [12; 13]. Moreover, higher salt levels are also known to alter photosynthesis via non-stomatal limitations, including variations in photosynthetic enzyme activity and changes in the concentration of chlorophyll and carotenoids [14; 15; 16]. In parallel, soil salinization evolves into a crucial matter which leads to land degradation [17], while there are estimations that about 20% (45 million ha) of irrigated land worldwide is affected by salinity [18]. Consequently, large areas are kept out of the production process rendering the necessity to improve this situation of vital meaning for the survival of the rapidly increasing population of Earth [19]. Temperate fruit trees, including apples, are salt-sensitive plants [20]. Apple cultivation under salt stress may result in a decline of fruit yield and quality. Several researchers suggest that salt stress impedes apple plant growth by decreasing water uptake, inhibition of photosynthesis, and by closing stomatal apertures [21; 22]. Various strategies have been proposed in order to moderate the negative impact of problematic soils on plant cultivation, such as the use of biostimulants and soil amendments (substrate additives). According to [PERSON] [23], \"a plantbiostimulant is any substance or microorganism applied to plants with the aim to enhance nutrition efficiency, abiotic stress tolerance and/or crop quality traits, regardless of its nutrients content\". Thus, photosynthetic bacteria have been extensively used in agriculture to improve plant growth and production quality [24], while it has been found that spraying _Rhodospseudomonas palustris_ GJ-22 strain on tobacco leaves can protect plants from tobacco mosaic virus infection [25]. Other microorganisms, such as plant growth promoting bacteria (PGPB), also have the capacity to increase yield and reduce biotic and abiotic plant stress without conferring pathogenicity [26; 27]. Additionally, products with lactic acid bacteria (LAB) are a common alternative in agricultural systems for improving soils and enhancing plant growth [28]. Regarding crop yield, seaweed extracts represent another perspective seeing that in many countries they are used as fertilizers [29]. It has been deemed that their effect derives from a combination of several constituents encompassing a number of macro- and micronutrients, plant growth regulators, and quaternary ammonium compounds [30; 31], while many studies have shown that the implementation of seaweed extracts contributed to better crop performance and elevated resistance to abiotic stress [32]. In this direction, betaines contained in seaweed products serve as a compatible solute that alleviates osmotic stress of plants induced by salinity and drought stress [30].