Since hydroponic systems are mainly established in protected environments (i.e. greenhouse, PFAL) foliar application of biostimulants can be applied regardless of outdoors environmental conditions while crops show high uniformity in size. Moreover, in liquid hydroponic systems, biostimulants could be added directly in the nutrient solution.

Different categories of biostimulants can be used with positive results in hydroponic systems. The use of digestate as a promising biostimulant has been recently reported [2, 3], since it is rich in both organic matter and nutrients and can be easily applied in a floating system. The results demonstrated that adding biostimulants it is possible to reduce the needs of inorganic fertilizer without imposing a negative effect on the yield and quality of lettuce. Moreover, the preliminary results demonstrated a lower nitrate level in leaves with the use of digestate, which is encouraging for the growers to satisfy EC regulation 1258/2011 for the acceptable limit of nitrates in lettuce. Consideration should be given in pH adjustment, the optimum range of which is 6-7.2 for nutrients availability, as well as in the physicochemical characteristics of the digestate.

Protein hydrolysates is a major category of plant biostimulants based on protein sources derived by partial hydrolysis [4]. An important advantage is the option to be applied either as foliar sprays or in the nutrient solution. In a study with green butterhead and red crisphead lettuce grown in floating system, the application of a legume-derived protein hydrolysates biostimulant applied both in the foliage and in the nutrient solution boosted their yield while having a positive impact on plant physiology and quality parameters [5]. Similarly, the use of amino acids in the nutrient solution, even at reduced minerals concentration, enhanced the yield as well as improved the nutritional quality and safety, especially by reducing nitrates’ accumulation in the tissues [6, 7].

Moreover, the beneficial effect of plant biostimulants has been established also in classic hydroponic systems with substrate mainly for fruit vegetables’ production. In particular, two sweet yellow peppers grown in rock wool substrate under reduced concentration of nutrients (30% of standard solution), with the addition of four biostimulants, resulted in the increase of yield and the improvement of their nutritional quality in terms of antioxidant activity, vitamin C and phenolic contents in fruits [8]. In another study involving hydroponically cultivated tomato under salt stress, a combination of amino acids inhibited the negative effects of salinity due to an increased accumulation of total soluble sugars [9].

In addition, Plant Growth-Promoting Rhizobacteria (PGPR) is a very attractive category of microbial biostimulants, which enhance nutrient uptake and increase important quantitative and qualitative parameters of the crops (i.e., yield, quality), as well as improve their tolerance to abiotic stress factors [10]. Even though the study of PGPR application is mainly focused under soil conditions, a few trials in hydroponic systems showed an increase of yield for a number of vegetables (tomato, cucumber, pepper) with the use of bacteria from the genus Pseudomonas and Bacillus [11-13].

A major concern in EU is the reduced consumption of fruits and vegetables, below the suggested amounts by World Health Organization, even though several projects aim to encourage young people to improve their eating habits. Therefore, crop biofortification programs have been proposed as a tool for increasing the daily intake of health-promoting substances. A recent review [14] has reported many examples of successful biofortification in hydroponic systems, and a key point is the simultaneous application of biostimulants on the availability and interactions of nutrients in the nutrient solution

Overall, hydroponic systems are expected to be in the foreground of the agriculture sector, especially for vegetable production, as an environmentally friendly technology and a valuable tool in terms of food security. A major advantage of plant biostimulants’ application in hydroponic systems is the flexibility for application either in the nutrient solution or as a foliar spray, which minimizes the cost; it is a simple procedure that allows to reduce nutrient concentration in the solution and enhance the yield with improved quality characteristics. However, further studies are required to clarify the optimum manner of biostimulants’ use in hydroponic systems (type, dose, application form, etc.) for enhancing crop performances in different growing conditions.