Gibberellins (GAs) are growth hormones strongly involved in a wide variety of physiological activities. Currently, gibberellins are commercially used to enhance phenotypic characteristics, earliness, and productivity of many vegetable and ornamental crops. In this work, the efficacy of supplementation of low levels of gibberellic acid (0, 10⁻⁸, 10⁻⁶, and 10⁻⁴ M GA₃) through the mineral nutrient solution of a floating system on yield and quality of leaf lettuce and rocket plants was tested. The marketability of plants was lost when 10⁻⁴ M GA₃ was added to the mineral nutrient solution. This study demonstrated that the addition of 10⁻⁴ M GA₃ exceeded the acceptable threshold for use in hydroponics production systems. Below the concentration of 10⁻⁴ M, the presence of GA₃ in the mineral nutrient solutions (MNS), especially at 10⁻⁶ M GA₃, stimulated plant growth and enhanced the yield. Various morphological and physiological traits were enhanced by GA₃ treatments (biomass accumulation, leaf expansion, stomatal conductance, water use efficiency (WUE), Nitrogen use efficiency (NUE), etc.), with superimposable trends in both lettuce and rocket. The addition of 10⁻⁶ M GA₃ to the nutrient solution of a hydroponic floating system can promote growth and quality of lettuce and rocket plants.

Mediterranean areas with intensive agriculture are characterized by high salinity of groundwater. The use of this water in hydroponic cultivations can lead to nutrient solutions with an electrical conductivity that overcomes the tolerance threshold of many vegetable species. Plant growth-promoting rhizobacteria (PGPR) were shown to minimize salt stress on several vegetable crops but the studies on the application of PGPR on leafy vegetables grown in hydroponics are rather limited and have not been used under salt stress conditions. This study aimed to evaluate the use of plant growth-promoting bacteria to increase the salt tolerance of leaf lettuce grown in autumn and spring in a floating system, by adding a bacterial biostimulant (1.5 g L⁻¹ of TNC Bactorr^S13 a commercial biostimulant containing 1.3 × 10⁸ CFU g⁻¹ of Bacillus spp.) to mineral nutrient solutions (MNS) with two salinity levels (0 and 20 mM NaCl). Leaf lettuce plants showed a significant reduction of growth and yield under salt stress, determined by the reduction of biomass, leaf number, and leaf area. Plants showed to be more tolerant to salinity in autumn than in spring. The inhibition of lettuce plant growth due to salt stress was significantly alleviated by the addition of the bacterial biostimulant to the MNS, which had a positive effect on plant growth and fresh and dry biomass accumulation of the unstressed lettuce in both cultivation seasons, and maintained this positive effect in brackish MNS, with similar or even significantly higher values of morphologic, physiologic, and yield parameters than those recorded in control unstressed plants.

Vegetable plants are more sensitive to salt stress during the early growth stages; hence, the availability of poor-quality brackish water can be a big issue for the nursery vegetable industry. Microbial biostimulants promote growth and vigor and counterbalance salt stress in mature plants. This study aimed to evaluate the application of plant growth-promoting microorganisms for improving salt tolerance of lettuce and tomato seedlings irrigated with different water salinity levels (0, 25, and 50 mM NaCl) during nursery growth. Two commercial microbial biostimulants were applied to the substrate before seeding: 1.5 g L⁻¹ of TNC Bactorr^S13 containing 1.3 × 10⁸ CFU g⁻¹ of Bacillus spp.; 0.75 g L⁻¹ of Flortis Micorrize containing 30% of Glomus spp., 1.24 × 10⁸ CFU g⁻¹ of Agrobacterium radiobacter, Bacillus subtilis, Streptomyces spp. and 3 × 10⁵ CFU g⁻¹ of Thricoderma spp. Many morpho-physiological parameters of lettuce and tomato seedlings suffered the negative effect of salinity. The use of the microbial biostimulants modified seedling growth and its response to salt stress. They had a growth-promoting effect on the unstressed seedlings increasing fresh and dry biomass accumulation, leaf number, and leaf area and were successful in increasing salinity tolerance of seedlings especially when using Flortis Micorizze that enhanced salinity tolerance up to 50 mM NaCl. The inoculation of the substrate with microbial biostimulants could represent a sustainable way to improve lettuce and tomato transplant quality and to use brackish water in vegetable nurseries limiting its negative effect on seedling growth.

Plant growth-promoting rhizobacteria have been applied to different vegetable crops but there is still no information on the effect of bacterial biostimulant application under variable nutritional level on lettuce seedlings and their performance after transplanting in the field. This study aimed to evaluate the efficacy of a bacterial biostimulant to enhance growth and quality of lettuce seedlings fertigated with increasing nutrient rates and to assess the efficacy of these treatments on lettuce head production. Lettuce seedlings were inoculated with 1.5 g L⁻¹ of TNC Bactorr^S13 (a commercial biostimulant containing 1.3 × 10⁸ CFU g⁻¹ of Bacillus spp.) and fertigated with a nutrient solution containing 0, 1, 2, and 4 g L⁻¹ of NPK fertilizer (20-20-20). At the end of transplant production, the plants were evaluated for greenhouse cultivation. The effect of fertigation rate on seedling height, dry biomass, dry matter percentage, and water use efficiency was evident up to 2 g L⁻¹ of fertilizer in the non-inoculated seedlings, whereas fresh biomass and nitrogen use efficiency changed up to 4 g L⁻¹ of fertilizer. The use of the bacterial biostimulant modified seedling growth and its response to nutrient availability. The inoculation of the substrate with Bacillus spp. promoted plant growth and allowed seedlings to reach the highest height and biomass accumulation. The physiological age of lettuce seedlings showed a strong influence on plant growth and production after transplanting. The bacterial treatment positively affected the yield and nitrate content of lettuce plants.

Molybdenum (Mo) is required in enzymes involved in a number of different metabolic processes, and is crucial for the survival of plants and animals. The influence of nutrient solutions containing four levels of molybdenum (0, 0.5, 1.5, and 3.0 µmol/L) on growth, yield, and quality of lettuce, escarole, and curly endive grown in a hydroponic floating system was evaluated. Biometric, nutrient, and quality analyses were conducted to assess the response of each species to Mo. The results demonstrated that molybdenum is essential for harvesting marketable plants. Lettuce, escarole, and curly endive plants differed significantly in their response to molybdenum fertilization. The increase of Mo concentration in the nutrient solution was not harmful for plants and had no influence on yield and morphological traits of the leafy vegetables; however, it significantly affected some quality characteristics. Mo fertilization raised the nutritional quality by increasing ascorbic acid content up to 320.2, 139.0, and 102.1 mg kg⁻¹ FW (fresh weight), and reducing nitrate content down to 1039.2, 1047.3, and 1181.2 mg kg⁻¹ FW for lettuce, escarole, and curly endive, respectively. The addition of Mo in the nutrient solution increased the Mo content of plants up to 0.50, 4.02, and 2.68 μg g⁻¹ FW for lettuce, escarole, and curly endive, respectively. Increasing Mo supply to lettuce, escarole, and curly endive up to 3.0 µmol L⁻¹ could lead to a higher nutritional quality with no significant morphological alteration or yield loss.

Hydroponics need water of good quality to prepare a balanced nutrient solution that could allow plants to reach their maximum yield potential. The rising difficulties in finding water with good quality have led to the compelling necessity of identifying sustainable ways to use saline water, limiting its negative effect on crop yield and quality. The exogenous supplementation of plant growth regulators, such as gibberellic acid (GA₃), can be effective in increasing plant growth and vigor, thus helping plants to better cope with salt stress. The aim of this study was to evaluate the feasibility to increase the salt tolerance of leaf lettuce and rocket grown in a floating system by adding GA₃ (10⁻⁶ M) to mineral nutrient solutions (MNS) with increasing salinity (0, 10, and 20 mM NaCl). Leaf lettuce and rocket plants suffered a significant reduction of growth and yield, determined by the reduction of biomass, leaf number, and leaf area, even with moderate salt stress (10 mM NaCl). The supplementation of exogenous GA₃ through the MNS allowed plants to substantially counterbalance salt stress by enhancing various morphological and physiological traits, such as biomass accumulation, leaf expansion, stomatal conductance and water and nitrogen use efficiency. The effects of salt stress and GA₃ treatment varied according to the species, thus indicating that this interaction may improve salt tolerance by activating different adaptation systems.