The Effect of Plant Growth Promoting Bacteria in Hydroponics Cultivation Conditions on Iron Concentration, Yield and Phenolic Compounds of Lettuce

Introduction Over time, water deficit and environmental pollution by traditional agriculture that forces the producer to contribute to competitive and sustainable agriculture. Leafy vegetables are beneficial to human health, therefore, to adapt an eco-friendly approach in some vegetables, the partial substitution (25–50%) of mineral NPK by biofertilizers in lettuce improves the yield and agronomic features and produces healthy plants for human nutrition as well. Lettuce (Lactuca sativa L.) from Asteraceae family is considered as one of the most popular salad vegetables as a cool season crop. PGPB (Plant Growth-Promoting Bacteria) are rhizosphere bacteria that improve plant growth through a broad range of processes, i.e., phosphate solubilization, biological nitrogen fixation, siderophore manufacturing, phytohormone manufacturing, antifungal activity, systemic resistance induction and plant-microbe symbiosis promotion. The promoting of growth and yield of horticultural crops such as cucumber, potato, tomato and spinach by plant growth promoting bacteria inoculation at nutrient solutions under soilless systems have also been reported. Material and Methods In order to study the effect of growth-promoting bacteria on the yield, iron concentration and phenolic compounds of lettuce (Lactuca sativa cv. New Red Fire) under floating systems, the experiment was carried out in a completely randomized design with three replications in the Research greenhouse of University of Zanjan, during 2020. Experiment treatments consisted of five levels of PGPB (Pseudomonas vancouverensis, Pseudomonas koreensis, Pantoea agglomerans, Pseudomonas putida, and one level of combined bacteria (Pantoea agglomerans+ Pseudomonas koreensis + Pseudomonas putida+ Pseudomonas vancouverensis)) and control plant (without bacteria treated). Application of bacteria was done in two stages, one step before cultivation as seed inoculation and the next step as root inoculation. Lettuce plants grown in hydroculture condition with Hoagland nutrient solution. Growth conditions were environmentally controlled at a relative humidity of 60/70 % day/night and temperature was maintained between 20 and 17 °C. At 40 days after transplanting date, the lettuce head were harvested. The freshly harvested lettuce head were immediately weighed separately of each plant for fresh weight determination. Leaf samples were dried at 72 °C for 48 h in a drying oven and kept for further investigations. Also, leaf number per