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

Abstract

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 plant, chlorophyll and carotenoids contents, Fe concentration, total phenol, total flavonoids and anthocyanin contents were measured. ResultsThe obtained results in the current study indicated that the application of PGPB on lettuce caused significant increase in growth, photosynthetic pigments and iron concentration. The maximum growth rate and photosynthetic pigments content was observed in combined four bacteria treatment, so that, an increase of 388.2% chlorophyll a, 439.8% chlorophyll b, 398.3% total chlorophyll, 246.3% carotenoids contents, 42.6% plant fresh weight and 22.2% number of leaves was obtained compared to control plants. Plant Growth-Promoting Bacteria (PGPB) can enhance growth and development of plants. PGPB have direct and indirect influences on plant growth process. The immediate promotion of growth involves either supplying the plant with a compound produced by the bacteria, i.e., phytohormones, or promoting certain nutrient uptake from the setting. Whereas, the indirect plant growth promotion happens when PGPB decreases or prevents the deleterious impacts of one or more phytopathogenic species. Plants inoculated with PGPB showed higher leaf iron concentration compared to control plant. Thus inoculation with combined four bacteria induced a 26.2 % increase of lettuce leaves iron concentration. The obtained results in the current study revealed that the inoculation with PGPB significant decreased the total phenol, flavonoid and anthocyanin contents. The maximum content of phenol (483 µg g-1FW), flavonoid (188.1 µg g-1FW) and anthocyanin (27.5 µmol g-1FW) were observed in control plants compared to treated plants. Conclusion According to the results of this research, the use of PGPB in the hydroculture system, on the one hand, led to a significant increase in iron absorption, the synthesis of photosynthetic pigments, and subsequently promote growth and increases lettuce yield. On the other hand, due to facilitating the growth conditions and increasing the absorption of nutrients for the host plants as a result of inoculation with PGPB, led to a decreases of phenolic compounds including total phenol, total flavonoid and anthocyanin contents.

Keywords

• anthocyanin
• hydroponics
• iron
• pgpr
• phenolic compounds