Titanium Nanoparticle and Humic Acid Applications Improve Seed Germination, Growth Development, and Phytochemical Contents of Lettuce (Lactuca Sativa) Plants

Nanofertilizers and nano-agrochemicals are widely used to increase yield and improve plant growth characteristics in the agricultural field. However, the impact of titanium dioxide nanoparticles (TiO₂NPs) on plants growing in humic acid (HA) amended soils has not been investigated before. Therefore, this study aims to determine the effects of TiO₂NPs and HA on the growth, yield, and nutritional values of lettuce plants. Lettuce seeds and plants were treated with TiO₂NPs suspension (0, 30, and 60 mg/L) and HA solution (0, 3, and 6 g/L) at different concentration levels. The plants were planted in the Randomized Complete Block Design (RCBD) under greenhouse conditions. Overall, TiO₂NPs and HA applications improved seed germination rate, plant biomass, and phytochemical contents of the plants. In particular, the integrated TiO₂NPs at 30 mg/L and HA at 60 g/L applications significantly increased seed germination (96.49%), plant yield (1123.0 g), and biomass (1159.0 g), as well as antioxidant activity (TAA) (5.25 μg/g FW) and nitrogen (5.08%) contents compared with the control treatment. TiO₂NPs and HA applications affected lettuce plant physiology by increasing the plant biomass, as well as biochemical pathways by increasing bioactive compound contents. Interestingly, the phytochemical contents of total polyphenol content (TPC), flavonoid content (TFC), and TAA contents of lettuce plants increased significantly; this indicates that the TiO₂NPs and HA applications induce the defense system of the plants. Thus, integrated TiO₂NPs and HA applications can be utilized to improve seed germination, growth, yield, and nutritional values of plant species that are nutritionally preferred and also to enhance plant resilience to different environments.