"Effects of Soil pH, Composition, and Rare Earth Elements on Plant Growth and Their Interactions with Herbivorous Insects."

Urbanization has significantly altered soil composition, leading to contamination by rare earth elements (REEs) such as lanthanum (La) and cerium (Ce), which can affect plant growth and interactions with herbivorous insects. This thesis investigates how soil composition, REE contamination, and pH variations influence plant development and plant-insect dynamics through soil-based, hydroponic, and insect feeding experiments. The research hypothesizes that both soil contamination and pH stress will negatively impact plant health and modify insect herbivory patterns. Six plant species were selected for soil-based experiments: Plantago major (Broadleaf Plantain), Solanum lycopersicum(Tomato), Poa pratensis (Kentucky Bluegrass), Taraxacum officinale (Dandelion), Lolium perenne (Perennial Ryegrass), and Medicago sativa (Alfalfa). Soils were collected from three urban parks near Turin—Mandria, Pellerina, and Parco Arte Vivente—with differing pH levels, nutrient compositions, and potential REE contamination. A total of 117 pots were prepared, each with five biological replicates per plant species and three control pots per soil type. The soil water content was maintained at 80% of the field capacity. Delays in plant growth and non-sprouting pots were observed, particularly in Mandria soil. At the end of the experiment, plants were harvested for biomass analysis, with root and shoot samples collected, and the rhizosphere preserved for further microbial and chemical evaluation. In the hydroponic experiments, S. lycopersicum and P. major were grown under three conditions: a control solution and two pH stress treatments (pH 8.2 with NaOH and pH 8.2 with carbonate). The hypothesis that pH stress would reduce plant growth was confirmed, with both shoot and root lengths significantly shorter under stress conditions. The carbonate stress, in particular, had a severe impact on root biomass. These results indicate that both pH and the type of stress play a critical role in plant development, with carbonate stress causing the most pronounced growth reduction. The interaction between rare earth elements and herbivorous insects was examined by growing Brassica rapa in soil treated with 1 μM, 1000 μM, and 10,000 μM concentrations of La and Ce. After seven days of treatment, third-instar larvae of Spodoptera littoralis were introduced to feed on the plants. Larval weight gain was measured after 24 hours to assess the impact of La and Ce contamination on insect herbivory. The results showed that higher concentrations of La and Ce significantly reduced larval weight gain, indicating potential toxicity or deterrence, which may have implications for insect-plant interactions in contaminated environments. Data analysis was performed using Python libraries (Pandas, Matplotlib, Seaborn, Pingouin) for visualization, and statistical analysis was conducted using one-way ANOVA with Tukey post hoc tests for non-parametric data. This research provides valuable insights into the ecological effects of urban soil contamination and its impact on plant growth and herbivorous insect dynamics. The findings offer practical applications for urban soil management and strategies to mitigate the negative effects of REE contamination on local ecosystems.