Background and Objective: In recent years, medicinal plants have attracted considerable attention from researchers due to their bioactive compounds and effective role in promoting human health. Field sowthistle (Sonchus arvensis L.) is one of the plants that holds particular importance owing to its diverse therapeutic effects, including antihypertensive, anticancer (due to the presence of compounds such as chlorogenic acid and chicoric acid), and antimalarial activities. A significant portion of these properties is attributed to the presence of phenolic and flavonoid compounds. The application of natural biostimulants and plant growth regulators is considered an effective approach to improving the phytochemical quality of medicinal plants within the framework of sustainable agriculture. This study was conducted to evaluate the effects of salicylic acid and the biostimulant Kitoplus on morphological characteristics and the accumulation of secondary metabolites in Sonchus arvensis L.

Materials and Methods: This research was carried out as a two-factor factorial experiment based on a completely randomized design with three replications in the research greenhouse of the Faculty of Agriculture, University of Zanjan. Seeds were sown in a 1:1 mixture of peat moss and perlite, and seedlings were grown under controlled conditions (18/25°C and 60–70% humidity) until the 3–4 leaf stage. They were then transplanted into pots containing 15% sand, 15% manure, and 70% soil. Foliar treatments with salicylic acid (0, 0.5, and 1 mM) and Kitoplus (0, 0.5, and 1%) were applied every 10 days in three sessions. Control plants were treated with distilled water. Two weeks after the final treatment, morphological traits including plant height, number of branches, internode distance, and leaf area were measured in treated and control plants. Soluble sugar content was determined using the phenol-sulfuric acid method with glucose as the standard, and results were expressed as mg of soluble sugars per gram of dry weight. Spectrophotometric measurements were performed at wavelengths of 380, 220, 325, and 260 nm to determine the contents of quercetin, gallic acid, chlorogenic acid, and rutin, respectively. Data analysis was conducted using SAS software version 9.1.

Results: The results of this study showed that the combined application of different concentrations of salicylic acid and Kitoplus significantly affected vegetative growth patterns and the accumulation of secondary metabolites in Sonchus arvensis L. Regarding morphological traits, increasing the concentrations of both treatments resulted in a reduction in the number of leaves, branches, and internodes, whereas the greatest plant height was observed with the 1% Kitoplus treatment, indicating stimulation of longitudinal growth and a potential shift in resource allocation toward stem elongation rather than lateral growth. In contrast, salicylic acid, particularly at a concentration of 1 mM, increased leaf area and improved the photosynthetic capacity of the plants, which may be attributed to enhanced stomatal activity and improved photosynthetic efficiency. From a biochemical perspective, the combined treatment of 0.5 mM salicylic acid and 1% Kitoplus significantly increased the concentrations of soluble and total sugars, indicating enhanced carbohydrate metabolism and increased accumulation of storage compounds in the plants. Regarding secondary metabolites, the results showed that all compounds evaluated, including chlorogenic acid, quercetin, gallic acid, and rutin, were significantly increased in response to the treatments. The highest chlorogenic acid content, 0.809 mg/g dry weight, was recorded under the 0.5 mM salicylic acid treatment, representing a 20.39% increase compared with the control. The highest quercetin content (2.964 mg/g dry weight), as well as the highest rutin content, was obtained with the 1% Kitoplus treatment, whereas the highest gallic acid content was observed under the combined treatment of 1 mM salicylic acid and 1% Kitoplus. Overall, the findings of this study indicate that the application of salicylic acid and Kitoplus as biostimulants can induce positive changes in morphological traits and significantly enhance valuable secondary metabolites. Therefore, their application may be considered an effective strategy for improving the quality and quantity of plant production.

Conclusion: The findings of this research demonstrate that the targeted application of salicylic acid and Kitoplus can significantly enhance the medicinal value of Sonchus arvensis L. through the modulation of vegetative growth and stimulation of secondary metabolite synthesis. This approach can be regarded as a novel and environmentally friendly strategy for improving the phytochemical quality of medicinal plants in sustainable production systems.