Background and objectives: Periwinkle (Catharanthus roseus (L.) G. DON) is a valuable medicinal plant from the Apocynaceae family, that contains a wide range of secondary metabolites with diverse applications in the pharmaceutical industry.The content of vincristine and vinblastine economically important metabolite is more important than the yield of the plant Researchers have been paying close attention to these alkaloids because of their high price but low presence in the plant. Biostimulants enriched with amino acids have been widely applied to various vegetable crops, demonstrating significant enhancements in nutrient utilization, overall yield, and the quality of the produce.

Nitrogen is the most essential element needed by the plant, which is involved in all the vital processes of plants. Therefore, the use of nitrogen fertilizers for plant production with optimal yield and quantitative and qualitative characteristics is essential. amino acids play a crucial role in enhancing plant nutrient availability and uptake, thereby improving physiological functions and contributing to overall crop quality and yield. Nitrogen availability plays a crucial role in the vegetative growth of plants and has a significant impact on the accumulation of alkaloids in their leaves and roots.

Higher contents of phenols and antioxidants activity in biostimulant amino acid-treated plants can be attributed partially to the indigenous positive effect of amino acids in plant processes. Amino acids are precursors of wide range metabolism secondary metabolites and is tightly link to protein, hormone, and carbohydrate synthesis

Methodology: The study aimed at investigating the influence of amino acid on the growth traits and phytochemical composition of periwinkle (Catharanthus roseus (L.) G. DON) medicinal plant in Greenhouse Conditions. It utilized a complete randomized design (CRD) with three replications during one consecutive growing season, 2024, in Medicinal herbs garden of Agriculture Research and Training Center, Hamadan. The experiment included five varying level of amino acid including 0, 2.5, 3, 3.5, 4 g-L applied via foliar spraying.

The aerial parts and roots were collected during the blooming period (mid-September). Morphological traits, such as plant height (from the crown to the tip of the main stem) and root length, were measured using a ruler in each replication, and then averaged. The number of flowers and fully grown leaves on each plant was tallied until the end of the growth cycle. To ascertain the fresh and dry weight of the above-ground portions, the plants were severed from the soil surface and weighed using a digital scale. Subsequently, the plants underwent a 48-hour drying process in an oven set at 70 degrees Celsius. Once their weight had reached a stable point, they were weighed again. Also the investigated, physiological and phytochemical traits included leaf chlorophyll, flavonoid, total phenol, antioxidant activity and tryptophan amino acid in the leaves of this plant.

Results: The effect of amino acid on the number of flowers and leaves, root and stem length, fresh and dry weight, chlorophyll a, b and chlorophyll content, phenol, flavonoid, and leaf antioxidant activity (DPPH) was significant (p < 0.01). With increasing levels of amino acid, the amount of all traits increased. The findings demonstrated that the amino acid treatment (3.5 g-L) yielded the highest average count of flowers and leaves, stem and root length, fresh and dry weight of the entire plant, chlorophyll a, antioxidant activity, leaf flavonoids and tryptophan amino acid. The largest quantity of chlorophyll b, total chlorophyll, leaf phenol were associated with the (4 g_L (treatment. Conversely, the control treatment displayed the lowest results. The mean comparison of the treatments showed that the highest number of flowers (8.50) and leaves (63.80), stem length (18.97 cm) and root length (12.98 cm), fresh weight (16.91 g) and total dry weight of the plant (3.36 g), antioxidant activity (48.21% DPPH SC),total flavonoid content (0.561 mg DW-1) and tryptophan amino acid (0.56 mg DW-1) were observed at the level of 3.5 g-L of amino acid.

Conclusion: Overall, applying (3.5 g_L) of amino acid showed the most significant effect on the growth traits and phytochemical composition of periwinkle medicinal plant in Greenhouse Conditions. Based on the results obtained, it can be concluded that the continuous application of amino acid as an organic fertilizer had a positive effect on the growth traits and phytochemical composition of periwinkle. This study's results suggest that applying amino acid biostimulants to leaves enhances periwinkle yields and holds significant promise for cultivating premium periwinkles with enhanced attributes, notably elevated levels of total phenolics, flavonoids, and antioxidant capacity.